Impedance control of antagonistically driven SMA springs

This paper presents impedance control of an antagonistic joint driven by SMA spring actuators. Based on the constitutive model of the SMA spring, the controller is designed to control the antagonistic joint angle as well as to achieve the desired joint damping and stiffness. The impedance control frame is a double loop, in which from the desired torque of the outer impedance control loop, an inversion force control law is proposed in the inner loop to control each actuator's force. The stability analysis is carried out using the ultimate bounded stability theory. Experiments are conducted to verify the effectiveness of the system. It is shown that the desired antagonistic joint angle can be obtained with small errors for both sinusoidal waveform tracking and set-point regulation, where the maximum position error is 0.8 deg and root-mean-squared error is 0.3 deg (with the frequency of 0.15Hz) during tracking control, and the maximum position error during set-point regulation is 0.04 deg at the steady-state.