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.
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
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
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
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.
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.
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
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.
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,"
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
"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."