Flexible Spacecraft Reorientations Using Gimbaled Momentum Wheels
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Flexible Spacecraft Reorientations Using Gimbaled Momentum Wheels 1 Kevin A. Ford 2 and Christopher D. 8al1 3
Abstract We study the reorientations of flexible spacecraft using momentum exchange devices. A new concise form of the equations of motion for a spacecraft with gimbaled momentum wheels and flexible appendages is presented. The derivation results in a set of vector nonlinear first order differential equations with gimbal torques and spin axis torques as the control inputs. Feedback control laws which result in smooth reorientations are sought with the goal of minimizing structural excitations. We pay special attention to a class of maneuvers wherein the magnitude of the momentum in the wheel cluster is held constant, resulting in a so-called "stationary platform maneuver." The advantage of this maneuver is that the platform angular velocity remains small throughout, thereby reducing the excitation of the appendages.
Introduction Momentum exchange devices are typically categorized as either momentum wheels (fixed spin axis) or control moment gyros (fixed spin speed). The gimbaled momentum wheel (GMW) is simply a combination of the two, i.e., a control moment gyro (CMG) with a controllable speed wheel. We begin the paper by presenting a new set of equations of motion for a rigid spacecraft with N gimbaled momentum wheels. A distinct advantage of this new set of equations is the explicit dependence upon the spin axis and gimbal axis input torques, which are arguably the control variables for the system. Certain restrictions on the GMW equations permit simplification to the momentum wheel or CMG case. New areas of interest in astronautics, IThis paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Presented as Paper AAS 97-723 at the AAS/AIAA Astrodynamics Specialist Conference, Sun Valley, Idaho, August 4-7, 1997. 2Formerly Ph.D. Candidate, Department of Aeronautics and Astronautics, Air Force Institute of Technology, Wright-Patterson AFB, Ohio. Currently Director, Plans and Programs, United States Air Force Test Pilot School, Edwards AFB, California. 3 Associate Professor, Aerospace and Ocean Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia. Formerly Assistant Professor, Department of Aeronautics and Astronautics, Air Force Institute of Technology, Wright-Patterson AFB, Ohio.
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such as simultaneous attitude control and energy storage [1], might benefit from this new form of the general equations. We then turn to the use of momentum storage devices to reorient a spacecraft from one rest condition to another (i.e. zero angular velocity at start and end of the maneuver). The rest condition (a stationary platform) is only possible when all of the angular momentum is contained in the cluster of GMWs. In the case of the gyrostat, Hall has shown that variation of the wheel speeds along a specific manifold in the space of rotor momenta results in a spacecraft reorientation which keeps body angu
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