Technical Papers

We provide technical papers in these categories. Click once to expand a category, then hold your mouse over a paper to see the abstract.

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Control Structure Interaction
- Adaptive Spatial Filtering for Aeroservoelastic Response Suppression
  Aeroservoelastic interactions have become a critical design consideration for meeting the increasingly demanding performance requirements imposed on aircraft designs. The traditional approach for establishing flight control system (FCS) stability margins is to use notch-filtering; this introduces phase lag that limits the FCS bandwidth, and may not be robust to changes in flight condition, aircraft configuration, or damage. We propose an adaptive spatial filtering approach that makes use of additional sensors to reduce aeroelastic interactions with the flight control system, allowing for increased control bandwidth, and greater robustness. A simple, computationally-efficient, and robust adaptation algorithm is used to optimize the spatial filtering as the system changes. A Lyapunov function is used to prove stability of the combined FCS and adaptive filter.
Deployables - Thin-Film Structures
- Dynamic Behavior of Thin Film Membrane Strips
  In this paper, results of work addressing the dynamic characterization of thin membrane strips is presented. While apparently ‘simple’, the behavior of such strips offers a wide range of interesting effects to be independently modeled and significant challenges for testing. It is felt that these results will offer a valuable insight into the considerable complexities involved with modeling and testing thin film membrane structures, both strips, and by extension, more complicated structures. The body of the paper concentrates first on analytical and numerical predictive techniques for membrane strips, followed by detailed discussions of the testing of thin film strips and the associated already developed test apparatus.
- Experimentally Characterizing the Dynamics of 0.5m+ Diameter Doubly Curved Shells Made From Thin Films
  The purpose of this paper is to present the results of a series of measurements made to experimentally determine the dynamics of doubly curved very thin film shells. Results for in-air tests of 0.5 m and 1.0m diameter 51micron thick shells are presented first, followed by details of in-vacuum and in-nitrogen tests of the 1.0m diameter sample.
- Modeling the Dynamics of Large Diameter Doubly Curved Shells Made From Thin-Films
  In this paper, the derivation of a relatively straightforward set of equations to predict the basic structural dynamic behavior of thin-film single surface shells was reviewed. These analytical results were shown to be close to numerical results for a variety of configurations with a variety of numerical modeling packages. The dynamic behavior of single surface thinfilm shells of realistic size and shape are primarily dominated by the shell terms. Fundamental modes, while sensitive to boundary conditions are expected to be relatively high given the flimsy nature of the base material. Once modal dynamics start, mode density is predicted to be dense and frequency separation between modes minimal. These general conclusions were independently confirmed with multiple different finite element packages including NASTRAN, STAGS, and ANSYS.
- Experimental Issues that Impact In-Vaccum Dynamic Characterization of Thin Film Membranes
  Results of work addressing methods to dynamically characterize thin membrane surfaces with discrete attachment points are presented. While apparently theoretically ‘simple’, the behavior of real membrane flats offers a wide range of challenges that complicate test/model validation. Differences between standard theory and testing include point instead of uniformly distributed edge loads, variations in achievable preload at corners, wrinkling, local defects, variations in preload caused by gravity induced sag, etc. This paper provides some information on additional test challenges such as changes induced by variations in vacuum level causing the expected changes in dynamic response amplitude, ambient temperature changes causing different rates of thermal induced contraction in the membrane versus tensioning structure, and absorbed moisture status of the base material. The latter two effects have been observed to cause significant variation in test results, and hence, unless carefully measured (or preferably managed) could cause significant difficulty in predicting the right on-orbit behavior.
- Demonstration of Dynamic Tailoring for Gossamer Structures
  A structure for demonstrating dynamic tailoring for gossamer structures is presented. This demonstration structure consists of a torus with stiffening “dimples” and an attached central membrane. As a performance metric for evaluating the tailoring approaches, three reaction mass actuators were used to drive the torus while the velocity of the central membrane was measured with a laser vibrometer. Four types of dynamic tailoring were demonstrated. Inert masses were attached to the torus at the disturbance input locations and random locations to increase the effective impedance of the structure. Lightweight tuned mass dampers were attached to the torus in an attempt to reduce the vibration in a narrowband region. Experimental data is presented for each of the dynamic tailoring approaches.
- Shape Memory Alloy Deployment of Membrane Mirrors for Spaceborne Telescopes
  The objectives of this research were to develop and refine a shape memory alloy (SMA) modeling approach, the martensite twin reorientation (MTR) subroutine, to predict the transient response of a spaceborne membrane optic SMA deployment actuator spine system. In concert with a commercial finite element solver, this application supports multidimensional, temperature-displacement transient predictions of the shape memory effect (SME) exhibited by SMAs through implementation of a phenomenological constitutive law. The scope of this study was to model the relation of input power magnitude and waveform to stress fields, reaction forces, and thermal fields for the figure acquisition of a gossamer reflector. Of particular interest is the stress and thermal field history of the polymeric membrane concentrator through the deployment to prevent mechanical and thermal failure. Thermal authority over the concentrator was found locally limited to the spine interface. Technology addressed through this thesis research is intended to foster and mature successive large, launch-packaged space vehicle programs.
- Development of Deployable Elastic Composite Shape Memory Alloy Reinforced (DECSMAR) Structures
  The objective of this research is to develop a novel, self-deployable truss architecture composed of carbon fiber reinforced plastic (CFRP) tape-spring elements and embedded shape memory alloy (SMA) flexures; this particular structural system is referred to as deployable elastic composite shape memory alloy reinforced (DECSMAR) and is representative of a concentrated, material deformation based deployable architecture. The scope of this study encompasses applying principles of rational boom design relevant to all deployable structures, first to define the design space of the individual CFRP tape-spring element, then to conduct an exercise for a point design of a 180 mm radius DECSMAR boom with correlation to experimental analysis, and finally to explore performance implications of scaling the truss radius. Characterizing the enhancement the SMA flexure features purchase and design issues for package envelop optimization are pertinent to individual CFRP tape-spring element and system wide design. Technology addressed through this research is intended to foster and mature successive large, launchpackaged concentrated strain structures.
- Solar Sail Topology Variations Due to On-Orbit Thermal Effects
  The objective of this research was to predict the influence of non-uniform temperature distribution on solar sail topology and the effect of such topology variations on sail performance (thrust, torque). Specifically considered were the thermal effects due to on-orbit attitude control maneuvers. Such maneuvers are expected to advance the sail to a position off-normal to the sun by as much as 35 degrees; a solar sail initially deformed by typical pre-tension and solar pressure loads may suffer significant thermally induced strains due to the non-uniform heating caused by these maneuvers. This on-orbit scenario was investigated through development of an automated analytical shape model that iterates many times between sail shape and sail temperature distribution before converging on a final coupled thermal structural affected sail topology.
- Experimental and Numerical Analysis of a DECSMAR Structure’s Deployment and Deployed Performance
  The objective of this research is to analyze the deployment and deployed performance of a recently developed, self-deployable truss architecture composed of carbon fiber reinforced plastic (CFRP) tape-spring elements and embedded shape memory alloy (SMA) flexures; this particular structural system is referred to as deployable elastic composite shape memory alloy reinforced (DECSMAR) and is representative of a concentrated, material deformation based deployable architecture. The scope of this study encompasses numerically and experimentally mapping the force profile through the deployment path of a 450 mm radius DECSMAR boom and then to numerically determine the effective continuum, deployed stiffness and strength properties, i.e., bending, shear, torsion, and axial moduli with corresponding critical loads, correlated to experimental analysis, of an equivalent radius, five-bay DECSMAR boom. Of particular interest were deleterious effects of the deployment sequencer on the force profile, the deployed performance attributable to the SMA flexure features, and consequences of flattening longeron ends to buy packaging efficiency.
- A Test Method to Assess the Foldability of Flexible Structural Materials
  A coupon level comparative test method was developed to assess the foldability of thin flexible materials used in deployable structures. The subject materials support tensile and compressive loads; they are not cloth-like. The non-destructive method consists of a tensile stiffness test and a compressive buckling test and reveals changes in coupon properties that could result from locally extreme strains incurred during folding. The test is intended to provide a standardized means to compare changes in material systems or fabrication processes as flexible material development efforts continue. The method was applied to nine identical rigidizable composite coupons folded ten times. Five of the coupons were folded to a 4 mm radius and four were folded to a 2 mm radius. The results did not reveal a measurable change in coupon behavior from the pre-folded state.
- Experimental and Numerical Identification of a Monolithic Articulated Concentrated Strain Elastic Structure’s (MACSES’s) Properties
  The objective of this research is to identify the effective continuum properties of a recently developed, deployable hierarchical truss architecture composed of carbon fiber reinforced plastic (CFRP) tubes and CFRP tape-spring hinge elements with embedded shape memory alloy (SMA) flexures; this particular structural system is referred to as monolithic articulated concentrated strain elastic structure (MACSES) and is representative of a concentrated, material deformation based deployable architecture. The scope of this study encompasses numerically and experimentally identifying the deployed stiffness and strength performance, i.e., bending, shear, torsion, and axial moduli with corresponding critical loads, of a 540 mm radius boom. Bending modulus to linear mass ratio was measured at 145 kNm3kg-1. Of particular interest were the sensitivity of joint composition to global properties and the acceptability of discontinuous load-paths. Developmental aspects of the MACSES architecture are reported in a preceding manuscript.
- Synchronous Deployed Solar Sail Subsystem Design Concept
  A solar sail concept has been developed from a common spiral fold pattern in order to enable a simultaneous mast and sail deployment. This novel concept utilizes the stored strain energy in a series of elastic spar members to enforce proper folding kinematics rather than relying on bulky mechanical joints. The critical inner and outer spar networks are secured to four elastically extendible masts anchored to a central drum. Deployment of the solar sail system is actuated by rotating the central drum around which the masts, spars, and film are wrapped. Tensioned radial cords deterministically unfold the membrane film under the authority of the resilient, spring-like spar members. Proper elastic behavior of the spars is an important facet to this design, and thus a significant effort was dedicated their development. This compact ground demonstration concept includes about 7.5 m2 of reflective membrane film for useful propulsion. Features of this robust, lightweight membrane structure may prove valuable to reducing mass and increasing deployment reliability of other planar subsystems such as sun shades, solar arrays, radiators, or antenna arrays.
- Deployable Booms and Antennas Using Bi-Stable Tape-Springs
  Deployable Booms and Antennas Using Bi-Stable Tape-Springs
- Design and Analysis of a Meter-Class CubeSat Boom
  Design and Analysis of a Meter-Class CubeSat Boom with a Motor-less Deployment by Bi-Stable Tape Springs
ESPA and Other Adapters
- Development and Structural Qualification of the EELV Secondary Payload Adapter (ESPA)
  The United States Air Force Research Lab is currently examining options to launch small satellites (<200 kg) more economically and efficiently. Most of the viable options include foreign sources, such as the Ariane launch vehicle, that are not available to Department of Defense (DoD) launches. This classification of satellite is quickly becoming a government and industry mainstay due to their ability to inexpensively demonstrate new technology, test prototype operational hardware, as well as perform space experimentation.
- ESPA: EELV Secondary Payload Adapter with Whole-Spacecraft Isolation for Primary and Secondary Payloads
  ESPA, the Secondary Payload Adapter for Evolved Expendable Launch Vehicles, addresses two of the major problems currently facing the launch industry:the vibration environment of launch vehicles, and the high cost of putting satellites into orbit. (1) During the 1990s, billions of dollars have been lost due to satellite malfunctions, resulting in total or partial mission failure, which can be directly attributed to vibration loads experienced by payloads during launch. Flight data from several recent launches have shown that whole-spacecraft launch isolation is an excellent solution to this problem. (2) Despite growing worldwide interest in small satellites, launch costs continue to hinder the full exploitation of small satellite technology. Many small satellite users are faced with shrinking budgets, limiting the scope of what can be considered an “affordable” launch opportunity.
- ESPA from Concept to Flight Hardware
  In August of 2003, flight hardware for the EELV Secondary Payload Adapter (ESPA) will be delivered to Cape Canaveral. The ESPA Ring and five spacecraft will launch on an Air Force Delta IV mission scheduled for March 2006. This flight, STP-1, will be the maiden voyage for a payload adapter that was conceived by the Air Force in 1995 to provide a secondary payload capability for Evolved Expendable Launch Vehicles (EELVs). ESPA was designed and flight qualified during the period of 1999 to 2002, for use with both Atlas V and Delta IV launch vehicles. The ESPA Ring provides an American counterpart to the Ariane adapter that has been exploited for European launches since 1990. It is now feasible for up to six secondary spacecraft to be placed in orbit whenever a Delta IV Medium or Atlas V (400 or 500 series) launch is configured with excess payload capacity. Since the majority of EELV launches in the foreseeable future have significant excess capacity, the small satellite community has a significant new option for access to space.
- EELV Secondary Payload Adapter (ESPA) Static Qualification Tests
  The U.S. Air Force Research Lab is examining options to launch small satellites (<200 kg e.g., 440 lb) more efficiently. This class of satellite is quickly becoming a mainstay due to its ability to inexpensively demonstrate new technology and prototype operational hardware. Most existing launch options, such as the Ariane launch vehicle, include foreign sources that are not available to Department of Defense (DoD) launches. A secondary payload adapter has been developed to allow small satellites to be launched with the primary payload. This is accomplished by using an adapter on the upcoming Evolved Expendable Launch Vehicles (EELV), DoD medium lift vehicles; more specifically, the Boeing Delta IV and the Lockheed Martin Atlas V launch vehicles. This adapter, known as the EELV Secondary Payload Adapter (ESPA), will take advantage of the primary payload’s unused volume and mass margins. As with any aerospace structure, the ESPA was subjected to a rigorous test program. Details of the static qualification tests will be presented as a four part series. This first installment will serve as the introduction.
- CASPAR: Low-Cost, Dual-Manifest Payload Adapter for Minotaur IV
  The Minotaur IV Launch Vehicle is being developed by the Air Force Rocket Systems Launch Program (RSLP) to utilize excess Peacekeeper missile motors and provide low-cost government launches to Low Earth Orbit (LEO). This vehicle uses three Peacekeeper stages, an Orion 38 motor, and avionics from heritage Minotaur I. Nominal capability for Minotaur IV is ≈4000 lbm to LEO. The fly-away cost is just over $20 million. The Composite Adapter for Shared PAyload Rides (CASPAR) Multi-Payload Adapter (MPA) will enable a Minotaur IV to launch two large satellites (1000-2000 lbm) for about $10 million each. The CASPAR MPA is being designed for projected Minotaur IV launch load environments with design objectives of light weight, integrated vibration isolation, low shock, and modularity. Low-shock separation systems are integrated for MPA and satellite separation events. Vibration isolation systems protect the payloads from the dynamic environment of the Peacekeeper motor stack, and isolation tuning will enable a range of payloads and facilitate modular designs.
- Design, Analysis, and Testing of the CASPAR Multi-Payload Adapter Bonded Composite Joint
  The CASPAR multi-payload adapter will provide a shared ride access to space for two 1,500-lbm satellites or up to four 400-lbm (ESPA-class) satellites. The adapter consists of two symmetrical carbon fiber composite shells joined by a Latching Lightband separation system that is integrated into its mid-plane. CASPAR co-manifests the payloads by placing one of the satellites, or small satellite pairs, atop the MPA while the other is stowed inside. The separation system is required to release the adapter’s upper shell, thus enabling the stowed satellite(s) to be expelled into orbit. The separation system interfaces with the composite shells via a bonded double lap shear epoxy joint. The design, analysis, and testing of the bonded joint is the focus of this paper.
- Adapter Ring for Small Satellites on Responsive Launch Vehicles
  A modular multi-payload adapter for small launch vehicles based on the ESPA ring is presented. The “Small Launch ESPA” is a scaled version of the EELV Secondary Payload Adapter (ESPA). This adapter ring for small launch vehicles features a 38.8-inch-diameter primary interface and is sized for Minotaur IV, Falcon 1e, Taurus, and Delta II. Modular features of the ring provide flexibility for a range of missions. Small-launch ESPA accommodates radial mounting for up to six low-profile small satellites on standard 8- or 15-inch-diameter interfaces. This “little ESPA” is particularly suited for the satellite class between CubeSats and “ESPA-class” spacecraft (180 kg). Launches with the ring on Minotaur IV and Delta II can accommodate secondary payloads up to 100 kg. Benefits to Operational Responsive Space (ORS) include simultaneous launch of several small payloads or testing of new technologies. Adapter rings would provide launch infrastructure for quick-turn missions. The paper describes launch stack configurations for two representative launch vehicles including payload volumes available based on fairing dynamic envelopes.
- CASPAR Payload Adapter Structural Failure Test Results And Analytical Predictions
  The CASPAR payload adapter was tested to structural failure at the Air Force Research Laboratory as part of a larger program intended to determine the ultimate capacity of three large composite structures and to evaluate failure analysis methods. This paper discusses the failure test design and results with comparison to analytical predictions.
- Structural Models and Dynamic Measurements of Satellite Launch Adapter Structures
  CASPAR, the Composite Adapter for Shared Payload Rides, is a Multi-Payload Adapter (MPA) with whole-spacecraft vibration isolation that was developed for the Minotaur IV Launch Vehicle. The CASPAR MPA can accommodate two 1500-lb satellites or four 400-lb satellites on a Minotaur IV launch. The CASPAR ShockRing is a satellite vibration-isolation system that has the form of a shock isolator with the functional capability to mitigate both vibration and shock loads. Both of these CASPAR structures were flight-qualified by test in 2008. Dynamic measurements of the MPA and the ShockRing, before and after testing, validated the models and the qualification of both structures. This paper documents finite element models and dynamic measurements of the CASPAR MPA and ShockRing structures. A brief summary of the qualification tests for both structures is included.
- Low-Cost Flat Plate Adapters for Dual Primary Payload Missions
  A dual payload flat plate adapter (FPA) supports co-manifested primary payloads allowing one rocket launch to support two satellites. The payloads may be part of a single mission requiring a tandem satellite launch or the payloads may be piggybacked on the same launch vehicle to maximize access to space for small satellites. Multi-payload adapters ensure launch vehicle excess space lift capacity is not wasted, providing small satellite manufacturers lower cost to orbit. Requirements for FPAs include designs that are optimized for both stiffness and weight as well as being low in design and manufacturing cost. This paper details two FPAs through design, manufacture, test and delivery.
- Space Access for Small Satellites on Rideshare Missions with ESPA and ESPA-Derived Payload Adapters
  Rideshare reduces launch costs while enabling access to space for the fast-growing small satellite community. Combinations of spacecraft are launched as secondary and tertiary payloads or as co-manifested primary payloads. Payload adapters designed for the launch environment are used to package the spacecraft on the launch vehicle. This paper reviews a subset of the adapter hardware with flight heritage and some new adapters in development.
Hexapods and Octopods
- Comparison of Multi-Axis Active Vibration Isolation Architechtures
  This paper compares two mechanical architectures useful for active vibration isolation and discusses factors which limit performance in each case. Nondimensional equations of motion for two types of single-axis isolators are developed. Parallel and series active-passive combinations are considered. Isolator models are mated to six degree-of-freedom representations of rigid body payload dynamics. Multi-Axis isolator transmissibilities and open loop transfer functions are examined for each case. The impact of actuator/sensor dynamics and reduced-order control on the achievable performance is studied. Additional dynamics, such as sensor signal conditioning and phase lag due to controller discretization, introduce constraints on the isolator design and further limit the achievable performance. The effects of sensor noise and controller simplification are demonstrated on a model of a three-axis isolation system.
- Multi-Degree of Freedom Parallel Actuation System Architectures for Motion Control
  This paper discusses multi-degree of freedom parallel actuator systems that can provide motion control authority, i.e. systems that provide pointing/positioning and/or active vibration isolation functionality using parallel, rather than serial/staged system level configurations. Such systems, commonly referred to as bi-, tri-, hexa- or octo-pods, are becoming more prevalent in aerospace and other high end applications. The sophistication of such systems has advanced to make them competitive with the more traditional systems for certain applications, especially where both pointing/positioning and vibration isolation are simultaneously required. In this paper, important considerations for specifying such a system are first discussed. Next, the way these specifications cascade down into system level and sub-system level design aspects/component selection is reviewed. The resolution of the resulting demands and issues are then illustrated with examples that cover a range from extremely precise to large stroke applications for payloads ranging from 10 to 7200 lbs, but the principles and systems discussed are capable of being extended to larger payloads.
- Compact Lightweight Six-Axis Point-and-Hold Positioning System
  A hexapod capable of precision positioning is described. The differences between serial and parallel motion control are presented, and the potential advantages of parallel systems realized as hexapods are highlighted. Actuation options for positioning hexapods are considered in light of a requirement for a high ratio of range to resolution and a need for zero power hold. For positioning of smaller payloads, piezoelectric-based step-and-repeat actuation becomes attractive. The merits of existing and new piezoelectric step-and-repeat actuators are evaluated. A point-and-hold hexapod designated PH1, and its performance, is described, along with several areas identified for possible design improvement. This motivates the development of advanced struts using similar actuation technology. Test results are presented, and a new hexapod, the PH2, is described. This system includes encoder-based feedback control of leg lengths, and a complete software-based user interface and control system. Hexapod test results and performance measurements are presented, and planned future enhancements are described.
- Hexapods for Precision Motion and Vibration Control
  Parallel kinematic manipulators offer several advantages over their serial counterparts for certain applications. Among the advantages are greater load carrying capacity, higher stiffness, reduced sensitivity to certain errors, and built-in redundancy. The hexapod is one form of parallel manipulator that is used increasingly in manufacturing, inspection and research. This paper highlights features of several hexapods developed for different applications. The ultimate hexapod would provide large motions for massive payloads in up to six degrees of freedom with high accuracy, resolution and repeatability. In practice, a range-to-resolution specification much greater than 60 dB is difficult to achieve, and most likely can be realized only at the expense of substantially reduced bandwidth and slew rates. High load carrying capacity restricts options in design of strut ends, also compromising precision. The current paper explores the limits of practical hexapods, through an overview of several existing systems and a discussion of important design issues.
- Precision Pointing Parallel Manipulator Design for Asymetric Geometries and Cryogenic Vacuum
  Parallel positioning systems such as hexapods are increasingly called upon for precise six degree of freedom positioning of a payload with asymmetric geometry or in harsh environments. For example, optical payloads may require keep-out zones to eliminate optical path obstructions and require significant deviation from the standard symmetric Stewart Platform geometry. The motion platform is no longer nominally coplanar with the base platform, nor aligned with the gravitational vector. Strut lengths and load forces may vary by an order of magnitude and can be subjected to manufacturing, deployment, and operational disturbances. Additionally, accurate positioning for high vacuum and cryogenic temperature conditions requires meticulous care to prevent material outgassing and seizing of joints and bearings. The work reported in this paper pertains to the selection and design of actuators, joints, feedback sensors, and cables for use in precision pointing hexapods with odd geometries and space simulation environments.
- Precision, Range, Bandwidth and Other Tradeoffs in Hexapods with Application to Large Ground Based Telescopes
  Hexapods can be an effective means of positioning optics of all sizes, including those within large ground-based telescopes. A hexapod is often a convenient geometry when multiple axes of positioning are required. The paper reviews several small and mid-sized hexapods built for different applications, and emphasizes experience with a threemeter-diameter unit built to position a large optical component. The discussion highlights design tradeoffs in precision, including repeatability, resolution and accuracy, range in multiple axes, and bandwidth of operation, and addresses test and verification of performance. The paper concludes with a discussion and presentation of hexapod concepts for secondary mirror positioning for Thirty Meter Telescope and Cornell Caltech Atacama Telescope.
- Simulation, Design, and Testing of a High-Performance Multi-Axis Hexapod for Vibration Isolation
  This paper describes the simulation, design, and testing of a high-performance six degree-of-freedom hexapod for the purpose of isolating sensitive payloads from low-frequency vibrations. Design criteria required the hexapod to support a generic payload up to 500 lb with an isolation plunge frequency of approximately 1 Hz. Simulations were performed using Matlab in order to determine the optimum geometry of the base and platform structures in order to provide the best combination of translation-rotation uncoupling, frequency spread, plunge frequency, and jitter. Based on these simulation results, hexapod base and platform structures were designed and fabricated based on a 50 inch-diameter platform size. All of the accumulators and pneumatic hardware were embedded into the base structure to allow for a totally contained system. Modal testing of the hexapod was performed in order to verify the modes predicted by the model.
- Practical Considerations of Joint Friction and Backlash in Large Ground-Based Telescope Secondary Optic Positioning Systems
  Secondary mirrors and lenses in several planned ground-based telescopes have masses on the order of 5000 kg and require a positioning system that is repeatable to one-tenth the pixel size of the optical sensors, nominally 10 micrometers or less. Hexapods, or Stewart Platforms, are frequently integrated into the support structure as six degree of freedom (DOF) parallel positioning and alignment systems. These systems are limited in resolution by friction in the 36 kinematic DOF necessary for properly constrained motion of the platform. The 30 passive DOF introduce unwanted friction and/or backlash. Backlash is generally unacceptable and elimination requires significant preloading of the joints, which in turn increases joint friction. This paper will: review various joint types; examine the backlash and friction tradeoffs involved in selecting joint type; compare with experimental data and previously published results; present methods for modeling both static and dynamic effects of friction; and suggest recommendations for general positioning system design. Considerations for both equatorial and altitude-azimuth telescopes will be discussed.
- Six Degrees of Freedom, Sub-Micrometer Positioning System for Secondary Mirrors
  Six Degrees of Freedom, Sub-Micrometer Positioning System for Secondary Mirrors
Optical Jitter Control
- Evaluation of Airborne Laser Beam Jitter Using Structural Dynamics Computer Codes and Control System Simulations
  Beam stabilization for the Airborne Laser Laboratory (ALL) is accomplished by inertially stabilizing an annular reference mirror attached to the beam expanding telescope on the Airborne Pointing and Tracking System (APT). The analysis reported here documents the calculation of the residual beam jitter due to vibrations of the ALL and required the use of the NASTRAN finite element computer code to determine the motion of the various optical elements in the beam expanding telescope. These motions were then combined in an analytical expression to calculate the optical path motion for both the high energy laser and autoalignment beams. The final step was the incorporation in the frequency domain of the effects of the two servo mechanisms that stabilize the telescope to an inertial reference and drive the beam steering mirror to align the autoalignment beam and the high energy laser beam to the annular reference mirror.
- Design, Installation, and Testing of a Damping Treatment for Jitter Reduction on a High-Power Optical Bench
  Passive damping hardware has been fabricated, installed, and tested on a large optical system required to meet a stringent RMS specification for closed-loop residual jitter due to ground and coolant flow excitations. Over the course of the study, constrained layer treatments, tuned-mass dampers, and link dampers were analyzed. It was shown through analysis that one type of treatment alone was not sufficient to solve the problem due to the number of harmful modes spread over a relatively wide frequency range. The final solution incorporated constrained layer damping treatments on an interface component between the mirrors and their mounts, link dampers between selected locations on the optical bench, and constrained layer treatments on some mirror support plates. Tests were performed before and after the application of the damping hardware to demonstrate its effectiveness and applicability to this class of problem. The primary focus of this paper is the dynamic testing of the optical system and design, fabrication, installation, and qualification of the passive damping hardware.
- Experimental Adaptive Beam Train Control with Multiple Beams and Disturbance Paths
  (Abstract available only) Acoustic and structure-borne disturbances can cause significant optical jitter in laser-based tracking, directed energy, and communications applications. Optical jitter negatively affects the accuracy and intensity of an outgoing laser beam on its target and is a considerable factor in mission ability. One method for mitigating the effect of these disturbances is the application of an Adaptive Filtering and Disturbance Feedforward (AFDF) technique. In this paper we present an adaptive control strategy using the filtered-X LMS algorithm to improve upon the performance of a traditional Type II servo loop in the presence of multiple beam path optical test bed. A system with two optical beam trains and disturbance environmental conditions is examined. The test bed consists of two beam paths, fast steering mirrors for disturbance and control inputs, and position sensing detectors (PSD) for use as reference and error measurements. A pseudo-reference is constructed using measurements from both a PSD and the fast steering mirror used for control. A second PSD is used as a "truth" measurement to gauge how well the control system improves line-of-sight jitter exiting the beam train. Real-time control experiments were carried out comparing the performance of the controller using AFDF to that of a traditional servo control system design. The results show that AFDF greatly reduces the error signal on which the servo loop is closed. Over a factor of two reduction in rms jitter at the "truth" sensor was achieved as compared to that achieved with the traditional design. For optical beam train applications, a factor of two improvement is significant and can be mission or task enabling. The results show that AFDF's ability to respond to and mitigate the effects of a complex broadband disturbance environment offers a more efficient and higher performance approach than conventional control system designs.
- Damping Treatment for Jitter Reduction on a High Power Optical Bench
  As part of a High Energy Laser program, a large optical laser system is required to meet a stringent RMS specification for residual jitter. Using MSC/NASTRAN, the optical jitter due to ground and coolant-flow excitations was predicted as a combination of the dynamic motions of several optics. The modal strain energy method was used both in identifying the best candidate locations for damping treatments as well as predicting damping levels. The final solution incorporated constrained layer damping treatments on an interface component between the mirrors and their mounts and link dampers between selected locations on the optical bench.
- Adaptive Feedforward Control for Actively Isolated Spacecraft Platforms
  Active vibration isolation systems are being considered to improve the performance of spacecraft instruments and sensors. Because of uncertainties inherent in on-orbit operation, adaptive control strategies and algorithms have relevance to these systems. In this paper, analysis of the algorithms, numerical simulation, and laboratory test data are used to evaluate adaptive feedforward control. Of particular interest are performance characteristics and limitations of the filtered-x LMS (FXLMS) algorithm and its finite impulse response (FIR) filter implementation. Combination feedback/feedforward control and the Augmented Error algorithm are two means investigated to extend the capabilities of FXLMS by desensitizing the algorithm to the specific dynamics of the plant. Several experiments were conducted on a laboratory testbed which serves as the prototype for a planned active vibration isolation flight demonstration.
- Satellite Ultraquiet Isolation Technology Experiment (SUITE): Electromechanical Subsystems
  Spacecraft carry instruments and sensors that gather information from distant points, for example, from the Earth’s surface several hundred kilometers away. Small vibrations on the spacecraft can reduce instrument effectiveness significantly. Vibration isolation systems are one means of minimizing the jitter. This paper describes the Satellite Ultraquiet Isolation Technology Experiment (SUITE). SUITE is a piezoelectric-based technology demonstration scheduled to fly in 2000 on PICOSat, a microsatellite fabricated by Surrey Satellite Technology, Ltd. The paper details the features of SUITE, with particular emphasis on the active hexapod assembly. A description of the PICOSat spacecraft begins the paper. Experiment goals are listed. The mechanical and electromechanical construction of the SUITE hexapod assembly is described, including the piezoelectric actuators, motion sensors, and electromagnetic actuators. The data control system is also described. The main features of the software used for real-time control and the supporting Matlab software used for control system development and data processing are summarized.
- Active Suppression of Acoustically Induced Jitter for the Airborne Laser
  The Airborne Laser (ABL) system has extremely tight jitter requirements. Acoustic disturbances are a significant jitter source. The first choice for mitigation will be passive approaches, such as acoustic blankets. There is, however, some uncertainty whether these approaches will provide sufficient attenuation and there is concern about the weight of these approaches. A testbed that captured the fundamental physics of the ABL acoustically induced optical jitter problem was developed and consists of a flexure-mounted mirror exposed to an acoustic field that is generated outside a beam tube; then it propagates within the tube. Both feedback and adaptive feedforward control topologies were implemented on the testbed using either of two actuators (a fast steering mirror and a secondary acoustic speaker located near the precision mirror), and a variety of sensors. This paper summarizes the results from these control topologies for reducing the acoustically induced jitter with some control topologies achieving in excess of 40 dB jitter reduction at a single frequency. This work was performed under an SBIR Phase I funded by the Air Force Research Laboratory Space Vehicles Directorate (AFRL/VS).
- Satellite Ultraquiet Isolation Technology Experiment (SUITE)
  An experimental active vibration isolation called Satellite Ultraquiet Isolation Technology Experiment (SUITE) is described in detail. SUITE is a piezoelectricbased technology demonstration scheduled to fly in 2000 or 2001 on board the PICOSat spacecraft. SUITE is designed to show that the effect of small vibrations on spacecraft instrument effectiveness can be reduced significantly. Control from the ground station is planned for the first year after launch. A description of the PICOSat spacecraft and the other considerations influencing the development of the flight hardware begins the paper. Experiment goals are listed. The mechanical and electromechanical construction of the SUITE hexapod assembly is described, including the piezoelectric actuators, motion sensors, and electromagnetic actuators. The data control system is also described, including the digital signal processor and spacecraft communication. The main features of the software used for real-time control and the supporting Matlab software used for control system development and data processing are summarized. Initial test results are presented.
- Damping SOFIA: Passive and Active Damping for the Stratospheric Observatory for Infrared Astronomy
  The Stratospheric Observatory For Infrared Astronomy (SOFIA) is being developed by NASA and the German space agency, Deutschen Zentrum f¨ur Luftund Raumfahrt (DLR), with an international contractor team. The 2.5-meter reflecting telescope of SOFIA will be the largest airborne telescope. Flying in an open cavity on a modified 747 aircraft, SOFIA will perform infrared astronomy at 41,000 feet while being buffeted by a 550-mile-per-hour slipstream. SOFIA requires tracking stability of 0.2 arc-seconds, and a 3-axis pointing control model has been used to evaluate the feasibility of achieving this level of stability. The pointing control model shows that increased levels of damping in certain elastic modes of the telescope assembly will help achieve the tracking stability goal and also expand the bandwidth of the attitude controller. This paper describes the preliminary work that has been done to approximate the reduction in image motion yielded by various structure configurations that use reaction masses to attenuate the flexible motions of the telescope structure. Three approaches are considered: passive tuned-mass dampers, active-mass dampers, and attitude control with reactionmass actuators.
- Optimal Sensing Strategy for Adaptive Control of Optical Systems
  Optical jitter degrades the pointing and imaging performance of precision optical systems. When a correlated measurement of the disturbance is available, improved control performance can be attained. In this research, an adaptive optimal sensing strategy for optical systems is proposed. An array of reference sensors makes it possible to estimate the disturbance and model the disturbance-to-reference paths. The least-square algorithm is applied for the disturbance model estimation. A sensor scoring algorithm is then used to select an optimal disturbance reference from the available reference signals. The optimal disturbance reference is comprised of sensors which are well correlated with the disturbance. This disturbance reference is then fed forward and used in an adaptive generalized predictive control design. The proposed technique is applied to an experimental test bed in which an array of accelerometer sensors measures the structural vibration of optical elements. Reduction of the structural vibration is attained using a fast steering mirror. Performance using optimally selected disturbance reference is shown to be better than a system in which a disturbance reference signal is chosen to be the sensor with the lowest score.
- Optimal Sensing/Actuation Strategies for Vibration and Acoustic Control of Optical Systems
  Optical jitter can result in the beam pointing inaccuracy and poor optical system performance. With a correlated measurement of the disturbance, improved control performance can be achieved. In this research, an adaptive optimal sensing strategy for optical systems is proposed. When an array of reference sensors is available, an optimal set of reference sensors that are coupled to modes of interests can be selected. The weighted reference signal from the optimal sensor set is then used in an adaptive control design algorithm. An adaptive generalized predictive control design algorithm combined with the proposed adaptive optimal sensing strategy achieves better performance than the control system using only one of the reference sensors. The overall algorithm is also advantageous in the presence of time-varying or uncertain disturbances. The proposed technique is applied to an experimental test bed in which multiple accelerometer sensors measure the structural vibration of optical elements. Reduction of the structural vibration of optical components is attained using a fast steering mirror which results in a reduction of the corresponding jitter.
- Adaptive Feedforward Controls for Adaptive Optics Systems
  Traditional adaptive optics control systems that use a Shack Hartmann wavefront sensor (SHWS) for zonal wavefront control employ a reconstructor matrix which maps wavefront slopes into actuator errors. Since there are typically more wavefront slope measurements (two per subaperture) than actuators, a pseudo inverse is required for the mapping. An influence function matrix, which maps actuator displacements into slope measurements, is first constructed, using either analytical models or test “poke” data. The reconstructor is the pseudo inverse of this influence function matrix, with appropriate measurement and state weighting. Integral compensation is applied to the estimated actuator errors. The resulting actuator commands drive the deformable mirror (DM) to the phase conjugate of the wavefront disturbance, within the spatial and temporal limits of the DM, SHWS, and servo. The topic of this paper is the application of adaptive feedforward control to wavefront control of atmospheric phase.
- Adaptive Filtering and Feedforward Control for Suppression of Vibration and Jitter
  This paper describes the use of adaptive filtering to control vibration and optical jitter. Adaptive filtering is a class of signal processing techniques developed over the last several decades and applied since to applications ranging from communications to image processing. A series of examples in vibration, motion and jitter control, including cryocoolers, ground-based active optics systems, flight motion simulators, wind turbines and airborne optical beam control systems, illustrates the effectiveness of the adaptive methods. These applications make use of information and signals that originate from system disturbances and minimize the correlations between disturbance information and error and performance measures. The examples incorporate a variety of disturbance types including periodic, multi-tonal, broadband stationary and non-stationary. Control effectiveness with slowly-varying narrowband disturbances originating from cryocoolers can be extraordinary, reaching 60 dB of reduction or rejection. In other cases, performance improvements are only 30-50%, but such reductions effectively complement feedback servo performance.
- Real-Time Optimal Sensing Strategies for Active Control of Optical Systems
  Optical jitter disturbances can be caused by structural motion of optical components due to vibration sources that have internal, external, or air-induced origins. In some applications, offline fixed gain controllers can be used to minimize jitter, but in many applications a real-time adaptive control approach would yield improved optical performance. We would like the capability to adapt in real-time to a system which is timevarying or whose disturbances are non-stationary and hard to predict. A potentially useful tool is a real-time adaptive control method. In this approach, real-time updating of reference sensors is provided to minimize optical jitter. The technique selects an optimal subset of sensors to use as references. The proposed technique is applied to an experimental test bed in which multiple proof-mass actuators generate structural vibrations on a flexible plate. These vibrations are transmitted to an optical mirror mounted on the plate, resulting in optical jitter. Accelerometers mounted on the plate are used to form the set of possible optimal reference sensors. Reduction of the structural vibration of optical components is attained using a fast steering mirror.
- Advanced Technique to Estimate the Number of Uncorrelated Disturbance Sources in Structural Systems
  In many control applications the number of input signals for use as reference sensors in feedforward control is restricted due to limitations. Techniques to identify an optimal subset of reference sensorsnis therefore beneficial in attaining improved closed-loop control performance. This optimal subset is a minimum number of sensors that convey the dynamics important in the performance path. To determine this subset, an estimate of the number of exogenous disturbance sources is first required. The proposed technique for estimating the number is based on the Principal Component Analysis (PCA). This classical statistical method is used in conjunction with Singular Value Decomposition (SVD). In practice, the dominant disturbance is evident but secondary disturbances important for the performance path of interest are not readily apparent. In order to better estimate the number of secondary disturbance sources, the addition of a control signal source to minimize the sensor response due to the dominant signal source is proposed. This improvement results in a better determination of the minimum number of reference sensors required for feedforward control of disturbances to structural systems.
- Optical Mounts for Harsh Environments
  Development and testing of a lightweight-kinematic optical mount with integrated passive vibration-and-shock mitigation technologies and simple/robust optical alignment functionality is presented. Today’s weapon and surveillance systems have optical sensor suites where static and dynamic alignment performance in the presence of harsh operating environments is required. Jitter and alignment stability is particularly challenging for larger optics operating within moving vehicles and aircraft where high shock and significant temperature excursions occur. The design intent is to have the mount be suitable for integration into existing defense and security optical systems while also targeting new commercial and military components. A mount suitable for moderate-sized optics and an integrated disturbance-optical metrology system are described. The mount design has performance enhancements derived from the integration of proven aerospace mechanical vibration and shock mitigation and other various technologies. Optical jitter, alignment, and wave-front performance testing of an eight-inch-aperture optical mount based on this design approach are presented to validate predicted performance improvements over an existing commercial off-the-shelf (COTS) design.
- SOFIA Telescope Modal Survey Test and Test-Model Correlation
  SOFIA Telescope Modal Survey Test and Test-Model Correlation
- Experimental Adaptive Filtering and Disturbance Feedforward Approach for Flexible Beam Train Control with Single Disturbance Path
  (Abstract available only) Laser beams are used in numerous military and commercial environments for applications such as tracking, directed energy weapons, and communications. They are subjected to a variety of acoustic and structure-borne disturbances that cause optical jitter, degrading the accuracy and intensity of the laser beam on the target. When a correlated measurement of the disturbance is available, improved control performance can be attained by employing an Adaptive Filtering and Disturbance Feedforward (AFDF) technique. In this research, an adaptive control stragegy using the filtered-X LMS algorithm is used to mitigate the effect of a broadband disturbance on a multiple beam path optical test bed. The test bed consists of two beam paths, fast steering mirrors for disturbance and control inputs, and position sensing detectors (PSD) for use as reference and error measurements. A pseudo-reference is constructed using mesurements from both a PSD and the fast steering mirror used for control. A second PSD is used as a "truth" measurement to gauge how well the control system improves line-of-sight jitter exiting the beam train. Special consideration was taken in designing the control output and reference filtering as well as the reference signal design. Tapped delay lines with logged-space taps were employed for optimal performance while minimizing computational burden. Experiments were carried out comparing the performance of the controller using AFDF to that of a traditional servo control system design. A factor of two reduction in rms jitter at both the error sensor and "truth" sensor was achieved as compared to that achieved with the traditional design. Up to four to five times reduction in optical jitter was obtained over some frequency ranges of interest. For optical beam train applications, a factor of two improvement is significant and can be mission or task enabling. The results show that AFDF's ability to respond to the actual behavior and disturbance environment instead of an anticipated on offers a more efficient and higher performance approach than conventional control system designs.
- Improved Jitter Control Using a Globalized Control Approach for Laser Relay Systems
  (Abstract available only) Many optically-based systems are sensitive to vibration disturbances from the environment or internal moving parts. These vibrations can cause significant optical jitter, negatively affecting accuracy and intensity of an outgoing laser beam on its target. These affects can be a considerable factor in mission ability or system performance in directed energy, laser-based tracking, and communications applications. In the present research, line-of sight jitter affecting laser relay systems used in directed energy weapons was examined. Unique to these systems is the transmission of vibration between two gimbaled telescopes and an intermediary optical bench on the structure. The relay mirror system examined in this effort is comprised of multiple gimbal mechanisms, electromechanically-driven fast steering mirrors, optical sensors, and an inertially stable platform. The combination of induced vibrations, error sensor noise, and structural resonances of the gimbaled system and optical components contributes to beam jitter. A more globalized control approach was utilized to improve upon the results tupically obtained using independent cascaded control loops. This integrated approach allows for the servos to be more informed by what is happening elsewhere in the system and improved jitter performance is achieved. Key to the improvements was an implementation of the Common Path/Mode (CPCM) approach and adaptive feedforward control. The simulation and experimental results were successful in demonstrating up to 85% and 74% reduction in line-of-sight jitter respectively over that attained using the baseline control system, and nuances of implementing the techniques in a multi-rate processing environment were learned. These results demonstrate that global techniques can be applied to complex optical beam train applications instead of solely relying on independent, cascaded control loops to give excellent reduction in line-of-sight jitter.
Smart Materials and Structures
- Mesoscale Integration of Actuation, Sensing, and Electronic Components
  Active control of physical systems requires sensing, actuation, and a control system connecting the two. Physical integration of the electromechanical components which perform these functions has the potential to reduce the total size, weight, and cost of active systems. Smart materials are well-suited to integration in certain active vibration suppression systems. An intermediate or mesoscale is suggested as the appropriate level for integration leading to near-term development of vibration suppression devices incorporating piezoelectric transducers. Five main areas – actuation, sensing, power amplification, signal conditioning, and control – are needed for an integrated system. The implications of combining each function with the four others are addressed. Actuation is shown to be the main driver for an integrated device incorporating all five functions.
- Passive Damping by Smart Materials: Analysis and Practical Limitations
  This paper examines the use of smart materials for passive damping. Among the smart materials considered are electrorheological and magnetorheological (ER and MR) fluids, piezoelectrics, electrostrictives, magnetostrictives, and shape memory alloys. The specific mechanism exploited for energy dissipation by passive of semi-passive means is described for each material. A distinction is made between internal and external energy dissipation. The external stimuli required for each of the semi-passive mechanisms are noted. Selected examples of damping results with each material are provided. Practical limitations for engineering design and implementation are considered, and recommendations are made for the more promising materials.
- Smart Material Actuator with Long Stroke and High Power Output
  The paper describes an actuator that makes use of high energy density smart materials, specifically piezoelectrics, in a non-standard way. Large displacements are produced, while high force capacity is retained, for a net high power output. Piezoelectrics are combined with a closed hydraulic system that acts as a transmission to convert smart material output to useful mechanical work. The paper reviews basic concepts in hybrid solid-fluid actuation. It then presents the design concept employed in the smart material – hydraulic hybrid technology. The basic design tradeoffs and major technical issues are discussed in the areas of materials, mechanical design, power delivery, electronics and control. This is followed by reviews of piezoelectric properties, subsystem and overall device test approaches, and results. Several prototype devices are presented. Test results quantifying actuator force and velocity are summarized. Potential applications in unmanned aircraft and elsewhere are discussed.
- A Case Study in Passive Piezoceramic, Viscous, and Viscoelastic Damping
  This paper presents a comparison of three approaches for the passive damping of the ASTREX structure. Open loop performance from specified disturbances (thrusters) to specified responses (line-of-sight) is considered. A computational model is used to select important eigenmodes on the basis of their contribution to the line-of-sight error. Important structural components are selected on the basis of their line-of-sight weighted modal strain energy fraction. Damped replacements for these components are designed using piezoceramic, viscous, and viscoelastic damping mechanisms. The components incorporated into the model and the relative improvement in structural damping response are compared.
- Experimental Characterization of Commercially Practical Magnetorheological Fluid Damper Technology
  As technologies for magnetorheological (MR) fluid hardware evolve towards commercial adoption, the appeal for simpler, cost-effective solutions becomes evident. While the skills involved in methods of manufacturing and cost reduction efforts lie with the manufacturing community, practical and cost-effective MR technologies must first exist. As part of a ‘whole approach’ MR solution, the MR damper technology presented in this paper illustrates the development of a fast-response, low-power, cost-effective solution. A competitive ‘whole approach’ active or semi-active MR solution can be viewed as system of: parameter sensing, intelligent control, power delivery, and MR hardware technology. The development of any one component should not successfully evolve without addressing the cost efficiency and commercialization of the other three. This research includes the characterization of a new prototype MR damper. This MR technology, comprising simple, commercial-off-the-shelf (COTS) components where possible, presents an attractive, practical and cost effective component of the ‘whole approach’ MR solution.
- ELITE-3 Active Vibration Isolation Workstation
  This paper describes the development and capabilities of ELITE-3, a product that incorporates piezoelectric actuators to provide ultrastable work surfaces for very high resolution wafer production, metrology, microscopy, and other applications. The electromechanical, electronic, and software/firmware parts of the ELITE-3 active workstation are described, with an emphasis on considerations relating to the piezoelectric transducers. Performance of the system and its relation to the smart materials is discussed. As the floor beneath a vibration-sensitive instrument supported by ELITE-3 moves, piezoelectrics are controlled to minimize the motion of the instrument. A digital signal processor (DSP) determines the appropriate signals to apply to the actuators. A PC-based interface allows reprogramming of control algorithms and resetting of other parameters within the firmware. The modular product allows incorporation of vibration isolator, actuator and sensor modules into original equipment manufacturer (OEM) products. The paper describes the system architecture, overall approach to vibration isolation, and various system components, and summarizes motivations for key design approaches.
- Precision Controlled Actuation and Vibration Isolation Utilizing Magnetorheological (MR) Fluid Technology
  Precision controlled vibration isolation utilizing magnetorheological (MR) fluid technology for potential space optical applications, such as surveillance and directed energy, is addressed. This research includes the design, development and preliminary testing of a semiactive, proof-of-concept, MR vibration isolator. Base disturbances designed to produce payload vibration responses were employed in a single degree-of-freedom test apparatus. The MR vibration isolator served as the load-coupling element between the payload and the base disturbance input. The three-parameter isolator consists of two passive spring elements combined with one MR damping element. The MR damper control algorithm uses relative rate between damper cylinderand piston to dynamically vary the effective coefficient of damping. The result of this technology is ability to tune isolation frequency within a given range. Through intelligent modulation of the damping element alone, dynamic changes in both apparent stiffness and damping of the isolator are achieved. For applications where the ability to vary stiffness and damping would improve pointing accuracy and jitter control, this technology holds great appeal.
- Piezoelectric Direct Drive Servovalve
  A single-stage servovalve using direct piezoelectric actuator drive is described. The single-stage servovalve design offers higher bandwidth than conventional two-stage valves. It takes advantage of the high energy density in piezoelectric materials while addressing the need for internal amplification of stroke. When used alone, the valve can regulate pressure, and when used in combination with a hydraulic output device it forms part of an effective servohydraulic actuator. Development of a direct drive prototype valve is described. Discussion includes design issues related to low stroke smart material actuators such as piezoelectrics. Component and subsystem testing and results are reviewed. Electronic drive and control of the piezoelectric and overall device along with performance in the control of fluid flow is discussed. The value of the new servovalve is shown in the combination of the valve with a hydraulic output device. Data are supplied for this servohydraulic actuator. The new actuator shows promise for a motion simulator application and more generally for motion control at higher bandwidth than is possible with currently available servohydraulics.
- Experimental Validation of a Novel Stictionless Magnetorheological Fluid Isolator
  Magnetorheological (MR) fluid damper design typically constitutes a piston/dashpot configuration. During reciprocation, the fluid is circulated through the device with the generated pressure providing viscous damping. The damper is also intended to accommodate off-axis loading; i.e., rotation moments and lateral loads orthogonal to the axis of operation. Typically two sets of seals, one where the piston shaft enters and exits the device and one between the piston and the cylinder wall, maintain alignment of the damper and seal the fluid from leaking. With MR fluid, these seals can act as sources of non-linear friction effects (stiction) and often possess a shorter lifespan due to the abrasive nature of the ferrous particles in the fluid. Intelligently controlling damping forces must also accommodate the non-linear stiction behavior, which degrades performance. A novel MR fluid isolator was designed, fabricated and tested that directly addresses these concerns. The goal of this research was the development of a stiction-free MR isolator whose damping force can be predicted and precisely controlled.
- Design and Testing of Piezoelectric-Hydraulic Actuators
  This paper describes design methodologies for construction of an actuator that uses smart materials to provide hydraulic fluid power. In the class of actuators described, hydraulic fluid decouples the operating frequency of the output cylinder from the drive frequency of the piezoelectric or other smart material. This decoupling allows the piezoelectric to be driven at high frequency, to extract the maximum amount of energy from the material, and the hydraulic cylinder to be driven at low frequencies to provide long stroke. However, due to fluid compressibility and structural compliance, the fundamental impedance match between the fluid and the piezoelectric make it difficult to convert energy from the piezoelectric into pressurized hydraulic fluid flow. The basic design tradeoffs and major technical issues are discussed in the areas of materials, mechanical design, and fluid-mechanical interface. Prototype devices and component measurements are presented. Test methods are described, and test results quantifying pump pressure and flow, and actuator force and velocity are summarized. The series of tests show the potential of these devices for high force long stroke devices powered by smart materials.
- Application of Smart Material-Hydraulic Actuators
  The application of a new class of actuators is considered. The actuators under development combine a high energy density smart material, specifically a piezoelectric material, with internal servohydraulic components. Large displacement outputs are produced, while the high force capacity of the stiff smart material is retained, for a net high energy output. The actuator is considered “power-by-wire” because only electrical power is provided from the vehicle or system controller. A primary motivating application is in unmanned combat air vehicles (UCAVs). The particular actuation needs of these vehicles are described and distilled to a set of relevant device requirements. Other potential applications are also highlighted. The new actuation architecture offers specific advantages over centralized hydraulic systems and has capabilities not present in electromechanical actuators (EMAs). A laboratory test facility designed to represent the loading experienced by a UCAV control surface is described. Key steps necessary to flight qualify the actuator are introduced.
- Active Damping and Vibration Control for Aircraft Fin and Appendage Structures
  This paper considers the value that vibration damping and control of aircraft fins and appendage structures can have in reducing loads and subsequent fatigue and possible failure. These structures often are subject to high loads resulting from wakes of upstream external stores. The vibration control methods were considered as part of a larger study focused on active flow control. The options for passive or active vibration control on a class of fin-type structures are reviewed, and one approach – active and passive damping using piezoelectric materials – is covered in greater detail. Piezoelectric transducer sizing for expected pressure loading and modeling of piezoelectric-based active damping control systems are discussed. Motivation for another possible techniques coupling active flow and vibration control is presented using arguments from adaptive filtering and feedforward control. Results are presented for bench tests with simulated disturbances, for low speed wind tunnel tests, and for high speed wind tunnel tests.
- Development of Smart Material--Hydraulic Pumps and Actuators
  Smart materials such as piezoelectrics and magnetostrictives produce mechanical power in a form that is improperly matched to many applications. When packaged in typical ways, these stiff materials can be used for the pressurization and pump stage in electrohydrostatic actuatos (EHAs). EHAs offer the advantages over traditional centralized hydraulic systems by providing local pressurization in a closed fluid system and eliminating the need for distributed, high-pressure fluid lines. Given inherent material power densities, smart material-based EHAs could produce higher power output compared to electromagnetic actuators. High frequency, low displacement smart material actuation, typically operated in the range of 500 Hz, but in some cases much higher, is rectified via fluid flow to produce larger output displacements at lower frequencies. Continuing previous research, this paper describes design approaches that address and attempt to minimize losses. Piezoelectric and magnetostrictive devices are compared, and the design and testing of magnetostrictive pumps is described in greater detail, with special considerations given to heat generation and improved efficiency.
- Magnetorheological-Fluid Damper with Integral Step-And-Repeat Actuator
  Magnetorheological (MR) fluids provide a means for controlling properties of dampers to achieve various performance objectives. Some applications that require dampers may benefit from an additional actuation capability. While MR fluid alone is not sufficient for actuation, it can be used in combination with other components to create an actuator while adjustable damping properties are retained. A concept for a device that serves as both a controllable damper and a linear actuator is described. Such a device might be useful in a suspension or leveling system, or a repositioning system incorporated in a recoil damper. Basic design considerations are reviewed and the particular step-and-repeat architecture, in which two MR damper subassemblies are used as variable holding clamps, is presented. Two candidate actuator cores are considered. A voice coil core was used in a prototype unit, and a magnetostrictive or piezoelectric core is suggested for a higher-force device. Prototype design features and results of testing are described. A test unit achieved speeds up to 10 mm/s. Several non-ideal effects were noted, and must be addressed to make the technology practical.
- Passive Vibration Damping with Magnetostrictive Composite Material
  This paper describes evaluation of an autonomous-material system tailored for free-layer vibration damping of structural elements. The magnetostrictive particulate composite (MPC) material described has moderate stiffness and minimal temperature and frequency dependence. The composite is created by curing Terfenol particles {Tb(1-x)Dy(x)Fe(2), 0.2<x<0.7} in a thermoset polymer resin system; during curing, the material is subjected to a constant magnetic field. The cured MPC, under vibratory loading, dissipates energy through hysteresis due to domain-wall motion within the particles. The material has an uncommon combination of stiffness and damping, with modulus near that of fiberglass and loss factor similar to many rubber formulations, and the material exhibits vibration damping capability over wide temperature and frequency ranges. Challenges for design are the material’s load-dependent damping capacity and its low ultimate strength.
- Smart-Material Based Hydraulic Pump System for Actuation of a Morphing Wing
  A magnetostrictive material is used as a high frequency driving element in a hydraulic pump that provides fluid power for actuation of a planned morphing wing. In the particular morphing concept, the pump serves as a centralized hydraulic power unit with distributed hydraulic lines to eight separate end-effectors in the wing, but the technology can also be used to create lightweight, compact electrohydrostatic actuators, which allow for distributed power-by-wire operation. The pump exceeds all flow rate and pressure requirements for actuation of the morphing wing. Magnetostrictive pump design and test results are described. Hydraulic and control systems are laid out in detail for a morphing wing demonstration on a small unmanned air vehicle. Looking towards future, non-experimental flights, information and test results are also provided on pump reliability and dual pump arrangements.
- SPIE Smart Structures Product Implementation Award: A Review of the First Ten Years
  The research field of smart materials and structures has been a distinct entity for two decades. Over the past ten years, the SPIE Industrial and Commercial Applications Conference has presented a Smart Structures Product Implementation Award at its annual symposium. This paper revisits the nine winning entries to date (1998-2007) and updates their status. The paper begins with a brief description of the original and current intent of the award and follows with a short overview of the evolution of smart structures, from research to products. The winning teams and their respective products are then described. The current status of the products is discussed based on publicly available information and input from the respective companies. Note however that it is not the purpose of the paper to rank the product winners in terms of success or sales. The paper concludes with an assessment of the larger trends in productization of smart structures technologies. The application “form” for the award as well as the evaluation criteria and suggestions for improving award application packages can be found in the appendix.
SoftRide Launch Load Alleviation
- Highly-Damped Exactly-Constrained Mounting of an X-Ray Telescope
  Instruments and machines requiring very high stability should be isolated from their normally less stable environment. Exact constraint mounting using six single-constraint flexures provides a stiff connection between the instrument and its environment while isolating the instrument from low frequency deformations of the environment, such as thermal expansion. Higher frequency disturbances, however, transfer through the flexures and excite environmental disturbances reaching the instrument. However, strict alignment requirements for the instrument preclude the use of low-frequency isolation, unless active methods are used. Therefore, the solution is to provide damping in parallel with the flexures to reduce the vibration amplitudes of the instrument. Flexures concentrate strain energy in blades, making them excellent candidates for damping treatments. A properly designed damping treatment across the flexures can provide as much as 8%-10% viscous damping to the isolation modes and will also help attenuate the instrument vibration modes. Thus, through the use of six damped single-constraint flexures, the instrument’s requirements for stability, alignment, stress, and vibration may be met.
- Launch Load Attenuation for Space Shuttle Experimental Equipment
  Experiments performed aboard the space shuttle often utilize sensitive scientific equipment which cannot withstand high launch loads without damage. It would be highly advantageous to reduce the severity of the dynamic launch environment so that space-qualification of such equipment would be faster and less expensive. This is the goal of the Soft Ride to Orbit program. This program has identified passive damping as one technology which will reduce loads seen by equipment and thereby provide a softer ride. Finite element structural modeling was used to predict both undamped and damped responses to simulated launch loads. The modal strain energy method was used to design the passive damping treatment. This treatment was manufactured and applied to the drawer. All analyses were verified by modal and vibration testing. It was found that predicted and tested frequencies, damping values, and vibration response levels agreed reasonably well, thus showing that passive damping may be designed into future equipment drawers to reduce launch loads on sensitive equipment.
- Soft-Ride to Orbit: Viscoelastic Treatments for Launch Load Attenuation
  The high level vibroacoustic environment, typical for launch vehicles, can result in excessive loads and fatigue problems for space payloads. Extensive analysis and testing requirements are imposed on the payload developer to ensure the flight worthiness of the payload element. To alleviate these requirements and reduce the long lead times involved in analytical verification, a soft ride capability was proposed for transporting experiment payloads to the Space Station. This paper presents the results of a study to determine the feasibility of using viscoelastic treatments to suppress structure-borne vibrations and provide soft-ride capability. Finite element analysis and the modal strain energy method are used to design a damping treatment for an equipment drawer with a representative payload mass. The response of the damped and undamped system to simulated launch loads are determined and compared with test results. The results showed good agreement and also confirmed the effectiveness of the damping treatment in controlling vibrations.
- Launch Vibration Isolation System
  One of the most severe environments that a satellite will be subjected to during its lifetime will occur during launch. This paper summarizes the results and status of research efforts in the area of satellite isolation from the launch environment. The objective of this effort was to reduce the launch-induced dynamic acceleration of the satellite by insertion of an isolator. Isolation issues involving the use of passive elements and launch vehicle system-level requirements will be discussed.
- Passive Isolation Systems for Launch Vehicles
  One of the most severe environments a satellite will be subjected to during its lifetime will occur during qualification testing and launch. This paper summarizes the results and status of research efforts in the area of satellite isolation from the launch environment. The objective of this effort was to reduce the launch-induced dynamic acceleration of the satellite by insertion of a passive isolator. Isolation issues involving the use of passive elements and launch vehicle system-level requirements will be discussed.
- On the Design and Testing of a Spacecraft Launch Vibration Isolation System (LVIS)
  The Air Force’s Phillips Laboratory has sponsored a program to isolate payloads from mechanical vibrations during launch. Called LVIS, for Launch Vibration Isolation System, the program’s goals are to reduce the RMS accelerations felt by an isolated payload by a factor of 5 compared to an unisolated payload. Its secondary goals are to use minimal launch vehicle services, fit within existing payload attach fittings’ dimension and mass envelope, and provide fail-safe operation. The LVIS must provide this mechanical isolation while at the same time not allowing its host spacecraft to “rattle” too much and make contact with the external payload fairing, which protects the payload against heat, aerodynamic, and acoustic loads during ascent. The LVIS program intends to accomplish this goal using an innovative suspension system which is specially designed to be compliant in the vertical and lateral directions, but stiff in the rotational directions to prevent payload fairing contact. An overview of the LVIS design and predicted performance will be given.
- A Magnetically Damped Vibration Isolation System for a Space Shuttle Payload
  A new vibration isolation system for a Space Shuttle payload is described. Designed for a large optical instrument to be launched aboard the next Hubble Telescope servicing mission, the system uses a set of eight telescoping struts to mount the payload to a shuttle pallet. Each strut is a combination of a titanium coil spring and a passive damper. The latter dissipates energy through eddy currents induced in a conductor moving in a DC magnetic field. The result is a simple, robust, all-metal isolation mount that is linear over a long stroke, relatively insensitive to temperature, and contains no fluids. The latter property is particularly important because it eliminates the need for friction-inducing seals as well as the possibility of leakage. Design of the system is described and strut-level test results are given along with predictions for system-level isolation under flight loads.
- Need for and Benefits of Launch Vibration Isolation
  Spacecraft designs are driven by the necessity of the spacecraft to survive being launched into orbit. This launch environment consists of structure-borne vibrations transmitted to the payload through the payload attach fitting (PAF) and acoustic excitation. Here we present a discussion on the need for and benefit of isolating the structure-borne vibrations. If the PAF were replaced with an isolator with the correct characteristics, the potential benefits would be significant. These benefits include reduced spacecraft structural weight and cost, as well as the increased life and reliability. This paper will present an overview of the problem of vibration on a launch vehicle payload and the benefits that an isolating PAF would provide. The structure-borne vibrations experienced by a spacecraft during launch are made up of transient, shock, and periodic oscillations originating in the engines, pyrotechnic separation systems, and from aerodynamic loading. Any isolation system used by the launch vehicle must satisfy critical launch vehicle constraints on weight, cost, and rattle space. A discussion of these points is presented from the perspective of both a launch vehicle manufacturer and a spacecraft manufacturer/user.
- On the Development of a Launch Vibration Isolation System
  The US Air Force’s Phillips laboratory has sponsored several programs to isolate payloads from mechanical vibrations during launch. This paper details a program called LVIS (Launch Vibration Isolation System). LVIS’ goals are to reduce the RMS accelerations felt by an isolated payload by a factor of 5 compared to an unisolated payload while using minimal launch vehicle services, fitting within existing payload attach fittings’ dimension and mass envelopes, and providing fail-safe operation. The LVIS system must provide axial isolation while at the same time not allowing its host spacecraft to “rattle” too much and make contact with the external payload fairing, which protects the payload against heat, aerodynamic, and acoustic loads. This challenging set of goals will be accomplished using an innovative suspension system specially designed to be relatively soft in the vertical and lateral directions and stiff in the rotational directions to prevent payload fairing contact. An overview of the LVIS design and predicted performance is given.
- Whole-Spacecraft Passive Launch Isolation
  A spacecraft is subjected to very large dynamic forces from its launch vehicle during its ascent into orbit. These large forces place stringent design requirements on the spacecraft and its components to assure that the trip to orbit will be survived. Reduction of launch loads would allow more sensitive equipment to be included in missions, reduce risk of equipment or component failure, and possibly allow the mass of the spacecraft bus to be reduced. This paper reports the design and testing of a prototype whole-spacecraft isolation system which will replace current payload attach fittings, is passive-only in nature, and provides lateral isolation to a spacecraft which is mounted on it. This isolation system is being designed for a medium launch vehicle and a 6500 lb spacecraft, but the isolation technology is applicable to practically all launch vehicles and spacecraft, small and large. The isolator significantly reduces the launch loads seen by the spacecraft. Follow-on contracts will produce isolating payload attach fittings for commercial and government launches.
- Payload Isolation System for Launch Vehicles
  A spacecraft is subjected to very large dynamic forces from its launch vehicle during its ascent into orbit which place stringent design requirements on the spacecraft and its components to assure that the trip to orbit will be survived. The severe launch environment accounts for much of the expense of designing, qualifying, and testing satellite components. Reduction of launch loads would allow more sensitive equipment to be included in missions, reduce risk of equipment or component failure, and possibly allow the mass of the spacecraft bus to be reduced. This paper reports the design and testing of a prototype whole-spacecraft isolation system which will replace current payload attach fittings, is passive-only in nature, and provides lateral isolation to a spacecraft which is mounted on it. This isolation system is being designed for a medium launch vehicle and a 6500 lb spacecraft, but the isolation technology is applicable to practically all launch vehicles and spacecraft. The feasibility on a small launch vehicle has been demonstrated with a system-level analysis which shows great improvements. Follow-on contracts will produce isolating payload attach fittings for commercial and government launches.
- GFO/Taurus Whole-Spacecraft Vibration Isolation System
  A whole-spacecraft isolation system for the GFO/Taurus mission was designed, fabricated, tested, and subsequently flown on February 10, 1998. This isolation system was designed to reduce dynamic responses on the GFO spacecraft caused by the resonant burn dynamic load introduced by the Castor 120 solid rocket motor. Longitudinal (flight direction) response of the GFO spacecraft center of gravity, due to the resonant burn load, was reduced by a factor of seven. The isolation system design was very nonintrusive to existing hardware, lightweight, and effective. Flight data indicates that the isolation system performed as designed. The GFO spacecraft had a successful launch and is currently operational on-orbit. Similar isolation systems are planned for other flights in 1998 and 1999. This whole-spacecraft isolation technology was highly successful for the GFO/Taurus mission.
- Isolation Systems for Electronic Black Box Transportation to Orbit
  Servicing the Hubble Space Telescope (HST) requires the safe transportation of electronic Orbital Replacement Units (ORUs) on the Space Transportation System (STS) to replace or enhance the capability of existing units. The delicate design of these electronic ORUs makes it imperative to provide isolation from the STS launch random vibration while maintaining fundamental modes above the transient load environment. Two method were developed and used exclusively, on Servicing Mission 2 (SM2), to isolate the ORUs from the environmental launch loads imposed by STS. The first load isolation system utilizes a refined open/closed cell foam design to provide the required damping and corner frequency, while the second method uses and innovative Viscoelastic Material (VEM) design. This paper addresses both systems as initially designed, including finite element model analysis of the VEM system. Vibration testing of prototype systems and modifications to the design resulting from test will be discussed. The final design as flown on HST SM2 with recommendations for future applications of these technologies in transporting electronic black boxes to orbit will conclude the paper.
- Passive and Active Launch Vibration Studies in the LVIS program
  A US Air Force-sponsored team consisting of Boeing, Honeywell Satellite Systems, and CSA Engineering has developed technology to reduce the vibration felt by an isolated payload during launch. Spacecraft designers indicate that a launch vibration isolation system (LVIS) could provide significant cost benefits in payload design, testing, launch, and lifetime. Simulations, which included models of a 6,500 lb spacecraft, an isolating payload attach fitting (PAF) to replace an existing PAF, and the Boeing Delta II launch vehicle, were used to generate PAF performance requirements for the desired level of attenuation. Hardware was designed to meet the requirements. The isolating PAF concept replaces portions of a conventional metallic fitting with hydraulic-pneumatic struts featuring a unique hydraulic cross-link feature that stiffens under rotation to meet rocking restrictions. The pneumatics provide low-stiffness longitudinal support. Two demonstration isolating PAF struts were designed, fabricated, and tested. Measurements matched analytical predictions closely. An active closed-loop control system was simulated to assess its potential isolation performance.
- Whole-Spacecraft Passive Launch Isolation
  A spacecraft is subjected to very large dynamic forces from its launch vehicle during its ascent into orbit. These large forces place stringent design requirements on the spacecraft and its components to assure that the trip to orbit will be survived. Reduction of launch loads would allow more sensitive equipment to be included in missions, reduce risk of equipment or component failure, and possibly allow the mass of the spacecraft bus to be reduced. These benefits apply to military as well as commercial satellites, This paper reports the design and testing of a prototype whole-spacecraft isolation system which will replace current payload attach fittings, is passive-only in nature, and provides lateral isolation to a spacecraft which is mounted on it. This isolation system is being designed for a medium launch vehicle and a 6500 lb spacecraft, but the isolation technology is applicable to practically all launch vehicles and spacecraft, small and large. The isolator significantly reduces the launch loads seen by the spacecraft. Follow-on contracts will produce isolating payload attach fittings for commercial and government launches.
- The Whole-Spacecraft Vibration Isolation System – Its Time Has Come
  Spacecraft typically are hard-mounted to the launch vehicle for launch. Therefore, all of the high dynamic loads from the launch are transmitted directly from the launch vehicle to the spacecraft. A whole-spacecraft vibration isolation system is an isolation system that vibration-isolates the complete spacecraft from the launch vehicle. Unlike many typical isolation systems, this isolation system must couple two dynamically rich structures in the same frequency range as the individual components. Therefore, a coupled-loads approach must be used for its design characteristics. This paper discusses two whole-spacecraft systems: a shear-type designed for large, liquid-fueled vehicle and an axial system designed for small, solid-fueled launch vehicles. A prototype shear-type system was designed, built and tested for a Delta II class launch vehicle. Coupled-loads analyses predicted that the RMS lateral accelerations on the spacecraft were reduced by a factor of two to six. The axial-type of system has been flown on two flights: the Taurus/GFO and the Taurus/STEX. Flight data shows that the dynamic responses on the spacecraft were reduced by up to a factor of 5.
- Whole-Spacecraft Vibration Isolation System for the GFO/Taurus Mission
  A whole-spacecraft isolation system for the GFO/Taurus mission was designed, fabricated, tested, and subsequently flown on February 10, 1998. This isolation system was designed to reduce dynamic responses on the GFO spacecraft caused by the resonant burn dynamic load introduced by the Castor 120 solid rocket motor. Longitudinal (flight direction) response of the GFO spacecraft center of gravity, due to the resonant burn load, was reduced by a factor of seven. The isolation system was designed very nonintrusive to existing hardware, lightweight and effective. Flight data indicates that the isolation system performed as designed. The GFO spacecraft had a successful launch and is currently operational on-orbit. A second flight of this type of isolation system occurred in October 1998. Similar isolation systems are planned for other flights in 1999 an 2000. This whole-spacecraft isolation technology was highly successful for the GFO/Taurus mission.
- Whole-Spacecraft Vibration Isolation on Small Launch Vehicles
  Small launch vehicles historically provide a very rough ride to spacecraft during launch. For the spacecraft to survive such a trip into orbit, one of two choices must be made: (1) redesign all structure, payloads, and systems on the spacecraft to withstand high launch loads, or (2) reduce the magnitude of the high launch loads. The latter is preferred because it allows the focus of the spacecraft design to be primarily for on-orbit performance rather than launch survival. Under contracts from the US Air Force Research Laboratory, Space Vehicles directorate, whole-spacecraft vibration isolation systems have been in development since 1993. This work has resulted in two whole-spacecraft isolation systems (SoftRide) that have been flown on Taurus launch vehicles in February and October of 1998. Full coupled-load analyses were used to predict their performances. This paper discusses the testing and results of the systems, including the typical 85% attenuation for the worst case resonant burn condition and 59% attenuation for a combination of static plus worst case burn resonant burn condition. Discussion of future endeavors towards isolation systems for larger vehicles is also within this paper.
- Whole-Spacecraft Vibration Isolation for Broadband Attenuation
  Launch vehicles impart high levels of vibration to spacecraft during launch. The vibration environments are defined over several frequency bands: (1) transient vibration < 80 Hz, (2) random vibration 20 to 2000 Hz, and (3) pyrotechnic shock 100 to 10000 Hz. Loads from transient vibration define spacecraft design of primary structures such as spacecraft bus, solar panel and antenna supports, instrument mounts, etc. Loads from random vibration define the design for spacecraft light structures such as antennas and solar panels, and shock loads define the design of electronic components and instruments. The spacecraft must survive the combination of all vibration environments. This requires spacecraft structures, instruments, and components to be designed to minimize vibration across a broad frequency range. Spacecraft are designed for the short launch to orbit, which is well beyond the requirements for on-orbit performance. A better choice is to reduce the magnitude of the high launch loads across all frequency bands and design smaller and less costly spacecraft.
- Multi-Axis Whole-Spacecraft Vibration Isolation for Small Launch Vehicles
  Small launch vehicles present an economically viable method for placing small satellites into orbit but would be even more attractive if they could provide a softer ride. Passive whole-spacecraft vibration isolation systems exist for small launch vehicles, and to date, two types of isolation systems have been designed. The first is a single-axis “SoftRide” axial isolation system that provides isolation for predominantly axial loads. The second type of isolation system is a multi-axis device that provides vibration isolation in three axes. This type of system is needed to alleviate dynamic launch loads on the Minotaur vehicle. This multi-axis “SoftRide” system inserts flexibility and damping in three orthogonal axes between the launch vehicle and the satellite. These isolation systems provide extreme reductions to shock and structure-borne acoustic loads. The multi-axis isolation system is a logical extension of the single-axis system and has the same qualities of being simple, passive, small, lightweight, reliable, and highly effective. Coupled loads analyses and flight telemetry data indicate that the new multi-axis vibration isolation system performed as expected and greatly reduced dynamic launch loads.
- Recent Launches Using the SoftRide Whole-Spacecraft Vibration Isolation System
  Passive whole-spacecraft vibration isolation systems have been developed for launch vehicles to greatly reduce the dynamic launch loads. To date, three types of isolation systems have been designed, and two types have been flown. The first is a single-axis “SoftRide” axial isolation system (SoftRide UniFlex) that provides isolation for predominantly axial loads. This type of system has been flown successfully three times on the Taurus/GFO mission in February 1998, the Taurus/STEX mission in October 1998, and the Taurus/MTI mission in March 2000. The second type of isolation system is a multi-axis device that provides vibration isolation in three axes. This multi-axis “SoftRide” system inserts flexibility and damping in three orthogonal axes. The dynamic launch loads with both axial and lateral components can be effectively mitigated. The multi-axis isolation (SoftRide MultiFlex) system has the same qualities of being simple, passive, small, lightweight, reliable, and highly effective. Coupled loads analyses and flight telemetry data indicate that both the patented UniFlex and MultiFlex isolation systems have provided a softer ride to orbit for satellites on small launch vehicles.
- Whole-Spacecraft Vibration Isolation Flown on the Minotaur Launch Vehicle
  Small launch vehicles present an economically viable method for placing small satellites into orbit but would be even more attractive if they could provide a softer ride to orbit. Passive whole-spacecraft vibration isolation systems have been developed for small launch vehicles to greatly reduce the dynamic launch loads. To date, two types of isolation systems have been designed. The first is a single-axis "SoftRide" axial isolation system that provides isolation for predominantly axial loads. The second type of isolation system is a multi-axis device that provides vibration isolation in three axes between the launch vehicle and the satellite. The result is that dynamic launch loads with both axial and lateral components can be effectively mitigated. Additionally, these isolation systems provide extreme reductions to shock and structure-borne acoustic loads. The multi-axis isolation system has the same qualities of being simple, passive, small, lightweight, reliable, and highly effective. Coupled loads analyses and flight telemetry data indicate that the new multi-axis vibration system performed as expected and greatly reduced dynamic launch loads for the satellites.
- Recent Achievements in Vibration Isolation Systems for Space Launch and On-Orbit Applications
  This paper presents successful recent applications of several vibration mitigation technologies to space and launch systems. These technologies include: wholespacecraft vibration isolation and shock protection during launch, an innovative fluid-free magnetic isolation system for the Hubble Space Telescope servicing mission, precision isolation and pointing platforms for on-orbit applications, and pneumatic isolation technology for 0-g ground test systems.
- Protecting Satellites from the Dynamics of the Launch Environment
  Reduction of the vibration and shock loads seen by spacecraft during launch would greatly reduce the risk of damage for the spacecraft and its instruments during their ascent into orbit, and would allow more sensitive equipment to be included. As the severe launch environment accounts for much of the expense of designing, qualifying, and testing spacecraft components, significant cost can also be saved. The basic concept of whole-spacecraft isolation is to isolate the entire spacecraft from the dynamics of the launch vehicle. This paper discusses two different systems: the SoftRide system, which is a lower frequency (10 – 50 Hz) isolation system and the ShockRing system, designed to attenuate higher frequency loads (70 Hz and above), including shock. All seven flights of CSA’s SoftRide systems have shown excellent loads reductions. Component tests performed on the ShockRing using a specially built pneumatic gun that can generate 10,000 g’s demonstrate substantial reductions of the shock being transmitted to the payload. Results from a system test consisting of a spacecraft simulator, payload attachment fittings, avionics section, and shock plate will be discussed.
- Active Vibration Isolation System for Launch Load Alleviation
  Payloads delivered to orbit by expendable launch vehicles experience high levels of vibration. This vibration can cause component failures or lead to extra weight that would otherwise be useful for added functions on orbit. Vibration isolation systems have been flown to protect various components as well as entire spacecraft, dramatically reducing launch loads and saving costs in redesign and tests. Future spacecraft and components may benefit from further load reduction through the use of higher performance active isolation systems. These active systems are capable of introducing compliance in selected axes, while maintaining required rigidity in others. They can also produce excellent isolation without large amplification. Passive and active vibration isolation systems were developed for the Vibro Acoustic Launch Protection Experiment (VALPE) and flew aboard sounding rockets. The paper describes the design and development of the isolation systems, actuation and isolation architectures and control strategies. Integration of two flight experiments is summarized. Ground test results are presented for passive and active systems.
- SoftRide Vibration and Shock Isolation Systems that Protect Spacecraft from Launch Dynamic Environments
  Reduction of the vibration and shock loads seen by spacecraft during launch greatly reduce the risk that the spacecraft and its instruments will be damaged during their ascent into orbit, and also allow more sensitive equipment to be included in missions. Protecting the satellite from these loads by wholespacecraft vibration and shock isolation systems has now been demonstrated. The basic concept of whole-spacecraft isolation is to isolate the entire spacecraft from the dynamics of the launch vehicle. This paper discusses two different systems: the SoftRide system and the ShockRing system. This paper discusses each of these types of systems and presents flight data that demonstrates their effectiveness.
- Satellite Component Load Reduction Using SoftRide
  Whole-spacecraft vibration and shock isolation systems (SoftRide) have been developed and flown to attenuate dynamic loads for several launch vehicles, and are currently in development for other launch vehicles and loading environments. To further enable responsive space, advanced knowledge of the dynamic environments of the launch vehicles and various size satellites could be gained by analyzing a wide array of actual satellite models that cover the size ranges under consideration for future missions. Coupled-loads analyses could be performed that will develop the characteristics (stiffness, damping, and strength requirements) of a whole-spacecraft vibration and/or a shock isolation system that will mitigate the effects of the dynamic loads for each combination of satellite and LV. From these characteristics, a family of three, four or five (if needed) isolation systems that will satisfy the requirements for all combinations could be developed and manufactured. The resulting isolators could be on-the-shelf and a software tool using key spacecraft and LV characteristics could determine which SoftRide system to install in the field.
- The Interstellar Boundary Explorer (IBEX) Flight Segment
  IBEX provides the observations needed for detailed modeling and in-depth understanding of the interstellar interaction (McComas et al. in Physics of the Outer Heliosphere, Third Annual IGPP Conference, pp. 162–181, 2004; Space Sci. Rev., 2009a, this issue). This paper describes the management, design, testing and integration of IBEX’s flight system, which successfully launched from Kwajalein Atoll on October 19, 2008. The payload is supported by a simple, Sun-pointing, spin-stabilized spacecraft with no deployables. The spacecraft bus consists of the following subsystems: attitude control, command and data handling, electrical power, hydrazine propulsion, RF, thermal, and structures. A novel 3-step orbit approach was employed to put IBEX in its highly elliptical, 8-day final orbit using a Solid Rocket Motor, which provided large delta-V after IBEX separated from the Pegasus launch vehicle. After orbit raising, IBEX transitioned from commissioning to nominal operations and science acquisition. The Systems Engineering and Mission Assurance teams supervised the design, testing and integration of all IBEX flight elements.
- Rapid Coupled Loads Analysis and Spacecraft Load Reduction using SoftRide
  The dynamic environment that a spacecraft will experience during a launch is unknown until a full coupled loads analysis (CLA) has been performed. By the time this has been done, the small satellite provider might not have time to mitigate problem areas. Unmitigated loads issues could increase the risk of failure of not only a component, but of the entire mission. A better approach is to have coupled loads analysis data available for real spacecraft of various sizes on multiple launch vehicles. To further ensure that any unknown vehicle loads will not be detrimental to the spacecraft, the installation of a SoftRide whole-spacecraft isolation system can reduce the transmitted loads. This is the approach currently under study for the Office of Responsive Space (ORS) by CSA. The analyses and results will drive the creation of a small set of SoftRide “sizes” which can be pre-manufactured and stored on-the-shelf in anticipation of a launch. A software tool will be able to identify the expected loads on the spacecraft and the correct size SoftRide for the mission when a user enters information about the payload into a database which contains results from the entire set of generic payload analyses.
- Whole-Spacecraft Shock Isolation System
  Spacecraft are subjected to shock loads in the several thousands of g’s level during their trip to orbit. These high shock loads usually result from a separation event. Shock loads are very detrimental to spacecraft components, instruments and electronics. A new type of shock isolation system is discussed. This shock system, referred to as the SoftRide ShockRing, is a whole-spacecraft isolation system. The standard SoftRide system is a lower frequency isolation system than the ShockRing, vibration isolating the spacecraft starting in the approximately 25 Hz range. The ShockRing is targeted at shock loads and is set to isolate above approximately 75 Hz. Component tests have been performed on the ShockRing using a specially built pneumatic gun that can generate 10,000 g's on the test article. Results from these tests demonstrate substantial reductions of the shock being transmitted to the payload. Results from a system test consisting of a spacecraft simulator, payload attachment fittings, avionics section, and shock plate are discussed.
Structures and Composites
- Dynamic Stability of Rectangular Sandwich Plates Under Pulsating Loads
  In a previous paper the senior author of this Note established an analogy between the dynamic stability of an homogeneous Euler column with pinned ends and a composite column with similar end conditions. The purpose of this Note is to show that the dynamic stability of both homogeneous and nonhomogeneous rectangular sandwich plates with hinged edges and subjected to pulsating periodic loads of the type Nx(r) = No + Nr cos()r is governed by a second-order, ordinary differential equation (the Mathieu equation) which differs only in the structure of the parmeters contained in it. Consequently, a complete analog for the sandwich plate is established, and the results that are applicable for the homogeneous plate carry over unchanged to the sandwich plate.
- Application of the Variational Theorem for Creep of Shallow Spherical Shells
  The variational theorem for creep is used to study the creep deflections and collapse times for simply supported thin shallow spherical shells under uniform external pressure. The initial and subsequent form of the stress resultants and displacements are obtained with the aid of the elastic theory of thin shallow spherical shells. The variational theorem for creep leads to a set of simultaneous ordinary differential equations with prescribed initial conditions. Numerical solutions are generated by means of the Runge-Kutta method. Theoretical predictions for creep deflections and collapse times are compared with experimental data for five test shells at each of three different pressure levels for Type 6/6 Nylon. It was found that the theory predicts creep deflections that are consistently larger than the experimental collapse times. The discrepancy between the theoretical predictions for creep deflections and collapse times and the experimental data is believed to be due largely to the deviations which occur between the theoretical and actual stress and displacement mode shapes with the passage of time. Closer agreement should be expected when more time flexibility is built into these mode forms.
- Dynamic Stability of Circular Cylindrical Sandwich Panels
  A theoretical study of the dynamic stability of simply-supported sandwich plates and circular cylindrical sandwich panels with dissimilar face-sheets and orthotropic cores is presented. The uniformly distributed periodic edge loads consist of steady components and periodic components which are harmonic, impulsive, or vary according to an arbitrary piecewise constant law. The parameters which determine the regions of dynamic stability are dependent on the static buckling load and the frequency of vibration. It was found that stable regions do exist when the steady components of the loads are greater than the static buckling loads. In these regions the oscillating components of the loads have a stabilizing effect on the structure. The stability diagrams may be used when the loads are tension, compression, uniaxial or biaxial. It was found that the dynamic stability equations for the sandwich plate and circular cylindrical sandwich panel have the same form as those for simply-supported, homogeneous plates and panels. Consequently, a complete analogy exists between sandwich and homogeneous plates and panels, and conclusions regarding the dynamic stability of the latter are applicable with change to the former.
- Dynamic Stability of Rectangular Sandwich Plates under Harmonic and Impulsive Compression
  The purpose of this investigation is to study the dynamic stability of simply-supported rectangular sandwich plates with dissimilar facesheets and orthotropic cores. The edgewise compressive loads, which may be either uniaxial or biaxial, consist of steady components and oscillation components, the latter which are either harmonic or impulsive. The equations of motion are obtained by means of Hamilton's principle. The regions of stability are presented for each type of lead, and numerical examples are given.. It was found that the dynamic stability of simply supported rectangular sanwich plates with dissimilar face-sheets under harmonic and impulsive edge compressions is analogous to that of homogeneous plates with similar edge supports and loadings.
- Optimum Design of Advanced Composite Structures for Static Loads
  The purpose of this paper is to present an efficient optimization method, based on an optimality criterion and a numerical search, for the minimum weight design of structures made from composite materials. The criterion can be stated as "The optimum design is the one in which the strain energy of each layer bears a constant ratio to its energy capacity." With the aid of this optimality criterion a recurrence relation is derived and incorporated into a program based on the displacement method of finite element analysis. This program includes such practical design considerations as contraints on stresses, displacements and sizes of the elements. The design procedures take into account multiple loading conditions and a strength criterion for the composite elements. A basic set of four fiber directions is chosen and the number of laminae in each direction is varied to obtain an efficient design. Several sample problems ranging from a simple rectangular plate to a wing structure consisting of both isotropic and composite elements are solved and the results presented.
- Optimization of Fiber Reinforced Composite Structures
  This paper presents an efficient optimization method, based on strain energy distribution and a numerical search, for the minimum weight design of structures made from fiber reinforced composite materials. The optimum design procedure takes into consideration multiple loading conditions and displacement constraints on the structure. Sample problems consisting of both isotropic and composite elements are solved and the results presented.
- Solution of a Large, Transient, Thermal and Thermal-Stress Problem with NASTRAN
  The efficient solution of large transient thermal problems with NASTRAN requires a considerable amount of engineering judgment and forethought. Such a problem will be described with particular emphasis placed upon the problem size and the solution techniques which were necessitated. Accuracy of the solution and the impact of mesh refinement on accuracy at various stages in the solution process was of utmost importance.
- Assessment of Structural Effects in Acoustic Transient Experiments
  Two methods are presented for simulating the wall pressure response of an acoustic fluid in a flexible container when a prescribed transient pressure is applied over a small portion of the fluid boundary. One method is experimental. A special purpose acoustic exciter is used to obtain the complex frequency response between input and output pressures. The function is then used with discrete Fourier transform methods to simulate response. The second method is analytical. A previously published hybrid finite element method is used to predict normal mode properties and frequency response of the coupled fluid structure system. A simple but necessary correction for residual compliance is used. Comparisons of prediction and experiment are given for two test tanks of different stiffnesses, along with analytical results for rigid-wall pools of identical geometry. Applications to unsteady condensation experiments are discussed.
- Estimation of Stresses Due to Imposed Base Motion Using In-Situ Modal Test Results
  Seismic or hydrodynamic load specifications that are imposed on nuclear plant equipment after installation can require costly requalification. This problem has recently been addressed by using modern', minicomputer based methods of dynamic testing (1). A method is described in Reference 1 for using the results of tests performed in-situ on installed plant equipment to predict the response of equipment to imposed base motion. The base motion can be specified arbitrarily and is usually taken to be that which is characteristic of some seismic or hydrodynamic event. The significance of the method is that it allows a mathematical model of the equipment to be constructed entirely from test data. The data can be obtained at relatively low cost and,- having been derived from tests of the actual equipment, is not subject to many of the uncertainties and assumptions of purely analytical methods such as finite element analysis. The model is suitable for prediction of response to base motion even though the base was fixed during the in-situ tests.
- Structural Optimization: A Practical Tool for Advanced Computer Aided Engineering
  A computer aided engineering (CAE) system will be more versatile if automated design capabilities are incorporated in addition to the interface with the response analysis capabilities. CAE systems for structural and mechanical engineering are often planned considering only generation of finite element analysis models. Practical automated design capabilities based on numerical optimization techniques are now available, therefore future plans for CAE system architecture and data base design should take them into account. The status of automated structural design technology is described and important aspects to be considered in the implementation procedure are discussed by introducing the concept of design models. Structural optimization capabilities are illustrated by typical examples obtained recently using ADS/NASOPT that works with MSC/NASTRAN.
- Experiences with Optimization Using ADS/NASOPT and MSC/NASTRAN for Structural Dynamics
  ADS/NASOPT is an optimization code which works with MSC/NASTRAN to produce optimal structural designs. The user creates a design model which defines design variables (element properties), constraints (limits on displacements, stresses, natural frequencies, buckling loads, or weight), and an objective, a function to be minimized. This paper describes experiences in using ADS/NASOPT in optimizing a complex structure subject to weight and frequency constraints.
- Design, Analysis, and Testing of the CASPAR Multi-Payload Adapter Bonded Composite Joint
  The CASPAR multi-payload adapter will provide a shared ride access to space for two 1,500-lbm satellites or up to four 400-lbm (ESPA-class) satellites. The adapter consists of two symmetrical carbon fiber composite shells joined by a Latching Lightband separation system that is integrated into its mid-plane. CASPAR co-manifests the payloads by placing one of the satellites, or small satellite pairs, atop the MPA while the other is stowed inside. The separation system is required to release the adapter’s upper shell, thus enabling the stowed satellite(s) to be expelled into orbit. The separation system interfaces with the composite shells via a bonded double lap shear epoxy joint. The design, analysis, and testing of the bonded joint is the focus of this paper.
- Parameter Estimation and Structural Dynamic Modeling of Electrical Cable Harnesses on Precision Structures
  The influence of electrical cable harnesses on precision spacecraft structural dynamics has never been extensively studied, consequently, models are inaccurate, isolation system requirements are overly conservative and program risk is increased. The mission of a research and development program managed by the Air Force Research Laboratory Space Vehicles Directorate is to develop test, analysis and modeling methodologies to include the influence of electrical cables in standard linear finite element models to increase model accuracy. This paper describes a system identification approach to derive cable specimen material and section properties from driving point mobility functions based on a shear beam model. The approach and its application to measurements is described.
- Structural Dynamic Effects of Cables on a Sparse Aperture Deployable Optical Telescope
  This paper will look at the quantitative effects of cables on a complex structure representative of stable optical metering structures. With the areal density of spacecraft decreasing while both the mass of cables and model accuracy requirements increase, it is increasingly important to understand the structural dynamic effects of cables. A ground based telescope test bed has been tested in both cabled and uncabled states. A system ID A, B, C, D model has been derived for both configurations and changes to modal parameters are examined. The effect of cables on the performance and stability of a controller designed around an uncabled structure is looked at. Changes to the structural dynamics were found to be dependent on the cable to base structure mass ratio. Small mass ratios on the order of ½% resulted in no appreciable modal changes. A mass ratio on the order of 3% resulted in a doubling of damping at specific modes, but no change in the natural frequencies. RMS responses varied, with piston decreasing from 8-19% and tip/tilt responses remaining mostly unaffected. At these small mass ratios the stability and performance of a low authority controller were unaffected.
- Fundamental Design of Tensioned Precision Deployable Space Structures Applied to an X-Band Phased Array
  The tendency of a tensioned structure to self-align with the load direction may be harnessed to create a precision structure that has potentially greater passive stability, more deterministic dynamics, and simpler metrology requirements than those of the traditional truss structure. Structural sti↵ness is derived from the nonlinear geometric sti↵ening of the tensioned structure, rather than from the mechanics of materials. With the absence of structural depth, the traditional structural precision issues related to thermal deformations are reduced to two-dimensions, and metrology and shape control may be simplified into the load management and planarity control of discrete tensioning points. The objective of the presented research is to develop a fundamental understanding of the mechanics for tensioned precision structures, in terms of a potential first application as an X-band phased array radar. Approximate analytical relationships are developed to determine sensitivities and then used as e↵ective tools for efficient system design. The e↵ect of boundary conditions on internal components of a non-uniform tensioned structure is investigated and discussed, as well as, strategies for passive error correction and the balancing of built-in stochastic error by tuned compliance. Designing for the e↵ects of thermal warping and tension pull-out of deformation are also discussed. The observed relationships are validated against a parametric finite element model of the proposed X-band phased array design.
Suspension Systems and Space Simulation
- CSA Pneumatic/Magnetic Suspension System Application Note No. 2: Comparison to Buoyancy Devices (Helium Balloons)
  Helium-filled balloons are sometimes used as suspension devices to simulate zero-gravity in dynamic testing of space structures. This application note compares the performance of balloons with that of the CSA pneumatic-magnetic suspension devices in terms of mass added to the test article by the suspension. Such added mass degrades the fidelity of the zero-G simulation by changing the normal modes of the payload.
- A Suspension System for Simulating Unconstrained Boundary Conditions
  A multipoint suspension system is described for supporting test articles such that their unconstrained modal properties will not be altered by stiffness, inertia, or friction forces from the suspension. Intended primarily for flexible space structures, it is suitable for test items with natural frequencies as low as one Hz. Using a combination of passive pneumatic and active electromagnetic subsystems, the suspension offers a wide payload range, near-zero stiffness, zero static deflection, small added mass, and zero friction. The electromagnetic system can also provide active cancellation of added mass, accurate ride-height control, and controlled disturbance input. The concept and hardware are described, test results are given, and applications experience from several installations is discussed. Current development efforts are described, aimed at testing of payloads such as solar arrays and large beam-pointing experiments.
- A Pneumatic/Electric Suspension Device for Very Low Frequency Dynamic Testing
  The work described in this presentation was performed by CSA Engineering under subcontract to Lockheed Missiles and Space Company as part of the NASA/LaRC Dynamic Model Technology (DSMT) Program. The program is focused on the development of a dynamically scaled model of Space Station Freedom. The DSMT model is particularly challenging with respect to micro-G simulation because of its large size (50 ft), low structural frequencies (.1.-Hz first mode), and the fact that a large number (10-20) of suspension points must be used to avoid member buckling in l G.
- Very Low Frequency Suspension Systems for Dynamic Testing
  Accurate simulation on earth of the unconstrained boundary conditions of space is a classic problem in dynamic testing. It has become both more difficult and more important with the advent of large, flexible structures. The planned hybrid-scale dynamic model of the Freedom Space Station is an important near-term example. A suspension system is needed to provide rigid-body translation frequencies on the order of 0.1-0.2 Hz for a 50-foot payload weighing about 3400 lb and having a number of highly flexible appendages. Requirements for the suspension system and the devices composing it are reviewed. Two alternate suspension devices have been explored, both analytically and through fabrication and testing of demonstration hardware. One is an all-mechanical passive device based on coil springs and called a zero-spring-rate mechanism. The other is a new hybrid concept using a combination of a passive pneumatic and an active electromagnetic system. Descriptions and test results for both are presented. Both have been found to meet the initial requirements.
- A Pneumatic/Electric Suspension System for Simulating On-Orbit Conditions
  Accurate simulation on earth of the unconstrained boundary conditions of space is a classic problem in dynamic testing. It has becomer more important and more difficult with the advent of large, flexible orbiting structures such as Space Station Freedom. A new suspension system is presented which is designed to provide an accurate ground simulation of on-orbit dynamics for structures with fundamental flexural frequencies as low as 1.0 Hz. Using a combination of passive pneumatic and active electromechanical subsystems, it offers a wide payload range, very low stiffness, zero static deflection, and zero friction. The electromechanical subsystem offers other useful features such as DC stiffness enhancement and active cancellation of the moving mass of the suspension device. The concept and hardware are described, test results are given, and the usage of several variants of the system in current aerospace programs is described.
- Defying Gravity with Active Test Article Suspension Systems
  A multipoint suspension system is described for supporting test articles such that their unconstrained dynamic properties will not be altered by stiffness, inertia, or friction forces from the suspension. Intended primarily for flexible space structures, it is suitable for test items with natural frequencies as low as 1 Hz. Using a combination of passive pneumatic and active electromagnetic subsystems, the suspension offers a wide payload range, near-zero stiffness, zero static deflection, small added mass, and zero friction. The electromagnetic system can also provide active cancellation of added mass, accurate ride-height control, and controlled disturbance input. The concept and hardware are described, test results are given, and applications experience from several installations is discussed. Current development efforts are described, aimed at testing of payloads such as solar arrays and large beam-pointing experiments.
- Simulation of the Zero-Gravity Environment for Dynamic Testing of Structures
  Simulation of unconstrained (free-free) boundary conditions is a longstanding problem in ground vibration testing of spacecraft. The test article weight must be supported without introducing constraining forces due to stiffness, inertia, or friction from the suspension system. High-fidelity simulation of the space environment requires that such constraint forces be kept small compared to forces inherent in the experiment. A multipoint, six degree of freedom suspension system for dynamic testing is described. Intended primarily for highly flexible space structures, it uses a combination of passive pneumatic and active electromagnetic subsystems. The suspension offers a wide payload range, near-zero stiffness, zero static deflection, small added mass, and zero friction. The electromagnetic system can also provide active cancellation of added mass, accurate ride-height control, and integrated disturbance input. Several versions of the system are described. The concept and hardware are described, test results are given, and applications experience from several industry, government, and university installations is discussed.
- Active Alignment and Vibration Control System for Large Airborne Optical System
  Airborne optical or electro-optical systems may be too large for all elements to be mounted on a single integrated structure, other than the aircraft fuselage itself. An active system must then be used to maintain the required alignment between elements. However the various smaller integrating structures (benches) must still be isolated from high-frequency airframe disturbances that could excite resonances outside the bandwidth of the alignment control system. The combined active alignment and vibration isolation functions must be performed by flight-weight components, which may have to operate in vacuum. A testbed system developed for the Air Force Airborne Laser program is described. The payload, a full-scale 1650-lb simulated bench, is mounted in six degrees of freedom to a vibrating platform by a set of isolator-actuators. The mounts utilize a combination of pneumatics and magnetics to perform the dual functions of low-frequency alignment and high-frequency isolation. Test results are given and future directions for development are described.
- Dynamic Behavior of A Low Inertia Gravity Off-load Passive Device
  A practical and passive constant force device to simulate free-free boundary conditions for ground testing of large deployable space structures is presented. The device features a virtually zero-stiffness response over several centimeters of travel, a mass contribution of less than 4 of the gravity offloaded mass, and an excellent dynamic response for frequencies exceeding 30 Hz, despite the zero-stiffness characteristic. The device is based on a commercially available, high energy density steel power tape spring, which is wrapped about two simple to manufacture drums, with one having a pulley feature. A string wrapped around the pulley is used to attach the device with offloaded object. Dynamic experiments of varying frequencies and travel amplitudes and quasi-static displacement experiments are shown to be in close agreement to a simple mathematical model for the device.
- Gravity-Offloading System for Large-Displacement Ground Testing of Spacecraft Mechanisms
  Gravity offloading of deployable spacecraft mechanisms during ground testing is a long-standing problem. Deployable structures which are usually too weak to support their own weight under gravity require a means of gravity offloading as they unfurl. Conventional solutions to this problem have been helium-filled balloons or mechanical pulley/counterweight systems. These approaches, however, suffer from the deleterious effects of added inertia or friction forces. The changing form factor of the deployable structure itself and the need to track the trajectory of the center of gravity also pose a challenge to these conventional technologies. This paper presents a novel testing apparatus for high fidelity zero-gravity simulation for special application to deployable space structures such as solar arrays, magnetometer booms, and robotic arms in class 100,000 clean room environments.
Testing and Test Systems
- Spectral Measurement of Angular Vibration
  Developments in airborne laser and optical systems have brought with them a class of vibration problems where the response quantities of interest are small (order of microradians) rotations of critical optical elements or their supporting structures. Prerequisite to analysis and solution of these so-called angular vibration problems is the ability to accurately measure very small rotations at frequencies as high as 1 kHz. Frequencies above 100 Hz are particularly important because the ability of a servo system to provide correction at higher frequencies is quite limited. One method of making such measurements is to difference the outputs of two linear accelerometers spaced a finite distance apart. This technique has important advantages with respect to sensitivity, bandwidth, transducer size, cost, and ruggedness. An expression for coherence between a differential acceleration signal and an applied for signal is also given in terms of single channel quantities. An experimental example is presented to show the effect of uncorrelated single-channel noise. A simple active compensation scheme is presented for elimination of the harmful effect of gain and phase mismatch between individual channels. The effectiveness of the method is demonstrated by an experiment.
- Low-Deflection Loss and Hysteresis Measurements on a Spacecraft Test Joint
  Passive damping has been demonstrated to be an efficient means for limiting the effects of on-board excitations on the dynamics of space vehicles. High-precision applications require these treatments to both be effective at very low excitation levels and not affect the dimensional stability of the structure under quasistatic and thermal-mechanical loads. This work documents a study of two important issues facing structures damped with viscoelastic materials: hysteresis and loss at low deflection levels. After identifying the most critical vibrational modes from a separate system-level analysis, a damping treatment was designed for the test joint using standard finite element techniques. A modal test using very low random excitation levels was performed on the resulting damped structure. Statistical methods were used to determine that the maximum displacement level of the free-free structure. Subsequently, hysteresis tests were performed on the same damped beam. Percent hysteresis was measured while the joint was loaded in three-point bending. Hysteresis behaviour during displacements as small as 150 nano-meters was recorded.
- Testing of a Viscous-Damped Isolator
  This paper documents component-level tests performed on viscous-damped isolators developed by Honeywell Satellite Systems for a spacecraft reaction wheel isolator system. Two types of component-level tests were performed on the elements: direct stiffness measurements (often called mechanical impedance) and transmissibility tests. Direct stiffness measurements indicated linearity, linear stiffness, damping, and hysteresis. A custom test apparatus was designed for accuracy and repeatability. Stiffness deviations as small as 5 percent could be detected, and loss factors as low as 0.01 could be resolved with the direct stiffness measurements. Motion transmissibility measurements determined high-frequency isolation and verified stiffness and damping near the predicted resonance of the sprung payload. Although the suspension system consisted of eight isolators, tests were performed on a single unit. Motion was constrained to a single degree of freedom using a system of air bearings sliding on rails. The air bearing design possessed less than 0.4 grams of friction allowing verification of isolation properties to above 300 Hz and enabled transmissibility to be accurately measured over 4 orders-of-magnitude of input excitation.
- Experimental Modal and Finite Element analyses, Modal Correlation, and Model Tuning for an Aircraft Brake Component
  A "torque tube," part of an aircraft braking system, was investigated both experimentally and analytically, as part of a larger project with the goal of developing finite element models for the prediction of aircraft brake dynamics to frequencies as high as 2 kHz. This paper presents a case history of this effort, and covers the following tasks: pre-test finite element analysis (FEA) including Guyan reduction; experiemental modal analysis (EMA) including data acquisition, parameter estimation, and development of a test-analysis model (TAM); correlation of FEA results with EMA; and tuning of the FE model. Correlation and tuning were used to improve the fidelity of the component models and hence their suitability for predicting overall response when joined together in a system model.
- Mini-Modal Testing of Wind Turbines Using Novel Excitation
  Modal testing of wind turbines can be fairly difficult because placing transducers on tall structures and providing low frequency excitation create problems. Moderate-size turbines are 100 to 200 feet tall, and their modal frequencies are very low, 0.1 to 5.0 Hz. In the mini-modal concept, only a limited number of response measurements are used in conjunction with a reasonably accurate finite element model to determine the modal parameters. Several techniques of low frequency excitation were explored, including impact, wind, step-relaxation, and human input. In tests using the mini-modal concept with human excitation, modal frequencies of large turbines have been determined in less than one day. As one application of these techniques, a prototype turbine was tested and two modal frequencies were found to be very close to integral multiples of the operating speed, which would cause a resonant condition. The design was modified to shift these frequencies, and the turbine was retested to confirm the expected changes in the modal frequencies.
- Dual-Channel FFT Analyzers
  Adding a second channel to a Fourier analyzer greatly increases its capability by allowing investigation of the relationship between two signals which occur simultaneously. A number of important two-channel frequency functions can be measured, all derived from the cross-power spectral function. These include frequency response, ordinary coherence, and sound intensity. This presentation reviews the definitions, physical significance, and some practical applications of these data types. Practical considerations for two-channel measurements with typical dual- or multiple-channel analyzers are discussed.
- Development of a Dynamic Pressure Response Calibrator
  Dynamic pressure measurements are made in all phases of testing, qualification, and production of turbine engine and rocket propulsion systems. These measurements require use of various sensors, including complex transducer/tubing configurations. Excitation devices used for frequency response characterizations are limited in ultimate peak pressure, bandwidth, and accuracy. In response to the need for an improved calibrator, a system has been developed for pressure levels ranging to 1.0 psi over the bandwidth of 2-2500 Hz. A prototype calibrator is capable of achieving accurate and controlled pressure for several sensor configurations, including flush-mounted, rake, and tube-type. Controlled pressure can be provided, in sine or random waveform, to a reference transducer and a transducer under test. For the frequency bands of 2-150 Hz and 1600-2400 Hz, acoustic pressure levels of 1.0 psi RMS can be attained. The application to acoustic-mechanical systems of a hybrid modeling technique known as admittance modeling is demonstrated. The technique combines test results with finite element analysis; here it is applied for a system with an acoustic pressure source driving various acoustic cavities.
- A Direct Complex Stiffness Test System for Viscoelastic Material Properties
  A test system designed specifically to acquire the complex moduli of viscoelastic materials in shear is described. Unique and innovative approaches in the mechanical design, temperature control system, and data acquisition methods provide a standard of accuracy that is rarely seen in dynamic mechanical properties of viscoelastic materials. The system operates on the principle of direct complex stiffness measurements. Unique sensors, hardware layout, and data acquisition and reduction methods maximize the frequency bandwidth and the dynamic range of stiffness, data acquisition speed, and temperature uniformity. Forced liquid convection temperature control also provides unparalleled speed and uniformity in specimen temperatures. Results are demonstrated and scrutinized using characterization software. Characterized data are stored in a database that provides the designer with the capability of searching based on the mechanical parameters commonly needed for damping designs. The end product is an end-to-end system capable of superior data accuracy and acquisition rates, and software that enables the most critical evaluation of results and ready storage in a manner that is efficient for damping design applications.
- Viscoelastic Material Properties in a High Pressure Environment
  Hardware representative of viscoelastic damping material in a cavity in a spinning jet engine blade was investigated. Specimens representing jet engine fan blades were analyzed, designed, fabricated and spun to establish that elastomer filled cavities can be designed for service in high-g environments. It was also shown that such systems can be analyzed using conventional finite element analysis. Spin rates of 7500 RPM were achieved which at a radius of 14 inches resulted in a g-level of 22,400 in the outer edge of a constrained viscoelastic material (VEM) damping treatment. Static strain readings were taken for the cavity walls. Dynamic testing was conducted and some excitation and response vibration data was acquired during spin. The elastic constants and elastomeric properties such as shear modulus, youngs modulus, bulk modulus, and Poisson's ratio of the VEM were also experimentally investigated in the laboratory. Initial results from these investigations are reported upon here.
- Complex Stiffness Measurement of Vibration-Damped Structural Elements
  As vibration suppression technology has matured, the application of viscoelastic materials in passive damping mechanisms has proven to be a reliable means towards improved structural dynamics. This paper discusses general steps required in characterization of viscoelastic damping elements and presents several successful passive damping devices as examples of the approach. Sample devices include a damper for the Hubble Space Telescope Solar Array 3, a damped strut built for the FORTÉ satellite a viscoelastic isolator, and a cocured viscoelastic/composite strut. When damping is built into a structure with a damped element, it is necessary to measure the element of stiffness to understand its effect on the system dynamics. The stiffness measurement is complex because of the level of damping. The complex stiffness function, with both real and imaginary components, characterizes the behavior of a damping device, a damped structural element, or a sample of viscoelastic material. Stiffness characterization of structural elements with viscoelastic damping is presented in terms of real stiffness and loss (imaginary stiffness/real stiffness).
- Image Stabilization Testbed (ISTAT)
  The Image Stabilization Testbed (ISTAT) is a high-bandwidth angular motion system for the simulation of missile dynamics with capability beyond that of current flight motion simulators (FMS). This paper describes the development and initial laboratory integration of the ISTAT. The intention is to mount a missile seeker and any associated inertial measurement sensors, and then allow ISTAT to replicate the dynamic boundary conditions at the base of the seeker resulting from both airframe vibrations (flexible body motion) as well as rigid body motion resulting from vehicle control forces or the flight environment. ISTAT will be driven by the output of deterministic simulations and will replicate the time history of the command signals. ISTAT makes use of high bandwidth hydraulic actuation and advanced feedback and feedforward control algorithms to deliver two- and three-axis motion control at frequencies from DC to greater than 500 Hz. The largest motions, achieved at lower frequencies, are about two degrees. The paper describes the motivation, the servohydraulic, mechanical, and electronic subsystems, control software and algorithms, and the software user interface for the testbed. An initial report on the system integration is also provided.
- Advanced Iso-Grid Fairing Qualification Test for Minotaur Launch Vehicle
  The development of the grid-stiffened fairing to be flown on the Minotaur launch vehicle has made significant progress over the past five years. During November and December 2002, this qualification structure was subjected to static qualification testing at the Air Force Research Laboratory (AFRL/VS) at Kirtland Air Force Base. The 6 m (20 ft) tall fairing was constructed of a carbon fiber composite grid structure that was over-wrapped to create a laminated skin. Upon completion of curing and machining, the fairing was cut in half to create the classic “clam-shell” fairing. Metallic joints were bonded and fastened to the fairing at all interfaces to complete the assembly process and simulate attachment of the base to a launch vehicle. Static qualification of the fairing tested the integrity of the fairing, thereby proving the design and manufacturing process. Loads were applied incrementally in a static loading scenario. The applied load envelope exceeded worst-case dynamic flight conditions with an added safety factor of 25%. At peak load the fairing must maintain structural integrity remaining within the defined dynamic displacement envelope.
- Structural Qualification of Unique Aerospace Structures
  The Air Force Research Laboratory, Space Vehicles Directorate (AFRL/VS), and CSA Engineering have developed a large scale structural testing facility on Kirtland Air Force Base, NM. This facility is capable of applying static and dynamic loads with up to 18 independent hydraulic actuators. Coupled with this servo-hydraulic load control unit is a fully integrated 256- channel data acquisition system (DAS). Configurable for strain gages, LVDT’s, or virtually any other strain gage-based or high level sensor. The DAS and load controller communicate and function simultaneously through software written by MTS Corporation. To date, AFRL and CSA Engineering have conducted four major static tests on developing launch vehicle technologies. These tests include two payload adapters, a large advanced grid stiffened composite fairing, and an advanced Payload Attach Fitting (PAF). Load levels for these tests have ranged from less than 13.3 kN (3 kip) on the Minotaur Multiple Payload Adapter to over 1.90 MN (425 kip) on the Boeing Delta IV PAF. Detailed test procedures, test design, and a testing overview for each static test configuration will be discussed.
- Testing of the Delta IV Payload Attach Fitting (PAF) with an Integral Composite Flange
  The Air Force Research Laboratory, Space Vehicles Directorate (AFRL/VS), has developed a unique composite Payload Attach Fitting (PAF) shell design. Alliant Techsystems (ATK) and the Boeing Company, Huntington Beach, CA, have developed manufacturing processes and a shell design that incorporates an all-composite forward flange with transverse reinforcement (Z-pinning) at key locations to prevent transverse separation and failure under high bending loads. This paper describes testing equipment, instrumentation, and procedures developed and executed by the AFRL in Albuquerque, NM to demonstrate the structural adequacy of the composite joint under increasing quasi-static load cycles near predicted failure. The overwhelmingly positive results obtained from this design and test program have led to new initiatives to develop modified designs for similar PAF structures.
- 2-D Biaxial Testing and Failure Predictions of IM7/977-2 Carbon/Epoxy Quasi-Isotropic Laminates
  In previous research, a series of a thickness-tapered cruciform specimen configurations have been used to determine the biaxial and triaxial strength of several carbon/epoxy and glass/vinyl-ester laminate configurations. The presence of a biaxial strengthening effect in quasi-isotropic, [(0N/90N/±45N)M]S, laminates have brought into question whether the cruciform geometry could be used to successfully generate two-dimensional strength envelopes. In the present study, a two-dimensional failure envelope for a IM7/977-2 carbon/epoxy laminate was developed at the Air Force Research Laboratory, Space Vehicles Directorate. Results are promising as they indicated that failure in the majority of the specimens occurred in the gage section. Multi-continuum theory (MCT) was used to predict and analyze the onset of damage. An accurate prediction of constituent failure at sampling points throughout the laminate provides a genesis for progressively analyzing damage propagation. Reasonable correlation between analytically and experimentally developed failure envelope suggests that the thickness-tapered cruciform specimen can be used to determine the biaxial strength of quasi-isotropic laminates.
- Design and Evaluation of a Reinforced Advanced-Grid Stiffened Composite Structure
  A composite grid-stiffened structure concept was selected for the payload fairing of the Minotaur launch vehicle. Compared to previous designs, this concept is lighter weight and requires reduced manufacturing costs. Various failure mechanisms were examined for the composite grid-stiffened structure. The controlling criterion for this design was determined to be joint peel-off failure. The identification of this failure mechanism and the assessment of bounding strains to control it, required extensive test and analysis effort. The final fairing design incorporated an undesirably thick skin to reduce the strain between the skin and ribs. This project investigated a means of controlling joint failure of the un-reinforced gridstiffened structure in an effort to reduce the skin thickness. Implementing lightweight foam inserts between the stiffeners on the interior of the fairing delayed failure to higher loads. A foam reinforced test panel, designed with equal mass to the current structure, withstood a substantially greater compressive force. The alternative concept also demonstrated an improved response, failing in global buckling instead of experiencing early failure due to joint strain.
- Test Design and Static Testing of the Atlas V CCB Conical InterStage Adapter
  A test facillity has been designed to qualify the Atlas V launch vehicle's Common Core Booster Conical InterStage Adapter. The composite conical ISA is being manufactured from an innovative tooling design that will be validated only after successfully completing the qualification test series. The structures, test hardware, and instrumentation used during test operations have been designed to meet all program requirements. Testing operations will include application of simulated launch loads witnessed by the conical ISA during flight. When completed, the test facility will function as an asset to the Air Force for future structural qualification tests.
- An Alternative to Pyrotechnic Testing For Shock Identification
  Flight components and structural systems often require qualification testing consisting of subjecting the payload to a specified shock response spectrum (SRS). Testing is used to qualify sensitive packages for flight, ensuring that the unit will survive and continue to function normally during and after being subjected to the anticipated environment. Traditional pyroshock testing is expensive, can over-test flight components, and is often not repeatable within desired limits. A compressed-air shock test system has been developed using a pneumatic gun to deliver repeatable and controlled transient loads, simulating high-shock flight conditions. This “shock gun” delivers a captive projectile through a barrel to provide the desired structural response. Measured accelerations are used to quantify the shock performance, typically using the SRS technique. Shock isolation systems have been directed at attenuating high-level, high-frequency loads associated with typical conditions such as launch vehicle fairing and stage separation events. The performance of the present system will be reviewed, and possible extensions of this test method, including ultimate limitations in frequency and amplitude, will be discussed in the paper.
- Failure Testing of Large Composite Aerospace Structures
  Large composite aerospace structures were tested to failure under flight-like static load conditions. Through rigorous test discipline and testing of relevant aerospace hardware, this research program provides previously unavailable experimental data to validate and augment current and advanced analytical techniques.
- An Inertially Referenced Non-contact Sensor for Ground Vibration Tests
  An Inertially Referenced Non-contact Sensor for Ground Vibration Tests
Vibration Isolation and Damping
- Analysis of Torsional Vibration in a High Bandwidth Angular Position Servo
  The method of finite elements is used to predict the torsional vibration characterisitcs of the capstan rotor of an instrumentation tape recorder. The usefulness of such results in designing a high bandwidth servo is discussed. Comparisons of finite element predictions, with results of forced vibration experiments on an actual capstan rotor, are presented, and agreement is found to be satisfacory.
- Finite Element Prediction of Damping in Beams with Constrained Viscoelastic Layers
  Vibration control in structures by means of viscoelastic material in constrained layers has gained wide acceptance, particularly in the aerospace industry. A key to increased use of damping technology is the ability to analyze and define the candidate viscoelastically damped structure accurately and efficiently in a project environment. This paper describes and establishes the validity of the modal strain energy approach, implemented with finite element techniques, for damped, laminated beams. The modal strain energy approach uses the modal strain, energy distributions, obtained by purely elastic analysis, to predict modal damping (loss) factors. These distributions may also be used by a designer as a tool to choose the best location and material for optimum damping. The approach described in the paper may easiy be extended to complex structures.
- Passive Damping Technology
  This paper presents a brief review of techniques for designed-in passive damping for noise and vibration control. Designed-in passive damping for structures is uaually based on one of four damping technologies: viscoelastic materials, viscous fluids, magnetics, or passive piezoelectrics. These methods are discussed and compared. The technology of using viscoelastic materials for passive damping is discussed in more detail than the other methods since it is presently the most applicable for surface treatments for noise control. Testing and characterization of viscoelastic materials and design methods for passive damping are discussed. An example showing the benefits of a passive damping treatment for an acoustic problem is presented.
- Sensitivity Analysis of Responses to Dynamic Loads
  Sensitivity analysis is a key element of structural optimization and optimization-based methods of parameter identification. Sensitivities of responses to dynamic loads pose special challenges because of the volume and complexity of data in a complete frequency spectrum or time record. An efficient method for computing the sensitivity of one or more peaks in a frequency spectrum or time domain is presented. The method accounts for shifts in peak location as well as changes in peak magnitude as design variables are varied. Brief examples are included.
- A Magnetic Tuned-Mass Damper for Buffet-Induced Airfoil Vibration
  Vibrations are an inherent problem for aircraft structures, especially military aircraft that perform high-speed maneuvers. A form of unwanted vibration called buffet occurs when an aircraft surface is directly exposed to an unsteady, vortex flow generated upstream during high-angle-of-attack maneuvers. In the case of the F/A-18 aircraft, buffeting of the twin vertical stabilizers excites the bending and torsional modes of these structures, and, over time, expensive fatigue failures occur. A tuned mass damper solution for this problem is proposed. Analysis indicates that an array of dampers employing eddy currents induced by rare earth magnets for the damping mechanism will provide sufficient damping to significantly reduce the dynamic response of the twin-vertical-tail buffet. A prototype damper was built, and it was determined by testing that this concept is a valid approach to solve this problem. The prototype was tested on a simple structure, selected so that the mass ratio with the prototype applied to the structure is equal to the mass ratio of the TMD array applied to the F/A-18 tail. The amplitude of the undamped response was reduced by a factor of 20 when the magnetic TMD was applied.
- Accurate Characterization of Passive Damping Materials with Database Storage and Retrieval on Different Computer Platforms
  An approach to successfully characterize a wide range of viscoelastic material data is discussed. Computer programs that implement the characterization process and allow the database storage and retrieval of materials based on dynamic properties are also discussed.
- Analysis and Design of Damped Structures
  Damping can be one of the more perplexing aspects of structural dynamics because it is much less intuitive and ``clean-cut'' than concepts like stiffness and mass. Nevertheless, analysts should not view damping simply as an unfortunate side issue in structural dynamics, but as an opportunity for creative design work aimed at suppression of unwanted vibrations. This paper presents a review of some basic concepts in damping and how damping is treated in finite element analysis. It also presents some new techniques that have been developed to support optimization of damping for structures.
- Passive Damping Design of a Thermoplastic Box-Platform
  Many space vehicles under consideration at the present time will contain sensors whose performance is affected by vibrations. One commom solution to vibration problems is to increase passive damping levels. A viscoelastically-damped thermoplastic box-platform representing a primary structural component with sensors was designed, built, and tested to demonstrate the feasibility of using advanced materials to both save weight and reduce the vibrations environment. The resulting structure was lighter in weight, stiffer, and achieved over an order-of-magnitude increase in passive damping when compared to a baseline aluminum structure.
- Temperature Control in Viscoelastic Dampers for Buildings
  Viscoelastic materials have been used successfully to reduce building deformations due to wind. Research has also indicated their suitability for counteracting small to moderate earthquake motions. Viscoelastic solutions are attractive in several ways. A completely passive solution is obviously desirable for earthquake dampers since power can't be relied on. Another advantage is that they work over moderately wide frequency ranges, unlike tuned devices. The large motions and forces associated with severe earthquake pose a serious problem. Large blocks of viscoelastic are needed to provide large forces. If the consequent heat isn't dissipated fast enough, the temperature may rise far enough that the material begins to lose stiffness and ceases ro provide the required damping forces.
- Demonstration of Solar Array Vibration Suppression
  An adaptive passive damping system is described for the low-order modes of spacecraft large solar arrays. Realistic system-level requirements are developed relating pointing error of a representative spacecraft to damping of its solar arrays. Performance is specified in terms of gain envelopes on open-loop frequency response functions for the structure with dampers. A family of remotely tunable, eddy-current tuned mass dampers (TMDs) is described which suppress several modes of a 430-lb solar array simulator in the frequency range of 0.1-1.0 Hz. The magnetic dampers are ground demonstration units, suitable for use in 1-G, but are designed around traceable, system-level dynamic requirements. The damper design is intended from the outset to be evolved into flight hardware. Tuning to TMD natural frequency and damping ratio to their optimum values is demonstrated through component-level base transmissibility test results. System-level tests using the solar array simulator demonstrate that resonant peaks for two target modes are reduced by 25-30 dB using an array of three dampers, each with a moving mass of about four pounds. Future work towards flight hardware is described.
- Experimental Modal and Finite Element Analyses, Modal Correlation, and Model Tuning for an Aircraft Brake Component
  A ``torque tube,'' part of an aircraft braking system, was investigated both experimentally and analytically, as part of a larger project with the goal of developing finite element models for the prediction of aircraft brake dynamics to frequencies as high as 2 kHz. This paper presents a case history of this effort, and covers the following tasks: pre-test finite element analysis (FEA) including Guyan reduction; experimental modal analysis (EMA) including data acquisition, parameter estimation, and development of a test-analysis model (TAM); correlation of FEA results with EMA; and tuning of the FE model. Correlation and tuning were used to improve the fidelity of the component models and hence their suitability for predicting overqll response when joined together in a system model.
- Implementation of the GHM Method for Viscoelastic Materials Using MATLAB and NASTRAN
  Representation of frequency-dependent viscoelastic material properties has long been problematical in the frequency domain and especially inthe time domain. The method of Golla, Hughes, and McTavish (GRM) addresses this problem with a Laplace-domain model of the complex material modulus, in which a number of parameters are determined by curve-fitting to experiemental data. The result is well suited to finite element formulations because the equations of motion retain the familiar second-order, constant-coefficent form, at the expense of some extra scalar degrees of freedom. This paper reports on an implementation of GHM in MATLAB, using FEM data imported from NASTRAN. Sample problems demonstate the efficacy and practiality of the method.
- Integral Passive Damping for Aircraft Fuselage Structure
  This paper describes the development of damped structure for the Supportable Technology for Affordable Fighter Structures (STAFS) Program. The study started with analytical design trade studies and progressed through a series of tests to characterize adhesive behavior, performance testing a a component level integrally damped panel concept, and full scale design integration. Analyses were performed on finite element panel models with viscoelastic elements in the bond areas to determine the sensitivities of configuration and adhesive type to overall damping achieved. The concepts studied showed that as much as 10 percent fabricated using super plastically formed-adhesive bonded 2095 aluminum, and tested to measure the comparative response improvement in the damped panels. A substantial weight savings was realized in comparison to the monolithic metal panels which would be required to withstand the acoustic environment.
- Interpretation of Active Dampers as Variable-Parameter Passive Devices
  This paper develops analogies between discrete devices that achieve damping by active and passive means. It describes the simplest models useful for inclusion in larger system design. Lumped-parameter models for representing passive dampers are given, and the similarity to local active feedback is shown. A model for active damper components containing piezoelectrics or similar materials is developed for force feedback compensation. This system is similar to the passive damper model when its parameters are allowed to vary. An active damper with direct feedback of the integral of measured force is represented by a model containing a single spring and a gain-dependent dashpot. Further aspects of the active damper are illustrated using a one-mode model of a vibrating structure. The effects of a feedforward modification to the system, and the analogy to changes in passive parameters, are illustrated by tracking the locus of the closed loop system poles with feedback gain.
- Passive Damping Technology Demonstration
  A Hughes Space Company study was undertaken to (1) acquire the analytical capability to design effective passive damping treatments and to predict the damped dynamic performance with reasonable accuracy; (2) demonstrate reasonable test and analysis agreement for both baseline and damped baseline hardware; and (3) achieve a 75% reduction in peak transmissibility and 50% reduction in RMS random vibration response. Hughes Space Company teamed with CSA Engineering to learn how to apply passive damping technology to their products successfully in a cost-effective manner. Existing hardware was selected for the demonstration because previous designs were lightly damped and had difficulty in vibration test; multiple damping concepts could be investigated; the finite element model, hardware, and test fixture would be available; and damping devices could be easily implemented. Bracket, strut, and sandwich panel damping treatments that met the performance goals were developed. The baseline, baseline with damped bracket, and baseline with damped strut designs were built and tested. The test results were in reasonable agreement with predictions and demonstrated that the desired reduction in dynamic response could be achieved.
- Active System for Vibration Isolation of Spacecraft Instruments
  This paper describes the development of a system designed to provide a stable, vibration-isolated platform for spacecraft sensors and instruments. The UltraQuiet Platform if made up of three subsystems: a six-strut, six-axis passive-active vibration isolation mount, a damped support bench, and specialized vibration isolation mounts which reduce transmission of narrowband vibration from individual noisy components on the quiet platform. this paper emphasizes the six-axis Stewart platform active isolation system, which uses a novel series approach to active-passive isolation within each strut. the stiff electromagnetic actuator and geophone velocity sensors in the struts are described. Control design and active vibration isolation performance are summarized. Vibration reduction of up to 20 dB was demonstrated over a 100 Hz bandwidth. The change in isolation performance which resulted when the system was mated with a highly flexible base structure in noted.
- Optimal Control Design for Systems with Collocated Sensors and Actuators
  Distributed sensors and actuators, placed in close proximity to one another, yield high bandwidth control systems that exhibit passivity characteristics that can be exploited in the design of robust structural control laws. In this paper, analogies between the Single-Input-Single-Output (SISO) case and Multiple-Input-Multiple-Output systems with collocated sensors and actuators are developed. The analogies are based on the fact that eigenvectors of complex symmetric matrices are orthogonal to their simple transpose, and that the eigenvalues of complex symmetric matrices are bounded by their real and imaginary components. These theorems are derived and applied to the analysis and control of nongyroscopic, noncirculatory mechanical systems. Transfer matrices of mechanical systems with collocated sensors and actuators are shown to be complex symmetric matrices whose eigenproperties are determined by the type of collocated feedback. These properties are derived for both the general damping case and for the case of modal dmaping. An optimal control technique based on the eigenproperties of complex symmetric systems is developed. The technique is derived in the paper and a design example is included.
- Active Vibration Isolation Using Adaptive Feedforward Control
  The structure and performance requirements for active vibration isolation control systems motivate the use of adaptive control. This is especially true for spacecraft platforms subject to uncertainties inherent in on-orbit operation. This paper is an initial investigation into adaptive control strategies and algorithms that may have application to isolation on spacecraft platforms. Analysis of the algorithms, numerical simulation, and laboratory test data are used to evaluate adaptive feedforward control. Of particular interest are the performance characteristics and limitations of the filtered-x LMS (FXLMS) algorithm. The Augmented Error algorithm and combination feedback/feedforward control are two means investigated to extend the capabilities of FXLMS by desensitizing the algorithm to the specific dynamics of the plant. Several experiments were conducted on a laboratory testbed which serves as the prototype for a planned active vibration isolation flight demonstration.
- Durability Patch: Repair and Life Extension of High Cycle Fatigue Damage on Secondary Structure of Aging Aircraft
  The Durability Patch Program addresses the repair and life enhancement of nuisance cracks of secondary structure due to resonant high cycle fatigue from aerovibroacoustics. For this type of damage, maintenance and repair costs are high. New cracks will emanate from the repair and eventually large areas of skin and substructure will have to be replaced. The Durability Patch consists of an elastic elliptical laminate overlaid and surrounded by a thoroughly integrated damping treatment. The bonded repair reduces static and dynamic stresses and crack tip stress intensities. Damping is maximized in order to enhance the life of adjoining structure with undetected damage. The life improvement goal is 600x. Finite element analysis results comparing static and vibratory stresses will be presented. High cycle fatigue and crack growth rates will be compared. The design and use of a miniature autonomous damage dosimeter to obtain service temperature and vibration environmental data at low cost will be described. Analysis and laboratory results will be presented.
- Vibration Mitigation for Wind-Induced Jitter for the Giant Magellan Telescope
  The Giant Magellan Telescope GMT is a planned large terrestrial telescope with a segmented primary mirror with a 24.5 meter overall diameter. Like most terrestrial telescopes, the GMT resides within an enclosure designed to protect the telescope from the elements and to reduce the effects of wind on the optical performance of the telescope. Wind impingement on the telescope causes static deformation and vibration in the telescope structure that affects the alignment and image jitter performance of the telescope. Actively controlled primary mirror segments and a secondary mirror can correct for the static and low frequency portions of the wind effects, but typically the actuators do not have the bandwidth to address higher frequency components of the wind environment. Preliminary analyses on the GMT indicate that the image jitter associated with wind effects meets budgeted allowances but without much margin. Preliminary models show that the bulk of the residual jitter arises from excitation of a small number of modes in the 9 to 12 Hz range. Therefore, as a risk mitigation effort to increase the margin on the wind induced jitter, passive and active vibration mitigation approaches have been examined for the GMT, which will be the focus of this paper. Using a finite element model of the GMT along with wind loading load cases, several passive and active vibration mitigation approaches were analyzed. These approaches include passive approaches such as tuned mass dampers targeting the worst offending modes, and constrained layer damping targeting all of the modes within the troublesome frequency range. Active approaches evaluated include two active damping approaches, one using several reaction mass actuators and the other using active strut type actuators. The results of the study show that although all approaches are successful in reducing the jitter, the active damping approach using reaction mass actuators offers the lightest weight, least implementation impact, and most adaptability of any of the approaches.
- Admittance Modeling: Frequency Domain, Physical Coordinate Methods for Multi-Component Systems
  Admittance modeling is a well-known but little-used technique for predicting the dynamic response of multi-component systems. Admittance functions (FRFs) measured or calculated for individual components are used along with in-service loads. FRFs are used directly; no estimation of modal parameters is necessary. The method does not require that the structural excitation be due to ideal sources; i.e., imposed forces or displacements described by explicit functions of time, independent of the structure on which they act. A typical application is the prediction of high-frequency vibration response of a sensitive payload, driven by its connections to an airframe which is acted on by unknown but repeatable dynamic loads. The underlying theory is summarized, a software package designed to render it practical is introduced, and results are given for several sample problems.
- Finite Element Prediction of Damping in Structures with Constrained Viscoelastic Layers
  An efficient method is described for finite element modeling of three-layer laminates containing a viscoelastic layer. Modal damping rations are estimated from undamped normal mode results by means of the modal strain enery (MSE) method. Comparisons are given between results obtained by the MSE method implemented in NASTRAN, by various exact solutions for approximate governing differential equations, and by experiment. Results are in terms of frequencies, modal damping ratios, and mechanical admittances for simple beams, plates, and rings. Application of the finite element-MSE method in design of integrally damped structures is discussed.
- Prediction of Damping in Structures with Viscoelastic Materials
  This paper describes an efficient method for finite element modeling of structures containing a viscoelastic material. Modal damping ratios are estimated from undamped normal mode results by means of the modal strain energy (MSE) method. Comparisons are given between results obtained by the MSE methods implemented in MSC/NASTRAN, by various exact solutions for approximate governing differential equations, and by experiment. Results are in terms of frequencies, modal damping rations, and mechanical admittances for simple beams, plates, and rings, as well as for an actual hardware application. Use of the finite element/MSE methods in design of integrally damped structures is discussed.
- Design Metholds for Viscoelastically Damped Plates
  Viscoelastic damping material is commonly used for vibration control in aerospace structures. Once of the most weight-efficient forms is a thin layer constrained between two metal face sheets to form a sandwich or integrally damped panel. Three methods are presented for efficient design analysis of flat sandwich plates. Each is intended to provide a different balance between accuracy, generality, and cost of use. The most general is the modal strain energy (MSE) method implemented in NASTRAN. A cheaper method usable for a variety of nondissipative boundary conditions is presented in terms of dimensionless charts. These are compiled from a large number of NASTRAN MSE analyses performed for various boundary conditions. Finally, a simple, inexpensive method based on a closed form solution is presented. It is strictly correct only for simply supported boundary conditions but can, with some judgement, provide useful results for other cases. Comparisons between the first and last methods are presented for simply supported boundary contitions as a means of testing the basic premise of the MSE method. They are presented for other boundary conditions to demonstrate the approximations involved in the simple, closed form method.
- Finite Element Design of Viscoelastically Damped Structures
  This paper describes four methods for the finite element analysis of structures containing a viscoelastic material. These methods fall into the categories of methods for damping treatment selection or methods for response prediction. The main emphasis of this paper is directed towards the Modal Strain Energy (MSE) method. The MSE method uses normal mode techniques and, therefore, is an efficient method for the design of viscoelastically damped structures. The MSE method, implemented by finite element techniques, will aid the analyst in 1) selecting the location of the damping treatment, 2) selecting the damping treatment, 3) predicting the modal damping factors, and 4) predicting the responses of the structure. A discussion of finite element modeling methods for structures containing viscoelastic materials is included. Two structures are discussed for which viscoelastic damping treatments were designed using the MSE technique. Comparisons of predicted and measured modal damping and frequencies are given.
- Design and Analysis of Damped Structures Using Finite Element Techniques
  This paper describes four methods for the finite element analysis of structures containing a viscoelastic material. These methods fall into the categories of either methods for damping treatment selection or methods for response prediction. The main emphasis of this paper is directed towards the Modal Strain Energy (MSE) method. The MSE method uses normal mode techniques and, therefore, is an efficient method for the design of viscoelastically damped structures. A discussion of finite element modeling methods for structures containing viscoelastic materials is included. An example of finite element design of a heavy, built-up structure is presented. The damping treatment for this structure was desired to damp over a wide frequency range. Comparisons of predicted and measured modal damping and response are given.
- A Viscoelastic Damping Treatment for Snow Skis
  Damping of the first few modes of vibration of a snow ski is an important factor in the ski's performance under certain snow conditions. An existing design for an undamped competition giant slalom ski was modified to incorporate integral viscoelastic damping. The undamped ski is essentially a tapered box beam using fiberglass and metal as the load carrying materials. It was modeled using finite element techniques and the model was verified by modal tests of a production ski of this design. A target value for damping was chosen based on subjective evaluation of existing skis. The modal strain energy method implemented by finite elements was used to evaluate a number of damping design concepts using viscoelastic material within the ski. The most likely candidate was chosen and the finite element model was then used to determine the viscoelastic layer thickness and material properties required to give the desired damping. Prototypes were fabricated and tested, both objectively to determine modal damping factors and subjectively to detrmine in-service performance. Comparisons of predicted and measured damping values are given and the difficulties of designing damping into weight-efficient structures are discussed.
- Measurement, Synthesis, and Use of Transmissibility Functions
  Transmissibility functions are frequency response functions between like variables (motion response/ motion input) as opposed to admittance functions which are frequency responses between conjugate variables (motion response/force input). While the two types of functions have substantially different properties, both are valuable in dynamic testing and analysis. Some properties of transmissibility functions are summarized and contrasted with those of the more widely used admittance functions. Synthesis of base transmissibility functions from measured, fixed-base, normal modes is demonstrated and a comparison is given between synthesized and directly measured functions for a scale model of an electrical cabinet. Applications to seismic qualification of nuclear plant equipment and performance prediction of isolator systems are discussed with examples.
- Design Analyses and Tests of Viscoelastically Damped Generic Space Trusses
  Design of demonstration and full scale viscoelastically damped generic space trusses is presented. The Modal Strain Energy method, implemented using finite elements, was used to design both truss systems. Two full scale trusses were built of different materials to demonstrate material damping. Discrete viscoelastic dampers were designed and installed on the lower diagonal members to provide additional damping. Comparisons of the predicted performances of both the demonstration and full scale trusses were made with experimental results. These trusses demonstrated the feasibility of the damping concept and verified the accuracy of the analytical design method.
- Analytical Design with Viscoelastic Materials for Passive Vibration Control
  Resonant vibration in the rotor, frame, or support structure of a large rotating machine can often be reduced by increasing the damping of the offending mode. The inclusion of a viscoelastic material into the machine's primary structure can be an effective way of achieving this end. However, the effective use of such materials requires specialized design techniques. Several methods are described for finite element analysis of structures containing viscoelastic materials. One in particular, the Modal Strain Energy method, allows modal damping factors to be predicted in much the same way as is commonly done for undamped natural frequencies and mode shapes. This in turn allows damping to be optimized at the design stage. The basis of each approach is reviewed and a design example is presented wherein the Modal Strain Energy method is applied. An add-on damping treatment is designed for a heavy steel weldment typical of the support structures in large rotating machines. Comparisons are given between analytical predictions and measurements of damping ratios, natural frequencies, and mode shapes.
- Application of MSC/NASTRAN and ADS/NASOPT to Noise Transmission Problems
  This paper addresses the problem of noise transmission from rotating machinery through a support structure to a foundation. The problem is attacked using dynamic finite element analysis with MSC/NASTRAN and optimization with ADS/NASOPT. First, the structure is automatically redesigned so as to drive resonant frequencies away from rotating frequencies. Then, a method is developed and illustrated for calculating the sensitivities of complex steady-state displacements to small changes in design variables. This information is passed to ADS/NASOPT so that it can redesign the structure to minimize the dynamic response directly. The methods are applied to a demontration problem.
- Design and Testing of a Sixty-Foot Damped Generic Space Truss
  Two sixty-foot, viscoelastically damped generic space trusses have been designed, fabricated, and tested for the PACOSS (Passive and Active Control of Space Structures) Program. They are demonstration structures which use integral viscoelastic damping for vibration control. Their purpose is to illustrate the application of the technology to a type of structure important in space applications. The Modal Strain Energy method, implemented by finite elements, was used to design the damping elements for the trusses. Damping due to the damping elements as well as hysteresis in the trusses' structural members was calculated by the finite element model. A modal test of one of the trusses was conducted to verify the model. Results of the analysis and test are compared and conclusions are drawn relative to the design method and to the use of viscoelastic damping for truss structures.
- Optimization Methods for Design of Viscoelastic Damping Treatments
  The modal strain energy method, implemented in finite elements, is the best method for designing viscoelastic damping treatments and predicting modal damping for complex structures. This mehtod uses real normal modes and estimates damping as a function of the viscoelastic material's strain energy and its loss factor. Designs in the past have been optimized manually, i.e., by running a number of analyses and using the results of each analysis to attempt manual improvements of the design. This paper reports on an automated optimization method which couples MSC/NASTRAN with the ADS optimizer. Design parameters are computed to maximize the effectiveness of modal damping treatments while observing weight constrains, frequency constraints, and side constraints. The design parameters considered are the viscoelastic shear modulus and thickness, and constraining layer thickenss for constrained layer treatments. This optimization software speeds up the design process, and allows engineers to explore more design concepts than they migh totherwise. Examples are presented to show the effectiveness of the procedure.
- Economical Prediction of System-Level Damping in Complex Structures
  The modal strain energy (MSE) method makes the. The design of a passive damping treatment is inherently an iterative process. However, experience and problem constraints usually eliminate some of the possible combinations of the damping treatment parameters, but many questions are only answered by an actual execution of the finite element model.
- Optimization of Structures with Passive Damping and Active Controls
  Vibration suppression may be approached in three ways: by optimal distribution of structural mass, by viscoelastic damping treatments, and by active feedback control systems. Software design tools that address all three approaches are needed for SDI structures so that each approach can be explooited with minimal weight penalties. This report documents development in structual optimization under dynamic loads, both steady-state and transient. New methods for these problems are derived and demonstrated on small-scale strucrural models. The methods address minimization of structural mass subject to constraints on peak responses in either domain, together with frequency constraints and side constraints on member sizes. The methods are exercised with and without active control systems. A follow-on effort is proposed which expands the initial optimization capability and incorporates optimization of damping treatments, links to a viscoelastic materials database, and a database for finite element analysis models, design models, responses and response sensitivities.
- Interactive Characterization and Database Storage of Complex Modulus Data
  Test data for viscoelastic damping materials often do not provide thorough coverage of temperature and frequency. A methodology for processing damping material complex modulus data and presenting it in a manner that is meaningful to the damping designer is discussed. The use of a computer program for characterization of complex modulus data is described. A database program for storage and retrieval of characterized complex modulus data is also described.
- Analysis and Testing of a Damping Treatment for a Multi-Component Space Structure
  A large space structure required at least 1% viscous damping for each of its four lowest global modes to reduce vibration response. Due to the complexity of the problem, two of the three components in the system were represented only by stiffness and mass matrices at a reduced set of points. The third component was represented by a finite element model. Damping designs were produced and their performance predicted by computing system-level modal strain energy using both the finite element model and the condensed stiffness matrices. The chosen design produced the required damping with less than 0.2% added weight.
- Optimized Designs of Viscoelastic Damping Treatments
  The modal strain energy method is a proven design tool for viscoelastic damping treatments. It provides a quantitative criterion for evaluation of candidate designs using finite element models coded for NASTRAN. This paper presents a method for optimizing damping treatments. As design variables, the method uses visocoelastic stiffness and layer thickness, and thicknesses of constraining layers or base layers. It seeks to maximize viscoelastic modal strain energy subject to constraints on weight or natural frequencies. Optimization of a damping treatment for a demonstration space structure is shown. Only four complete NASTRAN runs were required to produce a reduction in RMS response by a factor of more than seven for a PSD base input. Current research aimed at increased efficiency is discussed.
- Damping Design for a Disk Drive Head Flexure
  This paper describes the work done by 3M and CSA Engineering (under contract to 3M) that led to the design of a high-performance damper for load beams in disk drive head suspensions. 3M has supplied dampers for the past eight years. While the current damper designs do a good job of attenuating the first bending mode of the load beam, they do only a satisfactory job of attenuating the first torsion mode and a less than satisfactory job of attenuating the sway mode. If excited, this is the most severe mode with regard to operation of the disk drive servo heads. After the drive is manufactured, if a resonant condition occurs in the load beam of the servo suspension the drive is rendered useless. The purpose of this work was to design a damping treatment that attenuates all three modes in a satisfactory manner. The paper describes the four phases that are typical in the design of a damping treatment: mathematical modeling of the undamped system, incorporation of a damping treatment into the mathematical model, arriving at the optimum design for a damping treatment, and preparing and experimentally evaluating the design.
- Passive Damping Technology using Viscoelastics
  This paper presents a brief review of techniques for designed-in passive damping for space structures. Although the main topic of the paper is passive damping using viscoelastic materials, other methods are discussed and compared. The testing and characterization of viscoelastic materisla and design methods for passive damping are discussed. An example showing the benefits of a passive damping treatment for an actual space structure is presented.
- Smart Tuned-Mass Dampers
  Passive damping using tuned-mass dampers (TMDs) is well-known, weight-efficient approach to suppress vibrations of a single mode (or a group of modes). A TMD produces a high level of damping with a small amount of added weight if kept tuned to the frequency of the offending mode. A prototype TMD that will tune itself to an offending mode has been designed, built, and tested. It can keep itself tuned to the offending mode, even if that mode changes frequency. The prototype confirmed that a "smart" TMD could be built.
- Complex Stiffness Test Data for Three Viscoelastic Materials by the Direct Complex Stiffness Method
  An integral part of the Damping and Metal Matrix for Precision Structures (DAMMPS) program involves the acquisition and dissemination of complex stiffness data for several viscoelastic material (VEM) specimens. Three such specimens have been tested by the Lockheed Missiles and Space Company, Inc. DAMMPS team: 3M Y-966, Soundcoat DYAD-606, and 3M ISD-112. These particular materials were chosen to demonstrate usable loss factor amplitudes which span the anticipated frequency and temperature range of operation for the structure studied under the contract. Future testing will characterize these and other selected specimens in more detail to povide an accurate material properties database for the detailed design task. Test data were collected very efficiently utilizing the Direct Complex Stiffness (DCS) method and manipulated/reduced using the VEMINT program. In this paper, salient aspects of the test and data reduction procedures are presented, as well as several forms of the complex modulus data.
- VEM Characterization Program
  The development and use of an interactive computer program for the characterization of complex modulus data is described. The program uses the collocation process which accurately fits the real part of the complex modulus data and then uses the lack of fit of the loss factor to adjust the temperature shift function. This iterative method, which has converged when both the real modulus and the loss factor are fit simultaneously (the real directly and the loss factor indirectly), yields the most accurate estimate of the temperature shift function possible.
- VEM Database Program
  A three-tiered implementation of a viscoelastic material (VEM) database under development is described. Using low-level calls, searches for characterized VEMs may be conducted based on property contraints (e.g., modulus and loss factor at certain temperature and frequency) and/or other criteria (e.g., available thicknesses, type, etc.). 1. A graphical front end program that runs on a Macintosh personal computer is being written. It will be dynamically linked to VEM characterization and testing programs for data sharing. 2. A stand-alone program for UNIX machines using X windows is being written. Reports will be in the form of tables and X-Y plots. A similar program to run under MS-DOS is also being developed concurrently. 3. A VEM database engine which may be compiled and run on any computer that supports ANSI FORTRAN 77 is described. The engine consists of FORTRAN callable subroutines that search a VEM database created by a librarian program using VEM characterization data files.
- Measurement of the Mechanical Properties of Viscoelastics by the Direct Complex Stiffness Method
  Accurate material properties are essential for the design of viscoelastic damping treatments and material properties are often the predominant error source process when modal strain energy techniques are implemented. The trade-offs between various test techniques are discussed with primary emphases on a system developed at CSA Engineering for direct complex stiffness measurements on viscoelastic materials. Issues such as analog front-end design, temperature control, and system software are discussed.
- Analysis and Design of Damped Structures
  Damping can be one of the more perplexing aspects of structural dynamics becuase it is much less intuitive and 'clean-cut' than concepts like stiffness and mass. Nevertheless, analysts should not view damping simply as an unfortunate side issue in structural dynamics, but as an opportunity for creative design work aimed at suppression of unwanted vibrations. This paper presents a review of some basic concepts in damping and how damping is treated in finite element analysis. It also presents some new techniques that have been developed to support optimization of damping for structures.
- Passive Vibration Suppression
  This article discusses various vibration suppression technologies and their advantages and disadvatages. Since much of the passive vibration suppression to date has been accomplished using viscoelastic materials, this article will emphasize this technology. Applications of passive vibration sppression are ddiscussed in other articles in this issue and have been discussed recently in the following previous articles: "Damped Graphite/Tp Composite Platforms," Spring 1991; "Aerojet ALAS Program Utilizes Advanced Composites and Passive Damping," Winter 1992; "The AMASS Project," Winter 1992.
- A Simple Approach to Design, Installation,and Testing of Passive Damping for an Optical System
  Passive damping hardware has been fabricated, installed, and tested on a large optical system required to meet a stringent RMS specification for closed-loop residual jitter due to ground and coolant flow excitations. Over the course of the study, contrained layer treatments, tuned-mass dampers, and link dampers were analyzed. It was shown through analysis that one type of treatment alone was not sufficient to solve the problem due to the number of harmful modes spread over a relatively wide frequency range. The final solution incorporated constrained layer damping treatments on an interface component between the mirrors and their mounts, link dampers between selected locations on the opical bench, and constrained layer treatments on some mirror support plates. Tests were performed before and after the application of the damping hardware to demonstrate its effectiveness and applicability to this class of problem. The primary focus of this paper is the dynamic testing of the optical system and design, fabrication, installation, and qualification of the passive damping hardware.
- SPICE Finite Element Modal Correlation
  This paper presents the results of the correlation between the finite element modal test data for the SPace Integrated Control Experiment (SPICE) truss structure. For structural control experiments such as SPICE, the control system requires accurate knowledge of a large number of modes, thereby placing a greater demand on the modal test and on the fidelity of the finite element model. Excellent correlation results were obtained as demonstrated in the test-to-analysis orthogonality matrix, which was within 95% on the diagonal and 10% for off diagonals covering the first 25 modes over 60-Hz frequency bandwidth. Incremental model improvements were performed with the aid of design sensitivities and optimization. Component level testing, pretest analysis, and close analysis/test engineering collaboration were the keys to this success.
- Implementation of the GHM Method for Viscoelastic Materials using MATLAB and NASTRAN
  Representation of frequency-dependent viscoelastic material properties has long been problematical in the frequency domain and especially in the time domain. The method of Golla, Hughes, and McTavish (GHM) addresses this problem with a Laplace-domain model of the complex material modulus, in which a number of parameters are determined by curve-fitting to experimental data. The result is well suited to finite element formulations because the equations of motion retain the familiar second-order, constant-coefficient form, at the expense of some extra scalar degrees of freedom. This paper reports on an implementation of GHM in MATLAB, using FEM data imported from NASTRAN. Sample problem demonstrate the efficacy and practicality of the method.
- Design of Passive Damping Systems
  This paper presents a brief review of technicques for designed-in passive damping for vibration control. Designed-in passive damping for structures is usually based on one of four damping technologies: viscoelastic materials, viscous fluids, magnetics, or passive piezoelectrics. These methods are discussed and compared. The technology of using viscoelastic materials for passive damping is discussed in more detail than the other methods since it is presently the most widely used type of damping technology. Testing and characterization of viscoelastic materials and design methods for passive damping are discussed. An example showing the benefits of a passive damping treatment applied to a stiffened panel under an acoustic load presented.
- Cocured Viscoelastic Composites
  Conventional composite materials have high stiffness-to-weight ratios but exhibit little damping; many viscoelastic materials provide high levels of energy dissipation with minimal structural stiffness. The objective of this work was to combine these two material types to produce highly damped structural elements with favorable stiffness and weight characteristics. Cocuring refers to the inclusion of one or more layers of viscoelastic damping material sandwiched between composite plies prior to curing of the composite. Cocured viscoelastic/composite layups were studied experimentally at the material level, modeled analytically, and used to build optimized damped structural components. Measured cocured material properties were used in finite element models to design damped components which were built and tested individually and as part of a truss test structure. Load-carrying and highly damped struts and panels were fabricated. The curing process modified the viscoelastic behavior to some degree, but the materials retained significant, and predictable, damping capability.
- UltraQuiet Platform for Active Vibration Isolation
  This paper describes an active/passive system designed to provide a stable, isolated platform for vibration-sensitive equipment. The UltraQuiet Platform includes three subsystems: a six-strut, six-axis passive-active vibration isolation mount, a damped support bench, and specialized vibration isolation mounts which reduce transmission of narrowband vibration from individual noisy components on the quiet platform. This paper emphasizes the six-axis Stewart platform active isolation system, which uses a novel series approach to active-passive isolation within each strut. The stiff electromagnetic actuator and geophone velocity sensors in the struts are described. Control design and active vibration isolation performance are summarized. Vibration reduction of up to 20 dB was demonstrated over a 100 Hz bandwidth. Reduction in isolation performance which resulted when the system was mated with a highly flexible base structure is noted.
- Viscoelastic Struts for Vibration Mitigation of FORTE
  FORTE is a small satellite being developed by Los Alamos National Laboratory (LANL) and Sandia National Laboratories Albuquerque (SNLA). It will be placed into orbit via a Pegasus launch in 1996. Testing indicated that acceleration levels caused by structural resonances exceed component levels to which certain sensitive components had previously been qualified. Viscoelastic struts were designed to reduce response levels associated with these resonances by increasing the level of damping in key structural modes of the spacecraft. Four identical shear-lap struts were fabricated and installed between the two primary equipment decks. The struts were designed using a system Finite Element Model (FEM) of the spacecraft, a component FEM of the strut, and measured viscoelastic properties. Direct complex stiffness testing was performed to characterize the frequency-dependent behavior of the struts, and these measured properties (shear modulus and loss factor) were used to represent the struts in the spacecraft model. System-level tests were repeated with the struts installed and the response power spectral densities at critical component locations were reduced by as much as 10 dB in the frequency range of interest.
- A Passive Damper Exhibiting the Ideal Dashpot Characteristic, F=CV
  One of the most diufficult tasks in the structural control industry is providing linear, predictable, passive damping over a wide frequency range. This challenge has been worked around successfully in the past, but rarely has it been performed ideally. The subject matter of this paper takes a radical step toward attraining the goal of linear damping performance, while adding very low static stiffness to the system being damped.
- Durability Patch--Application of Passive Damping to High Cycle Fatigue Cracking on Aircraft
  Although high-cycle fatigue cracks in secondary structure are often termed "nuisance cracks," they are costly to repair. Often the repairs do not last long because the repaired part still responds in a resonant fashion to the environment. Although the use of visco-elastic materials for passive damping applications is well understood, there have been few applications to high-cycle fatigue problems because the design information, temperature, resonant response frequency, and strain levels are difficult to determine. The Damage Dosimeter and the Durability Patch are an effort to resolve these problems with the application of compact, off-the-shelf electronics, and a damped bonded repair patch. This paper will present the electronics and patch design concepts as well as damping performance test data from a laboratory patch demonstration experiment.
- Integrated Electromechanical Devices for Active Control of Vibration and Sound
  Advances in transducers and electronics have made possible integrated electromechanical devices for active vibration and noise control. This paper describes one such system which makes use of piezoelectric materials. An integrated device employing piezoceramic actuators and sensors, analog electronic signal conditioning, programmable control components, and a voltage amplifier is described. Issues driving design of each functional subsystem are addressed. The device is packaged using flex circuit technology and other electronics industry methods. The means of integrating transducers and other components are noted. Test results indicating the vibration suppression capability are presented, and the considerably greater possibilities for the more sophisticated control designs using the same system are summarized. Potential applications in active vibration and sound control are described, and uses of the broader technology, beyond the specific device design, are summarized.
- Reconfigurable Actuator-Sensor Arrays for the Active Control of Sound
  A reconfigurable actuator-sensor array is demonstrated for the active control of sound and vibration. The concept is motivated by applications in which shaped polyvinylidene fluoride films are used as error sensors for feedforward and feedback noise control. The advantage of the present concept, as compared to a fixed-shape sensor, is that the reconfigurable array can be adapted on line to account for uncertain structural dynamics, making it a more effective error sensor for applications in which the structural dynamics are not accurately known a priori. The array developed for this work consists of twenty-two polyvinylidene fluoride film sensors and four multilayer piezoceramic actuators connected to a set of reprogrammable electronics. The reprogrammable electronics consist of two 16 x 1 digitally-programmable gain boards and a digital signal processor with two analog inputs and outputs. The gains on the sensor array are set via the digital I/O of the digital signal processor. Feedback and feedforward control algorithms are then implemented to demonstrate the use of the actuator-sensor array for noise and vibration suppression. The results indicate that the reconfigurable array technique has merit for noise and vibration control.
- Admittance Modeling: Frequency Domain, Physical Coordinate Methods for Multi-Component Systems
  Admittance modeling is a well-known but little-used technique for predicting the dynamic response of multi-component systems. Admittance functions (FRF's) measured or calculated for individual components are used along with in-service response measurements which implicitly define the in-service loads. FRF's are used directly; no estimation of modal parameters is necessary. The method does not require that the structural excitation be due to ideal sources: i.e., imposed forces or displacements described by explicit functions of time, independent of the structure on which they act. A typical application is the prediction of high-frequency vibration response of a sensitive payload, driven by its connections to an airframe which is acted on by unknown but repeatable dynamic loads. The underlying theory is summarized, a software package designed to render it practical is introduced, and results are given for several sample problems.
- Analysis, Design, and Testing of Two Cocured Damped Composite Torsion Shafts
  Cocuring layers of viscoelastic damping materials with composite material systems offer the possibility of light-weight, stiff, highly damped sturctural components. The objective of this work was to design two cocured damped composite torsion shafts. The first shaft uses the extension-twist coupling mechanism of off-angle composite materials to enhance the damping performance of a damping material. Due to the extension-twist coupling, the shell center section moves axially when the shaft undergoes torsion deformation. The outer shell plies are oriented in the opposite manner so that its center moves in the opposite direction. The relative axial deformation between the two shells places the damping material into shear, providing damping. The second shaft uses a constraining layer embedded inside the shaft that floats between two layers of damping material. Finite element analysis was used to determine optimal damping material shear modulus and ply orientation to maximize shaft imaginary stiffness. Four shafts total were built and modal tests were performed. Torsion damping increased by factors from 5.8 to 20.0 and 6.1 to 10.9 over the undamped case for the extension-twist and floating contraining layer dampers, respectively.
- A Comparison of Two Cocured Damped Composite Torsion Shafts
  Cocuring layers of viscoelastic damping materials with composite material systems offers the possibility of manufacturing lightweight, stiff, highly damped structural components. The objective of this work was to design two cocured damped composite torsion shafts and compare their perspective performances. The first shaft uses the extension-twist coupling mechanism of off-angle composite materials to enhance the performance of a damping material. The second shaft uses a constraining layer embedded inside the shaft that floats between two layers of damping material. Finite element analysis was used to determine optimal damping material shear modulus and ply orientation to maximize shaft imaginary stiffness. Four shafts in total (two of each type) were built and modal tests were performed. Torsion damping increased by factors of 5.8 to 20.0 and 6.1 to 10.9 over the undamped case for the extension- twist and floating constraining layer dampers, respectively. While both damping concepts provide significant levels of damping, the performance of each was hindered due to the increase in shear modulus of the damping material as it was cocured with the composite material.
- A Tuned Mass Damper for Low Speed Vertical Machinery
  Excessive vibration was encountered during the prototype development of a large, low speed vertical process machine. The characteristic of this vibration was a discrete peak nearly 0.70 inches/sec, and was identified as a nonsychronous resonance. This paper describes the analysis, design, and testing effort that led to the creation of a unique damper which resulted in an order of magnitude reduction in the vibration. The application of this device, which is referred to as a Tuned Mass Damper, is thought to be unprecedented with rotating machinery.
- Vibroacoustic Modeling of a Launch Vehicle Payload Fairing for Active Acoustic Control
  Low frequency active acoustic attenuation is examined for the payload fairing of a representative small launch vehicle. The feasibility of using structural sensors and actuators for active acoustic control is assessed with a numerical model of the fairing. The results of the numerical analysis indicate that broadband damping decreases the interior overall sound pressure level between 4 db and 10 db, depending on the amount of structural damping that is added to the vibration modes. Singular value analyses demonstrate that point force actuators and in-plane strain actuators have the control authority for active acoustic control. Control studies indicate that the local velocity feedback reduces the average level of the fairing by roughly 50%, and that the force levels required of the control system are within the range of conventional point force and piezoceramic actuators.
- Active Structural-Acoustic Control for Composite Payload Fairings
  Launch loads are the prime driver of spacecraft structural design. Passive approaches for acoustic attenuation are limited in their low frequency effectiveness by constraints on total fairing mass and payload volume. Active control offers an alternative for low frequency acoustic noise attenuation inside the payload fairing. Smart materials such as piezoceramics can be exploited as actuators for structural-acoustic control. In one active approach, structural actuators are attached to the walls of the fairing and measurements from structural sensors and/or acoustic sensors are fed back to the actuators to reduce the transmission of acoustic energy into the inside of the payload fairing. In this paper, structural-acoustic modeling and test results for a full scale composite launch vehicle payload fairing are presented. These analytical and experimental results fall into three categories: structural modal analysis, acoustic modal analysis, and coupled structural-acoustic transmission analysis. The purpose of these analysis and experimental efforts is to provide data and validated models for active acoustic control of the payload fairing. In the second part of the paper, this closed-loop acoustic transmission reduction is implemented and measured.
- Development of Analytical Methods for Particle Damping
  Particle dampers are highly nonlinear auxiliary mass dampers whose energy dissipation, or damping, is derived from a combination of mechanisms including plastic deformations, external and internal friction, and momentum transfer. To complicate matters, the predominate energy dissipation mechanism may vary depending on parameters such as cavity fill ratio, vibration amplitude levels, ect. Research has indicated that particle dampers could be a viable option for extreme environment applications, such as elevated temperatures and/or under centrifugal loading. However, to date, the lack of robust design methodology has limited particle damper usage to “trial and error” applications. The objective of this effort is to develop the necessary design methodology to enable the successful design and application of particle dampers. Experimental and analytical efforts toward this goal are presented.
- Experimental Measurements of the Particle Damping
  Particle damping is believed to be a promising loss mechanism for engine turbine blades, as some particle damping materials have the capability to withstand the extremely high temperatures inherent in operational aircraft engines turbines. However, the ability to provide damping under centrifugal loads had never been shown experimentally. In this paper, initial measurement results from a series of particle damping configurations tested under true centrifugal loads are reported. The interplay of various parameters such as fill ratio, particle size, shape, and material on achieved damping levels were investigated with nine different particle damping configurations loaded up to 5,000 G's. Damping, in terms of frequency domain peak response reduction, was seen at this G loading for one configuration with no indication of reduction due to centrifugal load. The best performing particle damper, based on irregular tungsten carbide granules, was also proof tested to over 50,000 G's with no noticeable degredation. While damping still remains to be shown at centrifugal load levels comparable with real engines, these initial test results show that particle damping is worthy of further consideration.
- Hubble Space Telescope Solar Array Damper
  This paper describes the design of a solar array damper that will be built into each of two new solar arrays to be installed on the Hubble Space Telescope (HST) during Servicing Mission 3, currently scheduled for August 2000. Two "rigid" solar array wings will replace "flexible" wings currently providing power. The new arrays will also provide the capability for HST to survive re-boost to higher orbit. The objective of the damper is to reduce the dynamic interaction of these new wings with the HST spacecraft. The damper suppresses the fundamental bending modes of the deployed wings at 1.2 Hz (in-plane) and 1.6 Hz (out-of-plane). With the flight version of the damper, modal damping of 2.3% of critical is expected over the temperature range of -4 C to 23 C with a peak damping level of 3.9%. The unique damper design, a combination of titanium spring and viscoelastic damper, was developed using a system finite element model of the solar array wing and measured viscoelastic material properties. Direct complex stiffness (DCS) testing was performed to characterize the frequency- and temperature-dependant behavior of the damper prior to fixed-base modal testing of the wing at NASA/Goddard Space Flight Center (GSFC).
- Initial Structural-Acoustic Modeling and Control Results for a Full-Scale Composite Payload Fairing for Acoustic Launch Load Alleviation
  Launch loads are the prime driver of spacecraft structural design. Passive approaches for acoustic attenuation are limited in their low frequency effectiveness by constraints on total fairing mass and payload volume. Active control offers an alternative for low frequency acoustic noise attenuation. Smart materials such as piezoceramics can be exploited as actuators for structural-acoustic control. In one active approach, structural actuators are attached to the walls of the fairing and measurements from structural sensors and/or acoustic sensors are fed back to the actuators to reduce the transmission of acoustic energy into the inside of the payload fairing. In this paper, structural-acoustic modeling and test results for a full scale composite launch vehicle payload fairing are presented. These results fall into three categories: structural and modal, acoustic modal, and coupled structural-acoustic transmission analyses. The purpose of these efforts is to provide data and validated models that will be used for active acoustic control of the payload fairing. In the second part of the paper, this closed-loop acoustic transmission reduction is implemented and measured.
- Active/Passive Counter-Force Vibration Control and Isolation Systems
  Active counter-force control has significant advantages over the more traditional motion-based active vibration suppression for isolation of disturbance sources. In this paper, features of four specific actuators, two hybrid isolation struts, and three system level realizations are reviewed with a focus on vibration isolation/suppression to reduce cryocooler disturbance forces. All of the discussed hardware and systems are based on electromagnetic reaction mass actuators. The best measured tonal performance for all three systems discussed exceeds two orders of magnitude (40 dB) of vibration reduction. Control bandwidth can exceed 500 Hz. The necessary actuators are also robust, compact, and lightweight. Two of the systems were realized with miniature actuators weighing 3.8 ounces (107 grams) and 3.12 ounces (88 grams) respectively. Such systems have significant promise for addressing critical vibration isolation needs for upcoming space missions such as SIM, NGST, TPF, SBL, etc., through isolation of on-orbit noise sources such as cryocoolers and reaction wheels. They could also be quite useful for terrestrial applications in telecommunication, manufacturing, and semiconductor processing industries.
- Advanced Isolation Design for Avionics on Launch Vehicles
  Research on advanced vibration isolator designs and practical design techniques for Launch Vehicle (LV) manufacturers is discussed. Avionics of LVs have unique requirements for isolation since many generate heat and cannot use convection cooling. Nearly all isolation systems are ineffective thermal conductors unless custom modifications are performed, the cost of which can rarely be justified, particularly on expendable vehicles. While viscoelastic isolators offer simplicity and affordability, such materials with high loss factors also exhibit aggressive changes in stiffness with both temperature and frequency. Materials with unique formulations are introduced which have an order of magnitude higher thermal conductivity than today's materials of similar stiffness. This enables appreciable heat conduction with nominal temperature increases to isolated packages. New compounds introduced in this paper address past limitations. A software utility has been developed that greatly simplifies isolation design. The utility solves the equations of motion for a rigid body on flexible mounts and allows performance predictions using base vibration inputs.
- Cryocooler Disturbances Reduction with Single and Multiple Axis Active/Passive Vibration Control Systems
  In this paper two miniaturized, add-on, vacuum compatible, active vibration control systems for cryocoolers are discussed. The first, called VIS6, is an active/passive isolation hexapod with control authority in all six degrees of freedom. This capability is desirable when reduction of all cryocooler disturbance loads, including the radial loads, is required. Each of the six identical hexapod struts consists of a miniature moving coil electromagnetic proof mass actuator, custom piezoelectric wafer load cell, viscoelastic passive isolation stage, and axial end flexures. The first five disturbance tones are reduced over a bandwidth of 250 Hz. Load reductions of 30-40 dB were measured axially and radially. The second system, VRS1, is an active control system designed to reduce axial expander head disturbance loads. It works on the basis of a counter-force developed from an electromagnetic proof mass actuator. Error signals are provided from a commercial accelerometer to a standalone digital signal processor, and a filtered-x least means square control algorithm is implemented. Over the 500 Hz control bandwidth, 11 disturbance tones were reduced on between 14 to 40 dB.
- Overview- Design of Passive Damping Systems
  This paper presents a brief review of techniques for designed-in passive damping for vibtation control. Designed-in passive damping for structures is usually based on one of four damping technologies: viscoelastic materials, viscous fluids, magnetics, or passive piezoelectrics. These methods are discussed and compared. The technology of using viscoelastic materials for passive damping is discussed in more detail than the other methods since it is presently thewidely used type of damping technology. Testing and characterization of viscoelastic materials and design methods for passive damping are discussed. An example showing the benefits of a passive damping treatment applied to a stiffened panel under an acoustic load is presented.
- Effectiveness and Predictability of Particle Damping
  In this paper, recent results of ongoing studies into the effectiveness and predictability of particle damping are discussed. Efforts have concentrated on characterizing and predicting the behavior of a wide range of potential particle materials, shapes and sizes in the laboratory environment, as well as at elevated temperature. Methodologies used to generate data and extract the characteristics of the nonlinear damping phenomena are illustrated with interesting test results. Experimental results are compared to predictions from analytical simulations performed with an explicit code, based on the particle dynamics method, that has been developed in support of this work.
- Experimental Centrifuge Testing and Analytical Studies of Particle Damping Behavior
  In this paper, analytical and experimental studies of particle damping behavior are discussed. These studies have focused on the development of an analytical model to predict particle damping behavior and on determination of the effects of centrifugal loading on the behavior. An analytical model, based on the particle dynamics method, has been developed and is being correlated with results from experimental testing. A novel test facility is being established which enables laboratory based evaluation of the damping effectiveness of blade-like objects under centrifugal loading. Depending on the test article, this facility will be capable of exposing test specimens to centrifugal accelerations of up to 124,000 G’s.
- Flight Hardware for the Hubble Space Telescope Solar Array Damper
  The Hubble Space Telescope (HST) is currently operating with two flexible solar arrays (or "wings"), referred to as SA2, that were installed during Servicing Mission 1. These flexible solar arrays are to be replaced with two rigid solar arrays, SA3, during Servicing Mission 3B which is currently scheduled for May, 2001. The key requirements for these arrays are to: (1) increase long term power to support the HST mission, (2) improve the jitter performance while maintaining stability margin requirements, and (3) withstand re-boost loads without astronaut or ground intervention. Analysis of the original SA3 design showed that the Pointing Control System (PCS) stability margin requirements would be violated because of the modal characteristics of the SA3 fundamental bending modes. One of the options to regain the stability margins was to increase the damping of these modes. Damping of 1.5% of critical of theSA3 fundamental bending modes, at the HST system level, is needed to meet the stability margin requirements. Therefore, the development of a discrete damping device was undertaken to provide adequate damping of the SA3 fundamental bending modes for all operational conditions.
- Hubble Space Telescope Solar Array Damper for Improving Control System Stability
  Analysis of the rigid solar array, SA-3, design showed that the Pointing Control System stability margin requirements would be violated because of the modal characteristics of the SA-3 fundamental bending modes. A damping of 1.5% of critical of the SA-3 fundamental bending modes, at the HST system level, is needed to meet the stability margin requirements. A damper, consisting of a titanium flexure and viscoelastic damping material, has been designed, built, and tested, and is an integral part of the SA-3 mast. The viscoelastic material properties are temperature-sensitive, and Direct Complex Stiffness testing was performed to characterize the frequency-and temperature-dependant behavior of the damper. Fixed-base modal testing showedthat the optimized damper is expected to provide modal damping of at least 2.25% of critical (fixed base extrapolated to zero excitation force levels) over the temperature range of 0°C to 25°C.
- Design Methodology for Particle Damping
  Focused research in the area of Multi-Particle Impace Damping (MPID) has resulted in new methods of characterization and prediction. An analytical method has been developed, based on the particle dynamics method, that uses characterized particle damping data to predict damping in structural systems. A methodology to design particle damping for dynamic structures will be discussed. The complete design methodology has been validated in "proof-of-methodology" testing on a structural component in the laboratory.
- Life Cycle Testing of Viscoelastic Material for Hubble Space Telescope Solar Array 3 Damper
  During the March 2002 Servicing Mission by Space Shuttle (STS 109), the Hubble Space Telescope (HST) was refurbished with two new solar arrays that now provide all of its power. These arrays were built with viscoelastic/titanium dampers, integral to the supporting masts, which reduce the interaction of the wing bending modes with the Telescope. Damping of over 3% of critical was achieved. To assess the damper’s ability to maintain nominal performance over the 10-year on-orbit design goal, material specimens were subjected to an accelerated life test. The test matrix consisted of scheduled events to expose the specimens to pre-determined combinations of temperatures, frequencies, displacement levels, and numbers of cycles. To determine whether material degradation occurred during the exposure sequence, material performance was evaluated before and after the accelerated aging with complex stiffness measurements. Based on comparison of pre- and post-life-cycle measurements, the material is expected to maintain nominal performance through end of life on-orbit. Recent telemetry from the Telescope indicates that the dampers are performing as intended.
- Vibration Isolation for International Space Station External Cargo Items
  The International Space Station (ISS) has developed a common carrier platform for transportation of Orbital Replacement Units (ORUs). These carrier platforms can be adapted for flight on any Cross Bay or Sidewall Carrier in the ISS inventory. Recent random vibration analyses have shown that one ORU has its qualification and acceptance requirements exceeded. Since high frequency random vibrations are often the cause for failure in ORUs, studies were initiated to investigate and eliminate the structure and acoustic borne random vibration environment. These studies focused on load mitigation techniques that isolate the ORU. To achieve isolation from these ascent disturbances, solutions for the study article were investigated to produce an isolation design that ideally attenuates the random vibration levels at 50 Hz by a factor of two and those at higher frequencies even more. This paper chronicles the scope and parameters of the study and a design solution including recommendations for testing, that will provide an acceptable ORU environment no more severe than its qualification environment.
- A New Approach to Temperature Shift Functions in Modeling Complex Modulus Damping Data
  A new approach to determining the temperature shift function and its utility in smoothing, interpolating, and modeling a set of complex modulus data are described in detail and examples are given. A set of experimental data to characterize the dynamic mechanical properties of a viscoelastic damping polymer typically includes the real component of complex modulus and the material loss factor as well as the experimental frequency and temperature. The set of data must be made useful for design purposes. The present approach analytically represents the wicket plot loss factor as a function of the modulus magnitude. A ratio of polynomials is then used to model the complex modulus as a function of reduced radian frequency. The perpendicular distance from each experimental point to the wicket plot is used to calculate the temperature shift function.
- Active Vibration Isolation System for Launch Load Alleviation
  Payloads delivered to orbit by expendable launch vehicles experience high levels of vibration which can cause component failures or, more frequently, lead to extra weight that would otherwise be useful for added functions on orbit. Vibration isolation systems have been flown to protect various components as well as entire spacecraft, dramatically reducing launch loads and saving costs in redesign and tests. Future spacecraft and components may benefit from further load reduction through higher performance active isolation systems. These active systems are capable of introducing compliance in selected axes while maintaining required rigidity in others. They can also produce excellent isolation without large amplification. Passive and active vibration isolation systems were developed for the Vibro Acoustic Launch Protection Experiment (VALPE) and flew aboard sounding rockets. The paper describes the design and development of the isolation systems, actuation and isolation architectures and control strategies. Integration of two flight experiments is summarized. Ground test results are presented. Results of the experiments are provided, and recommendations for active isolation are offered.
- Fairing Noise Control Using Tubed-Shaped Resonators
  The potential for noise mitigation in composite Chamber Core fairings is investigated by using the walls of the fairing structure itself as acoustic resonators. This is the first documented application of long cylindrical tube- shaped resonators for fairing noise control. The theory and modeling of tube-shaped resonators for controlling fairing acoustic resonances is presented. The potential for noise mitigation in composite Chamber Core fairing using the walls of the fairing structure itself as acoustic resonators is investigated. Design criteria such as geometry, damping, spatial coupling, and robustness are considered for a variety of tube resonators. The results showed that a small number of tube resonators reduced the amplitude of low-frequency acoustic resonances by 10–12 dB in the test system and provided nearly 6 dB of reduction over the bandwidth from 0 to 400 Hz.
- Study of Free-Free Beam Structural Dynamics Perturbations due to Mounted Cable Harnesses
  Signal and power harnesses on spacecraft buses and payloads can alter structural dynamics, as has been noted in previous flight programs. The community, however, has never undertaken a thorough study to understand the impact of harness dynamics on spacecraft structures. The Air Force Research Laboratory is leading a test and analysis program to develop fundamental knowledge of how spacecraft harnesses impact dynamics and develop tools that structural designers could use to achieve accurate predictions of cable-dressed structures. The work described in this paper involved a beam under simulated free boundary conditions that served as a validation test bed for model development.
- Noise Transmission Studies of an Advanced Grid-Stiffened Composite Fairing
  Interior fairing noise is an important consideration for payload launch survivability and has been studied extensively since the beginning of the space program. This work presents acoustic transmission studies conducted by the Air Force Research Laboratory, Space Vehicles Directorate, on a composite, grid-stiffened, Minotaur payload fairing. These tests were performed in an acoustics laboratory and examined the effects of acoustic flanking paths, the thermal protection system, and melamine-type acoustic blanket treatments on fairing noise. The data showed that acoustic flanking paths significantly increase noise transmission, especially at low frequency. The bare fairing with thermal protection system provided approximately 14 dB of noise reduction over the 5000 Hz bandwidth relative to external levels. Acoustic blanket performance was measured as a function of bandwidth, surface area coverage, and mass. It was observed that small amounts of treatment (2 kg) significantly increased noise reduction (3.6 dB), even at low frequency.
- Viscoelastic damping for Noise Control in an Armored Military Vehicle
  Armored military vehicles have a harsh noise environment and often the noise levels are an afterthought for the vehicle designers. There are many noise sources such as the engine, transmission, cooling fans, and track noise. The thick armor plating and the fact that the noise sources are inside the vehicle exacerbate the issue. Therefore few paths exist for the noise to exit the vehicle, and the result is an extremely loud noise environment for crew and passengers. Much success has been seen in the use of active headsets for the crew, but this is not feasible for the embarked troops in an armored transport vehicle, so the noise control must be inherent to the vehicle. The development of a bulkhead panel for containing the vehicle’s engine is examined. The bulkhead panels serve the dual purpose of thermal and acoustic barriers. In addition to mass effects for reducing transmitted engine noise, viscoelastic layers are included in the panel to further passively attenuate noise transmission. The wide temperature swings seen in the panel during operation of the vehicle complicate the viscoelastic damping design. Test results from a panel prototype are presented for a testbed representing the military vehicle configuration.
- Viscoelastic Materials with Magnetically-Controllable Properties for Vibration Damping and Isolation
  Viscoelastic Materials (VEMs) and magnetorheological (MR) fluid devices are widely used for vibration damping and isolation. MR fluids are suspensions of metal particles in various carrier fluids that have properties controllable by imposition of a magnetic field, using mechanisms that suggest analogous manipulation of properties in more solid carrier or base materials. This paper studies the properties of composite materials that we call MR-VEM. MR-VEM offers the opportunity to change properties, including stiffness and material damping, by applying magnetic fields. The paper focuses on the experimental characterization performance of MR-VEM devices. Two properties were used as the basis for distinguishing samples: particle fill factor (the volume ratio of MR particles to the base VEM) and the magnitude of magnetic field applied while curing the MR-VEM elements. Applied magnetic field was varied during testing. Performance is studied through a range of experiments. Test data show a factor of five stiffness adjustability. Limitations are quantified. Overall, the material shows promise for applications requiring adjustability in effective stiffness.
- Particle Damping for Launch Vibration Mitigation: Design and Test Validation
  A non-intrusive damping technique was developed for use on a satellite cryogenic structure to reduce excessive stress levels predicted on a centrally located thermal isolator. The passive structure weighing nearly 53 lbs and consisting of several panel stages required a damper or system of dampers that could integrate to the exposed outer panel, referred to as the “cold stage panel”, and provide random vibration and shock attenuation during launch. The custom damper developed, and discussed in this paper, consisted of eight particle dampers mounted to specific locations on the cold panel to achieve the required amount of attenuation and reduce panel flexing modes contributing to undesirable loads on the thermal isolator. Unlike traditional damping technologies (viscoelastic, magnetic, viscous fluid, or piezoelectric), particle dampers can provide damping performance in any direction and over a wide frequency range, thus attenuating several modes with one damper design. Particle dampers are ideally suited for extreme temperatures and high G level applications due to their temperature insensitivity and robust design. Analysis predictions and test results presented show reductions in loads of up to 70%.
- A Comparison of Vibration Damping Methods for Ground Based Telescopes
  Vibration is becoming a more important element in design of telescope structures as these structures become larger, more compliant, and include higher bandwidth actuation systems. This paper describes vibration damping methods available for current and future implementation and compares their effectiveness for a model of the Large Synoptic Survey Telescope (LSST), a structure that is stiffer than most large telescopes. Vibration damping offers a mass-efficient means of reducing vibration response, whether from wind disturbances, telescope slewing, or other internal disturbances from translating or rotating components. The paper presents damping techniques including constrained layer viscoelastics, viscous and magnetorheological (MR) fluid devices, passive and active piezoelectric dampers, tuned mass dampers, and active resonant dampers. Basic architectures and practical implementation considerations are discussed and expected performance is assessed. With a goal of reduced settling time during the telescope’s surveys and considering practicalities of integration with the telescope structure, two damping methods were identified as most appropriate: passive tuned mass and active electromagnetic resonant dampers.
- A System for Suspending and Vibration-Isolating a Large Spacecraft for Testing in Vacuum
  A system is described for suspending from above a 15,900 kg (35,000-lb) payload for ground testing in a vacuum chamber. The system provides very low suspension frequencies to isolate the payload from ambient vibration. It also includes active capability to maintain a very stable ride-height and attitude of the payload. Passive magnetic dampers are included to suppress pendulum-mode lateral oscillations. Designated as the Large Suspension / Isolation System (LSIS) the system is described in terms of its design, analysis, fabrication, and component-level testing.
- Payload Isolation System for Ground Based Stability Testing: Design and Test Validation
  A ground based test adapter was designed, built, and tested to safely support a large payload and isolate it from ground borne vibration during system level stability testing. The first six suspension modes of the adapter, with the payload attached, are less than 3.5 Hz with critical damping between 0.5% and 2.0% over a very wide operating temperature range. The adapter consists of eight coil springs, eight magnetic eddy current dampers, and four lock mechanisms, all configured between two structural aluminum rings. The rings provide an interface to the payload (emulating actual interface specifications) and to the test facility at its base. Custom springs were sized to support the payload, provide isolation, and keep the self resonances of these springs, or “surge modes,” out of frequency ranges that may interact with the sensitive payload equipment. The dampers, in parallel with the springs, provide the isolation performance and the lock out mechanisms can be engaged to convert the adapter to a rigid mount. Leveling of the payload can be achieved by using height adjusters while the payload is being supported by the adapter.
- Reduction of Dynamic Response of a Wind Tunnel Sting Mount Using Co-cured Composite and Viscoelastic Materials
  Techniques have been investigated for increasing the damping in wind tunnel sting mounts to reduce unwanted vibrations and improve data quality. The development was carried out for a viable wind tunnel sting design using co-cured composite material and embedded viscoelastic material. As part of the investigation, an automated software toolbox for the design and optimization of the sting mount was developed. Based on the theoretical analysis results, it is found that with the added constraining layer of viscoelastic material in the sting’s body, a significant amount of additional damping can be achieved.
- Reduction of Dynamic Response of a Wind Tunnel Sting Mount Using a Hub Damper Unit
  This paper presents the design and prototype testing of a hub damper unit for reducing the dynamic response of a wind tunnel sting mount by adding passive damping. Several configurations for the hub damper were examined analytically before choosing the best performing design for prototype hardware development and testing. Fixed base laboratory testing showed that significant damping could be added to the sting mount with the hub damper. Subsequent testing of the hub damper in a wind tunnel confirms enhanced dynamic performance in an operational environment. Limitations in the wind tunnel test setup, however, led this enhanced performance to be less than might have been expected from the fixed base testing.
- Component Damping with Superelements
  Damping is a subject that is often misunderstood or misapplied in finite element analysis. This paper reviews the basics of viscous and structural damping, as well as viscoelastics and control system damping. Provisions for user specification of these forms of damping in MSC/NASTRAN are reviewed. The effects of each of these in the various dynamic solution sequences are discussed complex eigenvalues, transient response, steady-state frequency response. The implications of each of these for component mode synthesis are then reviewed. Methods are shown for handling two situations that the present solution sequences do not provide for: complex element strain energy and frequency-dependent component modal damping.
- Vibration Reduction and Power Flow Using Passive and Active Resonant Devices
  There are advantages to using resonant devices for vibration control. We use passive vibration absorbers, but sometimes it appears to be worth implementing active vibration control. The goals are to quantify limitations of passive resonant devices for vibration suppression, identify when to go active based on those limits, and to explain why passive or active was used in several cases.
- Active Damping and Vibration Control for Aircraft Fin and Appendage Structures
  Numerous vibration control techniques, employing both passive and active methods, have been developed and tested over the last several decades. Some of these techniques are in widespread use, while others have rarely or never left the laboratory. This paper considers the value that vibration damping and control of aircraft fins and appendage structures can have in reducing loads and subsequent fatigue and possible failure. These structures often are subject to high loads resulting from wakes of upstream external stores. The vibration control methods were considered as part of a larger study focused on active flow control The options for passive or active vibration control on a class of fin-type structures are reviewed, and one approach - active and passive damping using piezoelectric materials - is covered in greater detail. Piezoelectric transducer sizing for expected pressure loading and modeling of piezoelectric-based active damping control systems are discussed. Motivation for another possible techniques coupling active flow and vibration control is presented using arguments from adaptive filtering and feedforward control. Results are presented for bench tests with simulated disturbances, for low speed wind tunnel tests, and for high speed wind tunnel tests.
- Vibration Suppression for the Gemini Planet Imager
  Vibration Suppression for the Gemini Planet Imager
- Active Damping of the SOFIA Telescope Assembly
  The NASA/DLR Stratospheric Observatory for Infrared Astronomy SOFIA employs a 2.5-meter reflector telescope in a Boeing 747SP. The telescope is housed in an open cavity and is subjected to aeroacoustic and inertial disturbances in flight. To meet pointing requirements, SOFIA must achieve a pointing stability of approximately 0.5 arcseconds RMS. An active damping control system is being developed for SOFIA to reduce image jitter and image degradation due to resonance of the telescope assembly. Our paper discusses the history of the active damping design for SOFIA, from early concepts to the current implementation which has recently completed a ground and flight testing for proof-of-concept. We describe some milestones in the analysis and testing of the telescope assembly which guided the development of the vibration control system. The control synthesis approach and current implementation of the active damping control system is presented. Finally, we summarize the performance observed in early flight tests and the steps that are currently foreseen to completing the development of this system.
- Vibration Damping for the Segmented Mirror Telescope
  The Segmented Mirror Telescope SMT at the Naval Postgraduate School NPS in Monterey is a next-generation deployable telescope, featuring a 3-meter 6-segment primary mirror and advanced wavefront sensing and correction capabilities. In its stowed configuration, the SMT primary mirror segments collapse into a small volume; once on location, these segments open to the full 3-meter diameter. The segments must be very accurately aligned after deployment and the segment surfaces are actively controlled using numerous small, embedded actuators. The SMT employs a passive damping system to complement the actuators and mitigate the effects of low-frequency <40 Hz vibration modes of the primary mirror segments. Each of the six segments has three or more modes in this bandwidth, and resonant vibration excited by acoustics or small disturbances on the structure can result in phase mismatches between adjacent segments thereby degrading image quality. The damping system consists of two tuned mass dampers TMDs for each of the mirror segments. An adjustable TMD with passive magnetic damping was selected to minimize sensitivity to changes in temperature; both frequency and damping characteristics can be tuned for optimal vibration mitigation. Modal testing was performed with a laser vibrometry system to characterize the SMT segments with and without the TMDs. Objectives of this test were to determine operating deflection shapes of the mirror and to quantify segment edge displacements; relative alignment of λ/4 or better was desired. The TMDs attenuated the vibration amplitudes by 80 and reduced adjacent segment phase mismatches to acceptable levels.
- Advanced Isolation Design for Avionics on Launch Vehicles
  Advanced Isolation Design for Avionics on Launch Vehicles

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