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.
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.
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.
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.
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.
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.
We use cookies to analyze website traffic and optimize your website experience. By accepting our use of cookies, your data will be aggregated with all other user data.