The SL05 by Allied Motion achieves roughly half the resistance of competing products.
With the widespread adoption of minimally invasive surgery across many different specialty areas, the need has arisen for the small-scale tools to perform these precision-dependent procedures. Similarly, a rising demand for smaller, portable equipment has placed new constraints on device design. But as product size diminishes, so does the internal real estate for the components that enable it to function. As a result, motor designers have faced the ubiquitous medical device design challenge of shrinking the motor’s package size while maintaining functionality and the desired torque.
Making the Cut
Part of the solution to this quandary lies in the motor’s power density, according to Simon Pata, product line manager, brushless dc technologies, for Portescap (West Chester, PA; www.portescap.com), a Danaher Motion company. Featuring a high power density, Portescap’s specialty slotted brushless dc motors for surgical tools are offered with diameters ranging from 0.5 to 1.5 in. and are suitable for use in tiny penlike tools up to larger bone-cutting instruments. “Our motors are very light, which is linked to the power density, meaning we can have a lot more torque with a smaller motor,” Pata says. The combination of high torque, high efficiency, and a small package size is especially advantageous for portable applications because it enables efficient power consumption for battery-powered products.
Minimizing motor vibration is crucial in the operation of surgical tools because dire consequences can result from a slip of the hand or even the most minute inaccurate movement. Likewise, sizing the motor properly is important because it can affect the thermal characteristics of the motor. “The model is directly in the surgeon’s hand and [OEMs] do not want the motor to heat the surgeon’s hand, and cause some sort of discomfort,” Pata notes. “Power density is linked to temperature because if you try to have a very small motor and to get a lot of torque out of the motor, it will overheat.”
In addition to having a high power density, the ability to withstand sterilization is among the most essential features of a motor for surgical applications. Putting an electric motor into an autoclave raises many potential problems, however. Metal housings and components such as ball bearings are subjected to 100% humidity, a high temperature, and pressure variations for 20 minutes, making them vulnerable to rust, Pata points out.
“The tolerance in this industry is 500 to 1000 cycles, which a lot of motor manufacturers struggle to reach,” Pata says. “You’re going to face a lot of issues, so you need to seal the motor very properly if you want to survive it; this is a very tough environment.” Pata claims that Portescap has designed its motors with autoclavability in mind, and they can withstand exposure to such extreme conditions.
Like Portescap, Allied Motion Technologies Inc. (Englewood, CO; www.alliedmotion.com) has recognized the importance of being able to autoclave motors. The company offers a version of its SL05 motor that is designed to withstand 500 to 1000 cycles and features bearings that will not migrate or be flushed out during the process.
Although Allied Motion has followed the industry standard for autoclavability, it has diverted from the typical motor manufacturing path for the SL05 in other ways, namely in using a slotless motor design. While not the only manufacturer of slotless-designed motors, Allied Motion has developed a way of configuring the windings and coils that enables the motor to achieve roughly half the resistance of competing products, according to Dave Hawes, the company’s director of engineering.
“With the unique winding configuration that we have, we were able to do it in half the resistance, meaning that we were able to get two to three times the torque of the competition,” Hawes says. “Plus, with the materials and the configuration [we use], we run slightly cooler at the increased torque level.”
For example, Hawes explains that the SL05 would achieve almost 2½ oz-in. of torque if running at 50,000 rpm versus a competing motor, which would have about 0.8 oz-in. of torque while operating at the same speed. He adds that the motor can obtain speeds of more than 100,000 rpm.
Suited for use in powered surgical hand pieces and small medical power tools, the brushless dc motor can be used in a variety of applications that include arthroscopic surgery, oncology, ACL repair, and plastic surgery. The size-5 SL05 can even be retrofitted into other manufacturers’ size-5 motor applications, enabling OEMs to retain the hand tool’s design while essentially just adding a new engine, Hawes says.
Probing Ideas for Next-Generation Devices
Maxon's A-max motors are suitable for use in portable applications, such as a handheld ultrasound probe.
While minimally invasive surgery has had a profound impact on medical device development in recent years, it is not the only trend causing OEMs to scramble for small motors with big capabilities. A swelling need for portable products has placed the same demands on manufacturers.
Although the home healthcare movement has been a driving factor in the need for portability, the notion of better patient care within the hospital walls has influenced device development as well.
As a result, many types of diagnostic equipment have experienced dramatic size reductions in an effort to cultivate point-of-care diagnostics, thus potentially enhancing patient treatment.
One such type of equipment, the Ultrasound Probe manufactured by Interson Corp. (Pleasanton, CA; www.interson.com), represents a complete ultrasound imaging system built into a handheld probe that weighs only 7.5 oz. Users just need to download software to the exam room’s computer and connect the probe via the computer’s USB port, which provides the 0.5 A of power necessary to operate the probe. However, finding a motor that could run on the power supplied through the USB cables proved to be a distinct obstacle for the OEM.
The A-max series of motors manufactured by Maxon Precision Motors Inc. (Fall River, MA; www.maxonmotorusa.com) was able to meet such requirements. Able to operate in a temperature range of –30° to 85°C, A-max motors feature higher torque density than competing motors of the same size, according to Scott Hamilton, Maxon sales engineer. They also operate with maximum efficiency up to 85%, depending on their winding.
“With dc coreless motors versus conventional dc motors, you see a whole host of different things that come into play—one of which is that electrical noise is reduced to a low level,” comments Hamilton. “We have a low mechanical time constant, which means that we accelerate the speed much faster than a conventional motor. The efficiency is very high, which lends itself to portability because you’re able to use this in battery-driven applications very well, and it also increases the life of the product.” Hamilton adds that by altering its manufacturing processes and materials, Maxon has been able to make these motors cost-effective.
Because Maxon has witnessed a surging demand for miniature motors in applications such as the Interson probe, the company has launched a niche division, Maxon Medical, specializing in motors with diameters of 10 mm and smaller.
“We’ve seen a real push from the medical industry to entertain the smaller and smaller sizes, and we think that we have a very high specialty in this market,” Hamilton says. “The reason being that there are design, mechanical, and electrical constraints as to how small you can actually build something and still get a reasonable amount of power, torque, and speed out of the product.”