Extended state observer-based sliding mode control for an SEA-based lower limb exoskeleton

With the development of modern society, the advent of wearable exoskeleton devices has provided people with better experience and security in work and in life. However, since traditional rigid driving joints inherently lack impact cushioning capabilities, the impact force acts directly on the motors and human body, resulting in poor safety. Furthermore, especially for healthy wearers, since their motor function is intact and they are capable of independently engaging in various types of movements, the complexity and diversity of gait activities will also increase the uncertainty in control. To address the above issues, this paper proposes a compliant hip joint design for lower limb exoskeleton robots. This design employs a radial series elastic actuator (SEA) configuration, featuring an intuitive mechanical structure, convenient maintainability, and adjustable stiffness. Compared with traditional rigid driving joints, the SEA-based hip joint designed in this paper provides impact absorption and vibration cushioning capabilities, thereby enhancing the safety of human–robot interaction and improving control precision. Then an extended state observer-based sliding mode controller is designed to achieve precise trajectory tracking, where the extended state observer is used to mitigate external disturbances and modeling errors. The simulation results show that the proposed extended state observer-based sliding mode controller can achieve precise position control in the presence of vibrational and impact disturbances.