PLC5 PID Question

Does anyone know if .SO (as in PD20:0.SO) is always equal to .CV
(as in PD20:0.CV) when the PID is in automatic?

The reason for asking is I have come across a program that uses .SO in calculations but the PID has no hard M/A station nor any software faceplate associated with it. There is no logic which addresses SWM for the PID. It is in "auto" and I assume it is always in "auto" as I see no way of switching it to manual. It is just doing a calculation of a moving setpoint for another PID.

Any thoughts are greatly appreciated.

This is a classic case of what I am talking about in the tech support thread

The OP didn't post a link to the PLC5 PID information or explain any of the terms he is using. He didn't make it easy for us to help him. Ron B is going to have to guess at this one. Now I know I could answer the question if I just had the right information. There is little or anything about PIDs I don't know, but I don't know how AB implemented the PLC5 PID.

I think others feels the same way because no one has replied in over 3 hours.

Peter,

Here is a link
http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1785-rm001_-en-p.pdf

scottsull,

See attached PDF, this leads me to believe that it does (always)
track. Track the output PD20:0.OUT

Using the output of one controller to calc the setpoint of another
is usually called casade control not uncommon.
If I am wrong here I'm sure the AB guys will correct me.

Thanks Mickey

You have made it easy for me to help scotstull.

Scottsull, I think your intuition is right. Rockwell apparently has two variable for the current output instead of one. .SO appears to be the manual version of the CV. This must make it easier for Rockwell to go between manual ( open loop ) and automatic ( closed loop ) and back without having any discontinuities in the output ( bumpless transefer ).

I keep only one variable that has the current output on our product. This avoids the confusion that Scottsull has.

I still don't know what a M/A station is or why a PID would have a 'face plate'

When going from closed loop to open loop we use the last closed loop value as the new open loop value to avoid discontinuities in output. This is common practice.

M/A = Manual Auto station.

Intended to allow a purely manual CV control eg a Potentiometer, or the position feedback from a valve or positioner of some sort that is being moved by some purely local means to feedback to the PID loop, so that the loop CV can track the device, and when it next goes to loop control a bumpless transfer is achieved.

This is a "hardware manual" mode where there is some actual hardware feedback from the positioner which is being moved by some means other than the PID CV, as distinct from the "software manual" mode where the operator is writing to the CV and this is still controlling the device.

The "faceplate" refers to a pre-built standard ActiveX or software HMI object that gives the operator display and control over the PID loop.

The other difference, as mentioned before, is that the .SO value is unaffected by the clamps which constrain actual .CV output.

Ignoring the clamps, the simplest description is:
In Manual Mode, .CV tracks .SO
In Auto Mode, .SO Tracks .CV

Thank you all

Thanks to all who responded. I think with your help and a pointer to the documentation I now know what is going on in the logic.

Is there a way I am supposed to put an end to this thread or does it just go on and get archived for all others to view in the future?