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True negative derivative is called (negative) velocity feedback and is used to decrease response time at the expense of overshoot. The frequency response of the system goes up. The system becomes stiff but 'nervous'. Can be used to stabilize systems as the same stiffness can be had with a lower proportional gain.
This is just what one would expect as plain-ole' derivative feedback is used to lessen overshoot at the expense of slower response time.
Think of D as oil: it can be used to slow something down by being thick and greasy or it can speed something up by lubricating it.
Speculation: if the controller is configurable to use derivative on error _or_ derivative on process then the sign of the derivative at the summing junction must change, most modern controllers do this automatically in software but I suppose there are those that don't. The same is true if feed-forward on SP is available.

Not so simple ... it doesn't necessarily make the system unstable: think Nyquist diagram.

I'm not sure what this has to do with software, except to know to shout 'Hardware Error!' and go for a cup of coffee.
FWIW: dual slope and V/F are preferred conversion techniques for process control as there are no 'bad spots'. Also work a charm if synched to the line frequency.

Er, maybe a new audience is needed?
The only requirements for control are monotonicity and repeatability, everything else is icing on the cake (though some sort of linearity is _really_ nice to have).
--
Nicholas O. Lindan, Cleveland, Ohio
Consulting Engineer: Electronics; Informatics; Photonics.
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Guy Macon wrote:

The "D" part of PID refers to the presence of a rate-of-change term, often able to be interpreted as a derivative of a position (or similar) quantity. Derivatives are a concept deriving (!!) from the differential calculus. Either term refers to the notion of infinitesimal differences whose ratios are considered as they approach (under suitable existence conditions) some limiting value.
Thus either version of the acronym is fine. Elsewhere in this thread is a timely reminder about the danger of reliance upon statistics (eg: Google hits) to "prove" something. I've come to the conclusion that if the majority agree upon something, it's probably false (works great in the stock market, but you have to be careful about your audience when defining the principle as "the fallacy of democracy"......<grin>)
Geoff.
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Geoff wrote:

Please don't follow "Guy Macon wrote" with something that I clearly labeled (in the part you snipped) as being not my question, but rather a question typical of a student in a class I taught.

So you derive from deriving the derivative from the differential that one should not differentiate between differential and derivative? That's different.

Google hits prove commonness of usage on the World Wide Web. All else is derivative - an important difference.
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Guy Macon wrote:
...

Bravo!
Jerry
--
Engineering is the art of making what you want from things you can get.

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Guy Macon wrote:

My apologies.

Essentially yes. (Ignores irony). But then, I'm not only a mathematician, I'm also a linguistics freak....

<grin> As I intimated, "common usage" is to be distrusted. After all, the planet's population is now so large that virtually any human-behavioural parameter, via the central limit theorem, gets modelled as obeying a Gaussian distribution, whose *central* area dominates the sample results. I call the universal welcome currently accorded to this situation "the cult of mediocrity" and it is an example of positive feedback. Examples abound. Think about it.
The linguistics scene has "descriptive grammarians" (currently in the ascendant) versus "prescriptive grammarians" (started declining maybe 50 years ago). That's why I regularly find books, and even learned papers, which confuse "throes" with "throws", "pour" with "pore", and many more, since schools ceased to bother students with (horror!) rules, substantive examinations etc.
To pull things together: my derivative/differential fusion is based upon a return to fundamentals (mathematical and linguistic). I find that this approach is superior to all others I've tried. YMMV.
Geoff.
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Geoff wrote:
...

That really peaks (or is it peeks?) my ire. :-)
...
Jerry
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Are you sure you are not seeing the results from widespread usage of Micro$oft's Spelling and Grammar checkers, coupled with a liberal dose of good old plain ignorance??
The number of times my Australian spelling has been "automatically corrected", without my knowing it, just because Micro$oft's dictionary editors are not linguists (and/or can't spell) peaks _my_ire..!
Yes, they give us our own Dictionary (IMO, just to keep us off-guard) - but it is still crap.
BTW: This topic is now seriously off-topic. Have a nice day. ;-)
Cameron:-)
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Cameron Dorrough wrote:

Amen, both! (Errm, I'd pick "piques", but you knew that, you crank-winders!)
Regards, Geoff.
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of
but
Geoff, how right you are.. :-)
Cameron:-)
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Guy Macon <http://www.guymacon.com wrote in message

snip
Maybe it's just me, but shouldn't this be obvious to anyone who's had even basic physics in school?
-Lasse
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Lasse Langwadt Christensen wrote:

Only those who went to school back when they were still teaching how to apply basic physics to real-world problems.
To be fair, some schools do a great job of this, but I have personal experience of a person who got an EE degree from a state college without ever tumbling on to the fact that when you send current down a wire there has to be an equal current through a return path. :(
That engineer was put to work maintaining COBOL programs. This was in the '90s, not in the age of COBOL.
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I have the new issue and plan to read it on the plane tomorrow. Congratulations.
BTW, why is ESP so thin now? Embedded Systems Pamphlet ?????
I remember when it used to be 3 or 4 times longer, consistently.
John
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John Sampson wrote:

I tried to sell them an article a couple of months after the dot-com bubble burst. The word then was that advertising revenue was down, so they had to thin it out. The reduced page-count plus all the suddenly unemployed engineers writing stuff nixed my article for a while (but I have published there recently).
--

Tim Wescott
Wescott Design Services
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