Linear motor
Linear motors can be considered a structural variation of rotary motors; they can be viewed as a rotary motor cut radially and then flattened. With the rapid development of automatic control technology and microcomputers, higher demands are placed on the positioning accuracy of various automatic control systems. In this context, traditional rotary motors combined with a conversion mechanism to form linear motion drive devices are far from meeting the requirements of modern control systems. Therefore, many countries around the world are researching, developing, and applying linear motors, leading to an increasingly wide range of applications for them.
Compared with rotary motors, linear motors have the following main characteristics:
First, the structure is simple. Since linear motors do not require additional devices to convert rotary motion into linear motion, the structure of the system itself is greatly simplified, and the weight and volume are greatly reduced.
Second, it has high positioning accuracy. In places where linear motion is required, the linear motor can achieve direct transmission, thus eliminating various positioning errors caused by intermediate links. Therefore, the positioning accuracy is high. If microcomputer control is used, the positioning accuracy of the entire system can be greatly improved.
Third, it has a fast response speed, high sensitivity, and good follow-up performance. Linear motors can easily make their movers supported by magnetic levitation, thus ensuring that there is always a certain air gap between the movers and stators without contact. This eliminates the contact friction resistance between the stators and movers, thereby greatly improving the system's sensitivity, speed, and follow-up performance.
IV. Safe and reliable operation with a long service life. Linear motors can achieve contactless force transmission, with almost zero mechanical friction loss, resulting in fewer malfunctions, maintenance-free operation, and therefore safe, reliable operation with a long service life.
These characteristics enable linear motors to be used in the following three main applications:
1. It is used in automatic control systems, and there are many such applications.
II. As a drive motor for long-term continuous operation;
Third, it can be applied in devices that require a large amount of linear motion energy within a short time and distance.
Automation applications in the packaging industry
In the packaging industry, capping modules are a common type of filling equipment. They require simultaneous linear and rotary motion to tighten the caps, and force control is necessary throughout the process to ensure quality. Currently, two capping module technologies are widely used in packaging industry filling equipment: mechanical capping technology and semi-servo capping technology.
Mechanical capping technology, also known as first-generation capping technology, achieves linear motion through a mechanical cam and controls rotation and capping force through gears and magnetic rings. Because the entire mechanism's movement is mechanical, it suffers from drawbacks such as fixed rotational torque, lack of torque feedback, difficulty in adjusting speed and stroke, fixed contact force, a maximum stroke limited to 80-150mm, and the absence of feedback on the cap's vertical position and loading effect detection.
During operation, the vertical lifting height and the rotation angle of the rotating parts of this mechanism depend entirely on the level of mechanical design and machining, making it difficult to control the quality of the capping process. Because the overall mechanical design is relatively fixed, when customers need to change the movement distance and height, the mechanical position must be adjusted or the mechanical cam replaced, resulting in poor operability and making it unsuitable for small-batch production.
Currently, most domestic enterprises use this method, which is highly complex in design, difficult to install and debug, and requires a large amount of maintenance work for long-term operation. However, due to the low overall equipment cost, it is also used by many enterprise users who do not have high requirements for product efficiency, performance, and energy consumption.
50% servo capping technology, also known as second-generation capping technology, adds a rotary servo motor to the first-generation mechanical capping technology for capping, while the linear motion is still achieved through a mechanical cam. This technology has made breakthroughs in the control or adjustment of rotational force, torque, and speed, but it still retains the shortcomings of the first generation in terms of stroke, contact force, cap vertical position, and loading effect.
Because the rotating part uses a servo motor, the force and position of the capping mechanism can be better controlled, thus improving the quality of the capping. However, since the lifting part still uses a traditional mechanical cam structure, it is relatively troublesome to adjust the height when changing bottle shapes. Currently, some domestic equipment manufacturers have produced this type of capping module, and some companies with high requirements for capping quality have begun to adopt this technology and equipment.
The third-generation capping module, known as 100% servo technology, uses a linear rotary motor to completely replace the traditional mechanical structure, achieving full servo control of linear and rotary motion.
A linear motor opens the magnetic field of the stator of a traditional motor and rotates it laterally. By changing the current, it creates a linear magnetic field force on the internal moving part, thus directly achieving linear motion. This eliminates the need for various structures required to convert traditional rotary motion into linear motion.
Tubular linear motors have a simple structure, consisting of only two independent parts: a stator and a mover. They resemble cylinders in appearance, but their performance is several times that of electric cylinders. They can achieve high-speed, high-response multi-point control of position, speed, acceleration, and force.
Because of its axisymmetric structure, the tubular linear motor can be integrated with a traditional rotary servo motor to achieve linear-rotational motion. Based on the principles of linear and rotary servo systems, the position, speed, and force of both linear and rotary motions can be controlled. This linear-rotary motor has been successfully applied to capping modules in canning equipment, leading the development of third-generation servo capping technology.
The capping module, composed of a KERI LinMot linear rotary motor and a modular driver, is simpler in structure and greatly simplifies installation, debugging, and maintenance compared to traditional methods. In terms of performance, it achieves full servo control of the capping process, which not only greatly improves the quality of capping but also enables online adjustment. It is particularly suitable for enterprises with high requirements for product quality and frequent changes in product varieties.
Krones' Contiform3 filling equipment utilizes the Krones LinMot capping module. This module mounts 20 tubular linear motors (equivalent to 40 servo motors) on a 540mm cylinder, significantly improving capping efficiency and quality while substantially reducing energy consumption and eliminating the need for moving cables. It meets customers' stringent product quality requirements and allows for flexible small-batch production, garnering significant positive feedback in the filling industry market.
Application of tubular linear motors in the packaging industry
Applications on CNC machine tools
In recent years, the use of linear motors in CNC machine tools has become particularly popular internationally, for the following reasons:
High-speed and ultra-high-speed machining, developed to improve production efficiency and the quality of machined parts, has become a major trend in machine tool development. A responsive, high-speed, and lightweight drive system is needed, with speeds exceeding 40-50 m/min. The traditional "rotary motor + ball screw" transmission system can only achieve a maximum feed rate of 30 m/min and an acceleration of only 3 m/s².
The linear motor drives the worktable, which is 30 times faster and 10 times faster than the traditional transmission method, with a maximum acceleration of 10g; the stiffness is increased by 7 times; the worktable directly driven by the linear motor has no reverse dead zone; due to the small inertia of the motor, the linear servo system constructed from it can achieve a high frequency response.
In 1993, the German company ZxCell-O launched the world's first high-speed machining center with a linear motor-driven table, the HSC-240. The machine tool's spindle reached a maximum speed of 24,000 rpm, a maximum feed rate of 60 N/min, and an acceleration of 1g. At a feed rate of 20 m/min, its contour accuracy reached 0.004 mm. The American company Ingersoll followed suit with the HVM-800 high-speed machining center, with a maximum spindle speed of 20,000 rpm and a maximum feed rate of 75.20 m/min.
Since 1996, Japan has successively developed horizontal machining centers, high-speed machine tools, ultra-high-speed small machining centers, ultra-precision mirror machining machines, and high-speed forming machine tools using linear motors[1].
Zhejiang University in my country developed a stamping machine driven by a linear motor, and the Institute of Production Engineering of Zhejiang University designed a coordinate measuring machine with a parallel mechanism driven by a cylindrical linear motor [2]. In 2001, Nanjing Sikai Company launched a CNC linear motor lathe that was developed by itself and directly driven by a linear motor. At the 8th China International Machine Tool Exhibition in 2003, a machining center with a VS1250 linear motor launched by Beijing Electric Power Research Institute High Technology Co., Ltd. was exhibited. The maximum spindle speed of the machine tool reached 15,000 r/min.
Linear motor drive control technology
A linear motor application system not only requires a high-performance linear motor, but also a control system capable of meeting technical and economic requirements under safe and reliable conditions. With the development of automatic control technology and microcomputer technology, there are increasingly more control methods for linear motors. Research on linear motor control technology can be broadly divided into three aspects: traditional control technology, modern control technology, and intelligent control technology.
Traditional control techniques such as PID feedback control and decoupling control are widely used in AC servo systems. PID control, in particular, incorporates past, present, and future information from the dynamic control process, and its configuration is almost optimal, exhibiting strong robustness. It is the most fundamental control method in AC servo motor drive systems. To improve control performance, decoupling control and vector control techniques are often employed.
When the object model is fixed, unchanging, and linear, and the operating conditions and environment are constant, traditional control techniques are simple and effective. However, in high-precision, micro-feeding applications, changes in the object's structure and parameters must be considered. Various nonlinear effects, changes in the operating environment, and time-varying and uncertain factors such as environmental disturbances must be taken into account to achieve satisfactory control results. Therefore, modern control technology has attracted significant attention in the research of linear servo motor control. Commonly used 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. Currently, the main approach is to combine fuzzy logic and neural networks with existing mature control methods such as PID and H∞ control, leveraging their strengths and compensating for their weaknesses to achieve better control performance.
Application examples of linear motors in CNC machine tools
Piston turning CNC system
Linear motion mechanisms using linear motors have been successfully applied to CNC turning and grinding of irregularly shaped cross-section workpieces due to their fast response and high precision. For non-circular cross-section parts, which have the highest production volume, the Non-Circular Cutting Research Center of the National University of Defense Technology has developed a high-frequency response, long-stroke CNC feed unit based on a linear motor.
When used in a CNC piston machine tool, the worktable dimensions are 600mm × 320mm, the stroke is 100mm, the maximum thrust is 160N, and the maximum acceleration can reach 13g. Since the linear motor actuator and the worktable are fixed together, only closed-loop control can be used. The figure shows a simplified diagram of the control system for this unit.
Block diagram of linear motor position controller
This is a dual closed-loop system, with an inner velocity loop and an outer position loop. A high-precision optical grating ruler is used as the position detection element. The positioning accuracy depends on the resolution of the grating, and the system's mechanical errors can be eliminated through feedback, resulting in high precision.
Open CNC system using linear motors
A numerical control system is constructed using a PC and an open programmable motion controller. This system uses a general-purpose microcomputer and Windows as the platform, and a standard plug-in motion controller on the PC as the control core, thus realizing the openness of the numerical control system. The overall design scheme of the open numerical control system based on linear motors is shown in the figure.
Schematic diagram of an open CNC system based on linear motors
This system employs a design that inserts a motion control card into the expansion slot of a PC. The system comprises a PC, motion control card, servo drivers, linear motors, and a CNC worktable. The CNC worktable is driven by linear motors, and servo control and machine tool logic control are both handled by the motion controller. The motion controller is programmable and interprets and executes CNC programs (G-code, etc., supporting user expansion) as motion subroutines. The motion control card model is PCI-8132.
In today's industrial control technology, the PCI bus has gradually replaced the ISA bus and become the mainstream bus form. It has many advantages, such as plug and play and interrupt sharing.
