PID loop runaway

We have a machine that has two separate ovens on it. Both ovens have a separate thermocouple that goes back to a thermocouple card in a GE 90-30 rack. Within the GE program, thee is a single PID loop that controls the burner motor within the oven. Machine/program has been in place for 5-6 years running 24/5 so there is no issues with the programming portion up until the other day. Looking at the data trend for the oven temps ( PV,SP and CV)on the HMI it seems that at one point the system was running fine and then all of a sudden the PV (thermocouple)value locks up at one value on both ovens and stays there. The one oven it locked up below setpoint so over the next half hour you can see the the CV steadily increased to 100% ( it was gradual increase over that amount of time)until the oven was damaged and someone shut it down. AT the same time, the other oven PV value locked up at a value just below setpoint so luckily this one the CV decreased steadily. Any thoughts as to why this would happen. Could it just be a program glitch or could the card be going. Unfortunately we weren't able to monitor on-line while it was happening. Thanks

I have no experience with GE, but I would suggest that you add logic to detect a frozen value and automatically shut down the ovens. At least temporarily to prevent further damage until the root cause can be identified.

I would expect that if the thermocouple raw value did not change at all for a specific number of seconds, that would be a sure sign that the value from the analog input is no longer valid.

You did not mention if/when the thermocouple values went back to normal funcitoning, so I am assuming that there was nothing wrong with them...But...

You might also want to check how the analog module might be configured to deal with a loss of signal on its inputs. Perhaps it's configured to hold last state or something to that effect.

Paul

The PV variables were basically locked in until the operators finally shutdown the ovens. We will fire them in a few days so we will check everything out. We do have high temp limit controllers that we are readjusting to a lower temp that will kick the burners out if it gets to hot ( they have a separate thermocouple. I would assume if we fire up and the thermocouples test good that there may be an issue with the card itself disrupting something?

jcraft said:

AT the same time, the other oven PV value locked up at a value just below setpoint so luckily this one the CV decreased steadily.

Click to expand...

It's probably a typo; I suspect you meant to say that the 2nd PV value locked up at a value just above SP so the CV decreased steadily.

Given that the PI or PID reacted properly to the error between the locked PV and the SP with the reset action gradually attempting to correct the error, the suspected problem lies in the thermocouple input card.

I can't think of how a thermocouple itself could fail to provide a fixed value, it sounds more like a card problem.

You didn't mention what range of temperature these ovens work at, but thermocouples are notorius for drifting over time (heat alters the chemistry of the junction; the higher the temperature, the quicker the drift or failure).

But even drift doesn't 'freeze' at one value.

Good idea to adjust the highlimits.

Dan

I looked at this more in depth as the machine is shutdown currently for the repairs. Looking at the AI registers for all the thermocouples for the card they were all frozen at a certain value. I unplugged the plc and let it boot back up and the AI read what they wer supposed to. Definately a strange one.