Electrical and electronic devices, such as relay logic circuits, programmable controllers, or computers, normally control fluid power circuits. Fluid power systems can also 0be controlled with "Air Logic." These controls perform any function normally handled by relays, pressure or vacuum switches, time delays, counters, and limit switches. While the circuitry is similar, compressed air is the control medium instead of electrical current.

Environments high in dust or moisture are excellent places for air logic controls because practically no danger from explosion or electrical shock is possible even in these atmospheres. Water can splash on the controls with no effect on the operation. If there is danger of explosion, air controls can not ignite the materials involved.

Air logic can also be used on machines that have cylinders or fluid motors, but no type of electrical device. In such instances, two services are required because the machine is powered by air but controlled electrically. In cases where electrical and mechanical maintenance come under different labor grades, air logic is also ideal because different technicians work on different aspects of the machine -- one works on the circuit and the other handles the machine parts that are electrically driven.

Air logic does have its disadvantages; most common is the lack of understanding of how the components work and how to read the schematic drawing. If an air controlled machine fails, very few persons can work on it. Also, air logic with long control lines will have a noticeably slower cycle. Control lines longer than 10 to 15 ft fill and exhaust slowly when compared to electrical signals. In addition, air quality must be above average for long life.

Air logic controls are basically miniaturized 3- and 4-way air valves. The actions of the valves provide on or off functions like relays or switches. They also exhaust 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.

The following is an explanation of the basic logic components showing the ANSI logic symbol and ISO graphic symbol for a comparable directional control valve.

And, or, and not symbols

Figures 2-1 and 2-2 show two types of "and" elements, which must receive two inputs before it provides an output. This ensures that two functions have completed before there is a command to continue the cycle. This can also be described by saying "this input, this input, and that input must be present before getting an output. Connect "and" inputs in a series when using more than two inputs. The first "and" receives signals’ one and two while the output of this element hooks to one input of the second "and." The other input of the second "and" receives the third signal, making three inputs necessary before giving an output.

Some manufacturers supply both types of elements. This gives you Figure 2-1 "and" with Figure 2-2 designated as "yes." The difference in elements is that the "and" in Figure 2-1 uses the lower of the two inputs as an output. This is a passive element. In contrast, a "yes" element has two inputs which obtain an output, but the designer has the choice of which input pairs with the output. Using this feature can amplify a weak signal. The weak signal pilots the valve open while the through signal comes from a full pressure supply. The "yes" in this situation is an active element.

Figure 2-3 shows the symbol for an "or" element. A shuttle valve serves the same purpose as an "or" element. Both inputs to an "or" element provide an output. A pilot signal from two different sources can pass through to start the next function. This can also be described by saying this signal or that signal provides an output. An "or" element differs from an inline "tee" because an "or" passes either input to the output but does not allow the inputs to pass to eachother.

Stacking "or" elements allows for more than two inputs. Use an extra "or" element for each input after the first two signals.

Figure 2-4 shows the symbol for a "not" element, which is a normally open 3-way valve. An input signal or pressure supply will go through the valve until there is a pilot signal at port A. Pressurizing port A blocks supply and exhausts the output signal to atmosphere. "Not" elements will block a signal or supply as long as there is pilot pressure on the A port. The "not" always returns to a normally open condition without a pilot signal.

Replace a limit switch with a "not" element to indicate a cylinder is at the end of stroke. Pressure from the cylinder port goes to port A of the "not" element, holding it closed. As the cylinder moves to the work, pressure stays steady because of the meter-out flow control. When the cylinder contacts the work, the signal on port A drops, the "not" element opens and sends a signal to start the next operation. See Figure 2-21 and accompanying text to review a circuit using "not" elements to replace limit valves.

The cylinder can stop at any position and the "not" output signal will indicate its nonmovement. This will always happen whether the cylinder stopped where it should have or if it stalled by some other means. Because this can happen, take care when using a "not" element to replace a limit switch. In contrast, this feature can be advantageous when clamping different sized parts. Use a "not" element for applications where different work locations stop the cylinder.

Most manufacturers supply a different pilot ratio for a "not" element used as a limit switch. The valve function is the same but it shifts at much lower pressure. Some manufacturers make a special "not" element that mounts directly to a cylinder port. A port-mounted meter-out flow control used in conjunction with this special "not" element makes a compact installation.

Caution! Pressure control valves only show pressure buildup. When a positive location must be made, use limit valves.