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Old November 10th, 2004, 09:26 AM   #16
RMA
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Are those terms purely American, Tom, or particular to some branch of industry. I've been mainly involved with the chemical industry over the last 30+ years and I've never come across those terms. There again I've never seen pneumatic control in the chemical industry in that time either, although I bumped into it often enough in steel works - no EMR problems!
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Old November 10th, 2004, 09:29 AM   #17
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They are terms used by the instrument society of America (ISA)
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Old November 10th, 2004, 01:24 PM   #18
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if a picture is worth 1000 words, what about a picture AND 1000 words? ...

Greetings farid,

it sounds like the biggest part of your problem is understanding the “units” of the Integral term ... I’ve had some pretty good success using the following explanation in the “technician level” PID classes that I teach ... most of the students in those classes have the same problem that you seem to have ... they need to work around PID-controlled systems, but they don’t really have very strong math skills ... anyway ... let’s see if this approach helps you too ...

first of all ...you didn’t specify what brand and model of controller you’re using ... I’ll use the Allen-Bradley PID for our example ... specifically, I’ll be using the “ISA” or “Dependent Gains” equation ...

note: in addition to the most-common “ISA” and “Dependent Gains” equation, the PLC-5 and ControlLogix platforms also support the “AB” or “Independent Gains” equation ... we won’t go there for this initial discussion ... if you’re using the SLC or MicroLogix platforms, then the “ISA” or “Dependent Gains” is the only option available ...

first let’s start with a simple temperature control system ... now the natural thing to do would be to use “degrees” as the units for our process signal ... but to keep things as simple as possible, let’s use “PERCENT of FULL SCALE” instead ...

let’s say that the controller’s SP (Setpoint – or “target”) has been set for something like 10% of full scale for a very long time ...

let’s say that the controller has been regulating the CV (Control Variable – or controller “output”) at 25% for a very long time ...

let’s say that the system’s PV (Process Variable – or “temperature”) has been accurately controlled at the desired target of 10% for a very long time ...

you should already understand that every time the PID controller is executed, it calculates a value for E (Error – or “how far are we away from the target?”) ... in our example, the formula for this error calculation is “E=SP-PV” ... since E=10-10 then E=0% of full scale ... this “0” error value is the PID controller’s way of knowing that WHATEVER the value going out to the CV, it’s PERFECT ... in other words, we’re presently right on target ... let’s keep everything right where we’ve got it ...

and those are the conditions that we see at the left side of the trend shown below ...

[attachment]

so just to nail things down, we have a well-controlled system simmering along right on target ... the sun is shining ... the birds are singing ... life is lovely ...

now ... let’s do something really strange ... we’re going to do this just for EDUCATIONAL purposes only ... we’re going to physically JAM the system’s output valve (the real-world field device) in its present (25%) position ... now at first glance you might think that this will really “mess things up” ... but it won’t ... in fact, as long as everything else stays the same, the system will just keep on simmering along ... right on target ... think about it this way: we’ve already said that the CV has been running along at 25% for a very long time ... and even though we’ve physically jammed the valve at the 25% position, the system should stay right on target – just as long as nothing else changes ... and now the stage is set ...

first we’ll begin with the “P” or “Proportional” setting ... we’ll get to the “I” or “Integral” setting in due course ...

the CV starts out at position “A” on the graph ... that’s the 25% setting ... as soon as we reach position “B” on the graph, we suddenly increase the SP (the “target”) from its original setting of 10% to a new setting of 20% ... the PID controller sees this change ... and it calculates a new Error based on this equation: “E=SP-PV” ... since E=20-10 then E=10% of full scale ...

now let’s say that our controller’s “P” or “Proportional” setting is currently a nice round value of 2.00 ... the controller takes the E (10%) and multiplies that by the P setting (2.00) and then adds the result (20%) to the previous CV (25%) ... the result of this calculation (45%) becomes the new value for the CV ... and that’s what we see happening at position “C” on the graph ... specifically, the controller’s proportional action has instantly stepped the CV from 25% to 45% in an effort to drive the PV up to the new target value ...

now notice the little blue “yardsticks” on the graph ... notice that one of these “yardsticks” is used to measure the sudden increase in the SP ... this “yardstick” is 10% of full scale ...

now notice that it takes two “yardsticks” to measure the 20% jump in the CV between position “B” and position “C” ... just for discussion, if our controller’s P setting happened to be 1.00 instead of 2.00, then the increase in the CV from point “B” would only have been half as much ... specifically, it would have gone up one 10% “yardstick” rather than two ... since you said that you have a pretty good understanding of the PID’s proportional action, we won’t go into any more detail about it in this discussion ... now we’re ready to tackle the “I” or “Integral” setting ...

now remember that the system’s control valve has been physically jammed in the 25% position ... it cannot possibly move ... so even though the PID controller really and truly sends out a new value of 45% to the output valve, the valve (the field device) stays in the 25% position ... that means that the PV is going to keep right on tracking along at its 10% value ... you can see that happening on the graph ... specifically, the PV line stays completely horizontal ... even though the CV (in the PLC’s memory) does go up ...

so now the Proportional action has done as much as it can to drive the PV toward the target ... but ... since we’ve jammed the valve, the PV doesn’t change ... so the PID starts using the Integral action to try to increase the PV ... and that’s the “ramping” action that you see taking place in the CV after it reaches position “C” ...

on to the next post ...
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Old November 10th, 2004, 01:24 PM   #19
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and even more words ...

so now let’s look at the CV as it takes off from position “C” ... for discussion purposes, I’ve given you two possible paths for the CV ... let’s take the first one which leads up to position “D” ... this is the path that the CV would take if the controller’s “I” setting was 2.00 ... that setting would literally mean “2.00 minutes per repeat” ... in other words, it would take 2.00 minutes for the Integral action to REPEAT the Proportional action ... and that is what the two “yardsticks” are showing at position “D” ... notice that it took two minutes to make that amount of change in the CV ...

now let’s look at the CV again as it takes off from position “C” ... but this time we’ll take the second path ... the one which leads up to position “E” ... this is the path that the CV would take if the controller’s “I” setting was 4.00 ... that setting would literally mean “4.00 minutes per repeat” ... in other words, with this setting it would take 4.00 minutes for the Integral action to REPEAT the Proportional action ... and that is what the two “yardsticks” are showing at position “E” ... specifically, the Integral action caused the CV to increase the same 20% (two “yardsticks”) ... BUT ... it took twice as long (4 minutes rather than 2 minutes) to make the SAME amount of increase ...

so ... one thing that we’ve just learned is that for LESS Integral ACTION, we need to use a HIGHER Integral SETTING ... specifically, a BIGGER NUMBER gives us LESS ACTION ...

conversely ...

for MORE Integral ACTION, we need to use a LOWER Integral SETTING ... specifically, a SMALLER NUMBER gives us MORE ACTION ...

believe me this is NOT a trivial matter ... I’ve seen inexperienced technicians fight a PID tuning problem for days while changing the Integral setting in the WRONG direction ... just to nail it down: I refuse to let my students say “increase the Integral” or “decrease the Integral” ... because those statements are ambiguous ... do they mean “increase the Integral ACTION” or “increase the Integral SETTING”? ... taken the wrong way, the effect of their statement would be exactly the REVERSE of what they meant to do ...

warning! ... please remember that we’re basing this discussion on an Allen-Bradley controller using the “ISA” or “Dependent Gains” equation ... the “bigger-number-means-less-action” and “smaller-number-means-more-action” rule that we just nailed down is BACKWARDS if you use the “AB” or “Independent Gains” type of equation ... further, the Integral units in that equation are based on “repeats per minute” rather than “minutes per repeat” ... if you need help with this issue, please post again ... believe it or not, I’m trying to keep this current discussion as simple as possible ...

important exception to the “smaller-NUMBER-means-more-ACTION” rule: entering a value of “0.00” actually turns the Integral action OFF ...

summing up ... in an Allen-Bradley system using the common “ISA” or “Dependent Gains” equation ... the values of the Integral term are “minutes per repeat” ... in other words, the length of time that it takes for the Integral action to REPEAT the Proportional action ...

and so, farid, at this point I think that I have adequately answered your question ... but ... only you can be the judge of that ... if you need more information, please post again and give me another chance ... if that becomes necessary, it would be very helpful if you would tell me exactly what type of hardware you’re working with ... believe me, there are some BIG differences depending on the type of hardware and the type of equation that you decide to use ...

now ... here is the most important part of what I have to say ... if you want to do some more research on this subject, then I suggest that set up a “dummy” PID in a spare controller ... most people think that you can’t really learn anything about PID without a real-world process to control ... that is nonsense ... you can learn a LOT ... try this as a basic “getting started” experiment: ... set up a Trend Graph in RSLogix (ask if you don’t know how) and use it to monitor the memory locations for the PID’s “Setpoint” and “Process Variable” and “Control Variable” ... now ... think ... the PID is like a little man in a box ... he doesn’t know whether the PV (input) signal is coming from a real-world device or from YOU ... in other words, if you go to the PV location and manually type in a value for the temperature reading, then the PID will accept that value ... and he will respond by calculating a value for the CV ... now if you were to change the P setting ... or the I setting ... or the ... well you should have the idea by now ... basically I’m telling you that you can duplicate the experiment in my little drawing for yourself with very little effort ... and learn much more about PID on your own than I can possibly share on the forum ...

now ... it IS true that you cannot learn how to actually “tune” a loop without some type of process to control ... but then again, you CAN still learn a LOT about how the PID does its “magic” by just putting numbers “in” and seeing what numbers come “out” ...

finally ... have you looked at the “hotrod” simulator that my distinguished colleague Peter Nachtwey came up with in a recent thread? ... that might be something of interest as you continue your research ...
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Last edited by Ron Beaufort; November 10th, 2004 at 01:47 PM.
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Old November 10th, 2004, 01:45 PM   #20
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Thats bound to be one of the better and straight forward explainations I've read on PID. Thanks Ron
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Old November 10th, 2004, 04:11 PM   #21
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Thanks Ron, I get the feeling that I've just seen 30+ years of experience put into words. I've had a lot of experience in loop tuning in chemical plants with response times, in some cases, of 8 hours and more. I've never managed to explain to my co-workers why, or how, I was doing something as concisely as you've just done!
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Old November 10th, 2004, 04:27 PM   #22
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and a little bit more ...

Greetings Farid,

I had a few more minutes available so I’m going to add just a little bit more to the “Integral” discussion ... in your original post, you asked the following specific question:

Quote:
... why do I need the integral term ??
maybe I can help answer that for you ...

disclaimer: the figure below is a rough hand-drawn sketch ... it should be adequate for the purposes of this discussion but it is NOT to scale ... I’ll try to bring in some more accurate “real-world” data tomorrow if we need it ... anyway ...

here we have a system which we will try to tune using Proportional control only ... specifically, we will not use Integral action in this example ...

[attachment]

before position “A” the PV has been simmering along ... it has settled down at a certain value ... but the PV is not close enough to the desired SP ... so at position “A” we increase the Proportional setting to a new higher value ...

the PV starts to rise toward the SP ... but the PV starts to oscillate ... the oscillations die out and the PV settles down at a new steady value ... this new value is closer to the SP ... but it’s still not close enough ... so at position “B” we increase the Proportional setting to a new higher value ...

again the PV starts to rise toward the SP ... but the PV starts to oscillate ... the oscillations die out and the PV settles down at a new steady value ... this new value is closer to the SP ... but it’s still not close enough ... so at position “C” we increase the Proportional setting to a new higher value ...

again the PV starts to rise toward the SP ... but the PV starts to oscillate ... the oscillations die out and the PV settles down at a new steady value ... this new value is closer to the SP ... but it’s still not close enough ... so at position “D” we increase the Proportional setting to a new higher value ...

again the PV starts to rise toward the SP ... but the PV starts to oscillate ... and this time we’ve gone too far ... the oscillations do NOT die out ...

so this is the answer to your question ... by using Proportional-only control we can get closer and closer to the SP ... but we can never get quite there ... and if we try to get too close, the system will start to continuously oscillate ...

and so this is where we would decide to use some Integral action ... the Integral action will start to “ramp” the signal until it actually DOES reach the desired SP ... we’ve already seen that “ramping” action in another post above ...

incidentally ... the term “reset” which is often used for the “Integral” term comes from the old-timer’s technique of “tricking” a Proportional-only controller into accurately achieving the desired setpoint ... basically it works like this ... the controller’s knob is turned to a reasonable setting ... let’s say “350” degrees ... the Proportional-only controller finally settles down at a lower temperature ... let’s say “325” degrees ... the technician carefully slides the knob off of the controller and looks behind it ... there he finds a set screw which holds a little metal plate in place ... he loosens the set screw and turns the plate a little bit ... then he tightens the screw and replaces the knob ... the shaft on the controller has NOT been moved ... but now the knob points to “325” degrees ... bingo! ... the controller has now been “reset” to an accurate temperature ... now when the knob is finally turned up to the desired setpoint of “350” degrees, we’ll be very close to “dead-on-the-money” ... and that’s the way the thermostat on most of the ovens in homes and restaurants are calibrated too ... been there ... done that ...

in today’s controllers, the Integral action gives us almost exactly the same “resetting-of-the-knob” effect ... and that’s where the name “reset” comes from ... or at least that’s the way I’ve always heard the story told ...

and that seems pretty much in line with what Tom said in post #13:

Quote:
The tuning term's name is a historical artifact of the old names for a three mode controller - Proportional, Reset, and Rate. For the old analog or pneumatic single mode Proportional Only controllers the operators would manually get rid of droop by tweaking the setpoint adjustment until the droop disapeared - in effect "resetting" the setpoint.
the "droop" that Tom mentions can be seen in my drawing above ... it's the distance between the SP and the "settled out" PV value after each adjustment to the Proportional setting ...

PS to mordred and RMA ... thanks for the kind compliments ...
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Last edited by Ron Beaufort; November 10th, 2004 at 04:51 PM.
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Old November 10th, 2004, 07:36 PM   #23
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Thumbs up

hi Ron,

I couldn't wait to thank you for the time afforded to help me on this, I have never seen any thing more simple than this, one thing I can say , is that your students are lucky, god bless you..I can't thank you enough,therefore,I will continue reading what you wrote over and over (its like drugs loool),


I am using S7 200 PLC cpu 222, after reading your explaination(not all of it I couldnt wait to reply), now I dont know if whether its ISA or not , but Ron, I think your explaination will point me in the right direction in both ways , as the concept is the same, am I right ??


Keith,

you explaination is also a good thing.

thanks to all those who shares the fun.
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Old November 11th, 2004, 05:29 AM   #24
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Hi Ron,

from the data block in the s7 200 plc, the following has been generated by microwin PID wizard I will tell you what I'd undertand after reading ur thread and you tell me where I am wrong:


VD0 0.0 //Process Variable (this is the analog input(feed back from the process)

VD4 0.0 //Loop Setpoint (this is the target in you explaination, or our destination of where we wanna go)

VD8 0.0 //Calculated Loop Output (this is the CV in yoor explaination)

VD12 1.0 //Loop Gain ( the E or error is multiplied by this value and then added to the CV, this is the proportional right?)

VD16 1.0 //Sample Time ( this is the time for the loop to be executed(here every one second))

VD20 10.0 //Integral Time (this is the Integral SETTING,the more I increase this one, the slower the proprtional action will be repeated ).

VD24 0.0 //Derivative Time (if zero then its turned off)

VD28 0.0 //Integral Sum or Bias (yes..no..maybe..I don't know )

VD32 0.0 //Value of Process Variable stored from last execution. (this is kinda obvious I guess it stores the value from the last loop excution )

what do ya think Ron ?? did I waste your time for nothin' ??

regards.
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Old November 11th, 2004, 11:29 AM   #25
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Siemens? ... oh, well ...

Greeting Farid,

sorry ... I have ZERO experience with Siemens ... hopefully someone else can help you with your questions in that area ...

but MAYBE I can shed a little bit of light on this particular one ...

from Farid in post #23:
Quote:
I dont know if whether its ISA or not
suggestion: use a SAFE system for experimenting ... if you increase just the Proportional setting (let’s say you double it) ... and the Integral action ALSO increases (specifically it doubles too) then I’d be willing to bet that your Siemens is using the “standard” ... “ISA” ... “Dependent Gains” ... type of equation ...

again ... I have NO experience with Siemens ... but this experiment would work and “prove the point” with an Allen-Bradley system ...

and if you just had to “guess” which equation your system is using, smart money would go on the “good-old-fashioned-been-around-since-dirt” ISA type of equation ... keep in mind that ALL of my hands-on work with PID has been with Allen-Bradley systems EXCLUSIVELY so take this next statement with a grain of salt ... but ... as far as I know, the “AB” or “Independent Gains” type of equation is unique to Allen-Bradley ... if anyone out there knows of any other PLC manufacturer who uses this “new” type of equation, I’d sure appreciate it if you’d let me know ...

just a little bit of background on the two types of equations ... you can find both of these defined in Chapter #14 of the

PLC-5 Instruction Set Reference Manual

I’ll include a screen shot from the latest version (Publication 1785-6.1 dated November 1998) ... for anyone planning to do an in-depth study of this material, note the manual’s typographical error shown below ... (yes, this stuff will drive you nuts) ...

[attachment]

an earlier version (Publication 1785-6.1 dated February 1996) does not have this error ... (no, I NEVER throw out my old books when the new copies come out) ...

anyway ... the book also gives the formulas necessary to convert from one equation to the other ... and so ...

(important note: the following discussion is based on using an “integer” type control block for the PID ... the newer “PD” type control block allows the use of floating point numbers for the tuning parameter settings and thus gives a lot more flexibility) ...

when I’m asked the question: “which equation works better?” I usually answer with something along these lines ... suppose that we use ONE equation and tune it to properly control a certain system ... then suppose that we take the tuning values from that equation and convert them over to the equivalent tuning values for the OTHER equation ... and then we use that OTHER equation (with the equivalent values) to control the same system ... I’ve done this numerous times ... and when I print out the two trend line graphs, I can hold them up to the light ... and I find that the two traces are almost PERFECTLY superimposed ... and so ... as far as I’m concerned, either equation will work ok for most systems ... but ... notice that when we use the ISA equation, the MAXIMUM amount of Integral action that we could possibly obtain would require a (smallest number) setting of “0.01” ... let’s assume that our Proportional setting for both equations is “1.00” ... now converting the “0.01” Integral setting to its equivalent in the Independent Gains equation would give us an Integral setting of “1.667” ... now consider that when using the Independent Gains equation, we can INCREASE the Integral ACTION by INCREASING the Integral SETTING ... and we can go up as high as “32.767” for our Integral setting ... all of this boils down to the fact that we can get a LOT more Integral action by using the Independent Gains equation than we can with the Dependent Gains equation ... so if a system required a LOT of Integral action, then I could easily accept the argument that the Independent Gains equation would be the proper way to go ...

next ... some students like the idea that the Independent Gains equation will allow them to change the amount of Proportional action WITHOUT changing the amount of Integral action and Derivative action ... you can’t get the same effect with the Dependent Gains equation ... reason: even if you don’t like math, you should be able to tell from the formulas above that changing the Proportional action for the Dependant Gains equation will ALSO change the Integral action and the Derivative action ... specifically, the “P” term (“Kc”) is located OUTSIDE of the square brackets in the Dependent Gains equation ... (and thus the name: specifically, the amount of the Integral action and the Derivative action both DEPEND on the Proportional setting) ...

continuing on ... as I said, some students like the idea that the Independent Gains equation will allow them to change the amount of Proportional action without altering the other two actions ... they think that the PID will thus be a LOT easier to tune that way ...

simple analogy: suppose that we only have one shaker for the dinner table ... so we premix the salt and the pepper and put the mixture into the one shaker ... that would be like the “Dependent Gains” equation ... specifically, if you add more salt to your dinner then you’re going to automatically get more pepper too ... on the other hand, the “Independent Gains” equation would be more like having two separate shakers ... one for the salt and the other for the pepper ... easier to season ... easier to tune ...

still, I always force the students to do their hands-on loop tuning exercises using the “Standard – ISA – Dependent Gains” equation ... main reason: if the student ever has to tune a PID loop using an SLC or MicroLogix processor, then the Dependant Gains equation will NOT be available ... and so any “feel” that he’s developed for how to tune the loop will be of little or no advantage with the smaller systems ... I think it’s better to learn to drive a “clutch pedal” and “stick shift” than an “automatic” transmission ... the same reasoning applies to the “integer” and “PD” type control blocks ... it’s better to learn on the older “integer” type because there are tons of those still out there in the field ... but once we get used to doing it the “hard” way, we always work through the “easier” setups too ...

you said:
Quote:
did I waste your time for nothin' ??
no ... I’m just happy to have been able to help ...

you said:
Quote:
... your students are lucky
well, their bosses usually like the results ...

and finally:
Quote:
god bless you
thank you for the sentiment ... I can assure you that He has ... and He does ... every single day ...
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Old November 11th, 2004, 12:06 PM   #26
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If you are curious about the various forms of the PID equation based on manufacturer you might want to look at this link. It talks about a piece of software called Bestune, which is a PID loop analyzer and tuner. To come up with correct constants it need to know the PID form. So you tell it what controller you are using. This isn't an exhaustive list but it's a start and shows how SOME of the suppliers do this.

Keith
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Old December 14th, 2004, 03:36 PM   #27
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Would like to know more on the D in PID

Ron,
You did an excellent job in describing what the Integral setting does in the PID, and I was wondering if you could do the same with the Derivative? I am running RSLogix 5 version 3.21, and have an AB 5/20 processor. I am trying to loop tune a bunch of fans that we have on our system. A few of these fans seem to overshoot the SP all of the time and I am wondering if I could use the Derivative to stop this from happening. I have read in other posts that this will help to stabilize the loop, but I am not sure how it works or how to set it up. Thanks for the help.

Joel
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Old December 14th, 2004, 03:55 PM   #28
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Greetings Joel,

thanks for the kind compliments ... I’ll be glad to take a shot at the Derivative but it might be a day or two before I can get it all pulled together ... in the meantime, you can help me make it more specific to your system by answering a few questions:

1. are you using the “AB – Independent Gains” or the “ISA – Dependent Gains” equation for your PID? ...

2. are you using the “N-type” control block or the “PD-type” control block? ...

3. are you using “Setpoint scaling” – and if “yes” then what are the values of “Max Scaled Input” and “Min Scaled Input”? ...

those are likely to be the main points of confusion as we try to discuss this ... and if you can post your entire .RSP file that would really help nail things down ... I’ll understand if you can’t post it (proprietary code, etc.) but the more I know about your system, the more helpful the discussion is likely to be ... see you again in a day or two ...
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Old December 15th, 2004, 11:16 AM   #29
jvalle
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Hi Ron,

Thank you very much for the help. I have inserted my pid block configuration so that you can take a look at it. I hope that this helps. If you really would like my .RSP file I would need to send it to you because it is to large to post on here. Also Id like to give you a little info on the fan that I am trying to control. The fan is the main draft fan on a rotary kiln. Right now I have the fan pid reacting from an averaged draft signal that I am averaging in the plc. I did this because the fan has some pretty quick and drastic draft spikes, and I could not get the pid to work very well with the spikes happening. This was before I knew alot about setting up the pid. I am hopping now that I can get this to work alot better because the way it is right now it is very slow reacting to a change. Again thank you for your help and if you need anymore info please let me know.

Joel
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Old December 15th, 2004, 03:56 PM   #30
Ron Beaufort
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Greetings Joel,

I’ve opened the “PID Setup Screen” picture you that you posted ... this will help us get started ... your use of the “Dependent Gains” equation will make any discussion of the Derivative action a lot more useful to anyone who uses the SLC-500 platform ... the SLCs don’t support the “Independent Gains” ...

here are some more suggestions for you to consider ... try setting up a “Trend” in RSLogix5 and post the results (both a picture and data “snapshot”) to show us how your PID is responding now ... this post will give you some hints on how to set up the trend and capture the data ... I’ll be willing to bet that my distinguished colleague Peter Nachtwey would just love to get involved in this PID project ... basically: “if you post it – he will come” ... and that’s a GOOD thing ...

as for the .RSP file, have you zipped it and it’s STILL too big to post? ...

as for the picture you posted ... here are some possible “issues” to think about ...

(1) your setting for “Update Time” seems VERY high ... basically you’re telling the processor that you intend to execute the PID only once every 100 seconds ... I’ve got a hunch that this setting needs to be changed ... just as an "in the ballpark" tip: most fan control loops usually have their PID executed about once each half second or so ... specifically, a setting of “0.5” would be far more typical of a fan control loop than your current "100" setting ...

(2) since you didn’t post your .RSP file (yet) then I can’t tell for sure about this next one ... but I’ve got a hunch that the “trigger” that tells the PID when to execute is NOT set for “100” seconds ... specifically, I’ve got a hunch that your PID is being executed a LOT more often than the “Update Time” setting ... and the basic idea is that the two values (the “trigger” event and the “Update Time” setting) should both be set the same ... if they’re not the same, then the response of the PID is going to be “skewed” from what the specified tuning values would normally produce ... suggestion: read this post (and the one which immediately follows it) about PID “trigger” and “Update Time” issues ...

(3) your setting for “Proportional Gain” seems like it might be slightly low ... of course I can’t say for sure since I don’t know how your system responds ... but keep this particular possible “issue” in mind as you read the next one ...

(4) your setting for “Reset Time” seems VERY high ... the setting of “400” basically means that the Integral action will take a full 400 minutes (that’s over six hours) to “repeat” the response that the Proportional action produces ... and remember from issue #3 above, the Proportion setting is pretty low ... important: don’t forget that with the “Dependent Gains” equation you’re using, the BIGGER the “Reset Time” setting, the LESS response the Integral action will produce ...

(5) so basically the question now is: if you’re actually using the settings that we see before us for your PID, then how the heck is your system working at all? ... and I’ve got a hunch that the “trigger” timing is tied up in the answer somewhere ... about the only way for the settings that you’ve posted to give you any type of control at all would require that your PID is getting “triggered” into execution VERY VERY often ... a heck of a lot more often than the “100 seconds” that your current “Update Time” setting would indicate ... I’m just guessing here – but is your PID on an unconditional rung? ...

warning! ... please don’t just jump into the deep end of the pool and start changing things yet ... basic idea: if the loop is “sort of” working now, and you “fix” the “trigger” issue, then you’ll have to completely retune the loop ... likewise, if you “fix” the “Update Time” setting, then you’ll have to retune the loop ... the best course of action is to work all the way through this and have a step-by-step game plan before you start trying to change just one thing at a time ...

finally ... I sure would like to take a look at that .RSP file ...

PS edit ... for any SLC users who might be following this post ... the PID rung construction for the PLC-5 platform (like Joel is using) is quite different from the construction required for the SLC platform ... two specific differences: the rung for an SLC's PID should always be unconditional ... and ... the "trigger" which tells the PID when to execute is "built-in" to the SLC's PID (when using the "timed" mode) ... in the PLC-5 system, the PID requires that you provide some type of separate external "trigger" - usually a timer's Done bit ...
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Last edited by Ron Beaufort; December 15th, 2004 at 04:40 PM.
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