Conductive polymer composites-enable self-sensing twisted string actuators

Twisted string actuators (TSAs) are innovative linear soft actuators that can mimic natural muscle contraction. However, obtaining feedback on their stroke requires cumbersome external sensors that compromise their compliance. We addressed this by developing self-sensing strings using conductive polymer composites (CPCs). The CPC strings exhibit electrical resistance changes in response to strain, enabling stroke sensing by the strings themselves. We fabricated CPCs strings with varying concentrations of multi-walled carbon nanotubes (MWCNTs). Characterization revealed excellent conductivity, tensile strength exceeding 30 N, tunable strain sensitivity via MWCNTs concentration variation, as well as stable and repeatable strain-dependent resistance changes. Optimization via a high-strength core fiber enabled the CPCs strings to withstand over 6kg loading with enhanced stroke and minimal resistance variation. We achieved at least a 97.8% linear correlation between resistance change rate and contraction rate in twisting experiments. This research provides a self-contained stroke sensor compatible with high-load soft TSAs, overcoming limitations of external sensors. The simple yet effective sensing mechanism could spur adoption of TSA in robotics.