Beckhoff AI module - huge bouncing! Grounding issue?

[FNC]

I have a Beckhoff PLC with many AI modules. Most devices are loop powered using the same +24 VDC power as the Beckhoff Bus Coupler and all is working fine.

I have some panel mounted controllers powered by 120 VAC that spit out some outputs that go to my AI modules and they bounce like crazy! I have the controller send a simulation signal of 12 mA and my Fluke Voltmeter reads 12.0 solid. On the AI module it bounces quickly from 11.5 to 12.9 mA. This happens on 2 types of controllers from different manufacturers so I can't blame the controllers.

All I can think of is that these are 110 VAC devices that do not share the 24 VDC power as the Beckhoff modules. The controllers must create its own 24 VDC power to spit out powered 4-20 mA outputs. All other AI devices work perfectly.

Everything is grounded. All devices, including these controllers share a ground.

This is in a plant with VFDs nearby and it is noisy.

What can be causing this???

 

[I_Automation]

My first idea would be to put a signal conditioner in between set for 0-20mA input & 0-20mA output. I wouldn't use 4-20mA settings for the input as then a signal loss or wire break will still give the PLC 4mA.


Power the signal conditioner off your good 24V power supply.

 

[FNC]

I have one on order. I will be trying that soon!

At that point the 4-20 mA output will have the same 24 VDC power as the PLC AI module. I hope that fixes it as this has been totally unexpected.

 

[danw]

Fluke is battery powered and 'floats' so it doesn't have an earth reference. So it reads the controller output as steady.



The ground potentials between the 4 wire controllers and the AI vary enough to generate the noise.


I suspect a isolator module will fix it.


Most loop powered devices are inherently isolated, so using a common DC power supply makes everything work OK.

 

[FNC]

Isolation worked!

I used an isolator and it worked! Finally.


I have 4 analog outputs from the same controller that will go to an AI module on the PLC. DO I need 4 different isolators or is there a single device for isolating 4 outputs that is din mountable?

 

[I_Automation]

I used an isolator and it worked! Finally.


I have 4 analog outputs from the same controller that will go to an AI module on the PLC. DO I need 4 different isolators or is there a single device for isolating 4 outputs that is din mountable?

I have seen a 2 channel signal conditioner, but personally I always use single channel devices.



They do seem to have a higher than expected failure rate for a controls item, and setting up & replacing one at a time is easier.