FORUM ON BEER, HOMEBREWING, AND RELATED ISSUES
Digest Janitor: pbabcock at hbd.org
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Contents:
Re: Dumb HERMS Question ("Craig S. Cottingham")
Re: Dumb HERMS Question ("Jeff Dieterle")
RE: pumpkin ale ("Josh Knarr")
PID Controller ("A.J deLange")
HERMS: Liquid Return ("Dave Larsen")
PID - Heat Output ("A.J deLange")
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Date: Tue, 12 Aug 2008 22:53:22 -0500
From: "Craig S. Cottingham" <craig.cottingham at gmail.com>
Subject: Re: Dumb HERMS Question
On Aug 12, 2008, at 12:26, "Dave Larsen" <hunahpu at gmail.com> wrote:
> What exactly does a PID do? I know it has to do with temperature
> control, and has relays to turn things on and off. I assume that that
> is to turn on and off the heat. But, doesn't say a Ranco temperature
> controller do the same thing? Why would I need a PID?
The short answer: A PID is, in this context, a fancy temperature
controller. I don't know if the Ranco controller is a PID; assuming
it isn't, then a PID does the exact same thing the Ranco does, only
better.
The long answer: "PID" stands for proportional/integral/derivative,
which are the three inputs to the transfer function that turns the
input (the signal from the temperature probe) into the output (the
power to the heating coil).
The proportional stage scales the output proportional to the
difference between the measured value and the desired value. If the
HLT is cold, and it's supposed to be hot, then the output will be
large. If the temperature difference is small, then the output will
also be small. Note that the old bimetallic-coil-with-a-mercury-bulb-
switch thermostat most of us grew up with in our houses was a
degenerate case of a proportional controller; there were only two
output values -- on or off.
The integral stage sums the difference between the measured value and
the desired value over time, and produces an output proportional to
the accumulated error. The longer the measured value is not equal to
the desired value, the larger the output gets. This is important
because proportional-only controllers suffer from a condition called
offset; they eventually reach a point where the output is small
enough that you're putting in the same amount of heat as is being
lost through the walls of the HLT, the tubing, etc., and you *never*
reach the desired temperature. Add an integral stage, and you can
overcome the offset. (Integral controllers have their own problem,
called windup. Imagine that you're heating water at ambient ground
temperature, say 55 degF, to 155 degF. The difference between the
measured value and the desired value is going to be big -- on the
order of 50 to 100 degF -- for a long time, and the integral
controller will build up a very large error value. This can lead to
overshoot -- pouring enough heat into the HLT to bring the water to a
boil.)
The derivative stage tries to take into account how rapidly the
measured value is changing, and damp out wild swings in the output
value. Unfortunately, they tend to be sensitive to noise in the
measured value, and try to damp out changes that aren't really
happening.
There's a nice article at <http://en.wikipedia.org/wiki/
PID_controller>. It doesn't seem too technical to me, but then again
I understand this stuff somewhat already, so I may not be an good judge.
The bottom line is, you don't *need* a PID. However, if you're the
kind of person who is obsessed with precision and accuracy, you're
probably going to want one.
- --
Craig S. Cottingham
BJCP Certified judge from Olathe, KS ([621, 251.1deg] Apparent
Rennerian)
craig.cottingham at gmail.com
------------------------------
Date: Wed, 13 Aug 2008 08:27:52 -0500
From: "Jeff Dieterle" <djdieterle at hughes.net>
Subject: Re: Dumb HERMS Question
Date: Tue, 12 Aug 2008 12:26:46 -0700
From: "Dave Larsen" <hunahpu at gmail.com>
What exactly does a PID do? I know it has to do with temperature control,
and has relays to turn things on and off. I assume that that is to turn on
and off the heat. But, doesn't say a Ranco temperature controller do the
same thing? Why would I need a PID?
Dave
Tucson, AZ
Dave,
PID is the control algorithm used to match a match an output to a process
variable, Proportional, Integral and Derivative are the 3 types of gains
used. There are several control schemes this is simply one of them. For a
pid to function the output must be variable, not to practical in most
brewing rigs. However it is possible with pwm control of an electric element
or a modulating burner. Consequently most pids used in brew rigs are doing
an on/off type of control which is exactly what a Ranco does. So unless
you're going to get fancy with buner or element control or get a pid style
controller at a good price a Ranco will do the same thing at a lower cost.
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Date: Wed, 13 Aug 2008 10:22:26 -0400
From: "Josh Knarr" <josh.knarr at gmail.com>
Subject: RE: pumpkin ale
RE: Roasting Pumpking - Yes, I was planning on simply spreading it out
on a cookie sheet. Then I don't have to deal with the skin of it
contributing off flavors, etc. It seemed correct to me to bake it. One
of the janitors also pointed me to the archives which suggested
mashing it. You can also check the google archives but I figured I
would start a fresh topic.
http://www.google.com/search?q=pumpkin&domains=hbd.org&sitesearch=hbd.org
But basically I was thinking that so long as the pumpkin was spread to
pie-like thickness, it would do well for itself in the oven. If the
edges got burnt, more power to it since we're going for roasty.
RE: Raisins and Vanilla - I could see getting rid of vanilla, the
extract stuff always tastes a bit funky to me. Maybe it's just ACME's
fault. Raisins I think will add "dark sugary" flavors, probably on par
with molasses or maple syrup. The idea here is to hopefully get some
unfermentable flavor out of them.
------------------------------
Date: Wed, 13 Aug 2008 11:54:55 -0400
From: "A.J deLange" <ajdel at cox.net>
Subject: PID Controller
"PID" stands for "Proportional, Integral, Differential (or derivative)"
and means that the device sends a demand for heat (in a heating
application) which is in part proportional to the difference between the
process variable and the set point, in part a linear function of the
integral of that difference and in part a linear function of the
derivative of that difference with time. The relative amounts of the
three components are controlled through "tuning" of the controller - a
process which absolutely must be carried out in order for a PID
controller to operate properly with it's given load and heat source.
In proportional only control if the temperature of the load (the process
variable - PV) is less than the set point (SP) there will be some heat
output and the temperature of the load will rise until k1*(SP - PV),
with k2 the gain or "band", results in a heat output which just equals
the losses of the system and the PV will be regulated at whatever value
meets this condition. Note that this is not where SP = PV but at some
other value so regulation occurs at an offset. The loop is 0 order. If
we integrate (SP - PV) it will grow indefinitely but if we scale it by
another constant, k2, and subtract k2*(integral(SP-PV)) from the output
the new equilibrium is reached (we have said nothing about over and
undershoots at this point) is reached when (SP - PV) = 0 (i.e. SP = PV)
and k2*(integral(SP-PV)) = the amount of heat required to just replace
the losses. Not that SP = PV does not mean integral(SP - PV) = 0 unless
SP has always been equal to PV. With this second component, the system
regulates to SP = PV and will track changes in SP or load. If one wants
a quick response to a change in the load (e.g. ice cubes thrown into
mash tun) or to a change in the set point (step mash) the third
component which adds heat equal to k3*d(SP - PV)/dt i.e. the rate of
change of the difference between the set point and process variable is
added. This boosts heat output at times when the temperature of the load
is dropping rapidly
The art of tuning a PID conrtoller is the art of getting it to respond
to a step change in SP as quickly as possible without overshooting more
than a tolerable amount (1 degree?). The setting of k1, k2 and k3
depends on the heat source, how linear its response is to the
controller's output, how quickly it responds to the controller's output,
the thermal mass of the load (how long it takes it to respond to heat
input), heat losses from the load etc. It is problematical in brewing
that the tuning for water in a half full HLT is not the same as the
tuning a full HLT is not the same tuning for a 5 gallon mash which is
not the same tuning for a 10 gallon mash. I thus find PID control useful
only with close supervision and manual over-ride (or, to be strictly
accurate wrt my brewery, pedal override i.e. I use a foot switch for
this function). PID is fine for the HLT but for the mash tun I use the
controller more as a temperature recording device than I do as a controller.
A.J.
------------------------------
Date: Wed, 13 Aug 2008 09:03:12 -0700
From: "Dave Larsen" <hunahpu at gmail.com>
Subject: HERMS: Liquid Return
I've been doing some research about how to return the liquid from the
HLT to the mash tun. Some people use a manifold. Some people use
something similar to a sparge arm. Whatever you use, it seems to me
that you do not want to splash the liquid back into the mash tun, or
risk HSA. The way that I liked the best was simply to lay the return
hose on top of the mash. I guess there is the risk of channeling
that way. What way is best? How do you get the liquid back to the
mash tun without HSA or channeling?
Dave
Tucson, AZ
http://hunahpu.blogspot.com/
------------------------------
Date: Wed, 13 Aug 2008 12:07:57 -0400
From: "A.J deLange" <ajdel at cox.net>
Subject: PID - Heat Output
I forgot to put anything in my last post about how the controller
actually controls heat. Most of them put out a variety of electronic
signals which are used to control heat (or cold or anything else which
can be controlled electrically) in various ways. The device asks for
heat as a percetage of maxium i.e between 0 and 100 percent. In
proportional control it closes a circuit for x% of the time where x% is
the commanded heat level. For example, in my brewery I use steam. If the
controller wants 50% heat it closes a relay 50% of the time or 15
seconds as I have the cycle period (usually adjustable) set to 30
seconds. The relay is in a circuit with a 24V control transformer and a
24 V steam valve (the mash acts like a great big thermal capacitor and
smooths out the resulting heat pulses). Depending on the controller and
it's configuration it may have direct relay output or a logic level
signal designed to control a solid state (SSR) or other relay. For
direct control an analogue signal is set x% of the way between minimum
and maxium i.e. between 0 and 5 volts for a voltage control or between 4
and 20 milliamperes for a 4-20 loop. Various devices such as SCR's, gas
valves, steam valves and so on are capable of responding to these signals.
A.J.
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End of HOMEBREW Digest #5395, 08/13/08
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