Technology for Poets: Accelerometers

Do you like to only use technology while not caring how it works? Not me.

At least in general I like to know how things function and have learned that often confusing technologies fundamentally have simple explanations. The past few months the notion of motion has been nagging me because there are many common things around us that react to simple motion.

Today, I thought my way through a couple of articles and imagined an example of how I might teach someone how one type of motion detection works, and I think I found a good way to both put this answer into your head and satisfy your more important concern of why we care about such things.

So, why do we care about measuring motion? It’s simple because for lack of understanding this, we could set ourselves up for injury and even death.

Motion happens and our bodies can only tolerate a certain amount before injuries start to happen. Within the range of safe motions, we have all kinds of useful things going on that add value to our lives. Many of us drive or ride in various kinds of vehicles. This involves powered acceleration and stopping, but speeding up or stopping too fast can be uncomfortable or even deadly – right?

Thankfully – we have clever folks around us who understand how to measure such things and design our tools of transportation to not exceed these limits, but if you’re driving in a dense fog and can’t see the stopped car in front of you, you can have a life-ending deceleration which we all want to avoid, so other clever folks created and designed airbags to sense a quick stop and cushion us during such a crash, because the windshield does such a lousy job of slowing us down safely.

At the heart of this topic is a sensor known as an accelerometer. I know what you’re thinking at this point, because I had the same thought only a few hours ago. But now I don’t. You’re thinking, ‘I have no clue about such things nor am I likely to be able to understand something this technical’.

The good news is that just as I was wrong to think this, so are you. The answer in a general sense is actually quite easy. The details, I’ll grant are less than easy, so I won’t go there. Let me prove it to you.

Would it surprise you to know that most likely you weren’t even dressed this morning before your first use of an accelerometer? Some of you just imagined yourself still in your pajamas out driving your car and slamming on your brakes where your half-opened eyes nearly missed the red traffic light.

While that would certainly have brought your airbag accelerometers into use, it likely would not have been your first encounter with one.  No, your first use of an accelerometer today was likely when you picked up your smart phone and the thing reacted when you picked it up and presented you with the login screen.

Yea – our smart phones have very tiny accelerometers inside that sense such simple motions. You may have even turned your phone sideways to read something in a wider format and, ta-da, the phone sensed the change and flipped itself to landscape mode.  So, even if you’re still in your PJs, isn’t this intriguing?  How would you even begin to build something that could do this?

The good news is that we already understand the answer – in general.

Just imagine a flat piece of wood on a tabletop. It has a springy wire sticking straight up and a tennis ball stuck at the top. That wire has to be springy for this example to work for you. If you were to tap the ball and it moves back and forth some, then it will work for this demonstration.

This is our crude accelerometer. It’s sitting there. The ball is not moving, unless one of your kids came by and smacked it so you need to stop it from waving around before continuing.  Once it’s still, we can continue. the wood base, the wire and the ball all are part of the accelerometer, with nothing moving, you first try to flick your finger against the wood base. Did the ball move? No.  Well, that’s because your finger flick did not move the wood base. Next, try a book lying about a foot from the wood. Slide it with various degrees of speed against the wood base.

Okay, you already know what’s going to happen and you don’t really need to build this demonstration. When we hit the wood base with greater amounts of force, the ball is going to move a little for small amounts of force or a lot if you let your kids do it because they’ll really smack the thing and likely send it flying off the tabletop.

Thankfully, you’re only reading this demonstration and there is no model for your kids to mess with. If you could figure a way to measure the amount of swing each tap or smack caused, maybe how long it took to stop moving, you have the idea of how an accelerometer works.

Please don’t take your phone apart looking for a swinging ball, because there’s not one in there. Instead, there are a couple of very small circuits on one of the printed circuit boards designed in such a way that one parts slides slightly (microscopically actually) which changes a small amount of power passing through the device and produces a calibrated signal that reports direction and degree of motion.

For those of you who live in near a seismically active fault, think about the seismographs used to detect and measure earthquakes. They operate on the same principal but use different types of accelerometers.

For those of you who are technical and want to know the details of how someone created an accelerometer small enough to fit in a smart phone, think of very small, very sensitive, variable capacitors no larger than two wires with the distance between the two varying as one wire moves when the phone moves relative to one that doesn’t. The moving wire is actually a tiny spring. Varying capacitance = a change we can calibrate to and program device reactions to accommodate different features. Pretty cool – huh?

This is how your smart phone knows you picked it up. Unless your battery is dead. In that case you have a larger issue to address before you could log in anyway. So, just as it might be tough to move that board with tennis ball on a wire without moving the tennis ball, the same thing is going on inside your phone when you pick it up or try to use another car as a brake for your own and start your day with an airbag experience.

So, we have a new year to get started on.

Go plug your phone in to charge. Make yourself a big mug of coffee or tea. Get out of your PJs before you run out to work or the store and don’t even think about giving your car’s airbag accelerometer any reason to do its job. Just imagine a completely airbag deployment-free day.

Have a great 2023 everyone.

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