Fast stable contact transitions with a stiff manipulator using force and vision feedback

Ideal manipulator end-effector transitions from noncontact to contact states should be fast and stable with minimal impact forces and without bounce. These specifications, however are difficult to simultaneously achieve, especially for the most common manipulator force control configuration employing a wrist force sensor and a stiff manipulator position loop. In this paper the authors present a control strategy that uses high bandwidth vision feedback (30 Hz) in addition to force feedback (100 Hz) for contact transient control. A nonlinear control strategy is proposed that considers force and vision feedback simultaneously and then switches to pure force control when the camera-lens system becomes unable to accurately resolve the location of the end-effector relative to the surface to be contacted. Experimental results are presented which demonstrate that a stiff manipulator can quickly contact a stiff surface stably without bounce. Results also show that the magnitude of the impact force spike is directly related to the accuracy with which contact surfaces are visually observed