Proxy-based torque control of motor-driven exoskeletons for safe and compliant human-exoskeleton interaction

Exoskeletons are considered as promising devices for motion assistance of mobility impaired patients and motor ability augmentation of healthy people. Considering the interactive action between exoskeletons and human body, a safe and comfortable human-exoskeleton interaction is essential to achieve effective exoskeleton operation for human motion assistance. In this paper, a proxy-based torque controller (PTC) is designed for safe and performant interaction torque control of motor-driven exoskeletons under different scenarios. Firstly, the dynamics of commonly applied motor-based conventional stiff actuator (CSA) and series elastic actuator (SEA) for exoskeletons are modelled and analyzed. Then, the PTC with specified compensation is designed based on proxy-based sliding mode control (PSMC) strategy and the stability is theoretically proved for the closed-loop system of the two types of actuators. Meanwhile, an adaptive law is designed for online adjustment of the PTC parameter to produce an overdamped system response for the two actuators under unexpected interruptions. Following the adaptive law, the PTC robustly realizes a compliant torque control while keeping accurate torque tracking under normal operation. Experiments are conducted for two back-support exoskeletons with CSA and SEA, verifying the effectiveness of the proposed controller for safe human-exoskeleton interaction assurance under different scenarios.