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◾
Introduction to Industrial Automation
7.10. Write a program in Boolean language so that at the first press of a push button a machine
starts to operate, at the second press of the same pushbutton the machine stops, and at the
third press of the same button it starts again, and so on.
7.11. Write a program in Boolean language so that with a momentary signal Start (I0.0) a
machine (Q4.0) operates continuously and objects are counted (I2.0). The machine must
stop automatically 5 min after counting 100 objects.
7.12. Examine if the LAD program shown in the figure has any logical or syntax error. Justify
your answer.
S
R
I0.0
I1.0
Q2.0
Q2.0
Q3.0
Q3.0
7.13. Convert the logic-flow diagram of the figure to a LAD program equivalent from a logical
point of view, where A = Q1.0, B = I2.0 and C = I3.0.
No
Start
A = ON
A = OFF
?
B = "1"
C = "1"
Yes
No
Yes
?
7.14. In an industrial production line, objects are transferred along a conveyor belt which operates con-
tinuously as shown in the figure. It is desired to count objects passing in front of photocell (I0.0)
in time intervals of 1 min. It is also desired to store the last three measurements (objects/min) in
memory locations or in a data block. Write the required program in Boolean language for two cases:
a. Each measurement is caused by momentary pressing of button b0 (I1.1).
b. Three measurements are performed automatically every 15 min.
I0.0
Conveyor
b
0
I1.1
7.15. Complete the RLO which is created by the CPU of the PLC during the execution of the
instructions of the program in the empty rectangles (dash line) shown in the figure. Please
also complete the status of digital outputs or auxiliary bits at the end of first scan cycle and
Basic Programming Principles of PLCs
◾
351
at the end of the first scan cycle after time T2 (60 sec) has elapsed. The input devices (two
sensors) are in their normal status.
A
I1.1
AN M1.1
A
I1.0
=
Q7.0
A
I1.1
AN Q7.0
L
“60 sec”
SR
T2
A
T2
=
M2.0
ON
I1.0
O
M2.0
A
Q7.0
=
M6.0
A
T2
AN
M6.0
=
M3.0
BE
I1.0=NO contact of a sensor
I1.1=NC contact of another sensor
7.16. A machine operates with the aid of a direct starting motor in the START/STOP procedure
of which it is desired to insert an intermediate state for “confirmation”. In particular, the
following specifications have to be satisfied:
a. If the motor does not operate (C = OFF) and the START/STOP button is pressed, then
the message “Surely you want to Start?” is activated. By pressing the START/STOP
button a second time, the motor starts to operate (C = ON) and the message turns off,
or by pressing the button CANCEL the message turns off and the motor remains in its
stop state.
b. If the motor operates (C = ON) and the START/STOP button is pressed, then the
message “Surely you want to Stop?” is activated. By pressing the START/STOP but-
ton a second time, the motor stops to operate (C = OFF) and the message turns off, or
by pressing the button CANCEL the message turns off and the motor remains in its
operation state. Write the required program in any language according to the previous
specifications and for the I/O connections shown in the figure.
CANCEL
PLC
Inputs
Outputs
0 V
+24 V DC
0 V
+24 V DC
Q1.0
I0.4
C
Surely you
want to Start?
Q3.0
Q4.0
START/STOP
I0.2
Surely you
want to Stop?
352
◾
Introduction to Industrial Automation
7.17. A digital output (Q2.0) of a PLC must be energized when either one of two digital inputs
I1.0 and I1.1 or both, have been activated. Two different programmers wrote the following
LAD programs. Examine if the two programs are quite equivalent.
I1.0
Q2.0
I1.1
I1.1
Q2.0
Q2.0
I1.0
7.18. The conventional automation circuit, shown in the figure, implements the EXCLUSIVE-OR
function according to whether the relay C is energized when either button A or button B is
pressed, but not if both buttons are pressed simultaneously. After defining the I/O connec-
tion status in the PLC, write the required program in Boolean language for achievement of
the same function.
B
B
C
A
A
7.19. The figure shows an object transport conveyor and its manual takeover. The conveyor belt
M is desired to operate as follows:
a. Objects are coming accidentally. If only one object comes (with detection by photocell
PC
1
) the conveyor stops until a worker removes it, and then the conveyor starts again.
b. If two or more objects come near each to other, then the conveyor stops (after detec-
tion of an object from PC
1
), but after removing the object in front of photocell PC
1
the
conveyor remains in the OFF state. The conveyor starts again only if the next object
in front of photocell PC
2
is also removed. Therefore, the unnecessary operation of the
conveyor occurs for a very short time, and the transfer of the object from position PC
2
to PC
1
is avoided. Write the required program for a PLC.
M
Conveyor belt
PC
1
PC
2
7.20. The figure shows the conventional automation circuit for four motors which start with the
classical Y/Δ procedure and which also have two directions of rotation. The motors are
desired to operate either automatically via sensors (two sensors for each motor), or manually
via the handling buttons. One sensor activates the left rotation and the other the right rota-
tion, while their simultaneous activation is considered impossible. The symbol Y/Δ
↔
inside
Basic Programming Principles of PLCs
◾
353
the boxes expresses the required automation for starting such Y/Δ motors. Since, the multi-
plicity of motors and the uniformity of operations are suitable for using modular program-
ming, write the required program using any kind of program organization units.
N
STOP
4
Left
4
Μ
4
Sensor 7
“Α”
“M”
RS
4
Υ/Δ
Right
4
Sensor 8
STOP
1
Left
1
Μ
1
Sensor 1
Right
1
Sensor 2
R
“Α”
“M”
RS
1
Υ/Δ
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