Increasingly, scientific and military missions require the use of space-based optical systems. For example, new capabilities are required for imaging terrestrial like planets, for surveillance, and for directed energy applications. Given the difficulties in producing and launching large optics, it is doubtful that refinements of conventional technology will meet future needs, particularly in a cost-effective manner. To meet this need, recent research has been investigating the feasibility of a new class of ultra-lightweight think-skin optical elements that combine recent advances in lightweight thermally formed materials, active materials, and novel sensing and control architectures. If successful, the approach may lead to an order of magnitude reduction in space optics areal density, improved large scale manufacturing capability, and dramatic reductions in manufacturing and launch costs. In a recent effort, a one meter thin-film mirror like structure was fabricated. This paper provides an overview of tools used to model and simulate this structure as well as results from structural dynamic testing. In addition, progress in the area of non-contact global shape control using smart materials is presented.
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. In addition, 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 oftentimes possess a shorter lifespan due to the abrasive nature of the ferrous particles suspended in the fluid. Intelligently controlling damping forces must also accommodate the non-linear stiction behavior, which degrades performance. A new, unique 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. This paper presents experimental results for a prototype device and compares those results to model predictions.
A clear milestone has been reached in the development and demonstration of smart structures technologies for space applications. The success of recent space experiments not only demonstrates the feasibility of several new technologies, but also provides a glimpse of the various future opportunities available for research and development in the smart structures area. Three missions are discussed herein, as well as the role of the Air Force Research Laboratory (AFRL) and its government, industry, and academic partners in bringing them to fruition. The currently operating Vibration Isolation, Suppression, and Steering (VISS) space experiment and the Middeck Active Control Experiment Reflight (MACE-II), as well as the upcoming Satellite Ultra-quiet Isolation Technology Experiment (SUITE) are discussed in terms of notable achievements and lessons learned over the course of their execution. Directions for future research revealed by these experiments are also discussed, along with technology needs and transition opportunities for future operational systems.
KEYWORDS: Detection and tracking algorithms, Control systems, Adaptive control, Space operations, Sensors, Systems modeling, Actuators, Active optics, Analytical research, Control systems design
The Air Force Research Laboratory (AFRL) is sponsoring the Middeck Active Control Experiment Reflight (MACE II) Program. MACE II is a manned space experiment that evaluates the capabilities of adaptive control of flexible structures in the zero-g environment of the space shuttle's Middeck. MACE II has grown out of lessons learned from the original MACE flight and from AFRL sponsored structural control experiments. Previous experiments required extensive testing and 'tuning' for their particular test environment to meet their performance expectations. Such a process is too inefficient to be seriously considered for operational systems, especially space-based systems where access is limited. MACE II takes the next logical step by evaluating the capability of adaptive structural control algorithms AFRL has assembled a team of five small businesses and universities to develop and evaluate several adaptive control methodologies. In addition, AFRL has recruited a second science team led by the Massachusetts Institute of Technology to evaluate control system for time-varying and geometrically nonlinear systems. This paper is an overview of the AFRL science team only.
KEYWORDS: Satellites, Vibration isolation, Control systems, Digital signal processing, Passive isolation, Active isolation, Actuators, Electronics, Smart structures, Space operations
The Air Force Research Laboratory is currently conducting a number of space flight experiments with the goal of demonstrating and transitioning smart structures technology to the operational user. Three of these experiments have focused on approaches for providing high-performance on- orbit isolation to precision spacecraft payloads. This paper will describe the design and performance of two systems that are slated for a 2000 launch; the vibration isolation, suppression, and steering experiments and the satellite ultra-quiet isolation technology experiment. Additionally, this paper will provide an overview of a third program initiated in 1999, the miniature vibration isolation system.
KEYWORDS: Control systems, Space operations, Actuators, Sensors, Smart structures, Detection and tracking algorithms, Satellites, Electronics, Systems modeling, Performance modeling
The late 20th century has seen the evolution of smart structures technology form theoretical studies and bench level experimentation to demonstration on complex ground test articles and flight demonstration. This paper will discuss the role that the Air Force Research Laboratory (AFRL) and its government, industry, and academic partners have played in making this transition occur. The focus of the paper is on the flight experiments and large ground based demonstrations that have been led from these various activities. Directions for future research and activities that could motivate transition of the technology to operational system are also discussed.
Spacecraft require a variety of mechanisms to accomplish mission-related functions such as deployment, articulation, and positioning. Current off-the-shelf devices such as pyrotechnic separation nuts, paraffin actuators, and other electro-mechanical devices may not be able to meet future satellite requirements, such as low shock and vibration, and zero contamination. The Air Force Research Laboratory (AFRL), with corporate and government partners, has developed Shape Memory Alloy (SMA) spacecraft release mechanisms and hinges as alternatives. In order to meet future goals, the SMA devices have been designed to reduce shock and vibration, reduce parts, and eliminate pyrotechnics. This paper will focus on descriptions and results of on-orbit SMA mechanism experiments and applications. AFRL has flown SMA release devices as part of the Shape Memory Alloy Release Device (SMARD) experiment on MightSat I. The SMARD experiment, that compared the shock and release times of two SMA devices with those of current off-the-shelf devices, was conducted in May 1999 with extremely successful results. In addition, four AFRL funded SMA release mechanisms successfully deployed the Air Force Academy FalconSat spacecraft from the Orbital Sub-Orbital Program Space Launch Vehicle in January 00. AFRL has also conducted an on-orbit experiment with SMA hinges. The hinges were flown as part of the Lightweight Flexible Solar Array program, that was a joint AFRL/DARPA/NASA/Lockheed Martin program to develop innovative solar array technologies. Six SMA hinges were launched as part of the LFSA experiment on the Space Shuttle Columbia in July 1999 with successful results.
This paper presents a review of the Air Force Research Laboratory advanced controls technology experiment program. Representing the first space-demonstration of smart structures technology, the ACTEX-I program has met or exceeded all program goals at each stage, beginning with the program initiation in 1991 through launch in 1996 to the conclusion of the Guest Investigator program and program conclusion in 1999. This paper will provide a summary of the ACTEX-I program from the AFRL perspective, focusing on lessons learned from the program both positive and negative.
KEYWORDS: Sensors, Control systems, Interferometers, Actuators, Space telescopes, Error analysis, Digital filtering, Active optics, Digital signal processing, Active vibration control
Experimental results are presented for active vibration control of the Air Force Research Laboratory's UltraLITE Precision Deployable Optical Structure (PDOS), a ground based model of a sparse array, large aperture, deployable optical space telescope. The primary vibration suppression technique employs spatio-temporal filtering, in which a small number of sensors are used to produce modal coordinates for the structural modes to be controlled. The spatio-temporal filtering technique is well suited for the control of complex, real-world structures because it requires little model information, automatically adapts to sensor and actuator failures, is computationally efficient, and can be easily configured to account for time-varying system dynamics. While controller development for PDOS continues, the results obtained thus far indicate the need for an integrated optical/structural control system.
KEYWORDS: Genetic algorithms, Amplifiers, Sensors, Actuators, Control systems, Active vibration control, Analog electronics, Smart structures, Feedback control, System identification
This paper is concerned with the optimal tuning of digitally programmable analog controllers on the ACTEX-1 smart structures flight experiment. The programmable controllers for each channel include a third order Strain Rate Feedback (SRF) controller, a fifth order SRF controller, a second order Positive Position Feedback (PPF) controller, and a fourth order PPF controller. Optimal manual tuning of several control parameters can be a difficult task even though the closed-loop control characteristics of each controller are well known. Hence, the automatic tuning of individual control parameters using Genetic Algorithms is proposed in this paper. The optimal control parameters of each control law are obtained by imposing a constraint on the closed-loop frequency response functions using the ACTEX mathematical model. The tuned control parameters are then uploaded to the ACTEX electronic control electronics and experiments on the active vibration control are carried out in space. The experimental results on ACTEX will be presented.
This paper presents the analytical methodology and initial numerical simulation results for autonomous neural control of the Ultra-Lightweight Imaging Technology Experiment (UltraLITE) Phase I test article. The UltraLITE Phase I test article is a precision deployable structure currently under development at the United States Air Force Research Laboratory (AFRL). Its purpose is to examine control and hardware integration issues related to large deployable sparse optical array spacecraft systems. In this paper, a multi-stage control architecture is examined which incorporates artificial neural networks for model inversion tracking control. The emphasis in the control design approach is to exploit the known nonlinear dynamics of the system in the synthesis of a model inversion tracking controller and to augment the nonlinear controller with an adaptive neuro-controller to accommodate for changing dynamics, failures, and model uncertainties.
KEYWORDS: Sensors, Actuators, Control systems, Systems modeling, Mirrors, Space telescopes, Filtering (signal processing), Interferometers, Active vibration control, Device simulation
A spatio-temporal filter (STF) based active vibration suppression technique is presented. The STF approach is intended for use for stability and jitter compensation for the UltraLITE Precision Deployable Experiment -- a ground demonstration of a sparse array, deployable, large aperture, optical space telescope concept. This technique is well suited for control of complex, real-world structures because it requires little model information, autonomously accommodates sensor and actuator failures, is computationally efficient and the controller is easily updated to account for time varying system dynamics. An overview of the STF approach is given and experimental active vibration suppression results obtained on the Mirror Mass Simulator testbed at AFRL, Kirtland AFB are presented.
While significant theoretical and experimental progress has been made in the development of neural network-based systems for the autonomous identification and control of space platforms, there remain important unresolved issues associated with the reliable prediction of convergence speed and the avoidance of inordinately slow convergence. To speed convergence of neural identifiers, we introduce the preprocessing of identifier inputs using Principal Component Analysis (PCA) algorithms. Which automatically transform the neural identifier's external inputs so as to make the correlation matrix identity, resulting in enormous improvements in the convergence speed of the neural identifier. From a study of several such algorithms, we developed a new PCA approach which exhibits excellent convergence properties, insensitivity to noise and reliable accuracy.
KEYWORDS: Sensors, Matrices, System identification, Device simulation, Systems modeling, Adaptive control, Control systems, Computer simulations, Imaging systems, Principal component analysis
The need arises in adaptive closed-loop control to identify an efficient dynamic model of the system in real-time. It is well known that a general nu-input, ny-output, strictly proper system of order n possesses n(nu + ny) independent parameters. However, most on-line identification techniques identify many more parameters than these. For example, in an ARMA realization the number of parameters identified is at least n(1 + nu (DOT) ny), which is of third order in the size parameters. This paper presents a means of identifying only the minimum number of parameters, while avoiding non- convex optimization that results in local minima.
In choosing positions for sensors and actuators for structural control, the first step is usually to develop a model that describes the motion of the structure in response to an excitation. The next step depends on the type of sensors and actuators used. If displacement or acceleration sensors and shakers are used, the model serves as a guide to find locations on the structure where displacement is large for a given disturbance. If in-plane strain-based smart sensors and actuators are used, the model is used to identify locations with large in-plane strain. If the structure is relatively complex, there is a good chance that the initial model will not predict motion that agrees completely with the measured motion of the structure. This initial model is then typically adjusted so that the behavior it predicts agrees with a measured modal analysis of the structure. This process can be extremely time consuming, and while the reconciled modes often agree well with a modal analysis, there can be large errors with respect to in-plane strain. Prediction of in-plane is necessary for accurate location of smart sensors and actuators like piezoceramics. In this paper an experimental method is introduced which uses in-plane sensors to find good smart sensor and actuator locations to control acoustic excitation of a complex structure. Experimental results are also presented which demonstrate the proposed technique.
During launch, spacecraft experience severe acoustic and vibration loads. Acoustic loads are primarily transmitted through the shroud or payload fairing of the launch vehicle. In recent years, there has been a trend towards using lighter weight and extremely stiff structures such as sandwich construction and grid-stiffened composites in the manufacturing of payload fairings. While substantial weight savings can be achieved using these materials, the problem of acoustic transmission is exacerbated. For this reason, the Air Force Research Laboratory has been actively engaged in vibroacoustic research aimed at reducing the acoustic and vibration levels seen by payloads during launch. This paper presents experimental results for the simultaneous structural and acoustic cavity mode control of a sub-scale composite isogrid payload fairing structure. In this experiment, actuation is performed through the use of both an internal speaker as well as piezoceramic strain actuators located on the outer skin of the composite structure. Sensing is accomplished using a microphone as well as a piezoelectric strain sensor. The control approach presented in this paper is a decentralized frequency domain approach which makes use of a series of independent control loops. One loop uses the microphone and speaker, while additional loops use the piezoelectric sensors and actuators. The control algorithm consists of independent second-order Positive Position Feedback (PPF) controllers tuned to reduce the magnitude of each cavity mode. A PPF filter in conjunction with an extremely sharp bandpass filter is used on the structural mode of limit spillover. This approach leads to a substantial reduction in the acoustic transmission in the range of 0 - 800 Hz. Transmission coincident with the primary cavity modes of the system are reduced in magnitude by 26 and 9 dB respectively while the structural model that is responsible for the majority of transmission is reduced by approximately 7 dB.
Numerical simulation results are presented for a novel damage detection algorithm for detecting damage in smart structures. The method finds damage using a dynamically measured static flexibility matrix. Damage is found through a parametric correction of a large order static finite element model using a much smaller order measurement set. This is accomplished without any model reduction or eigenvector expansion step. The parametric correction is formulated to minimize a residual which is formed using the pseudoinverse relationship between structural flexibility and stiffness matrices. By posing the update problem in terms of measured flexibility matrices instead of experimentally determined modal data, the problems of modal correspondence, mode selection, and modal truncation are avoided. In this paper, numerical simulation results demonstrate that the technique is capable of locating and quantifying damage manifested as a change in local stiffness in the full order finite element model using a measured flexibility matrix obtained for a much smaller measurement degree- of-freedom set. This is accomplished using a non-iterative linear solution and is possible even when the damage is contained entirely in an element not touching the measurement degree-of- freedom set.
KEYWORDS: Control systems, Sensors, Actuators, Intelligence systems, Smart structures, Modeling, Amplifiers, Data modeling, Analog electronics, Systems modeling
This paper presents preliminary modelling and vibration suppression experiment results for the USAF Phillips Laboratory's Planar Articulating Controls Experiment (PACE) test bed. PACE is a two link flexible multibody experiment constrained to move over the surface of a large granite table. In this paper, an approximate analytical dynamic model of a single slewing flexible body with surface bonded piezoelectric sensors and actuators is developed using Hamilton's Principle with discretization by the assumed modes method. After conversion to modal coordinates, damping is added to the model by including experimental damping measurements. The model is then converted to state-space form for the purpose of control design. The model is verified by comparison of simulated and experimental open loop frequency response data. Both decentralized and centralized controllers are designed for vibration suppression of a single arm of the PACE test bed. The controllers presented in this paper include: a positive position feedback (PPF) controller for controlling the first mode of vibration, a decentralized controller which uses three independent PPF filters for suppressing the first three modes of vibration, and a multiple-input, multiple-output linear quadratic gaussian design. The experiments include both analog and digital implementations of these controllers.
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