Advantages of PLCs
Cost effective for controlling complex systems. Flexible and can be reapplied to control other systems quickly and easily. Computational abilities allow more sophisticated control. Trouble shooting aids make programming easier and reduce downtime. Reliable components make these likely to operate for years before failure.
CHAPTER - INTRODUCTION TO PLCS
PLC Origin
- Developed to replace relays in the late 1960s
- Costs dropped and became popular by 1980s
- Now used in many industrial designs
Relays and
Schematic
Symbols
no rm a llyo p e n
no rm a llyc lo se d
inp ut c o il
O R
O R
Logical Control
with Relays
115VACwall plug
relay logic
input A(normally closed)
input B(normally open)
output C(normally open)
ladder logic
A B C
Relay Logic
In a PLC
ladder
powersupply
+24V
com.
inputs
outputs
push buttons
logic
PLC
AC power
115Vac
neut.
A B C
light
Reading Ladder Logic
HOT NEUTRAL
INPUTS OUTPUTS
A B X
C D
E F
G
H
Y
Note: Power needs to flow through some combination of the inputs (A,B,C,D,E,F,G,H) to turn on outputs (X,Y).
A Ladder Logic Example
Note: When A is pushed, the output B will turn on, and the input B will also turn on and keep B on permanently - until power is removed.
A
B
B
Note: The line on the right is being left off intentionally and is implied in these diagrams.
Mnemonics
0 0 0 0 00 0 0 0 10 0 0 0 20 0 0 0 30 0 0 0 40 0 0 0 50 0 0 0 6
L D NL DA N DL DL DA N DO R
AB
CD
A B
C D
X
E N D
the m ne m o nic c o d e is e q uiva le nt tothe la d d e r lo gic b e lo w
S T0 0 0 0 7 XE N D0 0 0 0 8
N o te : T he no ta tio n sho w n a b o ve isno t s ta nd a rd A lle n- B ra d le yno ta tio n. T he p ro gra m to theright w o uld b e the A - B e q uivale nt.
S O RB S TX IC AX IO BN X BX IO CX IO DB N DO T E XE O RE N D
SFCs
Start
End
power up
power down
flash
Execution followsmultiple paths
Structured Text
i := 0 ;R E P E A Ti := i + 1 ;U N T IL i > = 1 0E N D _ R E P E A T;
PROCESS
Feedback fromsensors/switches
PLC
Connections toactuators
PLC in a Control Loop
Read inputs
PLC program changes outputsby examining inputs Set new outputs
Process changes and PLC pauseswhile it checks its own operation
THECONTROLLOOP
Power turned on
The Four Stages of a PLC Scan
Normally open, an active input x will close the contactand allow power to flow.
Normally closed, power flows when the input x is not open.
x
x
immediate inputs will take current values, not those fromthe previous input scan. (Note: this instruction is actually
xIIT
an output that will update the input table with the currentinput values. Other input contacts can now be used toexamine the new values.)
Ladder Logic Inputs
When power is applied (on) the output x is activated for the left output, but turned
An input transition on will cause the output x to go on for one scan
x x
OSRx
(this is also known as a one shot relay)
off for the output on the right.
When the L coil is energized, x will be toggled on, it will stay on until the U coil
Some PLCs will allow immediate outputs that do not wait for the program scan to
LU
IOT
end before setting an output. (Note: This instruction will only update the outputs using
is energized. This is like a flip-flop and stays set even when the PLC is turned off.
x
xx
the output table, other instruction must change the individual outputs.)
Note: Outputs are also commonly shown using parentheses ’-( )-’ instead of the circle. This is because many of the programming systems are text based and circles cannot be drawn.
Ladder Logic Outputs
Try to develop (without looking at the solution) a relay based controller that will allow three switches in a room to control a single light.
Sample Problem:
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