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> Sent by: foxboro-bounce@xxxxxxxxxxxxx 07/19/2007 12:41 PM Please respond to foxboro@xxxxxxxxxxxxx To foxboro@xxxxxxxxxxxxx cc 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 _______________________________________________________________________ This mailing list is neither sponsored nor endorsed by Invensys Process Systems (formerly The Foxboro Company). Use the info you obtain here at your own risks. Read http://www.thecassandraproject.org/disclaimer.html foxboro mailing list: //www.freelists.org/list/foxboro to subscribe: mailto:foxboro-request@xxxxxxxxxxxxx?subject=join to unsubscribe: mailto:foxboro-request@xxxxxxxxxxxxx?subject=leave