Abstract : AC servo systems are widely used in the field of manufacturing control. This paper analyzes the composition of position servo systems and mainly introduces the improved control algorithm of PID controller in digital position loop and parameter tuning method. Practical application shows that: selecting reasonable PID parameters can meet the requirements of fast response speed, high speed accuracy and strong robustness of the control system. Keywords : AC servo control; control algorithm; PID regulation AC servo systems have been more widely used in manufacturing control. The control requirements are reflected in fast response speed, high speed accuracy, wide speed range and good acceleration and deceleration performance. With the continuous development of computer technology, electronic technology and motor magnetic materials, AC servo control has gradually become the mainstream of motion control in the field of factory automation [1]. Various new control algorithms have emerged, such as adaptive control, field-oriented control and direct torque control, intelligent control, etc. However, the traditional PID control method is still the basis of other control algorithms due to its convenient implementation and reliability. I. System Composition Principle The overall structure of the system is shown in Figure 1. The system consists of four parts, namely microcomputer, servo control card, AC servo speed regulation system and sensor detection. The main control microcomputer is connected to the control card and can send position or speed commands via data lines, set PID adjustment parameters, and perform digital-to-analog (D/A) conversion. The analog signal is amplified by an AC servo amplifier to drive the servo motor. An incremental photoelectric encoder is mounted on the motor shaft end, providing feedback signals (A, B, IN pulses) to complete the position feedback of the position servo system, forming a semi-closed-loop system. Generally, the photoelectric encoder is mounted on the non-loaded shaft end of the motor for easy installation and to avoid adverse effects of mechanical component vibration and deformation on the position control system. In the position feedback loop, the sensing element—the incremental photoelectric encoder—transmits the real-time displacement (or rotation angle) change of the moving component via a long line in the form of A and B phase differential pulses to the field control station (PC) for encoder pulse counting to obtain digital position information. The main control microcomputer calculates the deviation between the given position and the actual position (i.e., the feedback position), and adopts a corresponding PID control strategy based on the deviation range. The digital control action is converted into analog control voltage via digital-to-analog conversion and output to the servo amplifier, ultimately adjusting the motor movement to achieve the desired positioning. II. Servo Control Methods The commonly used method in industrial control is the PID controller. Although with the development of modern AC speed control technology, various new control algorithms have emerged, such as adaptive control, expert systems, and intelligent control [2]. From a theoretical analysis, many control strategies can achieve good dynamic and static characteristics of motors. However, due to the complexity of the algorithm itself and the difficulty in model identification of the system, it is difficult to implement in actual systems. For traditional PID controllers, their greatest advantage lies in the simplicity of the algorithm, the ease of parameter tuning, strong robustness [3], strong adaptability, and high reliability. These characteristics make PID controllers widely used in the field of industrial control. For the controlled object in the CNC system, which is not complex, the PID controller can more easily achieve the expected effect. 1. Position Loop PID Control Algorithm In digital PID control systems, the purpose of introducing the integral element is to eliminate steady-state error and improve accuracy. However, at the beginning, end, or when the set value is increased significantly, integral accumulation will occur, causing large overshoot or even oscillation in the system, which is detrimental to the operation of the servo motor. To reduce the impact of integral correction on the dynamic performance of the control system during motor operation, integral separation PID control is appropriate. When the error between the actual position and the desired position of the motor is less than a certain value, the integral correction is restored to eliminate the steady-state error of the system. The integral separation PID control algorithm requires setting an integral separation valve ε. When |e(k)|>ε, i.e., when the deviation is large, PD control is used to reduce overshoot and enable the system to respond quickly; when |e(k)|≤ε, i.e., when the deviation is relatively small, PID control is used to ensure the position control accuracy of the servo motor. The discretized PID control formula is: where k is the sampling number, k=0,1,2…; Kp, Ki, and Kd represent the proportional, integral, and derivative coefficients, respectively. In practice, if the actuator requires an incremental control quantity, the incremental PID control formula can be obtained according to the stacking principle: 2. Position Loop Control Algorithm Flowchart 2 shows the control algorithm flowchart. 3. Tuning of Control System Parameters The main control microcomputer sends PID parameters to the control card to check whether the given parameters meet the requirements of the control system. This process needs to be implemented by parameter tuning. The main task of parameter tuning is to determine Kp, Ki, Kd and sampling period T. Increasing the proportional coefficient Kp makes the servo drive system more sensitive and faster in response, but too large a value will cause oscillation and increase the adjustment time. Increasing the integral coefficient Ki can eliminate the steady-state error of the system, but the stability will decrease. Differential control can improve the dynamic characteristics, reduce the overshoot and shorten the adjustment time. Common methods include the extended critical proportional method, the extended response curve method and the normalized parameter tuning method. These methods are derived from the use of the Ziegler-Nichols rule [4]. The AC servo system model can usually be considered as a first-order model with a delay element (a first-order element with hysteresis): The first-order response characteristic parameters K, L and T in the formula can be extracted from the S-shaped response curve shown in Figure 3. Determining these parameters is not difficult for practical systems. A step input excitation can be applied to the system to obtain the response curve, and then the characteristic parameters can be calculated from the curve. Therefore, the Ziegler-Nichols tuning rule can be used to obtain: [align=center] [/align] The choice of sampling period in a digital system is closely related to the system's stability. On the one hand, Shannon's theorem must be satisfied, i.e., ωs ≥ 2ωmax. However, the maximum frequency ωmax of the input and feedback in a practical system is difficult to determine. On the other hand, there is no precise formula for calculating the sampling period; it can only be selected based on empirical rules for engineering applications. For electromechanical control systems, a shorter sampling period is required, typically tens of milliseconds. III. Conclusion For AC position servo control systems, using a PC-based development platform and a conventional PID controller, as long as the parameters are properly tuned and the system's mechanical precision (motion shaft, gears, motor screw transmission) is controlled within a certain error range, the electrical control precision (encoder pulses) can be improved. This results in strong robustness and can achieve high-precision position control requirements in many situations.