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ELECTRO-PNEUMATIC CIRCUITS
Sequences with blocking signals
In this section we analyze how to construct electro pneumatic diagrams with blocking signals and also here we
use memory circuits to control the duration of these blocking signals.
We examine the following cycle:
A + / B + / B – / A –
1
2
3
4
Figure 25
Observing the flow chart, it is possible to detect blocking signals that are generated by limit switches:
•
a1
: activated by the positive stroke of cylinder
A
, it enables
Phase 2
for the positive stroke of cylinder
B
however
obstructs its negative stroke during
Phase 3
.
•
b0
: activated by cylinder
B
in rest position, it enables
Phase 4
for the negative stroke of cylinder
A
however
obstructs the positive stroke during
Phase 1
.
Phase 1:
stroke
A +
The safety condition required to execute this stroke; the piston rod/piston of cylinder
A
must have completed its
negative stroke.
Line 1:
with the activation of the
I.C.
button, the coil of relay
X
energizes and, via contact
x
, the
I.C.
latches.
Line 2:
contacts
x1
and
a0
are closed, both solenoid
B1
of the solenoid valve and the coil of relay
Y
on
Line 3
energize. The solenoid valve changes over and the piston rod/piston of cylinder
A
completes the positive stroke.
The command to solenoid
B1
remains active until the coil of relay
Y
is energized.
The equation for this motion is:
I.C. * a0 =
A +
Phase 2:
stroke
B +
Line 3:
The signal of
Line 2
energizes the coil of the relay
Y
activating contact
y
, latching the signal on solenoid
B1
.
Line 4:
upon reaching limit switch
a1
, via contact
z1
, solenoid
B3
energizes, the valve changes over allowing the
piston rod/piston of cylinder
B
to complete the positive stroke.
The output signal from limit switch
a1
acts as a blocking signal on cylinder
B
as it is present during both of its
strokes. The equation for this motion is:
a1 =
B +
Phase 3:
stroke
B –
Line 5:
upon completion of the positive stroke, cylinder
B
activates limit switch
b1
which energizes solenoid
B4
of the valve.
Line 6:
solenoid
B4
and the coil of relay
Z
energize simultaneously, inverting the state of all the contacts
z
,
z1
and
z2
.
The opening of contact
z
on
Line 3
interrupts the latching of relay
Y
and consequently interrupts the energizing of
solenoid
B1
of the valve. The opening of contact
z1
on
Line 4
de-energizes of solenoid
B3
of the valve. The piston
rod/piston of cylinder
B
can now execute the negative stroke.
The equation for this motion is:
b1 =
B –
Phase 4:
stroke
A –
Line 7:
upon the completion of the negative stroke, cylinder
B
activates limit switch
b0
which energizes solenoid
B2
.
The piston rod/piston of cylinder
A
can now execute the negative stroke. The equation for this motion is:
b0 =
A –
Once the negative stroke has been completed and limit switch
a0
activated, the cycle resumes automatically.
By pressing the
F.C.
button, the latching of relay
X
is interrupted therefore eliminating the possibility to energize
solenoid
B1
, the cycle therefore does not restart.
6
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ELECTRO-PNEUMATIC CIRCUITS