Motors for medical

June 07, 1999

Name an OEM application, and you'll very likely find some type of motor. But when it comes to the medical market, the omnipresent motor can take on some very unusual features.

At the March National Design Engineering Show, for example, Micro Mo Electronics (Clearwater, FL) unveiled what it calls the "world's smallest commercial gearmotor." Measuring just 1.9 mm in diameter (0.075 inch), the BL 1900 Series brushless motor, along with its matching planetary gearhead, is targeted at applications such as medical endoscopes and microsurgical tools.

Based on micromachining technology, the tiny system uses a bipolar neodymium permanent magnet for the motor, LIGA cast gears, and a sensorless electronic commutation system. The molded plastic microgears measure just 80 microns in diameter. This design produces a continuous output torque of 0.02 oz-in and an intermittent output torque of up to 0.042 oz-in. Continuous output speeds range up to 100,000 rpm.

But the motor is by no means a laboratory curiosity. Tests have yielded an average life of 20,000 hours, says Dr. Stephen O'Neil, vice president of Advanced Research and Planning for Micro Mo. Its ironless coil design reduces mass, extending the life of batteries that power these brushless mini motors.

"The medical community needed a very small, high-power motor that could also be produced consistently and cost-effectively," says O'Neil. Gone are the days, he adds, when the medical market--like the military--didn't care about costs, as long as performance and reliability were there. Now, cash-strapped hospitals want OEMs to produce equipment that delivers long life, portability, and cost efficiency.

The OEMs, in turn, want suppliers who can customize products to meet those goals. "About 75% of the time, we must offer special shafts or windings, pinions, or pulleys--or ancillary products like encoders, gearheads, or lubricants," says O'Neil.

The medical market is no place for "cookie-cutter" motors, say engineers at Pacific Scientific, another company active in the medical area. For example, to save space, XL 3000 syringe pumps by Cavro are built around Pac-Sci's patented Powerflex II stepper motors. To meet Cavro's special needs, Pac-Sci engineers specified alternate windings to deliver more torque and modified the shaft, which can be as large as 0.375 inch O.D., compared to just 0.250 inch for the typical NEMA 23 frame stepper.

To boost reliability, save space, and keep costs in check, Pacific Scientific uses injection molding in the manufacture of the front end bell/stator assembly of the Powerflex II. As the mold fills with engineering polymer, the windings are encapsulated, the front end bell becomes an integral part of the assembly, and the rear end bell and receptacle are formed. "No one else is building motors using this molding technique, which enabled us to eliminate several parts," notes Brad Trago, engineering manager.

Don Neumann, product marketing manager, adds that supplying motors for medical equipment involves a serious commitment of design and application engineering support. Life cycles for medical equipment tend to be long, given the rigors of FDA approval and the need for medical facilities to wring the most out of every dollar spent.

Even so, when vendors do succeed in getting their motors specified, the payoffs can be attractive. One automated blood/gas analysis device from Abbott Labs uses 23 Powerflex II motors. The object of the machine is to perform the maximum number of different tests at the lowest possible cost. And that calls for motors that are compact, reliable, and cost-efficient.

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