An HSA Cylinder Platform for Three DoF Prosthetic Wrist Motion

Handed Shearing Auxetic (HSA) structures have recently attracted attention as motor driven compliant elements for soft robotic platforms. Their deformation couples rotation, axial displacement, and structural compliance, which makes them useful not only as single actuators, but also as building blocks for multi DoF mechanisms. This paper presents TriHSA, a compact HSA cylinder platform developed for prosthetic wrist motion. The target is to generate the three main rotational motions related to the wrist and forearm, namely flexion and extension (F/E), radial and ulnar deviation (R/U), and pronation and supination (P/S). Instead of treating the HSA unit only as an isolated actuator, the study focuses on how the number, position, and driving combination of HSA cylinders change the motion of the output platform. A four cylinder arrangement is first evaluated on the prototype. Although it can provide bending motion, its twisting output is strongly constrained by the interaction among the cylinders, and a vertical shortening mode is also introduced. A three cylinder arrangement is then selected as the final layout, since it removes the redundant cylinder and keeps the useful wrist related motions. The prototype is experimentally characterized using an IMU based sensing setup. The measured range of motion is approximately ±28° in both F/E and R/U, and approximately ±19° in P/S. Dart Thrower's Movement is also tested to examine the coupled motion capability. The results show that the proposed HSA platform can provide a feasible compliant architecture for multi DoF prosthetic wrist motion.