Optimal disturbance rejection by LTI feedback control in a laser beam steering system

This paper presents an optimal control design and experimental implementation for pointing and disturbance rejection in a laser beam steering system. To compensate for broadband disturbances due to atmospheric turbulence in the optical path and mechanical vibration of the laser and optical components, the linear quadratic Gaussian (LQG) controller includes a stochastic disturbance model, as well as integral action for tracking low-frequency disturbances and commands. The experimental system consists of a two-axis tilt mirror actuated by piezo-electric actuators, 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 in desired bandwidths.