ANALYSIS AND SYNTHESIS OF DISTURBANCE OBSERVER AS A TOOL FOR NONLINEAR ROBUST CONTROL

Linear disturbance observer (DOB) approach has been successfully employed as a tool for robust control and disturbance rejection in the industry. While it is widely used, little has been done to develop the underlying theory. Motivated by this, we have analyzed the linear DOB in the state-space framework, and have derived a necessary and sufficient condition for robust stability. The lessons from it then lead to a new DOB structure for nonlinear systems in this paper. The proposed nonlinear DOB is a semi-global output-feedback inner-loop controller, which works in harmony with a (pre-designed) nominal outer-loop controller of any type. In particular, we prove that the proposed nonlinear DOB recovers not only the steady-state performance but also the transient performance of the nominal plant under plant uncertainties and input disturbances.