Originally Published MPMN March 2003

SPECIAL

Emerging Technologies

A survey of new products and processes that will affect medical device design

Motors supplier lends hand in development of next-generation prosthetic

When Motion Control Inc. (Salt Lake City; www.utaharm.com) began the design phase of its next-generation prosthetic devices, a key goal was to resolve some specific problems articulated by users of its products in the past. A better understanding of user needs and the integration of advanced-technology components led to substantial improvements in its motion-controlled wrist and hand prosthetics.

For example, users had complained about the hand experiencing a "lockdown" under certain circumstances. If the hand lost power for any reason while gripping a door or shopping-cart handle, it would clamp down, making it very difficult to release the grip safely. In the new design, Motion Control integrated a mechanical safety release that allows the user to manually release the drive mechanism and passively open the hand.

Because the hand operates on a variety of input voltages and requires rapid response times and high pinch forces, sourcing a suitable motor was also a key concern for the manufacturer. After considering several alternatives, the company chose a graphite-brushed motor from Maxon Precision Motors (Burlingame, CA; www. maxonmotorusa.com).

The prosthetic is designed for use with input voltages ranging from 6 to 18 V to accommodate various types of battery packs and to satisfy the product's speed and torque requirements. Graphite-brushed motors work efficiently within the specified voltage range, whereas competitive precious-metal-brushed motors are prone to burn out, according to Maxon. Other benefits of the motor include its light weight, compact dimensions, and quiet operation.

The motor is also instrumental in producing a rapid response rate in the hand without sacrificing pinch force. Traditional electric-motor gear trains sacrifice speed for torque, or vice versa. Motion Control uses an automatic mechanical two-speed transmission that senses a change in torque when the hand comes in contact with an object, and shifts gear ratios to achieve a high pinch force. When the object is released, the transmission automatically shifts up to a faster gear ratio. The use of a two-speed transmission, special gearing, and belt drive enables the hand to easily open and close.

The RE 16 motor from Maxon is mounted perpendicular to the axis of the forearm. The off-the-shelf component features no-load speeds greater than 14,000 rpm, stall torque up to 31 mN*m, and continuous torque exceeding 5 mN*m.

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Norbert Sparrow, Susan Wallace, and Zachary Turke

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