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issue: September 2003 APPLIANCE Magazine

Electronic Controls and Embedded Systems
Accelerating Medical Accuracy

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by Lisa Bonnema, Managing Editor

Some manufacturers talk about "thinking outside the box," while others - like Omron Corp.'s Healthcare Business Company - actually do it.

The Kyoto, Japan-based medical device maker not only looked beyond its current product offering when introducing the HEM-6371T portable digital blood pressure monitor, it also found a new application for an advanced sensing technology.

According to Osamu Shirasaki, senior engineer at Omron, the company wanted to design a wrist-style blood pressure monitor that could overcome the inaccurate measurements that most competitive models produced because of incorrect arm positioning. The solution was in Analog Devices, Inc.'s (ADI) ADXL202E accelerometer, an advanced inertial sensor with analog and PWM outputs that can measure both dynamic acceleration (vibration) and static acceleration (gravity).

Based on MEMS (Micro Electro Mechanical System) technology, the sensor is actually used to measure tilt, explains Paul Ganci, product line director for ADI's Inertial Products Group (Norwood, MA, U.S.) "You strap this monitor onto to your wrist like you would a watch, and the most accurate measurement is made if the blood pressure sensing device is at the same elevation as your heart," he explains. "By measuring the angle of your arm, the monitor actually guides the user to correct arm positioning, and then the blood pressure monitor takes the measurement. So they're increasing the accuracy and the repeatability of the measurement by using that tilt information to tell users where to exactly position their arms."

According to Mr. Ganci, ADI's accelerometers are designed with a mechanical sense element and electronic circuits, which condition the electronic output from the sense element. Attached to the 5-mm by 5-mm by 2-mm chip is a component called a proof mass, which is suspended on miniature springs. "When that mass moves in one direction or another, through electronic means, we're able to detect that motion and then convert it into an electronic signal," says Mr. Ganci.

In addition to measuring blood pressure at the correct angle, the accelerometer is also able to stop a measurement if a user's wrist shifts out of the proper position. "It is constantly sensing motion and position," Mr. Ganci explains. "Through software you can tell not only what angle the monitor is at, but also if it's moving significantly or if the angle has changed."

Although Omron looked at other accelerometer manufacturers, Mr. Shirasaki says ADI's solution was the clear choice in terms of performance, specifically its ability to accurately measure very small changes in inclination. "We found that Analog Device's accelerometer performance was most suitable to our device because of its high resolution and high sensitivity," Mr. Shirasaki confirms.

The portable wrist blood pressure monitor from Omron's Healthcare Business Company features the ADXL202E (inset) from ADI. The accelerometer integrates moving microscopic silicon parts and sophisticated signal conditioning to produce a complete two-axis sensor on a single monolithic integrated circuit (IC).

Mr. Ganci adds that the ADXL202E's low-power feature is another benefit, especially for portable devices. This feature, he explains, is a result of careful circuit design. "We have a process that integrates both the mechanical sense capability and the electronic circuits onto a single piece of silicon," he says. This, he adds, has also helped make the component a cost-effective option.

Design Feats

Mr. Shirasaki of Omron says the only design challenge the two companies encountered was finding a universal way to sense position, regardless of a user's individual body size. "It was difficult to estimate the relative vertical distance between the heart and wrist among a wide range of body sizes," he notes.

The answer, of course, was in software. "Using an advanced algorithm, we reached practical accuracy by not only using the angle of the forearm, but also compensating for the body angle by using the bi-dimensional acceleration signal," Mr. Shirasaki explains.

Mr. Ganci notes that there were few design challenges because of the accelerometer's simple construction. "We've tried to make the user interface for the design engineer as straight forward and easy to use as possible. With very few exceptions, people can treat these [accelerometers] pretty much like they would any other IC," he explains.

The real design feat, according to Mr. Ganci, was in the application itself. "Omron took a technology that we had developed and applied it in a way that we never thought about before," he says.

Designed initially for use in automobile front- and side-impact airbags, ADI's accelerometers have found uses in variety of other inertial sensing applications such as high-end washing machines, LCD projectors, consumer robots, gaming devices, and fitness equipment. The inertial sensor, for example, is used in a Nike pedometer device that straps onto a runner's shoe to measure how far and how fast the user is running. The portable blood pressure monitor, however, was a first for ADI.

"As MEMS technology becomes more mature and more prevalent, I think development engineers are becoming more aware of what its capabilities are and how they can use them in their product development," says Mr. Ganci. "In the early days of accelerometers that was a real challenge for us - getting people to just be aware that you could buy a sensor at a very aggressive price that would measure acceleration pretty darn accurately. That's less true these days. People are more aware and starting to do some real clever things with the technology."


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