What is in this article?:
- BOOK 2, CHAPTER 2: Air Logic Circuits
- Flip-flop circuits
- Anti-tie down air logic circuit, using logic symbols
- Anti-tie down air logic circuit, using ISO symbols
- Anti-tie down, non-repeat, flip flop air logic circuit
- Anti-tie down, non-repeat and flip flop air logic circuit with automatic cycling air drills
"Flip flop" elements, with their symbol shown in Figure 2-5 are double piloted 5-way valves that send supply air to either outlet port with a signal at pilot ports S or R. Supply can be system pressure or air from another logic element. The main use for a "flip flop" is to eliminate the first pilot signal to a directional control valve. This allows a second signal on the directional valve’s opposite pilot port to shift it back. "Flip flops," sometimes called "memory" elements, stay in their last shifted position even with no air supply. Whether the signal maintains or drops out, output from the "flip flop" stays the same.
The S and R signals stand for "set" and "reset." The "set" signal shifts the "flip flop" for a function; whether the signal continues or not, the element stays shifted. The "reset" signal returns the "flip flop" back to its original position until the next cycle.
"Flip flop" can also be used to set up a new cycle, allowing the operator to momentarily push the start buttons. Use this same "flip flop" to eliminate unwanted signals and set up the circuit for cycle completion as required.
Figure 2-6 shows another valve actually called a "memory" element, which is a normally closed 3-way valve with a built in shuttle valve. The shuttle valve uses the "memory’s" output air to hold it shifted once it receives an S "set" signal. A momentary "set" signal gives continuous pilot output. An R "reset" signal shifts the "memory" element to normally closed and exhausts output air. In addition, turning supply pressure off returns a "memory" element to its start position.
There are three different types of time delays in air logic control. Fixed- or adjustable-time delays are common in both normally closed and normally open configurations. Some time delays use an orifice and accumulator chamber for delays up to one minute. Some manufacturers use air actuated diaphragms and orifices that eliminate system pressure fluctuation inaccuracies.
A "one-shot" timer, shown in Figure 2-7, is sometimes called an "impulse timer." A "one shot" timer takes a signal and passes it on to the circuit. At the same time, the input signal goes through an orifice to an accumulator tank. The setting of the orifice and size of the accumulator give a certain time delay before the normally open 3-way valve closes. After a "one shot" times out and closes, it remains closed as long as it has an input signal.
Figure 2-7 shows an adjustable time delay before it loses its output. Leaving off the sloping arrow in the symbols makes it a preset time delay. Times range from to 2 or more sec on valves with preset time delays.
Many circuits uses "one shots" to eliminate opposing signals. When a valve receives a signal to extend to a cylinder, it resists a return pilot signal to itself until loss of the first pilot. Using a "one shot" element drops the extend signal shortly after iniatiation. However, when the short duratoinj signal meets a hard-to-shift valve, the time may not always be long enough to move the valve spool. The cycle will stall if the valve does not have time to shift. For best results, use a "flip flop" to drop unwanted signals after it performs a task. Figures 2-17 to 2-20 and accompanying text further describe "flip flop" valves.
Passing a signal through the element after timing stops is done with an adjustable, normally closed "time-on" time delay. Figure 2-8 shows the symbol for this element. A "time-on" delay is a preset fixed timer without the sloping arrow. Most anti-tie down circuits use a fixed time delay, thus forcing the operator to actuate both palmbuttons concurrently.
The symbol in Figure 2-8 shows an input A moving towards the blocked port of a 3-way directional valve. Signal A also moves to a meter-in flow control to fill an accumulator. After the accumulator is filled, pilot pressure shifts the 3-way valve, allowing air to pass on to the next operation. As long as the input signal stays on the time delay stays open.
Some brand of "time-on" delays use shop air to the normally closed port A of the 3-way valve while the signal to the timing section comes from another logic element or limit valve. This allows a strong passing signal to travel long distances or to quickly shift several other logic elements.
With a built-in accumulator tank, the time delay length is usually unjer one-to-one and one-half minutes. With added external accumulators, time delays up to 5 min are possible. The repeatability of long time delays using accumulators is poor. Often, diaphragm type timers go to 3 min with good repeatability.
With a normally open 3-way valve in place of a normally closed 3-way, the delay is "time off." Figure 2-9 shows the symbol for a "time off" delay timer. A continuous input to the supply gives an output until a set time after receiving a signal at A. When A receives a signal, the time delay starts and continues timing. When the accumulator fills, it closes the normally open 3-way valve and exhausts the signal. As before, a preset, non-adjustable time delay is available.
"Time-on" and "time-off" delays often are identical in appearance. The part number may be the only way to tell these units apart.
To get different functions, connect air logic elements together like the examples in Figures 2-10 and 2-11. These two common pairs might be familiar to anyone using air logic. A "nand" element, shown in Figure 2-10, uses an "and" to signal a "not." The term "nand" means "not this and this." As long as there are not signals at A and B, air passes. If signals A and B are present, the "not" closes and exhausts the output signal.
A "nor" element, shown in Figure 2-11, uses an "or" to signal a "not." The term "nor" means not this or this. As long as there is not a signal at A or B, air passes through the "not." If a signal is present at either A or B), the "not closes and exhausts the output signal.
Some other commonly used air logic elements include:
- Amplifiers to detect a low pressure signal (down to 3-in. water column) and send it on as an 80 psi signal.
- Pressure or vacuum sequence elements shift after reaching a set pressure or vacuum.
- Programmable controllers are combination elements that are used to design complex circuits with minimum knowledge of circuit design.
- Air-operated indicators show circuit condition and/or function. Several colors are available but none emit light.
The following text and images depict examples of air logic circuits, showing how some basic circuits perform machine control functions.