Quantitative PID Tuning Procedures

30.3. QUANTIT QUANTITA ATIVE PID TUNING PROCEDUR PROCEDURES  ES 

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Quan Qu antit titati ativ ve PID tun tuning ing proce procedu dures res

A   quantitative  PID tuning procedure is a step-by-step approach leading directly to a set of numerical values to be used in a PID controller. These procedures may be split into two categories:   open loop and   closed loop. An “open loop” tuning procedure procedure is impl implemen emented ted with the controller controller in man manual ual mode: int introduci roducing ng a stepstep-cha change nge to the controller controller output and then mathematica mathematically lly analyzing analyzing the results of the process variable response to calculate appropriate PID settings for the controller to use when placed into automatic automatic mode. A “closed loop” tuning procedure is implemente implemented d with the controller in automatic mode: adjusting tuning parameters to achieve an easily-defined result, then using those PID parameter values and information from a graph of the process variable over time to calculate new PID parameters. Quantitative PID tuning got its start with a paper published in the November 1942  Transactions  written by two engineers engineers name named d Zieg Ziegler ler and of the Ame Americ rican an So Society ciety of Me Mechan chanic ical al Engin Enginee eers  rs    written Nichols. “Optimum Settings For Automatic Controllers” is a seminal paper, and deserves to be read by every serious student of process control theory. That Ziegler’s and Nichols’ recommendations for PID controller settings may still be found in modern references more than 60 years after publication is a test testamen amentt to its impact in the field of industrial industrial control. control. Alth Although ough dated in its terminolog terminology y and references to pneumatic controller technology (some controllers mentioned as not even having adjustable proportional response, and others as having only discrete degrees of reset adjustment rather than continuously variable settings!), the PID algorithm described by its authors and the effects of P, I, and D adjustments on process control behavior are as valid today as they were then. This section is devoted to a discussion of quantitative PID tuning procedures in general, and the “Ziegler-Nic “Ziegler-Nichols hols”” meth methods ods in specific. It is the opinion of this autho authorr that the Ziegler-Nic Ziegler-Nichols hols tuning methods are useful primarily as historical references, and indeed suffer from serious practical impedimen impedi ments. ts. The most serious reservatio reservation n I hav havee with the Ziegl Ziegler-Ni er-Nicho chols ls methods (and in fact any algorithmic procedure for PID tuning) is that these methods tend to absolve the practitioner of responsibilit responsibility y for understanding understanding the process they intend intend to tune. Any time you provide people with step-by-step instructions to perform complex tasks, there will be a great many readers of those instructions tempted to mindlessly follow the instructions, even to their doom. PID tuning is one of  these “complex tasks,” and there is significant likelihood for a person to do more harm than good if  all they do is implement a step-by-step approach rather than understand what they are doing, why they are doing it, and what it means if the resul results ts do not meet with satisfacti satisfaction. on. Plea Please se bear this in mind as you study any PID tuning procedure, Ziegler-Nichols or otherwise.