September 1, 2000

4 Min Read
Potentiometers Provide Affordable Position Sensing : Dip Room Built in Record Time

Originally Published September 2000

Potentiometers Provide Affordable Position Sensing

The components are used as absolute position sensors in a radiation-therapy machine.

Because procedures such as angioplasty, laparoscopy, and laser eye surgery allow no room for error, modern medical equipment must be as precise as a surgeon's scalpel. Potentiometers, which convert rotary or linear motion into a voltage output, are one option available to design engineers to achieve precise positioning outcomes in medical equipment. At the instant that a system is powered up or that it recovers from a power loss, the potentiometer tells the system its exact position.

The 13 potentiometers used on the Clinac radiation-therapy machines—built by Varian Associates (Palo Alto, CA)—show how these components can be effectively used as absolute position sensors. Relatively inexpensive and, most importantly, immune to radiation, the potentiometers monitor the position of most of the machine's critical motions.

0009e28a.jpgVarian Associates specified potentiometers for the positioning requirements of its radiation-therapy equipment because the components are robust, withstand radiation, and are less costly than encoders.

Spectrol Electronics (Ontario, CA) provided Varian Associates with the potentiometers for their demanding application. Potentiometers were selected in place of other position sensors because of their inherent radiation hardness. Encoders with conventional ICs can also withstand 500,000 units of radiation, but they cost up to $1000 a piece. The less expensive potentiometers, on the other hand, can easily withstand a million units of radiation.

"We use potentiometers because they are robust, they can take the radiation, the number of interface wires is low, and they're relatively inexpensive," says Jeffrey Tuttle, senior engineer at Varian Associates.

One potentiometer regulates each of the four 70–80-lb aperture tungsten plates that move to restrict the x-ray or e-beam to the size and shape of the cancerous tumor being treated. The jaws can define fixed apertures from 0.5 cm to 40 x 40 cm. They can also be moved dynamically during exposure to modulate the beam's intensity in a process similar to the dodging and burning used in photo processing.

Spur gears link the potentiometers to the aperture-control motors. In order to withstand high shaft side loading, these potentiometers are equipped with ball bearings. Other potentiometers control the rotation of the collimator, the machine's gantry location, and the orientation of the couch upon which the patient lies. Potentiometers are also used to control the rotation of the aperture head and the entire beam section.

While Tuttle considered using other sensing techniques such as encoders to obtain the required accuracy and precision, he specified potentiometers because of their "low price, performance, and ability to provide absolute position even with the power off."—Karim Marouf

Dip Room Built in Record Time

Cardiac catheter supplier triples capacity without missing a beat.

Critical-care procedures require fast, well-coordinated action. A similar imperative underscored the recent installation of a cardiac balloon catheter facility: the firm had five weeks to increase annual production of its catheters from 900 to 2600 units.

0009e28b.jpgTo rapidly and effectively meet Class 10,000 cleanroom criteria in its production facility, Arrow International installed a dehumidifier manufactured by Munters Corp.

The necessity arose when Arrow International (Everett, MA), one of the top three producers of cardiac balloon catheters, acquired Boston Scientific and Bard Vascular Systems. During the transition, Bard's manufacturing capacity was to be relocated to the Arrow facility in Everett. In order to almost triple capacity, a new production room needed to be built. Five weeks before shutting down the Bard plant, Arrow contacted Grinnell Mechanical Contractors (Burlington, MA) to assist in the design of the new facility.

"Everett's enlarged dip room needed to meet Class 10,000 cleanroom guidelines," notes president Michael Grinnell. The relative humidity had to be reduced from 28 to 15%, he adds, and a constant temperature of 68° had to be maintained. To achieve these environmental conditions, Grinnell commissioned the Cargocaire Div. of Munters Corp. (Amesbury, MA) to design a suitable desiccant dehumidifier.

Based on the company's Cargocaire HCD-series dehumidifiers, the unit directs airflow toward a slowly revolving wheel impregnated with desiccant. The air is pulled through the wheel's corrugated honeycomb openings, where desiccants capture the moisture and release dehumidified air into the working environment. A second flow—a hot air stream called the reactivation air—removes and expels moisture from the desiccant. To meet Arrow's tight deadline, the unit was ordered through Munters' quick-ship program, which guarantees shipment within three weeks.

Installation and testing of the unit was scheduled to take two days, with certification slated to begin immediately afterwards. "It was a model installation," says Arrow facility manager Tony Lattorelli, who adds that the desiccant dehumidification unit was up and running, not in two days, but in just two and one-half hours!

The Bard facility closed on a Friday afternoon and the Everett production room was open the following Monday.—Norbert Sparrow

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