Impedance Compensation of Flexible Joint Actuator for Ideal Force Mode Control

To realize an ideal force control of robots interacting with humans, a very precise actuation system with zero impedance is desired. A flexible joint actuator may serve for this purpose. This paper presents the design of control algorithms to compensate for the impedance in the flexible joint actuator. To generate the torque as desired, a spring is installed between a motor and a human joint, and the motor is controlled to produce the proper spring deflection. When the desired torque is zero, the motor must follow human joint motion, which requires that the friction and inertia of the motor are compensated. Mechanical properties of a human body are not fixed, while they represent the load to the flexible joint actuator. The disturbance observer method is applied to make the flexible joint actuator precisely generate the desired torque under such time-varying conditions. Based on the nominal model preserved by the disturbance observer, feedback and feedforward controllers are optimally designed for the desired performance: i.e. the flexible joint actuator 1) exhibits very low impedance and 2) generates the desired torque precisely during interacting with a human. The effectiveness of the proposed design is verified by experiments.