PID Loop RSLogix 5000

I am trying to adjust my PID loop: The process variable is % solids in water. The control Variable is % VFD pump speed.

The problem with the PID loop is that it only looks at pump speed for the high limit and low limit. I want it to also take into account pump flow for the limits. I would somehow like to program it to only be able to go down to a certain flow rate - the % VFD limits fluctuate, sometimes 0% is a much different flow than other times.

Would this be solved by using cascading PID loops?

I Still would like to get a little direction on this. The problem is my low limit on my control variable is only pump speed. I would like to have the PID also reference another variable for a low limit - pump flow. Is it possible to tie in two different limits into this?

Thank you.

If your PID is jumping between High and low limis somethin is wrong with PID configuration.
Please download : http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/logix-wp008_-en-p.pdf
Read pages 7 & 8 for your second post.

I'd suggest looking at this:
http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/logix-wp008_-en-p.pdf

It sounds like you want to do multiloop selection, namely control %solids with the constraint of satisfying some lower flow limt. This will involve 2 PIDEs, one for %solids, the other for flow. The output of each PIDE goes to one high-select block; the output of the high-select block goes to the VFD and also to the CVPrevious field of each PIDE. The setpoint of the flow PIDE is set at about the low flow limit for your process.

Look over the above, think about it, and ask followup questions as needed if you think this is the path to take.

widelto said:

http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/logix-wp008_-en-p.pdf
Read pages 7 & 8 for your second post.

Click to expand...

... beat me to it...

Thanks for the suggestions.

"It sounds like you want to do multiloop selection, namely control %solids with the constraint of satisfying some lower flow limt. This will involve 2 PIDEs, one for %solids, the other for flow. The output of each PIDE goes to one high-select block; the output of the high-select block goes to the VFD and also to the CVPrevious field of each PIDE. The setpoint of the flow PIDE is set at about the low flow limit for your process. "

I am only familiar with ladder logic at this point. Is this method you describe above something that can be done with a standard PID?

LiftStation_18 said:

Is this method you describe above something that can be done with a standard PID?

Click to expand...

Unfortunately, no.