A low load cell signal

[Shohadawy]

You know that the voltage of a load cell is relatively small... can I connect the load cell directly to a PLC??? or Shall I make an amplification stage ???

 

[ericgibbs]

You know that the voltage of a load cell is relatively small... can I connect the load cell directly to a PLC??? or Shall I make an amplification stage ???

hi,
You will require to amplify a load cell signal before using it with a PIC.

A LC has an output in the order of 0 thru 10mV or 20mV at full load.

If you use the PIC's adc with the internal 5Vref, the LC signal will require 5000/10 = *500 gain for the max adc value.

What resolution in weight are you planning, also whats the max weight.?

 

[Shohadawy]

hi,
You will require to amplify a load cell signal before using it with a PIC.

A LC has an output in the order of 0 thru 10mV or 20mV at full load.

If you use the PIC's adc with the internal 5Vref, the LC signal will require 5000/10 = *500 gain for the max adc value.

What resolution in weight are you planning, also whats the max weight.?

Thanks alot for your reply .. but I'm using a PLC ( programmable Logic Controller ) not a PIC

 

[ericgibbs]

Thanks alot for your reply .. but I'm using a PLC ( programmable Logic Controller ) not a PIC

hi,
Does the PLC you are using have any ADC inputs.?

 

[Shohadawy]

well, not an integrated ADC .. but I will use an ADC module .... it has 12 bit resolution ( values from 0 to 4095 ) .... the load cell's output is 2mv/v

 

[ericgibbs]

well, not an integrated ADC .. but I will use an ADC module .... it has 12 bit resolution ( values from 0 to 4095 ) .... the load cell's output is 2mv/v

hi,

Is that 2mV/Volt or maximum output.?

What the weight range of the Load Cell.?

 

[Shohadawy]

No this is the output voltage per each volt of excitation voltage ... the cell weight range is from 0 to 100 kg

 

[ericgibbs]

No this is the output voltage per each volt of excitation voltage ... the cell weight range is from 0 to 100 kg

hi,
So IF the Vx=12Vdc then the 100kG FSD would be 24mV.
The Vext should be marked on the cell.?

Which external ADC do you plan to use.?

 

[Shohadawy]

as I've told you before I'm planning to use an ADC module .... it has 12 bit resolution ( values from 0 to 4095 )

 

[ericgibbs]

as I've told you before I'm planning to use an ADC module .... it has 12 bit resolution ( values from 0 to 4095 )

hi,
Thats what I said

Which external ADC do you plan to use.?

I know its a 12bit, whats the model number, some have inbuilt amplifiers.

 

[Shohadawy]

the model is Al-1116 .... it's a product from ALTUS company
This is the website of the company:
www.altus.com.br

 

[ericgibbs]

the model is Al-1116 .... it's a product from ALTUS company
This is the website of the company:
www.altus.com.br

hi,
I have downloaded the datasheet.
I would suggest the 0 to +5V input option.

The LC output will require to be amplified from its 2mV/Vx upto 0 to +5V, also a ZERO and SPAN control pot for the amp.

Are you using all 8 input channels.?

 

[Shohadawy]

hi,
I have downloaded the datasheet.
I would suggest the 0 to +5V input option.

The LC output will require to be amplified from its 2mV/Vx upto 0 to +5V, also a ZERO and SPAN control pot for the amp.

Are you using all 8 input channels.?

Thanks Eric ... Why do I need to amplify the signal ? ... The LC output is from 0 to 2mv/vx this values are inside the range 0 to 5v

 

[ericgibbs]

Thanks Eric ... Why do I need to amplify the signal ? ... The LC output is from 0 to 2mv/vx this values are inside the range 0 to 5v

The ADC will only give the full 12 bit output for +5V input.

That means to get a count of 4096 requires a +5V input.

If you say had a 10mV input the all you would get from
the ADC is 0.01 * 4096 =40 [40.96] counts,,, which is no use, the resolution is far too low.

If for example the LC had a 100Kg weight on it and gave say 10mV out, then
the LSB of the ADC would be 100Kg/40 = 2.5Kg

Do you follow OK.?

 

[Shohadawy]

ok eric ... What type of amplifiers can be efficient to use with PLCs??

 

[ericgibbs]

ok eric ... What type of amplifiers can be efficient to use with PLCs??

hi,
You need an amp that is stable and with a low noise figure.

Look at www.analog.com go thru their OPA collection, it depends upon the accuracy and stabilty your application requires.

What are you weighing, what resolution and accuracy do you need.?

 

[Shohadawy]

hi,
What are you weighing, what resolution and accuracy do you need.?

I'm weighing chickens ..... their weights may range between 800 and 1100 gm ....

 

[ericgibbs]

I'm weighing chickens ..... their weights may range between 800 and 1100 gm ....

hi,
It would be possible to use a standard PIC, with a 10bit ADC.
With an offset to give 500 thru 1500gm, you could weigh within +/-1gm.

They seem small chickens, are these live or dead [degutted] chickens.

We aim for 1500gm thru 2200gms, live weight.