Discrete-time Sliding Mode Control with Second-Order Decoupled Disturbance Compensator to Improve the Stability and Robustness

This paper presents a design method for the discrete-time sliding mode control (DSMC) with a second-order decoupled disturbance compensator (DDC). A DSMC with DDC has been widely used for industrial servo systems. The method has asymptotic convergence properties in the systems with slowly-varying disturbance and parametric uncertainty. The DDC can be modeled as a disturbance observer (DOB) with a first-order Q-filter structure. However, when the difference between the nominal plant model and the actual plant is large, this first-order Q-filter does not sufficiently minimize the effects of model uncertainties, which hinder applying a high DDC gain. Hence, high gains for high performance cannot be used. In this paper, a DSMC with a second-order DDC is developed to improve the system stability and robustness. The proposed method minimizes the effects of model uncertainties and increases the stability margin. As a result, high gains can be used for high performance. The developed method is applied to industrial servo systems and experimental results show enhanced stability performance, when compared to the DSMC with first-order DDC.