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Randy

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I have noticed alot of people asking about PID temp control.

I thaught I would write a little to let you all know what it is.

PID (Pulsed Incrument Diferential) control is for controling an analog output.
Only used if you have a proportional heat source.
Is is set by a time stamp for example say 5 seconds. Every 5 seconds a scan of the imput is taken ie mash tunn temp. If the mash temp is to high or to low it adjusts the output acordingly.

Example If usin a 4-20ma signal to control a steam valve or gas burner damper. 4 would be low heat or off and 20ma would be flat out.

If the temp is say 2 degrees below the valve would open say 0.04ma
If the temp is say 10 degrees below the valve would open 5 times the amount so 0.2ma

if the temp was say 10 degrees above it would close by 0.2ma

This is designed that eventualy the valve would eventualy be positioned so as to hold temp without having to move. Thus giving a much greater control.
 

JasonY

The Imperial Metric Brewery
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I work in process control, perhaps you have different terms for it all. Had a few too many today so here is my spool ... :p Waning rambling techy post below.

PID stands for Proportional (K), Integral (I) and Derivative (D) control. These refer to the three control actions within the algorithm (PID) inside your controller. These are a class of control algorithms which are used in about 90% of real world process control applications.

With all controllers you have a Set Point (SP), Present Value (PV) and an Output (OP). Essentially you plug this sucker into your HLT say and your PV is the actual temp, the SP is what you really want (say 76deg) and the output is what your burner is doing as Randy said 4mA (0%) is say min heat and 20mA (100%) is flat stick. The aim of the game is to move the output (your burner) to maintain a steady temperature.

Generally when tuning these controllers you enter tuning co-efficients for the three modes (K, I , D) .

The proportional action gives you an immediate kick in the output when the PV changes, so if your temp suddenly went up by 5% and K = 2 then you would get a 10% change in your output (1.6mA). So if you chucked 5L of cold water in the kettle the temp would suddenly drop -- well this baby will give the flame a kick in the ass.

Integral action continually works to bring the PV back to SP and (I) is specified in 'repeats per minute' or some such unit. A small value means it will aggresively wind the output up or down to try and bring the PV back (and overshoot if you go to hard). So after that 5L went in the temp is still low, this will progresively increase the heat.

Derivative well, this is best used to counter processes with a delay in the response when you change the output (dead time). Not really required for most brewing processes but you can use it if you feel the need. Beware. As soon as the controller sees the temperature changing it will kick the burner, how hard depends on how fast the temp is ging up of down -- if you throw a heap of cold water in then this will likely max out the burner immediately.

Working out these values is important to getting good control performance.

Of course there is the issue of sample speed of the controller, differing algorithms which change the interaction between the modes but that is another story and my glass has now emptied. :eek: sorry for the rant ...
 

GMK

BrewInn Barossa:~ Home to GMKenterprises ~
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Ben said:
my head hurts :)

i think i will just stick to drinking beer
Great Idea - think i will Join you in one... :chug:
 

Justin

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Yep, this last hour and a half has fried my brain more severely than I can recall occuring in the last few weeks. I've crammed it with info on PIDS and PICAXEs and now it hurts. Home James, and don't spare the horse!

APA. Yum. Also got to keg the oatmeal stout :D

Cheers.
 

sintax69

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sample speed of the controller is not really an issue with HB for chirst sake its just a bucket of water the termal mass of the bucket is going to dampin any need for fast sampling rates a reaction time of out to 5 sec would be fine I have seen problems with fast controllers stuffing valves up by reacting to quickly and constantly also watch out for a lot of PID on the market dont have Derivative (cheaper ones)

Remember keep it simple


Sintax
 

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