A possible approach for achieving robust tracking for nonlinear systems

The problem of achieving robust output regulation for nonlinear differential equations is discussed. Output regulation means that the closed-loop system is locally exponentially stable about an equilibrium, and that its output asymptotically tracks any reference signal produced by a fixed external generator. `Robust´ means that the tracking property continues to hold in the occurrence of perturbations in the plant parameters, as long as these perturbations are such that the local stability property is not lost. The author discusses the problems related to the extension to the nonlinear case of the robust regulator determined by B.A. Francis (1977) for the linear case. A possible path for carrying out this extension is also indicated