Tuesday, July 21, 2009

Something I Don't Understand About Technology

So I was just looking at this post on the Wired website, which lists "Five iPhone Apps That Replace Bike Hardware" (sadly, none of them is a 15 mm box wrench, but whatever). One of the apps uses a microphone and a little plastic tab attached to a spoke to measure speed by timing the sounds that the tab makes when it hits the frame, using the wheel size, and doing maths. That's cool (if slightly too complex), but here's what I don't understand: The iPhone and iPod Touch have accelerometers in them, right? (Right.) Accelerometers, if their name is to be credited, measure acceleration. So if you have a device that measures acceleration, and it is programmed to presume a velocity of zero when you turn it on, can't it pretty much keep track of your speed? How come my Touch can't act as a speedometer just by being on, without the aid of jury-rigged craziness? Isn't that what this future technology is all about? And where is my flying car?!!


Jess said...

think of an accelerometer as a motion sensor...when you turn your ipod from portrait view to landscape view, and the screen adjusts the image, that is the work of the accelerometer. Since your ipod would be mounted to your bike, it wont actually be moving all that much.

Rich said...

Theoretically, yes. Acceleration is the first derivative (yikes, I'm talkin' Calculus here) of velocity, so as long as you have the correct boundary conditions (which you've done in your example by forcing an initial velocity of zero) you can calculate the velocity.

The problem in practice is that the accelerometers in your iPod Touch probably aren't calibrated to provide any meaningful unit-based data. They're used to determine orientation and, I assume, relative acceleration (not absolute) for those games you have to whip it around for.

I've worked with high-precision calibrated accelerometers, and they are really expensive. But it's really fun to do drop tests of a 50 lb. device.

El Presidente de China said...

So what you're saying is, my iPod sucks?

Rich said...

Yes, that's exactly what I'm saying. You should give such a shoddy product to me, for free, for proper disposal.

Actually, the implementation of the accelerometers in the iPods & iPhone are quite awesome, given that it's a consumer product. They perform what they were designed to do, and then some. And they do so in an amazingly small space.

Accelerometers are often used to measure maximum shock (think shipping drop tests) or maximum "G's" that an object experiences. Something tells me that Apple didn't have a feature of measuring the shock of dropping the iPod onto concrete in mind when they designed the accelerometer functionality.

The thing is that using any accelerometer for the application you suggest, speed measurement, would be very difficult to implement. You'd have to account for things like bumpy roads, which make the measurement error huge. Also, going up & down hills would change the vector from the measurement axis, so the whole thing would somehow have to be self-leveling. How it handles turns would also have to be thought out.

Or you could buy a bike speedometer for $15.

Aw yeah...geekin' out with the Physics on the BBB.