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Title: Principles And Application Of Automatic Control
Author: C. H. Collins, Jr.
Source: 1964 Southwestern Gas Measurement Short Course (Now called ISHM)
Year Published: 1964
Abstract: Five elements are basic to pneumatically operated controllers. They are (1) A measuring element which measures the process variable to be controlled. The element can also position a recording pen on a chart or an indicating pointer on a scale. (2) A means of setting the desired control point on the measurement scale and a setting index to indicate the control point which has been set. (3) A detection system consisting of a differential linkage and a flapper-and-nozzle to detect the difference between measurement and set point. (4) A mechanism which feeds back a signal to the detection system to provide suitable control functions. And (5) an amplifying system, generally a pneumatic relay, which provides an output pressure of any value between three and fifteen pounds per square inch gauge. Usually, regulated air supply of seventeen to twenty pounds per square inch is required to operate this relay.

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