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from the Stupid glow bar, Knows only on-off state... department...
PID Control for Heater (glow bar)
Continuous process industries, DCS questions. topic
Posted by Isicom on 11 November, 2006 - 12:08 am
Hello Guys,

I'm working in a project where I have to program a controller for a heater (glow bar, heating rod). Of course I know the theory of PID. But I don't know how I can apply it for this project. For example: the heater "knows" only ON-OFF. The Question is, what's bout the PID-Set Value. Do I use it in a PWM-Simulation (e.g. PID-Value calculate me 8 --> PWM 8 => 20ms ON, 80ms OFF. PID-Value 2 --> PWM 2=> 80ms ON, 20ms OFF) or do I have it to use for new Temp-Calculation. For example PID returns me 30. My Actual Temp is 70C so new temp is 70C-30C=40C... and then if-then->
If (newtemp>settemp+5) Heater_OFF();
If (newtemp<settemp-5) Heater_ON();

What to know is, the Heater is very powerful. I made a Test: I switch the Heater on and let it on until 150C and then I shut it off. It increased in 33 sec from 24C until 150C (increase rate of 4C/s). And it "burned after" until 182C in 12 sec. In Cooling-Mode it have a cooling rate of -0,16C/s.

What are you thinking? Please help.


Posted by Bob Peterson on 12 November, 2006 - 9:47 am
Electric heating applications often use PID loops that vary the duty cycle of the heater to adjust the amount of heat introduced.

Pick a usable value for your time interval (say one second) and the output of your loop is going to determine what part of that interval the heater is energized.

For example: if the output of your loop requires 25%, the heater is energized for 250 milliseconds out of each 1 second interval.


Posted by Mitch Carr on 14 November, 2006 - 12:03 am
Yes, this is true but you first need to figure out what the characterisitcs of the load happen to be. If the glo-bars are simple linear resisitive loads then ignore what I am saying but if they have non-linear tempco's then you have to adjust for that with your output/power stage. Just like you wouldn't use type J linearization for a type K t/c, you can't use a linear output algorithm for a non-linear load.

There are a lot of questions that need to be answered first. Are the glo bars NiChrome, silicon carbide, moly-discilicide? Are they transformer coupled? It is common to use Scott T transformers to control glo bars in conveyor furnaces. You have no hope of controlling anything but NiChrome with a time proportioned output. OK, 99% or all electricaly heated loads are NiChrome but the gentleman said glo-bars which raises a lot of red flags.

Cheers, Mitch


Posted by Mitch Carr on 12 November, 2006 - 10:08 am
It has been a while but from what I recall glow bars are moly-discilicide and need to be phase angle controlled with true VxI power control. Once you get that sorted out, any good self-tuning PID controller should be able to sort things out on its own.

My suggestion is to call customer support at Eurotherm in Leesburg, Va. Your time is worth money and you will spend less byt installing the right equipment up front.

Mitch


Posted by Sidik on 13 November, 2006 - 11:22 pm
It is more simple using on-off state to cotrol the heat. When the PV less then SP then set the heater on until it reach SP. You can set the acceptable range for PV so the heater not toggle on of when it reach the SP. Cooling rate is slow enough, 1Deg C in 6 second so the heater will not on-off so often. Maybe the heater can set on-off fastly (less then 1 sec) but i think it's much better if we can set if more slowly.

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