Robust universal servomechanisms with improved transient performance

We consider the design of a robust continuous sliding mode controller (CSMC) that achieves asymptotic tracking and disturbance rejection for a class of nonlinear systems. Previous work has shown how to do this by incorporating a linear servo-compensator in the sliding mode design, but the asymptotic tracking is usually achieved at the cost of poor transient performance (compared to ideal SMC). Extending previous ideas from the design of "conditional integrators" for the restricted regulation problem in the presence of constant disturbances, we design the servo-compensator as a conditional one that provides servo-compensation only inside the boundary layer; achieving asymptotic tracking, but with improved transient performance. We give both regional as well as semi-global results for error convergence, and show that the controller can be tuned to recover the performance of an ideal SMC.