Best Book on PID ??

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

Best regards, Spehro Pefhany

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

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.

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).

Best regards, Spehro Pefhany

Walter Driedger has posted a PDF copy of the world famous Z-N paper on his web site:

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.

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

Best regards, Spehro Pefhany

Behold, Jim Thompson signalled from keyed 4-1000A filament:

PID?

Jim....you really oughtta see a doctor :-/

;-p

Ah, that's better!

Best regards, Spehro Pefhany

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.

There is PID without tears on Embedded.com (I think), which a simplistic view.

Then there is the text (Astrom) I have which goes from the basics to adaptive controllers etc. PID controllers Theory ,design tuning. Lotsa good stuff.

Cheers

For learning (i.e., some math but not dense page after page of it) look at "Control System Design Guide," George Ellis, ISBN 0-12-237461-4. I have (and enjoyed) the 2nd edition of the book; the ISBN is for the 3rd, published in 2004.

Web site, free companion software:

Sorry Tim ;D

PID with out a PHD is a very good beginning spot. Its where I had started, and wound up later with Astrom.

I never made the connection.

Cheers

view.

adaptive controllers etc.

the

True. Spent early years on petrochemical plant control&instrumentation. Safe pneumatics, 3-15psi range (Foxboro, Honeywell, Taylor etc). Air driven analogues of all the elecronic stuff that's about now, such as square root extractors, multipliers, mass flow computers etc. All process control by pneumatic PID controllers/recorders. The control courses all offered numerous pat equations for optimising the plant dynamics. We learnt to our dismay the equations looked neat in a textbook but useless on real plant. Most of the control systems ended up proportionally slugged down to near stupidity, with only the occasional bit of integral to wind down sticky setups. 'Reset' never had chance to get a look in. I never did identify one of those mooted ideal systems that could be classically tuned. The real world stuff had many processes at work in // and they -all- interacted. Lowest common denominator was the tuning rule. regards john

Well done. What's a few hundred k$ here and there. :-) Would hope the C++ programmers were terminated with maximum predjuduce. Best I've done is poisoning (ISTR) 10 tons of Platinum oxide catalyst. Was told the stuff is biblically expensive but they never brought up its value. At the time there was an explosion and fire and I'd been called out to overide some auto ESD trips allowing isolation of a plant segment. regards john

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

John

Proportional-Integral-Derivative

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)

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.

Cheers Terry

of which 1/3 provide a significant process improvement, 1/3 provide little or no process improvement and 1/3 make the process worse - due almost entirely to the inability of users to tune them correctly.

I wonder if autotuning has helped the situation any - probably not much, I imagine the installed base of non-auto-tuning units isnt going to be replaced in a hurry.

Cheers Terry

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.