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Linear Motors Take Center Stage PUBLIC ACCESS

Able to Produce High Forces without Sacrificing Speed or Precision, Linear Motors offer Significant Advantages Over ball Screws and Rack-and-Pinion Systems.

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Associate Editor

Mechanical Engineering 120(03), 92-93 (Mar 01, 1998) (1 page) doi:10.1115/1.1998-MAR-5

Linear motors offer significant advantages over ball screws and rack-and-pinion systems as they can produce high forces without sacrificing speed or precision. Driven by electricity, linear motors provide direct linear motion without the potential complications associated with pneumatic and hydraulic systems, and without mechanical linkages such as ball screws or rack-and-pinion systems. Linear motors are potentially effective in any application requiring linear motion, and some uses can derive a great many benefits from the technology. Linear motors cost more than their traditional counterparts; however, the price has decreased significantly in recent years because of increases in the volume sold. Also, manufacturers have begun to invest more heavily in tooling and equipment geared specifically for the manufacture of linear motors. As acceptance of linear motors has increased, so has the number of competitors; one expert estimated that the number of manufacturers has gone from four to 30 in the past four years.

IN THE PAST few years, linear motors have been gaining widespread acceptance in applications such as mac hine tools and semiconductor-manufacturing equipment, because technologies have become available that increase the effectiveness of these devices. Now the motors are catching on in other areas as well.

Driven by electricity, linear motors provide direct linear motion without the potential complications associated with pneumatic and hydraulic systems, and without mechanical linkages such as ball sc rews or rack-a ndpinion systems. "A permanent-magnet linear motor can be thought of as a permanent-magnet rotary motor whose stator and rotor have been cut along a radial plane and unrolled so th at th ey provide linear thrust," said Mark Wilson, vide president of engineering at Northern Magnetics Inc. in Santa Clarita, Calif. "The same forces of electromagnetism th at produce torque in a rotary motor are used to produce direct linear force in linear motors. The result is a flat linear motor that produces linear force, as opposed to torque, because the axis of rotation no longer exists."

Either the stator or the rotor must be extended to develop a constant force over a given stroke. The length of the extension is detennined by the length of the stroke required for the application. All linear motors are classified as short prim.ary or short secondary. The primary is the part of the nlOtor with the motor windings, which receives dc voltage. Depending on the type of lin ear motor, the windings are wound in slots in a steel lami nation stack or encapsulated in epoxy.

Linear motors can be used in a wide variety of applications. Depending on the design, they can generate forces less than 1 ounce or as great as 2,000 pounds, and speeds can range from 0.0001 to 250 inches p er second. A linear motor can produce accelerations greater than 10 g and can have submicron accuracy when coupled with the appropriate feedback device.

These motors have signifi cant advantages ove r ball screws and rack-and-pinion systems. Most signifi cantly, they can provide a very high force without sacrifi cing speed or precision. Because motion is transmitted by electromagnetic indu ction rather than the friction used in a rotary-driven device, the design is simple, whi ch enhances reliability. A linear motor is also completely noncontact. Under normal specified operating conditions, life expectancy is relatively high.

Only within the past few yea rs have linear motors been implemented on a widespread basis. "The main reason why linear motors were not accepted sooner was that th e ancillary technology wasn't there to support them," said Boaz Eidelberg, director of lin ea r- motor business development at Anotad Corp. in Hauppauge, N.Y. "Other technologies were needed so that linear motors co uld deliver the maximum possible benefit." For example, linear motors gained a signifi cant inroad with the introduction of the so-call ed recirculating bea ring, which helps nuke machines very stiff. "This stiffness is a prerequisite for linear motors to operate effectively," he added.

Another advance was comp uter numerical control (CNC). These controllers can eliminate linear tachometers with differentiating encoder signals; this is helpful because the feedback from such devices is noisy and can make systems difficult to stabilize. Once the supporting technology was readily available at a competitive price, linear motors began to gain a foothold. Since then, the CNC vendors and major bearing manufac turers h ave begun to design their products to support linear motors. Several are now shipping their equipment with such motors, and more are expected to follow suit.

Linear motors are potentially effective in any application requiring linear motion, and some uses can derive a great many benefits from the technology. Manufacturers of automation equipment showed interest early on, for exa.n1ple, because linear motors allow their existing products to operate both faster and more accurately. (By contrast, ball screws are highly accurate but speed is sacrificed.) In general, settling time is also lower for a linear-motor-based system than for competing te chnologies. As a result, linear motors can h elp machines generate the higher throughput that end users always demand.

Typical users in this area have been manufacturers of robots, assembly machines, and manufacturing equipment. Auto manufacturers h ave been quick to embrace linear-motor-based systems because these motors support flexible-automation initiatives. Instead of investing $50 million in a line that only makes one type of car, auto companies are d eveloping flexibleautomation machining centers that can manufacture several cars, simply by changing fixtures and software. This concept requires the machines to be extremely fast and use computer numerical control, which the linear motors allow.

The other major users of linear motors are semiconductor manufacturers. Unlike machine manufacturers, companies that make semiconductors place their highest priority on precision, because motions smaller than 1 miron are often needed. Moving through these small distances, the equipment also must be extremely accurate and smooth; any ripples could compromise the integrity of the entire lot. The linear-motor-based machine can use a combination of an air bearing and a laser interferometer as feedback to achieve a completely noncontact system with the necessary accuracy.

Emerging markets gradually accepting linear motors include manufacturers of control equipment for elevators, compressors, amusement-park rides, and oil-welldrilling equipment. Some machines are now being designed expressly for use with linear motors, which makes them much simpler and more reliable.

Not For All Applications

Although the motors can serve in almost any application that requires linear motion, they're not always the best choice. "Users need to conduct a thorough analysis of their systems," Eidelberg said. "The system must be very stiff and light, with as high a natural frequency as possible. This is essential for good smoothness and se ttling time. The design must be optimized for those things."

Linear motors are potentially simpler than any other linear actuator and deliver very high performance. However, if the other components in the system do not deliver equally high performance, the benefits of the linear motor will be lost. If the system does not have adequate stiffi1ess, for example, the linear motor might be no more effective than less expensive alternatives.

Linear motors cost more th'an their traditional counterparts, but the price h as decreased significan tly in recent years 'because of increases in the volume sold. Also, manufacturers have begun to invest more heavily in tooling and equipment geared specifically for the manufacture of linear motors. As acceptance of linear motors has increased, so has the number of competitors; one expert estimated that the number of manufacturers has gone from four to 30 in the past four years. Many firms are considering entering the market because of the opportunity to sell all of the related control equipment along with the motors.

"Although the cost of linear motors is generally higher,'" said Dan jones, a motion-control consultant based in Thousand Oaks, Calif., "the benefits that they offer often far outweigh the added cost." For example, a major manufacturer of borers changed the design of one particular model so that it worked with a linear motor instead of a ball screw. The price of the machine went up 15 percent-but the throughput increased 100 percent. "From a user standpoint," jones added, "the cost/ performance ratio makes the machine with the linear motor a much better value."

Copyright © 1998 by ASME
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