Flexible Spacecraft Rest-to-Rest Maneuvers with CMGs Parallel Gimbal Arrangement

A feedforward control algorithm for flexible spacecraft rest-to-rest maneuvers with CMGs (control moment gyros) parallel gimbal configuration is presented. Single-gimbal CMG cluster in parallel gimbal arrangement has singularity problem and output torque vector is constrained in a 2-DOF plane normal to gimbal axis. This CMGs parallel gimbal configuration is suitable in case that spacecraft has 2-DOF pointing requirement like a Japanese satellite called Astro-G (‘space VLBI’ = radio telescope observation satellite). A CMG guidance law presented in this paper is designed particularly for single-axis rest-torest rotational maneuvers for a flexible spacecraft with 2-DOF pointing requirement. Generally spacecraft with large flexible structure has a lot of large-mass flexible-modes. Therefore, uncertainity of high-order flexible-modes needs to be taken into account for highaccuracy controller design. Most researcher propose input preshaping algorithms with finite target flexible-modes and limited frequency tolerances, so if we apply these algorithms into an actual large flexible satellite, control errors might be theoretically inevitable. The NME (Nil-Mode-Exciting) profiler shown in this paper includes an extra-insensitive preshaping profiler which overcomes above-mentioned difficulties. The NME profiler is a preshaping profiler constructed from a pair of sampling functions (also known as sinc function), consequently feedforward profile generated from this profiler has no frequency response above a certain designed frequency. Therefore residual vibration at the end-point of maneuver can be highly reduced with minimum loss of maneuver agility. Flexible spacecraft dynamics model of Astro-G including 4-CMGs and 4-RWs is used to evaluate the proposed algorithm.