Enhancing Maximum Stroke of Twisted String Actuators by Adjusting Twisting Ratio

In robotics, twisted string actuators (TSAs) have attracted considerable attention owing to their high gear ratio, flexibility, and simplicity. However, TSAs face challenges such as control issues, limited lifespan, and limited stroke. Among them, their practical use is hindered by a limited stroke due to overtwisting, leading to hysteresis, efficiency, and lifespan issues. To avoid overtwisting, TSAs are confined to a narrow stroke range, approximately 30% of the original string length. This study introduces an innovative approach to enhance TSA stroke without introducing overtwisting. The key lies in adjusting the ratio of overlap and individual twisting (ROI) to strategically use the string that possesses the capacity of twisting. The method capitalizes on the observation that the maximum strokes of two TSA twisting methods—twisted single string and twisted looped single string (TLS)—are approximately equal. This is attributed to the locking point induced by overlap twisting in TLS. To mathematically model this phenomenon, this study develops a novel kinematic model of TSA accounting for the locking point. Additionally, we propose an optimization process, achieving a maximum stroke of 53.44% of the original string length, significantly surpassing the conventional limitation of 30%. This enhancement is achieved without introducing overtwisting, thereby avoiding hysteresis.