Robust output regulation of minimum phase nonlinear systems using conditional servocompensators

The design of robust output feedback controllers for output regulation of minimum-phase nonlinear systems with well-defined normal form is considered, with emphasis on their transient performance. Previous work has shown how to design such con trollers by incorporating a linear servocompensator in a continuous sliding mode design, but the asymptotic error regulation is usually achieved at the expense of poor transient performance. In this work, we present an approach to improve the transient performance. The servocompensator is designed as a "conditional" one that provides servocompensation only inside the boundary layer, effectively eliminating the performance degradation. We give both regional as well as semi-global results for error convergence, and show that the output feedback continuous sliding mode controller with conditional servocompensator can be tuned to recover the performance of a state feedback ideal sliding mode control.