I am designing a baby weighing scale. I am using 4 load cell and using excitation adjustment for sensitivity adjustment.

Procedure for sensitivity :

1.Place the weight(2 Kg) over the load cell 1.Disconnect the output of the other load cell
2.Note the adc count.
3.Similarly repeat the above step for all load cell
4.Check which load cell count for lower value and adjust the trim pot of the other load cell to match it.

The ADC count of the load cell before adjustment

Load cell 1 : 725
Load cell 2 : 730
Load cell 3 : 711
Load cell 4 : 735

I have matched it to load cell 3(i.e to 711 counts)
After this i connected all the load cell output parallelly and place the 2 Kg weight over the load cell one by one.
The adc count when the weight was kept over each load cell is below

Loadcell 1 : 463
Loadcell 2 : 455
Loadcell 3 : 451
Loadcell 4 : 465

Why the adc count is varying.I have adjusted the sensivity of the cell to match load cell 3.But the ADC count for load cell 1,2,4 are higher than the 3rd load cell

Can any one tell me why this is happening (variation in ADC count)?

 

Hi,

When you placed the 2kg weight on load cell1, is the entire 2kg weight is sensed by loadcell1. (If you place the weight on the pan, a part of the weight will be sensed by other load cells too ).

Look at the calibration certificates of the four load cells and see the sensitivity of each load cell. that will give you an idea on the signal variation and adjustment needed.

BTW, why do you want to use 4 load cells , when there are single point load cells which can support a pan of 400mm*400mm with just 1 load cell?

Connecting 4 load cells is similar to connecting 4 batteries in parallel. If the internal resistance of the 4 load cells (output resistance) is within tight tolerance, you can have current flowing from one load cell to another. so your combined signal voltage may not be the average of the signal of the 4 load cells.

Are you using 2 kg load cells in the scale?

Rgds
Sam

 

hi sam,

I am placing the 2 kg weight over the load cell. My platform size is more than 600 x 600.

I am using 10 kg load cell..

><b> If the internal resistance of the 4 load cells (output resistance) is within tight tolerance, you can have current flowing from one load cell to another. so your combined signal voltage may not be the average of the signal of the 4 load cells.</b> <

If current is flowing from one load cell to other how to avoid it..

 

Hi Sathiya,

You can use a summing (junction) box if you are not already using one. But most summing boxes will trim signals to a few % (<5% typically).

What is the tolerance on the signal (sensitivity) of your load cells? If you are using load cells with more than 5% signal tolerance and the output resistance are not matched , you may have little success even with junction box.

Regards
Sam

 

Hi Sathiya,

The best solution would be to use 4 loadcells with same outputs. The outputs of loadcells are provided on the data sheet. While a set of 4 with exact same output would maybe cost more, that should solve your problem