Abstract:  Aiming at the study of the predetermined trajectory control problem of the lower extremity exoskeleton, the kinematics and dynamics model of the two-degree-of-freedom mechanical leg was constructed. A transformation relationship between the load force and the expansion of the asymmetric hydraulic cylinder, and the rotation torque and motion angle of the exoskeleton was proposed, and the precise trajectory control of the exoskeleton was further realized by using the backstepping method. Firstly, the kinematics model of the two-degree-of-freedom lower extremity exoskeleton system was established by using the Denavit-Hartenberg (D-H) method. The relationship between the joint velocity of the exoskeleton end-effector was studied, and the mathematical model of the exoskeleton dynamics was derived by using the Lagrange dynamic equation. Then, by controlling the load force of the hydraulic cylinder, the corresponding torque was provided for the movement of the mechanical leg, and the motion posture of the lower limb exoskeleton was controlled. Secondly, aiming at the high precision requirement of the lower extremity exoskeleton movement, the state space equation of the valve-controlled asymmetric cylinder system combined with the whole lower extremity exoskeleton dynamics system was established by using the backstepping control theory. The hydraulic servo system of the lower extremity exoskeleton was controlled, so as to control the lower extremity exoskeleton to carry out the predetermined trajectory movement. Finally, through the joint simulation of AMESim software and Visual Studio software, the accuracy of PID control and backstepping control was compared and analyzed. The research results show that the maximum error of the backstepping control was 2 ° for the predetermined trajectory control of the lower extremity exoskeleton. Comparing with the traditional PID control, the error is reduced by 67 %. The designed backstepping controller had good control accuracy for lower limb exoskeleton.
Key words: electro-hydraulic servo system; asymmetric hydraulic cylinder; backstepping control; state space equation of lower limb exoskeleton system; Denavit-Hartenberg (D-H) method; AMESim; control accuracy