Experimental Physics and Industrial Control System
First, thanks for the responses, both on and off list about this.
To provide a bit more background, I've written two blog posts about the
project covering brewing theory, systems design, and what I'm trying to
accomplish. They also include a number of photos of the system. I still
need to write about the physical fabrication, electronics, and software.
http://www.mackenziegems.com/2015/12/25/cnc-beer-part-1-overview-and-theory/
http://www.mackenziegems.com/2015/12/28/cnc-beer-part-2-system-design/
The test I mentioned involved the hot liquor tank. (Part 1, first
picture, top center keg.) This is filled with water, and it contains a
copper coil that the beer wort passes through. (Part 2, third picture.)
I was asked offlist if I could move the temperature probes or add more
of them. What I'm trying to control is the temperature of the water near
the coil, which accounts for the probe placement. I'm trying to regulate
the temperature of the wort at the output of that coil, so the water
temperature near the coil is most important to me.
I know the probes themselves have a good deal of lag. They are digital
DS18B20 chips that take 750 ms to sample. But they are sealed inside
thin stainless steel tubes. Grabbing the outside of the tube with my
hand for, say, 10 seconds and letting go produces no immediate change.
Then, after I release it, I can watch as the temperature starts to
climb. I also imagine there's lag for convection to carry the hot water
up past the coils. And the bottom of the kettle is still hot, so even
after cutting off the flame, it still transfers heat to the water.
Matt Pearson suggested for larger setpoint changes using a larger
deadband that after a period of time resorts to a smaller deadband. That
sounds like it will work, and I know I can code it easily. I'm also
interested in whether there are generic algorithms that can compensate
for systems with lots of lag time. Eric Norum suggested a Smith
Predictor. After researching it, I think I understand how that works,
but it requires me having a mathematical model of what the system
behavior should be, which I'd need more research to develop. I also
received an offlist suggestion about a lead/lag block. If I'm not
mistaken, that uses a Laplace transform, yes? If so, again, I'll need to
do more math research.
I didn't hear anyone mention building CSS BOY on a Raspberry Pi. Has
nobody tried this? With inexpensive touch screens for the Pi, it seems
like one could make a small panel mount control console for under $150.
I'm definitely planning on trying this myself, but it's on the backlog
of things to do after getting the first batch of beer brewed.
Thanks,
Ryan
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