Moving part air logic controls

Electrical and electronic devices normally control fluid power circuits. Relay logic circuits, programmable controllers, or computers are common control methods. Another way to control fluid power systems is with moving part air logic. Air logic controls perform any function normally handled by relays, pressure or vacuum switches, time delays, counters, and limit switches. The circuitry is similar, but compressed air is the control medium instead of electrical current.

Environments with high concentrations of dust or moisture are excellent places for air logic controls. There is no danger from explosion or electrical shock even in such environments. Water can splash on the controls with no effect on their operation. If there is danger of explosion, air controls cannot ignite the materials involved. Another place to use air logic is on machines that have cylinders or fluid motors but no electrical devices. Machines powered by air and controlled electrically must be supplied with both utilities -- and require two crafts to work on them. With air logic, a single craft works on the circuit and the machine parts.

A disadvantage of using air logic control is a general lack of understanding about how the components work and how to read the schematic drawings. If an air-controlled machine fails, very few people have the skill to work on it. Also, air logic with long control lines will have a noticeably slower cycle. Control lines longer than ten feet fill and exhaust slowly when compared to electrical signals. Another thing: control air quality must be above average for long trouble-free life.

What are air logic controls?

Air logic controls are basically miniaturized 3-way and 4-way air valves. The actions of the valves produce on or off functions -- like relays or switches do -- plus exhausting of the spent signal. The symbols used for air logic are similar to electronic symbols. Some manufacturers use modified electrical symbols and ladder diagrams to show circuitry.

Following are explanations of the basic air logic components with figures showing the ANSI logic symbol, an equivalent ISO graphic symbol, and a generic cross-sectional view of the element.

AND element

Figures 19-1 and -2 show two types of AND elements. An AND element must receive two inputs before there is an output. This assures that two functions have been completed before there is a command to continue the cycle. Another way of saying this is that there must be a signal at A and B before getting an output at C. For more than two inputs, connect AND elements in series. The first AND receives two signals and its output connects to one input of a second AND. The other input of the second AND receives the third signal making three inputs necessary before passing an output. The first AND takes two inputs but any additional input requires another AND. (See Figure 19-14 for an example.)

From the cross-sectional view it is seen that air entering the A or B port will push the dual-seat poppet onto a seat and block flow. However, when a signal is present at both the A and B ports, the lowest pressure signal will pass to the C port. This element may allow a small amount of air to pass when it receives the first signal if the dual-seat poppet is off the seat on the side from which the signal is coming. In most cases this is not enough of a signal to start the next function. If this is ever a problem, use the YES element discussed next.

YES element

Some manufacturers supply both types of elements -- calling the element in Figure 19-1 an AND, and designating the element in Figure 19-2 as a YES. The difference between the elements is that the AND in Figure 19-1 is passive because its output is always the lower of the two inputs. In contrast, B is always the output of the YES element's two inputs. Using this feature can amplify a weak signal because it pilots the valve open at the A port while the through signal at B comes from a full pressure supply. A YES element is called active because there is a choice of which signal passes to the output.

From the drawing in Figure 19-2 it is easy to see that a signal at the A or B port cannot pass. A signal at A only shifts the dual-seat poppet; a signal at B is blocked in the at-rest condition. Any air that was present at C exhausts through the exhaust port. The flexible diaphragm above the poppet keeps air that is entering A from exhausting or producing an output. This logic element can be used to amplify a signal because of the approximately 10:1 area ratio difference between the A and B ports. This means a 10-psi signal at A can shift against a 100-psi input at B.