Hybrid position/force control system for Human Lower-limb Exoskeleton

This paper is focused on the dynamic modeling and design of hybrid position/force control system for the knee joint of Human Lower-limb Exoskeleton (HLE). The main object of this research is to develop a HLE for human to carry more loads. The dynamics of the human lower-limb analyzes the angle and torque of the knee joint in a gait cycle, and then gets the position and force output of the hydraulic cylinder. The highest peak torque of the knee joint and the force output of the hydraulic cylinder is extension in the stance state. So it is needed to research the force control of the support leg after controlling the position of the hydraulic cylinder. The controller based on fuzzy logic and PID is adopted to regulate the position of hydraulic cylinder of the knee joint because of the uncertainty of the human limb parameters and the actuator parameters. This control algorithm has good self-adaptation. On the other hand, Fuzzy PID is also adopted to control the force output of the hydraulic cylinder at the knee joint. There is a force sensor at the foot of the HLE to detect the efficiency of HLE carrying loads. The specific implementation methods of force control are going to research in the next research stage. The simulation results show the HLE could follow human gait more effectively under the control of Fuzzy PID and the following error is smaller than that of the common PID control.