Interference on T/C inputs to PLC
Interference on T/C inputs to PLC
(OP)
I have interference on analog inputs to a PLC via a thermocouple input module. There are 8 inputs, we are using 4 on this particular module. They are foil shielded type K T/C's. When the furnace zone approaches the set point temperature (~900°C) ONLY the 1st channel input starts fluctuating in the PLC analog input registers but not when verifying millivolts with a multimeter. I have tried; swapping modules - the same problem occurs with a new module, ferrite cores placed around the T/C close to the input card, Checking that shielding is only connected at the PLC shield grounding terminal, moving the location of the thermocouples (they are in close proximity to glo-bars). This system has worked fine in the past but intermittently presents this problem which seems to go away on its own. I suspect interference of some type causing problems in the MUX internal to the analog input card but cannot measure it. I have considered running the shielded T/C's in a conduit but this should be unnecessary with heavy foil shielding. Does anybody have an alternate solution to minimize this noise?





RE: Interference on T/C inputs to PLC
GE Fanuc suggests grounding the shield of analog input cables in the field, not in the panel. It's in one of their PLC I/O manuals. I've never done it myself, but you might want to lift the ground in the panel and try grounding the device end. If you try it, let me know if it works or not.
Have you checked the integrity of the cable on that particular thermocouple? Sometimes an intermittent loose connection can cause a floating input, which can produce erroneous readings. When your furnace temperature gets hot, you may have a loose connection due to expansion of materials located nearby.
Are you getting noise brought in on a cold junction compensation line?
RE: Interference on T/C inputs to PLC
RE: Interference on T/C inputs to PLC
Hope it helps you out.
bye
RE: Interference on T/C inputs to PLC
RE: Interference on T/C inputs to PLC
RE: Interference on T/C inputs to PLC
RE: Interference on T/C inputs to PLC
RE: Interference on T/C inputs to PLC
Regards.
RE: Interference on T/C inputs to PLC
Goodluck