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ELECTRO-PNEUMATIC CIRCUITS
Emergency control
As previously noted with pneumatic circuits, the emergency command can be executed in a variety of ways.
The emergency device is a manually operated contact, although in this case it is not a valve but an electrical
contact, (generally
NC
). The opening of this contact interrupts the power supply to the circuit, (the main circuit
and/or logic circuit located downstream from the command).
The
emergency
control may be utilized in the following ways:
• Stop during Cycle with restart after the command is removed. The system will restart from the stopped position.
• Stop during Cycle with restart after removing or resetting the command.
The system can restart from either the stopped or initial position, with the reset command which repositions the
piston rod/pistons of all the actuators to the cycle start position.
• Immediate repositioning of the piston rod/piston groups. The monostable 5/2-way solenoid valve with spring
return is used.
Restarts from the initial start position.
• Stops the piston rod/piston of the actuators in their present positions at the moment the EM is activated.
5/3-way CC solenoid valves are employed. The system can restart from its stopped or initial position with
the reset command which repositions the piston rod/pistons of all the actuators to the cycle-start position.
• Completion of the strokes and stop at the end positions. In this case bistable 5/2-way solenoid valves are used.
The system can restart from the current or initial position with the reset command which repositions the piston
rod/pistons of all the actuators to the start- cycle position.
The
Reset
phase is established by a return sequence, i.e. by repositioning (the various devices) back to the start
position. It is at the discretion of the machinery/equipment designer to select the most suitable emergency functions
based upon the required safety conditions.
We examine a circuit comprising two double acting cylinders controlled by a bistable 5/2-way solenoid valve.
Figure 20
Pos. 1:
Emergency Stop during Cycle function.
The cycle develops as observed in previous paragraphs. Activating the
EM
interrupts the power to all the limit switches
of the system thereby releasing the latching of relay
X
. The piston rod/pistons of the cylinders terminate their current
stroke and the cycle stops. Removing the
EM
command powers the limit switches and the cycle restarts, stopping
after the last phase. As relay
X
is no longer latching, the
I.C.
button must be pressed to restart the cycle.
A + = B1 = EM * I.C. * b0
B + = B3 = EM * a1
A – = B2 = EM * b1
B – = B4 = EM * a0
Pos. 2:
Stop during Cycle, restart from the Stop position or after Reset.
The connection of the
EM
command has been modified with respect to the previous case. When activated the cycle
stops, leaving the piston rod/piston to finish its stroke, the operator can then decide how to proceed.
By removing the
EM
command, the limit switch once again has power and the sequence restarts, stopping after
the last phase completes. As there is no latching relay to restart
X
it is necessary to press the
I.C.
button.
If the
EM
command is maintained, the actuators can return to their original positions via the
Reset
command,
which by being activated energizes the coil of relay
Y
, closing contacts
y
and
y1
, energizing solenoids
B2
and
B4
changing over their respective solenoid valves, reposition the piston rod/piston of cylinder
A
and
B
in the negative
end position. To restart the cycle, the
EM
command must be removed and the
I.C.
activated.
A + = B1 = EM * I.C. * b0
B + = B3 = EM * a1
A – = B2 = EM * b1 + EM * Reset
B – = B4 = EM * a0 + EM * Reset
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ELECTRO-PNEUMATIC CIRCUITS