Elastomeric Continuously Variable Transmission Combined With Twisted String Actuator

Abstract

Electric motors, with a fixed reduction ratio, have a large unused operating region when mimicking muscle movements owing to the difference between the force-velocity curves of electric motors and muscles. This unused region can be reduced by changing the reduction ratio according to the external force. However, the conventional continuously variable transmissions (CVTs) are large and heavy. Elastomeric CVT (ElaCVT), a new concept relating to CVT, is designed in this study. The primary purpose of ElaCVT is to expand the operating region of a twisted string actuator (TSA) and duplicate the force-velocity curve of the muscles by passively changing the reduction ratio according to the external load applied to the end of the TSA. A combination of ElaCVT and TSA (ElaCVT-TSA) is proposed as a linear actuator that mimics the characteristics of muscles. The deformation of elastomer changes the reduction ratio without the need for complicated mechanisms. This enables the CVT to be small and lightweight so that it can be applied to various robotic systems. The performance of the ElaCVT-TSA was evaluated by means of the experiments, and the results show that the reduction ratio was passively and continuously adjusted as the external load changed. The ElaCVT has a cylindrical shape with a length of 27 mm, a diameter of 24 mm, and weighs 12 g. The reduction ratio in the maximum velocity mode is approximately 2.31 times the reduction ratio in the maximum torque mode.