Extended Disturbance Observer based Double Integral Sliding Mode Control for Nonlinear Thrust Active Magnetic Bearing Systems

This paper develops a novel sliding mode control method for nonlinear thrust active magnetic bearing systems by proposing an original nonlinear extended disturbance observer and a modified double integral sliding surface. Without requiring the bound of disturbance to be known, the created disturbance observer can estimate both vanished and nonvanished disturbance. In order to increase the robustness and tracking precision of the system, a modified double integral sliding surface is presented and an extended disturbance observer based double integral sliding mode controller is constructed subsequently. On the basis of Lyapunov theory, it is proven that the norm of the estimation error is ultimately bounded and the sliding surface is reachable. The bound of disturbance estimation error can be lowered by appropriate choice of disturbance observer parameters and the robust performance of the system can be adjusted by selecting suitable controller parameters or nonlinear functions. Simulation on a nonlinear thrust active magnetic bearing system verifies the effect of the presented method under three kinds of tracking commands.