Design and Evaluation of a Novel Torque-Controllable Variable Stiffness Actuator With Reconfigurability

This article presents a novel reconfigurable torque-controllable variable stiffness actuator (RVSA) for knee exoskeleton. The concept of reconfiguring the pulley block is proposed to make the actuator work in different torque and stiffness ranges. The reconfigurability allows the actuator to achieve a variety of passive stiffness behaviors, namely softening, linear, and hardening behaviors. Compared with existing variable stiffness actuators based on the principle of changing the spring preload, an advantage of the RVSA is that the actuator can achieve a wider range of joint stiffness. Moreover, the RVSA model is established to study the stiffness characteristics and the ability of storing energy. The analysis results show that the RVSA can increase the number of pulley blocks to expand the stiffness range and improve the energy storage capacity. This article also introduces a method for estimating and controlling actuator torque without using additional torque sensors. Finally, the stiffness characteristic of the RVSA is tested, and its use in a rehabilitation exoskeleton is tested. Experimental results show that the proposed torque control method is effective, and it also has a good performance in practical applications.