Global Regulation of Flexible Joint Robots With Input Saturation by Nonlinear I-PID-Type Control

This brief addresses the global regulation of torque-driven flexible joint robots with input constraints. It is reported a nonlinear control scheme with bounded actions that guarantees global asymptotic stability despite input saturation, matched and unmatched disturbances, and parametric uncertainties. The control system has a double loop in a cascade configuration, where the outer loop has an integral (I) action driven by the joint deflection error. In addition, the inner loop has a nonlinear proportional-integral-derivative (PID-type) structure. Hence, an I-PID-type controller is obtained. The design methodology is based on a linear change of coordinates of the joint deflection and motor errors that allows the conclusion of global asymptotic stability via Lyapunov theory and the Barbashin–Krasovskii theorem. Sufficient conditions are explicitly stated and given in the form of matrix inequalities. Real-time experiments on a two-degrees-of-freedom flexible joint manipulator confirm the viability of the proposed controller, which exhibits better performance than the other two control algorithms.