Novel design and fabrication of a linear actuator based on shape memory alloy wire winding in hexagonal architecture

Bio-inspired actuators play a crucial role in the design of wearable robots and exosuits, offering promising capabilities for achieving human-like movement and enhancing the functionality of robotic devices. This paper presents the development of a novel linear actuator utilizing a single wire of shape memory alloy (SMA) wound in a hexagonal pattern. The actuator features three segments of hexagonal bases aligned along a central axle and incorporates an antagonistic spring. Two of the hexagonal bases are fixed, enabling SMA winding, while the remaining base is movable, enabling output motion. Six pulleys are strategically positioned on the faces of these hexagonal blocks to wind the SMA wire between the fixed and movable parts. When heated, the SMA wire contracts, resulting in linear motion of the movable part along the axle (maximum stroke of 14 mm, and maximum load of 10 kgf). The displacement and force exerted by the actuator are adapted by adjusting the length of the SMA wire and the configuration of the output pulleys. Upon cooling, the movable part returns to its initial position through the restoring force of the antagonistic spring. Moreover, system identification and PID control techniques are employed to enhance the actuator’s control performance. Experimental tests conducted in both open-loop and closed-loop controls validate the actuator’s ability to successfully follow desired trajectories. The importance of this work lies in the novel hexagonal architecture, which offers a vital and practical solution to deal with the limitations related to size and strain of the SMA wire while leveraging the unique properties of SMA technology.