Transcript Intro to PLCs

IENG 475 - Lecture 13
Logic Diagramming &
Introduction to
Programmable Logic
Controllers
7/7/2015
IENG 475: Computer-Controlled
Manufacturing Systems
1
Logic Diagramming


Methods
•
•
•
Ladder Logic (similar to wire logic)
ISO Pneumatic (Fluid) Logic
Logic Gates
Logic System Functions Required:
•
•
•
AND
OR
NOT
• Minimally:
•
•
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NAND
NOR
IENG 475: Computer-Controlled
Manufacturing Systems
2
Logical AND Function

Truth Table:
A
B
C
0
0
1
1
0
1
0
1
0
0
0
1
Pneumatic
A
=A•B
Logic Gate
B
A
B
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Ladder Logic
IENG 475: Computer-Controlled
Manufacturing Systems
A
B
3
Logical OR Function

Truth Table:
A
0
0
1
1
Pneumatic
B
C
=A+B
0
1
0
1
0
1
1
1
Logic Gate
Ladder Logic
A
A
B
B
A
B
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IENG 475: Computer-Controlled
Manufacturing Systems
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Logical NOT Function

Truth Table:
A
B
0
1
1
0
Pneumatic
A
=A
Logic Gate
Ladder Logic
A
A
A
(2/2 DCV won’t work)
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(ISO preferred)
IENG 475: Computer-Controlled
Manufacturing Systems
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Ladder Logic Rungs

A rung runs from the left (hot) rail to the right rail
(return), generally having only ONE output coil per
rung.

Mnemonic names (addresses) of inputs and outputs
are given at the top of the symbol.

The type of an output is given in the center of the
output symbol. (NO, NC)

Preset times/counts/other values are noted below
the output coil symbol.
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IENG 475: Computer-Controlled
Manufacturing Systems
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Logic Diagram Examples

Logic Gates (Network)
x
Timer
y

z
1s
Ladder Logic (Single Rung)
x
y
z
Timer
(on delay)
x
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y
IENG 475: Computer-Controlled
Manufacturing Systems
1s
7
PLC History 101


Pre-1968 electrical controls:
•
Hardwired Panels
•
•
•
•
Ladder Logic (electrical continuity)
Relays
Cams
Drum sequencers
Disadvantages:
•
•
•
•
•
Shut down line to change, debug, optimize control
Errors were difficult to locate, correct
Mechanical devices are prone to wear out
Electrical safety was difficult
“Real estate” for panel was expensive
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IENG 475: Computer-Controlled
Manufacturing Systems
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PLC History 102

1968 General Motors:
•
•
•
•
Use re-programmable computer to control system
Programmable using Ladder Logic
•
•
•
Concept is LOGICAL continuity rather than electrical continuity
Electricians would not have to be trained in a programming
language
Could be programmed off-line
Environmentally hardened
•
•
Operate without error in a high EMF environment
Sealed from dirt, dust
Electro-Optic Isolation
•
•
Separates computer from inputs & outputs
Modularization concept
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IENG 475: Computer-Controlled
Manufacturing Systems
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PLC History 103

Today:
•
•
•
•
Smaller
Cheaper
Expandable
More Capable
• Digital I/O modules
• Analog I/O modules
• High speed counters
• Communications
•
•
•
host - link
peer to peer
ASCII
• Speech modules
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Position control modules
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





open loop control
closed loop control
Machine vision modules
Bar code modules
PID control modules
Fuzzy logic control modules
RF - radio frequency modules
IENG 475: Computer-Controlled
Manufacturing Systems
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PLC System Components


Requirements:
•
•
•
•
•
•
Power Supply
CPU
Input block
Output block
Memory
Programming Unit
Options:
•
•
Expansion Units
Modules
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IENG 475: Computer-Controlled
Manufacturing Systems
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PLC System Diagrammed
Power Supply
Input
Block
RAM
ROM
EPROM
EEPROM
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CPU
Memory
Programming
Unit
IENG 475: Computer-Controlled
Manufacturing Systems
Output
Block
Dumb terminal
Dedicated terminal
Hand-held programmer
Micro computer
12
Electro-Optical Isolation

Purpose:
• Avoid direct electrical path between I/O blocks
and control circuitry

Inputs:
Input Block
Sensor
+
–
P
L
C
Output Block

Outputs:
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P
L
C
IENG 475: Computer-Controlled
Manufacturing Systems
~
Load
13
Questions & Issues
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Manufacturing Systems
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