Logix5000 PIDE cascade loop + feedforward

[josepss]

Hi erverybody. My question is related on how to use Feedforward to add a value to the CV of a PIDE intruction. I think is not easy and it is not documented enough in rockwell automation website.
Explanation of the process:
PIDE level controller of a boiler that its CV is the SP ( Scaled) of the Flow PIDE which feeds water to the boiler, in other words its a cascade control.
The problem is that the fluctuations of the steam comsumtion afects a lot in the beavior of the control and PIDE contants (Gain and integral) are not efficient for big and fast variations.
I think in a formula ( k x steam flow) to add to the feedforward of the level PIDE controller, but need some help in how to do it.
thanks.

 

[Mickey]

Go here for help:
http://www.controlguru.com/?p=38

 

[Peter Nachtwey]

I think you are on the right track

If you use K*ActualSteamFlow K must be a constant that converts steam flow units to output units. In this case you can use K*ActualSteamFlow as a bias or just add it to the output of the inner loop.

If the inner loop is scaled so the PV and SP are in ActualSteamFlow units then you can just add the ActualSteamFlow to the SteamFlowSetPoint from the outer loop. Warning, the units must be consistent. This way the inner loop will see any changes in the ActualSteamFlow immediately and react.

In both cases the output from the outer loop should never be large because the inner loop and the ActualSteamFlow distrubance rejection should provide most of the output. If the outer loop's output is more that %10 then you should try to find a better estimate for flow or a better K.

You could also visit
http://groups.google.com/group/sci.engr.control/topics

 

[josepss]

thanks for the reply. the casdade is working and everything is running the way you mention. The problem is that I wanted to use the FF word of the PIDE instruction, but didn't work: nothing was added to the CV out of the primary loop!.
I tried to understand the Feedforward instructions from the PIDE instruction user manual but was not possible. Do you know how to use FF?
Thanks again for your help