Re: PD(PI) Math Model (e.g.Tank Level) Still in need of CORRECTION!

pnachtwey@xxxxxxxxx wrote:
On May 21, 11:43 pm, Tim Wescott <t...@xxxxxxxxxxxxxxxx> wrote:
On Mon, 21 May 2007 17:49:15 -0700, Peter Nachtwey wrote:
JCH, are you listening?
This is another example of the nonsense that people think they can get
away with when they combine the plant and controller transfer functions
and convert the response directly to the time domain. There is no 0-100%
limit on the controller so the closed loop transfer function will be
happy allowing valves to pump fluid level up and heaters to cool a
temperature control system.
Peter Nachtwey
I'd like to get my hands on some of those heaters -- particularly with
summer coming on.

If he hasn't listened by now, I'm not sure that he ever will.

I just don't want anyone to think that what JCH is doing the right
way.
How about your class? What are you going to teach them? The last
thing the world needs is more people thinking their heater systems can
cool and gains can be infinite.

Peter Nachtwey


At least once a class meeting I remind them that while the analysis is linear, the real systems aren't. So you use the linear approximation to get close to a solution, or to get a solution that's close.

I do keep throwing out hints -- I was just thinking that this week's homework will have an example of treating a nonlinearity (friction in this case) as a disturbance, and using the known magnitude of the discrepancy to predict the worst-case deviation of the real system from the predictions of the linear model.

I _am_ saving the nonlinear analysis for later (or in this case for another class because I only have one quarter). But it's there in my book, and in a form that can be handled by someone with greasy hands and a pencil to do the math, instead of a form that requires access to a university math department and a super computer or two.

Note that I'm not dissing simulation here, or higher math: they're both excellent tools. But without understanding the underlying issues simulation is just a quicker way to fart around in the lab until you stumble on something that works on that day, at that temperature, with that particular choice of components. You need the understanding to guide the simulation, and to understand the results. On the other hand, for most people the higher math is a bar across the door rather than a window to let the light in. For those who do understand the math one can easily get lost in the beauty of it and forget to connect it back to the physical reality of the system that you're trying to get working.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
.

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