Force and position tracking: parallel control with stiffness adaptation

Force and position control strategies are aimed at handling the interaction of a robot manipulator with the environment. Among them, the parallel force/position control approach offers good performance in the face of uncertainty on the geometry of the contact surface. This article presents a new parallel control scheme which ensures tracking of end-effector position along the unconstrained directions and tracking of contact force along the constrained direction, in spite of uncertainty on the contact stiffness. The controller is of inverse dynamics type with a force feedforward action. Adaptation to the unknown stiffness coefficient is achieved by resorting to a suitable estimate update law driven by the force error. Experimental results on an industrial robot with open control architecture are reported.