Design, Modeling, and Control of a Variable Stiffness Actuator With Dual-Motor Load Sharing for Elbow Exoskeletons

Joint stiffness control is essential for safe and compliant human–robot interaction in rehabilitation exoskeletons. Variable stiffness actuators (VSAs), with controllable physical stiffness, offers a potential solution. However, existing VSAs generally do not allow both motors to contribute to power output simultaneously, which limits their performance in human–robot interaction. Therefore, the goal of this article is to develop a novel variable stiffness actuator with dual-motor load sharing (DMLS-VSA) and its stiffness and torque control scheme. The innovative DMLS-VSA mechanism features a power transmission mechanism (PTM) and a variable stiffness mechanism (VSM). The PTM enables the DMLS-VSA to have dual-motor load sharing capability, allowing the output power to be determined by both motors. Meanwhile, the VSM utilizes a modified planetary gear mechanism to achieve stiffness variation by regulating the effective lever arm length. In this differential configuration, the motion of both motors changes the output torque and stiffness. Consequently, a novel cascade PI controller, with the position-loop control term acting directly on the deflection angle and stiffness, is used to control the DMLS-VSA, preventing undesired movements during motion. The performance of the DMLS-VSA is verified through experiments on stiffness regulation, torque control, physical human–robot interaction, and the dual-motor load sharing capability, demonstrating its suitability for rehabilitation exoskeleton applications.