Download - Lecture 1 PLC
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Introduction to PLCIntroduction to PLC
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CHAPTER - INTRODUCTION TO PLCS
Advantages of PLCs
Cost effective for controlling complex systems.Flexible and can be reapplied to control other systems quickly and easilysystems quickly and easily.Computational abilities allow more sophisticated control.T bl h ti id k i i dTrouble shooting aids make programming easier and reduce downtime.Reliable components make these likely to operate for years before failure.
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PLC Origin
l d l l i h l- Developed to replace relays in the late 1960s
- Costs dropped and became popular by 1980s
- Now used in many industrial designs
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Relays and
Schematic
Symbols
input coil
normallyopen
normallyclosed
OR
open
OR
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Logical ControlLogical Control
with Relays115VACwall plug
relay logic
input A input Boutput C(normally open)p(normally closed) (normally open) (normally open)
l dd l i
A B C
ladder logic
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Relay Logicpush buttons
Relay Logic
In a PLC powersupply+24V
com.
inputs
PLC
ladder
inputs
outputs
logicA B C
115Vac lightAC power
neut.
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Reading Ladder Logicg g
HOT NEUTRALHOT NEUTRALA B X
C D G Y
E F H
INPUTS OUTPUTS
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).
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A Ladder Logic Example
A B
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.
B
N Th li h i h i b i l f ff i i llNote: The line on the right is being left off intentionally and is implied in these diagrams.
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Mnemonics
000000000100002000030000400005
LDNLDANDLDLDAND
AB
CD
the mnemonic code is equivalent tothe ladder logic below
0000500006
ANDOR
A B X
ST00007 XEND00008
C D
END
Note: The notation shown above isnot standard Allen-Bradleynotation. The program to theright would be the A-B equiva
SORBSTXIC AXIO Bg q
lent. NXBXIO CXIO DBNDOTE XOTE XEOREND
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Start
power up Execution followslti l th
power downflash
multiple paths
End
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S d TStructured Text
i := 0;REPEATi := i + 1;UNTIL i >= 10END_REPEA T;
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PLC in a Control Loop
PROCESS
Feedback fromsensors/switches
Connections toactuators
PLC
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PLC h
The Four Stages of a PLC Scan
PLC program changes outputsby examining inputs Set new outputs
THE Power turned on
Read inputsProcess changes and PLC pauseswhile it checks its own operation
CONTROLLOOP
Power turned on
p p
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Ladder Logic InputsLadder Logic Inputs
Normally open, an active input x will close the contactand allow power to flow.
x
Normally closed, power flows when the input x is not open.
x
xIIT
immediate inputs will take current values, not those fromthe previous input scan. (Note: this instruction is actually
IIT
an output that will update the input table with the currentinput values. Other input contacts can now be used toexamine the new values )examine the new values.)
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Wh i li d ( ) th t t i ti t d f th l ft t t b t t d
Ladder Logic OutputsWhen power is applied (on) the output x is activated for the left output, but turned
x x
off for the output on the right.
An input transition on will cause the output x to go on for one scan
OSRx
(this is also known as a one shot relay)
When the L coil is energized, x will be toggled on, it will stay on until the U coil
LU
is energized. This is like a flip-flop and stays set even when the PLC is turned off.xx
Some PLCs will allow immediate outputs that do not wait for the program scan to
U
end before setting an output. (Note: This instruction will only update the outputs usingthe output table, other instruction must change the individual outputs.)
IOTx
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.
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Sample Problem:
Try to develop a relay based controller that will allow three switches in a room to control a single light
p
three switches in a room to control a single light.