The history of linear motors can be traced back to the unsuccessful linear motor made by Wheatden in 1840, and in the following 160 years, linear motors have gone through three periods: exploration, experimentation, development and application, and commercialization.

From 1971 to the present, linear motors have finally entered the period of independent application, and the application of various linear motors has been rapidly promoted, and many devices and products with practical value have been made, such as steel pipe conveyors driven by linear motors, coal transporters, various electric doors, electric windows, etc. The maglev train driven by linear motors has reached 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 70s of the 20th century. At present, the main achievements include factory driving, electromagnetic hammers, stamping machines, etc. Although China's linear motor research has also made some achievements, there is still a big gap in its promotion and application compared with foreign countries. At present, many domestic research units have noticed this.

The current situation of linear motor application in CNC machine tools

In recent years, the use of linear motors for CNC machine tools has become particularly popular in the world, and the reasons are:

The development of high-speed and ultra-high-speed machining in order to improve production efficiency and improve the processing quality of parts has now become a major trend in the development of machine tools, and a responsive, high-speed, and lightweight drive system should increase the speed to more than 40~50m/min. The maximum feed speed that can be achieved by the transmission form of the traditional "rotary motor + ball screw" is 30m/min, and the acceleration is only 3m/s2. The linear motor drives the worktable, its speed is 30 times that of the traditional transmission method, and the acceleration is 10 times that of the traditional transmission method, up to 10g; The stiffness is increased by 7 times; The linear motor directly driven worktable has no reverse working dead zone; Due to the small moment of inertia of the motor, the linear servo system composed of it can achieve a high frequency response.

In 1993, the German company ZxCell-O launched the world's first worktable driven by linear motor, HSC-240 high-speed machining center, the maximum speed of the machine tool spindle reaches 24000r/min, the maximum feed speed is 60n/min, the acceleration reaches 1g, and when the feed speed is 20m/min, its contour accuracy can reach 0.004mm. Ingersoll in the United States launched the HVM-800 high-speed machining center with a maximum spindle speed of 20,000 r/min and a maximum feed speed of 75.20 m/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 machining machine tools, high-speed forming machine tools, etc. [1].

Zhejiang University in China has developed a stamping machine driven by a linear motor, and 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 its own CNC linear motor lathe with direct drive of linear motor, and in 2003, at the 8th China International Machine Tool Exhibition, the machining center obtained by VS1250 linear motor launched by Beijing Electric Power Institute High-tech Co., Ltd. was exhibited, and the maximum speed of the machine tool spindle reached 15000r/min.

How linear motors work

A linear motor is a transmission device that directly converts electrical energy into linear motion mechanical energy without the need for any intermediate conversion mechanism. It can be seen as a rotating motor split radially and flattened, as shown in Figure 1.

Fig. 1 Transformation process of linear motor

The side that evolved from the stator is called the primary, and the side that evolved from the rotor is called the secondary. In practical applications, the primary and secondary are manufactured in different lengths to ensure that the coupling between the primary and secondary remains unchanged within the required travel range. Linear motors can be short primary and long secondary, or long primary and short secondary. Considering manufacturing costs and operating costs, short primary and long secondary are generally used.

Linear motors work similarly to rotary motors. Take the linear induction motor as an example: when the primary winding is connected to the AC power supply, a traveling wave magnetic field will be generated in the air gap, and the secondary will induce an electromotive force and generate an electric current under the cutting of the traveling wave magnetic field, which will generate electromagnetic thrust when the current interacts with the magnetic field in the air gap. If the primary is fixed, the secondary will move in a straight line under the action of thrust; On the contrary, the primary line motion is done.

Drive control technology for linear motors

A linear motor application system must not only have a well-performing linear motor, 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 basically be divided into three aspects: one is traditional control technology, the second 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. Among them, PID control contains past, present and future information in the dynamic control process, and the configuration is almost optimal, with strong robustness, which is the most basic control method in AC servo motor drive system. In order to improve the control effect, decoupling control and vector control techniques are often adopted.

Under the condition that the object model is determined, unchanging and linear, and the operating conditions and operating environment are unchanged, the use of traditional control technology is simple and effective. However, in the high-performance case of high-precision micro-feeding, changes in object structure and parameters must be considered. All kinds of nonlinear influences, changes in the operating environment and environmental disturbances and other time-varying and uncertain factors can obtain satisfactory control effects. Therefore, modern control technology has attracted great 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 and neural networks with existing mature control methods such as PID and H∞ control to learn from each other's strengths and weaknesses to obtain better control performance.

Application examples of linear motors in CNC machine tools

Piston turning CNC system

The linear motion mechanism using linear motor has been successfully used in CNC turning and grinding of special-shaped section workpieces due to its characteristics of fast response and high precision. 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 response large-stroke CNC feed unit based on linear motors. When used for CNC piston machines, the table size is 600mm×320mm, stroke 100mm, maximum thrust is 160N, and maximum acceleration can reach 13g. Since the linear motor mover and the worktable are fixed together, only closed-loop control can be used, and Figure 2 shows a simplified diagram of the control system of the unit.

Fig.2 Schematic block diagram of linear motor position controller

It is a double closed-loop system with the inner ring being the speed ring and the outer ring being the position ring. A high-precision grating ruler is used as a position detection 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 high accuracy.

Open CNC system with linear motor

The CNC system is composed of a PC and an open programmable power controller, which is based on the general microcomputer and Windows, and the motion controller in the form of a standard plug-in on the PC is the control core, which realizes the openness of the CNC system. The overall design scheme of the open CNC system based on linear motor is shown in Figure 3.

Fig. 3 Schematic diagram of an open CNC system based on a linear motor

The system is composed of a motion control card inserted into the expansion slot of the PC, and the system is composed of a PC, a motion control card, a servo drive, a linear motor, a CNC worktable and other parts. The CNC worktable is driven by a linear motor, and the servo control and machine tool logic control are completed by the motion controller, which can be programmed to interpret and execute the CNC program (G-code, etc., support user expansion) in the form of motion sub-programs. The motion control card model is PCI-8132.

In today's industrial control technology, PCI bus has gradually replaced ISA bus and has become the mainstream bus form, which has many advantages, such as plug and play (plug and play), interrupt sharing, etc. The PCI bus has strict standards and specifications, which ensures that it has good compatibility and high reliability; High transmission data rate (132Mbps) or (264Mbps); The PCI bus is CPU-agnostic, clock-independent, suitable for various platforms, supports multi-processor and parallel work; The PCI bus also has good scalability, allowing for multi-level expansion through PCI_PCI bridge paths. The PCI bus provides great convenience for users and is the most advanced and versatile bus on the PC at present. The PCI-8132 is a 2-axis motion control card with a PCI interface. It can generate high-frequency pulse drive stepper motor and servo motor to control the motor movement of 2 axes and realize linear and arc interpolation. In CNC machining, position feedback is provided.

The system software is developed on the WINDOWS platform. The software adopts modular program design, which consists of user input and output interface, preprocessing module, etc. The user input and output interface realizes the user's input and the output of the system. The main function of user input is to allow users to input CNC code, issue control commands, configure system parameters, and generate CNC machine tool parts processing programs (G-code instructions). After reading the G-code instructions, the preprocessing module compiles and generates a program that can run the PCI-8132 motion control card, thereby driving the linear motor and completing linear or arc interpolation. The process of reading G code is to first set the parameters, and then read the G code, the program process is shown in Figure 4 below.

Figure 4: Flow chart of reading G-code programs

In this system, the PARKER406LXR series linear motors are used. For the two-coordinate CNC worktable, the 406T07 linear motor with a stroke of 550mm is selected in the X direction, and the 406T05 linear motor with a stroke of 450mm is selected in the Y direction.

Epilogue:

The high-speed machining center using linear servo motor has become a key technology and product for the research and development of major machine tool manufacturers in the world, and has achieved initial application and results in the automobile industry and aviation industry. However, the domestic research in this regard is still in its infancy, and the gap is still very large.