In this paper, the tracking control of nonlinear servo motor system subject to unknown disturbance is investigated. A disturbance observer and two control strategies are put forward to guarantee the output of the system can accurately track a given reference value, which can even be dynamic. Firstly, a nonlinear servo system model with unknown disturbance driven by a single motor is established. Then, a dual-sliding mode surface disturbance observer is designed, and the convergence of the observation error in finite time is rigorously confirmed. In addition, the PID tracking strategy is designed, as well as the stability of closed-loop system is proven by Lyapunov theory. Furthermore, a new tracking controller based on the output of disturbance observer is proposed, where a PID type sliding mode surface is designed. To illustrate the accessibility of the sliding-mode surface, an appropriate Lyapunov function is chosen. Finally, the effectiveness of the dual sliding mode observer and the two control strategies is verified by simulation results.
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