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Repetitive signals
We analyze the following example with repeated movement of the same cylinders in different Phases.
As an example, we observe the following sequence:
A + / B + / B – / A – / B + / B –
The piston rod/piston of cylinder
B
oscillates repeatedly. The limit switches
a1
and
a0
both operate the main valve
of cylinder
B
in order to execute the positive stroke. The limit switch
b0
in
Phase 3
operates the main valve of
the cylinder
A
to execute the negative stroke
A –
, while in
Phase 6
the limit switch
b0
operates the main valve to
enable the positive stroke
A +
. The movement of cylinder
B +
is a function of the actuation of limit switches
a1
and
a0
, the movements
A +
and
A –
are a function of the limit switch
b0
.
We divide the sequence into groups and determine how many lines and memories are needed.
A + / B + / B – / A – /
B + /
B –
U1 /
U2 /
U3 /
U4
4 lines and 3 memories are required.
Figure 96
Phase 1
:
Line U1
is active, the limit switch
b0
is activated and fed from the compressed air network.
The output signal from the limit switch
b0
fulfils two functions:
• when
b0 AND
Line U1
is TRUE, then the
I.C.
valve has air supply to Start the sequence
A +
• when
b0 AND Line U2
is TRUE, then the movement
A –
is permitted.
U1 * I.C. * b0
=
A +
Phase 2
: the piston rod/piston of cylinder
A
executes its positive stroke, then reaches and actuates the limit switch
a1
whose output through the
OR
function, operates the main valve of cylinder
B
, the piston rod/piston of cylinder
B
executes the positive stroke.
U1 * a1
=
B +
Phase 3
: the piston rod/piston of cylinder
B
actuates the limit switch
b1
whose output signal generates the pilot
signal
S2
, which operates memory valve and charges it over, interrupting
U1
and activating
U2
, the limit switch
a1
is no longer fed with compressed air, this means that the main valve of cylinder
B
(being monostable) returns
to its rest position, the piston rod/piston starts its movement in the negative direction
B –
.
b1
=
S2
=
U2
=
B –
Phase 4
: the limit switch
b0
fulfils the second of the afore-mentioned functions; when
b0 AND U2
is TRUE, then
the movement of the piston rod/piston of cylinder
A
towards the negative position
A –
is enabled.
U2 * b0
=
A –
Phase 5
: the limit switch
a0
is fed by
U2
and with its actuation, the signal
S3
is generated, which operates the
memory valve, interrupting
U2
and activating
U3
. The output of
U3
, via the
OR
function, operates the pilot port
of the monostable valve of cylinder
B
, which by changing over initiates the movement of the piston rod/piston of
cylinder
B
in the positive direction
B +
.
U2 * a0
=
S3
=
U3
=
B +
Phase 6
: when the limit switch
b1
is reached, the following conditions apply; when
b1 AND
Line U3
is TRUE,
then the signal
S4
is generated, which operates the memory valve, interrupting
Line U3
and activating
Line
U4
. Activating
Line U4
, the limit switch
a1
is no longer fed with compressed air which means the main valve of
cylinder
B
(being monostable) returns to its rest position, the piston rod/piston starts its movement in the negative
direction
B –
. When the signal from limit switch
b0 AND
Line U4
is TRUE, the signal
S1
is generated, resetting
the memory cascade.
U3 * b1
=
S4
=
U4
=
B –
5
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CIRCUIT TECHNIQUE