The Practical Application of Frequency Response Analysis to Automatic Process Control

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Abstract
The paper shows how the basic principles of frequency response analysis may be applied to automatic process control without the introduction of any of the mathematics which are used in the servomechanism field of study, and which have in the past discouraged its use in the process field. It is shown how the frequency response of any part of an automatic control system may be represented graphically, how the response of any simple element of a system may be calculated, and how the response of the individual elements thus obtained may easily be combined to give the characteristics of the whole plant. Alternatively, the frequency response of the plant may be plotted experimentally without interrupting its operation. By reference to a typical example, it is shown that all proprietary “three-term” controllers do not necessarily function as simple theoretical controllers, but that their performance may often be represented as that of an effective theoretical controller, and may, in any case, be shown as a frequency response diagram from which the necessary information is available to match the controller to any particular process. The method introduced is shown to give a convenient measure of plant controllability, and by means of plant analysis on this basis, it is suggested how automatic control may often be improved by better design of the process rather than of the controller. The design of automatic controllers is also treated, and it is shown how various design features may be compared in the light of their frequency response characteristics. It is suggested that future controllers should be designed to give a specified frequency response.

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