Assistance Torque Control Based on Musculoskeletal Hexagon Output Distribution for Upper Limb Exoskeleton

Abstract

Upper limb exoskeletons are a current focus of technology for rehabilitation and assistance. Although the exoskeletons are actively researched and various applications are being explored, it poses many challenges because of their robot-human fusion of dynamics. In particular, control strategy is one of main problems to solve. Important points of the exoskeleton control are safety, usability, and prevention of insufficient/excessive assistance. Control of physical HumanRobot Interaction (pHRI) without electromyographic (EMG) measurements is one of solutions for the first and the second points. However, the exoskeletons assist based on only payload information in conventional researches and they have disadvantages in third point. Since ease of output of muscles is affected by the limb posture, assistance should be determined based on both payload and musculoskeletal information of each user. Hence, this paper aims to realize musculoskeletal model-based assistance considering ease of output of muscles. For this aim, hexagon distribution (HD)-based assistance torque calculator is proposed. HD expresses human output characteristics of a hand tip by modeling muscles of an upper limb including bi-articular muscles as 6 muscle groups. In proposed control strategy, first, the maximum human output is derived by HD. Second, it is judged whether assistance is sufficient or insufficient. Finally, assistance torque is adjusted in the case of insufficient assistance. To verify effectiveness of the proposed method, an experiment is conducted. As a result, it is confirmed that the proposed method can assist users based on their musculoskeletal characteristics.