Abstract: The flexible ankle of the robot can reduce the impact of the ground force during walking. However, the flexible link not only increases the order of the control system, but also causes the internal oscillation of the system easily and increases the stability time, which can't satisfy the requirement of the rapidity and stability during the process of control. In this paper, the flexible ankle joint of the robot adopts a spring-lasso transmission method. On the basis of the singular perturbation theory, the order of the system is reduced. As a result, the system is more concise. According to the terminal angle error, the fast and slow controllers are both used for switching control. Herein, a sliding mode controller based on proportional switching is used in the slow controller which makes the terminal of the joint reach the desired position quickly and stably. The fast controller uses the state feedback controller to make the variable of spring converge to zero rapidly. Finally, the switching control is compared with the single PID control and the sliding mode control through Simulink simulation and prototype experiment. The experimental results show that although the three controllers all meet the need of rapidity, only the switching control can guarantee the stability of the terminal during multiple position controls, which meets the requirements of complete control objectives.

Key words: flexible joint, singular perturbation, sliding mode control, model order reduction, switching control