Impedance control with new inner torque loop for compliant joints and its performance analysis

For the purpose of enhancing compliance of robot joint, an impedance control strategy with new inner torque controller is proposed. Apparent inertia of the motor can be scaled down thanks to this new type of torque controller. Effects of different position (motor-side position θ and link-side position q) feedback strategies are explored through theoretical deviation, simulation and static stiffness comparison. Explicit expressions of static stiffness under both position feedback strategies are obtained with validity verified by Simulink tool. Compared with θ feedback strategy, static stiffness can exceed spring stiffness under q feedback strategy with high desired stiffness, which means wider variation range of static stiffness and this benefits robot applications that require wider variation range of impedance, such as rehabilitation robots and collaborative robots. Finally, several experiments with the compliant joint prototype verify the developed controllers and show the efficiency of the proposed control approach in terms of compliant behavior.