ROBUST-CONTROL FOR A NONLINEAR SERVOMECHANISM PROBLEM

This paper deals with the design of output feedback control to achieve asymptotic tracking and disturbance rejection for a class of nonlinea r systems when the exogenous signals are generated by a known linear e xosystem. The system under consideration is single-input single-output , input-output linearizable, minimum phase, and modelled by an input-o utput model of the form of an nth-order differential equation. We assu me that, at steady state, the nonlinearities of the system can only in troduce a finite number of harmonics of the original exosystem modes. This assumption enables us to identify a linear servo-compensator whic h is augmented with the original system. Moreover, we augment a series of m integrators at the input side, where m is the highest derivative of the input, and then represent the augmented system by a state mode l. The augmented system is stabilized via a separation approach in whi ch a robust state feedback controller is designed first to ensure boun dedness of all state variables and tracking error convergence; then, a high gain observer and control saturation are used to recover the asy mptotic properties achieved under state feedback.