Pneumatic Relay System

a system designed to perform algebraic and logic operations on pneumatic signals that assume a finite number of values (usually two)—for example, the ambient pressure, which is made to correspond to “0,” and the supply pressure, which is made to correspond to “1.”

The first pneumatic relay system for practical use was developed in the USSR in the early 1960’s from the universal pneumatic relay of USEPPA (universal system of elements for industrial pneumatic automatic control). Such pneumatic relays make possible the achievement of all the elementary logic functions and the storage of pneumatic signals. This permits construction of any single-cycle circuits (logic converters, encoders, decoders, and matrices) or multicycle circuits (with counters, registers, and so on). The appearance of the universal pneumatic relay marked the start of the introduction of pneumatic automatic control in machine and machine-tool building, power engineering, metallurgy, and other branches of industry in which the automation of cyclical processes had previously been accomplished by electrical automation.

All pneumatic relay systems can be divided into two basic groups according to their design: systems constructed from elements with moving parts and systems made up of elements without moving parts, in which the interaction of flows is used.

Systems of the first group may use all-purpose elements, by means of which several elementary logic functions are realized, or specialized elements, which perform only one specific function. Simpler, cheaper, and more compact equipment can be constructed using a pneumatic relay system having the second kind of elements, but such systems have a large assortment of elements, which is not always convenient when constructing actual control equipment. Systems made up of universal pneumatic relays are more flexible and allow for interchangeability of elements, but in this case each control device has some apparatus redundancy and is larger and more expensive than equipment using specialized pneumatic relays.

Most pneumatic relay systems consist of a universal pneumatic relay and a pneumatic element that performs the logic operation “OR.” A system that uses flow (jet) components is made up not of individual elements but rather of modules that can perform not only the elementary logic functions but also more complex functions. The most common in the USSR are combined jet-diaphragm systems (the first such system, called Tsikl, was developed in 1972), which efficiently blend jet modules to achieve complex logic functions and a variety of memory circuits and diaphragm amplifiers that produce the output signals, restore signal levels, convert nonstandard signals into standard signals, and perform the simplest logic functions.