Fellow Science Lovers,
Game update: Still no game updates, but I did send Scribes Emerge to beta readers! As soon as this little book goes to market, my attention will be focused squarely (and triangularly) on the game for a few months!
Oh, and here’s the cover:
 |
I did this one myself, except for the 3 circles which I copied from prior covers. I think it’s purdy, but I’m biased.
Measuring Celery?
In last month’s article, I explained how tiny gyroscopes figure out which way your phone is rotated. Now, we extend that discussion to accelerometers: how your phone knows when it changes speed. Why talk about this? Because Mallory and Leah’s mini drone also uses such a device. And because we use them everywhere in our universe. And it’s a cool technology to know about. 
Because this article builds on the last one, I recommend you read last month’s article for context. If not, I’ll include some reminders here.
Our cell phones use MEMS (Micro Electro-Mechanical Systems) to detect changes in movement along a single axis. Note that we’re talking about acceleration, not speed. For a quick refresher, here’s the difference:
Speed is the change in position over time. (100 mph–how fast some folks drive on the interstate. You know who you are.)
Acceleration is the change in speed over time. (9.8 meters per second per second–how quickly you speed up while falling)
For example: when you “fall” in love with someone, you develop feelings at a rate of 9.8 m/s in the first second, and 19.6 m/s in the next second, and 29.4 m/s in the 3rd second, and so on. The real speeds would be slightly slower due to air resistance, but still. This gets fast quickly, and the resulting collision after the first minute would be cataclysmic, unless limited by terminal velocity. I’d much rather glide into love with someone than fall. Somebody tell the poets and songwriters to update their wording. 
It Has Teeth
Anyhow, here’s how an accelerometer works. A structure in the sensor swings back and forth when the phone moves. This changes the gap between moveable teeth and fixed teeth, as shown below:
 |
See how C1 is a much bigger gap than C2? That makes the capacitance much lower on the left side of the fixed tooth than the right. (as distance goes up, capacitance goes down.) As we said in last month’s newsletter, capacitance is an object’s ability to store electric charge–a property that’s easy enough to measure in our circuit.
The sensor converts this change in capacitance into a measure of acceleration. This is similar to how MEMS gyroscopes work, except the Coriolis force isn’t being measured here–just direct movements.
With only 1 sensor, you measure acceleration along 1 axis (left-to-right). To measure along another axis, attach another assembly and rotate it 90 degrees. Like this:
 |
Now we can measure up-and-down movements. But what about the final axis? Take the same picture as above, but imagine the big gray piece as bending into and out of the page, and the purple teeth being situated in front of and behind the gray teeth. Sorry, it’s hard to show this without making a 3D model, but I think you get the idea.
Other Uses
Accelerometers are used in a lot more than phones:
- video game controllers
- virtual reality headsets
- step counters
- airbag deployment
- vibration monitoring – to predict when machinery may fail. More vibrations over time = increased wear.
- seismic monitoring
- navigation
- rocket and drone guidance
- image stabilizers in cameras
- hard drive fall protection-when your laptop falls, the hard drive will park its read heads to avoid data loss and disk damage. (called active hard-drive protection)
Other Types
I was going to list the types of accelerometers here, but I found at least 27 on a casual search. Instead, I’ll highlight one in particular: PIGA or Pendulous Integrating Gyroscopic Accelerometer. I found this one interesting for 2 reasons:
1. It integrates acceleration to calculate speed. (I don’t think most accelerometers do this.)
2. PIGA reminds me of “pigu” 屁股, the Chinese word for butt. I think of this as the butt accelerometer. Hey, I form word associations to remember vocabulary, and the sillier, the better.
Security Concerns
Some fear 3rd party companies are taking acceleration data from our phones without permission to infer a lot about our behavior: how we drive, how intoxicated we are, geographic location, etc. This kind of data theft is called an inference attack.
Even worse, many phones are vulnerable to having their accelerometers manipulated sonically. This article explains in great detail how such a hack is performed:
https://spectrum.ieee.org/smartphone-accelerometers-can-be-fooled-by-sound-waves
Quick summary: you can control an accelerometer if you make a sound that matches the resonant frequency of its moving assembly (the dark gray box with teeth in my drawings above). With such an attack, you could make someone’s airbags go off whenever you wanted!
This is hard to pull off, though. The attacker has to put a sound-emitter close enough to the target and must know the resonant frequency of that particular accelerometer. And you, as the driver, would hear this sound once it went off.
So yeah, we’re living in a techie world. It helps to understand how this stuff works and how it can go wrong.
More References
–https://www.youtube.com/
Writing update: While beta readers are reading, I’m updating the glossary and polishing the book description. When I get beta comments back, I’ll make final edits and finish formatting the interior pages. After that, I’ll order a proof copy of the paperback and hardcover from Amazon and inspect them. If those look good, I’ll order a batch of 100 copies and publish the book.
I’m actually formatting it myself this time, which will make this the first book I did all the design for. Turns out I’ve had a pro copy of Adobe InDesign CS6 all this time and didn’t realize it.
The longest part of the process now will be waiting for the proof copy to arrive in the mail. Amazon can take up to 2 weeks to ship author copies and proof copies, for whatever reason. In any case, more of the Scribeverse is coming soon!
 |