Disturbance observer design for a class of nonlinear system with application to active suspension system control

A systematic disturbance observer for a class of nonlinear affine systems subject to bounded exogenous input is designed. The main idea expounded upon is to design the “control law” for the observer dynamics such that the tracking error between the observer states and the original system states lies within a bounded set. It is then shown that, provided that the system satisfies some sufficient conditions, then the observer control authority will track the unknown exogenous input with the same order of magnitude of the error. In other words, the observer dynamics is designed to recreate the measured behavior of the original system to some order of accuracy. If this happens, then, under certain sufficient conditions, the control authority that reproduced this measured behavior will track the unknown disturbance input to the same order of accuracy. While the concepts developed in this paper can be used for a class of nonlinear system, the main application considered is the active suspension system control. The motivation is to eventually use the observed disturbance as a preview information for a cascaded suspension system.