ECET - 262
Introduction to Programmable Logic Controller(PLC)
Course Instructor
Akram Hossain
Professor
Department of Electrical and Computer Engineering Technology
Purdue University Calumet
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Definition of PLC
 Programmable Logic Controllers are solid state
devices that can be programmed to performed
sequential and discrete state operation on external
equipment
 They are designed to perform the logic functions
previously accomplished by electromechanical relays,
drum switches, mechanical and electronic timers and
counters, standalone digital PID controllers etc.
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Major Manufacturer of PLC
Allen-Bradley (A-B)
General Electric
Gould-Madicon
Texas Instruments
Square-D
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Reliance Electric
Siemens
 EATON
Schneider
Mitsubishi
Areas of PLC Applications
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Annunciators
Injection Molding
Cranes
Slitting
Auto Insertion
Assembly
Crushing
Sorting
Bagging
Motor Winding
Cutting
Stackers
Baking
Oil Fields
Digesters
Stitching
Blending
Painting
Drilling
Stack Precipitators
Boring
Palletizers
Electronic Testing
Threading
Brewing
Pipelines
Elevators
Tire Building
Calendaring
Polishing
Engine Test Stands
Traffic Control
Casting
Reactors
Extrusion
Textile Machine
Chemical Drilling
Robots
Forging
Turbines
Color Mixing
Rolling
Generators
Turning
Compressors
Security Systems
Gluing
Weaving
Conveyors
Stretch Wrap
Grinding
Web Handling
Heat Treating
Welding
History of PLC
The Hydromantic Division of the General Motors Corporation specified the design criteria for the first
programmable controller in 1968. Their primary goal was to eliminate the high costs associated with
inflexible, relay-controlled systems. The specifications required a solid-state system with computer
flexibility and able to:
Survive in an industrial environment.
Be easily programmed and maintained by plant engineers and technicians.
Be reusable.
Such a control system would reduce machine downtime
Provide expandability for the future.
Some of the initial specifications included the following:
1)
The new control system had to be price competitive with the use of relay systems.
2)
The system had to be capable of sustaining an industrial environment.
3)
The input and output interfaces had to be easily replaceable.
4)
The controller had to be designed in modular form, so that subassemblies could be removed easily for replacement
or repair.
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5)
The control system needed the capability to pass data collection to a central system.
6)
The system had to be reusable.
7)
The method used to program the controller had to be simple, so that it could be easily understood by plant
personnel.
PLC and Computer

A PLC and a computer both are electronic processor unit. The architecture of a PLC’s CPU is basically
same as that of a general purpose computer; however, some important characteristics set them
apart.

Unlike computer, PLCs are specifically designed to survive the harsh conditions of the industrial
environment. A well-designed PLC can be placed in an area with substantial amounts of electrical
noise, electromagnetic interference, mechanical vibration, and noncondensing humidity.

Distinction of PLCs is that their hardware and software are designed for easy use by plant
electricians and technicians. The hardware interfaces for connecting field devices are actually part
of the PLC itself and are easily connected.

The modular and self-diagnosing interface circuits are able to pin point malfunctions and moreover,
are easily removed and replaced.

Software programming uses conventional relay ladder symbols, or other easily learned languages,
which are familiar to plant personnel.

A computer can execute a complex programming task and also multitasking. An standard PLC is
designed to executes a single program in an orderly fashion. As PLCs are rapidly changing, modern
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PLCs have multitasking capabilities.
Why PLCs ?
Soft Manufacturing Process
Flexible Manufacturing Process
Retrofit Existing Process
Less Maintenance
Easy to Debug
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Why PLCs are so Popular?
 Programmable logic controller have made it possible to precisely control large
process machines and driven equipment with less physical wiring and wiring
time than it requires with standard electro-mechanical relays, pneumatic system,
timers, drum switches, and so on.
 The programmability allows for fast and easy changes in the relay ladder logic to
meet the changing needs of the process or driven equipment without the need
for expensive and time consuming rewiring process.
 Modem PLCs are "electrician friendly", PLC can be programmed and used by
plant engineers and maintenance electricians without much electronic and
computer programming background. They can programmed by using the
existing ladder diagrams.
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Advantages of PLC
A.
Flexibility
 Universal Controller - can replace various independent/ standalone controller.
B.
Implementing Changes and Correcting Errors
 Do not have to rewiring relay panel.
 Change program using keyboard.
C.
Large Quantity of Contact
 Large number of' Soft Contact' available.
D. Lower Cost
 Advancement in technology and open architecture of PLC will reduce the
market price.
E.
Pilot Running (Simulation Capability)
 A program can be simulated or run without actual input connection.
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Advantages of PLC
F.
Visual Observation.

Can observe the opening and closing of contact switch on CRT .

Operator message can be programmed for each possible malfunction.
G.
Speed of Operation

Depends on scan time -millisecond.

Asynchronous operation.
H.
Ladder or Boolean Programming Method.

I.
Reliability

J.
In general -very reliable
Simplicity of Ordering Control Sys. Components

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Easy for 'Electrician ,
One package with Relay, Timers, Control Block, etc.
Advantages of PLC
K.
Documentation

L.
Security

M.
Software lock on a program (Password)
Ease of Changes by Programming

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Printout of ladder logic can be printed easily
Ability to program and reprogram, loading and down loading
Disadvantages of PLC
A. New Technology

B.
Fixed program Application

C.
E.
Not adapted for very high temperature, high humidity level, high vibration, etc.
Fail-safe operation

Does not start automatically when power failure ( can be programmed into )

Not "Fail-safe" -Fail-shorted rather than OPEN
Fixed-circuit operation

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Not cost effective for single- function application
Environment Consideration

D.
Change from ladder and relay to PLC concept
Fixed control system -less costly
What is a Ladder Diagram?
Definition
A complete control scheme normally drawn as a series of
contacts and coils arranged between two vertical control
supply lines so that the horizontal lines of contacts appear
similar to rungs of a ladder. The control contacts (input
devices) are to left and coils (output devices) on the right.
Ladder diagrams are an industrial standard for representing
relay-logic control system
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PLC Series 5 Input/Output Chassis
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Allen-Bradley PLC Series 5 Processor Module
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PLC 5/25 Processor Mode of
Operation
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A-B Series 5 Power Supply
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120 VAC Input Module Connection
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120 VAC Output Module Connection
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Two Slot Addressing
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PLC Scan and Update Sequence
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How PLC Works?
Input
Map
00
Output
Map
0 00
I:001
01
1
I:001
O:001
01
00
03
04
0
02
1
03
0
04
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00
1
01
0
02
0
03
O:001
O:001
01
03
O:001/01
0
04
03
01
120 VAC
Input
Module
01
01
0
Ladder Logic in
Processor Memory
120 VAC
Output
Module
PLC Input & Output Mapping
M
START SW
STOP SW
RELAY COIL
M
SELF HOLDING FOR "M"
L1
CONTROL
POWER
I:
INPUT
START
STOP
N
ON LIGHT
00
00
01
01
02
02
03
03
MOTOR
RELAY
COIL
0
RACK
O:
OUTPUT
0
RACK
I:001
I:001
01
SLOT
01
SLOT
/
/
03
ADDRESS
03
ADDRESS
O:001
00
L1
04
04
MOTOR
ON LIGHT N
O:001/01
O:001
01
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01
01
O:001
03
PLC Program and Data Files
 Program Files
•
Reserved Files (File # 0)
•
SFC file (File # 1 )
•
Ladder file (File #2- 999) -File #2: Main Ladder File
•
Subroutine file (File #3 -999)
•
Selectable Interrupt file (File #3 -999)
•
Fault Routine File (File #3 -999)
 Data Files
•
Files which store data of the I/0 module.
•
Can be integer data, floating point (real) data, timer data, counter data, discrete
input data, discrete output data, etc.
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Input / Output Addressing
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General Instruction Format
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General Instruction Format
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Memory Organization for Data
Files
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Memory Map of File Types
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A-B PLC Series-5 Data
Organization
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Floating Point Files
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Timer and Counter Data Files
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Control Data File
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PLC Logical Addressing
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Status File
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TIME ON DELAY (TON)
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