Application and Prospect of Linear Motors in Machine Tools
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1 Introduction
With the rapid development of electronic technology, the emergence of a large number of high-speed multi-functional or dedicated microprocessors and signal processors (such as DSP), the application of linear motors in the field of machine tools ushered in its climax. From the 1996 Chicago Machine Tool Show IMTS`96, the 18th International Machine Tool Exhibition in Japan, to the 1999 Paris International EMO Expo and a series of international influential exhibitions, the United States Ingerell milling company, Germany's Siemens ( IFNI), Japan's three fine companies, the United States's Kollmorgen company and other international well-known companies to show people the power of DC motors applied to all types of machine tools, which indicates that the new era of linear motor development has arrived.
2 Application in the field of machine tools
2.1 Application in high-speed and ultra-high-speed precision machining
The high-speed and ultra-high-speed machining that has been developed in order to increase production efficiency and improve the quality of parts has now become a major trend in machine tool development. A responsive, high-speed, light-weight drive system, the speed should be increased to 40-50m/min above, acceleration and deceleration is also required to increase to 25-50m/s2, the traditional "rotary motor + roller screw" transmission form is obviously not This is determined by its own weaknesses, because the presence of the intermediate transmission link first reduces the stiffness, and the elastic deformation can increase the order of the system, thereby reducing the robustness of the system and degrading the servo performance. Elastic deformation is the root cause of mechanical resonance in CNC machine tools. Secondly, the presence of intermediate transmission links increases the inertia of the moving body, which makes the displacement and velocity response slow. In addition, such factors as the clearance dead zone, friction, and error accumulation make the highest feed rate that can be achieved in this conventional way to be 30 m/min, and the acceleration is only 3 m/s2.
The advantages of direct-current motor direct drive can precisely compensate for the shortcomings of the traditional transmission mode, its speed is 30 times that of the roller screw pair; the acceleration is 10 times that of the roller screw pair, up to 10g, and the rigidity is improved. 7 times; In addition, the linear motor directly drives the worktable, so there is no dead zone in the reverse direction; due to the small inertia of the armature, the linear servo system formed by it can achieve a high frequency response (eg 100Hz).
Through the above comparison, the application of the linear motor has a wide range of prospects in high-speed and ultra-high-speed precision machining. At present, the main requirements of the machine to meet the requirements of large thrust feed components are AC linear motors. From the excitation methods, they can be divided into permanent magnet (synchronous) and induction (asynchronous). The permanent magnet-type secondary (stator) is a permanent magnet. When applied to a machine tool, permanent magnets must be laid on the bed of the machine tool, and three-phase energized windings must be mounted on the lower part of the table to form the primary part of the linear motor. ). The inductive primary is the same as the permanent magnet, but its secondary is to replace the magnet with a grid, which is equivalent to deploying the "squirrel cage" of the inductive rotating motor along its circumference.
The permanent magnet type linear motor is superior to the inductive type in the unit area of ​​thrust, power factor, controllability, etc., but the price is relatively expensive, and installation, debugging, dust prevention, etc. are all inferior to the induction type. The X, Y, and Z axes of the high-speed horizontal machining center HVM800 produced by the Ingersoll Milling Machine Company of the United States adopt a permanent-magnet synchronous linear motor. The maximum feed speed is 76.2 m/min, and the acceleration α is =1 to 1.5 g.
Inductive linear motors are close to the level of permanent magnet motors in performance and, together with their own advantages, are becoming more and more popular. The typical example of application is the XHC240 high-speed horizontal machining center developed by Ex-cell-o GmbH of Germany. The three feed axes use Indramat's inductive linear motor to directly drive the feed parts. The fast moving speed is up to 60m/ Min, the maximum acceleration is 1g.
In the field of high-speed and ultra-high-speed machining, linear motors are widely used in high-speed milling machines, crankshaft lathes, ultra-precision lathes, grinding machines, and laser lathes. Now, the more popular research is to apply it to high-speed parallel mechanisms, that is, six. Axis and triaxial parallel machine tools control the tool through the expansion and contraction of multiple sliding plungers to achieve high-speed machining of complex surfaces. The University of Technology Zurich has developed a six-slide machine with a new structure, which applies linear drive technology in the efficient milling of the machine. In addition, both Lapik in Russia and Ingersoll in the United States have conducted research in this area and formed products.
2.2 Applications in Ultra-precision Machining Ultra-precision machining is a cutting-edge science in manufacturing. It plays a decisive role in the work, especially in the defense industry. The inertial guidance device gyroscope for cruise missiles, the key device waveguides for radar, and satellite instruments On the precision bearings, large integrated circuits, etc. are involved in superfinishing. In the ultra-precision machine tool, in order to carry out on-line compensation for machining errors in the machine, to improve the shape accuracy of the processing; for some special non-axisymmetric surface processing, all need micro-feed device, high-precision micro-feed device is now Become an important key device for ultra-precision machine tools.
At present, piezoelectric ceramic linear motors have a wide range of applications in precision micro-feed devices, the principle of which is the use of electrostrictive effects. The amount of deformation of the electrostrictive effect is proportional to the square of the electric field strength. It can achieve high stiffness without gap displacement; resolution of up to 1.0 ~ 2.5nm; and a large deformation coefficient; a very high frequency, its response time of up to 100μs. In order to increase the stroke, the motor is generally made of a plurality of crystals stacked and bonded together. The ulnar-type piezoelectric ceramic motor is composed of three individually controlled tubular ceramic piezoelectric devices, wherein A, B for radial expansion In order to clamp and release the motor shaft, and device C for axial expansion and contraction, so that the motor shaft produces axial displacement, to achieve linear motion step.
The DTM-3 large-scale diamond lathes and LODTM large-scale optical diamond lathes in the United States LLL National Laboratory and the OAGM2500 large-scale precision machine tools from Cranfieid in the UK have all adopted electrostrictive micro-feed devices.
Ultrasonic Motor (USM) is a new type of direct drive motor that is increasingly appreciated at home and abroad. It is also broadly speaking a piezoelectric ceramic motor that utilizes the inverse piezoelectric effect of piezoelectric ceramics to amplify the microscopic deformation of a material through resonance. Friction coupling is converted into a macroscopic motion of a rotor or a slider, and the piezoelectric ceramic can form a traveling wave in a single direction under a suitable voltage. If the rotor acts on the surface of the elastomer with the appropriate pressure, it will be driven by the friction of the particle. Change the traveling wave direction, then the rotor is reversed.
In ultra-precision machining, in order to process aspherical surfaces with high-precision shapes, the feed drive system of ultra-precision machine tools is required to have a high resolution, and the amount of movement per pulse is 0.01μm. There are several well-known foreign countries. The company owns the product, but bans it from China. At present, National Defense Science and Technology University, Harbin Institute of Technology, and Tsinghua University are conducting research in this area. Ultrasonic motor has the characteristics of small size, light weight, fast response speed, no electromagnetic interference and low speed and high torque. The stroke can replace the traditional electromagnetic motor in the range of 10cm. The step-driven ultrasonic motor with feedback closed-loop control has a step resolution of about 0.01 μm and is expected to replace the mechanical friction drive method. The surface acoustic wave linear USM developed by the University of Tokyo, Japan, has a step resolution of up to 5 nm. 2.3 Application in Profiled Section Processing
Due to the linear motion of the linear motor, the movement mechanism has been successfully used for computer control of special-shaped cross-section workpieces (car engine pistons, lobe-shaped bearing outer ring raceways, piston rings, cams, etc.) due to its fast response and high accuracy. Precision turning and grinding. Compared with the traditional method of “special†processing of profiled inner and outer circle contours, it has the characteristics of flexible programming modification and high processing precision, which is very suitable for the processing of multi-variety, low-volume products.
3 Application examples of linear motors on machine tools
The following is an example of machining an engine piston.
The piston is one of the most critical parts of the engine. In the reciprocating working state, the temperature rise of the head and the skirt is different, and the deformation is not uniform. Therefore, the cross-section of the piston is designed to be shaped (approximately ellipse) at room temperature.