Evaluation of Five Control Algorithms for Addressing CMG Induced Jitter on a Spacecraft Testbed

Abstract

Spacecraft often experience jitter from reaction wheels, control moment gyros (CMGs), or from motion of other internal parts. One may isolate fine pointing equipment by passive techniques, but active vibration control employing knowledge of the periodic nature of jitter can improve performance. Previous work by the authors and co-workers tested active isolation using a 6 degree-of-freedom Stewart platform. A new class of applications is laser communication relay satellites, which replaces radio frequencies communication by laser communications. Laser beam jitter can be corrected by control of tip and tilt in fast steering mirrors. This paper develops experiments testing five candidate jitter cancellation algorithms on the Bifocal Relay Mirror Spacecraft, Three Axis Simulator 2 testbed at the Naval Postgraduate School. Jitter results from the CMGs. Multiple period repetitive control (MPRC) and matched basis function repetitive control (MBFRC) are tested. Both use disturbance period information from Hall effect sensors for the CMG three phase brushless DC motors. Filtered-X LMS, adaptive linear model predictive control, and the Clear Box algorithm with Adaptive Basis Method are also tested. The best disturbance rejection resulted from the last of these choices, with a 66 % overall amplitude reduction. Concerning MPRC it was discovered that repeating an addressed period can be used as a technique to decrease sensitivity to accurate knowledge of the disturbance period.