Recently, soft actuators have emerged as crucial components in the design of bio-inspired applications such as: exosuits and wearable robots, providing promising capabilities to replicate human-like movement and improving the functionality of robotic systems. This paper presents the design of a novel artificial muscle utilizing a single wire of shape memory alloy (SMA) wound in a spiral pattern. The actuator consists of two spiral windings of a single continuous SMA wire in two parallel plans (plan A and Plan B). Two groups of roller bearings are arranged in two halves of hexagons shapes for the guidance of the SMA wire in the two spiral shapes, enabling SMA spiral winding, while a third group of roller bearings are movable in middle output stage. This output stage is strategically placed between the two SMA spirals, enabling output motion. When the SMA wire is heated, it contracts, resulting in linear motion of the movable part along the axle (maximum stroke of 25 mm, and maximum load of 7 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 under external loading. 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 key contribution of this work lies in the novel spiral 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. |
