Logix5000 PID minimum control value question

[defcon.klaxon]

Hi all,

I have a PID loop in an AB ControlLogix processor and what I'd like to do is make sure that it doesn't all the CV to fall below a certain value (say, 50%). In the setup dialog box under the Scaling tab, there is the ability to put min and max CV values but I'm a bit confused at what they're asking. It asks for Max (at 100%) and Min (at 0%). Is that as simple as the upper and low limits for the CV, or is there more to it than that? The at 100% and 0% makes me wonder if that's going to affect some sort of scaling, but maybe I'm just overthinking things. Thanks for the help!

 

[Robb B]

It's just that easy. If you specify a lower limit on the CV, it will maintain that value. I often use that as a redundant low setpoint for VFD well pumps, as the pump requires minimum speeds.

 

[defcon.klaxon]

It's just that easy. If you specify a lower limit on the CV, it will maintain that value. I often use that as a redundant low setpoint for VFD well pumps, as the pump requires minimum speeds.

OK great, thanks Robb. Yeah this is controlling a VFD and minimum speed is exactly what I'm trying to adjust.

 

[Robb B]

As I mentioned, you may want to put a minimum speed as a parameter in the VFD directly, in case someone or something overrides your PID values in the future. It hasn't never happened to me...