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Brushless Motor Technology Solves Design Team’s Dilemma

Tue, 05/31/2005 - 5:46am
The Project: To develop a next-generation airway clearance device that is easier to use and lighter in weight.The Solution: Incorporate a new motor design to improve the user interface, performance, and portability.
This effort resulted in a medical device that is 40 percent more efficient in usage of power with a 15 percent reduction in overall weight.Lonnie J. Helgeson is the manager of manufacturing at Electromed Inc., 502 6th Ave. N.W., New Prague, MN 56071. He has held key product development and production positions at several leading entrepreneurial companies over the last 24 years. Most of the products he has developed involve electronics and electrical/mechanical actuation. He has a degree from North Dakota State College of Science. Helgeson can be reached at 952-758-9299, ext. 2015, or lhelgeson@electromed-usa.com. Robbie Queen is a business development manager at Moog Components Group, 1995 NC Highway 141, Murphy, NC 28906. He has been with the company for 22 years and has served as both a design engineer and applications engineer. He has a bachelor's degree in industrial engineering and a master's degree in project management. Queen can be reached at 800-577-8685, ext. 256, or rqueen@moog.com.

By Lonnie Helgeson and Robbie Queen

This ultra-quiet Silencer brushless DC motor is used in the SmartVest system.
Electromed Inc., a leading designer and manufacturer of medical devices for the airway clearance market, was challenged to introduce a next-generation device that would be easier to use, lighter in weight, and transportable. The result of this effort is the MedPulse SmartVest Airway Clearance System Model 2000ez. The SmartVest System consists of an inflatable vest worn over the torso and an air pulse generator that supplies air pulses to the vest. It is these air pulses that produce the HFCWO—high frequency chest wall oscillation—to promote airway clearance, improve bronchial drainage, and create airflows that resemble a cough. This, in turn, will shear secretions from the airway, moving them upward, where they can be expelled by coughing or cleared through suction.

The engineering team at Electromed wanted a positive impact on product longevity and increased mean time between failures. The project began with a meeting to brainstorm the approved marketing specifications, which identified the design inputs. The motion control system was identified as a critical path, as it would require the longest lead-time to design, prototype, and finally procure for implementation into the manufacturing line. The existing motor was a DC brush servomotor, coupled with a pulse width modulated electronic drive. The team decided a brushless motor would give a higher energy density in a reduced form factor package, with the additional benefit of reduced weight and no brushes to replace.

Electromed needed a motor that would meet the demanding requirements of the device. This effort involved becoming familiar with different brushless motor and drive technologies. The supplier search led to several sources, ultimately resulting in the selection of Moog Components Group. Specializing in motion solutions for innovative medical applications, Moog Components Group joined the development team and produced a motor that met all requirements. The project required clear communications, good documentation control, and close cooperation between the companies to resolve technical obstacles.

Since the motor's audible noise and life were of prime importance to the new generation device, a brushless motor was selected. Brushless motors have much longer life and are typically much quieter than brush motors, but it is an entirely different technology. Once the replacement technology was selected, the real task of implementation began. The challenges encountered include the following:
1. Understanding the application
The primary task was to understand what the motor was expected to do. Working closely with the electronics supplier and Electromed, Moog Components Group obtained a working unit, established a test plan, and measured and recorded baseline performance parameters. From this, a "first-cut" design was recommended.
2. Prototype build and test

The MedPulse SmartVest Airway Clearance System Model 2000ez consists of an inflatable vest worn over the torso and an air pulse generator that supplies air pulses to the vest.
Based on test parameters, a prototype motor was constructed. This motor was substituted into the device, and testing was conducted at Electromed, Moog Components Group, and the electronics supplier. As brushless motors are electronically commutated, close cooperation between the motor supplier and the electronics designer was a must.
3. Debug and iterate
As is commonplace, testing revealed some areas for improvement and some unanticipated design criteria. These represented three technical demands for the motor. The first was shock loading. The mechanical mechanism of the equipment was of a cyclical, asymmetrical nature. The shock loading required Moog Components Group to rethink its bearing system—in particular how the bearing was housed and how the bearing handled these cyclical shock loads. The second technical demand involved sealing. The nature of the design required that a minimum amount of air be pulled through the motor. This requirement mandated the motor be sealed to prevent this from happening while at the same time keeping friction torque to a minimum value. Friction introduced by seals, sealed bearings, etc. can be quite high and could not be tolerated. The third technical demand involved the motor's custom mechanical features, including custom shaft and caps, as well as a custom wiring harness.

The technical abilities of Moog Components Group allowed the integration of the motor, drive, and user interface electronics to be completed in a parallel manner. Sharing CAD data files, which were transferred electronically, and utilizing conference calls, e-mails, and fax communications maintained the integrity of the concurrent design.

The conclusion of all this effort resulted in a medical device that is 40 percent more efficient in usage of power with a 15 percent reduction in overall weight. The thermal profile was reduced 18 percent and electromagnetic compatibility was improved as well.

There are several key factors that contributed to this project's success, which included ending within budget and on time. First, Electromed engineers had to overcome the technical challenges of integrating a new technology—brushless motor technology—into their design in order to meet their goals. As the motor supplier, Moog Components Group, had to contribute its motor expertise and experience and become an extension of the engineering group while working with an unfamiliar application. Lastly, the companies had to learn to effectively communicate and bring together a diverse set of skills and knowledge bases using the latest technology tools.

ONLINE

For additional information on the products and technologies discussed in this article, see Medical Design Technology online at www.mdtmag.com and the following websites:

• Electromed Inc. at www.electromed-usa.com

• Moog Components Group at www.moog.com/components
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