Robust fault detection in uncertain nonlinear systems via a second order sliding mode observer

A second order sliding mode observer-based robust fault detection in uncertain nonlinear systems is discussed. First of all, a second order sliding mode observer is presented. The reason why the second order sliding mode observer is used for fault detection is that the second-order S(t) (sliding surface) dynamics can sharply filter unwanted high frequencies due to unmodeled dynamics. The sliding condition is first derived such that the observer switching gain can be selected. The stability of the reduced sliding mode observer is then proved by assuming that the considered nonlinear system has a single output and two outputs, respectively. An example is employed to show that the proposed sliding mode observer can work very effectively.