Disturbance Rejection by Optimal Feedback Control in a Laser Beam Steering System

Abstract

This paper presents an optimal control design and experimental implementation for pointing and disturbance rejection in a laser steam steering system. The linear quadratic Gaussian (LQG) controller, which includes a stochastic disturbance model, as well as integral action, was designed and implemented to compensate for disturbances due to atmospheric turbulence in the optical path and mechanical vibration of the laser and optical components. The control design also considers the situation where the stochastic disturbances applied to the two beam axes are correlated and renders a multi-input-multi-output (2-by-2) output feedback controller. The experimental system consists of a two-axis tilt mirror driven by piezo-electric actuators for controlling the laser beam, a second actuated tilt mirror to generate disturbances, a position sensing device that senses the location of the beam on a target plane, and a real time computer for digital control. System identification is used to determine a state space model of the beam steering system for use in control system design. Experimental results are presented to demonstrate the effectiveness of the LQG optimal disturbance rejection for the prescribed stochastic disturbances.