Composite nonlinear control for discrete-time positioning servo systems

Abstract

A discrete-time composite nonlinear control scheme is proposed to achieve fast and accurate set-point tracking for servo systems subject to disturbances and actuator saturation. The control scheme incorporated a mechnism of disturbance estimation cum compensation into the framework of composite nonlinear feedback (CNF) control. The CNF control law consisted of a linear feedback part for achieving fast response and a nonlinear feedback part for suppressing the overshoot, so as to improve the transient performance. A reduced-order observer was adopted to estimate the unmeasurable state variables and unknown constant disturbance, and then used for compensation to remove the steady state error. Closed-loop stability with the proposed control scheme was analyzed theoretically. The control scheme was applied to a servo system modeled by a double integrator. Simulations were carried out, and a comparison was made with a previous CNF scheme, which resorted to integral control for rejecting disturbance. The results confirm that the proposed control scheme has a better performance robustness in set-point tracking tasks.