Twisted string actuation through a conduit: Undesirable behaviors and pre-twist effects

Twisted string actuation through a conduit enhances flexibility, one of the most important features of twisted string actuators (TSAs). It allows power transmission remotely along an arbitrary curved surface, such as the human body Exosuit, while avoiding interference with the natural movement of the wearer. However, TSAs on guided sliding surfaces or inside conduits have not been extensively investigated. This study experimentally investigates TSAs inside a conduit and its undesirable behaviors, namely, pullback and chatter and their causes. Furthermore, this paper proposes a pre-twist method to overcome undesirable behaviors by introducing a certain amount of advanced twists as an initial condition to compensate for any opposite directional twist. Also, investigated the relationship between the required amount of pre-twist and string parameters (radius, length, and stroke) and external conditions (deflection angle and external load). Finally, an online-required pre-twist estimation method is proposed to minimize the performance loss due to the present pre-twist. In addition, the feasibility of these approaches is validated through an experimental comparison study.