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Old 01-16-2013, 01:20 PM   #1
Robhansen
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Default Does turning off the contactor cause the PID to adjust itself?

Hi, sorry if this has been covered, I couldn't find it. I brewed with my new setup last weekend. It's a 3 vessel system with one 5500w element and one pump for now. I didn't put in a contactor in the element circuit, but am going to before I brew again. My question is if I just turn off the contactor and leave the PID temperature setting the same, will the PID re-adjust because the temp is not responding?
As I used the water from the HLT, I lowered the temp all the way down thinking that the element wouldn't fire. It continued to come on a few times until I switched to manual control at 0%. Is that normal?
Also will this contactor work? http://www.ebay.com/itm/DEFINITE-PUR...item43b698ca7f
Thanks

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Old 01-16-2013, 01:36 PM   #2
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If the process value from your sensor is below your set point on the PID, then it will close your output relay (allowing voltage to pass thru it). It does not sense whether or not the element is actually turning on. Its only input is the temperature sensor. It will continue to monitor the sensor, and once the process value is close to, or exceeds the set point, then it will open the output relay (turning off your element). Hopefully that answers your first question.

If I understand your 2nd question, your saying that you reduced the set point on the PID below the temperature that is currently in the vessel, and the element was still running?

This is possible, and the simple answer would be have you performed the "auto-tune" function on your PID?

If not, do so, it will re-adjust the factory defaults for the P, I and D gains in the controller to what it thinks works best with your enviroment. Keep in mind a PID controller uses an algorithm to determine when to turn on an output. It looks at the "current" error (Proportional error), sum of error over time (Integral), and the slope of error over time (Derivative).
When you run the auto-tune, it will calibrate those terms to provide you with a consistent ramp up/small overshoot/little steady state error.

What you may of seen when you made a sudden change (lowering the set point), the PID took some time to adjust, and waited for the error to stabilize

Hopefully this helps.

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Old 01-16-2013, 02:13 PM   #3
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Yes, I ran the auto-tune. I was concerned that if I just turned off the contactor, and did not run the temperature down, the PID would continue to "learn" even though the system was not heating.

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Old 01-16-2013, 02:30 PM   #4
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I definitely could see that the output was on, but whenever I turned contactor back on I didn't have rampant overshot or anything. I don't think I really had the PID temp up though and the element off much? Maybe just for a few seconds during like "oh crap forgot to turn a valve"

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Old 01-16-2013, 03:20 PM   #5
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It's a valid question... I don't know how quickly the PID learns when it's not in Auto-Tune mode... but if the PID is telling the element to fire, and the temperature is not going up (because you have the contactor turned off), the PID is going to start to learn that your setup has some serious issues getting up to temperature. So say the PID starts thinking that your system is just incredibly laggy and it adjusts its curves accordingly. Then you flip on the contactor, and all of a sudden the element actually fires and the temperature actually goes up. But the PID has 'learned' that it won't heat up correctly - will you see rampant overshoot until the PID re-learns that it actually can heat water?

A simple way to solve this may be to set your PID to manual mode when you have the contactor off for any period of time.

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Old 01-16-2013, 03:31 PM   #6
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I'm not a controls expert and I haven't had much PID use experience either (basically disregard everything I say until someone confirms). The only time the P/I/D values get modified is during an auto tune function or manually changing them in the programming mode. If your element has been manually disabled, the PID will follow its program to try to reach setpoint but will not modify hard coded P/I/D values to do so. Once you allow power to return, it should be business as usual.

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Old 01-16-2013, 03:48 PM   #7
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Quote:
Originally Posted by Bobby_M View Post
I'm not a controls expert and I haven't had much PID use experience either (basically disregard everything I say until someone confirms). The only time the P/I/D values get modified is during an auto tune function or manually changing them in the programming mode. If your element has been manually disabled, the PID will follow its program to try to reach setpoint but will not modify hard coded P/I/D values to do so. Once you allow power to return, it should be business as usual.
You are exactly correct.

The P, I & D terms are only changed by running the auto tune function, or by manually changing them. The PID doesn't change the way it reacts to a system by itself.

This is also the reason why you should re-run the auto tune mode anytime there is a change to the system (relocating the temperature sensor, changing a pump, changing vessel size or any orientations, etc..)

You can think of the controller as a mini computer, running a program (basically a 3 sets of equations which contribute to the overall stability of the system).

And I bet you all never thought that calculus you learned way back would contribute to making beer, but it does with a PID controller!
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Old 01-16-2013, 08:26 PM   #8
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So Kal's comment is what set me to my thinking... http://www.theelectricbrewery.com/co...part-2?page=11

He states the PID is in a learn mode for some period of time when it's brand new, which makes sense. If that's the case and you keep you contractor off for part of that initial learning cycle, it could learn bad values.

Or, are we saying that the PID never does any learning on its own without being in Auto Tune mode?

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Old 01-16-2013, 09:01 PM   #9
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Quote:
Originally Posted by BadNewsBrewery View Post
So Kal's comment is what set me to my thinking... http://www.theelectricbrewery.com/co...part-2?page=11

He states the PID is in a learn mode for some period of time when it's brand new, which makes sense. If that's the case and you keep you contractor off for part of that initial learning cycle, it could learn bad values.

Or, are we saying that the PID never does any learning on its own without being in Auto Tune mode?
I'm not sure where Kal got that information. I've never seen my PID change values from those it derived during the auto tune process. There isn't anything in the Auber PID SYL-2352 information stating it will learn or adjust outside of the auto tune operation.
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Old 01-16-2013, 09:12 PM   #10
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I shot off a question on this directly to Auber. I'll report back when I get an answer.

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