(A) study of methodologies for enhancement of life cycle of twisted-string actuators

Abstract

Twisted string actuator (TSA) is flexible, lightweight, and high power-to-weight ratio actuator widely used in robotics, especially in wearable robotics devices. However, the limited lifespan of the twisted string has been a key challenge that limits its application. There are no comprehensive studies that aim at improving the lifetime of a TSA and relate the reasons of string’s rupture to its material tribology. In this paper, we propose the effective methods to increase the lifespan of TSAs and also provide tribological analysis of the cause of the rupture of string. The proposed methodologies include pre-twisting, coating, lubrication, and the parallel string mechanism. The experiments consisted of conducting multiple trials using these individual methods, as well as their different combinations, to achieve maximum increase in life cycle of TSA. Pre-twisting is found to eliminate the impact force caused by initial offset between strings, resulting in improvement of life cycle by 20%. Coating the strings of TSA with skived PTFE tape provides a protective layer that reduces wear, and it extended the lifespan by 40%. Lubrication with synthetic grease reduced friction and enhanced lifetime of TSA by 100%. Combining these methods for a single-looped string resulted in an increase in life cycle of about 150%. The proposed parallel string mechanism divided the load evenly among multiple strings, increasing the lifespan of the TSA by 210%. Finally, combining all these methods further enhanced performance, resulting in a significant increase in the life cycle of more than 600%. Based on experimental results, the observations are made that the life cycle of the strings can be drastically improved by using methods to reduce wear and friction between the strings, as well as by minimizing the applied load.