As a crucial functional component of CNC machine tools, the characteristics of servo drives have always been a key indicator affecting the machining performance of CNC machine tools. In recent years, various servo drive technologies have been developed both domestically and internationally to improve the dynamic and static characteristics of servo drives. It is foreseeable that with the development of advanced technologies such as high-speed cutting, ultra-precision machining, and networked manufacturing, fully digital AC servo drive systems with network interfaces, linear servo systems, and high-speed electric spindles will become a focus of attention in the machine tool industry and represent the future development direction of servo drive systems.
I. AC Permanent Magnet Synchronous Servo Drive Device
Servo drive technology has gone through three stages: DC servo devices, DC brushless servo devices, and AC permanent magnet synchronous servo drives. With the increasing demands for high speed, high precision, and high efficiency in modern manufacturing, AC permanent magnet synchronous servo drives offer advantages such as high response, maintenance-free operation (no carbon brushes, commutators, or other wear-prone components), and high reliability. Utilizing microprocessor technology, high-power, high-performance semiconductor power device technology, and advanced motor permanent magnet material manufacturing processes, coupled with a favorable performance-price ratio, they have become one of the fundamental technologies for automation in the industrial field.
Currently, two development trends can be observed in servo drive devices:
1. Fully digital
Full digitalization is an inevitable trend in the future development of servo drive technology. Full digitalization includes not only the digitalization of the internal control of the servo drive and the digitalization of the interface between the servo drive and the CNC system, but also the digitalization of the measurement unit. Therefore, the full digitalization of the three loops within the servo drive unit, the fieldbus connection interface, and the digital connection interface from the encoder to the servo drive are important indicators of full digitalization.
With the continuous improvement of microelectronics manufacturing processes, the adoption of new high-speed microprocessors, especially digital signal processors (DSPs), has led to a geometric increase in computing speed. The digitization of the three-loop control (position loop/speed loop/current loop) within a servo drive is a crucial prerequisite for ensuring its high response, high performance, and high reliability. All control calculations of the servo drive can be performed by its internal DSP, achieving the requirements of high-speed real-time control of the servo loop. Some products even integrate the peripheral circuitry for motor control with the DSP core, enabling the implementation of new control algorithms such as speed feedforward, acceleration feedforward, low-pass filtering, and dip filtering.
Traditional analog control interfaces for servo drives are susceptible to external signal interference and have short transmission distances. The pulse-type control interfaces widely used in servo drive devices in my country are not true digital interfaces. These interfaces are limited by pulse frequency and cannot meet the requirements of high-speed, high-precision control. Adopting a digital control interface using a fieldbus is a necessary condition for servo drive devices to achieve high-speed, high-precision control. Therefore, in recent years, foreign companies have launched their own digital interface protocols and standards, such as FANUC's Serial Servo Bus (FSSB), Siemens' Profibus-DP, Mitsubishi's CC-link, and Xerox's SERCOS.
Full digitization has extended to the digitization of measurement unit interfaces. HEIDENHAIN of Germany has unified the subdivision functions of various encoder types, such as absolute, incremental, and sine/cosine encoders, into the EnDae2.2 encoder connection protocol. The subdivision process is completed internally within the encoder and then connected to the servo drive via a digital interface—this is true full digitization.
2. High performance
It features high precision, high dynamic response, high rigidity, high overload capacity, high reliability, high electromagnetic compatibility, high grid adaptability, and high cost-effectiveness.
At the 2005 Hannover Messe, FANUC of Japan launched its HRV4 servo control technology. The HRV4 servo system inherits and further develops the advantages of the HRV3, featuring the following characteristics: it employs nanometer-level position commands at all times, utilizing a high-resolution αi pulse encoder with 16 million revolutions per second (16 million rpm) to achieve nanometer-precision servo control; the HRV4 ultra-high-speed servo control processor can control motor speeds up to 60,000 rpm; and the HRV4 control algorithm reduces the maximum control current of the servo motor by 50% and motor heat generation by 17%, thus enabling the servo drive to achieve higher rigidity and overload capacity.
Companies are paying more attention to the dynamic characteristics of servos. For example, at the 2005 Hannover Messe, Heidenhain demonstrated the impact of encoders with different resolutions on torque ripple, revealing that increasing encoder resolution can significantly reduce torque ripple in servo drives.
The development of electronic power technology has enabled the switching frequency of power components in the main circuit of servo systems to increase from 2-5kHz to over 10kHz. The adoption of advanced devices such as high-power insulated gate bipolar transistors (IGBTs) and intelligent control power modules (IPMs) has significantly reduced the power consumption of the servo driver output circuit, improved the system's response speed and stability, and reduced operating noise. These advancements have not only laid the foundation for fully digital, high-speed, and high-precision AC servo systems but have also led to the miniaturization of AC servo systems.
II. Direct Drive Technology
Compared to traditional screw drive systems, the biggest advantage of direct drive technology, including high-thrust linear servo drives and high-torque servo drives, is the elimination of all mechanical transmission links between the motor and the moving/rotating worktable, effectively reducing the length of the machine tool feed transmission chain to zero. This "zero-transmission" method delivers performance performance that screw drives cannot achieve, such as acceleration exceeding 3g, which is 10 to 20 times that of traditional drive systems, and feed speeds that are 4 to 5 times faster.
Direct drive technology is the ideal drive mode for high-speed, high-precision CNC machine tools, attracting significant attention from machine tool manufacturers and experiencing rapid technological development. In recent years, dozens of international companies have showcased high-speed machine tools driven by linear motors, with some manufacturers increasing machine tool acceleration to 2-3g and rapid traverse speed to 150-240m/min. MAZAK will launch a supersonic machining center based on a linear servo system, boasting a cutting speed of Mach 8, a maximum spindle speed of 80,000 rpm, a rapid traverse speed of 500m/min, and an acceleration of 6g. This signifies that the second generation of high-speed machine tools, represented by linear servos, has overcome the drawbacks of linear motors, including heat generation, protection issues, and high cost, and is gradually moving towards practical application.
In late 2005, the author visited the Mori Seiki factory in Japan and was most impressed by the extensive use of self-made high-torque servo drives on Mori Seiki's machine tools. Compared with traditional worm gear drives, high-torque servo drives greatly simplify the structure of the machine tool's rotary axis, significantly increase the rotary table's speed, and greatly improve dynamic response.
III. High-speed electric spindle
An electric spindle is a product that integrates an electric motor and a spindle. It directly mounts the stator and rotor of the spindle motor inside the spindle assembly, with the motor's rotor becoming the rotating part of the spindle. By eliminating the transmission and connection between the gearbox and the motor, it achieves an integrated, "zero-transmission" spindle system. Therefore, it has advantages such as compact structure, light weight, low inertia, and good dynamic characteristics. It also improves the dynamic balance of machine tools and avoids vibration and noise, leading to its widespread application in ultra-high-speed machine tools. Electric spindles are generally driven using vector control frequency conversion technology.
Foreign high-speed machining spindle speeds are generally between 12,000 and 25,000 r/min, with the highest reaching 70,000 to 80,000 r/min.
IV. Current Status of Servo Drive Systems in my country
In the early 1980s, my country introduced, digested, and absorbed advanced foreign technologies, and during the Seventh, Eighth, and Ninth Five-Year Plans, it made significant progress in servo drive technology.
In the 1980s, my country spent a huge sum of money to import Siemens' servo drive technology. However, because the imported technology was outdated and obsolete, and there was no breakthrough in independent assimilation and absorption, industrialization was not achieved. This painful historical lesson taught everyone a fundamental truth: for strategic high technologies like servo drives, simply importing them with money is not feasible; blindly imitating foreign countries will only lead to falling behind, being vulnerable to attack, and being controlled by others; the only way out is to take the path of independent innovation.
Huazhong University of Science and Technology (HUST) is one of the birthplaces of my country's independently developed servo drive technology. During the Eighth Five-Year Plan period, the Department of Automation and the Department of Electrical Engineering at HUST began research and development of servo drives. In 1996, the Department of Automation collaborated with Huazhong Numerical Control to jointly develop a microcontroller-based analog-digital hybrid (analog current loop) AC servo drive and spindle drive (HSV-9 series). Later, they developed a DSP-based fully digital AC servo drive device (HSV-16/18/20) and put it into mass production. To date, more than 30,000 units have been produced and sold, and it has been recognized as a major national achievement and a key national new product. The servo drive technology developed by the Department of Electrical Engineering at HUST in collaboration with Guangzhou Numerical Control and Shanghai Kaitong Numerical Control has also been industrialized.
Since 2003, my country's economical CNC systems have begun to largely transition from stepper drive systems to servo drive systems. The servo drive technology, independently developed by the Chinese, has made significant contributions to the upgrading of my country's CNC system industry, and the servo drive industry itself has also made great progress.
Currently, my country's servo drive systems lag behind foreign products in terms of performance and reliability. Particularly in fully digital, high-performance servo drive technology, a significant gap remains between China and leading international companies, becoming a bottleneck hindering the development of my country's mid-to-high-end CNC system industry.
V. Features of Huazhong CNC Servo Products and Technologies
Since its establishment, Huazhong Numerical Control Co., Ltd. has maintained close cooperation with the Department of Control Science and Engineering of Huazhong University of Science and Technology in the development of servo products. It was the first in China to successfully develop and mass-produce the HSV-9 AC servo drive device based on a microprocessor. Later, it undertook the research and development tasks of seven national-level projects, including the National "15th Five-Year Plan" Key Project "Research, Development and Application of All-Digital AC Servo Drive System and AC Spindle Servo Control System", the National "15th Five-Year Plan" 863 Project "EQ6110HEV Hybrid City Bus Motor and Its Control System", the National Technological Innovation Project "All-Digital Intelligent AC Feed Drive Unit", and the SME Innovation Fund Project "Engineering Research of High-Performance Servo Drive System".
In recent years, Huazhong Numerical Control has developed a series of AC servo drive units and AC servo motors with independent intellectual property rights.
The HSV-16 series all-digital AC servo drive unit, as a new product, further improves and perfects product performance. The driver's hardware environment consists of a high-performance dedicated digital signal processor (DSP) for motor control, a large-scale field-programmable logic array (FPGA), and an intelligent power module (IPM). It is simple to operate, highly reliable, compact in size, and easy to install. It features self-testing and self-diagnostic functions, and comprehensive software and hardware protection against various fault conditions such as short circuits, overcurrent, overvoltage, undervoltage, pump-up, and overheating.
HSV-18D Series All-Digital AC Servo Feed Drive Unit and Spindle Unit
Designed using the latest technology, it is directly powered by AC 380V and can realize closed-loop or full closed-loop control of torque, speed and position for various AC motors, including AC permanent magnet synchronous motors, brushless DC motors and AC induction motors (AC asynchronous motors).
To improve product reliability, and considering the harsh operating environments for domestic users, Huazhong CNC's servo drive devices are manufactured according to European Level IV electromagnetic compatibility standards, which are twice as high as the national standard (European Level III electromagnetic compatibility). Huazhong CNC products can operate within a voltage range of ±20%, while the national standard requires a voltage range of +10% to -15%.
AC servo motors are one of the key components of CNC systems. In the past, they were mainly imported, which was not only expensive but also lacked variety and had long delivery cycles. Core technologies were controlled by foreign companies, and system technology upgrades were difficult. Motor costs, types, and delivery times severely restricted the development of the CNC system industry. To address this, Huazhong CNC began preparing for independent research and development of its AC servo motor series in 2000, investing in and establishing Shanghai Dengqi Electromechanical Technology Co., Ltd., to develop the GK6 and GK7 series of permanent magnet synchronous AC servo motors and the GM7 series of AC servo spindles with independent intellectual property rights.
In the early stages of development, extensive reference was made to advanced technologies both domestically and internationally, including those from Siemens (Germany), Yaskawa (Japan), Sanyo (Japan), and Panasonic (Japan). Combined with the needs of domestic users and the market, the GK6 series AC permanent magnet synchronous servo motors were successfully developed independently in 2001. This series of motors adopts a stator lamination stacked and exposed structure for direct heat dissipation without gaps; it uses ultra-high intrinsic coercivity rare-earth permanent magnet materials to form an air gap magnetic field; and it utilizes a domestically developed production process and equipment that allows for the integral magnetization of the rotor's oblique magnets, ensuring that the produced servo motors are free of magnetic adhesion, reducing losses, lowering magnetic field vibration and noise, and achieving higher performance. The integral machining process results in low motor vibration and smooth rotation. Currently, the rated torque of the products ranges from 0.6 to 1000 Nm, and the rated speeds are 1000, 1200, 1500, 2000, 3000, and 6000 r/min. The successful launch of this series of motors has provided Huazhong CNC and other domestic CNC companies with cost-effective, timely-delivered servo motors that meet their specific needs.
Building upon the successful development and commercialization of AC servo motors, and to better integrate them with CNC machine tools, the GM7 series AC variable frequency spindle motors were developed in 2004. Currently, the rated power of these products ranges from 2.2kW to 100kW. They employ a Class F special insulation structure, resisting surge current and corona discharge, resulting in long lifespan and high reliability. They feature an internal air duct and an integrated stator lamination disc pressing structure, achieving high heat dissipation efficiency and a compact size. The use of integrated machining processes and high-precision dynamic balancing technology significantly improves high-speed, high-precision, and reliable operation capabilities.
Huazhong Numerical Control boasts the most comprehensive range of AC servo motors and AC variable frequency spindle motors in China. Its production volume doubles annually, reaching a capacity of 20,000 units per year. In 2006, Huazhong Numerical Control's application for the National Development and Reform Commission's "Equipment Localization" special project—the industrialization of fully digital servo drives and motors—passed expert review. The company plans to invest 70 million yuan to build a new 10,000m² servo drive and motor production plant, aiming to achieve an annual production capacity of 100,000 sets of servo drives and motors.
Huazhong CNC has also developed a variety of dedicated servo products to meet market demands. Its high-power servo drive has been used in the balancing system of a tilting train carriage at a factory in Chongqing. Its switched reluctance motor controller has been applied to hybrid electric buses in Wuhan. The industrial sewing machine drive system, produced in cooperation with a German company, has been sold in batches. The GK6 and GM7 servo spindle motors have been mass-produced and are now used in all-electric injection molding machines manufactured by a well-known Chinese injection molding machine manufacturer. A servo drive with an output torque of 1000 Nm has been used in my country's first electric screw press developed by Ezhou Machine Tool Plant.
VI. Suggestions for Developing my country's Servo Drive System Industry
As crucial functional components of CNC machine tools, servo drives and servo motors significantly impact the performance of CNC systems. Servo drives and servo motors account for 1/2 to 3/4 of the total cost of a CNC system. Therefore, mastering independent servo drive and servo motor technologies is a decisive factor in a CNC system manufacturer's overall market competitiveness. Servo drives involve high-power electrical control, making them a weak link and bottleneck in reliability. Successful international CNC system companies all possess independent servo drive technology and self-sufficiency capabilities. Developing my country's independent servo drive technology and industry is imperative. The following suggestions are made:
1. Vigorously strengthen research on the production process technology of servo drive devices.
Building upon the achievements of the National 863 Program, the Innovation Fund for Small and Medium-sized Enterprises of the Ministry of Science and Technology, and the National "Tenth Five-Year Plan" key technologies, my country's control platform technology for servo drive devices has basically matured. The focus of product development is on reliability design and assurance measures, product structure and serialization design, and research on mass production processes. This will enable products to meet relevant national standards, satisfy rapidly growing market demand, and expand the market share of domestically produced servo drive devices.
2. Conduct research on performance evaluation and improvement technologies for high-performance servo drive devices.
Increase investment in testing methods for servo drive devices, conduct performance evaluations of servo drives, and provide clear technical trends, accurate product information, and reasonable procurement suggestions for the development of servo drive technology in my country based on independent, objective, scientific, and practical technical tests.
Based on this, support domestic enterprises in developing high-performance servo drive devices. Narrow the technological gap with foreign high-performance servo drive products in terms of high precision, high dynamic response, high rigidity, high overload capacity, high reliability, high electromagnetic compatibility, high power grid adaptability, and high cost-effectiveness, thereby improving the reliability of Chinese products.
3. Develop Chinese standards for digital interfaces of servo drives and CNC systems.
By using the common technology of digital interface specifications and standards for CNC systems and servo drives as a link, cooperation and alliances can be formed within the domestic CNC system industry. This will allow each sector to leverage its strengths, shorten development cycles, reduce development costs, and create synergy, thereby forming a combined force to compete with foreign CNC system manufacturers and enhance the competitiveness of my country's CNC machine tools and systems in the international market. Simultaneously, mastering the discourse power regarding digital interface protocols and standards for CNC systems and servo drives will improve my country's standing in international standardization organizations, regulate the domestic CNC market, influence international CNC companies, and establish technological barriers to protect the development of my country's CNC system industry. This can narrow the technological gap between my country's CNC system industry and that of foreign countries, changing the current situation of low-price, low-level competition among domestic CNC system manufacturers.
Increase investment in the development of high-thrust linear servo drive devices, high-torque servo drive devices, and high-speed spindle drive devices.
Direct servo drive technology represents the future direction of servo drive technology development. While CNC machine tools employing direct servo drive technology eliminate intermediate transformation stages, achieving so-called "zero transmission" and resulting in a more rational system structure, this new application technology still faces many practical technical challenges. These include the control system's resistance to interference from uncertain factors such as parameter perturbations and load disturbances; the forced cooling of the servo motor; the system's rapid energy absorption braking; and stringent dust and magnetic shielding measures. All these practical problems require further solutions and technological refinement.
