1. Methods of Obtaining Linear Motion The various motions in machinery can be broadly classified into linear motion, rotary motion, and curvilinear motion, among which linear motion and rotary motion are the two most basic. There are many ways to obtain linear motion: (1) Hydraulic cylinders and pneumatic cylinders: Hydraulic and pneumatic pressures drive hydraulic and pneumatic cylinders respectively to obtain linear motion, but oil tanks (oil pump stations), air compressors, etc. are required. The system structure is complex, and the motion is not precise or stable, but it is widely used. (2) Rotary motors and lead screws: Rotary motors convert rotary motion into linear motion through lead screw nuts. The structure is stable and reliable, and the transmission accuracy is high. It is currently the most commonly used method of linear motion. (3) Linear motors: Linear motors directly generate linear motion to drive the worktable. This method is the simplest and most ideal. It has fewer transmission links, does not require conversion through a lead screw from a rotary motor, and has no transmission gap (including backlash). Therefore, it has high accuracy and is known as "direct transmission", "backlash-free transmission", and "zero transmission". It has broad application prospects. 2. The principle and structure of a linear motor (1) The principle of a linear motor The principle of a linear motor is the same as that of a rotary motor. A rotary motor is expanded radially to become a linear motor, as shown in Figure 1. The stator is equivalent to the primary of the linear motor, and the rotor is equivalent to the secondary of the linear motor. When a three-phase sinusoidal current is passed through the primary, a magnetic field is generated in the air gap between the primary and secondary. Since this magnetic field is translational rather than rotating, it is called a traveling wave magnetic field. Under the action of the traveling wave magnetic field and the secondary, an electromagnetic thrust is generated, which causes the secondary to move linearly. In this type of linear motor converted from a rotary motor, when the secondary moves, the coupling between the primary and secondary will change, and the electromagnetic thrust will change accordingly. Therefore, the secondary should be shortened to keep the coupling constant. The length of the primary can be determined according to the required stroke length, as shown in Figure 1c. In principle, a long primary and short secondary or a short primary and long secondary are both possible. Considering that the manufacturing cost and operating cost of a short primary are relatively low, the short primary and long secondary method is generally adopted, as shown in Figure 1c. The linear motor shown in Figure 1 has only one primary and is called a single-sided linear motor. There is a large normal force between the primary and secondary of this type of linear motor. If a primary is installed on both sides of the secondary, the normal attraction force can cancel each other out and the thrust increases. This is called a double-sided linear motor, as shown in Figure 2a. It can also be a structure with one primary and two secondarys, as shown in Figure 2b. (2) Types of linear motors According to the different working principles of rotary motors, linear motors have corresponding types. In addition, linear motors have different structural forms to adapt to different applications, as shown in Figure 3. Linear DC motors: There are two types: frame type and voice coil type. The frame type uses a soft iron to close the excitation flux or permanent magnet flux, and the DC coil or frame moves. The voice coil motor adopts a coreless moving coil structure. When the voice coil is supplied with DC current, it moves in the fixed magnetic field. Because the moving coil has a small inertia, it is mostly used for small displacement high-frequency reciprocating motion. Linear induction motors: These use a three-phase AC coil as the primary winding, with the secondary winding potentially trapezoidal and connected by conductive plates or rails. Linear thrust is generated when the mover's speed differs from the primary winding's traveling wave speed; they are asynchronous motors. While the secondary winding structure is simple, it requires complex vector control methods. The primary winding generates significant heat, resulting in low efficiency. Linear synchronous motors: The primary winding is an AC coil that generates a traveling wave magnetic field, while the secondary winding has a fixed magnetic field. There are two types: excitation-type and permanent magnet type. When AC current is applied to the primary winding, the mover moves synchronously with the traveling wave magnetic field. Commutation methods can be divided into brushed and brushless types. Brushed commutation uses brushes, which is low-cost, but the brushes are prone to deformation, poor contact, wear, and aging, and produce sparks during operation. Brushless commutation uses electronic commutation, which is non-contact and safe and reliable. The primary coil of a linear synchronous motor can be enclosed in a steel core, aluminum core, or resin core. Using an aluminum or resin core avoids thrust fluctuations caused by changes in cogging magnetic reluctance, a phenomenon known as the cogging effect, resulting in smoother movement. Using a core made of stacked silicon steel sheets results in high thrust, high heat generation, and the cogging effect, requiring cooling. Single-sided flat plate types are commonly used in CNC machine tools. Linear stepper motors: The primary is an AC coil with equidistant cogging teeth, and the secondary is a steel plate with the same tooth pitch and cogging teeth as the primary. When three-phase AC current is applied to the primary, thrust is generated between the primary and secondary poles, causing stepping movement. This type of motor often uses open-loop control, has a simple structure, and is inexpensive, but its accuracy is low and its thrust is small. It should be classified as a type of synchronous motor. Linear piezoelectric motor: It utilizes the inverse piezoelectric effect of piezoelectric crystals, that is, when the crystal is excited by an external electric field, it deforms in certain directions. Its deformation is linearly proportional to the strength of the external electric field. Piezoelectric ceramic materials are usually used. Linear displacement is generated by controlling the strength of the external electric field. Since the stroke is very short, only a few micrometers, it is mostly used for micro-feeding with small thrust and high precision. Multiple piezoelectric crystals can be bonded together to expand the stroke to a few millimeters. Linear reluctance motor: The primary is a three-phase AC coil, which generates a traveling wave magnetic field when energized. The secondary direct-axis reactance and quadrature-axis reactance are not equal. Linear thrust is generated by utilizing the salient pole effect. This type of motor has a simple structure and low cost, but the thrust-current ratio is small, the heat generation is large, and the efficiency is low. In summary, for linear motors used in CNC machine tools, AC linear motors are of course the first choice. There are two main types: induction type and synchronous type. (3) Structure of linear motor Taking the permanent magnet linear synchronous motor as an example, its structural diagram is shown in Figure 4. As shown in Figure 4, it is a single-sided flat-plate permanent magnet linear synchronous motor, consisting of a primary, secondary, guide rails, position detection device, base plate, and control system. The primary is short, composed of a silicon steel core and AC coils, and has a cooling water system to prevent overheating. The secondary consists of a soft iron base plate and permanent magnets. The permanent magnets are composed of multiple pieces spliced ​​together and glued to the soft iron base plate; the shape and arrangement of the permanent magnets need to be designed. The primary and secondary are connected by two linear rolling guide rails to ensure smooth movement, straightness, and uniform air gap. The length of the secondary is determined by the required stroke and can be serialized, theoretically without limit. A linear position detection device using a grating or magnetic ruler is used, and the system is a closed-loop control. The motor base plate has mounting holes for fixing the motor to a machine tool or other device. When used on a machine tool, issues such as chip and cutting fluid protection must be considered. 3. Characteristics of Linear Motors For linear motion, linear motors can directly generate linear motion, while most rotary motors currently use lead screws and nuts to convert linear motion. Compared with the former, the characteristics of linear motors can be summarized as follows: (1) Simple structure: linear movement is achieved with a single moving part, and there is only friction at the guide rail. It is easy to maintain and has a long service life. In contrast, linear feed motion on CNC machine tools is mostly obtained through AC servo motors and ball screws, which have a relatively complex structure and many friction links. (2) Wide speed range: from a few micrometers to several meters per second, it has the characteristics of high speed and is suitable for high-speed cutting. In contrast, the speed range of AC servo motors and ball screws is relatively small. (3) Large acceleration: up to 10g. (4) Smooth motion: there is only a supporting guide rail between the primary and secondary stages, and no other mechanical links. (5) High precision and repeatability: because there are no intermediate transmission links, the system precision depends on the guide rail, position detection device and control system, and can reach the submicron level. (6) There are end effect and cogging effect, which cause thrust fluctuation. If the robustness of the control system is not strong, it will cause system instability and performance degradation. End effect refers to the fact that the two ends of the secondary pole of the linear motor are open, so the end magnetic field is distorted, affecting the integrity of the traveling wave magnetic field, increasing motor loss, reducing thrust, and thus causing thrust fluctuation. Cogging effect is caused by the non-uniform spatial distribution of the air gap magnetic field due to the slotting of the silicon steel core. The magnetic flux entering the primary winding core through the air gap is different at different spatial positions, which is caused by the change of cogging magnetic resistance. Both effects should be solved from the motor structure and control system. (7) The control system is difficult. Currently, a fully digital servo control system based on digital signal processor (DSP) can be used. (8) Installation is difficult. It can be directly installed on the machine tool, with position accuracy requirements, and there are problems such as chip protection, coolant protection and safety protection. (9) Although there are products on the market, the technology is not mature enough, and the cost is high and the price is expensive. Table 1 compares the transmission performance of rotary motors and ball screws with that of linear motors. Currently, the linear motors used in CNC machine tools and machining centers are mostly synchronous and induction types, each with its own performance characteristics. Table 2 compares their performance. It can be seen that synchronous linear motors have a simple structure, do not require secondary cooling, have high efficiency, and are easy to control, but they are more expensive, difficult to assemble, and require shielding of the magnetic field. However, with the development of new permanent magnet materials such as neodymium iron boron (NdFeB), they will become the mainstream in CNC machine tools and machining centers. 4. Application of Linear Motors In 1845, the Englishman Charles Wheatone invented the world's first linear motor. Due to poor manufacturing and low efficiency, it was not used and remained stagnant for a long time, monopolized by rotary motors. It was not until the mid-20th century that, due to the development of control systems, materials, and manufacturing technology, linear motors were put on the agenda and received attention from countries around the world. Linear motors have a wide range of applications, such as train drive (maglev train), material handling, machine tool operation, food and light industrial machinery, automatic plotters, hydraulic metal pumps, air compressors, electromagnetic hammers, household appliances, and semiconductor devices. From the perspective of CNC machine tools and machining centers, most of the linear motors used are produced abroad. Among them, well-known manufacturers include Anorad in the United States, Indramat in Germany, GE Fanuc Automation North America, Sulzer Electronics in Switzerland, and Siemens in Germany. There are no specialized manufacturers of linear motors for CNC machine tools and machining centers in my country. At present, linear motors used in CNC machine tools and machining centers can be divided into two categories: (1) High-precision, high-frequency response, and short-stroke linear motors. Most of them are voice coil linear DC motors. Due to the small inertia of the moving coil, a very high frequency response can be obtained. The feedback signal can be detected by eddy current sensors or grating sensors to achieve closed-loop control and obtain high precision. An example of this type of motor is the precision piston CNC lathe. Figure 5 shows the working principle diagram of this lathe. It consists of computer-controlled x and z-axis AC servo motors, linear motors and their position detection sensors, and a spindle rotation position detection sensor. The control of the x and z-axis AC servo motors constitutes the ordinary turning CNC system; the Z-axis AC servo motor, linear motor and its position detection sensor, and the spindle rotation position detection sensor constitute the piston turning CNC system. Currently, precision pistons are all convex-elliptical in shape, with an axial cross-section that is convex (barrel-shaped) and a radial cross-section that is elliptical or irregularly non-circular. Furthermore, the ellipticity of each radial cross-section along the axial direction is unequal, hence the name convex-elliptical piston, which requires CNC turning. During turning, the cutting tool is mounted on the moving coil shaft of the linear motor to obtain high-frequency reciprocating movement, which coordinates with the spindle rotation position to form an elliptical or irregularly non-circular shape. The machine tool produced by the Institute of Manufacturing Engineering of Tsinghua University in cooperation with Guangzhou Machine Tool Factory has a spindle speed of 3-2500 r/min, a linear motor pulse equivalent of 0.5 μm, a stroke of 1 mm, and a repeatability of 3 m. (2) High-thrust, long-stroke, high-precision linear motors mostly use permanent magnet AC synchronous linear motors and induction AC asynchronous linear motors. In some countries, linear stepper motors are also widely used. The thrust of this high-thrust, long-stroke, high-precision permanent magnet AC synchronous linear motor can reach 1500-2000 N, and can be larger or smaller, and can be serialized; the speed is 60-200 m/min, and can be higher or lower, such as up to 3000 m/min; the acceleration is 2-0 g, and can be large or small, adapting to the requirements of high-speed and ultra-high-speed cutting. At present, linear motors have been used in machining centers, laser cutting, plasma cutting and electrical discharge machining tools. 5. Development direction of linear motors Linear motors are important functional components and have received attention from industries around the world. Due to their wide range of applications, they are developing rapidly. The following development directions are worth mentioning: (1) Structural design: Functional components should be formed from modularization, standardization, and serialization. In addition to the main body of the motor, research should be conducted on dust prevention, chip prevention, cooling, antimagnetism, and safety protection to form a complete linear motor that is easy to install and adjust. (2) Technical performance: Improve dynamic performance and stiffness, reduce thrust fluctuations caused by end effects and cogging effects, and meet the requirements of thrust and thrust fluctuations through magnetic circuit design. Improve speed and acceleration to meet the requirements of high-speed and ultra-high-speed cutting. Accuracy is an important technical indicator. To improve positioning accuracy and repeatability, it is not only related to the motor structure and magnetic circuit design, but also to the position detection device and control system. Currently, optical gratings and magnetic rulers are widely used for position detection. Laser detection can be considered. (3) Control Technology: A linear motor is actually a linear motion servo unit, and the control system is an integral part of it. The control system includes both hardware and software aspects. Currently, the industrial control computer and digital signal processor (DSP) scheme are mostly adopted to form a hierarchical control between the upper and lower computers. The control strategy is also very important. Based on PID control, feedforward control, repetitive learning control and nonlinear control technologies have been developed. (4) Performance Testing and Quality Inspection: Linear motors are still under development and research. There should be corresponding testing methods and standards for their static and dynamic thrust, speed, acceleration, displacement, etc., and a special test bench should be designed and manufactured. (5) Commercialization: The scientific research and key research results should be transformed into commodities as soon as possible. Manufacturers should be selected for production and further promotion and application.