CNC machine tools and machining centers are important equipment in the manufacturing industry, and precision machine tools are indispensable for high-quality machining. Under normal operating conditions, the feed speed of the worktable of a precision machine tool is often required to be 1 cm/min, which necessitates that the precision machine tool servo system possess excellent low-speed characteristics. Therefore, low-speed performance, as an important indicator for evaluating the performance of machine tool servo systems, is increasingly attracting attention. Improving the low-speed performance of CNC machine tool servo systems is undoubtedly of great significance for improving machine tool performance, machining quality, and reducing costs. This paper takes the commonly used permanent magnet AC servo system of CNC machine tools as an example, analyzes the reasons affecting the system's low-speed performance, and provides corresponding solutions. Experimental results demonstrate the feasibility and effectiveness of the methods. System Structure [align=center] Figure 1 Structure diagram of CNC machine tool AC servo system[/align] Using TI's DSP-TMS320F240 as the control core, it mainly completes the calculation of the current loop, speed loop, 2/3 coordinate transformation, PWM generation and detection links, and the overall system coordination. The main circuit uses an IPM intelligent power module, and the controlled object is a permanent magnet synchronous motor with a rated speed of 2000 r/min. The speed detection uses a photoelectric pulse encoder that generates 2000 pulses per revolution. Reasons affecting the low-speed performance of CNC machine tool servo systems The influence of stator current and cogging effect Speed ​​fluctuation is an important technical indicator for measuring the low-speed characteristics of a servo system. This performance indicator is expressed by the speed non-uniformity, as shown in equation (1): Δω is the speed fluctuation, ω is the actual speed, Nmax is the instantaneous maximum speed during steady-state operation, and Nmin is the minimum speed during steady-state operation. Speed ​​disturbance is caused by torque disturbance. In actual operation, the torque Te of the servo system is not constant. Under medium and high speed conditions, the influence of torque disturbance on the operating characteristics of the system can be ignored. However, it has a great impact on high-precision servo systems that require stable operation at low speeds. This is because at low speeds, especially under no-load conditions, the control signal applied to the stator windings of the motor is very small. The magnitude of the disturbance signal can be compared with the control signal, or even exceed the normal control signal. The angular velocity output by the servo system will fluctuate under the action of the disturbance torque, disrupting the stability of low-speed operation. There are many factors that cause speed disturbances in the permanent magnet synchronous motor (PMSM) servo system. The influence of stator current In order to generate constant torque, the back electromotive force of the PMSM and the phase current input to the stator by the inverter must be sinusoidal. However, due to the combined influence of external factors, the three-phase stator current of the PMSM is not sinusoidal, but introduces a disturbance ΔI, as shown in equation (2). The generation of ΔI is caused by many factors. The physical shape of the permanent magnet and the existence of stator slots make the back electromotive force not ideally sinusoidal; the current input to the stator by the inverter contains high-order harmonics; current detection drift; current control has phase lag and other reasons that can generate ΔI, making the output torque unsatisfactory. The influence of cogging effect: Another important factor affecting the low-speed performance of CNC machine tool servo systems is the cogging torque generated by the cogging effect of the servo motor. Cogging torque is generated by the interaction between the rotor magnetic field and the stator core. The rotor of a permanent magnet synchronous motor is a permanent magnet, and the magnetic reluctance between the rotor and stator varies due to the different air gaps corresponding to the stator teeth and slots. When the permanent magnet synchronous motor rotates at a constant speed, these teeth and slots alternately pass over the magnetic poles. The periodic change in magnetic reluctance generates a periodic torque acting on the motor shaft. This periodic torque is the cogging torque, which is related to the position of the motor rotor magnetic poles and is a function of the amplitude and spatial position of the motor's permanent magnet magnetic field. It can cause periodic torque fluctuations in the system, affecting the low-speed performance of the servo system. The influence of dry friction: Dry friction on the actuator axis is another adverse factor affecting the low-speed characteristics of machine tool servo systems. When the system is running at medium to high speeds, the frictional force remains constant; at low speeds, friction is a function of the motor's angular velocity. Figure 2 shows the correspondence between frictional torque and motor angular velocity at low speeds. During low-speed operation, when the motor speed is greater than ωc, the frictional torque is constant, and the system motion is smooth. When the motor speed changes to a range between ωc and ωb, the frictional torque decreases, becoming less than Mc, and the output torque exceeds the load. The motor angular velocity increases until ωc, at which point the torque rebalances, but the acceleration continues to change. If the speed is less than ωb, the frictional torque exceeds the output torque, and the motor continuously decelerates until the next current sampling cycle. This results in the servo system performing jumpy tracking, and the actual system situation is more complex. Methods to Improve the Low-Speed ​​Performance of CNC Machine Tool Servo Systems Regarding the impact of stator current and cogging effect on the low-speed performance of the servo system, as mentioned above, there are currently many improvement methods. For example, regarding the impact on stator current, methods such as improving the distribution of the motor's spatial magnetic field, increasing current detection accuracy, reducing current detection drift, and real-time compensation for current control lag can be adopted. Regarding the impact of cogging effect, methods such as increasing the speed loop proportional gain, applying a specially designed robust regulator, and using a torque observer to compensate for torque disturbances in real time can be adopted. The method of variable speed loop regulator parameters is used in this system to overcome the speed fluctuation at low speed. Experiments have shown that this method is simple, feasible and effective. If speed fluctuation is considered, the angular velocity of the motor is assumed to be: ω=ω0+Δω (3) where ω0 is the average angular velocity and Δω is the angular velocity fluctuation. From the dynamic equation of the motion system, we know that: Let ΔT=jDΔω/dt, then the simplified AC servo system considering ΔT is shown in Figure 3. [align=center] Figure 3 Simplified block diagram of AC servo system considering ΔT[/align] According to Figure 3, the speed change caused by torque disturbance is shown in equation (5), where Kp is the proportional coefficient of the speed regulator and Ti is the integral time constant of the speed regulator. Increasing Kp and decreasing Ti can suppress speed disturbance and improve the performance of the system in steady state operation. However, in the actual system, the increase of Kp and the decrease of Ti are limited. Excessive increase of proportional and integral action will cause system oscillation and make the system unstable. In the actual system, we let Kp and 1/T1 change with the given speed of the system. The specific relationships are as follows: Kmax is the maximum value of the proportional coefficient, K1max is the maximum value of 1/T1, Kmin is the minimum value of the proportional coefficient, K1min is the minimum value of 1/T1, and nup and ndown are two specific speed setpoints. In the specific implementation of this method, the change in the speed setpoint must also be considered. When the setpoint speed changes from a large value to a small value, the proportional coefficient and integral coefficient should initially remain at their minimum values. Only after the error decreases to a certain extent can it be processed according to the above formula. The influence of dry friction mentioned earlier can be improved by improving lubrication conditions to reduce friction torque; increasing the system's moment of inertia to increase the proportional coefficient of the speed loop regulator in the dual-loop system; and increasing the damping ratio of the closed-loop system. This paper uses a variable structure control method. This method adds a rotor position loop while retaining the current loop and speed loop at low system speeds, adopting the structure shown in Figure 4. When the parameters of the position loop regulator are reasonably selected, this method can effectively overcome the influence of dry friction on the servo system motor shaft at low speeds, enabling the servo system to operate smoothly. Experimental Results Figure 5 shows the stator current waveform measured using the system shown in Figure 1 at a given speed of 1 r/min. The experimental results demonstrate that the method proposed in this paper effectively improves the low-speed performance of the system. [align=center]Figure 5 Stator current waveform during steady-state operation at 1 r/min[/align] Conclusion This paper analyzes the reasons affecting the low-speed performance of AC speed control systems and provides specific solutions, achieving better performance without increasing any hardware resources. Experimental results prove that the method is simple, effective, and feasible.