Design of a nonlinear disturbance observer

This paper presents a new disturbance observer based on the variable structure system theory for minimum-phase (with respect to the relationship between the disturbance and output) dynamical systems with arbitrary relative degrees. The model uncertainties and the nonlinear parts of the system are merged into the disturbance term and are regarded as a part of the disturbances. The upper and lower bounds of the disturbance are assumed to be known as a priori information. Simulation results are presented to show the robustness and effectiveness of the new disturbance observer. Experiment results show the practicality of the new observer.