Dynamic output feedback control of minimum-phase nonlinear systems

The synthesis of dynamic output feedback controllers for minimum-phase nonlinear systems is considered. The problem is addressed first for open-loop stable and then for general nonlinear systems, leading to dynamic one- and two-degree-of-freedom controllers, respectively. The derivation of the controllers is based on state-space considerations, essentially involving a combination of state feedback and state observers. An input/output interpretation of the control structures illustrates the importance of alternative state-space realizations of the inverse system for the controller implementation. The issue of closed-loop stability is discussed and precise internal stability conditions are derived.