The history of linear motors can be traced back to the unsuccessful prototype of linear motors produced by Wheatstone in 1840. For the more than 160 years, linear motors have experienced three periods of exploration, development, application and commercialization. From 1971 to the present, linear motors have finally entered the period of independent application. The application of various linear motors has been rapidly promoted, and many practical devices and products have been produced, such as linear motor driven steel pipe conveyors and coal conveyors. , various electric doors, power windows, etc. The maglev train driven by a linear motor has a speed of more than 500km/h, which is close to the speed of aviation flight. The research and application of linear motors in China began in the early 1970s. At present, the main results are factory driving, electromagnetic hammer, punching machine and so on. Although China's linear motor research has also achieved some achievements, compared with foreign countries, there is still a big gap in its promotion and application. At present, many research units in China have noticed this. In recent years, the use of linear motors for CNC machine tools has become particularly popular internationally. The reasons are: In 1993, ZxCell-O Company of Germany introduced the world's first HSC-240 high-speed machining center driven by linear motor. The maximum spindle speed of the machine tool reached 24000r/min, the maximum feed rate was 60n/min, and the acceleration reached 1g, when the feed speed is 20m/min, the contour accuracy can reach 0.004mm. Ingersoll of the United States immediately introduced the HVM-800 high-speed machining center with a maximum spindle speed of 20000r/min and a maximum feed rate of 75.20m/min. Since 1996, Japan has successfully developed horizontal machining centers, high-speed machine tools, ultra-high-speed small machining centers, ultra-precision mirror processing machines, and high-speed forming machine tools using linear motors [1]. Zhejiang University of China has developed a punching machine driven by a linear motor. The Institute of Production Engineering of Zhejiang University has designed a parallel mechanism coordinate measuring machine driven by a cylindrical linear motor [2]. In 2001, Nanjing Sikai Company launched a self-developed CNC linear motor lathe directly driven by linear motor. At the 8th China International Machine Tool Show in 2003, it exhibited the VS1250 linear motor from Beijing Electric Power High Technology Co., Ltd. The machining center has a maximum spindle speed of 15000r/min. How linear motors work A linear motor is a transmission that converts electrical energy directly into linear motion mechanical energy without the need for any intermediate conversion mechanism. It can be seen as a rotating electric machine that is cut radially and flattened. The side evolved from the stator is called the primary, and the side evolved from the rotor is called the secondary. In practical applications, the primary and secondary are manufactured to different lengths to ensure that the coupling between the primary and secondary remains constant over the desired range of travel. The linear motor can be a short primary long secondary or a long primary short secondary. In view of manufacturing costs and operating costs, short primary primary secondary is currently used. Linear motors work similarly to rotating motors. Taking a linear induction motor as an example: when the primary winding is connected to the AC power source, a traveling wave magnetic field is generated in the air gap, and the secondary is induced by the traveling wave magnetic field to induce an electromotive force and generate a current, which is in the air gap The magnetic field phase produces an electromagnetic thrust. If the primary is fixed, the secondary moves linearly under the action of the thrust; otherwise, the primary performs a linear motion. Linear motor drive control technology A linear motor application system must have not only a linear motor with good performance, but also a control system that can achieve technical and economic requirements under safe and reliable conditions. With the development of automatic control technology and microcomputer technology, there are more and more control methods for linear motors. The research on linear motor control technology can be basically divided into three aspects: one is traditional control technology, the other is modern control technology, and the third is intelligent control technology. Traditional control technologies such as PID feedback control and decoupling control have been widely used in AC servo systems. The PID control implies the past, present and future information in the dynamic control process, and the configuration is almost optimal and has strong robustness. It is the most basic control method in the AC servo motor drive system. In order to improve the control effect, decoupling control and vector control techniques are often used. The traditional control technique is simple and effective under the condition that the object model is determined, does not change, and is linear, and the operating conditions and operating environment are determined to be constant. However, in high-performance micro-feeding high-performance applications, changes in object structure and parameters must be considered. A variety of non-linear effects, changes in operating environment and environmental disturbances, such as time-varying and uncertain factors, can achieve satisfactory control results. Therefore, modern control technology has attracted a lot of attention in the research of linear servo motor control. Commonly used control methods are: adaptive control, sliding mode variable structure control, robust control and intelligent control. In recent years, intelligent control methods such as fuzzy logic control and neural network control have also been introduced into the control of linear motor drive systems. At present, it mainly combines fuzzy logic, neural network with existing mature control methods such as PID and H∞ control to learn from each other to obtain better control performance [3]. Application examples of linear motors in CNC machine tools Piston turning CNC system The linear motion mechanism using linear motor has been successfully applied to CNC turning and grinding of profiled workpieces due to its fast response and high precision. For the non-circular cross-section parts with the largest output, the Non-Circular Cutting Research Center of the National University of Defense Technology has developed a high-frequency large-stroke CNC feed unit based on linear motors. When used in CNC piston machine tools, the table size is 600mm × 320mm, the stroke is 100mm, the maximum thrust is 160N, and the maximum acceleration is up to 13g. Since the linear motor mover and the table are fixed together, only closed-loop control can be used. Figure 2 shows a schematic diagram of the control system of the unit. Open CNC system with linear motor The PC and the open programmable power controller are used to form the numerical control system. This system uses the universal microcomputer and Windows as the platform, and the motion controller in the form of standard plug-in on the PC is used as the control core to realize the opening of the numerical control system. The overall design of an open CNC system based on a linear motor is shown in Figure 3. The system consists of a scheme of inserting a motion control card into the expansion slot of the PC. The system consists of a PC, a motion control card, a servo drive, a linear motor, and a CNC workbench. The CNC table is driven by a linear motor. The servo control and machine logic control are all completed by the motion controller. The motion controller is programmable, and the NC program is interpreted in the form of a motion subroutine (G code, etc., to support user expansion). The motion control card model is PCI-8132. In today's industrial control technology, the PCI bus gradually replaces the ISA bus and becomes the mainstream bus form. It has many advantages, such as Plug and Play and interrupt sharing. The PCI bus has strict standards and specifications, which guarantees good compatibility and high reliability; high transmission data rate (132Mbps) or (264Mbps); PCI bus is independent of CPU, independent of clock frequency, suitable for each A platform that supports multiple processors and parallel operation; the PCI bus also has good scalability, and can be multi-stage extended through the PCI_PCI bridge. The PCI bus provides users with great convenience and is the most advanced and versatile bus on the PC. The PCI-8132 is a 2-axis motion control card with a PCI interface. It can generate high-frequency pulse to drive stepper motor and servo motor, control motor movement of two axes, realize linear and circular interpolation. In CNC machining, position feedback is provided. System software is developed on the WINDOWS platform. The software adopts modular programming and consists of user input and output interface and pre-processing module. The user input and output interface realizes the user's input and the output of the system. The main function of the user input is to let the user input the numerical control code, issue control commands, configure the parameters of the system, and generate the CNC machine tool part processing program (G code command). After the pre-processing module reads the G code command, it compiles and generates a program that can run the PCI-8132 motion control card, thereby driving the linear motor and completing linear or circular interpolation. The process of reading the G code is to first set the parameters and then read the G code. The program flow is as shown in Figure 4. Conclusion The high-speed machining center with linear servo motor has become a key technology and product that the major machine tool manufacturers in the world compete for research and development. It has achieved initial application and effectiveness in the automotive industry and aviation industry as a new generation of high-speed machining center. Direct drive servo actuators, linear servo motor technology has also entered the industrial application stage at home and abroad. However, domestic research in this area is still in its infancy, and the gap is still very large. This paper has made some discussions on the application of linear motors, and many technical problems have yet to be worked on in the future.
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High-speed and ultra-high-speed machining developed to improve production efficiency and improve part processing quality has become a major trend in machine tool development. A responsive, high-speed, lightweight drive system requires speeds of up to 40 to 50 m/min. The traditional "rotary motor + ball screw" transmission can achieve a maximum feed rate of 30m / min, acceleration is only 3m / s2. The linear motor drives the worktable, the speed is 30 times that of the traditional transmission mode, the acceleration is 10 times of the traditional transmission mode, the maximum is up to 10g; the stiffness is increased by 7 times; the table directly driven by the linear motor has no reverse working dead zone; Due to the small motor inertia, a linear servo system composed of it can achieve a high frequency response.
This is a double closed loop system, the inner ring is the speed loop and the outer ring is the position loop. A high-precision grating ruler is used as the position detecting element. The positioning accuracy depends on the resolution of the grating, and the mechanical error of the system can be eliminated by feedback to obtain higher precision [4].
The PARKER406LXR series linear motors are used in this system. For the two-coordinate numerical control workbench, the X-direction selects the 406T07 linear motor with a stroke of 550mm, and the Y-direction uses the 406T05 linear motor with a stroke of 450mm.