time delay circuit (application of time-dependent control valve)

TIME DELAY CIRCUIT

(APPLICATION OF TIME-DEPENDENT CONTROL VALVE)

As discussed in Section 12.6, pneumatic time delay valve is used in time based sequencing circuits to achieve the intended time delays between specific operations.

1. Circuit

Fig.13.13 illustrates a typical time delay circuit to provide the required time delay (say, 20 seconds) at the end of the extension stroke (so that the retraction stroke starts after the time delay set). This circuit provides continuous to and fro motion of piston, but with a set time delay only before the commencement of return stroke.

2. Operation

Extension: The compressed air is connected to the circuit through both sides of the valves V1 and V2. When the piston is at the left extreme position (as shown in Fig.13.13), the piston rod actuates the valve V4. The valve V4 applies pilot pressure to the valve V5 while the right hand side port is connected to the exhaust. Now the valve V5 allows compressed air on the blind end of the cylinder while the rod end is connected to the exhaust, hence the cylinder extends.

Time delay operation: At the end of the extension stroke, the piston rød actuates the valve V3 which connects the air supply to the right side of the pilot valve VS through a time delay valve V6. Due to the time delay valve V6, there is a time lag before which the actual supply is given to the valve V5, during which the cylinder remains in its extended position.

Retraction: At the end of this set time lag, the valve V5 is actuated due to signal from the time delay valve V6 with the left hand side pilot signal connected stroke of the cylinder, and the cycle is repeated automatically as long as the valve V2 is in open position. With the operation of the valve V2, the automatic operation of the cylinder is interrupted and when the compressed air is supplied to the right side of pilot of the valve V5 and the left hand side is being connected to exhaust through the valve V2, the cylinder is completed retracted.