Pointing control design based on the PID type-III control loop for two-axis gimbal systems

Abstract

Research has been focused on the pointing control of the two-axis gimbal system (TGS) which plays a significant role in many different tasks. The pointing accuracy is one of the most concerning performance in practical application, which is, to a large extent, affected by the pointing controller design. Often, the three-loop (current-velocity-position) control scheme is applied to suppress the disturbance and point reference signal. In this paper, the PID type-III control loop is proposed for TGSs and the controller parameters tuning rules are given explicitly. Concretely, the friction compensation based on the LuGre model is carried out first to the velocity inner loop. A greater system rigidity is achieved for the position loop. Secondly, based on frequency domain modeling, the PID type-III control loop is designed according to the position-controlled plant, which is characterized by the presence of three integrators in the open-loop transfer function to achieve zero steady-state position, velocity and acceleration error. Then, considering the system design complexity and practical application, the explicit tuning rules of the PID type-III control loop are derived using magnitude optimum criterion. The parameters are finally designed by stability margin and error suppression ability, which is more appropriate for practical systems. Besides, a pre-filter is also added outside the closed-loop structure when set-point tracking is carried out. Finally, to justify verily the efficiency of the proposed control law, the simulations and experiments in a two-axis pod gimbal system are investigated. The dynamic and stationary performances compared with another type-III control loop method, indicating that the proposed tuning rules have higher positioning accuracy.