Force Tracking Impedance Control with Variable Target Stiffness

In this paper, a novel force tracking impedance control strategy is presented in which target stiffness is varied on-line to regulate the desired contact force without any knowledge of the environment. Humans can control contact force by adjusting their arm stiffness. The contact force can be either increased by making one’s arm stiffer or decreased by reducing the arm stiffness. Furthermore, humans can keep the force tracking error within a certain range without any knowledge of environmental parameters as long as how much force they exert on the object is known to them. Analogously, the proposed control scheme achieves a contact force regulation control by adjusting the target stiffness of the impedance control. The new force tracking impedance control scheme does not require estimating environment stiffness or locations since the controller is adapted only based on the previous force tracking error between the desired and real contact force. Stability of the proposed scheme is discussed with a quadratic Lyapunov function. Extensive simulation studies with a 7 degree of freedom (DoF) robot manipulator using full arm dynamics are conducted to demonstrate the validity of the proposed scheme.