Best Book on PID ??

[John Larkin]

Eliminating that overshoot is one of the uses of properly applied
derivative.

I never could get derivative to work in this particular application.
The thermocouple is in a dewar maybe a foot downstream of the heater,
and the gas flow isn't fully mixed when it hits the t/c. The heater
emits four little jets of hot air, sort of like Stripe toothpaste. So
the temperature is literally noisy, and derivative goes sort of crazy
if set to any useful-to-the-dynamics level. That's a common problem in
flow systems. I designed all sorts of cute flow stirring devices (a
simple spring isn't bad) but couldn't get anybody interested in poking
them into the dewar.

Customers are the other big control problem.

John

 

[Spehro Pefhany]

I am frustrated with industrial PID controllers that force me to
choose between having the P & D terms based entirely on error or
entirely on the process measurement only. What I often need is a
separate gain and derivative term for the process measurement and
setpoint inputs to the controller.

Yes, especially if you're doing setpoint profile vs. time.

This is almost always superior the
gain and/or derivative based entirely on error (setpoint - process
measurement) or base entirely just on the process measurement and lets
me optimize (after I define that word for the particular loop) the
process disturbance response and setpoint change response,
individually. For critical tuning, I have to use the feed forward
connection and external math to to get all the factors I need. Why
must I choose chocolate or vanilla when I really want a swirl.

Best regards,
Spehro Pefhany

 

[John Larkin]

I am frustrated with industrial PID controllers that force me to
choose between having the P & D terms based entirely on error or
entirely on the process measurement only. What I often need is a
separate gain and derivative term for the process measurement and
setpoint inputs to the controller. This is almost always superior the
gain and/or derivative based entirely on error (setpoint - process
measurement) or base entirely just on the process measurement and lets
me optimize (after I define that word for the particular loop) the
process disturbance response and setpoint change response,
individually. For critical tuning, I have to use the feed forward
connection and external math to to get all the factors I need. Why
must I choose chocolate or vanilla when I really want a swirl.

Hmmm, guys, very interesting point. In most of the loops I've done,
the setpoint change just shoots through the error amp and the pid
stuff and sort of comes out OK, but I can see how that ought to be
optimizable.

John

 

[Spehro Pefhany]

Eliminating that overshoot is one of the uses of properly applied
derivative.

Part of the problem is that numbskull implementations don't properly
LPF the signal before taking the derivative so that ordinary process
and quantization noise makes the D term useless at the proper setting.

You didn't see that with analog controllers, because analog engineers
understand stuff like that. And it doesn't generally appear in the
textbooks.


Best regards,
Spehro Pefhany

 

[John Popelish]

:

Honeywell used to (and may still) have in some of their controllers a PID
algorithm that handled setpoint changes via integral action only. I think
that was called PID B, as distinguished from PID A which was the standard
ISA PID with independent gains.

Not exactly what you describe, but a step in the right direction.

Their current incarnation of DCS system that I work with has forms
A,B,C and D.

A bases P&D on error (most aggressive response to setpoint changes).

B bases P on error and D on process. Slightly less aggressive
setpoint response, but practically the same since D is used so
seldom. This is my preferred choice for inner loops in cascade
systems (where one loop provides a live setpoint for another loop, so
the setpoint is smoothly moving all the time).

C bases both P&D on process only. Uses only integral action to
approach new setpoint. This is my default for loops set strictly by
operators. The smooth ramp response to a setpoint step change allows
an operator to type in a wrong value, see the output heading off and
do an Oh shit! and make a correction before things get seriously
wrong.

D is an integral only algorithm.

The system I want replaces A, B and C with s single choice that
includes all those extremes and everything in between with the choice
of two proportional and two derivative values.

 

[Spehro Pefhany]

Aha!! The perfect pointer! Thanks!

Z-N tuning is an experimental controller tuning method. It tends to be
a bit aggressive for real systems (so, increase the P. band). I've got
a copy of the original 1942 paper around somewhere if you can't find
it online (shouldn't be that hard).

And what do you know, old "Nichols Chart" himself... last used one
about 40 years ago ;-)

...Jim Thompson

Best regards,
Spehro Pefhany

 

[John Popelish]

Z-N tuning is an experimental controller tuning method. It tends to be
a bit aggressive for real systems (so, increase the P. band). I've got
a copy of the original 1942 paper around somewhere if you can't find
it online (shouldn't be that hard).

Walter Driedger has posted a PDF copy of the world famous Z-N paper on
his web site:
http://www.driedger.ca/
http://www.driedger.ca/Z-N/Z-N.html

 

[John Popelish]

I never could get derivative to work in this particular application.
The thermocouple is in a dewar maybe a foot downstream of the heater,
and the gas flow isn't fully mixed when it hits the t/c. The heater
emits four little jets of hot air, sort of like Stripe toothpaste. So
the temperature is literally noisy, and derivative goes sort of crazy
if set to any useful-to-the-dynamics level. That's a common problem in
flow systems. I designed all sorts of cute flow stirring devices (a
simple spring isn't bad) but couldn't get anybody interested in poking
them into the dewar.

Customers are the other big control problem.

Dead time and noise definitely are limiting factors when applying
derivative. Usually the derivative effect has to include a low pass
roll off pole so that the phase lead of the derivative applies to a
narrow band of frequencies between the closed loop dominant pole and
the 1/(dead time) and noise frequencies.

Often you can do better by adding a second control loop that measures
the temperature very close to the heater that is not affected by that
dead time and noise, and control its setpoint (between judicious
limits) with the controller that watches the actual process that is
downstream.

 

[Clifford Heath]

It's the latter
messy stuff that most of the textbooks tend to ignore.

Has anyone here made use of fractional calculus (differentiation
& integration) in control loops?

But perhaps the math is too new for the applications to have been
tested...? There was also a time when roots and powers were always
integral, as differentiation/integration is even now to most folk
.

I'm interested to know what kinds of systems benefit from it, and
how you implemented it - pure curiosity of course.

Clifford Heath.

 

[Jim Thompson]

Okay, that's a nice HTML version. I've got a 5M PDF scan version of
the ASME paper here:

http://members.rogers.com/speff/Ziegler-Nichols.pdf


Best regards,
Spehro Pefhany

Click the "Download" button... and download the 757KB PDF version...
scanned, then OCRd into Word, touched up and then PDFd ;-)

...Jim Thompson

 

[Spehro Pefhany]

Walter Driedger has posted a PDF copy of the world famous Z-N paper on
his web site:
http://www.driedger.ca/
http://www.driedger.ca/Z-N/Z-N.html

Okay, that's a nice HTML version. I've got a 5M PDF scan version of
the ASME paper here:

http://members.rogers.com/speff/Ziegler-Nichols.pdf


Best regards,
Spehro Pefhany

 

[Rich Grise]

Proportional-Integral-Derivative

Thank you for this.

Many, many systems can be suitably controlled by using a feedback
controller combining the three terms. Either with an opamp or a DSP (or
for that matter hydraulic or mechanical controllers)

I had an assignment once to figure out why a particular heater controller
wasn't doing what it was supposed to do, and the nodes I was scoping just
confused me beyond all help. I think you've finally cleared that up for
me. Thanks!

Oh, and Astrom and Wittnemark's "Computer Controlled Systems" has a
pretty good treatment on PID. I have seen a better book, but alas forget
its title - its a chemical process control book.

Thanks again!
Rich

 

[Spehro Pefhany]

Click the "Download" button... and download the 757KB PDF version...
scanned, then OCRd into Word, touched up and then PDFd ;-)

...Jim Thompson

Ah, that's better!


Best regards,
Spehro Pefhany

 

[John Popelish]

Recommendations for Best Book on PID ??

Thanks!

You could probably write a good one. A PID controller is just a
follower amplifier (that forces a process measurement to follow a
setpoint). The PID controller tuning is just a lead lag network that
stabilizes that unity gain amplifier.

 

[Rich Grise]

"PID Without a PhD", and that's a "simplified" view, please. Inspired
by the directions given to union millwrights by control engineers who
aren't allowed to touch the equipment in many, if not most, mills.
Written by some schmo named "Wescott". Available through
http://www.wescottdesign.com/articles/pidwophd.html.

It certainly doesn't teach control theory, but it will let you twiddle
the knobs to get a working system most of the time (predicting how well
you'll like the result before you start requires control theory, however).

Ah! Finally! The crux! I see it all now!

It's Black Magic!

I have Astrom's adaptive control book, and I love it. Part of my
admiration is inspired by the fact that he devotes a whole chapter to
alternatives to adaptive control -- anyone who's writes a book about a
pretty new theory then tells you when you don't really need it has
integrity, in my view.

Cheers!
Rich

 

[John Popelish]

Ah! Finally! The crux! I see it all now!

It's Black Magic!

If you want to learn the twiddle method of tuning, I have written a
tutorial for non engineers that has been posted at:

http://www.tcnj.edu/~rgraham/PID/popelish.html

 

[Terry Given]

Hmmm, guys, very interesting point. In most of the loops I've done,
the setpoint change just shoots through the error amp and the pid
stuff and sort of comes out OK, but I can see how that ought to be
optimizable.

John

And of course in our smps error-amps the setpoint connection is
hard-wired, and causes the overshoot that makes us use soft-start
circuitry

Cheers
Terry

 

[John Larkin]

And of course in our smps error-amps the setpoint connection is
hard-wired, and causes the overshoot that makes us use soft-start
circuitry

Cheers
Terry

Are you saying that the people who design switcher chips don't know
much about control strategies? Shocking, Shocking!

John

 

[Phil Hobbs]

You could probably write a good one. A PID controller is just a
follower amplifier (that forces a process measurement to follow a
setpoint). The PID controller tuning is just a lead lag network that
stabilizes that unity gain amplifier.

I'd agree if we were talking about PI controllers, but PID are somewhat
different--the D term is there to compensate for slow transducers such as
motors and heaters.

The slow transducers put a few wrinkles in practical control systems that are
different from ordinary amplifers: windup in motors and asymmetrical slewing
in heaters. The D term will turn the 2-pole response of a motor into 1-pole
so that it can be stabilized, but the settling behaviour won't be anything
pretty unless some sort of (nonlinear) windup control is in there somewhere.

Cheers,

Phil Hobbs

 

[Terry Given]

Are you saying that the people who design switcher chips don't know
much about control strategies? Shocking, Shocking!

John

I am saying (albeit indirectly) that setpoint weighting/removal is not
so easy/obvious in the analogue world. Actually its pretty
counter-intuitive if you dont do the maths.

Cheers
Terry