Reactionless drive mechanism and its stability problems

Abstract

The purpose of this paper is to characterize the residual reaction torques of a reactionless drive system, and to evaluate the impact of the drag force (produced by the eddy current in the reactionless drive mechanism) on the stability problems. The Routh-Hurwitz criterion is used to derive the stability conditions for a closed loop control system employing the reactionless drive mechanism. It shows that if the drag force exists in the reactionless drive system, the closed loop control system becomes unstable when the natural spring-mass frequency of the reaction mass is greater than the natural springmass frequency of the mirror. I. Introduction To achieve high precision pointing and line-of-sight (LOS) jitter control, high bandwidth steering mirror alignment control techniques are usually used in a n optical tracking system. When driving the steering mirror, reaction torques acting on the base structure will be introduced. Transmission of reaction torques to the base structure-can be eliminated by adding a counter reaction inertia to the existing steering mirror inertia. If the dynamics of the steering mirror and the reaction mass are identical, no net forces or turning moments will be transferred from the mirror motion to the base structure. This reactionless drive concept in the steering mirror designs becomes vital for large optics systems. Because, the reaction torques acting on the base structure may excite the beam director structural modes and the excited structural modes will cause severe LOS jitter.