Albert Bosse

Optical fiber sensors for spacecraft applications

Optical fiber sensors offer a number of advantages for spacecraft applications. A principal application is strain sensing for structural health monitoring, shape determination, and spacecraft qualification testing. This paper will review the results of recent work at the Naval Research Laboratory where optical fiber strain sensors have been used on spacecraft structures and ground test hardware. The sensors have been both surface mounted to the structure and embedded in fiber-reinforced polymer composites. The issue of potential strength reduction of high-performance composites due to embedded optical fiber sensors and leads has been studied, low-cost fabrication of tubular struts with embedded sensors has been demonstrated, and a novel technique for fiber ingress-egress from composite parts has been developed. Applications of fiber sensors discussed in this paper include distributed dynamic strain monitoring of a honeycomb composite plate and a lightweight reflector during acoustic qualification tests, ultrahigh-sensitivity static strain and temperature measurements for precision structures, and on-line system identification of a lightweight laboratory truss.

SUMO: spacecraft for the universal modification of orbits

SUMO, or Spacecraft for the Universal Modification of Orbits, is a risk reduction program for an advanced servicing spacecraft sponsored by the Defense Advanced Research Projects Agency and executed by the Naval Center for Space Technology at the Naval Research Laboratory in Washington, DC. The purpose of the program is to demonstrate the integration of machine vision, robotics, mechanisms, and autonomous control algorithms to accomplish autonomous rendezvous and grapple of a variety of interfaces traceable to future spacecraft servicing operations. The laboratory demonstration is being implemented in NRL’s Proximity Operations Test Facility, which provides precise six degree of freedom motion control for both the servicer and customer spacecraft platforms. This paper will describe the conceptual design of the SUMO advanced servicing spacecraft, a concept for a near term low-cost flight demonstration, as well as plans and status for the laboratory demonstration. In addition, component requirements for the various spacecraft subsystems will be discussed.

Robustness of positive real controllers for large space structures

The robustness of a continuous positive real controller design is established by linking the positivity theory to the standard singular value robustness tests. By application of the singular value test to a model of the deviation from positivity induced by actuators, computational delays, etc., the global stability of the control design can be assured, even with significant modeling errors due to modal uncertainty. Both theoretical and experimental verification of this stability result is presented.

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.

Modal Filters and Neural Networks for Adaptive Vibration Control

An adaptive control algorithm is investigated for the vibration suppression of a space truss structure using modal filters for independent modal space control and a neural network for online system identification. The modal filters are computed off-line using measured frequency response functions and estimated pole values for the modes of interest. They are used to conduct transformation of response measurements from physical coordinates to modal coordinates. The time histories in the modal coordinates are then processed in real time by the neural network to extract estimates of modal parameters, namely, natural frequency, damping ratio, and modal gain. To examine the performance of the adaptive control approach, a controller was designed using the modal filters and implemented on a laboratory 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 neural network to adaptive control was 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 neural network, adaptive control of individual modes in a time-varying system is possible. Eventually, this type of adaptive controller will help develop a control system that can maintain desired closed- loop performance characteristics under significant modal parameter variations.

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.

Strain-displacement mapping for a precision truss using Bragg gratings and identified mode shapes

A procedure for mapping strain measurements to nodal displacements for a 3.74 meter laboratory truss is presented and validated using experimental data. Assumptions of small displacements and a linear displacement-strain relationship were used to develop the strain- displacement mapping. Due to the assumed discrete nature of the space truss structure, the transformation depends only on kinematics and hence only geometric data is required for the mapping. The procedure is therefore valid for quasi-static deformations as well as dynamic deformations. Estimated displacements for several nodes are compared with truth measurements obtained from both a laser interferometer system and accelerometers. It is shown that the accuracy of the predicted displacement for a limited number of sensor depends not only upon the deformation state but also which degree of freedom is being estimated.

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%.

Control of the ultraLITE precision deployable test article using adaptive spatio-temporal-filtering-based control

Experimental results are presented for active vibration control of the Air Force Research Laboratorys 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.