Twisted String Actuation Module for Compact Robotic Finger With Extended Stroke, Reduced Hysteresis, and Bidirectional Operation

Twisted String Actuators (TSAs) are promising alternatives to conventional gear-based transmissions due to their high reduction ratios and compact form factors. However, practical limitations such as nonlinear hysteresis, limited stroke, and inherently unidirectional motion hinder their deployment in robotic systems. In this work, we propose a novel bidirectional TSA mechanism that addresses all three limitations simultaneously through an antagonistic configuration, asymmetric axis shift (AAS), and pre-tension tuning. This mechanism enables reliable bidirectional actuation by compensating for asymmetric contraction-extension behavior, suppresses hysteresis via adaptive tensioning, and extends the effective stroke. We implement the proposed design in a continuum finger module and derive a compact kinematic model for control. Extensive experiments validate the effectiveness of the approach, demonstrating the attenuation of the hysteresis, accurate bidirectional bending control across a wide range (±180°), and the feasibility of integration into multi-finger grippers for dexterous manipulation. The results suggest that the proposed actuator design serves as a practical and scalable solution for compact robotic systems requiring precise and reversible motion.