Putting Linear in Motion

Oct. 4, 2007
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-conditio
ning 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.

A company that builds systems with rotary motors doesn’t just buy a linear motor and slap it in. Linear motor systems bring with them an entire area of knowledge. This knowledge encompasses a wide range of disciplines, from mechanical design to programming.

It’s not that machine builders don’t want to learn. In fact, they’re always learning—and that includes learning about linear motors. “In my business, you must keep up with the trends,” says Flowers. “I’m very familiar with linear motor capabilities and areas of use. In fact, we’re considering linear motors on a new design we’re mulling over.”

After the Sale

Serviceability is another factor. Machine builders want to be confident they can provide the level of service their customers need. When a critical production system goes down, nobody wants to hear, “I need to research this.”

How does a machine builder think of it? There you are, with two choices. The first is a system with which you have a great deal of experience. The second is a system you haven’t seen perform nearly as many times. You know that, down the road, support for design modifications and those inevitable malfunctions will fall squarely on your shoulders. With which system would you feel more confident in going forward?

Generally, a machine builder has far more experience with rotary systems and consequently less uncertainty than with linear. There’s a lot to be said about working within your comfort zone. The 1982 business blockbuster In Search of Excellence (Peters and Waterman) gained a cult following that still is strong a quarter century later. It devoted an entire chapter to the concept, “Stick to your knitting,” that is, focus on what you do well.

Does It Add Up?

Plain old economics provides yet another barrier. Machine builders need a return on the substantial investment in implementing and supporting linear technology. For most applications, that return isn’t quite there because the demand isn’t quite there. Yet some day, it will be. The machine builders we talked to are preparing for that day.

Mark Lamendola is a frequent contributor to Control Design, with many years of experience working on and writing about industrial automation issues. He holds an MBA, EE degree, and is an IEEE member. You can reach Mark at [email protected].

Performance Parameters

There are as many types of linear motors as there are rotary motors. The physics are the same, however, the linear, direct drive nature of linear motors introduces differences in the way the two types of motors are specified and applied. Most users are familiar and comfortable with rotary motor specifications because they have been used in practical applications for more than a century. But linear motors have only been commercially available for about the past 20 years and used primarily in niche applications. As linear motors gain in popularity, the need to understand their specifications and how they relate to the familiar rotary motor terms becomes more important.

Linear motor parameters can be confusing and difficult to understand, especially when different manufacturers use different methods to formulate and present them.

On ControlDesign.com we’ve posted a white paper written by Parker-Trilogy design engineer Jack Marsh that develops specifications specifically for three-phase, brushless linear servo motors based on fundamental characteristics. It also develops relationships between certain motor parameters and shows how some are dependent on winding type and others are not. The theoretical formulations are compared to experimental results to verify their validity.

This paper uses first principles, specifically the Lorentz Force Law and Faraday’s Law, to establish linear motor parameter formulations that are consistent and convenient to measure. It also develops the relationships between certain motor parameters, shows how some values differ depending on the winding type of the motor, and shows how some parameters are not dependent on the winding type. Differences between peak-of-sine and RMS measurements will be examined. Simple ways to measure linear motor parameters will be shown. The formulas and relationships then will be compared with experimentally measured values to show their accuracy.

Go to ControlDesign.com/trilogy to download the paper.

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