A Multimodal Twisted-Winching String Actuator with a Passive Automatic Transmission: Design and Validation

This paper presents a novel single-motor actuator that integrates a twisted string actuator (TSA) and a winch mechanism with passive automatic clutches. This approach enables seamless transitions from large displacement winching to high force twisting without additional motors. The actuator leverages the characteristics of its epicylic-style gear transmission system to passively transition from high displacement winching to high force twisting when the load on the string exceeds a force threshold. A slip clutch integrated with a unidirectional bearing ensures complete untwist and unwinch cycles, returning the system to its original state. A mechanical dial with a high-reduction gearbox permanently encodes the system's twisted state, ensuring precise and repeatable untwisting cycles. Mathematical models characterize the clutch torque thresholds, force output, and string kinematics. Experimental validation demonstrates the actuator's ability to achieve efficient, reliable transitions and force amplification. This design eliminates the need for additional motors and simplifies control. It offers a compact, efficient solution for tendon-driven robotic applications, specifically those that require large initial displacements followed by high force outputs.