Rockwell Controllogix PID Question

[BobLfoot 301]

I know this type of item has been beat like a dead horse but I am going to ask the question yet again because today I am having a senior moment and can't recall the answer and my attempt to search it out have been fruitless. THE GIVENS; 1. I have a electrical heating element with RTD feedback and a CLGX PID is regulating the applied current. 2. PID is configured for Dependent Gains. 3. The tuning constant Reset Time (TI) was 0.2 mins/repeat two months ago. 4. The tuning constant Reset Time (TI) is now 0.5 mins/repeat when I just got online. THE QUESTION: I know the I portion of PID is integral and means how much historical error do I incorporate. But does 0.5 mins/repeat mean more historical error or less than 0.2 minutes per repeat? And which is more likely to regulate to SP faster?

[Guest]

Greetings Bob ... with the DEPENDENT gains equation: entering a HIGHER number for the Integral setting gives LESS Integral action ... and, of course ... entering a LOWER number for the Integral setting gives MORE Integral action ... (so the PID works harder to push the PV toward the Setpoint) ... exception: entering a setting of ZERO turns the Integral action OFF ... note that the HIGHER/LOWER rules above are backwards when using the INDEPENDENT gains equation ... going further: the LOWER number would make the PID "drive" harder - and "faster" ... my question: why did the number get changed? ... someone else trying their hand at tuning perhaps? ... finishing up: the answers to your questions are covered here http://www.ronbeaufort.com/WhatIsI.pdf available on the Sample Lesson page of my website ... Figures 1 and 3 are pretty much on target for the specific questions that you asked ... Edited 12 Jun 2009 by Ron Beaufort

[BobLfoot 301]

Thanks for the great answer Ron. I use the term "Historical Error" because the one prof who taught PID explained it in "lay" terms as follows: "P is the here and now Gain ; I is the look back at what happened or historical error ; D is the predictive of the future gain. " SO if I were to switch to independent gains then 0.5 is faster than 0.2 at reaching setpoint all other things being equal {which I know they probably aren't} And yes I think somebody was playing.

[Guest]

as you say, other things usually AREN'T equal ... the Proportional settings will stay the same – but the numbers used for the Integral and the Derivative settings go haywire ... here are some "conversion" formulas that should help show the relationships between the two equations ... for example, in "Ron-talk" you would read these as: "Pind" means "Proportional setting for INDependent" ... "Iind" means "Integral setting for INDependent" ... "Ddep" means "Derivative setting for DEPendent" ... etc. ... disclaimer: I didn't get much sleep last night – so I apologize in advance if I've made any typos here ... (but I think that I've got it right) ... if you want to double check, you can find the basic ideas behind the conversions in the PID chapter of the Instruction Set Reference Manual for the PLC-5 system ... hope this helps ...

[BobLfoot 301]

It very well will when I get a chance to play later this week. We're down for Summer Maintenance for the week. so no headaches to keep the line running. Just a different set of headaches of parts not ordered, etc.