chapter 01 welcome to plcs
TRANSCRIPT
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Chapter 1
Welcome to the World of
Programmable Logic Controllers
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Objectives
Define PLC.
Explain where the PLC came from. Explain why their use is valuable.
Explain where they are used.
Detail what PLCs can do. Explain how PLCs know what they are
supposed to do.
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What Is a
Programmable Controller?
A programmable logic controller, usually
called a PLC or programmable controller, is a
solid-state, digital, industrial computer.
Simply, a programmable controller is a
computer, much like a desktop personal
computer.
A PLC is an industrially hardened computer.
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Where Did the
PLC Come From? (1 of 2)
In the 1960s, electromechanical relays,
timers, counters, and sequencers were the
standard.
Many control panels contained hundreds of
these devices and a mile or more of wire.
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Where Did the
PLC Come From?(2 of 2)
Reliability was low and maintenance costs
were high.
Cost was high to modify or upgrade control
panels.
In 1968 the General Motors Hydramatic
division specified a device that would become
what we know today as the programmable
logic controller.
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Early PLCs
Only relay replacers
Did not have timers or counters No sequencer instructions
No math instructions
No data manipulation instructions
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Why a PLC? (1 of 2)
Easily changeable
Programmable Reliable
Smaller
Fast switching
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Why a PLC? (2 of 2)
Able to withstand harsh factory environment
Consumes less power Easier to troubleshoot
Easy to install
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Why Use a PLC?
The question why use a PLC? should really
be rephrased to why automate?
The PLC is the tool that provides the control
for the automated process.
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Automating Helps a
Manufacturing Facility (1 of 2)
Gain complete control of the manufacturing
process
Achieve consistency
Improve quality and accuracy
Work in difficult or hazardous environments
Increase productivity
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Automating Helps a
Manufacturing Facility (2 of 2)
Shorten lead time to market
Lower cost of quality, scrap, and rework Offer greater product variety
Allow a quick changeover from one product
to another
Control inventory
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A PLC Upon First Glance
A black box with wires bringing signals in and
other wires sending signals out
Some sort of magic being done inside that
somehow decides when field devices should
be turned on or off
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Actually There Is No Magic
The PLC is a computer and someone has to
tell it what to do.
The PLC knows what to do through a
program that was developed and entered into
its memory.
Without a set of instructions telling the PLCwhat to do, it is nothing more than a box full
of electronic components.
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What Makes a PLC Work? (1 of 4)
The heart of any computer is the
microprocessor.
The microprocessor, also called the processor
or central processing unit (CPU), supervises
system control through the user program.
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What Makes a PLC Work? (2 of 4)
The processor reads input signals and
follows the instructions that the programmer
has stored in the PLCs memory.
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What Makes a PLC Work? (3 of 4)
As a result of the solved program, the PLC
writes information to outputs, or field-
controlled devices, to turn them on or off.
When the PLC is running and following the
programs instructions, this is called solving
the user program. The PLC is running or in RUN MODE.
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What Makes a PLC Work? (4 of 4)
The user program (ladder program) is the list
of instructions that tells the PLC what to do.
The library of instructions available to the
PLC is called the instruction set.
The instruction set determines how much
flexibility the programmer has.
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Common PLC Inputs
Pushbuttons
Selector switches Limit switches and level switches
Proximity sensors
Photo switches Relay contacts
Motor starter contacts
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An Overview of a PLC System (1 of 2)
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An Overview of a
PLC System (2 of 2)
Incoming signals, or inputs, interact with
instructions in the user program to help the
PLC determine when an input instruction is
either true or false.
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Conventional Circuit
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Representation of
a PLC Program
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PLC Ladder Program Rung
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Series 90-30 and
90-20 Hand-held Programmer
Image courtesy of GE Fanuc Automation
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Interfacing a PC
to an Omron CQM 1 PLC
Image courtesy of Omron Electronics, Inc.
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Correlating Ladder Program
Rung to Actual PLC Wiring
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Programmable Controller
Block Diagram
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Product Sensed in
Position Will Send an Input Signal
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Allen-Bradley SLC 500 Fixed PLC
Image courtesy of Allen-Bradley, a Rockwell Automation business
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Allen-Bradley SLC
500 Modular PLC
Image courtesy of Allen-Bradley, a Rockwell Automation business
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SLC 500 Power Supply
and a Four-slot Rack
Image courtesy of Allen-Bradley, a Rockwell Automation business
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Installation of an I/O Module
Image courtesy of Allen-Bradley, a Rockwell Automation business
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Allen-Bradley
SLC 500 Modular Processor
Image courtesy of Allen-Bradley, a Rockwell Automation business
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Limit Switch Interface
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Output Module Wiring to
a Motor Starter Coil