Anti-disturbance Control For Nonlinear Systems Based On Disturbance Observer

The problems of anti-disturbance control for nonlinear systems with disturbances have been a hot topic in the past few decades. As an effective disturbance rejection method, disturbance observer based control(DOBC) has been widely applied in many control fields. The basic idea of that is to design an observer to estimate external disturbances, and the disturbances can be compensated in the feed-forward channels. Recently, some research results have been presented about DOBC applied to nonlinear systems. In this paper, a series of anti-disturbance control approaches based on disturbance observer are proposed for some classes of nonlinear systems with different disturbances. The main contents are stated as follows:1. A disturbance rejection approach based on disturbance observer is proposed for a class of nonlinear systems subject to mismatched disturbances. The mismatched disturbances are described by exogenous systems and satisfy partially-known information. The disturbance observer is designed to estimate the mismatched disturbances. By integrating disturbance observer with back-stepping method, the disturbance observer plus back-stepping(DOPBS) controller can be constructed to reject the disturbances. It is proved that all the signals in the closed-loop system are asymptotically stable. Finally, simulation examples are given to verify the correctness and effectiveness of the proposed scheme compared with existing methods.2. Anti-disturbance control problem is studied for a class of nonlinear systems with bounded disturbances. The system disturbances satisfy derivative-bounded condition, which further extends the range of disturbances description. In order to estimate the bounded disturbances, a nonlinear disturbance observer is designed. By introducing disturbance estimations into virtual control laws in back-stepping method, the disturbance observer plus back-stepping(DOPBS) control strategy is proposed to reject the disturbances. And the semi-global uniformly ultimate bounded(SGUUB) stability for the composite closed-loop system can be achieved. Finally, the numerical simulation result shows that the correctness and effectiveness of the proposed approach by the comparison with existing methods.3. A class of nonlinear systems with unknown nonlinear dynamics and bounded disturbances are considered. The range of systems description is further extended. The unknown nonlinear functions are approximated by fuzzy logic systems. The adaptive disturbance observer is designed to estimate the derivative-bounded disturbances. Based on disturbance estimation, the adaptive fuzzy controller is constructed to reject disturbances via back-stepping method. The adaptive fuzzy control scheme based on disturbance observer is proposed. With such a method, the composite closed-loop system is semi-global uniformly ultimate bounded(SGUUB). Finally, the simulation example is given to demonstrate the correctness and effectiveness of the proposed method.