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CIRCUIT TECHNIQUE
Techniques to eliminate blocking signals
The signals generatedby the limit switches remainactive for all the time they remainactuatedandenableprogression
through the various stages of the sequence. However the long duration of these signals can present a problem in
those situations where they prevent the repositioning of the device upon which they are acting; in this case they
are defined as
blocking
signals.
These signals must be identified on a schematic flow chart to avoid possible “conflicts” which could obstruct the
continuation of the cycle.
To eliminate the blocking signals any of the followingmay be applied:
• remove themechanical control which operates the limit switch
• control the air supply of the limit switch
• control the signal they generate.
Consider the following sequence:
A – / B+ / B – / A+
1
2
3
4
Figure 75
Drawing the flowchart.
Phase 1
presence of the
I.C.
command and limit switch
a1
A – I.C. * a1
=
A –
Phase 2
presence of limit switch
a0
B+ a0
=
B+
Phase 3
presence of limit switch
b1
B –
b1
=
B –
Phase 4
presence of limit switch
b0
A+
b0
=
A+
Phase 1
:
A –
To activate “start” (of movement) requires the output of the limit switch
a1
to be actuated together with the Start
Cycle command
I.C.
(Inizio Ciclo). To accomplish this, either place two valves in series or their two respective
outputs in a logic AND element. The piston rod/piston of cylinder
A
completes the negative stroke.
A –
=
I.C. * a1
Phase 2
:
B+
Once limit switch
a0
has been reached by the piston rod/piston of cylinder
A
, a signal is transmitted enabling the
positive stroke of the piston rod
/piston of cylinder
B
.
B+
=
a0
Fase 3
:
B –
Once limit switch
b1
has been reached by the piston rod/piston of cylinder
B
, a signal is transmitted enabling the
negative stroke of the piston rod/piston of cylinder
B
.
If theactuatingdeviceof the limit switchwere tobeabidirectional roller lever, the cyclewould stop. The command
given by limit switch
a0
that generates themovement of
B+
will still be present on the valve of cylinder
B
.
Consequently, the output signal from limit switch
b1
will be unable to change the state of the main valve to
executemovement
B –
. In this case the signal
a0
is
blocking
and therefore highlighted by a red circle.
B –
=
b1
Fase 4
:
A+
Presumingwe have resolved the problem of the previous step.
Once the piston rod/piston has reached the limit switch
b0
of cylinder
B
, a signal is transmitted enabling the
positive stroke of the piston rod/piston of cylinder
A
. The cycle has completed, but it is unable to restart as the
output signal from limit switch
b0
is
blocking
themovement of cylinder
A
. The blocking signal is highlighted by a
red circle as in the previous case.
A+
=
b0
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CIRCUIT TECHNIQUE