chapter 01 welcome to plcs

8/13/2019 Chapter 01 Welcome to PLCs

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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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The processor reads input signals and

follows the instructions that the programmer

has stored in the PLCs memory.


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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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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



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SLC 500 Power Supply

and a Four-slot Rack



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Installation of an I/O Module



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Allen-Bradley

SLC 500 Modular Processor



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Limit Switch Interface


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Output Module Wiring to

a Motor Starter Coil

chapter 01 welcome to plcs

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