A fuzzy PID-controlled SMA actuator for a two-DOF joint

Shape memory alloy (SMA) actuator is a potential advanced component for servo-systems of aerospace vehicles and aircraft. This paper presents a joint with two degrees of freedom (DOF) and a mobility range close to ±60° when driven by SMA triple wires. The fuzzy proportional-integral-derivative (PID)-controlled actuator drive was designed using antagonistic SMA triple wires, and the resistance feedback signal made a closed loop. Experiments showed that, with the driving responding frequency increasing, the overstress became harder to be avoided at the position under the maximum friction force. Furthermore, the hysteresis gap between the heating and cooling paths of the strain-to-resistance curve expanded under this condition. A fuzzy logic control was considered as a solution, and the curves of the wires were then modeled by fitting polynomials so that the measured resistance was used directly to determine the control signal. Accurate control was demonstrated through the step response, and the experimental results showed that under the fuzzy PID-control program, the mean absolute error (MAE) of the rotation angle was about 3.147°. In addition, the investigation of the external interference to the system proved the controllable maximum output.