Development of an EMG-based Elbow- Exoskeleton with Twisted String Actuation

Abstract This paper presents an exoskeleton for upper extremity rehabilitation of stroke patients. In order to make rehabilitation widely available, the goal is an inexpensive, portable, and easy-to-use therapy for paralysis of the arm. The exoskeleton is actuated by twisted string actuators (TSA). To reduce the length of the TSA a motor with hollow shaft was chosen, as this allows the string to run through the motor. The control of the exoskeleton is designed for predefined elbow flexion exercises. The start of the movement is triggered by a threshold value in the sEMG of the m. biceps brachii. To evaluate the controller, the exoskeleton performed a flexion movement (5° to 115°) in 2 s and then an extension back to 5° in the same time after a 1 s wait. To simulate different force requirements, additional weights up to 1500 g were attached. During both, the flexion and extension movement, that angle tracking error was sufficiently small and the steady state error was zero. The angular velocity of the elbow joint showed pronounced oscillations with increasing weights. The results show that TSAs can be used to actuate elbow exoskeletons. Thus lightweight, ergonomic, simple and small rehabilitation devices can now be built on this basis. The elasticity of the system must be considered in future work. sEMG must be further implemented to control more sophisticated functions of the rehabilitation device.