Re: [foxboro] Feedforward control question

I would think this application would be multiplicative, as you have 
described, though I though you had to connect all your feedforward inputs 
to the LOADx inputs on the FFTUNE if you wanted it to figure out the 
feedforward delays and lags.

I strongly second the motion for better/more friendly how-tos on FFTUNE 
and FBTUNE blocks.  These have been around for some time, but I've never 
used them because I have not had the time to sit down and do some 
simulation trial-and-error to try and figure out why they did not behave 
as I expected.  I don't know too many others who have used them, either. 
If they work well, it's  a shame, and potentially lost $$$ opportunities 
to get the most out of our systems.


Some of Foxboro's competitors now have similar offerings (and more), and 
they often have much-friendlier automated tools to help set them up 
(Emerson comes to mind).  I'd settle for some better documentation and 
availability of training on these blocks.


Corey Clingo
BASF Corporation






Gregory A Hurwitt <gregory.hurwitt@xxxxxxxx> 
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07/19/2007 12:41 PM
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Subject
[foxboro] Feedforward control question





We have a new distillation tower where the steam to the reboiler will be
controlled by a sump temperature to steam flow cascade loop.  The tower
will be run at various different rates; we would like to use the tower 
feed
flow rate as a feedforward control signal to the sump temperature
controller.  (Control blocks are PIDA's running in a ZCP270.)

The PSS for the PIDA block states that "Multiplicative feedforward
compensation for a load flow is commonly used in a composition or
temperature control loop."  Based on this and other statements in the 
PSS's
and Integrated Control Block Description documents for the PIDA/FFTUNE
blocks, I plan to:

1.  Connect the feed flow signal to the MULTIN parameter of the 
temperature
controller.  Define the range and units of the feed flow signal using the
HSCIN, LSCIN, and EIN parameters.

2.  Build an FFTUNE block and connect it to the temperature controller by
setting the FFTUNE's PIDBLK parameter equal to <PIDAname>.BLKSTA.

3.  Since the feedforward signal will be connected to the PIDA, I will not
connect it to one of the LOADx parameters of the FFTUNE.

Any comments/corrections on #1-3 above or on the general topic of
multiplicative vs. additive feedforward for this application?

Where do I go from here to "train" the FFTUNE?  The documentation is short
and dense.  Is there a more user-friendly "white paper" or other resource?

Should I also be considering the use of a FBTUNE block in conjunction with
(or prior to the implementation of) the feedforward signal and the FFTUNE?
If I can get reasonable tuning of the steam flow and sump temperature 
loops
under one (or more) typical load condition(s), can I get by without 
FBTUNE?

______________________________________________________________________________________


Greg Hurwitt
Engineering Associate

E-Mail:  gregory.hurwitt@xxxxxxxx
BASF Corporation
Freeport, TX


BASF - The Chemical Company






 
 
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