Design and Control of the Compact Cable-driven Series Elastic Actuator Module in Soft Wearable Robot for Ankle Assistance

Abstract

For assisting the daily lives of the elderly or patients with abnormal walking by hemiplegia, wearable robots which are constructed with relatively soft materials such as cable-driven method are being actively developed and partially commercialized. The main advantage is that the flexible power transmission system could distribute the inertia of the distal area, which is essential for stabilizing the gait balance. However, in flexible power transmission system, it has disadvantage that it cannot directly measure the tension applied to the patients. Therefore, tension measurement device on the assisting point is required for generating precise assistance on the patients. But adding the tension measuring device is challenging because it increases complexity and inertia. The main factor for inaccurate force transmission is the unpredictable friction from the transmission cable's varying curvature in human motion. In this paper, a methodology of design and control of reaction force-based series elastic actuator is proposed to generate the precise assistive force at the actuating module while maintaining the overall weight small. The mechanical elements of Series Elastic Actuator are designed with the optimal specifications required for stroke patient assistance. The compactness and weight reduction are also guaranteed by designing the mechanical elements with optimal specifications, which enables assisting the patients with minimal gravitational load. Furthermore, periodic friction compensation is done through experiments by using feedforward compensator, which adjusts the corresponding loss, linked with the estimated gait phase. Therefore, even in the presence of friction loss in the cable, application of precise torque at the end-effector became possible.