Nonlinear disturbance observer based control for systems with arbitrary disturbance relative degree

The existing nonlinear disturbance observer based control (NDOBC) is only applicable for the systems whose disturbance relative degree is higher than or equal to the input relative degree. The disturbance attenuation problem of nonlinear systems with arbitrary disturbance relative degree is presented in this paper via a novel NDOBC approach. A systematic method is proposed to design the disturbance compensation gain. It is shown that the disturbance can be eliminated from the output channel in steady-state with the proposed method. The input-to-state stability of the closed-loop system is also investigated rigorously by regarding the disturbance as the input of the augmented system. Simulation example of a missile demonstrates that the proposed method provides an effective way in rejecting disturbances with arbitrary disturbance relative degree.