Robust tracking control of uncertain nonlinear systems with unknown control directions

A global robust tracking control design procedure is proposed for a class o f uncertain nonlinear systems. The key point is that the signs of multiplic ative uncertainties, the so-called control directions, are not assumed to b e known a priori. The class of systems can be of arbitrary dynamic order an d the unmatched additive uncertainties need not satisfy the global Lipschit z condition. It is proved that under the proposed control, all closed-loop states are bounded and the tracking error converges to any prescribed small neighborhood of the origin. The results of this paper enlarge the class of uncertain nonlinear systems for which global robust tracking control can b e designed. (C) 2001 Elsevier Science B.V. All rights reserved.