PID-Like Robust Control of Non-Minimum Phase Robotic Manipulators

This paper proposes an output-feedback tracking controller for non-minimum phase nonlinear systems with unknown uncertainties and external disturbances, where not all states are measurable, and the zero dynamics are unstable. The approach combines a backstepping-based stabilizing state-feedback law with a cascade extended high-gain observer (EHGO): A fast observer estimates the measured output and its derivatives to generate a virtual output, while a slower observer estimates the unmeasured controlled output, its derivatives, and the lumped disturbance and uncertainty. For relative-degree-two systems, the controller exhibits a PID-like control. This feature is rarely achieved for non-minimum phase systems. Simulations of a wafer-handling robot modeled as a flexible-joint robot demonstrate accurate tracking, rapid disturbance rejection, smooth inputs, and improved robustness compared to existing methods.