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Project Help and Ideas » Weight Scale Project help

April 04, 2011
by abd445
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Hello,

I am trying to implement and modify the weight scale project from nerdkits but the problem is that my weight scale doesn't have signals labelled as E or S, instead I have G Y R B and I don't know how to find out which ones are excite and which ones are sense signals. Also, I am trying to display the weight on a LCD but I don't know the conversion factor from voltage to its corresponding weight.

Can anyone please help me with this?

Here are the pictures of the weight scale:

Alt Text Alt Text Alt Text

April 04, 2011
by abd445
abd445's Avatar

Here are the link to the images:

http://img545.imageshack.us/i/dsc02183yb.jpg/

http://img812.imageshack.us/i/dsc02182u.jpg/

http://img15.imageshack.us/i/dsc02181iy.jpg/

April 04, 2011
by Ralphxyz
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For "I don't know the conversion factor from voltage to its corresponding weight." I can help you with that.

Measure two or three known weights and get the voltage reading of each weight.

You probable should try 100#, 150# and 200# but that might not be to easy unless you have a weight room.

You will probable be able to make up a scale using 10#, 35# and 50# but if you are going to be weighing a 220# object the curve might be off a bit when you get to the top.

If you have a multi meter take some ohm readings between the four wires and post the readings maybe we will spot a pattern to help you identify which is which.

Ralph

April 06, 2011
by abd445
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Ralph,

I've tried measuring the voltage (using different combinations of the four wires as sense or excite), but it just shows a constant value on the LCD. Regardless of what weight is applied on the scale, the output on the LCD is always the same. Do you have any suggestions for measuring the voltage? I will post the resistance readings as soon as I get them.

Thanks!

April 11, 2011
by abd445
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Hey Ralph,

Thank you for your patience, I am sorry but I am new at this but I really want to learn, so please bear with me.

I am still confused as to from where am I supposed to measure the voltage when I put a weight on the scale?

Also for measuring resistance, do you want me to put some weight on the scale and then take the ohm readings and repeat this for different weights?

April 11, 2011
by abd445
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Okay, I have figured out that Blue and Black wires are the sense wires. I tried measuring the voltage between those wires by putting different weight on the scale, but it was too low. So I connected the sense signals to the AD622 amplifier but I encountered another problem which is that the voltage output was constant at 4.8V when the scale was off and when I put 200 lbs on it, it changed to 5.1V. the change is really small to come with a conversion factor so I am not sure what to do?

The way i figured out the sense wires was: First I did a continuity check with ground and found out that the white wire was ground. So I hooked up the Blue and Black wire to a multimeter and found out that voltage was changing when I applied pressure on the scale. So I guessed red wire was the E+. Do you think this might be correct?

April 11, 2011
by Ralphxyz
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Yes that sounds correct.

You have to use a op-amp to amplify the signal coming from the strain gage.

That will boast it up to the 0 - 5 volt signal required by the ADC.

My strain gage thread list the op-amp I ended up using and Mongo recommended others.

Ralph

April 19, 2011
by abd445
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Hey Ralph,

I did amplify the voltage signal and found out that the conversion is something like 0.2V for every 10lbs. But this when used in the program gives a 2.5lbs accuracy. Do you think I might be missing something here?

April 21, 2011
by Ralphxyz
Ralphxyz's Avatar

abd445, explain [quote]"But this when used in the program gives a 2.5lbs accuracy."[/quote]

If you can get a dependable 0.2v per 10lbs I assume measured with a multimeter but then get a +-2.5v from your program I would say it is something in your program.

I do not have my straingage project setup so I can not verify anything.

I do remember that I did not get a linear reading and I think that is where I stopped.

I got something like .2v for 10lbs and .4v for 20lbs but it curved as the weight got higher and did not make a straight line.

So at 160lbs I did not get what I was expecting.

I had communications with another guy doing a similar project and he said that he got linear results using the same process you are describing.

Ralph

April 27, 2012
by bdpf15
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Can anyone send me a schematic of the wiring layup including resistor and capacitor values. I have no idea on how to do this part.

Thanks

April 27, 2012
by Ralphxyz
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Actually if you learn how to setup a Op-Amp you will know what resistors and capacitors to use!

I'll see what I might have to help you further but first thing you have to know how to amplify a weak signal.

abd445 above was using a AD622

Here is one of my discussions about using a Op-Amp with the strain gage, there are even a few schematics.

You can search the forum for some other discussions.

Ralph

May 03, 2012
by bdpf15
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Thanks Ralph, I have some reading to do.

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