Shalom Fisher

Application of actuators to control beam flexure in a large space structure

This paper describes a flexible-body control methodology for performing a nonlinear slew of a large space structure and simultaneously regulating the associated flexural vibrations by linear methods. Degree of controllability methods are used as guidelines for the positioning of proof-mass actuators on the structure. The goal is to determine the importance of location of the actuators on regulator performance, and the utility of the degree of controllability methods. A numerical simulation is made of a 20 deg slew maneuver of the spacecraft laboratory experiment (SCOLE). The SCOLE model treated here is the spacecraft version. It includes three bodies: the Space Shuttle, a reflector antenna, and a flexible beam of length 39.62 m connecting them. In the simulation, the beam vibration is fully coupled to the dynamics of the slewing motion. Regulation of the beam vibration is addressed by means of proof-mass actuators on the beam, and by vernier thrusters on the Shuttle and antenna bodies. Repeated simulations are made with different actuator placements. The results show that with the actuators placed within a region of strong control effectiveness, damping and flexural amplitude are changed only slightly by changes in actuator location. However, the damping is significantly reduced for actuator location in regions of low-control effectiveness, although the amplitude of the vibrations changes only slightly.

Satellite vibration measurements with an autodyne CO 2 laser radar

Vibration signatures of the Low Power Atmospheric Compensation Experiment satellite were obtained with a ground-based CO(2) laser radar. The laser radar operated in a cw mode and used autodyne receivers to extract relative target velocity information between a germanium retroreflector located at the base of the satellite and a retroreflector array located at the tip of an extended forward boom. Time-frequency analysis algorithms were applied to the vibration data to investigate the correlation between excitations and modal structure. The resultant analysis suggests that vibration modes of an on-orbit spacecraft can be suppressed with simple open-loop techniques.

Strain-based shape estimation algorithms for a cantilever beam

Strain-displacement mappings based on linear and quadratic curvature assumptions are derived, compared for a numerical model and applied to a 4.37 m tapered composite boom with a circular cross-section. Displacement estimations are obtained for both the vertical and horizontal directions with displacement estimation errors of less than 0.2 mm in the vertical direction and 1 mm in the horizontal direction. Limitations on strain displacement algorithms for long booms are discussed as well as strain sensor noise effects on estimation accuracy.

Structural damage detection using real-time modal parameter identification algorithm

A new approach to conducting structural damage detection is described. The approach employs a real-time modal parameter identification algorithm implemented in a digital-signal-processor-based data acquisition system. Because the modal parameter extraction process is conducted in real time, the algorithm is capable of identifying changes in structural properties attributable to structural damage as soon as they occur. Using the algorithm and a laboratory truss structure, it is demonstrated experimentally that continuous, real-time monitoring of anomalies attributable to structural damage is feasible. This monitoring capability will provide an early warning to an operator so that proper measures can be taken before a catastrophic failure occurs. The results of the damage detection study using the truss are presented along with the description of the real-time modal parameter identification algorithm.

Implementation of local feedback controllers for vibration supression of a truss using active struts

This paper describes the design and implementation of local feedback controllers for active vibration suppression of a laboratory truss referred to as the Naval Research Laboratory (NRL) space truss. The NRL space truss is a 3.7 meter, 12-bay aluminum laboratory truss used as a testbed to explore smart structures technologies for future Navy spacecraft missions. To conduct real-time control and data acquisition for the implementation of controllers, a digital signal processor based system is used. Two piezoceramic active struts are employed in this experimental study. Each strut is instrumented with a force transducer and a displacement sensor. Modal strain energy computed using a refined finite element model was used to select the optimum locations of the two actuators to ensure controllability of the first two structural modes. Two local feedback controllers were designed and implemented, an integral force feedback and an integral plus double-integral force feedback. The controllers were designed independently for each active strut using classical control design techniques applied to an identified model of the system dynamics. System identification results and controller design procedure are described along with closed loop test results. The test results show up to a factor of 1/110 attenuation of the truss tip motion due to sinusoidal resonant input disturbances and up to 100 times increase in damping of the lower frequency modes of the truss.

Feasability of adaptive vibration control of a space truss using modal filters and a neural network

An adaptive algorithm is proposed for the control of a large space truss structure which uses modal filters for independent modal space control and a simple neural network that provides an on-line system identification capability. The modal filters are computed off-line using measured frequency response functions and estimated pole values for the modes of interest, and provide a coordinate transformation that yields modal coordinates from physical response measurements. The time histories for the modal coordinates are then processed in real time by the neural network, which models a single degree of freedom system transfer function and provides estimates of modal parameters, namely, frequency, damping ratio and modal gain. The modal filters are used to implement independent modal space control on a 3.74 meter space truss using a single reaction-mass actuator and 32 accelerometers. The performance of the modal filter based controller is compared to that of a local rate feedback controller using the same actuator. The applicability of the adaptive filter to adaptive control is demonstrated by real time estimation of the modal parameters of the truss with and without control. Because the modal filter control gain can be adjusted to maintain a desired closed loop damping ratio, which is tracked by the adaptive filter, adaptive control of individual modes in a time-varying system is possible. The goal of this work is to field a control system which can maintain desired closed loop damping ratios for mode frequency variations as high as 10%.

A GEOMETRIC APPROACH TO STRAIN-DISPLA CEMENT MAPPING FOR SPACE TRUSS STRUCTURES

A strain-displacement mapping algorithm has been developed to estimate the deformation of space truss structures using the measurements of strains of individual struts. The algorithm relies solely on the geometry of the structure and does not require any knowledge of the material properties or dynamic characteristics of the structure. This algorithm is expected to be useful in estimating deformation of long booms which are employed to enhance the accuracy of the Global Positioning System based attitude determination systems. To examine the performance of the algorithm, experimental studies were conducted using a laboratory truss. The results indicate that the algorithm is capable of estimating accurately the displacements at various locations in the truss under static, dynamic and thermal loads. In order to reduce the number of strain gauges required, a sensor placement strategy was also investigated.

Vibrations of the Low Power Atmospheric Compensation Experiment satellite

The Low Power Atmospheric Compensation (LACE) satellite dynamics experiment has measured vibrations of an orbiting satellite from a ground site and has observed the excitation of satellite vibrations by a sequence of boom movements. The preprogrammed boom movements were initiated by commands from a ground control site and observed by the Massachusetts Institute of Technology Lincoln Laboratory Firepond laser radar facility located in Westford, Massachusetts. In the tests, a narrow-band heterodyne COi laser radar, operating at a wavelength of 10.6 /Ltm, detected vibration-induced differential Doppler signatures of the LACE satellite. Augmentation of vibration amplitudes was achieved through timing of repeated boom movements. Evidence of open-loop vibration damping by this method of repeated boom movements was also obtained, although the data were not conclusive since only a single attempt at open-loop damping was observed. The tests have demonstrated the feasibility and advantages of using relatively low cost ground-based observation techniques for vibration measurements and health monitoring of orbiting structures and for improving the accuracy of mathematical models for the structural dynamics of light, flexible space structures.

Ladar measurements of satellite vibrations with post-heterodyne detection autodyne receivers

Vibration signatures of the Low Power Atmospheric Compensation Experiment (LACE) satellite were obtained using a ground based CO2 laser radar. The laser radar operated in a cw mode and utilized autodyne receivers to extract relative target velocity information between a germanium retroreflector located at the base of the satellite and a retroreflector array located at the tip of an extended forward boom. Time-frequency analysis algorithms were applied to the vibration data to investigate the correlation between excitations and modal structure. The resultant analysis suggests that vibration modes of an on-orbit space craft can be suppressed using simple open-loop techniques.

Ladar measurements of satellite vibrations

Vibration signatures from the Low Power Atmospheric Compensation (LACE) satellite were obtained using a ground-based coherent CO2 laser radar facility. Analysis of pulsed CW laser radar measurements of the satellite indicates the presence of complex time-varying vibration modes. These data represent the first observations of satellite vibration modes from a ground-based laser radar.