1 research outputs found
Overtwisting and Coiling Highly Enhances Strain Generation of Twisted String Actuators
Twisted string actuators (TSAs) have exhibited great promise in robotic
applications by generating high translational force with low input torque. To
further facilitate their robotic applications, it is strongly desirable but
challenging to enhance their consistent strain generation while maintaining
compliance. Existing studies predominantly considered overtwisting and coiling
after the regular twisting stage to be undesirable non-uniform and
unpredictable knots, entanglements, and coils formed to create an unstable and
failure-prone structure. Overtwisting would work well for TSAs when uniform
coils can be consistently formed. In this study, we realize uniform and
consistent coil formation in overtwisted TSAs, which greatly increases their
strain. Furthermore, we investigate methods for enabling uniform coil formation
upon overtwisting the strings in a TSA and present a procedure to
systematically "train" the strings. To the authors' best knowledge, this is the
first study to experimentally investigate overtwisting for TSAs with different
stiffnesses and realize consistent uniform coil formation. Ultra-high
molecular-weight polyethylene (UHMWPE) strings form the stiff TSAs whereas
compliant TSAs are realized with stretchable and conductive supercoiled polymer
(SCP) strings. The strain, force, velocity, and torque of each overtwisted TSA
was studied. Overtwisting and coiling resulted in approximately 70% strain in
stiff TSAs and approximately 60% strain in compliant TSAs. This is more than
twice the strain achieved through regular twisting. Lastly, the overtwisted TSA
was successfully demonstrated in a robotic bicep