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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. 

While numerous studies have been conducted, developing a compliant robotic gripper capable of replicating human hand grasping and manipulation capabilities is still challenging. This paper presents the design, fabrication, and preliminary testing of an anthropomorphic soft robotic gripper driven by twisted string actuators (TSAs). Termed as STAR–2, it is a second generation TSA-driven soft gripper from the Smart Robotics Laboratory at the University of Nevada, Reno. The novel design facilitated a monolithic structure comprising of a 3-degrees-of-freedom (DOF) thumb and four fingers each with 2-DOFs. On account of using tendon-based actuation and the large footprint required for the thumb, the design employed meticulously planned tendon routing within the monolithic structure. Preliminary results showed STAR–2’s enhanced ability to demonstrate grasp taxonomies and dexterity over STAR–1.