KEYWORDS: Space operations, Robotics, Machine vision, Satellites, Sensors, Control systems, Analytical research, Aerospace engineering, Space robots, Algorithm development
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.
Infrared projection systems commonly use a collimating optical system to make images of a projection device appear far away from the infrared camera observing the projector. These `collimators' produce distortions in the image seen by the camera. For many applications the distortions are negligible, and the major problem is simply shifting, rotating, and adjusting the magnification, so that the projector image is aligned with the camera. In a recent test performed in the Kinetic Kill Vehicle Hardware-in-the-Loop Simulator facility, it was necessary to correct for distortions as small as 1/10th the size of the camera pixels across the field of view of the camera. This paper describes measurements and analyses performed to determine the optical distortions, and methods used to correct them.
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.
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.
KEYWORDS: Digital filtering, Actuators, Control systems, Neural networks, Sensors, Feedback control, Algorithm development, Adaptive control, System identification, Linear filtering
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%.
KEYWORDS: Digital signal processing, Feedback control, Transducers, Sensors, Signal processing, Control systems, Actuators, Space operations, Aerospace engineering, Data acquisition
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.
KEYWORDS: Sensors, Fiber Bragg gratings, Interferometers, Digital signal processing, Laser systems engineering, Antennas, Data acquisition, Associative arrays, Analytical research, Global Positioning System
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.
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