Integrated adaptive-robust control of robot manipulators with joint stick-slip friction

Abstract

An integrated adaptive-robust approach along with a smooth adaptive robust friction compensation strategy are applied to tracking control of robot manipulators with joint stick-slip friction. The designed controller takes advantage of both adaptive and robust approaches: namely, the ability to learn like adaptive controllers and the ability to reject disturbance and to handle various uncertainties including stick-slip friction, like robust controllers. The uncertainties due to unknown robot link parameters, unknown viscous friction coefficient, and unknown maximum static friction, which are linear in parameters, are compensated by the integrated adaptive-robust control method. On the other hand, the exponential friction (used to model the Stribeck effect: the downward bend of friction torque at low velocities), which are nonlinear in parameters, and external disturbances, are compensated by a robust compensator with a self-learning upper bounding function when no a priori knowledge on the exponential friction and disturbances are available. The employed robust compensators produce smooth control action, and avoid motion intermittency, which are commonly associated with stick-slip friction. The proposed controller guarantees global asymptotic stability of the closed-loop system, as proved by Lyapunov's direct method.