In air logic control there are several different types of time delays. Fixed or adjustable time delays are common in both on delay (or normally closed) and off delay (or normally open) configurations. Some time delays use a fixed or variable orifice and an accumulator chamber to produce delays as long as one minute. Some types use air-actuated diaphragms and orifices to eliminate inaccuracies due to supply pressure fluctuations. (Most types depend on guesses or stopwatch setting procedures.)
The symbol and cross-sectional view in Figure 19-7 is for a one-shot timer (also called an impulse generator.) A one-shot timer takes a signal at A and passes it on to the circuit. At the same time, input signal A goes through an orifice to an accumulator tank. The setting of the orifice and size of the accumulator tank gives a certain time delay before the normally open 3-way valve under the accumulator tank is piloted closed. After a one-shot times out and closes, it remains closed as long as the input signal at A stays on.
The symbol in Figure 19-7 shows an adjustable time delay. Leaving off the sloping arrow in the symbol makes it a preset non-adjustable time delay. Times range from one half second to two or more seconds on valves with preset time delays.
Many circuit designs use one-shots to eliminate opposing signals. When a valve receives a signal to shift, the opposite pilot signal has no effect until loss of the first pilot signal. A one-shot element drops the first signal shortly after initiation, thus making the valve ready to accept the opposite signal. One problem is that if the short-duration signal meets a hard-to-shift valve, the time period may not be long enough to move the valve spool. When the valve does not have time to shift, the cycle stalls. For best results, add a flip-flop to drop an unwanted signal after it performs its task
Time-on delay element
Figure 19-8 shows an adjustable, normally closed time-on delay symbol and cross-sectional view .A time-on delay passes a signal through the element after timing stops. The symbol without the sloping arrow is a preset, fixed time-on delay. Most anti-tie-down circuits use a fixed time delay to force the operator to actuate two palm buttons concurrently.
The symbol in Figure 19-8 shows supply air going to the blocked port of a 3-way directional valve. A signal also goes through a fixed or variable orifice to fill an accumulator tank. After the accumulator tank fills, pilot pressure shifts the 3-way valve to allow supply air to pass to the next operation. If the input signal stays on, the output stays on.
With an integral accumulator tank, time delay length is usually around one to one-and-a-half minutes. With added external accumulators, time delays up to five minutes are possible. The repeatability of long time delays using accumulators is poor. Diaphragm-type timers often go to three minutes with good repeatability.
Time-off delay element
With a normally open 3-way valve in place of a normally closed 3-way, valve, the timer becomes a time-off delay. Figure 19-9 shows the symbol and cross-sectional view for a time-off delay element. A continuous input to the supply port gives an output until a set time after receiving a signal. The input signal starts to fill the accumulator tank through a fixed or variable orifice and when it is full, it closes the normally open 3-way valve and exhausts the signal.
Time-on and time-off delays often are identical in appearance. Checking the part number may be the only way to tell these units apart.