Robust nonlinear integral control

It is well known from linear systems theory that an integral control law is needed for asymptotic set-point regulation under parameter perturbations. The note presents a similar result fur a class of nonlinear systems in the presence of an unknown equilibrium due to uncertain nonlinearities and dynamic uncertainties. Both partial state and output feedback cases are considered. Sufficient small-gain type conditions are identified for existence of linear and nonlinear control laws. A procedure for robust nonlinear integral controller design is presented and illustrated via a practical example of fan speed control.