Modular shape memory alloy linear actuator driven elbow exoskeleton with geometry based passive cooling design and performance assessment

Abstract

Abstract Shape Memory Alloy (SMA) actuators offer high power-to-weight ratios and a compact form, making them attractive for wearable assistive robotics. However, their slow thermal response and limited modularity hinder repeated-use assistive tasks. This letter introduces a modular SMA linear actuator that combines geometric optimization with a flexible, reconfigurable design. A hexagonal wire arrangement increases inter-wire spacing by 70% over conventional linear bundles, alleviating thermal boundary-layer effects. Numerical simulation predicts a 35–50% reduction in cooling time constants, a trend experimentally validated at a representative inter-wire pitch, indicating improved thermal cycle efficiency in the demonstrated regime. The modular architecture supports parallel and series configuration, by which output force and displacement are designed to scale independently, as shown here at the single-module level. A dual-channel current-control method ensures thermal uniformity, and a PID algorithm sampling at 33 Hz provides precise, thermally balanced closed-loop control on the benchtop. In a cable-driven elbow exoskeleton, surface electromyography from 17 subjects showed neuromuscular offloading of 13.2–44.7% across movement phases and loads. The nearly load-invariant assistance is hypothesized to be congruous with progressive rehabilitation protocols, pending clinical validation. These results indicate that the proposed actuator balances thermal efficiency with modularity for quasi-static and repetitive wearable assistive applications.