This article presents a controller for global joint position tracking control of torque-driven robot manipulators involving dynamic Dahl friction. The control law has been designed using a novel theoretical Hamiltonian inspired framework, which allows global uniform asymptotic stability (GUAS) of the nonautonomous nonlinear closed-loop system. An observer is provided to estimate the nonmeasurable state of internal friction of the Dahl model, preserving the GUAS attribute in the closed-loop system. The performance of the proposed control scheme is illustrated through experiments on a two-degree-of-freedom direct-drive robot arm.

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.

An important subject that arises in the position control of robot manipulators is the local or global asymptotic stability of the closed–loop system equilibrium point, especially when an integral action is added to the control loop. For rigid joint robot manipulators, global nonlinear PID controllers have been introduced few years ago. However, for the case of flexible join robot manipulators, the design and analysis of such nonlinear PID laws is much more challenging. Only local asymptotic stability results have been reported. Thus, a novel global regulator for flexible joint robots is presented in this document. The proposed controller considers a nonlinear integral action and it requires only motor position measurements because an estimator subsystem is used to replace the motor velocity measurements. In other words, a nonlinear proportional-integral-“derivative” (PI“D”)-type controller is introduced. According to the closed–loop system analysis, conditions on the controller gains are established. Thus, the global asymptotic stability is concluded. Finally, experimental results on a two degrees-of-freedom serial flexible joint robot are shown and discussed.