NONLINEAR OUTPUT-FEEDBACK TRACKING USING HIGH-GAIN OBSERVER AND VARIABLE-STRUCTURE CONTROL

We consider a single-input-single-output nonlinear system which can be represented by an input-output model. The system, which can be transf ormed into the normal form, is required to be minimum phase. The model contains unknown bounded disturbances. We assume that the reference s ignal and its derivatives are bounded. A high-gain observer is used to estimate derivatives of the tracking error while rejecting the effect of the disturbances. We design a globally bounded output-feedback var iable structure controller that ensures tracking of the reference sign al in the presence of unknown time-varying disturbances and modeling e rrors. We give regional as well as semiglobal results. We do not requi re exponential stability of the zero dynamics nor global growth condit ions. (C) 1997 Elsevier Science Ltd.