A Wire-Driven Series Elastic Mechanism Based on Ultrasonic Motor for Walking Assistive System

Abstract In order to improve the elderly people’s quality of life, supporting their walking behaviors is a promising technology. Therefore, based on one ultrasonic motor, a wire-driven series elastic mechanism for walking assistive system is proposed and investigated in this research. In contrast to tradition, it innovatively utilizes an ultrasonic motor and a wire-driven series elastic mechanism to achieve superior system performances in aspects of simple structure, high torque/weight ratio, quiet operation, quick response, favorable electromagnetic compatibility, strong shock resistance, better safety, and accurately stable force control. The proposed device is mainly composed of an ultrasonic motor, a linear spring, a steel wire, four pulleys and one rotating part. To overcome the ultrasonic motor’s insufficient output torque, a steel wire and pulleys are smartly combined to directly magnify the torque instead of using a conventional gear reducer. Among the pulleys, there is one tailored pulley playing an important role to keep the reduction ratio as 4.5 constantly. Meanwhile, the prototype is manufactured and its actual performance is verified by experimental results. In a one-second operating cycle, it only takes 86 ms for this mechanism to output an assistive torque of 1.6 N·m. At this torque, the ultrasonic motor’s speed is around 4.1 rad/s. Moreover, experiments with different operation periods have been conducted for different application scenarios. This study provides a useful idea for the application of ultrasonic motor in walking assistance system.