Robust adaptive motion and force control of robot manipulators in unknown stiffness environments

Robust motion and force tracking control of robot manipulators in the presence of parametric uncertainties, in both the robot dynamics and the contact surface or external disturbances, is considered. Several approaches are adopted to compare their advantages. In the absence of time-varying disturbances, a continuous adaptive motion and force tracking control algorithm is developed to deal with the unknown robot parameters and the unknown surface parameters, such as the stiffness and friction coefficient. No prior information on the parameter uncertainties is assumed. Exact motion and force tracking control is ensured without any persistent excitation being satisfied. A discontinuous term is introduced in the control law to guarantee stability and tracking performance under the presence of time-varying external disturbances. Finally, under an assumption that the bounds on the modelling error are usually known, the control law is smoothed with a modified adaptation law to avoid a possible chattering problem induced by the discontinuous control term and, at the same time, to achieve robustness to unmodelled dynamics