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Linear Vs. Rotary

Linear Vs. Rotary

In the next few weeks, linear motors will face their ultimate litmus test.

After years in the eye of a swirling technical storm, surrounded by doubters who say they're too expensive, supported by a handful of loyal users who say they're worth it, linear motors may finally be poised to capture a larger share of the growing linear motion market.

This month, Baldor Electric Co.'s Linear Motor Division (Santa Clarita, CA) will unveil a linear motor with a price tag that's less than half that of current technology, thus putting it squarely in competition with less expensive solutions such as belt-driven stages and ball screws.

"We're effectively redefining the market for linear motors," says Gavin Johnson, sales manager for linear motors at Baldor Electric. "We think we ought to be able to take linear motors from 5% of the linear motion market, which is where they are today, to about 50%."

Not everyone is so sure. "I think linear motor technology is great stuff. It's getting better and cheaper all the time. But a lower price doesn't guarantee a home run, particularly if there is any compromise at all in performance," says George Gulalo, president of Motion Tech Trends, a consulting company that specializes in technical analysis and market research in the area of motors, drives, and motion systems. "The old adage is, 'If it works, don't fix it.' The reality is that engineers have a lot of money tied up in their designs, and they aren't necessarily going to risk a form factor change in a component that represents maybe 5% of the total cost of that design," Gulalo adds.

The leading force behind the new motor design, Baldor Engineering and Operations Manager John Heilig was a natural choice to head up the development team. A mechanical engineering graduate from the University of Houston, he's spent his entire professional career around motor technology and is a self-described "motorhead." He says "I've always been interested in learning how things work, then making them work better." And, he's not one to give up on a design-even when people say it can't be done.

Conventional wisdom about linear motors is the lower the cost, the lower the performance. "Sure, you can bring linear motor system costs down, but you might end up turning out so little force that the motors are worthless," says Bob Novotnak, division manager for Aerotech (Pittsburgh, PA), another maker of linear motors.

Undaunted by that kind of skepticism, Baldor's engineers started out by kicking around the idea of a low-cost linear stepper motor, but eventually gravitated toward the development of an ultra-low-cost, three-phase linear servo. No easy task, given that materials and processing account for about 60 to 70% of the cost of a linear motor. Traditional linear motors typically incorporate magnets at a cost alone of about $5 per inch of track for a 1,000N motor. With tracks reaching two to three meters in length, the costs add up.

Magnet-Free Track Key

Heilig's team devised a scheme whereby they could completely eliminate the magnets from the linear motor's track. "The only magnets in this motor are in the moving coil assembly," says Heilig. "There are no magnets in the stationary plate. The magnet track is simply made up of teeth that complement the tooth structure of the forcer."

A magnet-less track, however, presented a perplexing design issue-mainly because permanent magnets are largely responsible for producing higher flux densities that produce high acceleration and speed. But by developing a proprietary tooth structure for the movable forcer and the track, Heilig says his design can supply sufficient force. "The shape and size of the teeth maximizes the force yet keeps our manufacturing costs to a minimum," Heilig says.

To further minimize the motor's cost, Baldor engineers are also employing high-efficiency sintered metals for the stack material in the forcer. They claim that the sintered materials minimize thermal losses. The result is a "hybrid core" model that challenges all existing assumptions about linear motor costs. Its $800 price tag includes both the forcer and track together-as compared to $3,000 and up (and up) for typical linear motors.

That savings did not come without compromise: Baldor acknowledges that the new hybrid core line won't produce as much peak force as other linear motors, let alone rotary models. Whereas most iron core models produce a peak force ranging from 3 to 5x that of their continuous force, Baldor's new motors will be in the 2.5 to 3.5x range.

Plans are for the new motor to ultimately reach lengths of as much as 2.5 to 3 meters. The company says it has already quoted models offering velocities of 4 m/sec and peak force of 602N (as compared to 5 m/sec and 801N for Baldor's conventional linear motor).

Baldor engineers want the new hybrid core motor to compete with ground ball screws, rolled screws, and acme screws in applications that include automated assembly, transfer lines, load/unload systems, and semiconductor processing. "There are a lot of mechanisms that don't need high acceleration forces," says Johnson of Baldor. "They're doing a lot of back-and-forth motion at high speeds, and they're using belt-drive systems or pneumatic actuators. We hope to capture some of that market."

By lowering the entry cost of a linear motor system to approximately $800, Baldor executives believe they have eliminated the economic justification that has served as a barrier to its success. "For 95% of users, the major factor limiting the acceptance of linear motors has been cost," says Johnson. "We constantly hear, 'This is great, but it's too expensive,'" he notes.

Now, at long last, engineers won't be able to say that anymore. As a result of Baldor's announcement, linear motors will now be expected to go head to head with rotary motors and ball screws, pneumatic cylinders, and belt drives in applications ranging from transfer lines to test instruments.

The company's executives, of course, are well aware that linear motors haven't had the impact on the industrial market that some analysts predicted during the past decade. Still, their confidence in the technology is high. "The last couple of years, sales of linear motors throughout the industry have been bumping along the bottom," says Johnson. "But we're in a depressed market at the moment. Ultimately, we expect linear motor sales to take off."

But Will Engineers Buy?

Still, questions remain as to whether droves of engineers will embrace the technology, despite Baldor's dramatic price reductions. Consultant Gulalo, for one, is skeptical. "It turns out that the rotary motor is relatively efficient and can do lots of work, so there isn't necessarily a compelling need to change. The designs we're talking about have a lot of inertia, and if engineers are going to make a change at all, it's more likely going to be for a performance-related reason than because they can save a few bucks."

To underscore the point, Gulalo describes a recent study on motor pricing he conducted. "A supplier asked us to examine the price elasticity of a particular motor that they thought they could make cheaper," he says. "Turns out in some applications the supplier could have reduced the price of the motor to zero dollars-in essence they would give it away-and engineers still would not use it in their designs."

Even some linear motor makers themselves have questioned whether a lower-cost linear motor is capable of capturing 50% of the linear motion market. "It would be nice to get linear motors into cost-sensitive applications, but it may not happen in the very near future," notes Aerotech's Novotnak. "We probably aren't going to see that happen in the next five years."

Novotnak believes that linear motor costs will ultimately fall, but only as magnet and coil technology improves. Lower costs are especially dependent on better manufacturing techniques, which ultimately raise manufacturing yields and, therefore, reduce costs, he says. Aerotech is pursuing an alternative strategy, focusing on generating higher forces in linear motors by employing a bifurcated motor design that enables the company to squeeze overlapping coils into less motor volume. The company's linear motors cost between $4,000 and $30,000 for single-axis air bearing systems and are aimed at the medium to high end of the market. Aerotech's linear motors produce continuous forces as high as 1,200N, with peak forces reaching 4,500N, he says.

Novotnak and others in the linear motor community know that efforts to bring their products to the mainstream won't be well received in some quarters. Some end users who have tested linear motors question its value in high-end applications, let alone low-end machinery.

Design Engineers Weigh In

"Our customers aren't as willing to accept linear motors because they don't understand them," says Philip Szuba of Lamb Technicon (Chesterfield, MI), which tested linear motors during the development of a horizontal machining center.

Szuba points to a study performed at the University of Technology in Aachen, Germany, in which researchers concluded that linear motors consistently drew far more current, dissipated more heat, and produced less force than conventional alternatives, such as rotary motor-ball screw combinations. "The power draw is often three times as great, even at identical accelerations," he says. Baldor contends, however, that there's a large contingent of industrial applications that don't need huge peak forces. "Linear motors have always tended to specialize in the top end applications with 5-10 Gs of acceleration," Johnson says. "But there are a lot of mechanisms that now use belts and inexpensive ball screws and don't need high G's."

Many experts believe that resistance to linear motors will subside if costs are reduced sufficiently. Already, they say, many engineers have successfully used linear motors in high-end applications, and would gladly use them more if prices allowed.

Hydraulics out: Enduratec uses linear motors on its medical test systems.

"If linear motor prices dropped, more applications would definitely open up," notes Russell Kaiser, vice president of engineering for Landis Gardner (Waynesboro, PA), a maker of grinding machines. "It wouldn't take a major cost change for them to really start taking off."

Landis Gardner engineers, who used linear motors to develop a new grinding machine, believe that linear motors actually offer cost advantages in many cases. Linear motor systems, they claim, are composed of fewer parts and are therefore simpler to assemble. Unlike rotary motors and ball screws, linear motors need only a table, encoder, and the motor itself.

Similarly, engineers from Cincinnati Machine (Cincinnati, OH) plan to increase their use of linear motors in the next few years, mainly because they've used them successfully in their HyperMach and HyperMach RT machining centers. The company's engineers say that linear motors enabled them to build machines five times faster than state-of-the-art machining centers and ten times faster than conventional systems. "Linear motors gave us more aggressive acceleration," says Randall Von Moll, project manager for Cincinnati Machine.

For reasons such as those, even the staunchest opponents of the technology say they plan to keep an open mind on the topic. "If the suppliers can solve the technical problems and get the volumes up, then the costs will come down," says Szuba of Lamb Technicon. "And if the costs come down significantly, then we can absolutely foresee applications for linear motors."

How Baldor's New Hybrid Core Linear Motor Stacks Up
Parameters Units Existing motor LMBL14A-HC0 New hybrid core LMHB46B-3C0B (Preliminary)
Continuous Force lbs 60 90
N 267 401
Continuous Current Amps 4.2 9.5
Peak Force @ 10% Duty lbs 180 135
N 801 601
Peak Current @ 10% Duty Amps 12.5 19.0
Continuous Power Watts 140 239
Attractive Force lbs 800 820
N 3560 3650
No. of Poles 14 2
Moving Member Length in 15.35 14.96
mm 390.00 380.00
Moving Member Width in 3.00 4.32
mm 77.00 109.61
Moving Member Height in 1.30 1.90
mm 34.00 48.25
Moving Member Weight lbs 7.65 15.50
kg 3.48 7.04
Magnet Track Weight lbs/in 0.27 0.35
kg/m 4.83 6.26
Cooling Convection Convection
Resolution &1 mum &1 mum
Force Constant* lbs/amps 14.4 9.5
N/amps 64.1 42.3
Back EMF Constant* V/i/s 1.63 1.07
V/m/s 64.1 42.3
Resistance* at 25s/Be Ohms 5.8 1.9
Resistance* at 25C Ohms 8.1 2.6
Thermal Resistance Watts/ degrees C 1.40 2.39
Inductance* mH 20.8 22
Electrical Time Constant msec 3.59 8.31
Max Accel @ 3% dc (Peak Force/Coil mass) g 24 9
Max Speed ips 200+ 160
mps 5+ 4
Source: Baldor Linear Motor Divison
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