Load Cell Connection

The load cell documentation cannot be seen from Fargo, USA. What does the documentation of the load cell say about this?

Press for Rimshot

Well, that sort of worked...

How about: Press button in next window for Rimshot.

That is dependent on the supporting Input modules of the PLC manufacturer.

Doing a google search shows that Omron has hardware that supports this function.

http://www.omron-ap.com/product_info/F159/index.asp

If you have an Omron PLC you may be able to plug the load directly into the module. However, most manufacturers PLC modules for mV input do not supply the stable excitation voltage required for load cells.

Obviously, you will need to check your PLC manufacturer, and to ignore the scrappy replys from less-than-helpful commentors.

"Obviously, you will need to check your PLC manufacturer, and to ignore the scrappy replys from less-than-helpful commentors."

It is not obvious at all. If it were obvious, the documentation of the two devices would already have been referenced and this thread would not have been posted.

Further, I found the response from redfred to be refreshingly accurate; I may give him a GA vote just because he fussed.

If you want a crude and low-resolution method to connect a strain-gage directly to a PLC analog input, a thermocouple input would work.

Most load cells need an excitation voltage of 10volts and return a signal voltage typically around 3mV/V. Assuming that you PLC can deliver a stable 10V output (most PLC output at 24V +/- 1V depending on the attached load) you should be able to return 30mV at full range of the load cell. With a 0-10V input to the PLC this works in theory but you are only using 1/300th of the input range. This will be converted into a digital signal with let us assume a 12bit A-D converter. So instead of being able to resolve to 4096 steps you will only be able to resolve to 14 steps .

Now for the snags.

You cannot just use a dropper resistor to reduce the PLC output voltage to 10V as this will follow the voltage variations. A properly regulated 10VDC supply with low ripple is required. (This costs out at about the same price as an amplifier which has a regulated supply included)

You are probably not going to use the load cell over its full range, if you do you are in danger of overloading and destroying it. So if allow a safety factor of 2 your maximum output is 15mV. (reducing you resolution to 7 steps)

The output of your load cell is supposed to be linear, but in fact is isn't, a problem that would be corrected by the amplifier, but you don't have an amplifier. So you should analyse the non-linear input (you are looking for a six or seven function sine curve that you can derive using Fourier analysis) and then write a program to apply a correction (that might be quite hard as you only have a PLC, but it would be relatively easy in software like C++), but your input and resolution are so crude that this may not be worth the effort.

Yes you can connect a load cell to a PLC, now then come all the buts.

But the PLC analog input card has to have the capability to read the low level voltage signal.

But the PLC will need a special summing program since most load cells work several at a time averaging their measurements.

But the load cell faults are almost impossible to detect directly from a PLC input module.

And finally the accurate ranging of the load cell measurements are not easy to input into the PLC.

So the answer is Yes you can do it, and No you should not unless it is an experiment.

What you would be saving as less load cell summing card and transmitter you will pay out extra in the PLC part.