Asymptotic Regulation of Minimum Phase Nonlinear Systems Using Output Feedback

We consider a single-input single-output (SISO) nonlinear system which has a well defined normal form with asymptotically stable zero dynamics. We allow the system´s equation to depend on bounded uncertain parameters which do not change the relative degree. Our goal is to design an output feedback controller which regulates the output to a constant reference in the presence of constant unkown input disturbances. The disturbance vector fields satisfy geometric conditions which ensure that the system is transformable into the so called disturbance-strict-feedback form. The integral of the regulation error is augmented to the system equation and a robust output feedback controller is designed to bring the state of the closed-loop system to a positively invariant set. Once inside this set, the trajectories approach a unique equilibrium point at which the regulation error is zero. We give regional as well as semi-global results.