Need help with Micrologix 1200 PID and SCALE

The SCP and PID are in Lad 3 which was already in the program and it was blank. I just added the rungs.

So I added the PID and and the SCP are in LAD 3

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I will say it again , you need to call that subroutine with a "JSR" instruction in ladder 2.

Have you done that?

Post your .rsp file zip it first.

Did you add a jsr in ladder 2 like Mickey showed in his post above? If not and there was not one than ladder is not being executed and your scp will not work even though you can see the input value changing and your manual output entry updating. It can be a little misleading when seeing things while online even xic and xio bits will light up in a ladder not being called by a JSR.

I will definitely add that tomorrow at work.

Your PID Loop

Also, it is good practice to have your PID instruction write to a variable rather than directly to the physical output. Often it is a good idea to set the analog output to zero when the PID loop is not running.

In the example shown we are writing the PID output to N7:21; then when the PID loop is off (state of loop is false, or manual mode for forcing your output which is a great tool as well) we can simply write 0% to the output (or any other value if poking a manual %out to your physical output). We are doing something a bit different in the example shown but the idea is the same: Set the physical output to some known value when PID loop is off... Assuming that you want to shut down the loop and start again from 0% output.

Hopefully this will help a bit. Boy, I miss working in a manufacturing environment, right now I work for an equipment OEM (Original Equipment Manufacturer) and its a bit different ....

Let us know how it goes ....

@ MICKEY and bce123, So I put the JSR instructions in the main ladder (LAD2) to jump to the main air dryer program in LAD 3 and PID control in LAD 4. The air dryer program cycle is about 3 hours. Will the PID loop be executed repeatedly without waiting for the main program to be done?

@L5K_Edit, I did change the PID output indirectly to an N file. We don't have an MANUAL/AUTO mode for this air dryer system; however, there is a manual valve for the heat exchanger.

@ MICKEY and bce123, So I put the JSR instructions in the main ladder (LAD2) to jump to the main air dryer program in LAD 3 and PID control in LAD 4. The air dryer program cycle is about 3 hours. Will the PID loop be executed repeatedly without waiting for the main program to be done?

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If the "JSR" is unconditional and you have no condition on the rung with your PID instruction then yes it will execute every time the rung is scanned depending on the "Loop Update" and "Time Mode" settings of your PID instruction.

Thank you!

Gentleman,

Thank you all very much for your help. The program is working properly now. I just need to get the PID tuned. I truly appreciate your time and knowledge. I would totally buy you all around of beer if I could.

Sincerely
NQT