Well, I'm reading through the last link I posted, and I have a few things I'd like to correct/clarify....
Step 4 - Control Action : IMHO they have this wrong. A positive result of the two equations (SP-PV and PV-SP) will drive the output ON, So SP-PV (Setpoint higher than Process Variable) would be positive in,for example, a heating application, when you need to turn the heat on to raise the PV to the SP.
This is a Forward-Acting loop, and the most common, and used in heating, flow control, etc.
A Reverse-Acting loop (PV-SP), puts the output ON to drive the PV down to SP, like you would need to do for cooling, or some flow control applications, e.g. blending or dosing.
Step 6 - Loop Update Time : They say this is not critical in most applications. I beg to differ - it is one of the most critical parameters you need to set.
Not only do you need to set this parameter accordingly, you need to make sure the PID instruction itself is scanned at that same interval.
A PID instruction does calculations based on time - i.e. "what has happened since I was last scanned" - getting a mismatch between instruction scans and what you put in the loop update time will make your PID difficult to tune, and sometimes unstable.
Best way to do this in Logix5000 is to put your PID instructions in a Periodic Task.
possibly more to come as I read on......
