Abstract: Servo motors have superior performance compared to stepper motors. With the development of modern motor control theory, servo motor control technology has become an important component of CNC machine tool systems and is developing towards AC, digital, and intelligent directions. Keywords: CNC system, servo motor, direct drive. In recent years, servo motor control technology has been developing towards AC, digital, and intelligent directions. As the actuator of CNC machine tools, the servo system integrates power electronic devices, control, drive, and protection. With the advancement of digital pulse width modulation technology, special motor material technology, microelectronics technology, and modern control technology, it has undergone a development process from stepper to DC and then to AC. This article briefly discusses its current technical status and development trend. [b]I. CNC Machine Tool Servo System[/b] (I) Open-loop servo system. The open-loop servo system does not have a detection feedback device and does not constitute a motion feedback control loop. The motor works according to the command pulses issued by the CNC device. There is no detection feedback and processing correction process for motion errors. Stepper motors are used as driving devices. The position accuracy of the machine tool depends entirely on the step angle accuracy of the stepper motor and the transmission accuracy of the mechanical parts, making it difficult to achieve relatively high precision requirements. The speed of a stepper motor cannot be very high, and the speed of the moving parts is limited. However, stepper motors are simple in structure, highly reliable, and low in cost, and their control circuits are also simple. Therefore, open-loop control systems are mostly used in economical CNC machine tools with low precision and speed requirements. (II) Full closed-loop servo system. A closed-loop servo system mainly consists of a comparator, a servo drive amplifier, a feed servo motor, a mechanical transmission device, and a linear displacement measuring device. It has the function of detecting and feedback correction of the movement of the machine tool's moving parts, and uses a DC servo motor or an AC servo motor as the drive component. A grating or inductive synchronizer directly mounted on the worktable can be used as a position detection device to form a high-precision full closed-loop position control system. The linear displacement detector of the system is installed on the moving part, and its accuracy mainly depends on the accuracy and sensitivity of the displacement detection device, resulting in relatively high machining accuracy. However, various nonlinear factors such as the stiffness of the mechanical transmission device, friction damping characteristics, and backlash have a great impact on the stability of the system, making the installation and debugging of the closed-loop feed servo system more complicated. Therefore, it is only used in high-precision and large CNC machine tools. (III) Semi-closed-loop servo system. The working principle of a semi-closed-loop servo system is the same as that of a fully closed-loop servo system. Both use a servo motor as the drive component. A pulse encoder, brushless rotary transformer, or tachogenerator built into the motor can be used as the position/speed detection device to construct a semi-closed-loop position control system. The system's feedback signal is taken from the motor shaft or lead screw. The mechanical transmission device in the feed system is outside the feedback loop, and its stiffness and other nonlinear factors have no impact on system stability. Installation and debugging are relatively convenient. The positioning accuracy of a machine tool is related to the accuracy of the mechanical transmission device. CNC devices have functions such as pitch error compensation and backlash compensation. When the accuracy of the transmission device is not very high, the compensation function can be used to improve the machining accuracy to a satisfactory level. Therefore, semi-closed-loop servo systems are widely used in CNC machine tools. [b]II. Superior Servo Motor Control Performance[/b] (I) Good Low-Frequency Characteristics. Stepper motors are prone to low-frequency vibration at low speeds. AC servo motors do not exhibit this phenomenon and operate very smoothly. AC servo systems have resonance suppression capabilities, which can cover insufficient mechanical rigidity. Furthermore, the system has an internal frequency analysis function that can detect mechanical resonance points, facilitating system adjustment. (II) High control precision. The control precision of AC servo motors is guaranteed by the rotary encoder at the rear end of the motor shaft. For example, in Panasonic's fully digital AC servo motors with a 17-bit encoder, the motor rotates once for every 2^17 = 131072 pulses received by the driver, meaning its pulse equivalent is 360°/131072 = 9.89 seconds. This is 1/655 of the pulse equivalent of a stepper motor with a step angle of 1.8°. (III) Strong overload capacity. Stepper motors do not have overload capacity. To overcome the inertial torque of the inertial load at startup, a motor with a rated torque much larger than the load torque must be selected, resulting in wasted torque. AC servo motors have strong overload capacity. For example, the maximum torque of the servo motor in Panasonic's AC servo system reaches three times the rated torque, which can be used to overcome the inertial torque at the moment of startup. (IV) Fast speed response. Stepper motors require 200 to 400 milliseconds to accelerate from a standstill to the rated speed. AC servo systems have a faster speed response. For example, the Panasonic MSMA 400W AC servo motor only requires a few milliseconds to accelerate from a standstill to its rated speed. (V) Good torque-frequency characteristics. The output torque of a stepper motor decreases as the speed increases, and the torque drops sharply at higher speeds. Therefore, its maximum operating speed is generally 300 to 600 RPM. AC servo motors have constant torque output, that is, they can output rated torque within their rated speed (generally 2000 RPM or 3000 RPM). III. Prospects for Servo Motor Control (I) The development of servo motor control technology promotes the high speed and high precision of machining technology. Since the 1980s, CNC systems have gradually used servo motors as driving devices. AC servo motors feature a brushless structure, requiring virtually no maintenance and having a relatively small size, which is beneficial for increasing speed and power. Currently, AC servo systems have largely replaced DC servo systems. In modern CNC systems, the replacement of DC servo systems with AC servo systems and the replacement of hardware control with software control have become the development trend of servo technology. This has led to the development of AC digital drive systems applied to servo feed and spindle devices in CNC machine tools. With the development of microprocessors and fully digital AC servo systems, the calculation speed of CNC systems has greatly increased, and the sampling time has been greatly reduced. The shift from hardware servo control to software servo control has significantly improved the performance of servo systems. For example, the servo control loop of the OSP-U10/U100 network CNC system is a high-performance servo control network that achieves distributed configuration and network connection of various servo devices and components for autonomous control, further enhancing its control capabilities and communication speed over machine tools. These technological advancements have improved servo system performance, reliability, ease of debugging, and increased flexibility, greatly promoting the development of high-precision and high-speed machining technology. Furthermore, the development of advanced sensor detection technology has also greatly improved the dynamic response performance and positioning accuracy of AC motor speed control systems. AC servo motor speed control systems generally use brushless rotary transformers, hybrid photoelectric encoders, and absolute encoders as position and speed sensors, with response times of less than 1μs. Servo motors themselves are also developing towards higher speeds, achieving rapid feeds of 60m/min or even 100m/min and accelerations of 1g when combined with the aforementioned high-speed encoders. To ensure smoother motor rotation at high speeds, the motor's magnetic circuit design has been improved, and high-speed digital servo software ensures smooth, creep-free rotation even at speeds less than 1μm. (II) AC linear servo motor direct drive feed technology has matured. CNC machine tool feed drives include two types: "rotary servo motor + precision high-speed ball screw" and "linear motor direct drive." Traditional ball screw technology is mature, has high machining accuracy, and relatively low cost for achieving high speeds, so it is currently widely used. High-speed machine tools using ball screw drives have a maximum moving speed of 90m/min and an acceleration of 1.5g. However, ball screws are mechanical transmissions, and the elastic deformation, friction, and backlash between mechanical components cause motion lag and nonlinear errors, making it difficult to further increase the movement speed and acceleration of ball screw pairs. Since the 1990s, linear motor direct drive feed drive has been used in high-speed, high-precision large-scale machine tools. It offers advantages over ball screw drives, including higher stiffness, wider speed range, better acceleration characteristics, lower moment of inertia, better dynamic response performance, smoother operation, and higher positional accuracy. Furthermore, linear motor direct drive eliminates the need for intermediate mechanical transmissions, reducing mechanical wear and transmission errors, and minimizing maintenance. Compared to ball screw drives, linear motor direct drive offers 30 times the speed, 10 times the acceleration (up to 10g), 7 times the stiffness, and a maximum response frequency of 100Hz, with significant room for further development. Currently, both drive methods will coexist in the field of high-speed, high-precision machine tools for a considerable period, but the trend indicates that linear motor drives will account for an increasingly larger proportion. All indications suggest that the application of linear motor drives in high-speed and high-precision machine tools has entered a period of accelerated growth. References: [1] Research on AC servo motor control technology, China Test Technology, Zheng Lieqin, 2006.5. [2] CNC machine tools and their use and maintenance, Machinery Industry Press, Lu Bin, 2005.2. [3] Servo motor control technology, Electromechanical Technology, Zhang Chusheng, 2006.5.28.