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Jan
15

Five Weeks in Five Minutes

Time is precious so here is your chance to learn in five minutes what took me five weeks of investigation. While most of these thoughts were banging around in my mind for last couple of decades, they might never have congealed if not for some triggering thoughts from my colleagues Terry Blevins and Willy Wojsznis and some knowledge discovery in my favorite laboratory, the virtual plant. All of this stuff has been discussed to some degree in last year’s blogs with more detail available in my Control magazine articles and Control Talk columns, the book New Developments in Bioprocess Modeling and Control, and Advanced Application Notes 1-4. The notes and presentations based on my ISA books as they become available are free for the downloading at http://www.modelingandcontrol.com/2009/03/application_notes.html

(1) All the most popular tuning rules reduce to the same equation for the controller gain for maximum load rejection.

(2) While the ultimate performance of a loop is proportional to the dead time squared, the actual performance is set by the tuning (reset time and controller gain).

(3) Nearly all studies on the beneficial effect of improving loop dynamics retune the controller for better performance. If the controller was not retuned, there would be no immediate recognizable benefit in most cases.

(4) You can estimate the amount of dead time you can add before the loop performance deteriorates for unmeasured disturbances by comparing the present controller gain to the maximum controller gain for maximum load rejection.

(5) I would be out of a job if there was no dead time or disturbances, because barring any extenuating circumstances the controller gain could be set higher than you have ever seen or the control valve just sequenced to predetermined positions.

(6) Continuous temperature, concentration, and pH control loops on large well mixed volumes are best treated as “near integrators” for tuning.

(7) The use of dynamic reset limiting and a delayed external reset can provide dead time compensation that is easier to implement and more robust than a Smith Predictor. If the valve position PV for single loops and the secondary loop PV for primary loops is used for external reset, it prevents the controller from outrunning the valve or secondary loop and the dead time compensation is more accurate.

(8) If the model dead time used for the Smith Predictor is 100% larger than actual, the Smith predictor can break out into rapidly growing oscillations. A model dead time that is too large besides too small can cause instability in this predictor.

(9) The controller gain setting must be significantly increased beyond the normal maximum controller gain to realize the benefit from dead time compensation.

(10) A zero discharge flow causes the mass to increase as a batch progresses, which causes concentration and pH control to have an integrating response. The integrating process gain here is inversely proportional to level. For vessel pressure control where the vent valve pressure drop is large or critical, the pressure response’s integrating process gain is proportional level because the vapor space volume is decreasing. However, for temperature control where there is significant heat release and cooling capability, vessel level has little effect on the controller gain except when it is above or below the heat transfer surfaces (e.g. coils) because the effect of more mass is cancelled out by more heat transfer area covered by liquid.