Re: Exploring the FOPDT Model With a Parameter Sensitivity Study


"Doug Cooper" <newsgroup@xxxxxxxxxxxxxxx> wrote in message
news:6YTNh.41$JR7.1@xxxxxxxxxxxxxxx
The 50+ articles in the controlguru.com table of posts
(http://www.controlguru.com/pages/table.html) represent perhaps 40% of a
complete textbook.

It is all on-line. There are no pop-ups or in-your-face advertising. And
it is all free. I post a new article twice a month and expect the project
to continue for a few more years.

I currently have over 4000 unique visitors every month at
www.controlguru.com. I know of faculty from more than a dozen
institutions who are migrating bit-by-bit from my textbook (currently
used in 125 schools) to this online resource because of its practical
orientation. I conclude from this that there is benefit from my efforts.

Boy Howdy. I've used your methods and practices successfully, Doug.

With that said, I acknowledge that electrical engineers, mechanical
engineers and chemical engineers all approach control from a different
perspective for valid reasons.

The www.controlguru.com site has a chemical/process orientation. The
primary focus is on PID control and related architectures. Applications
of interest include processes with streams comprised of gases, liquids,
powders, slurries and melts. Final control elements for these
applications tend to be valves, variable speed pumps and compressors, and
cooling and heating elements. Industries that operate such processes
include chemical, bio-pharma, oil and gas, paints and coatings, food and
beverages, cement and coal, polymers and plastics, metals and materials,
pulp and paper, and consumer and personal care products.

The control system at one of these plants may have from 20 to over 250
PID loops and from 50 to over 1000 measurements. Often, the controls
people have a different supervision structure from the production staff
trying to meet schedule. A control person seeking to tune a loop 'might'
receive permission to make a couple of bump tests. On the other hand,
they may be told to take a hike and be forced to dig out what they can
from the data historian.

Permission to bump a loop in a huge food process plant is frowned upon where
I am now. HAACP concerns are the most frequent reason I'm given when I ask
to bump a loop so I can approximate a FOPDT fit. With so many other
processes that are dependent on the loop you would like to test operations
people are justified in there hesitancy to allow it. This holds true for new
processes as well, although I do manage to get approval for it *sometimes*.
Make the product with minimal waste and make it now.

They are almost never given permission to conduct extensive testing. It
is simply too expensive and sometimes even dangerous. A couple of bump
tests can take much of a shift in many plants, and they are using
expensive feedstock and consuming utilities the whole time. For one loop.

The PID controllers in these production facilities tend to be a
horrifying mix from several of the roughly 20 major manufacturers. Each
manufacturer uses different terminology and formulates the PID algorithm
in its own fashion. Tuning these controllers is a challenge too often
addressed with trial-and-error fiddling by the operators, technicians and
engineers at these facilities.

Err, yep. I don't call it "fiddling" however. My preferred term is "knob
dicking."

www.controlguru.com works to address the needs of these individuals.

It most certainly does.

Most all processes comprised of gases, liquids and such are overdamped
and self regulating. The time constants have units expressed not in
milliseconds or rarely even seconds, but most often in minutes (and
sometimes hours). Dead time, measurement noise, and nonlinear behavior
are always complicating issues. If underdamped behavior shows up in a
trend plot, it likely means that an upstream controller is cycling and
needs to be checked out. It is almost never the characteristic behavior
of the process itself.

No loop in these plants is truly first order plus dead time (FOPDT). Not
a single one. In fact, if modeled from first-principles including the
thermo, transport and kinetic equations, we would likely find such
processes to be accurately described with tenth or even twentieth order
differential equations.

But in my experience, 85%+ of PID loops when the application is a process
with streams comprised of gases, liquids and such can be well-tuned using
the FOPDT approximation. If we introduce non self regulating overdamped
processes (also discussed on www.controlguru.com), we are up to maybe 90%
+ of the processes.

If readers start at the top of the table of posts
(http://www.controlguru.com/pages/table.html), they will find, presented
in a logical order and free of any charge, the details, methods and
practices I believe provide the best general methods for the application
domain I am focused upon.

I hope readers will visit and judge its value as a resource for
themselves.

And I know my posts on this newsgroup announcing the publication of the
next installment of the e-book are valued by some of the readership. So
it is a practice I intend to continue.

Taking personal time to help people in their endeavors is a wonderful
thing, and definitely not something that should be frowned upon. And for
that I thank you, sir. Keep up the good work.


Doug Cooper



.

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