Externally limiting the control value in a PID

I work in the cement industry, and I have a infinitely variable valve controlling product flow to a pump in my reclaim system. The valve is controlled by a PID which adjusts the valve based on the outlet pressure of the pump.

Material flow is never stable, so I have a limit on the actual output instruction to the valve which closes it should the pump current rise above a preset level. I also have a MOV instruction between the PID and the output instruction limiting the aperture to the valve to a maximum of 59 degrees.

This works, but it isn't great because when the valve closes due to pump overcurrent and the output pressure of the pump drops, the PID drives the control variable past the external limit.

A better solution would be to externally limit the control variable in the PID, but I don't know if this is possible or how to do it. I'm using RSLogix5000 20.03 with a 1769-L35E processor.

Can anybody help me out with this?

Thanks

The PID instruction has CV high and low limits that you can set. I would use these.

Paully's5.0 said:

The PID instruction has CV high and low limits that you can set. I would use these.

Click to expand...

Thanks, that's better.

I'm using the tieback value as a manual control, so I'll have to intervene if the maintenance guys need to run it through it's full range, but this give us better control n automatic.

Assuming you are measuring the motor current and can use the PIDE function block, the solution is right at hand. See the section on 'multiloop selection' here: http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/logix-wp008_-en-p.pdf

What you're trying to do is called constraint control; your objective is to control outlet pressure subject to an upper current constraint on the motor.

Basically, you'll have 2 PIDEs. One is set up to control outlet pressure as is presently done and the other is used to control motor current. The two PIDE outputs go to a low select block whose output goes to the valve and to the CVPrevious tag of both PIDEs.

I hope this describes what you're looking for. Good luck and have fun!