On the design of robust servomechanisms for minimum phase nonlinear systems

We consider a single-input single-output nonlinear system which has a well defined normal form with asymptotically stable zero dynamics. We study the design of a robust output feedback controller, that incorporates a linear servo compensator, to achieve asymptotic tracking and disturbance rejection. We determine the least amount of precise information about the plant that is needed to design the controller. Such design will be robust to plant parameters and uncertainties which are not used in the design calculations. We also study the robustness of the design to errors in determining the linear servo compensator. Such errors could arise from approximating a general continuous nonlinear function by a polynomial one, or from errors in determining the frequencies of the exogenous signals. We give regional as well as semiglobal results