The British had invented the linear motor in 1845, but the linear motor's air gap at that time was too large, resulting in low efficiency and it could not be applied. Kollmorgen was also introduced in the 1870s, but its development was limited by its high cost and low efficiency. It was not until the 1970s that linear motors were gradually developed and used in some special fields. In the 1990s, linear motors began to be used in the machinery manufacturing industry. Now some technologically advanced machining center manufacturers in the world begin to use them on their high-speed machine tools. Well-known enterprises such as DMG, Ex-cell-O, Ingersoll, CINCI ATI, GROB, MATEC, MAZAK, FANUC, SODICK have successively launched high-speed high-precision machining centers using linear motors.
With the development of direct drive technology, the comparison between the linear motor and the traditional "rotary servo motor + ball screw" drive has attracted industry attention.
As a world leader in linear products, HIWIN has developed and produced linear motors in recent years after achieving success in ball screws and linear slides, and has achieved good results in the field of high speed and high precision.
The following mainly refers to HIWIN's advanced high-speed silent screw SUPER S series (DN value up to 220,000) and HIWIN's linear motor to make some comparisons on several main characteristics, to provide a reference for related companies.
Speed ​​comparison:
The linear motor has considerable advantages in speed. The linear motor speed reaches 300m / min and the acceleration reaches 10g; the ball screw speed is 120m / min and the acceleration is 1.5g. From the comparison of speed and acceleration, the linear motor has a considerable advantage, and the speed of the linear motor will be further improved after successfully solving the heat problem, but it is difficult to limit the speed of the "rotary servo motor + ball screw" Increase more.
In terms of dynamic response, linear motors also have an absolute advantage because of problems such as motion inertia, clearance, and mechanism complexity.
For speed control, the linear motor has a faster response and a wider range of speed regulation. It can achieve the highest speed at the moment of startup and can quickly stop when running at high speed. The speed regulation range can reach 1: 10000.
Precision comparison:
In terms of accuracy, the linear motor reduces the problem of interpolation lag due to the simple transmission mechanism. The positioning accuracy, reproduction accuracy, and absolute accuracy are all higher than those of the "rotating servo motor + ball screw" through the position detection feedback control, and are easy to implement.
Linear motor positioning accuracy can reach 0.1μm. "Rotary servo motor + ball screw" up to 2 ~ 5μm, and requires CNC-servo motor-gapless coupling-thrust bearing-cooling system-high-precision rolling guide-nut seat-table closed-loop transmission of the entire system Some parts should be lightweight, and the grating accuracy should be high.
If you want to achieve high stability, the "rotary servo motor + ball screw" should be driven by two axes. The linear motor is a high-heat component and strong cooling measures must be taken. To achieve the same purpose, the linear motor has to pay a greater price. .
price comparison:
In terms of price, the price of linear motors is much higher, which is why the linear motors are more widely used.
Energy consumption comparison:
The energy consumption of the linear motor when providing the same torque is more than double that of the "rotary servo motor + ball screw". The "rotary servo motor + ball screw" is an energy-saving and force-increasing transmission component. The reliability of the linear motor is controlled by the control system Stability has a great impact on the surrounding area. Effective magnetic isolation and protective measures must be taken to isolate the impact of strong magnetic fields on the rolling guide and the adsorption of iron dust and magnetic dust.
Through the following example, it is easier for everyone to understand some characteristics of linear motor and "rotary servo motor + ball screw":
A Japanese company's ultra-high-speed gantry machining center. X, Y axis is driven by linear motor V = 120m / min. Why doesn't the company apply "rotating servo motor + ball screw (HIWIN SUPER S series)"? Although the DN value of SUPER S has experienced a speed increase from 70,000 to 150,000 to 220,000, but due to the soft ribs of pure mechanical transmission, the increase in linear speed, acceleration, and stroke range is always limited. If the product of Φ40 × 20mm is selected, vmax = 110m / min, because nmax = 5500r / min, the speed is very high, and the stroke range is obviously impossible to be too long because of the critical speed Nc. If the product with large lead Φ40 × 40mm is used, Vmax = 220m / min, which obviously cannot meet the occasion with high positioning accuracy. Achieving a DN value of 220,000 reflects HIWIN's design and manufacturing standards from one side. If we choose Φ40 × 20 (double-headed) mm products and use them under the condition of n≈4000 ~ 5000r / min, V = 80 ～ 100m / min, the safety, reliability and working life can all be higher than expected. In fact, so far, in high-speed and high-precision CNC metal cutting machine tools (except CNC forming machine tools), the speed V≥120m / min is still successful examples of using SUPER S series drives. In fact, the best application of "rotating servo motor + ball screw" is: mid-range high-speed CNC equipment and some high-end CNC equipment that require V = 40 ~ 100m / min, acceleration 0.8 ~ 1.5 (2.0) g, and accuracy of P3 or above .
Application comparison:
In fact, the two driving methods of linear motor and "rotary servo motor + ball screw" have their own weaknesses despite their advantages. Both of them have their own optimal scope of application on CNC machine tools.
The linear motor drive has unique advantages in the following CNC equipment fields: high speed, ultra high speed, high acceleration and large production batches, requiring many positioning movements, and frequent changes in speed and direction. For example, production lines of the automotive industry and IT industry, the manufacture of precision and complex molds.
Large-scale, ultra-long-stroke, high-speed machining center, and "hollow-out" processing of light alloy, thin-walled, and high-metal removal components in the aerospace manufacturing industry. For example, the "Hyper Mach" machining center (46m) of CINCI ATI in the United States; the "HYPERSONIC 1400L ultra-high-speed machining center of Japan MAZAK".
High dynamic characteristics, low-speed and high-speed follow-up, and highly sensitive dynamic precision positioning are required. For example, a new generation of high-performance CNC electric machining machines represented by Sodick, CNC ultra-precision machine tools, a new generation of CPC crankshaft grinding machines, cam grinding machines, CNC non-circular lathes, etc.
Light load, fast special CNC equipment. For example, "DML80 Fine CutTIng" laser engraving and punching machine of DMG in Germany, "AXEL3015S" laser cutting machine of Belgium LVD company, "Hyper Cear510" high-speed laser processing machine of MAZAK, etc.
The German DMG company is famous for mass production of various high-performance CNC equipment. The linear motor is used earlier in its servo feed system, and the adoption rate is also very high (all marked "Linear" after the machine model). There are three types of drive configuration:
All coordinate axes are equipped with "rapid" CNC equipment driven by linear motors. For example: DMC85V Linear, DMC75V Linear, DMC105V Linear, DMC60H Linear, DMC80H Linear and DML80-Fine CutTIng laser processing machine.
Hybrid drive type. For example: DMF500 Linear moving column type large vertical machining center, equipped with a linear motor on the X axis (stroke 5m), V = 100m / min; and the use of "rotating servo motor + ball screw" on the Y and Z axes, V = 60m / min.
Each coordinate axis is equipped with a "strong type" machining center of "rotary servo motor + ball screw". For example: DMC63H high-speed horizontal machining center, V = 80m / min, acceleration 1g, positioning accuracy 0.008mm. In addition, there are DMC80H and DMC100H, DMC125H (duo BLOCK) and DMC60T.
The use of both driving methods in the German DMG company also shows that they have their own advantages. Linear motors have a lot of room for improvement. In the future, linear motor technology will become more mature, output will increase, costs will decrease, and applications will be more extensive. "Rotary servo motor + ball screw" drive still has its broad market space. Linear motors will become the mainstream drive method in high-speed (super high-speed) and high-end CNC equipment, while "rotary servo motor + ball screw" will continue to maintain its mainstream position in mid-range high-speed CNC equipment.
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