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View Full Version : PLC5/40E PID CV stuck at 20%


p_logix
September 10th, 2016, 04:42 AM
Hello I am troubleshooting a PID. My set output percentage from my SCADA is 60%. However when my PID is in manual my CV is stuck at 20%???

I can put a number in my cv manually and my analogoutput to my field servo changes. It looks like the PID instruction froze???

Any help would be appreciated.

Thanks

Ron Beaufort
September 10th, 2016, 08:38 AM
It looks like the PID instruction froze???

this is just a GUESS ...

maybe you've conditioned the PID rung (with an XIC, XIO, etc.) since you've figured that you won't be "using" the PID anyway while in the "manual" mode ... that is a VERY common programming technique ... (it's also usually not the best way of programming) ...

so ...

if my GUESS is correct, then the poor old PID (bless its little heart) isn't able to move your Set Output value into its Control Variable location ... (the PID instruction has to be executed as TRUE by the processor in order to do that) ...

TIP: most programmers have their PID rungs being executed as TRUE on a TIMELY basis (in other words, "triggering" it once each half second or so) ... then - if they don't "need" the PID's output signal for some reason - they just don't "use" the CV signal that the PID produces ...

if this doesn't help, consider posting your ENTIRE project file (RSP extension) ... maybe we can spot something ...

good luck with it ...

robertmee
September 10th, 2016, 11:47 AM
What do you mean when the PID is in manual? If you are setting the SWM bit in the PID to put it in manual then it's doing exactly what it's supposed to. The CV will lock into the last value, and you can change it at will...That's the purpose of manual in the PID.

danw
September 10th, 2016, 12:21 PM
Robertmee has spelled it out, it's working like it's supposed to.

Manual Mode is the opposite of Automatic Mode.

Manual mode holds the loop's output at whatever it is when the transfer from automatic occurs (in manual my CV is stuck at 20%) and then responds to whatever form of manual adjustment is available (put a number in my cv manually and my analogoutput to my field servo changes).

It's working like it's supposed to, according to conventional control terminology.

Ron Beaufort
September 10th, 2016, 01:33 PM
once again - this is just GUESSWORK since we haven't seen the OP's program file ...

but ...

I'm pretty sure that the figure below shows the general ideas involved ... it would be well to keep in mind that the platform in question is a PLC-5 - and that there are significant differences between how that platform's PID is handled - when compared to the PID instruction for the SLC/MicroLogix platforms with which many people are more familiar ...

and along those lines, the PID instruction in the PLC platform normally gives its output signal in the range of 0 to 4095 ... so the value 819 shown at N7:75 (the CV) represents 20% of that range ...

also ...

as long as the PID rung stays FALSE (as shown) a value can be manually typed into the CV (N7:75) and the output signal to the field device will indeed change ...

but ...

if a value happens to be manually typed into the Set Output % location instead, then the output signal to the field device will NOT change ... which (if I'm interpreting the OP's post correctly) would be the condition that he is witnessing with his system ...

another big question is whether the OP is actually using the "Software Manual" mode - or the "Tieback" manual mode ... yes, the PID instruction for the PLC-5 platform has TWO (not just one) manual modes ... I'd say that fact might be contributing to the confusion here ...

finally - it would be nice to see the OP's actual program code - so that this "guesswork" could be eliminated ...
.

p_logix
September 10th, 2016, 01:53 PM
Thank you Ron and others.I managed to get it going last night. Y'all hit the nail on the head. When the Scada controller on the hmi was placed in manual they set a bit in the Plc. My PID in the configuration was set to manual. So once I put my PID in auto the cv updated. we had a power blip and these are redundant plc5s. I think I had a processor switch occur and the programmer must of not used first scan logic to place the PID in auto.


Sent from my iPhone

p_logix
September 10th, 2016, 01:55 PM
One other thing that was strange was my PID had only a configure tab? What am I missing?

p_logix
September 10th, 2016, 02:01 PM
This is once I placed in auto. It was built with an integer control block. What got me was the faceplate was in manual at the HMI. They entered in say 60%. The Set output value % would change but not affect the Current CV I figured it should match. So I had to change the mode in the configuration tab to auto.Then the faceplate in manual with a manual output would change the Current CV%. Thanks again!!!
41906

Ron Beaufort
September 10th, 2016, 02:04 PM
One other thing that was strange was my PID had only a configure tab? What am I missing?

the PID instruction for the newer PLC-5 models can be set up with a PD-type control block (example: PD9:0) ...

or ...

as yours was with the older N-type control block (N9:797) ...

the PD-type has a few more bells and whistles ...

glad you got it figured out ...