Disturbance rejection for nonlinear systems with mismatched disturbances based on disturbance observer

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, which enter the system in the different channels with the control input. The disturbance observer is designed to estimate the mismatched disturbances, which can be introduced separately from the controller design. By integrating disturbance observer with back-stepping method, the disturbance observer plus back-stepping (DOPBS) controller can be constructed to reject the mismatched disturbances. And the asymptotically stability for the closed-loop system can be achieved. Finally, simulation examples are given to demonstrate the feasibility and effectiveness of the proposed scheme compared with existing methods.