Negative-imaginary Control For Laser Beam Pointing Stabilization

Laser beam stabilization is a technology currently used in aircraft/spacecraft targeting, surveillance and free space laser-based communication systems. However, all kinds of dynamical disturbances in natural environment, such as temperature changes, atmosphere turbulence and mechanical vibrations caused by various sources, may influence the performance of laser beam pointing. Quanser’s laser beam stabilization platform uses a single-axis, fast steering mirror driven by a DC motor and a high-resolution position sensor detector to imitate the laser beam pointing process. Recently, this laser beam stabilization system is found to be a negative-imaginary system with a pole at the origin. Based on negative-imaginary theory, this thesis investigates the disturbance rejection strategy for laser beam stabilization systems, and the main contents are as follows:Firstly, the concept of negative-imaginary systems, together with two control methods, namely, positive position feedback control and integral resonant control, are introduced; the working principle of Quanser’s laser beam stabilization platform is briefly described, then the system model is shown to be a negative-imaginary system with a pole at the origin.Next, an improved positive position feedback controller is designed. The controller parameter constraints are derived by using negative-imaginary stability theorem and the principle of positive position feedback control, and these constraints can make the system reach the maximum damping ratio with the minimum gain, then those controller parameters are optimized to meet the pole placement requirement. Comparisons are made with the original PID controller within the laser beam pointing stabilization platform, and the disturbance rejection effectiveness of the proposed positive position feedback controller is verified via simulation.Finally, an improved integral resonant controller is designed. Via negative-imaginary stability theorem and the principle of integral resonant control, the range of the controller parameters with robust stability is firstly obtained, and then the controller parameters are optimized to place the closed-loop poles properly. Compared with the practical PID controller via simulation, this improved integral resonant controller is demonstrated to compensate for vibration sources and stabilize the laser beam position more effectively.