Switched position-force tracking control of a manipulator interacting with a stiff environment

This work proposes a control law for a manipulator with the aim of realizing desired time-varying motion/force profiles in the presence of a stiff environment. In many cases, the interaction with the environment affects only one degree of freedom of the end-effector of the manipulator. Therefore, the focus is on this contact degree of freedom, and a switching position-force controller is proposed to perform the hybrid position-force tracking task. Sufficient conditions are presented to guarantee input-to-state stability of the switching closed-loop system with respect to perturbations related to the time-varying desired motion-force profile. The switching occurs when the manipulator makes or breaks contact with the environment. The analysis shows that to guarantee closed-loop stability while tracking arbitrary time-varying motion-force profiles, the controller should implement a considerable (and often unrealistic) amount of damping, resulting in inferior tracking performance. By redesigning the manipulator with a compliant wrist and employing the designed switching control strategy, stable tracking of a motion-force reference trajectory can be achieved and bouncing of the manipulator while making contact with the stiff environment can be avoided.