# load cells to plc?



## garfield (Jul 30, 2009)

I am wanting to weigh a 5 gallon pail of liquid soap. I want to feed this to a plc to determine that the correct amount of soap comes out on each cycle. So I have a 24 vac solenoid that is energized that feeds an injector. I want to have a feedback from a load cell to the plc to determine how much the weight of the 5 gallon bucket has changed. If it is out of spec then it would alarm to let us know that the injector is clogged / worn solenoid failed, pump failed etc. Is it feasible to buy load cells that feed directly to the plc without the normal scale interface? I would want a touch screen to display the values, reset alarms, logging etc. There is not any need for certifications etc. I could just do a draw on a beaker to calibrate it.


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## joebanana (Dec 21, 2010)

garfield said:


> I am wanting to weigh a 5 gallon pail of liquid soap. I want to feed this to a plc to determine that the correct amount of soap comes out on each cycle. So I have a 24 vac solenoid that is energized that feeds an injector. I want to have a feedback from a load cell to the plc to determine how much the weight of the 5 gallon bucket has changed. If it is out of spec then it would alarm to let us know that the injector is clogged / worn solenoid failed, pump failed etc. Is it feasible to buy load cells that feed directly to the plc without the normal scale interface? I would want a touch screen to display the values, reset alarms, logging etc. There is not any need for certifications etc. I could just do a draw on a beaker to calibrate it.


Just what is it you're asking? :blink:


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## garfield (Jul 30, 2009)

Good question joebanana. I guess is it feasible and or cost effective to feed directly from load cells to a plc? What type of input card would i need?


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## ScooterMcGavin (Jan 24, 2011)

The easiest thing would probably get a load cell that puts out a 4-20ma signal and then get a 4-20ma input card for your PLC. Just do a google search for "4-20ma load cell" and you'll get a bunch of hits.


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## brightspark1 (Feb 17, 2016)

I generally use Mettler Toldeo weigh scales and use either the optional profibus card or a 4-20mA output card for communications to the PLC, 
I like the Mettler scales due to the fact they're very user friendly.


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## oliquir (Jan 13, 2011)

there are some strain gage analog cards. also there are some strain gage to 4-20ma converters. i also already done it with just a 15vdc power supply and a millivolt analog input if precision is not critical. you can also use the cheapest scale display with communication or 4-20ma output (some are under 2-300$)
here a cheap transmitter form ebay
http://www.ebay.com/itm/121333591608


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## John Valdes (May 17, 2007)

The load cells I worked with used a millivolt signal, not a milliamp signal.

So I would think the plc would accept an analog signal and you scale it?


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## JRaef (Mar 23, 2009)

garfield said:


> ... Is it feasible to buy load cells that feed directly to the plc without the normal scale interface?...


The reason for the "scale interface modules" is that the way a load cell works is that it has to be "excited" by a voltage source FIRST, usually 10-15VDC, then it gives a signal output. But that signal is very low energy, not linear and can sometimes vary with temperature, so it need amplification, interpolation and often temperature compensation in making it linear at 4-20mA or 0-10VDC. The scale interface modules are essentially a highly filtered power supply and smart transducer that interprets the raw load cell output into a linear usable output proportional to the range of the load cell, with offset and gain adjustments. You can do all of that yourself with separate components, the scale interface module just makes it easier and all in one unit.

If you find a load cell that has the 4-20mA or 0-10VDC output already available, it is essentially most of what you need, but you will likely still need a good quality filtered power supply for it.


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## Tornado (Dec 27, 2015)

As others have stated, load cells will generally output a millivolt signal. This signal can be read by a thermocouple input module on the PLC. I know some manufacturers TC input modules have a setting called "raw mV" just for this purpose. Again, a load cell that conditions the mV signal to a 4-20mA or 0-10vdc output would be preferable and can be connected to a standard analog input module on the PLC.


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