Different Perceptions of PID Control in the Mechatronic and Process Control Worlds

It is not uncommon for graduate students on the mechatronics side of the control world to treat the Proportional plus Integral plus Derivative (PID) controller with a certain amount of disdain. This is not surprising since most control texts from this end of the control world treat PIDs as simple, basic structures, to be quickly replaced by more advanced methods. To that end, these texts devote only a handful of pages to the subject. It seems that – at least in the mechatronics world – PIDs are considered too simple for much interest in academia while practicing engineers do not seem to care why they were working.This is a far cry from the treatment of PIDs in the chemical and bio-process control worlds (CPC and BPC, respectively). At this end of the control spectrum, PID controllers are studied in far more depth obtaining entire books or book series. Despite this volumetric expansion of material, it seems that in the latter worlds, many of the issues and concerns one sees in the mechatronic world are treated as obscure corner cases. Depending upon the teaching text, issues of sampling and digital representation may have been completely omitted. There were other surprises. While PIDs were almost universal and standard, they were almost never unified or standardized. Furthermore, what seemed to limit performance was not the structure of the controller itself, but the lack of accurate system/process models based on repeated physical system measurements.However, the mechatronic and process PID goals and foibles were not that different once one considered the different system, time constant, and measurement constraints. We will discuss these issues with the goal of getting a more unified view of PIDs across our application domains. We will provide a handful of common PID forms and show how they are related, so that we can approach any PID structure with the same analytical approach. We will finally look forward to how PIDs can be used, not only as a fundamental teaching tool for explaining control outside of our research circles, but as a critical component for advanced control methods.