Changing from SLC to CMPLX

If I am taking a program that worked fine in a SLC and moving it to a CompactLogix, is it safe to assume that the GAIN numbers in the PID should be about the same? (Plus or minus for small adjustments of course).

I realize that I have to set up a periodic task and match the loop update time, but I'm just asking about the 3 gain adjustments.

I am using the regular ladder logic PID (not the PIDE).

IF (that's a big IF) the triggering/timing is set up correctly ... AND ...

IF (another big IF) the I/O scaling is set up correctly ... AND ...

IF (yet another big IF) you select the Dependent Gains equation for the new PID ...

then yes, the Proportional, Integral, and Derivative settings should be the same ...

suggestion: post the existing RSS program file for the SLC system ... in most cases, it won't be too hard for us to check the original program to make sure that it was set up correctly ... if it wasn't, then you're basically going to be back to square one when you make the transition ...

good luck with your project ...

Thanks for replying Ron (I was hoping you would)...

I'm glad to hear that I should be setting the gains to "dependent" since I really thought "independent" was what I wanted...shows me that I shouldn't ALWAYS trust me gut feeling, haha.

This setup is to control the steam that heats water in a heat exchanger. Previously, they had the PID loop set up on an unconditional rung in the SLC, although I do not remember what they had the loop update time set at. In my new program, I put the PID loops in a periodic task that update once every second. I set the loop updates to 1 second as well. They had the Proportional at .5 and the Integral at .01 (no Derivative). So, those are the numbers I was planning on using when I installed the system (I don't install for another couple of weeks).

I have got the scaling setup as follows: I'm using a 1769-IF4 input card. I have the signal setup as "Scaled For PID" coming in. I believe, if I understand this right, that this will make it 0-16383. I have this as the Process Value to the PID loop. The output card I'm using is a 1769-OF4. From what I understand there is no "add-on profile" for this card in Logix5000, so I'll have to set it up as a generic card according to the manual. I haven't done that part yet, so I'm not sure about how to scale the output since it will not have a "configuration" tab on the menu.

I will see about sending you the program (although I'm not sure if I can due to company rules and such).

Just a side note, but with Logix level processors, I scale everything (Process Value and Setpoint) into actual engineering units (reals), and just use them in the PID. Just make sure to set the fields correctly in the "Scaling" tab.

Another hint, EVERY TIME you open up the PID configuration screen, get into the habit of going to the "Configuration" tab. It's probably a bug in Logix, but if you don't go to that tab, and let the fields populate, it will warn you about not having a loop update time set when you close the dialog.

here are a few more details we need to know in order to fully answer your original question (will the old tuning numbers work?) ... this will help us nail down the previous "timing/triggering" operation ...

question (1) ... how was the SLC's PID set up for "Time Mode" ? ...
(a) for STI ... or ...
(b) for Timed ...

question (2) ... was the PID rung located in an actual STI file? ...
(a) yes ... or ...
(b) no ...

question (3) ... if the PID was located in an actual STI file, how often was the STI set to execute? ...
(a) ___ (x10 ms) ... or ...
(b) ___ (x1 ms) ... or ...
(c) STI was not used ...

I'll try to help with the I/O scaling later as time permits ...

note to rdrast:

It's probably a bug in Logix,

Click to expand...

yep, and a highly annoying one at that ...