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Topic: Weight based system (Read 511 times)previous topic - next topic

cameronasmith

Apr 02, 2014, 05:10 pm
Hi Guys,

I have a weight scale (only the sensor) connected to an INA125P to measure money, and to my Arduino analog pin 0.

The good news is that it is working! I can put a dollar note on the scale and it will give me a good reading. However, I have a question about weighing coins as well. The scale should read like 100 coins, but the scale of the analog system seems to be only small enough to take in like 10 coins. After that it goes off the scale.

* The analog number ranges from 422 (me not touching it) to 900 (me pressing down on it)

Can anyone recommend any suggestions? I have thought of the following ideas if someone could share some light that would be great:

1. Chanding the 10hm resistor when counting coins on the INA125P

2. I heard that the Ardunio has some software syntax that you can change analog inputs, but I'm not sure about that.

Also, can anyone recommend how to stop ripples in the weight system? For example, the reading I get is:

421..422, 425, 421, 421, 421, 423

Any suggestions would be appreciated.

Thanks,

Cameron.

Robin2

#1
Apr 02, 2014, 07:52 pm
I'm a bit confused. I have no idea what a dollar bill weighs but I imagine it is very much less than a coin so I would expect you would have to change the physical range of the scale (a stronger spring or shorter lever) to weigh coins.

Assuming you are using the default 5v reference for the Arduino IDC 422 equates to just over 2 volts and 900 equates to just over 4.3 volts. That seems to be the range produced by your scale and, as you say, it is wasting some of the resolution of the ADC. But that is a different matter from the range of weights the scale can measure.

If you discovered that it runs past 1023 before the scale is fully depressed that would suggest that the upper end of the scale output exceeds 5v and a voltage divider (or adjustment of the amplifier gain) could be used to match the output to the ADC. But clearly the scale does not produce an excessive output.

Without having the specifications for the load cell I can't think of anything else to say.

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

cameronasmith

#2
Apr 02, 2014, 07:57 pm
Hi Robin,

The scale is from an existing weight scale that weights dollar bills and coins, and it only has one scale inside. I checked the voltages of the original scale and it was 5volts going to the scale.

What do you think if I lower the amplification? I'm thinking that might work? What do you think? Maybe, some sort of variable resistor chip controlled by the Arduino.

BTW: 1 USD bill weights 1 gram, and 25 cents weights 5 grams (aprox)

Cheers,

Cameron.

Robin2

#3
Apr 02, 2014, 11:34 pmLast Edit: Apr 02, 2014, 11:36 pm by Robin2 Reason: 1
While there may be room for more sophistication it seemed to me from your original post that the Arduino ADC was giving a number of about 420 when there is nothing on the scale and 900 when it has it's maximum load.

That suggests that you are getting the full range of what the scale is capable of.

Your original question was about weighing 100 coins (500 grams?) and it seems to me the scale can't do that.

You said in your original post "After that it goes off the scale." but that isn't true if the maximum reading is "900 (me pressing down on it)".

Changing the amplification won't change this unless of course you are trying to work beyond the capabilities of your amplifier.

If you have a suitable meter try measuring the voltage at the input to the amplifier with nothing on the scale and with a heavy (maximum) load on it.

...R

PS, if I was you I would ask the moderator to move this Thread to the Project Guidance section of the forum. ...R
Two or three hours spent thinking and reading documentation solves most programming problems.

Chagrin

#4
Apr 03, 2014, 12:32 am
Instrumentation amps and op amps never output a full scale, 0 to VCC range. For the INA125P specifically it's a minimum of .4V to VCC - 0.9V, typical.

You have to be aware that the load cell is never going to show a perfect, 0V differential output between its signal wires; you're always going to see a little offset. If you set the gain of the InAmp too high you're going to be magnifying that offset excessively. From experience I can say that the resistance setting the gain needs to be precise to 2 ohms or less. The Instructable's suggestion of using a 10 ohm resistor is naive -- you need to fiddle with that value to see what works best for your load cell. Ideally you'd use a 50 ohm, multiturn pot for this; you would start at 50 ohms and slowly decrease the resistance right until the point you see the output start to change -- that's your ideal gain setting.

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