Electronics 101: How Accelerometers Work

What's that? Accelerometers? I like how you bridged that there. Yeah, it wasn't that. That was a really good segue. That's that's the tom Merritt segue where you get me mad by pointing out my segue. Oh nice, Yeah, tom Merritt always hated that, which is awesome, which why you always do it whenever soon anyway, So tom Merritt decide we're

gonna talk about accelerometers to day. And accelerometers are in a lot of different products, um particularly a lot of handheld mobile gadgets, as it turns out, because they provide a handy guide to things like orientation and also just movement in general. So to understand what an accelerometer is, I guess we need to talk about what acceleration is. Okay, we can do that, Okay, Yeah, I just wanted to point out that we were going to make a return to our electronics one oh one. Yes, yes, a long

return to electronics one oh one. This time we're talking specifically about accelerometers. And it's funny because, uh, um, on our first few podcasts on this in the series, we actually sort of went into a lot of different components. Um, and we we sort of recently went back into it. It's probably electronics to a one with the Arduino board. Um. But yeah, this is a component that you could actually

hook up to them. They're not particularly expensive components, and they're found in all kinds of things, as you pointed out, including a lot of things you wouldn't necessarily think of, um, laptops, laptops. Yes, um yeah. I remember being uh sort of astounded when I heard that Apple was going to put a device inside their laptops that would detect when it the laptop had been dropped, so it could park the hard drive to minimize damage. And I thought, wow, that's cool. I

wonder what that is. Of course, now they're almost in everything, so it's kind of hard to avoid accelerometers. But I think I think you're right. I think acceleration would be a good, um, good place to start. So acceleration is the time rate of change in velocity? Yes, it is, and uh and you you might say, well, what what was that was? Velocity? Velocity is two things. It's a thing speed and the direction it's moving in it. And it has two vectors, speed and direction. You cannot have

velocity without both of those. Oh man, I'm suddenly transported back to my physics senior physics class in high school. So, so acceleration is the time rate of change in that she aange and velocity so um, in a way you would, And you measure it by saying it's a distance over time over time. Yes, so it's it's measured in feet per second per second or meters per second per second. So for example gravity, gravity's force of acceleration is at thirty two ft per second per second or nine point

eight one per second per second. So yes, are you using the just curious are using the same texas instruments? Power point presentation that was one of my main sources. UM. The only reason I mentioned that is if you really want a uh basic introduction to accelerometers and the things we're talking about, that's a great place to start. It's free and you can find it online. It's really you know,

and I trust Texas. It's a it's a PDF format and it's clearly it was obviously a slide show, yes originally, and you can go through and it's quite quite simple to follow along. It does get into a little more technical detail than what will cover here, mainly because um it uses some really handy graph fixed to explain which Chris is showing off to me to the benefit of

no one out there. But it has some really handy graphics to explain things like measuring acceleration and observing acceleration, which obviously in an audio podcast we cannot take advantage of, so we'll be skipping over that stuff. So accelerometers are really all about detecting a change in motion, and uh there two basic types of accelerometers. There's a kind that will just uh detect um a dynamic change in acceleration,

and then there's the static acceleration. Now static acceleration, you're thinking, well, how can you have something that's static and accelerating at the same time. Yes, because static basically the definition of static is it is not changing. Yeah, an acceleration is a change in time, uh, the time rate of change in velocity. So how can you have static acceleration? That's really talking about measuring um the amount of stack acceleration due to gravity. Yes, so gravity is going to have

this constant poll. Uh. And like we said before, three two ft per second per second is the standard um. That's the acceleration of gravity. So on Earth anyway, So that's that's always going to be there as long as whatever the devices is on Earth. I mean, granted, if you take that device into outer space that changes or you go to a different planet, then that will change the They will still be static, it'll just be a different force because you know, not all planets have the

same gravity as Earth. Right, So where did you want to go from here? Well, I was gonna talk If you're telling you about dynamic acceleration, then what you're measuring is how fast or how how much how much change is undergoing in the velocity of a particular object over

uh an amount of time. So um, that would be something that would give you readings on uh, how quickly something is changing in velocity, uh, for stuff like our handheld devices, and that would be a good example that might be, um, the the laptops, Like it tells you that it's changing very quickly from stationary to moving really fast and that needs to shut down the hard drive.

Although really I would say that the accelerometers that are in laptops are probably similar to the ones that are on things like, um, the iPad or other tablet devices also smartphones, which are the static ones, and that's where where it's uh. The reason for the stag accelerometer is to kind of measure the difference uh that's going on due to gravity, so that you can determine the orientation of that particular device. Yeah. Now they do make accelerometers

will measure things in two axes. Basically you're you know, horizontal in your vertical and then just oversimplifying again obviously if you tilt it, you're still measuring in two axes. But but there are three axes axis accelerometers as well, so you can you can measure a lot more effectively as you could imagine within a three D space plane. Yeah, So, um, it depends on your application. UM, for something like a Nintendo WE remote, you're definitely gonna want a three axis accelerometer.

Same thing with the other various motion controlled game controllers that are out there. But it depends. You know, you may not necessarily need that if you can, if you're measuring acceleration on something that is moving on a flat plane, UM, to access would be fine, to access should be fine, or you could. I read one guy that suggested that if you really needed three D and didn't have a three D, you could mount to two D basically one and at at a right angle from the other to

measure the third axis. But that you know, And and while there's the static and dynamic accelerometers, there's also the there's also another major division, which is analog versus digital.

So an analog accelerometer is going to have an tinuous voltage output that's proportional to its acceleration UH, and then digital accelerometers will use UH well, most of them use pulse width modulation for the output, which has a square wave of a particular frequency, and the amount of time the voltage is high will be promoted proportional to the

amount of acceleration. So you know, again you're talking about measuring this and in terms of electricity, in terms of voltage, and that's what tells the sensor, Hey, this thing is moving in this particular way. It's the change in voltage. So you've got you've got the accelerometer and you've got a sensor that together are telling the device what this information actually means. And uh, then within these these major categorizations,

we have several different ways of actually registering acceleration. Yes, so we can talk about some of those. For example, there are capacitive alerometers. Now that's where you're talking about change in capacitance across the surface. Now, capacitance is materials ability to hold an electromagnetic charge. Well, you know, like capacity. Yes,

so it's an easy pneumonic device. Yes, and you one very common form of capacitance involves having these parallel plates that are a certain distance apart, and then if you change the distance, that changes that the capacitance of those surfaces. Uh. The distance has a a very strong correlation with the capacitance of those um plates. So if they move closer together,

the capacitance is going to change. So if you create a an accelerometer that will allow these these parallel plates to either move closer together or move further apart than The accelerometer can measure that change in capacitance and then interpret that through the sensor as a various form of acceleration. It may be the tilt of the device, the orientation of that device. That's how you would see it. When you like turn your smartphone sideways and it automatically changes

the view. That's how you perceive this. But on the back end, it's all being done through this change in voltage. That's just one version, though there's also piezo electric accelerometers rather which now you may not be familiar with piezo electric, uh that that term we're specifically talking about a an

interesting phenomenon. There's certain there's certain elements, there's certain kinds of materials that when they are when their shape is changed in a certain way, they will emit um electricity or if they are uh subjected to electricity, their shape will change yeah um and and basically in some cases, like the application of stress will make this make this effect happen. Um cistals are often used. Crystals and watches are a very common way of of h illustrating this principle.

That's how they work. They actually do change shape and uh when stress is applied, and then they will admit electricity or vice versa, which kind of interesting. UM. So this would be a piece of electric crystal that would be mounted to some sort of mass and there'd be a voltage output UM whenever acceleration changed. There's also piezo resistive. Those are surfaces that have some sort of electrical resistance and that will change relative to whatever acceleration is uh

is placed upon that object. UM. There's some that involve magnetic fields, the magneto magneto resistive resistive accelerometers. Yeah, those will change in the presence of any sort of magnetic field. There's also the Hall effect accelerometers, which they detect motion by sensing changes in magnetic fields, but only in corridors.

I'm kidding, I walk alone. Uh. Anyway, There's also the there's UM some that detect changes in heat, and the changes in heat and become interpreted as changes in acceleration. And Yeah, there's all these different kinds that are out there on the market. So the kind you use is really dependent upon what sort of device or um. Uh, what's our purpose accelerometer is going to have? UM? So it's it's an interesting world. I mean, it's interesting kind of um uh component that can be used in lots

of different applications. Uh, it's I think it's actually pretty clever to use it in things like smartphones and tablets as a way to help, uh, determine the orientation of that device so that it gives you the proper view. I mean, it's also used useful in things like let's say that you have a device that has UM a camera in and you're using it for an augment and reality purpose. The accelerometer might help the the your processor

in your in whatever device you're using. For example, I'm just gonna use smartphone because I've seen that most frequently in smartphones. Let's say you've got a flat map out on a table, and you've got your augmented reality application running on your smartphone, and it creates this three D virtual realm that's that sits on top of that physical map that you're looking at through the lens of your smartphones camera. The accelerometer might be feeding information to the

CPU that helps the the smartphone. No how you're holding your phone so that it's displaying the three D model properly. Because if it doesn't know the right orientation, then you might be looking at let's say it's a building. You might be looking at a building and it's tilted at a really weird angle because it hasn't the phone is unable to figure out how it's positioned in relation to the map. Now, some of that is done by the

camera itself. A lot of that actually is done by the camera itself, but the accelerometer may also feed additional information to the CPU in order to get the most accurate results. So that's just one example. I mean there's

plenty of others. There's things like, uh there, I remember there was a um, I think it was an iPhone app that came out shortly after the iPhone debut, which was, uh, it would measure how long in the air your iPhone was when you would toss it up in the air and catch it, which to me sounds like a terrible, terrible idea. It sounds like a great way to to create Apple iPhone turnover, Apple turnover. Uh. But yeah, I

mean cars. Cars. Yes, that's a very obvious use, especially for things like even even if you're not talking about within a car itself, like like for for the end driver. Let's say that you want to do things like test a car for the stresses that it will undergo when

it's in a crash situation. You use accelerometers in that as well to kind of and accelerames within crash test dummy, since you can determine if the forces that a human would uh would experience within a test or within a crash situation rather if they would be you know, strong enough to cause injury or even death. In fact, we have how crush test dummies work at House of Works dot com that goes into more detail on that. Yeah, yeah, well,

I mean you can there. They were also talking recently again about how UH car manufacturers have been putting boxes in cars for years now to gather information, uh, sort of like the boxes that you see in airplanes. UM, that will tell you more or less, you know, the information that it collects all kinds of information, but basically the speed at which you were traveling. Um, you know, it will give you an idea the direction you were

headed in. So if they need that information for to determine what happened in an accident, UH, they could pull that from the their uh the box and they can you know, they use these these instruments to tell but you know, an accelerometer is not necessarily enough depending on what you want to do. And that's that's actually interesting.

I mean they are everywhere now, but you're starting to see certain applications for which UM accelerometers are being augmented with gyroscopic devices, things like the the WE remote when they and introduced the motion plus the six excess controller

for the PS three. Yeah, yeah, yeah, these are elements where the gyroscope also plays a big role in determining the orientation and the pitch, the role the yaw, all that kind of stuff that also gets that also gets uh measured by the gyroscopes, which, by the way, we have an article how gyroscopes work and how stuff works dot com. Yes, and when when we wrote about the space industry, which it's come up a couple of times about the gyroscopic controls that they used to use back

when we were trying to reach the moon. Uh not us personally, No, we never tried. Well I tried, but my cardboard box never actually took flight. But yeah, so I mean between the two of them, uh, an accelerator axcet acceleramat Earth everything and a gyroscope can be very very accurate in terms of what measurement can you know the measurements that you can take on on uh, what

direction and what velocity? Yeah, actually, actually acceleration is going on to give you a much more accurate response in your electronic gear and some other things that you have to consider with an accelerometer. We talked about the number, uh, the the access number like how many Yeah, so we had that in there, but there's there's more than just that. You also have to consider the maximum swing, which is that's how much force the accelerometer is is essentially uh

rated to to measure. Now for something like your smartphone or your tablet, it doesn't have to be particularly it doesn't have to be able to withstand huge amounts of of of acceleration. We think of acceleration often in terms of the Earth's gravity. So, frank example, if you've ever heard, oh man, he was pulling six gs on that turn, that's six times the acceleration force of Earth's gravity that

that person was was experiencing. And um, you know, if you're on the Space Shuttle, you might be experiencing ten g's which, by the way, uh, if you do that for too long. Um, it will kill you because as as you experience these these higher g s, you will actually have the blood forced from your head and uh down your body, and that can actually deprive your brain

of oxygen. Which is why if you ever watch a test pilot or you ever go through test pilot training for supersonic flights, they often are told that they have to uh, they have to sort of strain so they force blood back up to their brain to avoid blacking out. So it's it's like where you actually hear I'm going where they're they're really trying to force blood back up there so that they can withstand these g's that are

uh they're undergoing for the purposes of a supersonic flight. Um. Actually, I saw a great MythBusters where Adam Savage totally blacked out because he was doing a supersonic flight and they did this really powerful turn and uh and it was it was too much for him. And you know, let's let's be honest, the dude it was the first time going on a on a flight like that. I mean I would have happened to me, just probably faster because I think Adams in better shape than I am, so um, anyway,

for your accelerometer, you have to have it. You know, you have to figure out how much force it's going to be able to to measure. And for something that's simple like smartphone or tablets, something that's not going to withstand high amounts of acceleration, it might be plus reminds one point five g's, so one and a half times the the gravitational force of that we feel on Earth. But if you wanted something more substantial, um, it might

be plus or remindus to g's. And it's only when you're starting to look at something that it's gonna have lots of sun stops and starts that you're looking at plus or minus five gs or more. Uh Like just something like the space shall clearly you're gonna have to have plus or minus ten g's. It's not gonna because the forces that that experiences are much greater than anything that we're going to have on our phones. For most

of us. Anyway, maybe if you get really mad and you throw it really hard, um, in which case you might want to pursue a career in the major leagues. Uh So the maximum swing uh is one of the other things you have to consider. Also, the sensitivity of the accelerometer. UM, not like that, but in general you wanted to be really a sensitive device because if it's not sensitive, it's not gonna pick up uh a slight changes in velocity, it's not really doing you any good. UM.

And then also the bandwidth of the accelerometer. You have to figure out how frequently the accelerometer is checking the uh the changes in velocity over time, So for example, might be fifty hurts, so fifty times a second the accelerometer is checking to see about changes in velocity. UM. You know it's uh if you're looking at something that's checking the acceleration of some of a device that's vibrating at a really high frequency, then obviously you're gonna have

to have a much higher bandwidth. You're gonna have to be checking that those changes much more frequently than fifty hurts. So one other UH factor that you might want to consider, depending on what you're doing with the the device, UM is the impedance or buffering. Basically, UH, that's beyond it's really we're getting to the point where we're beyond the

scope of electronics one and one UM. But depending on the type of project you you need you might have to consider this because uh, analog and digital accelerometers handle impedance differently. So, um, you know, if you're really getting into the fine points of an electronics project, UH, take a look at this before you run out and buy a component to add to your to your project, because it may be a factor for you, and it may not. I mean, if you're doing something uh really simple, then

it may not necessarily be an issue. I remember, actually I think we talked about this a Make project where, um, you know, Make magazine from Riley Publishing, they had taken apart. They showed a video where they took a part a WE remote and basically uh installed it on a in a box, um, and put it on a roller coaster where they were measuring the acceleration using the basically the

accelerometer already in the board. UM. So I mean this is talking about impedance things that they were already using a known device that they just hooked it up to. They hacked it apart and uh put it on a device where they could measure these things, and it was it's kind of cool to be able to do that. So I mean, if you're doing something where you're already working with like un if you're building it from scratch, then this is something you have to look into. Yeah, yeah, definitely,

so so um something to to investigate more thoroughly. Um As we said, there are many, uh, pretty good sources of information out there. That Texas Instruments document is one, and and there are some others out there, so yeah, they Uh, I wanted to also mention that, you know, we we've talked a little bit about other We've talked extensively about other devices that use accelerometers in a way. You can even think of a seismometer as having an accelerometer.

It's got a a mass that's that's that's kept in in separation with the rest of the device, and it actually measures the the movement and uh like the it's not technically the speed, it's the magnitude and the direction of the movement. So it's not exactly an accelerometer, but it's a similar concept. Yeah, because we're talking about we're talking about magnitude as opposed to speed. Those are two

different things. Now, you might move a great distance, but do it slowly, uh, and that you know, but but a seismometer would still pick that up. Um as opposed to moving a small distance, but moving really really quickly. Again, a size mometer will pick that up. But it's it's looking at you know, it's looking for the the strength or the amount of movement, not the speed of movement necessarily, right, right,

And and I could see instances where you would want both. Yeah, sure, you know, but for something like you know, the geographical surveys and and keeping track of those things. Um, you're right. I mean, the Earth is considerably larger than a lot of other things for which you would want, uh, seismological information and acceleration information, so they're probably a lot less concerned with the acceleration, except you know, it would help you.

It probably would help you determine the speed with which like a fault is snapping. Yeah. Well, I mean they can measure the difference of time between the two different waves that that kind of gives an indication of that. But the I was going to say that, Yeah, until I build my planet size compute, Yeah, it's not really

a concern. Maybe I already have, but yeah, I mean, these these devices being able to measure this information uh is very helpful for a number of real world applications, and um, without it, we wouldn't have the versatility that we do with our objects are devices that we have today? Yeah, definitely, So you know you would have maybe a manual like can you imagine being so primitive as to have a tablet device where you have to manually change the view

so that it's portrait to landscape? What is this stone Age? Well, you know in the Stone Age, all you had to do was turn the tablet. Yeah, and then of course it wouldn't change to turn your head, carve a new one, U turn sideways. This not little right, and that was brought to you by Jonathan Strickland, Capeman extraordinaire. I'd like to wrap this discussion now. Yeah, we'll have to come

up with some other components. Uh, specific components that you might find in electronics projects, might not yep, if you do, please let us know, sure do something specific. Like if you're thinking, guys, I really need you to do a full episode on capacitors, let us know. I don't know that we could squeeze a full episode of capacitors, but

maybe we can. Well, we've talked about them a little bit in our previous Electronics one on one series, so it's kind of like a battery, but it ain't all right, So that wraps up our discussion on capacitors, also accelerometers. If you guys have any other either components or actual electronics you want us to talk about, let us know. You can send us an email. That address is text stuff at how stuff Works dot com, or you can

contact us on Facebook and Twitter are handled. There is tech stuff H s W. Chris and I will talk to you again really fast, I mean soon. Be sure to check out our new video podcast, Stuff on the Future. Join how Staff Work staff as we explore the most promising and perplexing possibilities of tomorrow. The House of Works iPhone app has arrived. Download it today on iTunes, brought to you by the reinvented two thousand twelve camera. It's ready, are you