Struggling with PID tuning


 
M

Marlin Schrock

New member
Trying to dial in the PID settings on a uds. I have a heatermeter (4.0 hardware version), Pit probe just under meat rack. Roto-Damper with digikey standard wiki recommended blower fan. I also have a brick divider in my charcoal basket to give the fire "structure".

I started off this morning with P only of 8: (disregard it showing a I of 1.8% and the P% as that wasn't at the time of the chart below it.)


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I thought that looked stable.

Looking at the wiki, I read:

P Here's the easiest place to start. Think about what the acceptable temperature range is for operation. At what point of the pit temperature dropping do you want BWOOP BWOOP giver 'er all she's got, Captain! Divide 100 by that number. That's going to set the lower limit on the P value. For a setpoint 225F with a lower temperature limit of 215, P = 100/(225-215) = 10. That's your ballpark. The faster your grill can change temperatures by changing the output value, the smaller this number should be. In my graph the P was 3 because the blower can really turn things around quickly in there. With just the servo, if I opened it fully it would still take a long time before anything actually happened. Fast response, low P. Slow response, high P.
Click to expand...

I feel dense here. So i'll ask: Am I to reduce the P setting by half? Or does the pic above look good, and I should leave P at 8 and proceed to working on the I?
 
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Nah you're way beyond that. What you've found there is you "Ku" ultimate gain. You've got relatively steady oscillations with a relatively fixed period of what about 10 minutes (600 seconds)?
P = Ku / 1.7 = 8 / 1.7 = 4.7
I = Ku / (T / 2) = 8 / (600 / 2) = 0.026
D = Ku * (T / 8) = 8 * (600 / 8) = 600

I've always thought the D value computed way off with these tunings. I'd set it for somewhere between 5 and 15 and then watch and see if the temperature keeps changing direction before it crosses the setpoint. If it does, it is too high. If it doesn't and goes way low before the blower is kicking in, then turn it up some.
 
Yeah it doesn't matter where you measure it as long as it is one complete cycle. Two setpoint crossings, two highs, two lows, etc.
 
Bryan Mayland said:
Yeah it doesn't matter where you measure it as long as it is one complete cycle. Two setpoint crossings, two highs, two lows, etc.
Click to expand...

But Peak to Peak is generally a measure of magnitude, and measuring from the negative peak to the positive peak is not a full cycle, and also measuring on the wrong scale.
 
Right, technically speaking the correct term is not measuring "peak to peak", which is an amplitude. Measuring the time from the a relative maximum extrema, a "peak", to the next relative maximum extrema "peak" is acceptable slang in my book. It is open to misinterpretation as you say if someone measures time from a top to a bottom (which is a half cycle).
 
I've came to the conclusion that I have better success with no fan and only servo damper, "naturally aspirated". The key reason is, in my opinion, the uds has fire and food in a single chamber with air intake dumping into the common chamber. So, I made a damper that attaches via 1" pipe to the smoker.

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not perfect, but better than any of my attempts with fan. B=0 P=18 I=0.06 D=20

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I was having issues to with the blower and servo. But I also at the same time, that I got the servo, I went from .5 inch inlet pipes + ball valve to 3/4 inch without a ball valve. I was getting similar results. So. I got another ball valve and I barely have it open(same as I had before when I just had a blower) and it works great with the servo and blower. I do have a huge blower though and I never run it higher then 30% at max.

If you try a ball valve

To get it set correctly start your UDS the same as always without using the blower or Servo attached and let it settle to 220 by closing the ball valve until its stable. I choose 220 as I usually don't smoke lower then that. When the UDS is stable close the ball valve a tad more until you see the temp start to lower, put a mark at that position on the valve. That's the spot you leave the valve at when you use the Blower and servo. I also use a larger blower(28CFM) as the stock one was not effecting the temp fast enough, so I went with a somewhat larger then what I really need, but have been using it for 4 years now. To set the blower max speed you just start it at 100% and you keep lowering the max speed until you see the temperature start to increase, you may need to lower it even more though. That's how I got the 30% max, when I run with the blower at 40% or higher I get a cooling effect on the temperature and then I get a huge spike in temp when to many coals lite and its a pain after that to control.
 
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