Air logic valve combinations
To get different functions, connect air logic elements together like the examples in Figures 19-10 and 11. They illustrate two common pairs that might be familiar to anyone using air or electronic logic.
A NAND output, Figure 19-10, uses an AND to signal a NOT. As long as there are no signals at A and B, air passes. If signals are present at A and B, the NOT closes and exhausts the output signal. The term NAND comes from the phrase: not A and B.
A NOR element, Figure 19-11, uses an OR to signal a NOT. When there is no signal at A or B, air passes through the NOT element. If a signal is present at either A or B, the NOT closes and exhausts the output signal. The term NOR comes from the phrase: not A or B.
Other air logic components:
Amplifiers detect low-pressure signals (as low as a 3-in. water column) and send them on as an 80-psi signal. They can be used with gap sensors to detect whether a part is breaking an air-bleed signal.
Pressure or vacuum sequence elements shift after reaching a set pressure or vacuum level. They can be used to indicate an actuator has completed its stroke. They should not be used when the actuator positively has to finish its task before the next operation starts.
Air-operated indicators show circuit conditions and/or functions. Several colors are available, but none emit light.
The drill circuit in Figure 19-12 uses most of the components discussed in this chapter -- plus other valves from other parts of this book. For safety, this circuit requires both of the operator's hands to be on the palm buttons simultaneously before a part can be clamped and drilled.
When the operator pushes Start 1 and Start 2 palm-button-operated, normally closed 3-way air valves, they send a signal to the AND1 and OR1 elements. One OR1 output goes to the N.C. time delay and it starts timing. At the same time, both signals from the palm-button valves satisfy AND1, so it sends a signal through NOT2 to the flip-flop to start the cycle. The output from NOT2 also goes to NOT1 and closes it. This keeps the N.C. time delay (which will time out in 0.5 to 1.0 sec) from sending a signal to block the start signal from AND1 to the flip-flop. When the flip-flop shifts, it sends a signal to A+ to start the clamp extending. The same signal also goes to LV1, where it is blocked. When the clamp reaches the part, it mechanically shifts LV1.
After the clamp cylinder extends fully, pressure in its rod end drops the inlet pressure to Start 1. If the operator still had the palm buttons depressed, the loss of air at Start 1 would be the same as releasing that button. Releasing either Start 1 or Start 2 causes AND1 to stop sending a signal to NOT2, which drops its signal to the flip-flop and to NOT1. When NOT1 loses its signal, the N.C. time delay sends a signal through NOT1 to close NOT2, thus preventing a later signal from the palm buttons through AND1 from giving another start signal. This same scenario also requires the operator to let up on both palm buttons anytime they are not depressed simultaneously within a 0.5- to 1.0-second delay. Now the anti-tie-down circuit also is anti-repeat.
LV1 sends a signal to start all three drills on their drill cycle. When the drills leave their home positions, they put out a signal that goes to AND2, AND3, OR2, and OR3. When AND2 and AND3 receive all three drill signals, they send a signal to the flip-flop that shifts it back to its home condition. Its output now goes to the inlet of NOT3. Any or all of the outputs of OR1 and OR2 hold NOT3 shut.
The three drills continue forward until they finish their operations. They then retract automatically (as commanded by their own internal valves). As each drill reaches home position, it drops its run signal to OR1 or OR2. When all drills have retracted fully, NOT3 opens and sends a signal to A- to unclamp the part.