Nonlinear generalization of scaled H∞ control:: global robustification against nonlinearly bounded uncertainties

This paper proposes a novel approach to the problem of L-2 disturbance attenuation with global stability for nonlinear uncertain systems by placing great emphasis on seamless integration of linear and nonlinear controllers. This paper develops a new concept of state-dependent scaling adapted to dynamic uncertainties and nonlinear-gain bounded uncertainties that do not necessarily have finite linear-gain, which is a key advance from previous scaling techniques. The proposed formulation of designing global nonlinear controllers is not only a natural extension of linear robust control, but also the approach renders the nonlinear controller identical with the linear control at the equilibrium. This paper particularly focuses on scaled H-infinity control which is widely accepted as a powerful methodology in linear robust control, and extends it nonlinearly. If the nonlinear system belongs to a generalized class of triangular systems allowing for unmodelled dynamics, the effect of the disturbance can be attenuated to an arbitrarily small level with global asymptotic stability by partial-state feedback control. A procedure of designing such controllers is described in the form of recursive selection of state-dependent scaling factors. Copyright (C) 2004 John Wiley Sons, Ltd.