Design of Fully Soft Actuator with Double-Helix Tendon Routing Path for Twisting Motion.

Soft actuators have been widely studied in recent years because of their ability to adapt to diverse environments and safely interact with humans. Their softness broadens their potential range of medical applications since they can provide inherent safety. Among the various motions a soft robot can perform, \"torsion\" can maximize the efficiency of motion in confined spaces like the human abdominal cavity. This paper presents a fully soft actuator with a double-helix tendon routing path for large-angle torsional motions. The double-helix tendon routing enables the actuator to generate large twisting deformations, while also avoiding buckling generally associated with the torque imbalance in small diameter soft cylinder structures. A sequential casting method was developed for cylindrical structures with internal double-helix pathing. A parametric study of the actuator’s twisting angle and the axial contraction with respect to different design parameters was conducted, including the wire tension and path pitch. From the results, when the tendon was pulled with 40 N after the pitch was decreased, the axial contraction of the soft actuator was reduced by half and the torsional angle was doubled up to 600° without buckling.