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Optimum Settings For Automatic Controllers
the American Society of Mechanical Engineers, 1942.

First published in "Transactions of the A. S. M. E.", November 1942.

An exact facsimile on Adobe ® is available for download.
"Optimum Settings For Automatic Controllers" – is the name of one of the most important publications in the history of automation, instrumentation, and control systems. Written by Ziegler and Nichols and published in the November 1942 issue of the Transactions of the American Society of Mechanical Engineers, it gave the "hit and miss" art of tuning controllers a practical basis. Ziegler and Nichols developed and published tuning rules for pneumatic PID controllers while working in Rochester, NY for Taylor Instruments, now a part of ABB. Although developed for pneumatic controllers, their rules are still widely used as a comparison for other methods. When Nichols died in April 1997, at age 82, and Ziegler not long after on December 9, at age 88, a chapter in industrial automatic control came to an end.
The ASME graciously granted permission for me to reproduce "Optimum Settings for Automatic Controllers", © 1942, on my website, walter(at)********(dot)ca. To generate this reproduction, a photocopy of the printed original has been scanned, run through an optical character recognition (OCR) program, formatted in MS Word, and edited to duplicate the original as closely as possible. The diagrams were scanned and embedded in the Word file in .jpg format. Finally, this MS Word "forgery" was converted into .html and .pdf formats.

I thank the ASME for allowing this important work to be made so freely available. The society can be reached at www.asme.org. I would also like to thank my daughter, Mika ********, for her patience with the OCR work.

Optimum Settings for Automatic Controllers

By J.G. ZIEGLER¹ and N. B. NICHOLS² • ROCHESTER, N. Y.

In this paper, the three principle control effects found in present controllers are examined and practical names and units of measurement are proposed for each effect. Corresponding units are proposed for a classification of industrial processes in terms of the two principal char-acteristics affecting their controllability. Formulas are given which enable the controller settings to be determined from the experimental or calculated values of the lag and unit reaction rate of the process to be controlled. These units form the basis of a quick method for adjusting a controller on the job. The effect of varying each controller setting is shown in a series of chart records. It is believed that the conceptions of control presented in this paper will be of assistance in the adjustment of existing controller applications and in the design of new installations.
A purely mathematical approach to the study of automatic control is certainly the most desirable course from a standpoint of accuracy and brevity. Unfortunately, however, the mathematics of control involves such a bewildering assortment of exponential and trigonometric functions that the average engineer cannot afford the time necessary to plow through them to a solution of his current problem.
It is the purpose of this paper to examine the action of the three principal control effects found in present-day instruments, assign practical values to each effect, see what adjustment of each does to the final control, and give a method for arriving quickly at the optimum settings of each control effect. The paper will thus first endeavor to answer the question: "How can the proper controller adjustments be quickly determined on any control application?" After that a new method will be presented which makes possible a reasonably accurate answer, to the question: "How can the setting of a controller be determined before it is installed on an existing application?"
Except for a single illustrative example, no attempt will be made to present laboratory and field data, to develop mathematical relations, or to make acknowledgment of material from published literature. A paper covering the mathematical derivations would be quite lengthy as would also a paper covering laboratory and field-test results. Work on these phases of the subject is still under way, and it is expected that the results will be published at a later time when convenient. It is believed advisable to publish the present paper without delay in order to make the information available for use by the many persons interested in the application of automatic-control instruments. To these persons the present subject matter is of much greater interest than the other phrases of the study which are being omitted.
To simplify terminology wewill take the mostcommon type of control circuit in which a controller interprets the movement of its recording pen into a need for corrective action, and, by varying its output air pressure, repositions a diaphragm-operated valve. The controller may be measuring temperature, pressure, level, or any other variable, but we will completely divorce the measurement portion of the control circuit and speak only of the pen movement in inches; 1 in. of pen movement might represent 1 or 1000 deg F, or a flow of 1 or 1000 gpm. The actual graduation will be of no moment in a study of control.
Ourcontroller will translate pen behavior into behavior of a valve; the relation between the two behavior patterns is determined by the setting of each control effect. The term valve covers any similar device, i.e., a damper or rheostat which must be operated by the controller in order to maintain correct process conditions.

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