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Miniature motors deliver big performance
Engineers apply knowledge from building miniature bearings to create world
By Mark A. Gottschalk, Western Technical Editor -- Design News, May 5, 1997
Mountain View, CA--A new line of miniature dc brushless motors--reportedly the world's smallest--offers engineers the opportunity for big breakthrough applications at a micro scale. The tiniest of the motors, about the size of a fly, weighs 350 mgm, and is just one-quarter the diameter of its nearest competitor. Initially developed for a new generation of small gas analyzers, they may also make possible micro-drives and pumps for minimally invasive surgery, miniature motion-control systems, and even hummingbird-sized remote-control aircraft.
Their secret is, of course, size. The baby of the motor family measures just 3 mm in diameter. A 5-mm size is available, and both 4-mm and 2-mm models are in the works. Each employs a slotless, brushless design in which the cylindrical outer housing serves as the stator. Coils attach to the inner surface of the housing, and high-flux neodymium-iron-boride magnets mount to the rotor shaft.
Power requirements are appropriately Lilliputian. The motors will run on 2 to 9 Vdc at currents from 20 to 200 mA. Users can easily use batteries as a power source by connecting them to a three-phase controller.
The motors will operate at speeds as high as 110,000 rpm. Yet, while spinning at less than 5,000 rpm, they can be re-versed simply by switching the direction of the rotating stator field. "Their inertia is so low that they change direction almost instantaneously," says Jeff Perkins, president of RMB Miniature Bearings, manufacturer of the tiny motors. The company calls the motors smoovyTM, a name that hopefully conjures up images of small, smooth, and movement.
Much of the size and smoothness arises from RMB's own miniature precision bearings. Motors are offered in three grades--High, Standard, and Economy. In the High-Grade 3-mm motor, two 1.6-mm diameter cartridge bearings support the shaft. Each cartridge contains seventeen 200-micron-diameter stainless-steel balls with a roundness tolerance of 0.0002 mm. "They are so small they float in water," says Perkins, "surface tension will support them." Standard and Economy motors respectively receive ruby and sleeve bearings--and a lower price tag to match.
The bearings determine a smoovy's operational life, as they do for any brushless dc motor. Engineers predict that High Grade motors will last for 10,000 hrs, Standard and Economy grade motors somewhat less. To date, prototypes with the stainless-steel ball bearings have run for more than 4,000 hrs at 30,000 rpm.
Engineers chose the slotless motor design because it offers several advantages in very small sizes, explains Albert Birkicht, product manager for smoovy motors at RMB Miniature Bearings' main facility in Switzerland. "The main one is that they are easier to make," he says.
And building them indeed presents most of the challenge. Working tolerances between the inner diameter of the coil and the magnet measure 0.05 mm. The coils are 26 to 50 microns in diameter. Birkicht notes that if a worker drops one during assembly it can be nearly impossible to find the ends of the wires again. RMB considers many of its tiny-assembly secrets a competitive advantage, and is loath to reveal them.
The company might be particularly well suited to creating motors this size. Its primary business is manufacturing extremely precise, miniature bearings used in watches and the like. RMB engineers essentially invented the miniature precision ball bearing in the late 1950s. But recently the future of this market came into question, and the company began to think of ways to diversify. But diversify how?
"We watched motor manufacturers struggle to make motors this size," says Perkins. "That's when we realized that maybe our core competency wasn't simply in small bearings, but rather in making small, precise things." RMB engineers decided that they could more easily learn how to make a motor than someone else could learn how to make their motors small--and smoovy was born. "An experienced motor manufacturer could possibly come up with as good an electromechanical design," says Birkicht, "but I doubt they could actually produce it."
Initially, smoovy was targeted towards a small, portable gas analyzer, a-bout the size of a ball-point pen. It would run an axial-flow pump to draw in air samples. The company, however, is still looking for a partner to supply suitable miniature sensors to turn this idea in-to a product.
Motors just represent the beginning of an expected series of tiny devices. Already, engineers have created drivetrain components for the 5-mm smoovy motor--a planetary gearhead in ratios of either 1:25 or 1:125. And in the lab are gears for the 3-mm motor's drivetrain. Too small for conventional technology, they are being manufactured using the LIGA X-ray lithography process. RMB is also designing miniature motor controllers, as well as tiny sensors that attach to the motor's end to provide closed-loop capability.
How small can the motors get? Power is a function of volume, and at some point bearing drag will prohibit the motor from doing much work. "Two millimeters diameter is probably the limit with the current technology," says Birkicht, "below that it gets much more difficult."
Additional details…Contact RMB Miniature Bearings, 29 Executive Parkway, Ringwood, NJ 07456, (201) 962-1111, FAX (201) 962-1101, email: 104764.246@compuserve.com.
Other Applications
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Surgical instruments
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Micro drives and actuators
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Miniature pumps
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