Greg: Last month, we introduced you to the motivation and signals for a proportional-integral-derivative (PID) controller to do what can be an incredible job of providing basic and advanced regulatory control. There is unfortunately a considerable lack of understanding of what the PID can do and should do, complicated by confusion from academia, suppliers and users not being on the same page. The key to spreading the beneficial knowledge gained over decades is to get the leading experts in industrial applications to document their knowledge, and develop consistent terminology and essential concise explanations of functionality in the ISA 5.9 PID Algorithms and Performance Technical Report. Here we begin to get to the heart of the matter by having one of the experts who wrote the section on PID Algorithms explain the informational objectives he sought to resolve and the resulting knowledge gained.
Peter Morgan, what are points of confusion that you resolved in the PID Algorithms section?
Peter: While the PID algorithm in its various forms has been the workhorse of virtually all closed-loop control applications for nearly a century, the fact that there's been no standard convention for naming each of the algorithm forms has led to confusion and sometimes heated debate, when the adopted form name suggests the property of the algorithm. A good example of the latter is the common use of the form name “non-interacting” for the parallel algorithm. This name, suggesting the behavior of the algorithm, for gain, reset and derivative parameter adjustments, rightly expresses the behavior of the algorithm from a time domain perspective, but not from a frequency domain perspective, when, if gain is adjusted, the phase shift across the controller changes at a given frequency, requiring compensating adjustment in reset and derivative parameters during the tuning process.