RSLogix v20 - PIDE Cascade setup.

KristienP

Member
Join Date
Jan 2015
Location
Nova Scotia
Posts
44
Looking for some advice. I haven't setup a cascading PID loop since a classroom 4yrs ago..
Attached PDF is what I have programmed now. Typical field values are in RED.

SETUP: Booster Pump with Suction Pressure and Discharge Pressure. Want to maintain a Discharge Pressure SP. Suction Pressure only matters if it goes below the Suction Pressure SP, at which point it should ramp the Booster Pump down in an attempt to raise the Suction Pressure. There is also a Low Low Suction Pressure that shuts the Booster Pump down.
 
Cascade control isn't suited to this problem. What you have here is a constraint control problem; the PIDE is perfectly suited for this.
See the 'Multiloop Selection' section of this document:
http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/logix-wp008_-en-p.pdf


Basically, the suction pressure PIDE will be reverse acting, the discharge pressure PIDE direct acting, and you'll select the lower of the 2 PIDE outputs to go to the pump speed control (which is also fedback to both PIDEs).
Good luck!
 
Last edited:
Thanks for the tip!
Multiloop does appear to be better suited. I haven't seen that setup before. Odds are the suction pressure will never get that low... but the control has still been requested.

They originally stated just a shut down at low suction pressure, but a conversation with the engineer determined he actually wanted a ramp down at LOW followed with a shutdown at LOW LOW.
 

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