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Q: I think it’s cool how my smartphone’s screen flips around depending on how I’m holding it and what I’m doing. How does it know up from down? -Michael Fong, Vancouver

WE ASKED | Bill “the Engineer Guy” Hammack, professor of engineering at the University of Illinois

What’s at work here is something called an “accelerometer.” Built into the circuitry is a tiny device that detects changes in orientation and tells the screen to rotate. The device used in smartphones is called a MEMS accelerometer, an acronym for Micro Electro Mechanical System. MEMS can be found everywhere from smartphones to cars to earthquake and animal research.

How does a basic accelerometer work? Imagine, says Hammack, a ball (a “seismic mass”) attached to a spring inside a housing. “If we move the housing up, the ball lags behind and stretches the spring,” he says. “If we measure how much that spring stretches, we can calculate the force of gravity.” From there, we can figure out the acceleration. Add an accelerometer on each of three axes and you can imagine how you’d plot acceleration in three dimensions and also determine the orientation: horizontal or vertical.

MEMS accelerometers are tiny, at 500 microns (0.5mm). They’re etched from a silicon wafer such that the spring, the weight and the scale are one piece. The spring, says Hammack, becomes “a small cantilevered beam, like a diving board,” etched in the silicon.

More MEMS? MEMS can be found in the new 4G LTE BlackBerry PlayBook.

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