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BobLfoot

PID Basics

10 posts in this topic

Having read the rather lengthy discussion of gurus like Peter and Ron in a post about PID and going nearly crosseyed I have to ask. SOOO - When is someone going to write PID for Dummies. THe book intended for the tech who wants to add a PID proportional control to a single loop process and not get a PHD while elarning to tune his loop. The help files for the AB PID instruction have enver been really useful and RSTUNE has always been an expense. Bottom Line. My plant has several PLC's with PID Instructions in them. From time to time we get asked to add more. How do I know I am adding them correctly and that what we have is tuned for optimum performance?

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Could'nt agree more Bob... And what would be betrter would be real world examples.. ie we have a flow sensor and variable speed drive with the variable speed dirve conrtolling the flow rate... where do you start, do you bump the drive and measure the response on the flow ? how do you measure the response ? do you need to use specific software or is it through observation ? What does plant constatnt mean ? What do you set the time base of the PID at ? etc..

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And I know setup will require some Math, but please remember this is PID for dummies. Keep it in grade school algebra at worst.

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Greetings to BobLfoot and to Sleepy Wombat ... to continue the discussion ... if you don’t have ANYTHING to go by, just set it for 0.50 second and get on with your life ... here’s more ... the “natural period of oscillation” of the system shown in this "rerun" post is about 3.54 minutes per cycle ... the “book” recommends a PID update setting of one/tenth of that value ... so about 0.354 minutes - about 21.24 seconds ... personally, I never shoot for less than 20 executions/updates per cycle ... so in this example, I would set the update time for about 0.177 minutes - or 10.62 seconds ... this setting should be “fast enough” to give us “satisfactory” control ... anything faster is (theoretically at least) simply wasting processor scan time, etc. and over-burdening the processor with extra/unnecessary calculations ... but now then ... in many (most?) cases, we also use the PID to make certain handy “scaling” calculations for us ... things like the “Scaled PV”, etc. ... if we only update the PID once every ten seconds or so, the operators might get the impression that the “dial-is-stuck” because the numbers on their displays don’t change fast enough to suit them ... so the machine works fine with the recommended update - but the humans usually want something a little more “real-time” on their HMI screen ... so everything considered, an “update” setting of more than about 1 second is probably going to be asking for complaints ... are we having fun yet? ... I’d like to add more to this discussion - but my wife and I have to go give blood this afternoon ... the Red Cross is calling and BEGGING for a donation ... they say they’re literally to the point of postponing surgeries for people because they’re so short ... if that’s a plug for the Red Cross - then so be it ... finally ... the chart below has helped a lot of people ... no, it’s not the pot of gold at the end of the rainbow ... just a handy “getting started” point ... PS Edit ... I'll add this much just before I go ... I have this recurring fantasy where someone from Allen-Bradley calls me up some day and makes me an offer that I can’t refuse ... basically they want to hire me (full-time and with benefits) to write a complete PID book (“for Dummies” if you insist) for all of their processor families ... and I don’t even have to move away from Charleston to some distant land of ice and snow ... I always hear the “Somewhere Over the Rainbow” theme song from “The Wizard of Oz” playing in the background whenever I have this little daydream ... oops! gotta go ... the phone’s ringing again ... maybe it’s really them this time ... Edited by Ron Beaufort

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Of my OWN money, $75 to $100 sounds fair. Spending "company" money, double that (it becomes a "shop reference" if the company tilts for the cost). I suspect your biggest obstacle would be advertising. Can get pricey, and your "target" audience is rather small in terms of trying to make mass-marketing cost-effective. On the other hand, think of all the systems integration houses and the people therein who would do immoral things to obtain such a reference! Okay, I've probably expended or exceeded my allowance of 'PID High Priest' attention time.

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Great Response Ron. I've read it thru and letting it bounce around the empty space I call a brain. I'm sure I'll have questions in the coming weeks. Appreciate the offer and I will take you up on it someday soon. Hope Peter chimes in soon too.

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Thanks again for your lamens learned approach to PID... I raised the quesitons to get the ball rolling, and yes what you have said is what i do in practice generally due to time contraints, but have never been able to collate the toughts and process so elequently as you have seemed to...BIG THANKS mate......very intested in the calc's you use , Pity, cloning isn;t avail yet, i am sure that a lot of people in the land of OZ would enjoy your teaching methods...mmm mabey you should do the world circuit.... I always thought that your blood would be worth bottling.

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I would like to know: 1. Does anybody follow the www.controlguru.com website? There is a good example about a heat exchanger there. There are simple equations there that allow one to calculate Kc, Ti and Td. The trick is to calculate gain ( Kp ), Tp and dead time. 2. How much is it worth to you to have your PIDs tuned? What if your PIDs can be tuned over the internet using goto meeting? 3. How much are you willing to learn. A better question would be how many hours do you think it should take? PIDs are not for dummies. The trick is to educate the dummies so they aren't dummies anymore. It should be 10 times faster than the closed loop or plant time constant, which ever is shorter. I prefer faster. I know this isn't much help without knowing what a time constant is. Everyone wants a quick fix. It isn't possible. If your plant has PIDs to tune why not hire somebody to do it? BTW, I too have been asked to write a book by a publisher, but as Ron pointed out, it isn't worth it unless you do it for marketing or can use the book in a training class. I have enough material for about 3 books. I agree that example are necessary. Would you work through the examples using Scilab, Matlab or Mathcad? What about the examples on www.controlguru.com?

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An entire quote from Peter on PID's and NO MATH - That deserves a medal - LOL. Thanks for the response Peter. I will check out controlguru.com and spend a little time with my systems. Guess PID's are like any other "higher level" PLC command. To understand them takes a little effort. Guess I'll just have to knuckle down and jump in. The wizard will have to relinqish his cloak for the robe of a PID acolyte.

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Will check control guru examples .. about to go away and commission again....

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