Overview of Linear Motors for Machine Feed System

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 in the cutting zone no longer rises, but the cutting force will decrease and the tool wear will also decrease. This improves the surface quality and machining accuracy of parts while increasing productivity.
In general, cutting speeds and feed rates 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 speed must be matched with the high-speed spindle to reach 60m/min or higher: (2) Acceleration should be large in order to achieve the required high speed within the shortest time and stroke, at least 1 ~ 2g: (3) better dynamic performance, can achieve rapid servo control and error compensation, with high positioning Accuracy and stiffness.
For a long time, the feed system of CNC machine tools is mainly "rotary servo motor, ball screw". The highest feed rate that this kind of feed system can achieve is 90 ~ 120m/min, and the maximum acceleration is only 1.5g. At the same time, due to the existence of a series of intermediate links such as couplings, screws, nuts, bearings, and brackets from the motor shaft to the worktable, these components must be started, accelerated, decelerated, reversed, and stopped. The elastic deformation, friction, backlash, etc. generated by the mechanical elements will cause the feed motion to lag behind and many other nonlinear errors: these intermediate links also increase the inertial mass of the system and affect the rapid response to the motion command. In addition, the screw is an elongated rod, under the action of force and heat, will have a deformation, affecting the processing accuracy.
In order to overcome the shortcomings of traditional feed systems, simplify the machine tool structure, and meet the requirements of high-speed precision machining, people began to study new feed systems. Linear motors are the most promising fast feed systems. It eliminates all intermediate transmission links between the source power and the workbench components so that the length of the machine tool feed chain is zero, which is called "direct drive" or "zero drive."
2 Principles and Classification of Linear Motors
The so-called linear motor is the equipment that uses the principle of electromagnetic action to directly convert electrical energy into linear kinetic energy. In practical applications, in order to ensure that the coupling between the primary and the secondary remains unchanged throughout the entire stroke, the primary and the secondary are generally manufactured to different lengths.
Similar to a rotary motor, a linear motor generates a magnetic field in an air gap after passing a three-phase current. If the end effect is not taken into account, the magnetic field is sinusoidally distributed in a straight line direction, except that the magnetic field is translated rather than rotated. Traveling wave magnetic field. The interaction between the traveling magnetic field and the secondary generates electromagnetic thrust, which is the basic principle of linear motor operation.
Due to 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 than the rotary motor. The linear motor can be divided according to the working principle: linear DC motor, linear induction motor, linear synchronous motor, linear stepping motor, linear piezoelectric motor and linear reluctance motor: According to the structural form can be divided into flat plate type, U shape and cylinder formula.
3 advantages and disadvantages of linear motor analysis
The linear motor is characterized by direct linear motion. Compared with the indirect linear motor "rotary motor, rolling screw", the advantage is (the specific performance is shown in the following table): (1) Without mechanical contact, the transmission force is in the air There is no friction other than the guide rail produced in the gap: (2) Simple structure, small size, linear drive with a minimum number of parts, and only one moving part: (3) The stroke is theoretically unlimited, And the performance will not be affected by the change of travel: (4) It can provide a wide range of speed, from a few micrometers per second to a few meters per second, especially high speed is a prominent advantage: (5) great acceleration, maximum Up to 10g: (6) Smooth motion because there is no other mechanical connection or conversion device other than a linear guide or air bearing that acts as a support: (7) Accuracy and repeatability are high because the influence accuracy is eliminated In the middle link, the accuracy of the system depends on the position detection elements, and there are suitable feedback devices up to the sub-micron level: (8) Simple maintenance, due to few parts, no mechanical contact during movement, from Greatly reduce the wear and tear of parts, with little or no maintenance and longer service life. Linear motor and "rotary motor, ball screw" transmission performance comparison table Performance Rotary motor + ball screw Linear motor
Accuracy (μm/300mm) 10 0.5
Repeatability (μm) 2 0.1
The maximum speed (m/min) 90 ~ 120 60 ~ 200
Acceleration (g) 1.5 2~10
Static stiffness (N/μm) 90~180 70~270
Dynamic stiffness (N/μm) 90~180 160~210
Smoothness (% speed) 10 1
Adjustment time (ms) 100 10~20
Life (h) 6000 to 10000 50000
The disadvantages of the linear motor are: First of all, 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. This is the unique end of the linear motor. Edge Effect." The structural characteristics of the linear motor determine the end effect is unavoidable. Second, the control of the linear motor is difficult, because during the operation of the motor load (such as the workpiece weight, cutting force, etc.) changes, the perturbation of the system parameters and various interference (such as friction, etc.), including the end effect are directly On the motor, there is no buffering or weakening. If the control system is not robust, it will cause system instability and performance degradation. Other disadvantages include difficult installation, magnetic isolation, low efficiency, and high cost.
In the manufacturing industry, AC linear motors are mainly required to meet the requirements of high-speed machining center feed systems. AC linear motors can be divided into inductive and synchronous two categories. Although synchronous linear motors are more expensive than inductive linear motors, difficult to assemble, and require shielded magnetic fields, they have high efficiency, simple structure, no secondary cooling, convenient control, and easier attainment of the required high performance. The emergence and development of ferro-boron (NdFeB) permanent magnetic materials, permanent magnet synchronous linear motor 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
Foreign linear motor development
Development History
The starting point for the development of the linear motor is not much later than that of the rotary motor. Soon after the appearance of the rotary motor in the world, the prototype of the linear motor appeared, but the development process of the linear motor is tortuous.
In 1845, the British Charles Wheastone invented the world's first linear motor, but this type of linear motor was inefficient due to its large 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. Prof. ERLaithwaite from the United Kingdom is a pioneer in the development of modern linear motors. He emphasized the basic research of linear motors, and the research team headed by him has achieved many important results. The representative is Professor Yamada I of Japan. He has written several books on linear motors. Since the 1970s, linear motors have become more widely used in applications 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 in machining centers as feed systems. Because the direct drive feed system has the advantage and potential that the traditional feed system cannot match, it has once again received the attention of various countries. According to relevant reports, the sales of linear motors and drives in the United States in 1997 were 45.53 million U.S. dollars, and it is expected to reach 107.72 million U.S. dollars in 2002.
As an electromechanical system, the linear motor simplifies the mechanical structure and complicates the 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 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 different powers, which are widely used in various fields.
The German company Indramat produces both inductive linear motors and permanent magnet linear motors in more than 50 models. The permanent magnet type features high efficiency (up to 1.72N/W) and high thrust density. According to reports, its product speed can reach 600m/min, and the thrust is 22kN.
In order to reduce the price of linear motors, Trilogy has introduced a linear encoder module (LEM). It uses the magnetic field of the motor to provide position feedback, regardless of travel. Can work in harsh environments, providing commutation signal and full-stroke sensor, resolution and repeatability of 5μm.
The products of other linear motor manufacturers have their own characteristics. For details, please refer to Liu Jinling et al., “High Frequency Linear DC Motors” (published in “Micro Motor”, Issue 4, 1993).
Application in machine tools and machining centers
The application of linear motors in high-speed machining centers and other large-stroke CNC machine tools is still a matter of recent years. Machines that install linear motors must have advanced CNC systems, high stiffness and natural frequencies, and the quality of moving parts should be as small as possible in order to fully exploit the capabilities of linear motors. In addition, the direct drive feed system design in the machine tool must also consider the cooling and heat dissipation issues. In order to prevent the chips and various powders from being attracted by the open magnetic field of the linear motor, it is also necessary to adopt magnetic isolation and anti-magnetic measures. In addition, the linear motor is not self-locking like a screw, and if the motor is mounted vertically, balancing weights and braking must also be considered.
The cooperation of Ford, Ingersoll and Anorad in the mid-1980s initially enabled the application of linear motors on machine tools. Ford expects the machine to be both high-speed, high-precision, and highly flexible. The result of the collaboration was Ingersoll's introduction of the "High Speed ​​Module" HVM800, which is equipped with Anorad's permanent-magnet linear motors for excellent performance.
German Ex-Cell-O company exhibited the XHC240 high-speed machining center of the world's first linear motor-driven workbench in 1993 at the European Machine Tool Fair in Hannover, Germany, using an inductive linear motor developed by the German company Indramat. The speed of each axis is up to 80m/min, and the acceleration can reach 1g. Afterwards, many manufacturers have introduced machining centers that install linear motors. According to statistics, the sales of machine tools using linear motors in 1997 were 300, and it is expected that it will increase to 3,000 by 2005. After 10 years, 20% of CNC machine tools will be equipped with linear motors.
In addition to cutting machine tools, other machine tools such as laser cutting, plasma cutting, EDM and other equipment have also begun to use linear motors.
Research on Domestic Linear Motors
Although there are many units of linear motors in China, there are three major universities that study linear motors as machine tools or machining center feed systems: Guangdong University of Technology has established the “Ultra High Speed ​​Machining and Machine Tool Research Laboratory”, which mainly focuses on research and development. Ultra-high-speed electric spindle and linear motor high-speed feed unit. They studied a linear induction motor and developed a 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 length of 800mm. Since the late 1990s, Shenyang University of Technology has studied permanent magnet linear synchronous motors and has manufactured prototypes with a thrust force of 100N. The other focus of their research was on the motor control method and servo system, and published many papers on this. Tsinghua University Institute of Precision Instruments and Mechanical Engineering Institute of Manufacturing Engineering successfully developed a high-frequency DC linear motor, stroke up to 5mm, cut-off frequency greater than 250Hz, thrust up to several hundred newtons, used to drive a transverse convex lathe Frame, in the actual processing to obtain a better application effect. The long-travel 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, research on linear servo motors, especially linear servo motors in machine tool feed systems, is still in its infancy. Researchers and funding are obviously insufficient, progress is slow, and there is a widening gap with foreign countries. It is already imminent. In order to break the monopoly of foreign technology, we must take the path of combining technical tracking and independent development and strengthen the research of basic and key technologies.
5 Development Trends and Research Directions
development trend
The current development of linear motor direct drive technology presents the following trends:
The linear servo motor used in the machine tool feed system will be dominated by the permanent magnet type:
Integrate motors, encoders, guide rails, cables, etc. to reduce the motor size for ease of installation and use:
Modularize the various functional components (rails, encoders, bearings, connectors, etc.):
Pay attention to the development of related technologies such as position feedback components, control technologies, etc. This is the basis for improving the performance of linear motors.
research direction
The research goal of the linear motor is to improve the motor performance and meet the application requirements. The main features of the linear motor include speed, acceleration, thrust and its fluctuation, positioning accuracy, repeatability of positioning, mechanical characteristics (speed-thrust characteristics), transient performance (speed response) and thermal characteristics.
As an electromechanical system, to improve performance is nothing more than to start with the structure and control.
Structural design
The linear motor includes primary and secondary magnetic circuit structure and support, sensing measurement, cooling, dust prevention, protection and other mechanical structures.
Magnetic circuit design The most important task of the magnetic circuit design is to make the motor's thrust and thrust ripple reach the design requirements.
The calculation of the magnetic field distribution within the motor is the basis of the magnetic circuit design. Because of the particularity of the structure, the linear motor has an end effect, which causes the distortion of the magnetic field. At the same time, a magnetic material such as a silicon steel sheet is used to polymerize the magnetic circuit, the media boundary is crooked, the magnetic circuit is complicated, and the nonlinearity is strong. If the traditional equivalent magnetic circuit method or graphical method is used for calculation, a large error will occur, and it may even be impossible. Therefore, numerical solutions are commonly used today—mainly using finite element method (FEM) to calculate the magnetic field distribution of a linear motor to further calculate the thrust, its fluctuations, and vertical forces. At present, there are many excellent electromagnetic field FEM software available on the market. Therefore, the key point of using FEM to calculate the electromagnetic field of a linear motor is to establish an accurate finite element model.
Reducing thrust ripple is a key and difficult point in magnetic circuit design. The causes of thrust ripples are: high harmonics in primary current and back EMF, non-sinusoidal air gap flux density, cogging, and end effects. By optimizing the shape and arrangement of permanent magnets, reducing the magnetic flux density of permanent magnets, adopting core-free and multi-pole structures in the primary, increasing the number of slots, adding air gap, etc., thrust ripple can be reduced, but some measures may cause other performance. The weakening, so the design should be comprehensively consider the design requirements to achieve the best results.
Mechanical Structure Design The mechanical structure involves many problems. Here we only emphasize the research on the cooling system, because this problem is easily overlooked. In fact, the thermal characteristic is an important characteristic of the linear motor. When the same model of motor has the peak value of the cooling when it is not cooled, the motor cooling system has a great influence on the performance of the motor. Starting from the cooling system Optimizing the design is a quick way to improve motor performance. The analysis of the thermal characteristics of the motor generally uses the finite element method. Based on the calculation results, the cooling design is optimized.
Control Technology Research
Control technology is another key and difficult point in the design of linear motors.
When the linear servo system is running, the load is directly driven, so that the change of the load directly affects the motor: the external disturbance, such as the quality of the workpiece or the tool, the change of cutting force, etc., also directly acts on the motor without attenuation: the change of the motor parameter is also direct. Influencing the normal operation of the motor: The linear guide has friction: The linear motor also has cogging and end effects. These factors all make it difficult to control the linear motor. In the control algorithm, these disturbances must be suppressed or compensated, otherwise the instability of the control system will be easily caused.
In general, the controller design 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 may impose different requirements on the control algorithm, so appropriate control methods should be used according to specific conditions. 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 and artificial intelligence ( Such as artificial neural network system) control etc.
It can be seen that the control algorithm of the linear motor has a large amount of calculation, and the real-time performance in the practical application of the high-speed machining feed system is very strong, and therefore a high requirement is put forward for the entire numerical control system. To meet this requirement, high-performance hardware should also be used while optimizing the control algorithm. In the high-speed machining center feed system, all-digital drive technology is usually used. The PC is used as the basic platform, and 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 rotary motors has been developed more mature. So in the experimental research stage, in order to establish an 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 a servo controller of a linear motor, which can reduce the cost and cycle of development, and special for development. The linear motor servo controller also has guiding significance.
Experimental Research
Theoretical research is the basis of design, but to determine the performance of the motor, in the final analysis, it depends on the specific test. The performance test technology for rotary motors has matured and has been standardized, but there is no uniform method for performance testing of linear motors. Therefore, it is a very important topic to study the efficient and accurate linear motor performance test method, and it can also promote the theoretical research. The key point of the experimental research is accurate measurement of various parameters such as speed, acceleration, static force, dynamic force, displacement, temperature, etc. If necessary, special test benches are also designed.
According to the results of theoretical calculations, the design scheme is optimized, and the prototype is manufactured on this basis. Then the performance of the prototype is tested to verify the correctness of the design. A linear motor with excellent performance is often manufactured through repeated calculations and tests.

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