Development of a soft knee exosuit with twisted string actuators for stair climbing assistance

Stair climbing remains one of the toughest tasks of the elderly in activities of daily livings (ADLs). However, most of the existing stair climbing assistance devices are large and heavy, such as the automatic stair climbing wheelchair, the rigid exoskeleton and so on. They all lack applicability in the daily life. Based on this starting point, in this paper, a soft knee suit (Hitexosuit) has been developed to assist people climbing. It is a lightweight and wearable assistance suit for human lower extremity in climbing the stairs. This paper finds out the certain phase of stair climbing motion that requires the most muscle strength and locates the target muscle that contributes most when people climb the stairs through the biomechanics analysis. Furthermore, this research draws inspiration from the human muscles, which contract to generate force for knee extension and help people lift their body to go upstairs. Based on that bionic principle, a new type of artificial muscle is developed. It simulates human muscle contraction by shortening the length of steel wire to give extra strength to human lower extremity. And textiles have been adopted in the suit, soft straps and belts to make up the structure of the suit instead of rigid and heavy materials. In addition, to minimize the whole weight of the exosuit system further, we develop a lightweight and portable actuator using twisted strings. This twisted string actuator (TSA) weighs 390g and the whole exosuit weight is 3.5kg with two TSAs. The exosuit is tested with three different users and the mean assistance efficiency is 29.8%. It will improve the living quality of the aged by guaranteeing them more confidence in ADLs and self-caring.