Putting Linear in Motion

ControlDesign.com

The Technology Is Sound. The Application Often Is Ideal. Why Don’t We See Linear Motors Everywhere?

By Mark Lamendola

Early adopters tend to brave the uncertainty of new technologies as they pave the way for eventual widespread acceptance. The replacement of rotary motion translation with linear motors is one of those technologies. Progress has been slow but deliberate.

To understand where the industry stands today and why that is, the first step is to understand the compelling reasons for choosing a linear motor in the first place.

Linear Motors Fill a Void

“With linear motors, we’re able to reproduce accurate, high-velocity, non-sinusoidal motion, including triangle, square, random, and data acquired for the road or test track, without the use of hydraulics,” says Doug Boals, vice president, engineering sales, Roehrig Engineering, Lexington, N.C. Roehrig makes automated damper dynamometers and spring raters.

Did that mention of hydraulics come out of left field? No. In some cases, a rotary motor isn’t even an option. “Linear motors can provide relatively high forces and velocities that, until recently, only servo hydraulic systems could reach for us,” adds Boals.

Pick and Place

Baldor’s St. Louis Stamping Plant was able to automate movement of stacks by implementing linear motors. Photo courtesy of BALDOR

So, linear motors can fill a requirement that rotary motors can’t. And, as Boals points out, they do it without the installation and facility costs associated with hydraulics, including pumps, hoses and hard lines, heat exchangers and related chillers/coolers, accumulators, and fluid-conditioning equipment. “This allows our units to be portable as well, since only an electrical connection is required to run a test system,” he says. “In addition, linear motors are about twice as efficient as a comparable hydraulic system, thereby reducing daily operating costs.”

Consider an example application of such a motor. At Baldor’s St. Louis Stamping Plant (see figure), machines move stacks of laminations—steel parts for making the stators and rotors—from conveyors onto pallets or into boxes. Laminations are stamped out on high-speed presses, and the stacks weigh up to 51 lb. Automating the movement of these stacks from the presses with rotary motors proved so challenging that this operation was left as a manual one for quite some time. With linear motors, however, automation became feasible to implement.

Automation supplier B&R Automation specifies linear motors as a part of certain systems it provides. “We use the linear motors primarily when very precise and dynamic moves must be initiated at very low mass,” says Dario Doko, B&R sales engineer.

Slow Pace of Adoption

While it appears that a linear motor approach has great advantages, actual implementations are confined to a narrow slice of the potential user base. Why is this?

End users typically are cautious about using a technology they don’t already have. There’s also the issue of using the right tool for the job. If you only look at the advantages of a linear motor, you don’t get the whole story about its suitability. For example, a linear motor lacks a holding brake for applications for which gravity is a factor.

Those traditional rotary motors have advantages, too. Linear motors aren’t automatically better than their rotary counterparts. “Clearly there are some very good reasons to use linear motors,” says Kenneth Flowers, vice president and owner, Machine Tool Builders, Machesney Park, Ill. “The applications include those needing high speed and high accelerations. Linear motors make sense where many high-speed repetitions of motions—oscillations—are required. But let’s not forget that rotary motors have served us well over the years, and still have a firm hold on the market.”

There are many applications, adds Flowers, for which rotary motors are the only motion method that works. “For example, rotary axes, or very heavy vertical axes, or axes that have loads to carry that demand high torques and low speeds,” he says. Flowers believes linear technologies will improve and eventually become very prevalent. But, he says, they will never totally replace rotary motors.

Next-Generation Design

What about upgrading an existing application to a linear motor? “Our business is recontrolling and rebuilding machinery, and it’s often impractical to modify the machine to such a large extent to adapt linear motor technology,” says Flowers. “When you design a new machine with a clean sheet of paper, adapting linear technologies is much easier than modifying a machine already equipped with rotary motors.”

In applications involving forces, rotary motors are more appropriate. Where you have low inertia and high speed, linear motors typically outperform rotary motors. At this point, you could conclude that if the customer is open to using linear motors, then the machine builder’s decision is straightforward. But such a conclusion would be incomplete and often wrong.

Decisions can’t be based on technical factors alone. Those factors are the only part of the iceberg you can see. The real story lies beneath the surface.