Toward the robust control of high-bandwidth high-precision flexible optical systems

It is well known that control-structure-interaction (CSI) phenomena limit the stability and performance of controlled flexible structures. Most CSI research focuses on rigid body control of flexible structures with relatively low closed loop control bandwidths. This research examines the CSI phenomena associated with high bandwidth high precision control of a reaction actuator mounted to a flexible support structure. In particular, CSI using a high gain proportional integral-derivative (PID) controller is examined as it relates to certain design parameters. Rapid small angle line-of-sight repositioning and precision line-of-sight stabilization against a variety of disturbances are performed using both a single and multimode model. The single flexible mode model consists of a three-mass lightly damped translating system, referred to as the modified benchmark model. The multimode model consists of a single-axis reaction steering mirror mounted to the tip of a flexible truss-like support structure. CSI analysis of the reaction actuator control problem is performed first on the single-mode model as a function of flexible mode location, then with the multimode model as a function of inertial and elastic coupling between the reaction steering mirror and the flexible support structure. The analysis describes the CSI effects on both stability and performance of a high gain line-of-sight PID controller. This research introduces the idea of bicollocated control of a reaction actuator. It also shows that control of a reaction actuator in the classical 'collocated' sense may lead to nonminimum phase zeros in the plant model and may result in an unstable closed loop system. The critical parameters affecting both stability and performance of high gain reaction actuator controllers on flexible systems are shown to be actuator natural frequency, actuator inertia, uncoupled flexible support structure model inertia, and coupled system model reaction inertia.