Design of a wearable rehabilitation robot for the shoulder abduction using petal mechanism driven by SMA actuators

Abstract

A wearable rehabilitation robot for shoulder abduction, which is frequently used in activities in daily living and where the rehabilitation treatment is important, is necessary for patients. In the case of shoulder abduction, not only does a large torque that can lift the arm more than 10 Nm, but also minimizes the compressive force to the shoulder joint and follows scapulohumeral rhythm (SHR). Shoulder abduction robots using various actuators such as tendon-driven, pneumatic, and motor-gear systems have been developed, but many studies have focused only on the purpose of lifting the arm with a large torque without considering SHR or required force/energy increases rapidly in a high degree. In this paper, a wearable rehabilitation robot for shoulder abduction using a petal mechanism was designed to solve these problems. This mechanism converts linear motion into rotation motion and has not only a high mechanical advantage but also minimizes the compressive force applied to the shoulder joint. Additionally, to satisfy the SHR, a double-petal mechanism was applied that displaced the two petal mechanisms by mimicking the scapula and humerus, respectively. When the same force is applied to the two actuators in a static condition, the angle ratio of the double-petal mechanism maintains close to 2, and it is confirmed that a maximum torque output of 12.6 Nm is generated at 90 degrees. After that, because of evaluating the rehabilitation performance of the proposed mechanism through user tests, not only did sEMG decrease by 34.63 % on average when actively supported but also trajectory errors with SHR are reduced.