Not All Control Loop Cycles Are Due to Tuning

If a control loop has dead band in it, usually in a valve, but occasionally somewhere else, like in a transmitter, a loop with reset action in the controller will cycle, and
following the typical tuning rules will result in the controller being grossly detuned, plus the cycle will persist. For more information you may refer to a booklet "Controller Tuning and Control Loop Performance, a Primer", which is available in the US for $17, if prepaid with a check or money order.
If you want more information on this booklet, refer to: http://members.aol.com/pidcontrol / or e-mail me for a four page brochure. It has sold 30,035 copies. A page about the companion software ($11) is also described at that site.
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Dwstclair wrote:

So what do y'all think -- should I tail end every one of these irritating advertisements with a link to appropriate articles on my web site, or should I just ignore them?
--

Tim Wescott
Wescott Design Services
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Tim Wescott wrote:

I'm almost inclined to overlook one ad every six months or so. I'd not even wrote "almost" if St. Clair deleted the old one when he posted anew. Messages have a long lifetime on this group.
Aah, go ahead and append. :-)
jerry
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Engineering is the art of making what you want from things you can get.

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Given that David's post *is* relevant to this group, not sent every day, week or month and that the book is actually very helpful to newbies in the field, I would say just ignore them - after all, you don't want to mistaken for a Spammer, do you?
Having said that, if he bothered to actually contribute to this group rather than just advertising in it (without even bothering to prefix his subject line with "ADV:" or similar), I'm sure his posts would be more warmly received by those of us that have been here for a while and seen them with monotonous regularity... :-)
JM2cW,
Cameron:-)
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St Clair's experience and contributions to this trade eclipse any concerns of mine that he may be making a buck educating us.
Back to the topic. Don't confuse a too slow cycle time with control loop instability
"CHOICE OF HEAT CYCLE TIME ON A TIME-PROPORTIONING TEMPERATURE LOOP This is a controller parameter with a range adjustable typically between 0.1 and 100 sec. A 0.1 sec time (10 operations per second) would soon destroy a magnetic contactor, so in this case you would select typically 20 sec. On slow loops e.g. extruder zones there is so much metal mass that the temperature would not rise and fall noticeably in sympathy with these slow on and off pulses of power. On the other hand consider a fast responding application like radiant heating of moving sheet. You would need a cycle time say 0.2 sec to avoid the alternate under and overheated sections of sheet that slow cycling gives. A 10 second cycle would show large temperature swings. Be on the alert to recognise this as too long cycle time not control loop instability.
If you are using tungsten lamp heaters, you would not use magnetic contactors; you will need silicon controlled rectifiers (SCRs) with phase-angle control in the manner of a lamp dimmer."
This is extracted from article 17 on the list of articles at: http://www.eurotherm.com/training/tutorial/tutor.htm
Art
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He sends one every month, cycling around a few standard topics. He also sends them to other groups. And he never as far as I know contributes anything to this group. I think he should join in or stop. You dont see me emailing the group every month on the virtues of proper design and the use of design tools, but this time I will at least include a URL.
--
Francis
www.controldraw.com
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Cameron Dorrough wrote:

Tim: I'd like to see your links posted where appropriate. Usenet's primary goal is to share information, not as an advertizing medium. As for David, a quick google shows over a thousand posts to various groups, with little information conveyed, only advertizing. Spam is often in the eye of the beholder.
--
jeff


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Jeff Lowe wrote:

While I don't have as many articles posted as I'd like I post links where appropriate. Working with customers and participating in the group have suggested even more topics; now I just need time to incorporate them.
--

Tim Wescott
Wescott Design Services
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