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Old February 6th, 2003, 03:02 AM   #1
russrmartin@hotmail.com
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Control Logix PID block not working

Hello. I have a PID block in a control logix 5555 that is not functioning. The processor is acting like it is very bogged down, as it changes ladders etc very slowly. We have a PID block that simply will not work. The output is a constant 100% when the block is enabled. The process variable display only line at the bottom of the PID block shows us an input of -327.0000. However, when we highlight the tag for the process variable above in the PID block and monitor it, it shows us that we are at about 290, which is correct. This is a temp application. I have looked at the input, and it is correct. After going thru the scaling process, it shows the 290 degrees just fine. I am baffled as to why I can monitor the actual process variable tag used in the PID and see a value of 290, and yet the PID block will not update it and set the output value accordingly. Be advised that the rung is true, and the enable bit for the PID is set. I have looked at other blocks that are functioning ok, and the value of both process variable display and by monitoring the PV tag are the same. Also, if the rung is not true, I can change the process varialbe display at the bottom of the PID block to whatever I want and that value stays. However, as soon as the rung goes true, the -328.lakd is written in again. Anyone have any ideas? Thanks in advance.

Russ
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Old February 6th, 2003, 08:31 PM   #2
Ken Roach
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Time for the cavalcade of troubleshooting questions, led by the two Great Troubleshooting Questions:

1. Have you called RSI technical support ? What did they say ?
2. If possible, ZIP up your file and attach it to a post. That way others can maybe help diagnose. At the very least, do a printscreen, save as a GIF file, and post a picture of the offending PID block.

Now for the rest of the questions:

3. Is this the only PID block out of several that is not working ? How many PID blocks are in the program ?

4. Are the PID blocks in a Periodic Task or in a subroutine of Continuous Task ?

5. Are these PID instructions in ladder diagram language, or are they PIDE instructions in function block language ?

6. What is the Loop Update Time setting on this block ?

Your description of the controller seeming "bogged down" suggests that maybe these PID blocks are being executed faster than they need to be... you're going to have to figure out what the appropriate execution period for each block is.
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Old February 25th, 2012, 10:38 PM   #3
pankz7
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hello sir, I am having some temperature controlled loop and want to use PID block given in my software for PLC(PhoenixContact 130 ETH, doesnt matter). I am inputting Gain,TI,TD,PV(scaled temperature from 4-20mA),SP (SetPoint constant) to the block.Now what should be the "CYCLE TIME" value for the block.It doesnt affect the operation of PID at run timt, If it does how would I interpret it..?
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Old February 26th, 2012, 08:27 AM   #4
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I've seen this before - too often...

I've seen this kind of thing before . I don't know what causes it but the solution is pretty easy. In cases I've seen, the data in the PID loop is invalid - ie. INF or similar.

What I've done to solve this is to create another PID loop identical to the one that's not working. Double check to make sure that all of the parameters match. Once this loop is in the PLC add a COP instruction to copy the new loop to the bad loop. This will clear the garbage in the bad loop. Delete the copy and the new loop and you should be fine.

Good luck.

(8{)} (: .)
(Yosi)
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Old February 26th, 2012, 10:16 AM   #5
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Quote:
Originally Posted by (8{)} ( .) View Post
I've seen this kind of thing before . I don't know what causes it but the solution is pretty easy. In cases I've seen, the data in the PID loop is invalid - ie. INF or similar.

What I've done to solve this is to create another PID loop identical to the one that's not working. Double check to make sure that all of the parameters match. Once this loop is in the PLC add a COP instruction to copy the new loop to the bad loop. This will clear the garbage in the bad loop. Delete the copy and the new loop and you should be fine.

Good luck.

(8{)} (: .)
(Yosi)
Yosi,

That may be a good solution, but I imagine (hope!)the OP hasn't been waiting by the computer for 9 years for your response.
Either he's found a solution by now or he's gone off and found honest employment somewhere else.

@pankz7
Your question is only very loosely related to a VERY old thread. Try posting your question in a new thread with more information, you'll probably get a better response. I have no experience with Phoenix Contact software, but for those that do, to answer your question about cycle time, they will need details about what kind of system you're trying to control (hydraulic pump, heat/cool, etc.)
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Old February 28th, 2012, 01:17 PM   #6
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Funny.

I have no idea how that happened. I must have pushed the >> button or something. Oh well... Thanks for the heads up

Cheers,

(8{)} (: .)

Yosi
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Old February 28th, 2012, 05:31 PM   #7
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Hey no worries, mate.

Actually no harm in resurrecting a dead thread if a good solution comes out of it.

Cheers,
Dustin
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Old March 1st, 2012, 10:05 PM   #8
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Thanks sir but I dont know how to generate the thread please guide of I an new to this website..
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Old March 1st, 2012, 10:14 PM   #9
dcooper33
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Quote:
Originally Posted by pankz7 View Post
Thanks sir but I dont know how to generate the thread please guide of I an new to this website..
Look near the top of the page when you first log onto the Live Q&A. Right above the list of threads is a button that reads "start a new thread". It does exactly what it says.

Cheers,
Dustin
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