I. Introduction
In recent years, there has been a growing interest in the design of feedback-control laws for mechanical systems subjected to nonholonomic constraints. Due to Brockett's necessary condition for asymptotic feedback stabilization [1], a nonholonomic system cannot be asymptotically stabilized to a equilibrium by any smooth or continuous pure-state feedback law. To overcome this limitation, several approaches have been proposed in last decade (see [18], [20], and the references therein). Configuration of a wheeled mobile robot.