1 Introduction High speed, precision and modularity are the development directions of modern manufacturing technology. The new cutting theory believes that when the cutting speed reaches a certain level (about 500m/min), the temperature of the cutting zone no longer rises, and the cutting force will decrease instead, and the tool wear will also decrease. In this way, the surface quality and machining accuracy of the parts can be improved at the same time as the productivity. Generally speaking, the cutting speed and feed rate of high-speed machining are an order of magnitude higher than those of conventional machining. Therefore, high-speed spindle and rapid feed system are two key technologies for realizing high-speed machining. The following new requirements are put forward for the feed system: (1) The feed rate must be matched with the high-speed spindle, reaching 60m/min or higher; (2) The acceleration must be large so that the required high speed can be achieved in the shortest time and stroke, at least 1~2g; (3) The dynamic performance must be good, and it can realize rapid servo control and error compensation, and has high positioning accuracy and rigidity. For a long time, the feed system of CNC machine tools has mainly consisted of "rotary servo motors and ball screws." The highest feed speed achievable with this system is 90–120 m/min, and the maximum acceleration is only 1.5g. Furthermore, due to the series of intermediate components such as couplings, lead screws, nuts, bearings, and supports between the motor spindle and the worktable, the elastic deformation, friction, and backlash generated by these mechanical components during start-up, acceleration/deceleration, reversal, and stopping actions cause lag in the feed motion and many other nonlinear errors. These intermediate components also increase the system's inertial mass, affecting the rapid response to motion commands. In addition, the lead screw, being a slender rod, deforms under force and heat, affecting machining accuracy. To overcome the shortcomings of traditional feed systems, simplify machine tool structure, and meet the requirements of high-speed precision machining, researchers began to study new feed systems. Linear motors are the most promising rapid feed systems. They eliminate all intermediate transmission links between the power source and the worktable components, making the length of the machine tool feed transmission chain zero—this is the so-called "direct drive" or "zero transmission." 2. Principle and Classification of Linear Motors A linear motor is a device that uses the principle of electromagnetic interaction to directly convert electrical energy into linear motion kinetic energy. In practical applications, to ensure that the coupling between the primary and secondary windings remains constant throughout the entire stroke, the primary and secondary windings are generally manufactured with different lengths. Similar to rotary motors, linear motors generate a magnetic field in the air gap when three-phase current is applied. If end effects are ignored, the magnetic field is sinusoidally distributed in the linear direction; however, this magnetic field is translated rather than rotated, hence it is called a traveling wave magnetic field. The interaction between the traveling wave magnetic field and the secondary winding generates electromagnetic thrust, which is the basic principle of linear motor operation. Due to the above correspondence between linear motors and rotary motors, each type of rotary motor has a corresponding linear motor, but the structural form of linear motors is more flexible than that of rotary motors. Linear motors can be classified according to their working principle as: linear DC motors, linear induction motors, linear synchronous motors, linear stepper motors, linear piezoelectric motors, and linear reluctance motors; and according to their structural form as: flat plate type, U-shaped type, and cylindrical type. 3. Analysis of the Advantages and Disadvantages of Linear Motors The characteristic of linear motors is that they directly generate linear motion. Compared with "rotary motors and ball screws" that indirectly generate linear motion, their advantages are (see the table below for specific performance details): (1) There is no mechanical contact, and the power transmission is generated in the air gap. There is no friction except for the guide rail. (2) The structure is simple and the size is small. It achieves linear drive with the fewest number of parts and there is only one moving part. (3) The stroke is theoretically unlimited and the performance will not be affected by the change of stroke. (4) It can provide a wide speed range, from a few micrometers per second to several meters per second. High speed is a prominent advantage. (5) The acceleration is very large, up to 10g. (6) The movement is smooth because there are no other mechanical connections or conversion devices except for the linear guide rail or air bearing that plays a supporting role. (7) The accuracy and repeatability are high because the intermediate links that affect the accuracy are eliminated. The accuracy of the system depends on the position detection element. With a suitable feedback device, it can reach the submicron level. (8) The maintenance is simple. Due to the small number of parts and the lack of mechanical contact during movement, the wear of the parts is greatly reduced. It requires little or no maintenance and has a longer service life. Comparison of Linear Motor and Rotary Motor + Ball Screw Transmission Performance Table Performance Rotary Motor + Ball Screw Linear Motor Accuracy (µm/300mm) 10 0.5 Repeatability (µm) 2 0.1 Maximum Speed ​​(m/min) 90～120 60～200 Maximum Acceleration (g) 1.5 2～10 Static Stiffness (N/µm) 90～180 70～270 Dynamic Stiffness (N/µm) 90～180 160～210 Stability (%Speed) 10 1 Settling Time (ms) 100 10～20 Lifespan (h) 6000～10000 50000 The disadvantages of linear motors are: Firstly, the distortion of the magnetic field at the end of the linear motor affects the integrity of the traveling wave magnetic field, increasing the linear motor's losses and reducing thrust. Furthermore, there is a significant thrust fluctuation, which is the unique "edge effect" of linear motors. The structural characteristics of linear motors dictate that end effects are unavoidable. Secondly, linear motors are difficult to control because during operation, changes in load (such as workpiece weight, cutting force, etc.), system parameter perturbations, and various disturbances (such as friction), including end effects, directly affect the motor without any buffering or mitigation mechanisms. If the control system is not robust, it can lead to system instability and performance degradation. Other disadvantages include difficult installation, the need for magnetic shielding, low efficiency, and high cost. In manufacturing, AC linear motors are primarily used to meet the feed system requirements of high-speed machining centers. AC linear motors can be divided into two main categories: induction and synchronous. Although synchronous linear motors are more expensive, more difficult to assemble, and require magnetic field shielding than induction linear motors, they offer higher efficiency, simpler structure, no need for secondary cooling, easier control, and are more likely to achieve the required high performance. Furthermore, with the emergence and development of neodymium iron boron (NdFeB) permanent magnet materials, permanent magnet synchronous linear motors will gradually become the mainstream. Therefore, the proportion of permanent magnet AC synchronous linear motors in high-speed machining centers will continue to increase. 4. Development and Application of Linear Motors The Development History of Linear Motors Abroad The development of linear motors did not begin much later than that of rotary motors. Shortly after the emergence of rotary motors, the prototype of the linear motor appeared, but its development was a winding road. In 1845, the Englishman Charles Wheastone invented the world's first linear motor, but this model suffered from low efficiency due to an excessively large air gap, and was unsuccessful. By the mid-20th century, advancements in control, electronics, and materials technologies provided theoretical and technical support for the development of linear motors, ushering in a new stage of development. Professor E.R. Laithwaite of Britain was a pioneer in the development of modern linear motors. He emphasized fundamental research, and his research group achieved many important results. Another notable figure is Professor Hajime Yamada of Japan, who authored several books on linear motors. After the 1970s, the applications of linear motors became even more widespread, including automatic plotters, liquid metal pumps (MHD), electromagnetic hammers, light industrial machinery, household appliances, air compressors, and semiconductor manufacturing equipment. Since the 1990s, with the introduction of the concept of high-speed machining, linear motors have begun to appear as feed systems in machining centers. Due to the unparalleled advantages and potential of direct-drive feed systems compared to traditional feed systems, they have once again attracted attention from various countries. According to reports, the sales of linear motors and drive units in the United States reached $45.53 million in 1997, and were projected to reach $107.72 million in 2002. As an electromechanical system, linear motors simplify mechanical structures while complicating electrical control, aligning with the development trend of modern electromechanical technology. Anorad Corporation of the United States is the world's most famous manufacturer of linear motors. The company launched a brushless DC linear motor in 1988 and obtained a US patent. The company mainly produces permanent magnet synchronous linear motors, forming a series of products with different structures and power ratings, widely used in various fields. Indramat of Germany produces both induction linear motors and permanent magnet linear motors, with more than 50 models. Permanent magnet linear motors are characterized by high efficiency (up to 1.72 N/W) and high thrust density. Reports indicate that their products can reach speeds of 600 m/min and thrust of 22 kN. To reduce the price of linear motors, Trilogy introduced the Linear Encoder Module (LEM). It utilizes the motor's magnetic field to provide position feedback, independent of stroke. It can operate in harsh environments, providing commutation signals comparable to full-stroke sensors, with a resolution and repeatability of 5 µm. Other linear motor manufacturers offer products with unique features; for details, please refer to "High-Frequency Response DC Linear Motors" by Liu Jinling et al. (published in *Micromotors*, Issue 4, 1993). Applications in Machine Tools and Machining Centers: The application of linear motors in high-speed machining centers and other large-stroke CNC machine tool feed systems is a relatively recent development. Machine tools equipped with linear motors must have advanced CNC systems, high rigidity and natural frequency, and minimize the mass of moving parts to fully utilize the linear motor's capabilities. Furthermore, the design of direct-drive feed systems in machine tools must consider cooling and heat dissipation. To prevent chips and various powders from being attracted by the open magnetic field of the linear motor, magnetic shielding and anti-magnetic measures must be implemented. Furthermore, unlike lead screws, linear motors are not self-locking. If the motor is installed vertically, factors such as counterweights and braking must be considered. The collaboration between Ford, Ingersoll, and Anorad in the mid-1980s initially realized the application of linear motors in machine tools. Ford wanted machine tools that were high-speed, high-precision, and highly flexible. The result of this collaboration was Ingersoll's "high-speed module" HVM800, which featured permanent magnet linear motors from Anorad on all three axes, achieving excellent performance. In 1993, the German company Ex-Cell-O exhibited the world's first high-speed machining center with a linear motor-driven worktable, the XHC240, at the European Machine Tool Exhibition in Hanover, Germany. It used induction linear motors developed by the German company Indramat, achieving axis movement speeds of up to 80 m/min and accelerations of up to 1g. Subsequently, many manufacturers launched machining centers equipped with linear motors. Statistics show that 300 machine tools using linear motors were sold in 1997, and this number was projected to increase to 3,000 by 2005. Ten years from now, 20% of CNC machine tools will be equipped with linear motors. Besides cutting machine tools, other machine tools such as laser cutting, plasma cutting, and EDM equipment will also begin to use linear motors. Domestic Research Status of Linear Motors Although many institutions in China are researching linear motors, three universities are primarily focusing on their application as feed systems for machine tools or machining centers: Guangdong University of Technology established the "Ultra-High-Speed ​​Machining and Machine Tool Research Laboratory," mainly researching and developing "ultra-high-speed electric spindles" and "high-speed feed units for linear motors." They are researching linear induction motors and have developed the GD-3 type high-speed CNC feed unit for linear motors, with a rated feed force of 2kN, a maximum feed speed of 100m/min, a positioning accuracy of 0.004mm, and a stroke of 800mm. Since the late 1990s, Shenyang University of Technology has been researching permanent magnet linear synchronous motors and has manufactured a prototype with a thrust of 100N. Another focus of their research is the control method and servo system of the motor, and they have published several papers on this topic. The Institute of Manufacturing Engineering, Department of Precision Instruments and Mechanics, Tsinghua University, has successfully developed a high-frequency response DC linear motor with a stroke of up to 5 mm, a cutoff frequency greater than 250 Hz, and a thrust of several hundred Newtons. This motor is used to drive the transverse tool post of a cam-type variable piston lathe and has achieved good application results in actual machining. Currently, research is underway on a long-stroke permanent magnet linear servo unit, with a rated thrust of 1500 N, a maximum speed of 60 m/min, a maximum no-load acceleration of 1 g, and a stroke of 600 mm. It should be noted that in China, research on linear motors, especially linear servo motors for machine tool feed systems, is still in its early stages. Researchers and funding are significantly insufficient, progress is slow, and the gap with foreign countries is widening. Strengthening research is now urgent. To break the foreign technological monopoly, it is necessary to combine technology tracking with independent development, and strengthen research on basic and key technologies. 5. Development Trends and Research Directions Development Trends Currently, the development of direct drive technology for linear motors shows the following trends: Linear servo motors used in machine tool feed systems will primarily be permanent magnet type; Integrating the motor, encoder, guide rail, and cables reduces motor size, facilitating installation and use; Modularizing functional components (guide rail, encoder, bearing, connector, etc.); Emphasis is placed on the development of related technologies, such as position feedback elements and control technology, which are fundamental to improving linear motor performance. Research Directions The research goal of linear motors is to improve motor performance to meet application requirements. The main performance characteristics of linear motors include speed, acceleration, thrust and its fluctuation, positioning accuracy, repeatability, mechanical characteristics (speed-thrust characteristics), transient performance (speed response), and thermal characteristics. As an electromechanical system, improving performance can be approached from two aspects: structure and control. Structural Design Linear motors include primary and secondary magnetic circuit structures, as well as mechanical structures for support, sensing and measurement, cooling, dust prevention, and protection. Magnetic Circuit Design The most important task of magnetic circuit design is to ensure that the motor's thrust and thrust fluctuation meet design requirements. Calculating the magnetic field distribution within a linear motor is fundamental to magnetic circuit design. Due to its unique structure, linear motors exhibit end effects, causing magnetic field distortion. Furthermore, the use of soft magnetic materials like silicon steel sheets to aggregate the magnetic circuit results in intricate and complex medium boundaries, leading to a highly nonlinear and nonlinear magnetic circuit. Traditional equivalent magnetic circuit methods or graphical methods would introduce significant errors, or even be impossible to implement. Therefore, numerical methods—primarily the finite element method (FEM)—are widely used to calculate the magnetic field distribution of linear motors, thereby further calculating thrust, its fluctuations, and vertical force. Many excellent electromagnetic field FEM software programs are available on the market; therefore, the key to calculating the electromagnetic field of a linear motor using FEM lies in establishing an accurate finite element model. Reducing thrust fluctuations is both a key focus and a challenge in magnetic circuit design. Causes of thrust fluctuations include: high-order harmonics in the primary current and back electromotive force, non-sinusoidal air gap magnetic flux density waveforms, cogging effects, and end effects. Thrust fluctuations can be reduced by optimizing the shape and arrangement of permanent magnets, reducing permanent magnet excitation flux density, using a coreless and multi-pole structure in the primary winding, increasing the number of slots, and increasing the air gap. However, some measures may weaken other performance characteristics. Therefore, design requirements should be comprehensively considered to achieve the best results. Mechanical Structure Design: Mechanical structures involve many issues. Here, we only emphasize the study of the cooling system, as this issue is easily overlooked. In fact, thermal characteristics are an important characteristic of linear motors. The peak thrust of a motor with cooling is twice that without cooling, so the quality of the motor's cooling system has a significant impact on its performance. Optimizing the cooling system is a shortcut to improving motor performance. The analysis of motor thermal characteristics generally uses the finite element method, and the cooling system is optimized based on the calculation results. Control Technology Research: Control technology is another key and challenging aspect of linear motor design. Linear servo systems directly drive the load during operation, meaning load changes directly affect the motor. External disturbances, such as changes in workpiece or tool quality and cutting force, also act directly on the motor without attenuation. Changes in motor parameters directly impact its normal operation. Linear guides exhibit friction, and linear motors also suffer from cogging and end-effector effects. These factors all contribute to the difficulty of controlling linear motors. Control algorithms must suppress or compensate for these disturbances; otherwise, the control system is prone to instability. Generally, the controller design should meet the following requirements: high steady-state tracking accuracy, fast dynamic response, strong anti-interference capability, and good robustness. Different linear motors or applications place different demands on control algorithms, so appropriate control methods must be adopted based on specific circumstances. Currently, the control strategies used for linear servo motors mainly include traditional PID control and decoupling control, as well as modern control methods such as nonlinear control, adaptive control, sliding mode variable structure control, H∞ control, intelligent control such as fuzzy control, and artificial intelligence (such as artificial neural network systems) control. It can be seen that the control algorithm of linear motors involves a large amount of computation and has strong real-time requirements in practical applications of high-speed machining feed systems, thus placing high demands on the entire CNC system. To meet these requirements, high-performance hardware should be used while optimizing the control algorithm. High-speed machining center feed systems typically employ fully digital drive technology, using a PC as the basic platform and a DSP for interpolation and servo control. Although controlling linear motors is much more difficult than controlling rotary motors, their electromagnetic characteristics and operating principles are basically similar, and the servo control technology for rotary motors is relatively mature. Therefore, in the experimental research phase, to quickly establish an experimental system and verify the feasibility of the design, we can also modify the servo controller of the rotary motor into a servo controller for the linear motor. This can reduce the development cost and cycle, and also provide guidance for the development of dedicated linear motor servo controllers. Experimental and theoretical research are the foundation of design, but ultimately, determining the performance of the motor still relies on specific experiments. The performance testing technology for rotary motors is mature and standardized, but there is no unified method for testing the performance of linear motors. Therefore, researching efficient and accurate performance testing methods for linear motors is also a very important topic, and it also promotes theoretical research. The key to experimental research lies in the accurate measurement of various parameters such as velocity, acceleration, static force, dynamic force, displacement, and temperature. If necessary, a specialized test bench must be designed. The design scheme is optimized based on theoretical calculations, and a prototype is manufactured. Performance tests are then conducted on the prototype to verify the correctness of the design. A high-performance linear motor often requires numerous repeated calculations and experiments to manufacture.