Variable Structure Compensation PID Control for Lower Extremity Exoskeleton

In this paper, a portable lower extremity exoskeleton is designed to assist the operator in weight-bearing walk. The whole exoskeleton has an unthropoputhic structure, and each leg has six joint DOFs. The structure is the simplification of human joints in consideration of weight loss, the valve-controlled asymmetrical hydraulic cylinder is used as actuator in knee joint to bear load while walking. The variable structure compensation PID control method, (VSCPID), is proposed to implement the position tracking of knee joint. This method combines traditional PID control and sliding-mode control by adding the variable structure part associated with sliding surface to the PID control, so the controller can achieve robustness without the requirement of accurate system model. Meanwhile, the global stability of this approach can be proved through Lyapunov methods in the presence of bounded disturbances. The contrast experiments between the proposed control and PID control illustrate that VSCPID control can improve the tracking performance and has good robustness. In addition, the sigmoid function is employed in VSCPID to solve the chattering problems encountered in the experiments.