Active-Type Continuously Variable Transmission System Based on a Twisted String Actuator

Abstract

This letter presents an active-type twisted string actuator (TSA)-based continuously variable transmission (CVT). The proposed mechanism improves the advantage of the intrinsic CVT nature of TSA by compensating its nonlinear transmission ratio (TR) with the active and continuous conversion of the TR during contraction at a constant load as well as variable load. To change the TR, the proposed CVT adjusts the ratio of rotation between individual twists and overlap twists, which are two different types of string-twisting patterns. The performance of the TSA-based active CVT was experimentally assessed with the fabricated prototype and compared with a mathematical model. This adjustment in the rotation ratio between individual and overlap twists is realized using a planetary gear. By adjusting the rotating speed ratio between the sun and ring gear, the ratio of rotation between individual and overlap twists can be adjusted. Unlike the conventional TSA-based CVT, the proposed CVT can actively and continuously change the TR independent of the external load, even during operation. Consequently, it allows an increase in the operating range of the TSA while maintaining a high contraction speed.