May 19, 2007
Originally Published MPMN May 2007
The Driving Forces behind Motor Development
RoHS compliance and environmental requirements play important roles in motor construction
This hybrid stepper by Nippon Pulse is one of many RoHS-compliant motors entering the market since the directive went into effect last July.
Motors are integral parts of a multitude of medical devices. From robots to micropumps, motors are among the critical mechanisms responsible for making these devices work. And yet, despite their ubiquitous use in a number of industries, motors—with the exception of those for the automotive industry—barely seem to register a blip on the radar in terms of reports, market analyses, and news items. Noting this neglect on its Web site, the Motion Control Association (MCA), a nonprofit organization recently formed to promote, educate, and serve the motion control industry, wonders, “How can a multi-billion-dollar market remain virtually hidden except to those customers, manufacturers, and suppliers that are an intimate part of it?”
Luckily, these motor manufacturers and suppliers aren’t sweating their low-key industry presence. As the association points out, the motor market (under the umbrella of motion control) continues to prosper, thanks to the influx of countless new devices. In the healthcare sector, the demand for motors that are smaller and faster than their predecessors continues to keep suppliers scrambling. Also generating a flurry of activity is the EU’s recently imposed Restriction of Hazardous Substances (RoHS) Directive. This factor, as well as the need for motors that can withstand harsh environments, is discussed in this article. A buyers guide listing suppliers of motors follows on page 45.
Some motors used in medical applications have to be engineered to withstand extreme temperatures or processes, such as these all-stainless-steel motors from Leeson Electric.
For many companies, competing in the global marketplace is an essential ingredient in the recipe for success. However, the institution of the RoHS Directive in July 2006 has forced device companies that cater to European clients to adapt their practices and products to comply with the regulation—and to do it quickly in order to remain a contender. Since compliance is being monitored, those companies that don’t adjust to meet RoHS standards may just get burned.
Oriental Motor (Torrance, CA; www.orientalmotor.com) is one supplier that’s modifying products to retain its European presence in the RoHS era. In addition to offering more than 2800 standard RoHS-compliant components, the firm can accommodate special orders for parts that do not conform to the standard. The company even includes RoHS as an option in its Web site’s search criteria to hasten accurate product selection.
Included in its RoHS-compliant offerings are RK-series five-phase step motor and driver packages. Standard, IP65-rated, and geared motor types are available, with geared step motor types including tapered hob, planetary, and harmonic. To interface to the universal driver as step or geared step motors, 42-, 60-, and 85-mm frame sizes are offered. The 85-mm step motor interfaces to a slightly larger gearhead of 90 mm for all gearhead types, while the IP65-rated step motor is provided in only the 60- and 85-mm frame sizes. This motor’s basic 0.72° angle can be electronically divided into a maximum of 250 microsteps. Moreover, microstepping used within the driver independently of the controller’s output frequency enables smooth rotation as well as low-vibration, low-noise operation at low speeds. The driver features angle accuracy control, a high starting frequency response, and 16 resolution levels for both microstep low-speed operation and high-speed full-step operation.
The Nippon Pulse America Inc. (Radford, VA; www.nipponpulse.com) NEMA Size 11 hybrid is another RoHS-compliant stepping motor entering the market. Suited for medical applications requiring high torque in a small package, the compact motor measures 28 mm square by as little as 32 mm long. The bipolar and unipolar versions are among the small- est stepper motors offered that can accurately execute 1.8° steps, according to the company.
Stack lengths of 32, 45, and 51 mm dictate the motor’s maximum holding torque, which ranges from 42 to 117 mN•m. Additional features include single- or double-ended shafts, four or six leads, a step resolution of 1.8° when in full-step mode, and precision-honed stators and ground rotors for tight air gap and maximum performance. Customizations include coil voltage, encoder mounts, single- and double-shaft details, and lead wire–connector assemblies.
Oriental Motor offers this RoHS-compliant stepping motor, along with more than 2800 standard RoHS-compliant products.
RoHS compliance is a major consideration for design engineers—but it’s not the only one. Many medical devices and equipment will be exposed to harsh environments including washdowns and sterilization, or may be designed into a product that interacts with fluids. Because of such circumstances, motors integrated within these applications must also be able to withstand the conditions they could experience.
Attempting to meet this need, Leeson Electric (Grafton, WI; www.leeson.com) has laid claim to the industry’s first line of permanent dc motors with all-stainless-steel construction. Configured with all exterior components engineered from 300-series stainless steel, the motors are designed for use in washdown environments, owing to the material’s corrosion-resistant nature. The motors can withstand frequent washdowns with corrosive solutions containing lye, bleach, nitric acid, alkaline, and concentrated surfactants. Only exposure to hydrochloric and sulfuric acids will harm the surface finish of the stainless-steel motors, according to the company.
Constructed with a steel frame and aluminum shields on each end, most standard motors typically have painted surfaces to withstand rust and corrosion. And while the endshields may not rust in washdown environments, they may start to corrode, says Chris Medinger, stock product manager for Leeson. Meanwhile, the paint on the exterior of the frame may start to wear away over time or if nicked, which, in turn, can induce rusting, he continues. No part of the Leeson motors’ exterior is painted or coated; however, suitable interior coatings are applied to the armature, frame, and magnet to provide protection against corrosion.
A number of additional features safeguard the motors against moisture. By using exterior fasteners, the number of entry points for moisture has been minimized. The motors have also been designed so that there are no holes in the frame for attaching a nameplate and conduit box mounting screws are unnecessary. Moreover, bearing lock screws are located inside the motor and double-sealed bearings are prelubricated with moisture-resistant high-temperature grease. Moisture and chemical resistance is further reinforced by the employment of Viton sealing components.
Like washdown environments, sterilization processes put motors to the test. Since many medical devices require sterilization, integrated motors also need to be able to withstand the intense heat and pressure applied in the process. Autoclaving, for example, can require the device to undergo heat extremes of 121°C without failing.
Maxon Motor UK (Finchampstead, Berkshire, UK; www.maxonmotor.co.uk) has modified a line of motors for such situations. The company offers 6-, 16-, and 22-mm electronically commutated brushless motors capable of withstanding autoclaving. Rendering the motors suitable for sterilization processes consists of coating the magnets and motor bodies in addition to constructing the shafts from a higher grade of stainless steel than previous models. Hall sensors are positioned internally rather than externally for further protection.
Copyright ©2007 Medical Product Manufacturing News
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