Pneumatic time delay valves
Pneumatic time delay valves are used to delay operations in time-based sequences. As in an electrical system, it is also possible to form in pneumatics an on-delayed and an off-delayed time valve, with a normally open or normally closed contact.
- As a step-enable condition within a sequence control
- As a reaction time, e.g. time-controlled two-hand start
- To compensate transitions like pressure / no pressure. This is especially important for large systems which have eventually long pneumatic lines.
On-delay time valve
Logic symbol on-delay
Compressed air flows into the pressure accumulator via the control connection 12. Since the air is compressible, a certain amount of time passes until the switching pressure has built up and the actuator (in this case a 2/2-way valve) switches through.
The pressure increases according to an e-function. It is possible to determine a time constant T and thus determine the switching time.
Off-Delay time valve
Technical implementation in pneumatics:
Compressed air flows into the pressure accumulator via the control port 12 - but now unthrottled. The timer switches immediately. When the control pressure at port 12 drops, the air escapes throttled from the pressure accumulator. That`s why it takes a while until the pressure falls below the switching value.
Application: Time-controlled two-hand locking
In this case, unlockable is only possible if button S1 and button S2 are pressed simultaneously within a certain time. This control is also available as a module as shown here.
|1 The two-pressure valve (logical AND) opens when both buttons S1 and S2 are pressed.
2 Via the logical OR valve, the pressure within the accumulator rises until a sufficient switching pressure is achieved to switch the 3/2-way valve within the control block. This corresponds to a switch-on delay.
3 The 3/2-way valve within the control block only opens when the two-pressure valve (AND function S1 and S2) switches on faster, than as a sufficiently large control pressure has built up via the OR function (S1 or S2).