Non-cylindrical wrapping rods and compliant adapters for multi-mode twisting string actuation

In this paper, improvements of the basic twisting string actuation commonly found in the literature are demonstrated based on the addition of two types of passive elements in order to optimise mechanical efficiency. First, the motion speed is increased with non-cylindrical rigid wrapping. Secondly, passive compliant load adjusters are added to improve the overall mechanical advantage of the transmission in reaction to varying loads. A mathematical model of the new system is first detailed in the paper and provides numerical simulations subsequently validated with experimental results. The impact on the important characteristics of the transmission of the design parameters are shown in details and the simulations highlight several improvements of our proposed design compared to existing twisting wire systems. Finally, this novel transmission with a compliant adjuster is used to drive an underactuated robotic finger to demonstrate its effectiveness.