Modern Control Methods Applied to a Line-of-Sight Stabilization and Tracking System

Modern control methods are used to develop compensators for a precise optical positioning system designed to track a commanded line-of-sight (LOS) position while rejecting the vibrational environment of an F-16 fighter aircraft, the vehicle upon which the device will be mounted. The physical system being controlled consists of two gimbals that effect large changes in the LOS position, and a mirror assembly that is used, because of its fast dynamic response (but limited range of motion), to reduce LOS jitter (i.e. angular disturbances above 5 Hz that cause blurring). Compensators are designed that include models of the motion to reject and of the motion to track. These models become part of each compensator and enable them to distinguish aircraft vibration from aircraft maneuvers, rejecting the former and tracking the latter. The vibrational disturbance that must be rejected has a magnitude of 560 μrads rms, which is magnified by the optical system to 1400 μrads of LOS motion. Simulation of closed-loop performance with a nonlinear dynamic model of the system demonstrated that LOS jitter is reduced to about 130 μrads rms. A tradeoff between stabilization and tracking is demonstrated. Compensator robustness to unmodelled disturbances is increased using the Loop-Transfer-Recovery technique.