Confusing: AO Card vs Loop Calibrator output - Why does my AI see these differently?

Hello, I'm confused today. I have a pump controller that has it's own 24vdc supply and takes a 4-20ma input from a level sensor to drive a pump.

I have a PLC that I want to read the same level sensor into for logging, trending and other business.

I have this set up as an analog input reading into the PLC, and analog output from same PLC to the pump controller.

So, My expected output is working, some level gives me expected milliamp output, great. Input of course works like a charm, all seems well.

I hook the AO into the pump controller, it reads a near-constant 'level', much different than what I am expecting, no matter what I do to the AO's 'percieved milliamp output' it seems the pump controller is altering the signal.

I hook up my loop calibrator into the pump controller and viola! Exactly what I'm expecting...

SO, I make the PLC generate 15ma and loop calibrator do the same. The pump controller 'likes' my loop calibrator and 'hates' my AO from the PLC...

My question is WHY are these so different? Does the loop calibrator have some resistance immunity that the AO card dosen't have? Is the AO signal being overwhelmed by the 24VDC from the pump controller whereas the calibrator is not?

I'm stumped today. Any help greatly appreciated. This has been a tough time getting one simple 4-20ma loop circuit to work between both devices. Splitting (the old-fashioned trick) never worked either.

Thanks All!!!!

EDIT: MicroLogix 1400 PLC, Standard AB Analog I/O modules, Logix 500, Alltek Calibrator, Grundfos CU-300 pump controller

just a wild guess ...

maybe your PLC's analog output is actually a VOLTAGE output signal - rather than a CURRENT signal ...

many analog output cards are built as an EITHER/OR type of arrangement ...

other types have a DIP switch - or set of JUMPERS - etc. - to decide what type of output signal (voltage vs. current) they send out ...

if this doesn't help, you probably need to tell us more about your specific hardware ...

Hi Ron, no, I've double-checked the I/O config in Logix. When I commissioned the cards, I deliberatly set them up as 4-20ma output instead of 0-10VDC. I hook up my meter and also verified the current output scales correctly.

Thanks

It's a ground loop caused by different ground potentials between the PLC AO and the pump controller AI.

Use a loop isolator module to isolate the two devices.

Could be a ground loop. If more than one device in the 4-20 mA loop is grounded, say the power supply and the controller input, then the current will take the path of least resistance.

EDIT: Gee, Dan - great minds think alike! Only old minds think slower, I guess.

Thanks Guys. I bought the output cards thinking they would provide the isolation I needed.

The AO into the pump controller is the only 'device' in the loop.

The entire system shares the same ground but looking around, I see no stake into earth. Not sure if that hurts.

EDIT, there does seem to be a connection to earth, I see it.

OK, so it DOES appear to be a ground issue. I tied the drain wire to ground on the AO card AND the ground terminal in the pump controller and now I get a "Sensor Failure" alarm in the pump controller. I was thinking tying the two grounds together would help, it sure did, now I get direct confirmation that something is wrong!

SO, can anyone recommend one a particular ground loop isolator module?

Thanks!

The Automation Direct FC-33 will do the job. The FC-11 is a little more iffy because it is loop-powered (might be too much load on your signal circuit).

http://www.automationdirect.com/adc...gnal_Conditioners/DC_Voltage_or_Current_Input

If you want to keep it in the AB family and someone else is paying for it, then look at these AB isolators:
http://www.ab.com/en/epub/catalogs/12768/229240/229268/10202439/10202447/