With the development of direct drive technology, the comparison between linear motors and the traditional "rotary servo motor + ball screw" drive method has attracted industry attention.
The British invented the linear motor in 1845, but its large air gap resulted in low efficiency, making it unusable. Kollmorgen also introduced one in the 1870s, but its high cost and low efficiency limited its development. It wasn't until the 1970s that linear motors gradually developed and were applied in some specialized fields. In the 1990s, linear motors began to be used in the machinery manufacturing industry. Now, some of the world's most technologically advanced machining center manufacturers are using them in their high-speed machine tools. Well-known international companies such as DMG, Ex-cell-O, Ingersoll, CINCIATI, GROB, MATEC, MAZAK, FANUC, and SODICK have all launched high-speed, high-precision machining centers using linear motors.
As a world leader in linear products, HIWIN has achieved success in ball screws and linear guides. In recent years, it has independently developed and produced linear motors, and has achieved good results in the field of high speed and high precision.
The following comparison mainly refers to HIWIN's advanced high-speed silent ball screw series (DN value up to 220,000) and HIWIN's linear motors, and makes some comparisons on several key characteristics to provide a reference for relevant industry players.
Accuracy comparison:
In terms of accuracy, linear motors reduce interpolation lag due to their simpler transmission mechanism. Their positioning accuracy, reproducibility accuracy, and absolute accuracy are all higher than those of "rotary servo motors + ball screws" through position detection feedback control, and are easier to implement.
Linear motors can achieve a positioning accuracy of 0.1μm . The "rotary servo motor + ball screw" system can achieve a maximum accuracy of 2~5μm. It is required that the transmission parts of the entire closed-loop system, including CNC, servo motor, backlash-free coupling, thrust bearing, cooling system, high-precision rolling guide, nut seat, and worktable, be lightweight and have high grating accuracy.
To achieve high stability, a "rotary servo motor + ball screw" system requires dual-axis drive. Linear motors are high-heat-generating components and require strong cooling measures. To achieve the same goal, linear motors require a greater investment of resources.
Price comparison:
In terms of price, linear motors are much more expensive, which is why they are not more widely used.
Speed comparison:
In terms of speed, linear motors have a significant advantage, reaching speeds of up to 300 m/min and accelerations of 10g; ball screws, on the other hand, achieve speeds of 120 m/min and accelerations of 1.5g . Comparing speed and acceleration, linear motors have a considerable advantage, and their speed can be further improved once the heat generation issue is resolved. In contrast, the combination of a rotary servo motor and a ball screw is limited in terms of speed and cannot be significantly increased further.
In terms of dynamic response, linear motors also have an absolute advantage due to issues such as motion inertia, backlash, and mechanism complexity.
In terms of speed control, linear motors offer fast response and a wider speed range, allowing them to reach maximum speed instantly upon startup and stop quickly during high-speed operation. The speed range can reach 1:10000.
Energy consumption comparison:
Linear motors consume more than twice the energy of a rotary servo motor and ball screw when providing the same torque. Rotary servo motors and ball screws are energy-saving and power-enhancing transmission components. The reliability of linear motors is affected by the stability of the control system and has a significant impact on the surrounding environment. Effective magnetic shielding and protection measures must be taken to isolate the influence of strong magnetic fields on the rolling guide rails and to prevent the adsorption of iron filings and magnetic dust.
The following example will make it easier to understand some of the characteristics of linear motors and "rotary servo motor + ball screw":
A Japanese company's ultra-high-speed gantry machining center uses linear motors to drive the X and Y axes at a speed of V=120m/min. Why doesn't this company use a "rotary servo motor + ball screw (HIWINSUPERS series)"? Because although the DN value of the Supers series has increased from 70,000 to 150,000 and then to 220,000 rpm, the inherent weakness of purely mechanical transmission limits the increase in linear speed, acceleration, and stroke range. If a Φ40×20mm product is used, then vmax=110m/min, and since nmax=5500r/min is very high, the stroke range is clearly limited by the critical speed Nc and cannot be too long.
If a product with a large lead of Φ40×40mm is used, then Vmax=220m/min, which obviously cannot meet the requirements for high positioning accuracy. The ability to achieve a DN value of 220,000 reflects HIWIN's design and manufacturing standards. If we choose a Φ40×20 (double-headed)mm product, using it at n≈4000~5000r/min and V=80~100m/min, its safety, reliability, and service life will all exceed expectations. In fact, to date, there are no successful examples of using the SUPERS series drive in high-speed, high-precision CNC metal cutting machine tools (excluding CNC forming machine tools) with speeds V≥120m/min. Actually, the best application scenarios for "rotary servo motor + ball screw" are: mid-range high-speed CNC equipment and some high-end CNC equipment requiring V=40~100m/min, acceleration 0.8 ~ 1.5 ( 2.0 )g, and accuracy of P3 level or higher.
Application Comparison:
In fact, while linear motors and "rotary servo motors + ball screws" each have their advantages, they also have their own weaknesses. Both have their optimal application ranges on CNC machine tools.
Linear motor drives have unique advantages in the following CNC equipment fields: high-speed, ultra-high-speed, high-acceleration, and large-volume production applications with numerous positioning movements and frequent changes in speed and direction. Examples include production lines in the automotive and IT industries, and the manufacturing of precision and complex molds.
Large, ultra-long stroke high-speed machining centers are used for "hollowing out" integral components with light alloys, thin walls, and high metal removal rates in the aerospace manufacturing industry. Examples include the "HyperMach" machining center (46m) from CINCIATI in the United States and the "HYPERSONIC1400L ultra-high-speed machining center" from MAZAK in Japan.
This requires high dynamic characteristics, responsiveness at both low and high speeds, and highly sensitive dynamic precision positioning. Examples include the new generation of high-performance CNC EDM machines represented by Sodick, CNC ultra-precision machine tools, the new generation of CPC crankshaft grinders, cam grinders, and CNC non-circular lathes.
Lightweight, high-speed specialized CNC equipment. Examples include the "DML80 FineCutting" laser engraving and drilling machine from DMG Germany, the "AXEL3015S" laser cutting machine from LVD Belgium, and the "HyperCear510" high-speed laser processing machine from MAZAK.
The German company DMG is renowned for its mass production of various high-performance CNC equipment. It was an early adopter of linear motors in its servo feed systems, and their adoption rate is very high (all marked "Linear" after the machine tool model). The company offers three configurations for the two drive methods:
All axes are equipped with "high-speed" CNC equipment driven by linear motors. Examples include: DMC85VLinear, DMC75VLinear, DMC105VLinear, DMC60HLinear, DMC80HLinear, and DML80-FineCutting laser processing machine.
Hybrid drive type. For example, the DMF500Linear large vertical machining center with moving column is equipped with a linear motor on the X-axis (travel 5m) with V=100m/min; while the Y and Z axes use "rotary servo motor + ball screw" with V=60m/min.
These are "high-powered" machining centers where all axes are equipped with rotary servo motors and ball screws. For example, the DMC63H high-speed horizontal machining center has a speed of 80 m/min, an acceleration of 1 g, and a positioning accuracy of 0.008 mm. Other models include the DMC80H, DMC100H, DMC125H (duoBLOCK), and DMC60T.
The simultaneous use of both drive methods by the German company DMG demonstrates their respective advantages. Linear motors have significant room for improvement; as the technology matures, production volume increases, and costs decrease, their applications will become more widespread. However, considering energy conservation, emission reduction, and green manufacturing, as well as the inherent characteristics of the two structures, the "rotary servo motor + ball screw" drive still has a vast market potential. While linear motors will become the mainstream drive method in high-speed (ultra-high-speed) and high-end CNC equipment, the "rotary servo motor + ball screw" method will continue to maintain its dominant position in mid-range high-speed CNC equipment.
Note: This article is from China Machinery & Electronics Network.
