How Does An Accelerometer Work In A Smartphone? Bill Hammack, The Engineer Guy, Explains [FULL TEXT]

 @redletterdave
on May 23 2012 5:01 PM

We barely notice them, but they're one of the most important features buried in the everyday smartphone experience. Accelerometers are devices that can measure acceleration (the rate of change in velocity), but in smartphones, they're able to detect changes in orientation and tell the screen to rotate. Basically, it helps the phone know up from down.

Despite the accelerometer's regular use for games, videos, and other smartphone activities, few people know how the gadget actually works, or how engineers were able to cramp such a small but important piece of technology, which can detect motion in three directions, into a millimeters-thick smartphone.

That's where Bill Hammack comes in. Hammack, a.k.a. The Engineer Guy, is a professor at the Department of Chemical & Biomolecular Engineering at the University of Illinois at Urbana-Champaign, but the 50-year-old college professor also has a second life on YouTube, breaking down how various components of everyday technology work, from digital cameras to fiber optic cables.

On Tuesday, Hammack released his newest video, which explains what he believes to be one of the coolest features of today's smartphones. He not only describes how accelerometer technology works, but he also goes into further detail on how engineers translated the three-dimensional technology to work inside a tiny, thin smartphone. The video is lifted somewhat from Hammack's own book, Eight Amazing Engineering Stories: Using the Elements to Create Extraordinary Technologies, but the YouTube video explains how the entire process works in much better detail. Here's the full text from Hammack's video, which is also embedded at the bottom of the article.

Hammack introduces the accelerometer: I think this is one of the coolest features of today's smartphones, Hammack begins. It knows up from down. Built into the circuitry is a tiny device that can detect changes in orientation, and tell the screen to rotate. Now let me show you what it looks like using an old iPhone. There it is. It's an accelerometer. I'll tell you how this kind of chip works and how it's made, but first, some basics of accelerometers.

Hammack explains the accelerometer's components: They have two fundamental parts: A housing attachment to the object whose acceleration we want to measure, and a mass that, while tethered to the housing, can still move, Hammack said, producing 3D graphics to illustrate his point. Here, it's a spring with a heavy metal ball. If you move the housing up, the ball lags behind stretching the spring. If we measure how much that spring stretches, we can calculate the force of gravity. You can easily see that three of these could determine the orientation of a three-dimensional object. While lying with the z-axis perpendicular to gravity, only the ball on the x-axis spring shows extension. Turn this on its side so that z-axis points up and only the accelerometer along the spring on that axis stretches.

Hammack explains the difference of accelerometers in smartphones: So, how does this phone and this chip measure changes in gravity? Hammack asks, reading the mind of his audience. While more complex than the simple ball and spring model, it has the same fundamental elements. Inside the chip, engineers have created a tiny accelerometer out of silicon. It has, of course, a housing that's fixed to the phone, and a comb-like section that can move back and forth. That's the seismic mass equivalent to the ball. The spring in this case is the flexibility of the thin silicon tethering to the housing. Now clearly, if we can measure the motion of this central section we can detect changes in orientation.

To see how that's done, examine each three of the fingers on the accelerometer. The three fingers make up a differential capacitor: That means that if the center section moves, then current will flow. Engineers correlate the amount of flowing current to acceleration.

Hammack details how accelerometers are built: This accelerometer fascinates me, but even more amazing is how they make such a thing, he said. It would seem nearly impossible to make such an intricate device as the tiny smartphone accelerometer. At only 500 microns (1/50th of an inch) across, no tiny tools could craft such a thing. Instead, engineers use some unique chemical properties of silicon to etch the accelerometer's fingers and H-shaped section.

To get an idea of how they do this, let me show you how to make a single cantilevered beam, like a diving board, in a solid chunk of silicon. Empirically, engineers noticed that if they pour potassium hydroxide (KOH) on a particular surface of crystalline silicon, it would eat away at the silicon until it forms a pyramidal-shaped hole. This occurs because of the unique crystal structure of silicon. To make a pyramidal hole in silicon, engineers cover all but a small square with a mask impervious to the KOH. Now, it only etches within the square shape cordoned off by the mask. The KOH dissolves silicon faster in the vertical than in the horizontal direction. This is why it makes a pyramidal hole.

Now, to make a cantilevered beam, engineers follow these steps: First, mask the surface except for a U-shaped section. At first, the KOH will cut two inverse pyramids side-by-side. As the etching continues the KOH begins to dissolve the silicon between these holes. If we wash it all away at just the right point before it dissolves the silicon just underneath the mask, it will leave a small cantilever beam hanging over a hole with a square bottom.

Hammack explains what accelerometers mean to the world of engineering and technology: Engineers make smartphone accelerometers using these same methods, but as you can picture, it takes a series of detailed masks to create the intricate structure of a smartphone accelerometer. While complex, a key point is that the whole process can be automated. This is absolutely essential in the miniaturization of technology. Engineers now make all sorts of amazing things at this tiny scale. Microengines with gears that rotate 300,000 times a minute, nozzles in ink-jet printers, and my favorite, micromirrors that focus light in semiconductor lasers.

I'm Bill Hammack, The Engineer Guy.

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