Can I use half of the fs range on a load cell to get better resolution?

BrockSamson

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I have a 10k load cell with signal conditioner that I need to read on a PLC. I can buy a Micrologix that only go up to 12 bit (4095). So the resolution I could get with that is about 2.5 lbs. I want a resolution of 1 lb.

I know I could get an analog card or another load cell but I am already over budget for this.

I only need to read up to 2000 lbs max from the load cell. Is there a legit way to set the full span in a plc for half the 0 to 10v signal? That way I could get double resolution on the load cell?
 
Depending on the signal conditioner you may be able to adjust the scaling.

You might be able to set the input at 0-20V and output 0-10V, this would give the PLC half the signal.

Another way to read half the signal is to wire 2 resistors at the input in series (of the same value) wire the signal conditioner output across the 2, but wire the PLC input across ONE resistor. The PLC will read the divided half-voltage.
 
I assume that its possible to set the 4-20ma (0 to 4096) to a desired span. If you reduced your span then you can increase your resolution?
 
Why not adjust your load cell conditioner to 20mA on 2000lbs?



That should give you 0.488lbs resolution. Might as well calculate the span to give you 0.5 to make it neat and get a bit of buffer to show they are going above where they need to be.



This is, I might add, something that in microcontrollers is possible (by changing the value you're comparing your input to) and used a lot but not something you see in PLC's.
 
I just realized a typo.

Set your signal conditioner to 0-5V input and 0-10V output, this would give the PLC a scaled reading of half the value, and the signal conditioner would consider anything 5.01-10V as the max 5V input.

Like said above, this changes the scaling of the output - just adjust it so your 2000# = 10V.
 
Changing the resolution in the PLC is only half the equation. If you don't change the resolution in the load cell it doesn't matter what you do in the PLC. It is going to produce a signal based on a load and if that signal is a 12 bit signal that's what your resolution is going to be regardless of what kind of input you connect it to.
 
If you amplify the signal so 10k becomes 2k, you will surely have more resolution in the PLC but the real accuracy will continue to be what the 10k cell indicates in its specifications, which will probably be worse than 1 lb.

it would be better to change the cell for another with full scale = 2k
 
yeah,

when I said:I assume that its possible to set the 4-20ma (0 to 4096) to a desired span. If you reduced your span then you can increase your resolution?

I was talking about the instrument
 
lfe said:
If you amplify the signal so 10k becomes 2k, you will surely have more resolution in the PLC but the real accuracy will continue to be what the 10k cell indicates in its specifications, which will probably be worse than 1 lb.

it would be better to change the cell for another with full scale = 2k
Click to expand...

Just looked at accuracy and it is plus/minus 25lbs so you are right. I will have to keep that in consideration.
 
Cardocea is on the money. On many load cell amplifiers you can zero & span them so therefore if you zero to 4ma (0lb)then add load of 2000lb and set the output to 20ma (if it will let you) then you will get the resolution you require, however if the accuracy of the load cell and the amplifier are not up to the resolution you require then your up the creek without a paddle.
 
For loadcells, in the converter you usually must specify the mV per V ratio of the loadcell.
As an example, if the loadcell outputs 4mV/V you can force the converter to double the gain by specifying 2mV/V.
It wont increase the accuracy of the loadcell, but it will make the PLC use more of the 12 bits it uses before scaling. Remember that you have to manipulate the scaling in the same way. If you have done the above for a 2.5 lbs loadcell, you should set the scaling to 0..1.25 lbs
 

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