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1 Tiny barometers in cell phones could tell you how high you are on Fri Nov 29, 2013 10:01 pm


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The modern smart phone is packed full of goodies that help it figure out where it is. Internal accelerometers provide orientation, and WiFi and GPS signals combine with it to provide geographic location. We never need be lost again.
Until you walk into a large building. The GPS signal disappears, and the WiFi may only tell you accurately that you are in the building. Even if you were to use WiFi access points as navigational beacons, they do tend to get shifted about and new access points added as networks evolve. This makes positioning a bit of a lottery. Clearly navigational systems for inside buildings require an alternative approach. Now, research from Japan hints at a new sensor that may well be destined for the smart phone of the future.
One obvious approach to navigation is to use inertial guidance. Unfortunately, inertial guidance systems rely on sensing changes in acceleration, and they use that to calculate a new position. Every calculation has a small error, and, since each new calculation relies on the results of the last calculation, this error grows as fast as the national debt during war-time.
A group of Japanese scientists realized that one can also use pressure changes to help determine position. As a person climbs stairs or takes the lift, the barometric pressure changes, and an internal navigation system can use this to determine which floor they are on. Unfortunately, barometers, as they are currently designed, are not well suited to miniaturization. The problem is that their sensitivity drops with decreasing size—by the time the barometer fits in your cell phone, it might be able to distinguish between the top and bottom of Mount Everest, but not much more.
In a clever bit of work, researchers have figured out a new way to make a barometer. In their system, they put a tiny cantilever over the opening of an equally tiny chamber. The cantilever is not designed to seal the chamber; rather, it acts as a pressure sensitive valve, slowing the flow of air between the chamber and the outside world. Whenever there is a pressure difference between the two, the cantilever bends, and the rate of flow between them increases. Once the pressure is equalized, the cantilever relaxes.
By making the cantilever out of a piezoresistive material, the electrical resistance of the cantilever changes as it bends, providing a direct measure of the bending. With a bit of calibration work, this can be turned into a barometer.
The cool thing about this design is that the smaller the device is—especially the size of the air gap between the cantilever and the entrance to the chamber—the more sensitive it is. In the researchers' first attempt, the barometer was only 2mm square, with the cantilever coming in at about 100 micrometers in size. The performance of several barometers, with air-gap sizes ranging from one micrometer to just under six micrometers, were compared. The researchers showed that, indeed, as the gap size decreased, the sensitivity of the barometer increased. Additionally, as the chamber size increased, the range of pressure changes that could be measured increased.
The time response of the barometer came in at a leisurely 20s, which doesn't seem ideal. But the researchers claim that this is good, as it's about the response time you want for walking navigational systems.
It is important to note that this does not measure the absolute pressure, just changes in atmospheric pressure. This is good in that it is independent of local weather—as long as it keeps track of changes, it can calculate what floor you are on. On the other hand, it is going to suffer from the same weakness as inertial navigational systems in that the positional errors are going to add up.
The researchers don't address these problems in the paper. And that is to be expected: this is a first demonstration of a new barometer. But there is hope that by combining a barometer with an accelerometer, the two accumulated positioning errors can be checked against each other and corrected (at least for elevation).
I am skeptical that a barometer can be used to improve location accuracy. But, that said, I am very sure that such a sensor will find a myriad of other useful applications.]

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