The history of linear motors can be traced back to the unsuccessful little linear motors produced by Wheaton in 1840. In the following 160 years, linear motors have experienced three periods of exploration experiments, development and application, and commercialization.
From 1971 to the present, linear motors finally entered the period of independent application. The application of various types of linear motors has been rapidly promoted, and many useful devices and products have been made, such as linear motor-driven steel pipe conveyors and coal conveyors. All kinds of electric doors, electric windows and so on. With the use of linear motor-driven magnetic levitation trains, the speed has exceeded 500km/h, approaching the speed of aviation flights.
The research and application of linear motor in our country started from the early 1970s. At present, the main achievements are factory driving, electromagnetic hammer, punching machine and so on. Although China's linear motor research has made some achievements, compared with foreign countries, there is still a big gap in its promotion and application. At present, many domestic research institutions have taken note of this [1].
The Status Quo of Linear Motors in CNC Machine Tools
In recent years, the use of linear motors in CNC machines has become particularly popular in the world. The reasons are:
The high-speed and ultra-high-speed machining that has been developed in order to increase production efficiency and improve the machining quality of parts has become a major trend in the development of machine tools. A responsive, high-speed, and lightweight drive system is required to increase the speed to 40 to 50 m/min. The maximum feed speed that can be achieved by the traditional "rotary motor + ball screw" transmission mode is 30m/min, and the acceleration is only 3m/s2. Linear motor drive table, its speed is 30 times the traditional transmission mode, acceleration is 10 times the traditional transmission mode, up to 10g; stiffness increased by 7 times; linear motor directly driven worktable without reverse working dead zone; Since the motor inertia is small, the linear servo system formed by it can achieve a high frequency response.
In 1993, ZxCell-O Company of Germany introduced the world's first HSC-240 type high-speed machining center driven by a linear motor. The maximum speed of the machine spindle reaches 24000r/min, the maximum feed rate is 60n/min, and the acceleration reaches 1g, when the feed speed is 20m/min, the contour accuracy can reach 0.004mm. Ingersoll of the United States subsequently introduced the HVM-800 high-speed machining center. The maximum spindle speed is 20000r/min and the maximum feed speed is 75.20m/min.
Since 1996, Japan has successively developed horizontal machining centers using linear motors, high-speed machine tools, ultra-high-speed small machining centers, ultra-precision mirror-processing machines, and high-speed forming machines [1].
A press machine driven by a linear motor was developed at Zhejiang University in China. The Institute of Production Engineering of Zhejiang University designed a parallel mechanism coordinate measuring machine driven by a cylindrical linear motor [2]. In 2001, Nanjing Sikai Company launched a self-developed digital linear motor lathe directly driven by a linear motor. At the 8th China International Machine Tool Exhibition in 2003, it exhibited the VS1250 linear motor introduced by Beijing Electric Power High-Tech Co., Ltd. Machining center, the maximum speed of the machine tool spindle up to 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 rotary motor cut in radial direction and developed into a plane, as shown in Figure 1.
Figure 1 The transformation process of a linear motor
The side evolved from the stator is called the primary, and the side evolved from the rotor is called the secondary. In practice, the primary and secondary are manufactured to different lengths to ensure that the coupling between the primary and the secondary remains constant over the desired range of travel. The linear motor can be a short primary length secondary, or it can be a long primary short secondary. Taking into account the manufacturing costs, operating costs, generally adopt the short primary length secondary.
The working principle of a linear motor is similar to that of a rotary motor. Take a linear induction motor as an example: When the primary winding is connected to an AC power source, a traveling wave magnetic field is generated in the air gap. Under the cutting of the traveling wave magnetic field, the secondary induces an electromotive force and generates a current. The current and the air gap The magnetic field acts to generate electromagnetic thrust. If the primary is fixed, the secondary performs a linear motion under the action of thrust; otherwise, the primary performs a linear motion.
Linear motor drive control technology
A linear motor application system must not only have a linear motor with good performance, but also must have a control system that can achieve technical and economical requirements under safe and reliable conditions. With the development of automatic control technology and microcomputer technology, more and more linear motor control methods. The study of linear motor control technology can be basically divided into three aspects: First, the traditional control technology, the second is the modern control technology, and the third is the intelligent control technology.
Traditional control techniques such as PID feedback control and decoupling control have been widely used in AC servo systems. The PID control contains 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.
Under the condition that the object model is determined, unchanged, and linear and the operating conditions and operating environment are fixed, the use of traditional control techniques is simple and effective. However, in high-performance applications where high-precision microfeeds are used, changes in the structure and parameters of the object must be considered. Various nonlinear effects, changes in the operating environment, and environmental disturbances such as time-varying and uncertain factors can achieve satisfactory control results. Therefore, modern control technology has attracted much attention in the research of linear servo motor control. Common control methods include: 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, PID, H∞ control and other existing mature control methods to learn from each other to obtain better control performance [3].
Examples of the linear motor applications in the NC machine tool
Piston Turning CNC System
Due to its fast response and high accuracy, the linear motion mechanism using linear motors has been successfully used in CNC turning and grinding of special-shaped cross-section workpieces. For the non-circular 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 digital feed unit based on a linear motor. When used in CNC piston machine tools, the table size is 600mm × 320mm, stroke 100mm, the maximum thrust of 160N, the maximum acceleration of up to 13g. Since the linear motor mover and the table have been fixed together, only closed-loop control can be used. Figure 2 shows a schematic diagram of the control system of the unit.
Figure 2 Block diagram of linear motor position controller
This is a double closed loop system where the inner ring is the speed loop and the outer loop is the position loop. High-precision scales are used as position detection elements. 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 accuracy [4].
Open CNC system with linear motor
Using a PC and an open programmable operation controller to form a numerical control system, this system uses a general-purpose microcomputer and Windows as a platform, and a motion controller in the form of a standard plug-in on a PC as the control core, and realizes the opening of the numerical control system. The overall design of an open CNC system based on a linear motor is shown in Fig. 3.
Fig. 3 Schematic diagram of an open CNC system based on a linear motor
The system is composed of a program for inserting a motion control card in an expansion slot of a PC. The system consists of a PC, a motion control card, a servo drive, a linear motor, and a numerical control table. The numerical control table is driven by the linear motor, the servo control and the logic control of the machine are all completed by the motion controller, the motion controller is programmable, and the numerical control program (G code, etc.) is interpreted and executed in the way of the motion subroutine. 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, interrupt sharing, and so on. The PCI bus has strict standards and specifications, which ensures that it has good compatibility and high reliability; the transmission data rate is high (132Mbps) or (264Mbps); the PCI bus has nothing to do with the CPU and has nothing to do with the clock frequency. The platform supports multi-processor and parallel work; the PCI bus also has good scalability, and multi-level expansion is possible through the PCI_PCI bridge. The PCI bus provides users with great convenience. It is the most advanced and universal bus on the PC. PCI-8132 is a 2-axis motion control card with PCI interface. It can generate high-frequency pulses to drive stepper motors and servo motors, control motor motion of two axes, and realize linear and circular interpolation. In NC machining, position feedback is provided.
System software is developed on the WINDOWS platform. The software adopts a modular program design and consists of user input and output interfaces and pre-processing modules. The user input and output interface implements user input and system output. The main function of the user input is to allow the user to input numerical control codes, issue control commands, perform system parameter configuration, and generate a numerical control machine tool part program (G code instructions). After the preprocessing module reads the G code instruction, it compiles and generates a program that enables the PCI-8132 motion control card to run, thereby driving the linear motor to perform 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 FIG. 4 .
Figure 4 reads the G code program flow chart
Choose PARKER406LXR series linear motor in this system. For the two-axis NC workbench, the X-direction uses the 406T07 linear motor, the stroke is 550mm, and the Y-direction selects the 406T05 linear motor with a stroke of 450mm.
Conclusion
The high-speed machining center adopting a linear servo motor has become a key technology and product that all major machine tool manufacturers in the world have been competing for research and development, and has achieved preliminary application and results in the automotive industry and aviation industry as a new generation of high-speed machining centers. Directly driving servo actuators, linear servo motor technology has also entered the stage of industrial application at home and abroad. However, domestic research in this area is still in its infancy, and the gap is still very large. This article has made some discussion on the application of linear motors, and many technical problems have yet to be resolved in the future.
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