PIDE for unstable heater

yulia

Member
Join Date
Jun 2017
Location
DE
Posts
3
Hello all!
I am trying to figure out how to tune PIDE for an unstable heater.
This is a startup tuning. My project was to remove old temp controllers and use something robust and cheaper to substitute temp controller.

So i am using 1734-IT2i thermocouple input module. I am also using PIDE function with SRTP to drive SSR.

I have 8 heaters in the machine, and i have managed to tune other 7, but this one is like impossible to tune. So this heater has design issues: it has small mass, and there is a thermocouple sensor in the same heater element. Also there is a vacuum turns on and blowing and heater while machine is running.

I tried to use Autotune on cold startup and while running the machine, and that number kind of worked, with +/- 3 degrees oscillation while running, but when i just turn the heater on, without vacuum and machine is not running, the oscillation is +/- 20 degrees. (P= 3.576, i=0.601, d=0 <- forgot what d was)

I also tried manually tune and seemed like i got +/- 3 degrees in oscillation on startup while machine is not running, and when i start the machine the oscillation is +/- 20 degrees.(P=60, i=30, d=0)

Do you think it is possible to stabilize it? If yes, what values or what do I need to change?

The previous temp controller was able to control heater with +/- 5 degrees oscillation. Is it a good thought to try to plug previous temp controller PID values? But temp controller methods are different...

Maybe I can disable it when it is trying to overheat?
Burnt 2 heaters already...

I would really appreciate your help. Really struggling here.
Thanks!

o_Oo_Oo_Oo_Oo_Oo_O
 
Are you controlling the element temp, vac temp or blower air temp?
Normally this would be a cascade loop. Adjust element temp to get air temp.

What technique did you use to tune manually?

Do you know fan speed or vac speed? If so, bit of feed forward in there
 
I am controlling element temp. So you are saying cascading PIDE would help?

Also I noticed when I compare thermocouple input temperature I see it is a few degrees faster/higher than the input temp i see in PIDE funciton block...

For manual PID tuning I used some book but I don't have it on me now. I am going to try use this info here: https://www.eurotherm.com/pid-control-made-easy.
Don't know vac speed =( But maybe I can find it out. What is your technique of using vac speed?
 
I don't know how this would normally be handled, but my first thought would be to pipe a "vacuum ON" input back to the PLC (if not already there) and use a 2nd set of PID parameters for when the vacuum is running.

If you like that idea, then think hard about what you're going to say when you're asked "why does your new controller need this vacuum signal where the old one didn't? " It could be a hard question to answer, and my guess is that the old system was phase angle fired power control? (This helps with low thermal mass).

IMO it is not unreasonable for the system to demand inputs relating environmental variables. In the best and smoothest-running winders I have seen, they employ a sonic range sensor to feed drum diameter into an equation which modifies the P of the PID, to get a smooth operation regardless of full/empty spool or big/small spool.
 
UPD: I could finally find good PID values using this manual: https://www.watlow.com/-/media/documents/white-papers/tuning-pid-loops.ashx
but this manual was saying start with P value =60, and raise it every time the temperature is not stable, however my stable P value (when running) was 9.5. Duh.

So the heater with small mass when machine is running had +/- 1-2 degrees oscillation, and when machine was not running +/- 5 degrees. And I also have a transition swing like +/- 9 degrees when it starts and stops. But I added logic which shuts off output when the t is getting higher than setpoint so it won't overheat.

Thanks everyone for your inputs!
 
Is it a good thought to try to plug previous temp controller PID values?

To do this you will need to know what the PID equation is running in the previous controller, as well as what the units used are. Then its just a little math to get them into the right values for the PIDE instruction (though we would need to know which mode this is running in)
 

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