A Local Optimization Technique for Attitude Motion Tracking Using Control Moment Gyroscopes
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A Local Optimization Technique for Attitude Motion Tracking Using Control Moment Gyroscopes1 Giulio Avanzinf and Guido de Matteis' Abstract A control technique is proposed for satellite attitude slew maneuvers, where the reference trajectory is specified as a function of time. A feedforward steering law, capable of avoiding singular states, is generated by a local optimization technique where, in the framework of integration methods, an inverse simulation problem is solved. A novel tracking controller is developed to account for model uncertainties and external disturbances. Physicallimitations of the system in terms of maximum command rates and angles of gimbals and/or maximum command torque can be precisely accounted for in the determination of the control profile. Finally, the inverse simulation algorithm appears highly efficient in terms of computational time.
Introduction This paper addresses the problems of command generation and tracking control of a rigid spacecraft using control moment gyroscopes (CMG) for attitude maneuvers. CMGs are momentum exchange devices used as internal torque actuators for spacecraft maneuver control and precision pointing [1]. The control torque is generated by orienting the spin axis of a flywheel spinning at a constant rate. In single gimbal CMG devices (SGCMG) the spin axis can rotate about a gimbal axis mounted to the platform, and input torque is applied to the gimbal to produce an output torque perpendicular to both the gimbal and wheel spin axes. SGCMG systems have received much attention in the last three decades due to favorable torque amplification characteristics and relative mechanical simplicity in comparison with other torque-generating devices. 1Presented as AAS Paper 01-106 at the 11th AAS/AIAA Spaceflight Mechanics Conference, Santa Barbara, CA, February 11-15,2001. 2Department of Aerospace Engineering, Polytechnic of Turin, corso Duca degli Abruzzi 24, 10129 Turin, Italy. 3University of Rome "La Sapienza," Department of Mechanics and Aeronautics, via Eudossiana 18,00184 Rome, Italy.
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However SGCMGs have some disadvantages. Gimbal rate variations may excite resonant structural vibrations [2, 3] and this phenomenon, although somewhat more manageable than in variable-speed reaction wheels, is a serious problem for highly accurate attitude control of flexible spacecraft [4]. Next, apart from limitations associated to the maximum amount of momentum that can be exchanged, i.e., the socalled external singular state, the main drawback of SGCMGs is due to the presence of a quite complicated system of internal singular states, where the gimbal configuration is such that the torque can be generated only in a plane [5]. In that circumstance, when it is not possible to produce a command torque in the singular direction, highly undesirable path deviations from the assigned trajectory may result. A complete analysis of the SGCMG system topology including a detailed description of geometric properties of singular states is reported in a renow
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