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Old March 7th, 2005, 10:36 PM   #16
Ron Beaufort
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Greetings Jnelson:

Why divide by 500 and not 600 for instance?

simple answer:

the Allen-Bradley PID equation for the PLC-5 requires that its input must be ranged between 0 and 4095 ... on the other hand, the PID equation for the SLC and MicroLogix platforms requires an input range of 0 to 16383 ... before writing this thread, I decided to make the information useful to as many people as possible by working in units of “percent of full scale” ...

background information:

the actual Hotrod’s thermocouple transmitter is calibrated so that 0 degrees Fahrenheit results in a 4mA signal to the analog input card ... likewise 500 degrees results in a 20mA signal ... when working with the PLC-5 system, we often use a 1771-IFE analog input module ... this is configured so that a 4mA signal gives a data reading of 0 ... and a 20mA signal gives a data reading of 4095 ... this corresponds exactly to the PID’s required input signal range ...

when using the SLC-5/04 processor, we often use a 1746-NI4 analog input module ... this is configured so that the transmitter’s 4mA signal gives a data reading of 3277 ... and a 20mA signal gives a data reading of 16384 ... in order to properly scale this for the SLC’s required PID input range, the data from the analog input module is usually processed by an SCP (Scale with Parameters) instruction in the ladder logic program ... the data is thus scaled for a range from 0 to 16383 before being presented to the PID ...

actually I put quite a bit of thought into how to present these various scaling issues ... in the end, I decided that the “percentage of full scale” approach would be the most flexible method ... and hopefully would be understood by the majority of readers ...

and now a specific answer to your question based on the PLC-5 configuration:

suppose that we take a 75 degree input signal from the Ramrod (Hotrod simulator) and divide it by the “600” example that you mentioned in your question ... the result would be 12.5% ...

but ...

for a 75 degree input reading, the PID would actually be seeing an input signal of 614 (raw data) ... the PID would interpret this as a 15% input reading (614 divided by 4095 equals 0.15) ... we would obtain the same results as the PLC if we divided the 75 degree input by 500 ... but not if we divided by an arbitrary value such as 600 ...

and so if we were to use the “600” divisor that you mentioned, then all of the math in our examples would not correspond correctly with the values which the PID would actually calculate ... specifically, “12.5%” is not the same as “15%” ... that would be a bad thing ...

finally ... I think that I’ve answered your question ... but if not, please post again and give me another shot at it ...

2-B ?
Best regards, ----+----] [----+------------( )----
Ron | |
PLC Boot Camp - Retired | 2-B |

I once was lost, but now am found, was blind, but now I see.

Last edited by Ron Beaufort; March 7th, 2005 at 10:41 PM.
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Old March 7th, 2005, 11:01 PM   #17
Peter Nachtwey
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Just curious

What is RAMROD? I know it is a simulator but is it a spreadsheet, a product or what? I write simulators using Mathcad or Excel.

Second, what parameters can you change? It must have a system gain, a dead time and a couple of time constants. For instance, I modeled the Hotrod using a system gain, a dead time and two time constants.
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Old March 8th, 2005, 10:30 AM   #18

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Very usefull explanation ! Is it possible to have this information in a pdf ?

Thnx !!!
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Old March 11th, 2005, 08:35 AM   #19
ayman metwally
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Mr Ron,

I have a question and I hope you have the time to help me finding the answer.

I really understood this thread, and I got the concept that why “Proportional” control only will not eliminate the error.

In your example “the oven”, the temperature won’t just “keep right on climbing” because at some definite point - the amount of heat leaving the oven eventually BALANCES the amount of heat entering the oven ... it EXACTLY balances ... and that’s what causes the steady state condition.

I quote some words from your post, and they are perfectly clear to me.

Now, the problem is that my mind can’t apply this fact on other “Process variables” e.g. level, flow, pressure and so on.

To be more clear: I understand that the oven will continue to loose HIGER amount of heat due to the continuous INCREASE of temperature difference until the system achieves a balance or steady state condition, so PV (i.e. temperature) will stop changing which in turn will cause a constant error, but what would prevent other “PVs” from just climbing and climbing until they reach SP [using “Proportional” only]? Can you give me other example?

And finally, I have to thank you gain for this great post.
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Old March 11th, 2005, 08:45 AM   #20
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Ron Beaufort:

As always, you have done a masterful job of explaining complex ideas in a simple manner. I wanted to take just a minute here to THANK YOU, and APPLAUD you!

Now, where can I buy your book?
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Old March 11th, 2005, 12:55 PM   #21
jolio ST

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I enjoy your explaination! This is even better than my school notes. I gotta keep a tab on this thread. I'm sure students will benefit a lot from your teachings. You should just qualify well being a lecturer. Or are you one? Or are you a writer as well??

Sherine T.
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Reactive Load: Fuzzy Power Factor.
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Old March 11th, 2005, 07:27 PM   #22
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I am sure I am not alone in telling you I appreciate the obivious amount of time you put in to helping the less knowledgeable. I have had PID's explained to me many times, many ways. Never has it made more sense. I only wish that I had some knowledge to give to you in return. I could teach you how to do a good 'whip' over a doublejump on a dirtbike. It's alot like computers, there's always the chance for a crash.


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Old May 12th, 2005, 06:29 PM   #23
Ron Beaufort
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PDFs on PID ...

greetings to all,

since I first posted my “What Is P?; and I?; and D?” trilogy, quite a few people have asked for a handy .PDF file of each ... I’ve finally pulled those together for anyone who is still interested ... each one is about 20 pages long and roughly 1/2 Meg in size ... since this is way too big a load (and most of it is a "rerun") to impose on Phil’s server, I’ve posted them on our company website ... please note that the website is still “under construction” so don’t expect any glitz or glitter ... but an index to the “Sample Lesson” material is available as a selection near the bottom of the first page of the “Training” section ... this is the first time I’ve ever tinkered around with putting anything on a website, so if the links don’t work correctly, please let me know and I’ll do my best to fix them ... my boss is finally thinking along the “let’s just hire someone else to do the website” lines ... that’s NOT going to break my heart at all ...

anyway ... since the PDFs are so big, they might take awhile to download ... sorry - but that's the price you pay for the "overkill" approach ... and incidentally, the one on Integral action has quite a bit of detail added beyond my original forum post ...

for those of you who have asked, thank you for the kind compliments ... I hope that you’ll find this material helpful ...

2-B ?
Best regards, ----+----] [----+------------( )----
Ron | |
PLC Boot Camp - Retired | 2-B |

I once was lost, but now am found, was blind, but now I see.

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