Abstract: Combining the advantages of traditional PID control and sliding mode variable structure control, a composite control of PD and sliding mode variable structure for AC servo system is proposed, which makes the system have good robustness. This paper analyzes the mathematical model of AC motor and focuses on the design of three closed-loop system controllers. Simulation experiments show that the controller can improve the static and dynamic performance and robustness of the control system. Keywords: AC servo system; sliding mode variable structure control; PID AC servo system is increasingly widely used in the joint drive of robots and manipulators, as well as precision CNC machine tools. The main reason is that compared with DC servo motor, AC servo motor has the advantages of small size, strong overload capacity, large output torque, no brush wear, and no need for frequent maintenance. Moreover, since there is no influence of brush voltage drop, the performance requirements of AC servo system, from the perspective of dynamic and static characteristics, are mainly: (1) good fast tracking performance, that is, the system responds to the input signal quickly, the tracking error is small, the transition time is short, and there is no overshoot (or the overshoot is small) and the number of oscillations is small. (2) High steady-state accuracy, meaning a small steady-state deviation between the system output and the given value, high positioning accuracy, and a considerable positioning (braking) torque. AC servo systems have very high requirements for control strategies, which cannot be met by traditional control methods (such as PID controllers). This is mainly due to uncertainties in AC servo systems, such as time-varying parameters, load disturbances, and the severe nonlinear characteristics and strong coupling of the AC motor itself and the controlled object. Therefore, an ideal control strategy not only needs to meet the above dynamic and static performance requirements, but also should suppress the influence of various nonlinear factors on the system, have decoupling capabilities and strong robustness, and require no precise mathematical model. For details, please click: Research and Design of Composite Control for AC Servo Systems