Force-free control of low drag resistance for humanoid robot joint

Abstract A method for the force-free control of humanoid robot joints is presented that meets human–machine cooperation functions of a humanoid robot. To ensure accurate torque control, the current loop of the permanent magnet synchronous motor was initially optimised and its dynamic response improved with a compensating back electromotive force. The motor speed is then estimated by sampling the voltage and current, the values of which are used in the calculation of the joint angle and the gravitational-force compensation. Finally, a method to evaluate the dynamic force compensation is applied that then yields the motor output offset, the total gravitational load, the partial inertia force, and the partial friction moment. Experimental results show that this method reduces the drag torque to less than 1 Nm. This method can be widely applied to a variety of robotic joints.