ROBUST SERVOMECHANISM OUTPUT-FEEDBACK CONTROLLERS FOR FEEDBACK LINEARIZABLE SYSTEMS

We consider a single-input-single-output nonlinear system which has a uniform relative degree equal to the dimension of the state vector. Th e system can be transformed into a normal form with no zero dynamics. We allow the system's equation to depend on bounded uncertain paramete rs which do not change the relative degree. Disturbances are assumed t o satisfy a strict-feedback condition which allows us to use a time-va rying, disturbance-dependent transformation to transform the system in to an error space where disturbances and uncertainties satisfy the mat ching condition. The nonlinear functions in the error space are used t o choose an internal model which is augmented with the system, and a r obust state feedback control is designed to drive the error to a posit ively invariant set that contains the origin. We then show the existen ce of a zero-error manifold inside this set which attracts ail traject ories inside the set. To implement this control using output feedback, we saturate the state feedback control outside a compact set of inter est and estimate the state using a high-gain observer. The output feed back controller recovers the robustness and asymptotic tracking proper ties of the state feedback controller.