1 Introduction High-speed, precision and modularization are the development direction of modern manufacturing technology. The new cutting theory holds that when the cutting speed reaches a certain level (about 500m/min), the temperature in the cutting zone no longer rises, and the cutting force is reduced, and the tool wear is also reduced. This improves the surface quality and machining accuracy of the part while improving productivity.
In general, the cutting speed and feed rate for high speed machining are an order of magnitude higher than conventional machining. Therefore, the high-speed spindle and rapid feed system are two key technologies for high-speed machining. The following new requirements are imposed on the feed system: (1) The feed rate must match the high-speed spindle to 60 m/min or higher: (2) The acceleration should be large, so that the required high speed can be achieved in the shortest time and stroke, at least 1~2g: (3) Dynamic performance is good, fast servo control and error compensation can be realized, and high positioning is achieved. Precision and stiffness.
For a long time, the feed system of CNC machine tools is mainly "rotary servo motor, ball screw". The maximum feed rate that can be achieved by this feed system is 90-120m/min, and the maximum acceleration is only 1.5g. At the same time, since there are a series of intermediate links between the motor shaft and the table, such as couplings, screws, nuts, bearings, brackets, etc., when the feeding parts are to be activated, accelerated, decelerated, reversed, stopped, etc. The elastic deformation, friction, backlash, etc. generated by mechanical components can cause the lag of the feed motion and many other nonlinear errors: these intermediate links also increase the inertial mass of the system and affect the fast response to motion commands. In addition, the lead screw is an elongated rod, which will deform under the action of force and heat, affecting the machining accuracy.
In order to overcome the shortcomings of the traditional feed system, simplify the machine structure and meet the requirements of high-speed precision machining, people began to study the new feed system, which is the most promising rapid feed system. It eliminates all intermediate transmission links between the source power and the table components, making the length of the machine feed drive chain zero, which is called "direct drive" or "zero drive".
2 Principle and classification of linear motors The so-called linear motors are devices that use the principle of electromagnetic action to convert electrical energy directly into linear motion kinetic energy. In practical applications, in order to ensure that the coupling between the primary and secondary remains constant throughout the stroke, the primary and secondary are typically manufactured to different lengths. A linear motor is similar to a rotating motor. When a three-phase current is applied, a magnetic field is also generated in the air gap. If the end effect is not considered, the magnetic field is sinusoidally distributed in a straight line, but the magnetic field is translated rather than rotated. For the traveling wave magnetic field. The traveling wave magnetic field interacts with the secondary to generate electromagnetic thrust, which is the basic principle of linear motor operation. Since there is the above correspondence between the linear motor and the rotary motor, each rotary motor has a corresponding linear motor, but the linear motor is more flexible in structure than the rotary motor. Linear motors can be divided into linear DC motors, linear induction motors, linear synchronous motors, linear stepping motors, linear piezoelectric motors and linear reluctance motors according to their working principles: they can be divided into flat plates, U-shaped and cylindrical according to the structure. formula.
3 Advantages and Disadvantages of Linear Motors Linear motors are characterized by direct linear motion, which has the advantage of “rotating motors, rolling screws†that indirectly produce linear motion (see the table below for specific performance): 
(1) Without mechanical contact, the transmission force is generated in the air gap, and there is no friction other than the guide rail: (2) The structure is simple, the volume is small, the linear drive is realized with the minimum number of parts, and there is only one moving part. : (3) The stroke is theoretically unrestricted, and the performance is not affected by the change of the stroke: (4) can provide a wide range of speeds, from a few micrometers per second to several meters, especially high speed is one of them Outstanding advantages: (5) Acceleration is very large, up to 10g: (6) The movement is stable, because there is no other mechanical connection or conversion device except for the supporting linear guide or air bearing: (7) Accuracy and repeatability are high, because the intermediate link affecting the accuracy is eliminated. The accuracy of the system depends on the position detection component. There are suitable feedback devices up to the sub-micron level: (8) Simple maintenance, due to fewer components, no mechanical movement Contact, which greatly reduces the wear and tear of the parts, with little or no maintenance and a longer service life.
Comparison of linear motor and "rotary motor, ball screw" transmission performance 
The shortcomings of the linear motor are: firstly, the distortion of the magnetic field at the end of the linear motor affects the integrity of the traveling wave magnetic field, so that the loss of the linear motor increases, the thrust decreases, and there is a large thrust fluctuation, which is the end of the linear motor. Effect (Edge Effect). The structural characteristics of linear motors determine that end effects are unavoidable. Secondly, the control of the linear motor is difficult because the load (such as workpiece weight, cutting force, etc.) changes, system parameter perturbations and various disturbances (such as friction), including the end effect, are directly affected during the operation of the motor. There is no buffering or weakening on the motor. If the control system is not robust, it will cause system instability and performance degradation. Other disadvantages include difficulty in installation, magnetic isolation, low efficiency, and high cost.
The main requirement of the high-speed machining center feed system in the manufacturing industry is the AC linear motor. AC linear motors can be divided into two types: inductive and synchronous. Although the synchronous linear motor is more expensive than the inductive linear motor, it is difficult to assemble, and needs to shield the magnetic field, but the efficiency is high, the structure is simple, the secondary is not cooled, the control is convenient, and the required high performance is more easily achieved, and With the emergence and development of iron-boron (NdFeB) permanent magnet materials, permanent magnet synchronous linear motors will gradually develop into the mainstream. Therefore, the proportion of permanent magnet AC synchronous linear motors in high-speed machining centers will become higher and higher.
4 Development and application of linear motors The development of linear motors in foreign countries The starting point of linear motor development is not much later than that of rotating electric motors. Shortly after the emergence of rotating electric motors in the world, the prototype of linear motors appeared, but the development of linear motors is tortuous. of.
In 1845, the British Charles Wheastone invented the world's first linear motor, but this linear motor was inefficient due to excessive air gap and was unsuccessful. By the middle of the 20th century, the development of control, electronics, materials and other technologies provided theoretical and technical support for the development of linear motors, and linear motors began to enter a new stage of development. Professor ER Laithwaite of the United Kingdom is a pioneer in the development of modern linear motors. He emphasized the basic research of linear motors. The research team led by him has achieved many important results. The representative is also Professor Yamada I of Japan, who has written many books on linear motors. After the 1970s, linear motor applications were more widely used, such as automatic plotters, liquid metal pumps (MHD), electromagnetic hammers, light industrial machinery, home appliances, air compressors, and semiconductor manufacturing equipment. After the 1990s, with the concept of high-speed machining, linear motors began to appear as machining systems in machining centers. Since the direct drive feed system has the advantages and potential unmatched by the traditional feed system, it has once again received the attention of all countries. According to reports, sales of linear motors and drives in the United States in 1997 were $45.53 million, and are expected to reach $107.72 million in 2002.
As an electromechanical system, linear motor simplifies the mechanical structure and complicates electrical control, which is in line with the development trend of modern electromechanical technology.
The American company Anorad is the world's most famous manufacturer of linear motors. The company introduced a brushless DC linear motor in 1988 and obtained a US patent. The company mainly produces permanent magnet synchronous linear motors, which form a series of products with different structures and different powers, which are widely used in various fields.
Germany's Indramat produces both inductive linear motors and permanent magnet linear motors in more than 50 models. The permanent magnet has the characteristics of high efficiency (up to 1.72 N/W) and high thrust density. According to reports, its product speed can reach 600m / min, the thrust is 22kN.
In order to reduce the price of linear motors, Trilogy has introduced the Linear Encoding Module (LEM). It uses the magnetic field of the motor to provide feedback of the position, independent of the stroke. It can work in harsh environments and provides the same commutation signal as the full-stroke sensor with 5μm resolution and repeatability.
The products of other linear motor manufacturers have their own characteristics. For details, please refer to Liu Jinling et al., "High-frequency DC linear motor" (published in "Micro-motor", No. 4, 1993). The application of linear motors in machine tools and machining centers in high-speed machining centers and other large-stroke CNC machine feed systems is still a matter of recent years. Machine tools equipped with linear motors must have an advanced CNC system, high stiffness and natural frequency, and the mass of moving parts should be as small as possible in order to fully utilize the capabilities of linear motors. In addition, the design of the direct drive feed system in the machine tool must also consider cooling and heat dissipation issues. In order to prevent chips and various powders from being attracted by the open magnetic field of the linear motor, magnetic isolation and anti-magnetic measures must also be taken. In addition, linear motors are not self-locking like screw bars. If the motor is mounted vertically, balance weights and braking should also be considered.
The cooperation between Ford, Ingersoll and Anorad in the mid-1980s initially led to the application of linear motors on machine tools. Ford hopes that the machine tool is both high speed, high precision and high flexibility. As a result of the cooperation, Ingersoll introduced the "High Speed ​​Module" HVM800, which is equipped with Anorad's permanent magnet linear motor for good performance.
Ex-Cell-O Company of Germany exhibited the XHC240 high-speed machining center of the world's first linear motor drive table at the European Machine Tool Exhibition in Hannover, Germany in 1993, using the inductive linear motor developed by Indramat of Germany. Each axis has a moving speed of up to 80m/min and an acceleration of up to 1g. Since then, many manufacturers have introduced machining centers that install linear motors. According to statistics, in 1997, the sales volume of machine tools using linear motors was 300, and it is expected to increase to 3,000 by 2005. After 10 years, 20% of CNC machines will be equipped with linear motors.
In addition to cutting machines, other machines such as laser cutting, plasma cutting, and EDM have begun to use linear motors.
Research on Linear Motors in China Although there are many units for linear motors in China, there are three universities that study linear motors as machine tools or machining center feed systems: Guangdong University of Technology established "Ultra High Speed ​​Machining and Machine Tool Research Office" The main research and development of "super high speed electric spindle" and "linear motor high speed feed unit". They studied the linear induction motor and developed the GD-3 linear motor high-speed CNC feed unit with a rated feed force of 2kN, a maximum feed rate of 100m/min, a positioning accuracy of 0.004mm and a stroke of 800mm. Since the late 1990s, Shenyang University of Technology has conducted research on permanent magnet linear synchronous motors and manufactured prototypes with a thrust of 100N. Another focus of their research is on the control of the motor and the servo system, and has published several papers on this. The Institute of Precision Engineering and Mechanical Engineering of Tsinghua University has successfully developed a high-frequency DC linear motor with a stroke of up to 5 mm, a cut-off frequency of more than 250 Hz, and a thrust of several hundred Newtons. It is used to drive a transverse knives for a medium-convex piston lathe. The frame has achieved good application results in actual processing. The long-stroke permanent magnet linear servo unit is currently under study. The rated thrust of the motor is 1500N, the maximum speed is 60m/min, the maximum acceleration of no-load is 1g, and the stroke is 600mm.
It should be noted that in China, the research on linear motor, especially the linear servo motor in the machine tool feeding system, is still in its infancy. The researchers and funds are obviously insufficient, the progress is slow, and the gap with foreign countries is getting bigger and bigger. It is already imminent. In order to break the foreign technology monopoly, we must take the road of combining technology tracking and independent development, and strengthen the research of basic and key technologies.
5 Development Trends and Research Directions The current development of linear motor direct drive technology presents the following trends:
The linear servo motor for the machine feed system will be dominated by permanent magnets:
Integrate motors, encoders, rails, cables, etc. to reduce motor size for easy installation and use:
Modularize the various functional components (rails, encoders, bearings, connectors, etc.):
Focus on the development of related technologies, such as position feedback components, control technology, etc., which is the basis for improving the performance of linear motors.
Research direction Linear motor research goals are to improve motor performance and meet application requirements. The main properties of linear motors include speed, acceleration, thrust and its fluctuations, positioning accuracy, repeat positioning accuracy, mechanical properties (speed-thrust characteristics), transient performance (speed response) and thermal characteristics.
As an electromechanical system, to improve performance can be achieved from both structural and control aspects.
Structural design Linear motors include primary and secondary magnetic circuit structures as well as mechanical structures such as support, sensing measurement, cooling, dust protection, and protection.
Magnetic Circuit Design The most important task of magnetic circuit design is to make the thrust and thrust fluctuations of the motor meet the design requirements.
The calculation of the magnetic field distribution in the motor is the basis of the magnetic circuit design. Due to the particularity of the structure, the linear motor has an end effect, causing distortion of the magnetic field, and a soft magnetic material such as a silicon steel sheet is used to aggregate the magnetic circuit. The boundary of the medium is zigzag, the magnetic circuit is complex, and the nonlinearity is strong. If the calculation is carried out by the traditional equivalent magnetic circuit method or graphic method, a large error will occur, which is even impossible. Therefore, the numerical solution is generally used at present - mainly by using the finite element method (FEM) to calculate the magnetic field distribution of the linear motor, thereby further calculating the thrust and its fluctuation and the vertical force. At present, there are many excellent electromagnetic field FEM software available on the market, so the key point of using FEM to calculate the electromagnetic field of linear motors is to establish an accurate finite element model.
Reducing thrust fluctuations is a key and difficult point in magnetic circuit design. Thrust fluctuations are caused by high-order harmonics of primary current and back electromotive force, non-sinusoidal air gap waveform, cogging effect, and end effect. Thrust fluctuations can be reduced by optimizing the shape and arrangement of the permanent magnets, reducing the permanent magnet excitation magnetic density, using a coreless and multi-pole structure, increasing the number of slots, and adding air gaps, but some measures can cause other performance. The weakening, so design should consider the design requirements to achieve the best results.
There are many problems involved in the mechanical structure design of the mechanical structure. Here we only emphasize the study of the cooling system, because this problem is easily overlooked. In fact, thermal characteristics are an important characteristic of linear motors. The same type of motor has twice the thrust peak when cooling, so the motor cooling system has a great influence on the performance of the motor. Start with the cooling system. Optimizing the design is a shortcut to improve motor performance. The analysis of the thermal characteristics of the motor is also generally based on the finite element method, and the cooling is optimized based on the calculation results. #p#分页头#e#
Research and control technology of control technology is another important and difficult point in the design of linear motors.
The linear servo system directly drives the load during operation, so the load changes directly to the motor: external disturbances, such as workpiece or tool quality, cutting force changes, etc., also directly affect the motor without attenuation: the motor parameters are also directly changed Affects the normal operation of the motor: the linear guide has friction: the linear motor also has cogging and end effects. These factors all contribute to the control of linear motors. These disturbances must be suppressed or compensated in the control algorithm, otherwise it will easily cause instability of the control system.
In general, the design of the controller must meet the following requirements: high steady-state tracking accuracy, fast dynamic response, strong anti-interference ability, and good robustness. Different linear motors or different applications require different requirements for the control algorithm, so the appropriate control method should be used according to the specific situation. At present, the control strategies adopted by linear servo motors mainly include traditional PID control and decoupling control. Modern control methods such as nonlinear control, adaptive control, sliding mode variable structure control, H∞ control, intelligent control such as fuzzy control, artificial intelligence ( Such as artificial neural network system) control.
It can be seen that the control algorithm of the linear motor has a large amount of calculation, and the real-time performance is strong in the practical application of the high-speed machining feed system, so high requirements are imposed on the entire numerical control system. To meet this requirement, high-performance hardware should be used while optimizing the control algorithm. In the high-speed machining center feed system, the full digital drive technology is usually adopted, and the PC is used as the basic platform, and the DSP implements interpolation and servo control.
Although the control of linear motors is much more difficult than that of rotating motors, their electromagnetic characteristics and operating principles are basically similar, and the servo control technology of rotating motors has been developed. Therefore, in the experimental research stage, in order to establish the experimental system as soon as possible to verify the feasibility of the design, we can also transform the servo controller of the rotary motor into the servo controller of the linear motor, which can reduce the cost and cycle of the development. The linear motor servo controller is also instructive.
Theoretical research on experimental research is the basis of design, but to determine the performance of the motor, in the final analysis depends on the specific test. The performance testing techniques for rotating electric motors are well established and standardized, but there is no uniform method for performance testing of linear motors. Therefore, it is also an important subject to study the high-efficiency and accurate linear motor performance test method, which also promotes theoretical research. The key point of the experimental research is the accurate measurement of various parameters such as speed, acceleration, static force, dynamic force, displacement, temperature, etc., and design a special test bench if necessary. According to the theoretical calculation results, the design scheme is optimized, and the prototype is manufactured on the basis of this, and then the performance of the prototype is tested to verify the correctness of the design. A linear motor with excellent performance is often manufactured after repeated calculations and tests.
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