Accelerometer Sensor

[Pesticida]

Hello,

I make a Accelerometer Sensor Circuit for a Elevator,and I'm using a ADXL311J with a Rail to Rail OP Amp (AD6809) and 5 Volt Ref,50 Hz Bandwith with 0.1uF.
When I read now the Data on The Pic Microkontroller I see just a very little change,for Example :

when the Sensor is Idle I Read 130 thats mean I have 5 Volt/256 = 0.019mV per Unit then I have 0.019*130=2.47 Volt
2.413V-Vref/2=-0.03V
-0.03V/0.312V=-Acceleration 0.096g

Ok now when the Elevator is in movement I can Read the value 131 thats mean like the Formulas on the Top I have -0.035g Its this possible or I make somewhere a mistake?!

Thats mean i have (-0.096g) - (-0.035g)=0.061g
-0.061g*10 Seconds = Speed -0.61 Meters in 10 Seconds

-0.61 * 10 Seconds = Distance 6.1 Meters

Thank you for any Information

Regards Pesticida

[skimask]

Take the accelerometer and tip it over on it's side (or back, or front, or the other side) and take another reading. It should either read A minimum or maybe go to A maximum.

Just because something is MOVING doesn't mean it's ACCELERATING.
An elevator only ACCELERATES to get MOVING (or DECELERATES to come to a stop and/or reverse the direction it's MOVING).
Once it's MOVING, it's probably no longer ACCELERATING.

I'd doubt an elevator takes 10 seconds to ACCELERATE to it's maximum speed. 5 seconds, maybe, 10 seems like quite a long time to ACCELERATE.

[Pesticida]

Hi Skimask,

The Mnimum Value I have is 130 and the Max Value is 167 .
= 3,173 Volt - 2,5 V = 0,673 V ~ +-2g

What You mean what is the Best way to found the Speed and the ~ Distance?

Thanks

Regards Pesti

[skimask]

Hi Skimask,
The Mnimum Value I have is 130 and the Max Value is 167 .
= 3,173 Volt - 2,5 V = 0,673 V ~ +-2g
What You mean what is the Best way to found the Speed and the ~ Distance?
Thanks
Regards Pesti

So you get 130 when?
and you get 167 when?

673mV for 2g worth of acceleration is not a lot to measure or play with.
If you're trying to make a speed/distance sensor, I hope you're reading the part as fast as you possibly can and not just a few times a second.

And in your first post, the math is all wrong...

5 Volt Ref,50 Hz Bandwith with 0.1uF.

At 3v, the output is 174mV/g. At 5v, you should have 290mV/g, with ZERO g at 2.5v
So, first off, you have to set the thing so it's sitting level, sensing nothing, right at 2.5v, and/or trim the output to read 2.5v using an external means.
Turn it 90 degrees one way in the axis you are measuring, the output should drop 290mV (1g), turn it 90 degrees the other way from the original 'idle' position, the output should rise 290mV (1g).

-0.061g*10 Seconds = Speed -0.61 Meters in 10 Seconds
-0.61 * 10 Seconds = Distance 6.1 Meters

All wrong...
1g acceleration = an acceleration of 9.8 meters per second each second
If you measure 1g of acceleration continuously for one second, at the end of that second, your accelerometer will be traveling at 9.8 meters per second, but will have covered only 4.9 meters (assuming a constant rate of acceleration).

Therefore, .061g of acceleration = .5978 meters per second - per second.
At the end of 10 seconds, the speed will be 5.978 meters per second (assuming a constant rate of acceleration), and the object will have moved 2.989 meters (again, assuming a constant rate of acceleration)...

I don't want to say that it's basic math, but if you're playing with PICs, math like this should come easy once you know what to look for.
http://en.wikipedia.org/wiki/Acceleration

[sayzer]

I think our friend Pesti missed an important point that Ski made.

Accelerometer "does not" give you usable data while it is moving at a constant speed.

It gives you usable data while its speed is increasing or decreasing.

[skimask]

I think it might be easier to think of it this way...

You are an accelerometer. You are blind folded sitting in a car. It's completely quite. The car isn't moving.

How can you tell if the car starts moving? You get pushed back in the seat a little bit for little awhile.
How can you tell if the car is speeding up? You get pushed back in the seat for a bit for a little while.
How can you tell if the car is slowing down? You get 'pulled' forward out of the seat a little bit for a little while.
If you don't get pushed back in the seat or 'pulled' forward out of the seat, did the car change speed?