Re: PID Control theory and application
- From: Curly Surmudgeon <Curly.is.not@xxxxxxxx>
- Date: Sun, 03 Feb 2008 21:54:49 -0800
On Sun, 03 Feb 2008 20:15:53 +0000, Ian Smith wrote:
["Followup-To:" header set to alt.coffee.] On Sat, 2 Feb 2008,
EskWIRED@xxxxxxxxxxxxxxxxxxx <> wrote:
This is important too, for a couple of reasons. One is because the
beans are lofted into a fluid bed by a fan. The beans lose mass and
gain volume during the process, so adjusting the fan's static pressure
is useful. Unfortunately, that also changes to volume of air per unit
of time, so the temperature of the air changes drastically when the fan
is adjusted. I used to be more restricted in fan adjustments, before I
installed the PID on the heating coil.
I found that proportional control (with a relatively narrow band) was
plenty accurate enough for fluid bed coffee roasting. The offset from set
point is small enough that you don't need integral, and the whole system
reacts fast enough that you don't need derivative.
http://www.astounding.org.uk/ian/roaster/index.html
Graphs of bean mass temperature on that page, including
http://www.astounding.org.uk/ian/roaster/realroast.png. Note how close
the measured temperature (yellow line) is to the target (magenta). That's
pure proportional control, so I never bothered to program integral or
derivative.
Since all the control required can be done with the heater element, I
don't control the fan (though there is fan control in the hardware - the
processor has PWM control of the fan speed - I just never use it).
regards, Ian SMith
Good job.
Earlier a comment was made on optical vs. mechanical relays and that
optical was much more reliable. That is true, if properly designed.
Optically coupled 4-layer switches have an infinite lifespan as long as
load spikes are controlled and power factor nulled. This isn't as easy as
it sounds, motors and heater coils are inductive.
My point is that mechanical switching isn't necessarily of poor lifespan
though. When you're only switching a few watts lifespans of decades are
possible. Remember those old round wall thermostats for controlling
heaters? They were mechanical, most had mercury switches, a few just
silver/cad-oxide and both were highly reliable. The sensing element was a
bi-metallic coil of flat wire spring which would wind and unwind according
the temperature variation. In fact I don't remember ever seeing one fail,
only replaced for digital controllers with their additional, programmable,
features.
The poster of this parallel topic/thread made the point that mechanical
wasn't capable of rapid switching. They were right however with good
modeling a bang-bang controller with mechanical switching can work
reliably for long periods of time.
-- Regards, Curly
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