Enhanced Modeling of Twisted String Actuators with Low-Torque Motors Accounting for Strings’ Friction and Opposing Torque

Twisted string actuators (TSAs) have shown strong promise in emerging applications, such as soft robotics and assistive robotics. To construct compact and lightweight TSAs, it is often inevitable to use low-torque and low-speed motors. An accurate TSA model can facilitate the appropriate design of motor-string components and enable TSA’s reliable operation. However, existing models often neglect the twisted strings’ friction and the exerted opposing torque on the motor, which would result in significant discrepancies in predicting the dynamic behaviors of TSAs with low-torque and low-speed motors. This work presents an enhanced model to accurately capture TSA’s dynamics by accounting for the aforementioned phenomena. The theory of torsional closed-wrapped helical springs is used to capture the friction between the twisted strings. The total elastic potential energy of the strings considering string curvature is used to derive the total opposing torque exerted by the twisted strings. The proposed model is experimentally identified, validated, and compared with an existing TSA model to confirm its superior accuracy.