The booklet was originally issued in 1983 as an internal report in the DuPont Company, to help engineers and technicians, who have no special training in feedback control, understand the basic considerations and limitations. It handily broke all records at the DuPont Company for number of requested copies (over 1200) when issued. That report was released to the public and published in 1990. It subsequently sold almost 17,000 copies. In 1992 it was expanded for DuPont to about twice the size. This report also broke distribution records in the Company, with over 2500 copies requested when issued. The part unique to DuPont was deleted and more was added to make the Second Edition, which was published in Dec., 1993.

SECOND EDITION

The second edition of "Controller Tuning and Control Loop Performance" has been extended in both directions from the first. Sections have been added for the very beginner and for the somewhat more experienced. It is about twice the size. Sections have been added on the what-to-do and how-to-do-it of tuning, to help the person who may have never done it before. Then interspersed throughout are paragraphs that extend some of the non-math concepts to the realm of math, or at least algebra. These sections explaining concepts in math (sometimes frequency response terms) are clearly identified to make them easy to skip. This Second Edition still stands on its own, of explaining the essence of feedback control, without referring to math. I hope these new references will help any reader who wants to bridge the gap from the nonmath to the math. I have tried to make this second edition appeal to readers whose background may not be the chemical processing industries. I know I can only partly succeed in this broadened scope, for all of my 40 years in the automatic control business were in that industry. I hope this booklet meets what I perceive as a need for more information on the beginning end of training on the subject of controller tuning and control loop performance.

This booklet on controller tuning and control loop performance stops where most books and courses on the subject begin. Too often this subject is introduced with math unfamiliar to the reader. That does not have to be - there are simple concepts to help those unschooled in the math to know and understand the basics of control system tuning, to appreciate the limitations and to know what can be expected.

TUNING CONTROLLERS IS MOSTLY SCIENCE. It consists of fitting the time and amount parameters of the controller to the time and amount parameters of the process. An open-loop test of the process yields the needed parameters, and simple tuning rules based on these parameters have proven to apply well to a large portion of industrial control loops. Tuning parameters can also be determined from a closed-loop test, though the test is not as thorough. For a large family of loops it is possible to predict what is likely to happen to performance when the process changes or when the tuning adjustments are set differently.

ALL CONTROL LOOPS WILL CYCLE IF THE CONTROLLER GAIN IS HIGH ENOUGH. The period of this cycle is called the natural period, and it largely determines the potential performance of the loop. The shorter the period the better. The natural period in turn is closely linked to the apparent (or real) dead time in the loop. It is paradoxical that the natural period is not determined by the largest lags in the loop, but rather by the dead time and the smaller lags. The potential performance of a loop is limited by certain lags in the loop, and trying to eke out better performance through tuning is often an exercise in futility.

TUNING RULES ARE DESIGNED TO GIVE REASONABLY TIGHT CONTROL. This may not always be the objective. Many, and perhaps most, loops do not need to be tuned tightly. However all loops need to be tuned as part of the process of putting the controller into operation. Most loops respond to changes in tuning parameters much like the response curves given in chapter 2. Consideration should be given, when contemplating retuning a loop, to what the justification for the effort is, and whether the desired improvement can reasonably be expected from tuning.

THERE ARE A FEW TYPICAL GREMLINS WHICH CAUSE LOOPS TO NOT BEHAVE IN THE TYPICAL FASHION. It is important to recognize these, for failure to do so can result in detuning a loop, not to mention a loss of faith in the scientific approach. You will not have to tune many loops before you run into one of these gremlins.

BE PATIENT IN LEARNING THE METHODS WHICH ARE NEW TO YOU. They do work, and the method of understanding loop performance will allow you to converse with others on a common ground, sharing your experiences. Otherwise tuning is just one isolated hit-or-miss experience after another.