3 point temp control

remullis

Member
Join Date
Oct 2012
Location
Georgia
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179
We are currently running 3 point temp control of a kiln and recently had a thermocouple failure which caused issues with our heat up cycle. In this instance operations says the thermocouple failed low which is normally not the case; normally its high. To eliminate both scenarios I would like to take the middle number of the 3 temperatures and always revert to that as my set point for control. If I fail low or high on a TC it would select the middle temp range to control a gas valve. I would like to do this in controllogix 5000.
 
We are currently running 3 point temp control of a kiln and recently had a thermocouple failure which caused issues with our heat up cycle. In this instance operations says the thermocouple failed low which is normally not the case; normally its high. To eliminate both scenarios I would like to take the middle number of the 3 temperatures and always revert to that as my set point for control. If I fail low or high on a TC it would select the middle temp range to control a gas valve. I would like to do this in controllogix 5000.

So you are monitoring the temp of your oven with 3 different TCs? And you would like to use whichever temperature is in the middle as your set point? Why not just have an absolute value instead of using the TC as your control? I wouldn't want my control to fluctuate based off the reading on a TC, I would have a static control so my temp. remains the same, right?
 
So you are monitoring the temp of your oven with 3 different TCs? And you would like to use whichever temperature is in the middle as your set point? Why not just have an absolute value instead of using the TC as your control? I wouldn't want my control to fluctuate based off the reading on a TC, I would have a static control so my temp. remains the same, right?


I assume he means that he's using it for feedback.
 
Since there are only three values, the "brute force" approach to finding the middle one is not too bad (example attached). Something like this would be necessary if the platform does not include a sort instruction.

midof3.jpg
 
Guys,

sorry for the late response to my own question. I was able to simulate this with similar code as Mispeld is showing in the example. I am not familiar with the SRT instruction but I will also simulate this method. I appreciate the input.
 
The P_AInMulti Pax object (Add-on Instruction) should handle everything you are looking for
 
If the reading of the thermocouple goes to the maximum value when it fails it is due the connected device is designed in this way to protect the kiln against overtemperatures, this protection is usually always present in isolated temperature controllers, but not in PLC analog inputs or remote acquisition modules ... although it would be highly recommended that they have it.

I have gone through the same dilemma, I had installed in the past some kiln with up to 14 thermocouples and 14 regulation zones.

I used to do the average of all the temperatures and if any of the measurements came out in more than a margin of the average then it generated a thermocouple failure alarm and to regulate that zone then I used the temperature of the nearest thermocouple instead.

.. but I do not know how to do that with AB PLC.

I also to ensure that the measurement of the open/fail thermocouple went to zero and did not stay in a floating value then I connected a 4000 ohm resistance in parallel in the terminals of the analog input on each of the thermocouples.
 
I used to do the average of all the temperatures and if any of the measurements came out in more than a margin of the average then it generated a thermocouple failure alarm and to regulate that zone then I used the temperature of the nearest thermocouple instead.
This is how I'd do it. Haven't programmed any kiln but bed temperatures of BFB/CFB boilers. Fault the measurements that deviates too much from average and exclude faulted measurements from average calculation.
 

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