Decoupled error dynamics design for discrete-time sliding mode control in industrial servo systems under control input saturation and disturbance

This paper presents a design framework for the discrete-time Sliding mode control with Decoupled disturbance compensator and Auxiliary state (SDA) method in industrial servo applications. In particular, the discrete-time SDA method is formulated to provide an excellent tracking performance even in the presence of control input saturation and disturbance. Moreover, it preserves the separation principle of the sliding mode dynamics and the disturbance estimation error dynamics of the original discrete-time Sliding Mode Control (SMC) with Decoupled Disturbance Compensator (DDC) method. However, it can be problematic due to the unpredictable transients in the error states under control input saturation. This paper investigates the error dynamics of discrete-time SDA after a transition from a saturated region to an unsaturated region, which is called the decoupled error dynamics. The decoupled error dynamics can be designed by shaping a “hidden” sliding manifold which is activated only after the transition. Based on the analysis, a systematical design methodology for the decoupled error dynamics is proposed, which prevents the recurrence of the control input saturation and provides fast convergence of the error states. The effectiveness of the proposed design methodology is shown experimentally using an industrial linear motor system.