Programs 269
LD
ANI
OUT
K
LD
OUT
END
X404
X405
C460
4
C460
Y433
(*Sixth rung*)
(*Four bottles counted.*)
(*Seventh rung*)
(*End rung*)
The Siemens program in instruction list is:
A
O
A
AN
AN
(AN
ON
)
=
AN
=
A
O
=
A
O
LKT
SR
A
=
AN
=
A
AN
CU
LKC
A
R
=
END
14.4 Control of a process
I0.0
Q2.0
I0.1
Q2.2
F0.0
I0.4
I0.5
Q2.0
Q2.0
Q2.1
I0.2
F0.0
F0.0
I0.3
Q2.2
2.2
T0
T0
F0.1
F0.1
Q2.2
I0.4
I0.5
C0
4
I0.5
C0
Q2.3
(*First rung*)
(*Second rung*)
(*Third rung*)
(*Fourth rung*)
(*2 s allowed for capping.*)
(*Fifth rung*)
(*Sixth rung*)
(*Four bottles counted.*)
(*End rung*)
The following is an illustration of the use of a sequential flow chart for
programming. The process (Figure 14.20) involves two fluids filling two
containers: when full their contents are then emptied into a mixing
chamber, from where the mixture is then discharged. The whole process is
then repeated.
270 Programmable Logic Controllers
Limit switches
Pump 1
1
Pump 2
3
Fluid 2
Fluid 1
2
Valve 1
4
Valve 2
Mixer
Limit switch 5
Valve 3
Figure 14.20 The mixing operation
Figure 14.21 shows the type of valve that might be used in such a
process. It is solenoid operated to give flow through the valve and then,
when the solenoid is not activated, a spring returns the valve to the closed
position.
Figure 14.21 Valve
Figure 14.22 shows the sequential function chart program. When the
start switch is activated, Fill 1 and Fill 2 occur simultaneously as a result
of the actions of pumps 1 and 2 being switched on. When limit switch 1 is
activated then Fill 1 ceases, likewise when limit switch 3 is activated Fill
2 ceases. We then have the containers for fluid 1 and fluid 2 full. The
action that occurs when both limit switch 1 and 3 are activated is that the
containers start to empty, the action being the opening of valves 1 and 2.
When limit switches 2 and 4 are activated then the containers are empty.
The next stage, the mixing of the liquids is then determined when limit
switch 2 and limit switch 4 are both activated. After a time of 100 s the
moxing ceases and the mixed liquids empty through valve 3. When limit
switch 5 is activated the program reaches the end of its cycle and the
entire sequence is then repeated.
Programs 271
Start
Start switch activated
Fill 1
Fill 2
Pump 1
Pump 2
Limit switch 3
Limit switch 1
Full 1
Full 2
Limit switch 1 AND 3
Empty 1
Empty 2
Valve 1
Limit switch 2
Valve 1
Limit switch 4
Emptied 1
Emptied 2
Limit switch 2 AND 4
Mix
liquids
Mixer
Time elapsed 100 s
Mixed
liquids
Valve 3
Limit switch 5
End
Figure 14.22 The mixing operation program
Problems
1 This problem is essentially part of the domestic washing machine
program. Devise a ladder program to switch on a pump for 100 s. It is
then to be switched off and a heater switched on for 50 s. Then the
heater is switched off and another pump is used to empty the water.
2 Devise a ladder program that can be used with a solenoid valve
controlled double-acting cylinder, i.e. a cylinder with a piston which
can be moved either way by means of solenoids for each of its two
positions, and which moves the piston to the right, holds it there for
2 s and then returns it to the left.
272 Programmable Logic Controllers
3 Devise a ladder program that could be used to operate the simplified
task shown in Figure 14.23 for the automatic drilling of workpieces.
The drill motor and the pump for the air pressure for the pneumatic
valves have to be started. The workpiece has to be clamped. The drill
has then to be lowered and drilling started to the required depth. Then
the drill has to be retracted and the workpiece unclamped.
Solenoid 1
Moves drill up or down
Motor
Solenoid 2
Solenoid 3
Limit switch
3 normally
closed, open
when piece
clamped
Limit switch 4
normally
open, closed
when piece
clamped
Limit switch 1,
open when drill
up, closed when
down
Limit switch 2,
normally closed,
opened when
drill at required
depth
Clamp
Workpiece
Figure 14.23 Problem 3
4 What are the principles to be observed in installing a safe emergency
stop system with a PLC?
5 The inputs from the limit switches and the start switch and the outputs
to the solenoids of the valves shown in Figure 14.24 are connected to
a PLC which has the ladder program shown in Figure 14.25. What is
the sequence of the cylinders?
Programs 273
a–
b–
a+
B
A
A–
A+
B–
B+
c–
c+
C
–V
C+
C–
Figure 14.24 Problem 5
Start
A+
Start
B+
a+
b+
C+
c+
A–
c+
B–
a–
b–
END
Figure 14.25 Problem 5
C–
b+
274 Programmable Logic Controllers
6 The inputs from the limit switches and the start switch and the outputs
to the solenoids of the valves shown in Figure 14.26(a) are connected
to a PLC which has the ladder program shown in Figure 14.26(b).
What is the sequence of the cylinders?
a–
IR 1
IR 2
Start
A+
IR 1
IR 2
IR 1
IR 2
a+
B+
IR 1
IR 2
IR 1
IR 2
IR 1
IR 2
a–
IR 1
IR 2
b+
a+
A–
A
B–
A–
A+
b–
IR 1
IR 1
b+
IR 1
IR 2
B
a+
IR 1
B+
IR 2
b–
IR 1
B–
(a)
IR 2
IR 2
a–
END
(b)
Figure 14.26 Problem 6
IR 2
Programs 275
7 Figure 14.27 shows a ladder program involving a counter C460,
inputs X400 and X401, internal relays M100 and M101, and an
output Y430. X400 is the start switch. Explain how the output Y430
is switched on.
X400
M101
M100
X400
M101
C460
RESET
X401
C460
K10
M100
OUT
Y430 M100
Y430
C460
END
Figure 14.27 Problem 7
Appendix: Symbols
The following are the main symbols encountered.
Ladder programs
Semi-graphic form
Full graphic form
P
P
N
N
S
S
Reset coil
R
R
Retentive memory coil, the
state of the associated variable
is retained on PLC power fail
M
M
A horizontal link along which
power can flow
Interconnection of horizontal
and vertical power flows
Left-hand power connection
of a ladder rung
Right-hand power connection
of a ladder rung
Normally open contact
Normally closed contact
Positive transition-sensing
contact, power flow occurs
when associated variable
changes from 0 to 1.
Negative tranistion-sensing
contact, power flow occurs
when assoaciated variable
changes from 1 to 0
Output coil: if the power flow
to it is on then the coil state is on
Set coil
Appendix: Symbols 277
Function blocks
Semi-graphic form
Full graphic form
AV_WEIGHT
AV_WEIGHT
Horizontal and vertical lines
Interconnection of horizontal
and vertical signal flows
Crossing horizontal and
vertical signal flows
Blocks with connections
Connectors
AV_WEIGHT
AV_WEIGHT
Commonly encountered blocks:
BOOL is Boolean signal, INT is integer, REAL is a floating point number,
ANY is any form of signal
Up-counter counts the number
of rising edges at input CU.
PV defines the maximum
value of the counter. Each
new rising edge at CU
increments CV by 1. Output
Q occurs after set count. R
is the reset.
Down-counter counts down the
number of rising edges at input
CU. PV defines the starting
value of the counter. Each
new rising edge at CU
decrements CV by 1. Output
Q occurs when count reaches.
zero.
Up-down counter. It can be
used to count up on one
input and down on the other.
BOOL
CTU
CU
Q
BOOL
R
INT
PV
BOOL
CTD
CD
Q
BOOL
LD
INT
PV
BOOL
BOOL
BOOL
BOOL
INT
CV
CV
CTUD
CU
QU
CD
QD
R
LD
CV
PV
BOOL
INT
BOOL
INT
BOOL
BOOL
INT
278 Programmable Logic Controllers
On-delay timer. When input IN
goes true, the elapsed time at
about ET starts to increase and
when it reaches the set time,
specified by input PT, the output
Q goes true.
Off-delay timer. When input IN
goes true, the output Q follows
and remains true for the set
time after the input Q goes
false.
Pulse timer. When input IN goes
true, output Q follows and remains
true for the pulse duration
specified by input PT.
BOOL
TON
IN
Q
BOOL
TIME
PT
ET
TIME
BOOL
TOF
IN
Q
BOOL
TIME
PT
TIME
BOOL
TP
IN
Q
BOOL
TIME
PT
TIME
ET
ET
Logic gates
A
A
AND gate
&
Inputs
Output
Output
Inputs
B
B
A
OR gate
m 1
Inputs
Output
B
A
1
Input
A
NAND gate
Inputs
Output
Output
&
A
>1
A
Output
A
Output
Inputs
B
B
A
XOR Gate
Output
Inputs
B
Output
Inputs
A
Input
B
NOR Gate
Output
Inputs
B
Not gate
A
Inputs
B
=1
Output
A
Inputs
B
Output
Appendix: Symbols 279
Sequential function charts
Start step. This defines the step
which will be activated when the
PLC is Cold-started.
Start
Transition condition. Every transition
must have a condition. One that always
occurs should be shown with the
condition TRUE.
Step in a program
Every step can have an associated
action. An action describes the
bahviour that occurs when the step
is activated. Each action can have a
qualifier: N indicates the action is
executed while the step is active. If
no qualifier is indicated it is taken
to be N.
Step
Step
Action
Qualifier
D: time delayed action which
starts after a given time.
Selective branching
Parallel branching when the
transition occurs
Convergence when both
transitions occur
Simultaneous convergence
Instruction List (IEC 1131-3 symbols)
LD
LDN
ST
S
R
AND
ANDN
OR
ORN
N
Start a rung with an open contact.
Start a rung with a closed contact
An output
Set true
Reset false
Boolean AND
Booleand NAND
Boolean OR
Boolean NOR