In optical systems, where the positions of such components as mirrors must be controlled in six degrees of freedom (DOFs), magnetic bearings can impart forces and torques to the suspended component without rigidly constraining its DOFs, and obviate the complex flexures typical of conventional designs. Attention is here given to a magnetically suspended linear bearing employing variable-reluctance actuators to control three rotational and two translational DOFs, as well as to a linear motor charged with control of the sixth DOF. Applicability to vibration isolation and optical line-of-sight stabilization are noted.

A straightforward approach is presented for the design of low weight/power/cost, space-base inertial line-of-sight (LOS) stabilization systems with microradian-level residual LOS jitter and wide-angle search (WAS) capability. The system employs a strapdown pointing configuration which obviates the telescope's gimballing for WAS. The LOS is steered through large angles with a gimballed steering optical element. A fine-steering mirror furnishes low-jitter operation through the operation of servo that nests the two mechanisms electronically.

The present F-O rotation sensors (FORS) are all-solid state devices for measuring rotations and rotation rates in inertial space that may reach the 0.003 deg/hr (1-sigma) accuracies required for NASA's Saturn-orbiting Cassini mission. Attention is presently given to the mission, inertial reference unit, and FORS instrument optoelectronic component requirements envisioned for such spacecraft applications.

The three methods presented for calculating the image rotation of any plane-mirror optical system are based on a novel optical kinematic construction, the 'line-of-sight' reference frame (LOSRF). By using the LOSRF, image rotation due to fixed or movable mirrors, derotation mirrors, and vehicle orientation is calculated precisely. The first two of the methods yield the total image rotation angle irrespective of the source of the rotation, while the last furnishes the amount of rotation imparted to the system by each of its components; total image rotation is the sum of these individual rotation angles.

A two-stage active control approach was developed addressing the figure control problem for a spaceborne FIR telescope, the Precision Segmented Reflectors Focus Moderate Mission Telescope (FMMT). The first active control stage aligns the optical segments based on images; attention is here given to the second stage, active figure maintenance control system, which maintains the alignment of the optical elements between initializations to hold the mirror figure steady while obtaining data and fixes translational and rotational changes of the optical segments induced by long-term thermal drifts of the support structure. Errors are expected to be 10-100 microns at the nodes of the primary backup structure over the course of an orbit. An rms performance of 0.8 microns of wavefront error can be expected during the maintenance function based on specified nominal sensor noises, actuator accuracies, and system environments. A performance of less than 0.3 microns rms can be expected, based on advanced components.

A general framework is presented for the matrix-form, linear optical model analysis of controlled optomechanical systems; the models are conjoined with linear models of structures and controls to compute system performance as a function of optics, structures, and control parameters. Covariance analysis, optimization, and estimation/simulation are used. Attention is given to a tolerancing example for the Hubble Space Telescope's Wide Field and Planetary Camera, which involves the creation of a linear model of residual pupil shear.

Controller-structure interaction (CSI) affects the stability margin of actively-controlled large flexible structures, degrading performance to the point where instability is generated. Perturbation theory is presently employed to identify those structural modes that are affected by CSI, in order that they be filtered out of the controller's path. Numerical results are presented for the cases of a simply-supported beam, a space-based laser, and a truss beam.

Due to control processor limitations, the design of reduced-order controllers is an active area of research. Suboptimal methods based on truncating the order of the corresponding linear- quadratic-Gaussian (LQG) compensator tend to fail if the requested controller dimension is sufficiently small and/or the requested controller authority is sufficiently high. Also, traditional parameter optimization approaches have only local convergence properties. This paper discusses a homotopy algorithm for optimal reduced-order control that has global convergence properties. The exposition is for discrete-time systems. The algorithm has been implemented in MATLAB and is applied to a benchmark problem.

As part of a technology development program for realizing a space based submillimeter telescope, two different approaches to the absolute phasing of a segmented primary mirror using focal plane measurements have been implemented for feasibility. The method of optimization by simulated annealing evaluates the image quality of a point spread function after all the telescope segments have been randomly moved. It accepts each iteration which improves the image quality, as well as a random number of iterations which do not, thus keeping the Strehl in the initialization procedure from falling into local maxima. Methods for determining the annealing schedule, and the step size for random segment movements are presented and discussed. Using phase diversity and a model for the telescope imaging system, a nonlinear least squares algorithm has also been implemented which parameterizes each of the segment actuator movements. Using multiple out of focus images, the segment actuator positions are estimated using an iterative procedure. Nonlinear least squares, although computationally intensive, offers a large savings in the actuator movements over simulated annealing and pairwise phasing methods for large numbers of segments. These algorithms have been integrated into a general simulation program which models the behavior of the telescope under anticipated space conditions.

The spillover modes of a large flexible structure with high modal density can be stabilized by residual-mode filtering, as is presently illustrated for a reduced-order model of the Passive and Active Control of Space Structures program's Dynamic Test Article by means of an estimator and regulator that were developed via linear Quadratin Gaussian and loop-transfer recovery methods. The implementation of this controller on the full-order system generated spillover instabilities; the residual-mode filter was added to the control system without modification of the original controller, and stabilization was achieved.

The rapid retargeting and precision pointing (R2P2) simulator is an adaptable hardware testbed for space-based optical systems requiring pointing agility and extreme pointing precision. The simulator is capable of performing an entire pointing, tracking, or repointing scenario with full scale angular dynamics including both external and control-generated disturbances. Vehicle or spacecraft dynamics, including flexible body effects, are faithfully represented in one plane, while line-of-sight (LOS) dynamics are represented with limited out-of-plane motion. This paper describes the simulation capabilities of R2P2 which permit the achievement of full scale angular dynamics of the simulated vehicle and the simulation of structural dynamic effects. The two processes used to simulate these aspects of the system are referred to as torque allocation and structural mode simulation. The torque allocation process ensures that the angular accelerations of the bodies of the real system are faithfully reproduced on the simulator. The structural mode simulation process reproduces the effects of critical structural modes on the pointing of the vehicle and the pointing of the LOS. Together, these processes generate a testbed that is capable of representing a wide range of vehicle dynamics and structural dynamics.

The USAF's Advanced Space Structure Technology Research Experiment, 'ASTREX', employs a large precision-beam expander structure for the development and testing of vibration and slewing technologies. Attention is given to the test article's air-bearing system's gimballing, and the ASTREX's reaction-control, control-moment gyro, and sensor and actuator systems.

This paper presents analytical and experimental results in actively damping flexible structures with reaction mass actuators. A two degree of freedom spring-mass model of a flexible structure is analyzed and the key parameters of actuator mass participation and pole-zero separation are related to the maximum damping achievable from rate feedback control. The main conclusion of the paper is that the larger the pole-zero separation the larger the amount of damping that can be imparted to a structural mode. Laboratory experiments conducted on an 8-foot truss structure support the analytical predictions.

New frequency response measurement procedures, on-line modal tuning techniques, and off- line modal identification algorithms are developed and applied to the modal identification of the Advanced Structures/Controls Integrated Experiment (ASCIE), a generic segmented optics telescope test-bed representative of future complex space structures. The frequency response measurement procedure simultaneously uses all the actuators to excite the structure and all the sensors to measure the structural response so that all the transfer functions are measured simultaneously. Structural responses to sinusoidal excitations are measured and analyzed to calculate spectral responses. The spectral responses in turn are analyzed as the spectral data become available and, which is new, the results are used to maintain high quality measurements. Data acquisition, processing, and checking procedures are fully automated. As the acquisition of the frequency response progresses, an on-line algorithm keeps track of the actuator force distribution that maximizes the structural response to automatically tune to a structural mode when approaching a resonant frequency. This tuning is insensitive to delays, ill-conditioning, and nonproportional damping. Experimental results show that it is useful for modal surveys even in high modal density regions. For thorough modeling, a constructive procedure is proposed to identify the dynamics of a complex system from its frequency response with the minimization of a least-squares cost function as a desirable objective. This procedure relies on off-line modal separation algorithms to extract modal information and on least-squares parameter subset optimization to combine the modal results and globally fit the modal parameters to the measured data. The modal separation algorithms resolved modal density of 5 modes/Hz in the ASCIE experiment. They promise to be useful in many challenging applications.

An integrated beam control demonstration (IBCD) is being fabricated and tested under the direction of the Naval Surface Warfare Center for the SDIO. The IBCD demonstrates the key technologies required for implementing a three-mirror, wide field of view (WFOV), Advanced Beam Control System for a space-based laser. This paper describes an overview of the IBCD and progress in the fabrication and testing of the WFOV beam expander, the outgoing wavefront sensor, the deformable mirror, the dynamic steering mirrors, the wavefront control subsystem, and the high speed diagnostic interferometer. The results include photos of the IBCD and hardware assemblies and evaluation of wavefront control performance.

This paper presents new experimental results which pertain to the control of flexible link manipulators. Flexible link manipulators have the peculiar characteristic of possessing an infinite number of degrees-of-freedom, but (usually) only one actuator per link. This is one of the main distinctions, along with the presence of significant kinematically induced nonlinearities, between flexible manipulator control and control of other flexible bodies such as large spacecraft. For such systems, one safe and reliable strategy is to maximize the modal damping. To increase the system modal damping one can use feedback control and in particular this paper reports results of feedback control experiments which use modified outputs as the feedback signal. These outputs are noncollocated but have the jw axis pole-zero interlacing property possessed by systems under collocated control. The modified outputs offer the potential of giving increased damping in a feedback configuration without introducing the dangerous non-minimum-phase zeros normally associated with non-collocated control. Experimental results are presented which show that it is practical to use these modified outputs.

The present multidisciplinary telescope-analysis approach, which encompasses thermal, structural, control and optical considerations, is illustrated for the case of an IR telescope in LEO; attention is given to end-to-end evaluations of the effects of mechanical disturbances and thermal gradients in measures of optical performance. Both geometric ray-tracing and surface-to-surface diffraction approximations are used in the telescope's optical model. Also noted is the role played by NASA-JPL's Integrated Modeling of Advanced Optical Systems computation tool, in view of numerical samples.

Future flight projects at JPL such as the Large Deployable Reflector (LDR) require close design effort between controls, dynamics, structures, optics, and thermal disciplines. The Figure Control Simulation (FCSim) program has been developed under the Precision Segmented Reflector (PSR) program of NASA to perform analysis on a particular moderate mission of interest, a telescope with a 3.65 m primary mirror broken into 7 segments, designed for 100 - 800 micrometers wavelengths. Simulation code is designed to handle both the flight system and a ground-based test system. The optical analysis, performed using the Controlled Optics Modeling Package (COMP) can handle multi-spectral images, extended sources, image smear due to jitter of the structure, and surface irregularities of the panels. Various dynamic disturbances are included as well. One simple method of aligning the panels is presented. Results obtained with FCSim are presented.

Recently there has emerged a new class of sensors, called spatial filters, for structures which respond over a significant gauge length. Examples include piezoelectric laminate PVDF film, modal domain optical fiber sensors, and holographic sensors. These sensors have a unique capability in that they can be fabricated to locally alter their sensitivity to the measurand. In this paper we discuss how these sensors can be used for the implementation of control algorithms for the suppression of acoustic radiation from flexible structures. Based on this relationship between the total power radiated to the far field to the modal velocities of the structure, we show how the sensor placement to optimize the control algorithm to suppress the radiated power.

This paper presents a position monitoring system incorporating high birefringent optical fiber and white-light interferometry, where the measurement process takes place within the optical fiber medium. With a novel scheme of relative position detection, the effects of temperature variation can be largely eliminated. A position monitoring range of 200 mm, accuracy of about 3 mm with temperature stability to within less than 0.5 percent was experimentally demonstrated.