Re: Iterative Technique for PID Controller Tuning

Paul M wrote:
On 13 Apr 2007 14:10:57 -0700, "George" <george.buckbee@xxxxxxxxx>

I have been following the PID threads and had a lot of things to
contribute, but knew that I would be sucked into defending my
statements and cracking open books trying to follow math and models
taking up time I did not have or want to spend doing so.

But that would have been good.


While I appreciate Peters desire to nail PID down with models, I don't
appreciate the way he responded to my simple innocent post at the
start of this thread.

You and Pieter stepped in front of Tim's and Joreg's pot of coffee comment, with your gain tweaking comment. If I were looking at this from OP's point of view I would expect more than tweak gains and drink coffee. Some one should have, I did, point the OP in the right direction but by this time the OP probably figured there was no intelligent life here and went away just like I did many year ago. The comments about tweaking gains and wait waiting by drinking coffee got me going. Actually, it was Tim giving the Control Station guys a bad time that got me going. This thread came to soon after that one. I have seen so much useless and wrong info this news group that a link to the www.controlguru.com site is a beacon of light and definitely more useful than the tweaking and coffee. You just happened to step in pile of stuff that had been growing for years.

I wanted to succinctly explain some things. Your
post goes a long way towards that. Thanks. I've had no need for models
of PID controls and little need for the math in what I do. It's not
that these things are not important, they just do not play a big part
in commissioning a PID loop historically. Note, I did not say "no
part".

Just because you don't have the need to know about better PID tuning techniques does meant that the next guys doesn't need to know more. Everyone's system and needs is different and it is wrong to assume they just need to know about tweaking and coffee.


The other thread on level control I found amusing. There were some
very simple answers that should have been given.

A simple answer was given. I admitted and that I should have made sure my program was the same as what was posted for the others to look at. This caused some confusion. Still Pieter's gain was way too high. Comment about using little integrator gain without knowing about the system was wrong. I saw no reason to blame the integrator for any level control problems mentioned by John.

BTW, I wish Pieter luck with his auto tuning project on the level control. It isn't needed for such a simple problem but at least he will have a model to test against. Then his program gain be applied to more difficult problems like this type 1 under damped second order system. One can see that the effort put into auto tuning will have huge paybacks when tuning just a few systems.

Instead there was
more math and models that had little to nothing to do with the
application.

That is true. What do you want me to say. We weren't provided with numbers for his application. We could have easily modified the program to fit his application if we had the values. Since we didn't have the model for John's application what are we do to? We can supply formulas and a simple simulator where he could eventually find the right numbers to use.

It's this aspect of engineering that turns a the building
of a local municipal project from the $100k job it should be to a $1M
"project".

BS. Fred Thomasson has a reference or book that must have formulas worked out for level control. I can work them out myself and it didn't take but a couple minutes to modify what I already had. Fred had that rule about ALV that was handy. I had my model that would give me exactly what the maximum error would be. These problems are trivial.
Add dead time, non-linear flow control valves, tanks shaped like a cone, horizontal cylinder or sphere and things get to be interesting.

Peters taunt to me about how intuitive or Iterative tuning
fails to further the science and offers him no help assumes he is
entitled to this and it is the only worthwhile way. This is not true.

I don't see sections on tweaking and drinking coffee in my control books .. I will stand by that statement and yes it is a taunt or a challenge. however you want to look at it. It isn't aimed directly at you alone. Perhaps I am complicit because I have not objected much over the years when I have known better for a long time. The goal is to make people crack open books and follow the math and models so the next time someone asks a question they get an answer better than tweaking and coffee. At least they should be pointed in the right direction. Then the people asking question will see there is a hint of intelligent life here.

There are plenty of gain tweakers out there that think every system is the same and will tune the same way. Whatever tweaker thinks he has learned tuning one system may not work on the next. My "test" was trying to prove my point about taking a more systematic approach. There are too many different types of plants. It makes me cringe when I see tweakers giving advice on tuning other peoples systems without knowing anything about the other people's systems.

The challenge is still there. Try manually tuning this type 1 under damped second order system without looking at my solution. Give it a shot. I know you can do it eventually. I think the time would be better spent cracking open the books and following the math and the models.

I couldn't figure out what JCH was trying to prove on that thread either. Sorry JCH. I think you need to show your work step by step and add more comments.

Peter Nachtwey
.

Relevant Pages

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