Abstract: This paper analyzes and studies the servo system of electrical discharge machining from the perspective of positioning accuracy. By analyzing the working characteristics and positioning error of the servo drive of the system, it proposes to use software to test and improve the servo performance of the system and improve positioning accuracy by correcting speed commands. Keywords: CNC control system, positioning accuracy 1 Introduction With the rapid development of the aerospace, electronics and automotive industries, a large number of precision parts with complex structures and novel materials have emerged, which has put forward higher requirements for machining processes and technologies. In recent years, both domestic and foreign companies have been committed to developing high-end CNC electrical discharge machining tools to achieve multi-axis linkage for machining complex and high-precision workpieces. The widespread application of microcomputers in modern industrial control has made it inevitable to develop machining control systems and application software based on PC hardware and software environments and fully utilize computer system resources. Because computer control is implemented in the machining process, and computer technology is used to optimize the control system, establishing integrated parameter adaptive control devices and databases, productivity and machining accuracy are greatly improved. Therefore, the CNC numerical control of electrical discharge machining is an inevitable trend. The feed servo system, as the execution device of CNC control commands, directly reflects the machine tool's ability to track motion axes and achieve positioning. It can be said that the machining characteristics, productivity, and machining accuracy of a machine tool are mainly determined by its feed servo system. A CNC servo system is shown in Figure 1. Therefore, in the research and development of CNC electrical discharge forming machines, we analyzed the characteristics of the feed servo system and the resulting positioning errors, and proposed measures to reduce positioning errors and improve servo system performance to achieve high-precision machining of workpieces. 2 Feed Servo System and Characteristics The feed servo system of a CNC machine tool is a precise position tracking and positioning system that can achieve position and speed control. Figure 2 shows the functional block diagram of a servo system for one coordinate axis of a CNC machine tool. The position adjuster receives instructions from the CNC device (position movement information allocated to each coordinate axis after interpolation by the interpolator for each machining program segment), adjusts it to have a certain dynamic change pattern, and uses it as an instruction for the speed control unit. The speed control unit controls the torque and speed of the motion actuators (motors) according to this instruction to drive the mechanical transmission components and the worktable, thereby converting speed into position. For CNC machine tools, the time constant of the servo system is a crucial factor affecting its performance. When the computer sends the commanded speed value to the servo system, the actual speed of the motion axis cannot simultaneously reach the commanded speed; instead, it rises exponentially until it reaches the commanded speed. A large time constant will make the transition process relatively slow. When the actual speed of the motion axis approaches the commanded speed, the area below the commanded speed line represents the commanded displacement value, while the area below the actual speed curve of the motion axis represents the actual displacement value traversed by the motion axis. The difference between the two areas represents the following error D of the actual movement of the motion axis relative to the commanded displacement value. The value of the position following error D indicates the degree to which the actual position value lags behind the command. Increasing the loop gain value can effectively improve the control accuracy and positioning accuracy of the system, and reduce the following error value. For machine tools with multi-axis linkage, it is necessary to ensure that the values ​​of each axis are equal; otherwise, the loop gain will participate in contour shaping, causing changes in the machined shape. For linear motion, there is a parallel offset between the actual contour and the command contour; while for motions where the command contour is circular or parabolic, actual contour deformation will occur. 3 Measures to Improve Machining Accuracy When machining parts, relative motion is required between the tool and the workpiece to obtain the required shape. This is achieved through a position control loop (Figure 3) that controls the speed of a servo motor. Based on the stored geometric dimensions and speed data, the control device provides command values ​​to adjust the feed unit. The position command value of the input signal of the position control loop is compared with the actual position detected by the measuring instrument, and the position command value is changed over time by an appropriate command value adjuster. The error of the position control is used to control the actuator of the feed device. In the process of researching and developing the servo system of CNC EDM forming machine, several measures to improve machining accuracy were proposed in response to factors affecting the performance of the servo system: (1) Optimization of PID controller parameters In a continuous control system, the PID controller can improve the stability and accuracy of the system: (a) Appropriately increasing the proportional coefficient KP can speed up the response speed of the system, which is beneficial to reducing the steady-state error in a system with steady-state error; (b) Increasing the integral time constant can reduce overshoot and oscillation, making the system more stable; (c) Differential control can improve the dynamic characteristics of the system, such as reducing overshoot, shortening the adjustment time, allowing for increased proportional control, reducing steady-state error, and improving control accuracy. In the process of establishing and debugging the servo system, the dynamic performance and motion response curve of each motion axis were tested using test software; according to the test servo system response process, the KP, KD and KI of each motion axis were adjusted to improve the system response performance; each motion axis has the same servo performance, so that the feed drive device with lower dynamic performance can achieve the same accuracy as the drive device with higher dynamic performance, thereby improving the contour accuracy of the workpiece machining. (2) Determination of actual following error: Based on the relationship between the differential control action and the rate of change of the deviation, the expected error that the system parameters may produce is calculated and corrected. The advance correction effect is used to improve the dynamic performance of the servo system. By obtaining the encoder feedback signal, the actual following error value of each motion axis of the system is obtained, the loop gain of each motion axis is calculated and its value is written into a fixed memory unit; accordingly, the drive voltage of the motor is adjusted so that the worktable tracks the command speed with the largest possible acceleration, thereby improving the tracking performance of the system and ensuring good dynamic characteristics of the entire servo feed system. (3) Smoothing correction of command value: The error is compensated in software by measuring the servo error in real time. That is, the acceleration control position command model is replaced by the speed control position command model. The magnitude of acceleration and deceleration is determined according to the sign and magnitude of the error and the required accuracy. For example, the motor acceleration and deceleration motion control law with the speed-time curve as S-shaped is replaced by the motor acceleration and deceleration motion control law with the speed-time curve as trapezoidal. Meanwhile, for the control of the table speed with low dynamic performance, a larger starting acceleration (i.e., a larger starting torque) is used to improve dynamic performance, ensuring that each motion axis has the same servo performance, reducing dynamic contour deviation, and improving machining accuracy. In the research and development of the CNC EDM forming machine, we combined the optimization of PID controller parameters, the determination of actual following error, and the smooth correction of command values. During the establishment and debugging of the servo system, the KP, KD, and KI parameters of the servo system for each motion axis were adjusted to improve dynamic performance and motion response curves. Based on the measured actual following error values ​​of each motion axis, the position error limits were compared, and the position response accuracy of the servo system was improved by combining the smooth correction of command values. By adopting the above control strategies and compensation methods, the servo accuracy was significantly improved, and the real-time position error during machining was reduced to the micrometer level, meeting the accuracy requirements of EDM. 4. Conclusion The dynamic characteristics of the servo feed system are an important factor causing machining errors. Based on adjusting the performance of the servo system, reasonable control of the command speed to make the transition process of the speed curve smoother can effectively improve machining accuracy.