Shape Memory Alloy-Based Spring Bundle Actuator Controlled by Water Temperature

This paper proposes a novel artificial muscle, shape memory alloy (SMA) spring bundle actuator (SSBA), which can be applied to robotic arms while replacing the motor-gear mechanism. Artificial muscles applicable to robotic arms should have characteristics, such as high contraction strain comparable to that of skeletal muscles, high load capacity, fast actuation frequency, and force/position controllability. To improve the low efficiency and slow cooling of the Joule heating, which is widely used for SMA heating, the SMA spring bundle is heated and cooled using hot and cold water. A faucet-like valve was developed to control the temperature of water supplied to the SSBA, and the force generated by the SSBA with respect to the water temperature change was confirmed to be very linear compared to that of the conventional SMA actuators. Owing to this linearity, the force control of the SSBA was easy by using a simple proportional-integral-derivative controller. A bundle weighing 12 g and consisting of 24 SMA springs was able to actuate with a contraction strain of more than 50% and actuating frequency of 1 Hz under a mass condition of 10 kg. Further, it could generate 130 N of force with respect to a temperature change from 28 °C to 82 °C. The flexion-extension motion of an arm of 1-Hz actuating speed was possible by applying two SSBAs antagonistically to an arm of one degree-of-freedom. This proposed artificial muscle offers the possibility of lightweight and safe robotic arms.