A twisted string actuator using a shape memory alloy for dual actuation

Abstract

A Twisted String Actuator (TSA) is a type of linear actuator that converts rotational motion into linear motion. Due to its lightweight structure and high energy efficiency, TSA has been extensively studied in various fields. In particular, the speed-mode TSA offers the advantage of increasing both actuation range and speed through a simple mechanism. However, a critical drawback of the speed-mode TSA is its relatively low force transmission efficiency. To overcome this limitation, this study proposes a novel actuator concept: the SMA-TSA, which replaces the traditional TSA string with Shape Memory Alloy (SMA). By incorporating SMA, which can generate actuation independently, the SMA-TSA enables dual actuation. This provides an additional driving force alongside the conventional twisted-string mechanism. We designed the SMA-TSA to facilitate dual actuation and experimentally validated its performance. The results demonstrated that the SMA-TSA achieved an additional 14.42 mm of contraction and force output of 95.96 N. Furthermore, when applied to a robotic finger, the SMA-TSA increased the finger tip force by approximately 6.38 N, suggesting its potential for future applications in robotics. © 2025