Harnessing Both Tensile and Torsional Forces of Twisted String Actuators for Three-Dimensional Helical Deformation

A thorough exploration of actuator capabilities can lead to the discovery of new deformation modes. Traditional twisted-string actuator (TSA)-based systems typically utilize only the tensile force generated by string twisting while neglecting the torsional force. As a result, a single actuator often produces only planar deformations such as bending. In this study, we propose the twisted string helical hose (TSHH) mechanism, which innovatively harnesses both tensile and torsional forces to achieve three-dimensional helical deformation through a single input. The TSHH only consists of a motor, string, and hose, all of which are standard commercial components, making the system highly reproducible. We investigated the deformation principle and developed a kinematic model to support parameter optimization. Based on the optimized parameters, we designed a soft gripper capable of gently grasping objects of various shapes and sizes. This work not only demonstrates the promising potential of TSHH in soft robotic applications but also inspires TSA-based systems toward greater design potential.