Author Topic: PID controller settings?  (Read 2837 times)

Offline stratocasterdave

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Re: PID controller settings?
« Reply #15 on: February 08, 2013, 08:29:16 AM »
Sorry ya had negative results on the auto tune.  Your set up is really coming together though.  Keep us updated.

Thinking outside the box, I wonder if you couldn't put some sort of submersible pump or something in the water tank.  Circulating that water might increase efficiency?  Just an idea.



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Offline Wayne Harris

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Re: PID controller settings?
« Reply #16 on: February 08, 2013, 08:34:34 AM »
The circulation pump is a solid idea. 
But if you look close on the left side of the tank, you will see a  black cord going into the tank, at the end is a small little circulation pump.
Having tested both with and without that little pump, it makes a huge difference.  It is amazing how still water can contain very seperate thermal zones.
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Offline stratocasterdave

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Re: PID controller settings?
« Reply #17 on: February 08, 2013, 08:39:14 AM »
Awesome!  You can use almost you it as sous vide system!   ;D

Offline Mike Richards

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Re: PID controller settings?
« Reply #18 on: February 08, 2013, 09:34:06 AM »
What values did you get for P, I, and D?  I have a controller, but when I let it run its auto-tune for a while it kept resetting.  I stopped mid-tune once and saw that it had an enourmous I and D values.  I'm really convinced that for the kinds of setup people use here, the values really should be a high P (which, depending on your controller might be a small number) and medium I and practically no D.  I'm putting a new vat together in the next couple of weeks and will run auto-tune on mine again to see what happens.

Good luck!
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Offline Wayne Harris

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Re: PID controller settings?
« Reply #19 on: February 08, 2013, 11:39:30 AM »
Good to know.  I would have to go back and check what my actual PID values are. 
Right now, I am considering a lot of tweaking to the whole setup.  Mainly centered around where the dang thermostat should go for the PID.  The water bath? or the cheese milk?

To this point, the thermostat has been in the water bath.  And while that works great if what i want is the water bath temp to get to particular temp, however, i really only care about the milk. So, I am going to play a bit.
In addition, I will revisit my PID values tonight. I really don't remember what they are. I do remember moving my overshoot coeffeicent from .2 to .5, but thats really the only change i have made..


And for those that think this is overkill, i would agree. But for me, this kind of tinkering is half the fun.
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Offline Mike Richards

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Re: PID controller settings?
« Reply #20 on: February 08, 2013, 12:22:32 PM »
If it's legal to do, I'd want the thermocouple/thermistor/temp probe in the milk itself.  As you said, it's great that the water is the right temperature, but that doesn't mean the milk is.  If you are worried about putting the detector in the milk, I think the closest you could get would be to get a surface mounted temperature probe and put it on the water side of the milk container.  The stainless, with the temp probe mounted on the water side, should be pretty close to the temperature of the milk there at the edge.

If this is overkill, then most techincal profession are overkill...  :P
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Offline stratocasterdave

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Re: PID controller settings?
« Reply #21 on: February 08, 2013, 02:24:52 PM »
Wayne, I used my PID for two different setups.  One was a infusion mash system (a micro-brewing system).  It is now used to control an oven.  For the homebrew system my values were P=77, I=660, D=155.  On the oven setup I was P=47, I=825, D=206.

Each of the values were derived with the autotuning.  I  remember the oven system took at lease 4 hours to tune.  You might want to check with someone with more experience in value setting.

Dave

Offline Mike Richards

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Re: PID controller settings?
« Reply #22 on: February 08, 2013, 02:59:35 PM »
Something to consider when looking at other people's values is that the way the numbers are used might not be the same across systems.  I don't remember exactly what it was, but when I looked at and started messing with my values, I was surprised to find that one of my numbers was the reciprocal of what made sense (it was 1/(the number I thought it should have been) ).
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Offline stratocasterdave

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Re: PID controller settings?
« Reply #23 on: February 08, 2013, 03:24:23 PM »
This might help.  On my controller:

P = Controlls the output of the controller in degree units.  So, if you have P at say 100, the proportional band is 10 degrees.  In other words, when the sensor is 10 degrees below the proportional band, the controller will output.  When the temp is 5 degrees below set point, the output is 50%  When the temp is equal to set, there is 0 output.  And so on.

I = Time.  For me this is in seconds.  This controlls the output of your controller based on the difference the measured and set temp integrated with time.  A larger number would mean slower action.  For you, if temp fluctuates often, increase your I number.

D = Derivative time.  Contributes the output power based on the rate of temp change.  For you you can adjust your number to adjust for temperature overshoot.  Controllers out put based on the rate of change rather than the net amount of change.

Maybe this is stuff you already knew though.  So, you can try to adjust your values on your own.  Autotune would be the easier in my opinion.

Offline kdttocs

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Re: PID controller settings?
« Reply #24 on: March 08, 2013, 08:54:23 PM »
PID values are completely unique to the system being used, even the temps being used at (direct/indirect heat, doubleboil, recirc.. A duel setup for example that does cheesemaking and Sous Vide will require different PID values. It's all a combination of volume/heating capacity, power of heating system, time, etc (that an unscientific list :) ).

Autotune MUST be run to completion. It's the best way to get as close as possible to the correct settings. In my setup I found that once autotune runs, it helped to de-tune it as my temps kept creeping up.

Also, there's no better way to maintain temps in large vats at home than direct heating on a recirculating waterbath setup. I'm working on a vat that has recirculating waterbath through a waterheater element in a pipe where output temps are monitored by the PID right after the heating element.


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Offline Mike Richards

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Re: PID controller settings?
« Reply #25 on: March 08, 2013, 09:13:15 PM »
Also, there's no better way to maintain temps in large vats at home than direct heating on a recirculating waterbath setup.

That's a lot of confidence. ;)
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Offline kdttocs

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Re: PID controller settings?
« Reply #26 on: April 03, 2013, 06:03:11 PM »
Thought I would update this thread with some more info I've learned recently. PID controllers can run in PID, PI, and PD configurations simply by setting the value to 0. PD is very uncommon though. When tuning your PID I've found it's best to work only with PI and set D to 0. D essentially adds a predictive element and can only be used when using an RTD probe. If you're using a thermocouple, the erratic temp reads causes any value you set to D to get confused.

With autotune settings I had difficulty tuning my PID when used on a turkey roaster with a small water pump. I had problems with temp creep and sever overshooting. This last week I threw out any autotune settings set and started with some simple values:

P: 2% (.02)
I: 2000
D: 0

What resulted was a very quick and short response to small deviations to the target temp. This worked when trying to hold a water bath at 90F and at 134.6F. I made a 60 hour sous vide short ribs and then following day made a caerphilly. I only modified the PID values slightly to slow down the reaction to temp changes. It should be noted that I use a double-boil in the turkey roaster rather than direct heating to the milk.

Here are my current values. These allowed me to keep agitated water within +- 0.1F of 134.6F. The lack of a significant D value does result in an overshoot by a few degrees when heating up. This can easily be overcome either by prewarming and letting it stabilize or by starting with cold milk as the milk will lower the overshot temp before the milk has a chance to overheat.

P: 5% (.05)
I: 5000
D 5

I threw in 5 for D to see what it would do. I'm not sure it did much as it may be way too small to notice. I've kept it nonetheless.

Here's my setup... yes water and electricity don't mix. This was when I first tested before I dremeled the case to hold everything.