ACCELEROMETER HELP

[nakamori]

Hi, Here's the deal, I'm in a high school advanced physics class, and I am doing a project for JSHS. Here's the Project; "Is it possible to measure the gravity coming from the parking lot to find the number of cars in the parking lot using an accelerometer?"  I'm thinking that I need one with high sensitivity, just based on super sensitive scales, where blowing on one lightly would change the reading.  What I need help with is; Do any of you think it is possible?  The fact that I don't have an accelerometer isn't helping either....    If anyone could help me out i'd be really really really grateful. Thank you for your time!

[mccPaul]

I'm not a physicist, but surely blowing on an accelerometer will not in fact change the amount of gravity acting on it? If you move an accelerometer, you can measure changes in velocity. If the accelerometer doesn't move relative to the gravity acting on it, then you would be able to measure a change in gravity (assuming someone in the distant future makes one that is sensitive enough). With today's devices if the accelerometer is stationary relative to the gravity then you just see 0 on the outputs.

Why don't you just use an accelerometer to count the cars entering an leaving the car park? You may need to amplify the movement of the road as the cars pass by, but it will be a lot easier than measuring gravity.

Paul.

[nakamori]

That's a good idea, do you know of a place where I could aquire a device that could measure when a vehicle is passing by? (hopefully for a low LOW price or free, from someone/somewhere) Also, I need to find someone to be my "expert" for this JSHS project, if you would know someone who knows more than most about this subject please see if you can send me their e-mail. 

[mke_et]

Make it.

Just make a big coil and sit it by the entrance. Make an LC circuit so that it is resonant. Tie it to a PLL.

Then just tie the PLL 'correction voltage' to your A/D circuit. You can then watch as cars pull in and probably even see a difference from motorcycles, small cars, big cars, and busses.

[nakamori]

sorry, I just spoke with my instructor, he says that I need to use an accelerometer, and, I was thinking piezoelectric would be a good kind to use, thoughts? suggestions? help? X P

-Thanks

[mke_et]

Hmm, so you HAVE to use an accelerometer.

Do you have any restrictions on how you have to use it?

I mean, you could (or someone could) make a giant 'tipper' with a sensor on it. A platform that is a giant teeter-totter. It only moves maybe 1/4" or so (6mm). As a vehicle drives in it pushes one end down. As it drives across it tips the other way.
You can use the acelerometer to detect the rocking.

Hey, you could even put an air hose across the driveway. Make it with a "T" so that as vehicles drive across it, that makes the leg of the "T" push a piston up with an accelerometer on it.

Lots of things you can do to use an accelerometer in an unconventional way...

Using to measure gravity changes as an object with the mass of a vehicle goes by I think is not something you could ever make practical.

[Alban]

Hello,

This sensor could be used like in the Wii and the iPod Touch.

You put something that moves when the car passes (like a turnstile) and detect the movement or the position.

Alban.

[mke_et]

Uh, practical?

Consider this. As an aid to undersea navagation, Russian boomers are rumored to have a device built into the subs that detects undersea mountains by gravity changes. To do this, in a many hundred foot long sub they hang two HUGE (many ton) lead balls with mirros on them. They bounce a laser beam between the mirrors, and from the interference of the reflection determine when the sub passes over a mountain top.

And you want to detect the mass of automobiles?

You'd be better off trying to detect the metal in a huge induction coil...

[rocco]

Hi Nakamori,

Is it possible? yes.
Today? no.

The mass of the earth is aproximately161,900,000,000,000,000,000,000 pounds. The mass of a typical car may be around 2000 pounds. That is, the mass of a car is one part in 81,000,000,000,000,000,000.

Despite the fact that the center of gravity of the car is much closer than the center of gravity of the earth, the effect of the cars mass on the local gravity won't come close to getting off of the noise floor.

You need to wait until the 64-bit analog-to-digital converters come out.