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Topic: Load cell issues (Read 1 time) previous topic - next topic

Having Issues with my load cell. This is part of a bigger project but this bit I am stuck on. I am using a kmart cell ( tho I am sure that is not the issue) from a 0 to 5000g scale.// I say its not the issue because the Kmart cells seems to get the resolution it needs with less voltage and a very less amplifier.
When i output the voltage to the ADC on A0, the biggest spread I get is 14. For instance no load is 100, half a kg is 114. I am using a dedicated loadcell amplifier that puts out 4.73v for maximum input.//alsoconfident about my amplifier
Tried mapping the output but still cant get the resolution i need// I want to be at least gram accurate.
MY Question: Hardware issue eg my load cell?
                     Hardware issue eg the ADC on the arduinomega 2560?
                     Software issue eg I have very poor skills (i am basically reading analog input and converting to digital?
were do you think I am going wrong?
Mick

Erdin

Do you have a multimeter to measure the voltages ?
You could measure the voltage at A0 and check if that matches what you are reading in your sketch.

We need to know a lot more.

Can you give a link to your load cell ?
How is it connected ? You could even upload a photo so we can see it.
Can you upload your sketch between code tag ? If you write a post, there are buttons above the input text field. The '#'-button is for the code tags.
Do you use USB to power your Arduino board ? That is okay for now.
Do you use the internal reference ?



dc42

How exactly have you connected the amplifier to the load cell and the Arduino? What load cell amplifier are you using?
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

I am at work and cant take a photo
the load cell is from honkong. A voltage regulator and amplifier.

It puts out o to 4.3v, 9 ma current
I am getting the full 4.3v at full excitation (hope thats a word)
I have the output in A0 and i am using the example sketch for reading and printing an analog value
i need to increase the "span" between 0 output and desired output

link to loadcell

http://www.ebay.com.au/itm/350764184926?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

Using nine volt battery and unaware of the significance of the internal reference

Erdin

If the output of the loadcell amplifier is 0 to 4.3V, I assume it is working.
So you only need to read that in a sketch.
Did you also connect the GND of the amplifier to the GND of the Arduino ?

If the value of analogRead() is correct, you can use floating point math to calculate the weigth.
Code: [Select]

// example code, not tested.
float weigth;
int adc_value;

adc_value = analogRead(...);
weight = adc_value /1024 * 5.0 / 4.3 * 5.0;

Sanddune600

I just did a simular project are work didnt use arduino but I will let you know what I had a hard time with is how to calibrate it for me I ended up doing it as follows


to zero
if zero button is pressed    zerocalibration = inputpin  //to zero have your scale fully assembled this will start you at 0


scaleoutput = (inputpin - zerocalibration)    //this will go in your loop


to calibrate I had a 1# weight for calibration and used this method
if calibration button is pushed      scaledivide = (scaleouput * 1)  //put the wieght on here

now to get your wieght
weight = scaleoutput / scaledivide





hope this helps and is understandable

Thanks for that
Scale all good now, just a matter of adjusting voltage to the bridge and the gain of the amp.
Next I want to tie the load cell output to the speed of the conveyor feeding it.

Sanddune600

are you weighing a package going across a convoyer if so how fast are your packages going across on my project I had to be within .001 pounds (I think thats about .5 grams) accurate and weigh packages coming out of a machine at over 90 pieces per minute if this is what you are doing I can go into detail some of the things that helped me

I would be very interested in that if you would like to share it with me. What i have is edible nuts on a vibrating feed conveyor flowing into a scale/hopper. This drops at 100 grams into a machine making plastic tubes. A bag maker. I need to control the speed/feed ratio to dribble the last 5 grams.
I would have thought that after the initial calibration to zero, the scale would be linear for the rest of the day. I have some commercial equipment and thats my experience with that.
I also assume i will need some sort of PID compensation to get the last drops out
please contact me!
Mick

Sanddune600

I am not quite picturing how your setup is where is the load cell in this is a bag produced and put on a platform that is being suspended by the load cell how many packages are you producing per minute


for my project in order to get a accurate result I had to average my package while it was on a 1ft square convoyer suspended by the loadcell I put a sensor on the end of the convoyer and while my package is seen by the sensor I took as many samples as I could and divided by how many samples I took

Hard to describe but i will have a go
An 18 inch long conveyor vibrates, transporting nuts, which drop of the end into a small hopper.
when the hopper has 100g in it
the conveyor stops;
then the hopper opens and the nuts fall out ( into a pipe that has a tube of plastic around it, sealed at the bottom. nuts go in, top gets sealed and cut, becoming the next bottom)
repeat 20 times a minute
I have this working, without a variable speed control for conveyor. The problem is the amount of nuts in free-fall when the conveyor stops makes the system very inaccurate.
i want to control the speed of conveyor proportional to the increase in weight, so the last 5 grams travel along very slowly, and i would like to adjust for average error from freefall. I would also like an over weight alarm for when three caramel macadamias stick together and fall over at the 99 grams weight, making the final bag 125 grams.

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