ROBUST ADAPTIVE OUTPUT-FEEDBACK CONTROL OF NONLINEAR-SYSTEMS WITHOUT PERSISTENCE OF EXCITATION

We consider a recent method for adaptive output feedback control of no nlinear systems. The method deals with a single-input-single-output mi nimum phase nonlinear system represented by an nth-order differential equation. A Lyapunov-based design that employs parameter projection, c ontrol saturation, and a high-gain observer has been shown to achieve semiglobal tracking. One drawback of that recent result is that tracki ng error convergence is shown under a persistence of excitation condit ion, which is uncommon in traditional adaptive control results where p ersistence of excitation is needed to show parameter convergence but n ot tracking error convergence. In this paper, we prove tracking error convergence without persistence of excitation. We then proceed to prov ide two other extensions. First, we show that the adaptive controller is robust with respect to sufficiently small bounded disturbance. Seco nd, by adding a robustifying control component, we show that the contr oller is robust for a wide class of, not-necessarily-small, bounded di sturbance, provided an upper bound on the disturbance is known. (C) 19 97 Elsevier Science Ltd.