copyright © 2002 delmar thomson learning chapter 1 welcome to the world of programmable logic...

Copyright © 2002 Delmar Thomson Learning

Chapter 1

Welcome to the World of

Programmable Logic Controllers


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.


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.


Where Did the PLC Come From?

• In the 1960s, electromechanical relays, timers, counters, sequencers were the standard.

• Many control panels contained hundreds of these devices and a mile or more of wire.


Where Did the PLC Come From? (cont’d.)

• Reliability was low and maintenance costs 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.


Early PLCs

• Only relay replacers• Did not have timers or counters• No sequencer instructions• No math instructions• No data manipulation instructions


Why a PLC?

• Easily changeable• Programmable• Reliable• Smaller• Fast switching


Why a PLC? (cont’d.)

• Able to withstand harsh factory environment

• Consumes less power• Easier to troubleshoot• Easy to install


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.


Automating Helps a Manufacturing Facility:

• Gain complete control of the manufacturing process.

• Achieve consistency.• Improve quality and accuracy.• Work in difficult or hazardous

environments.• Increase productivity.


Automating Helps a Manufacturing Facility:

(cont’d.)

• 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.


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


Actually There Is No Magic

• The PLC is a computer and someone had 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 PLC what to do, it is nothing more than a box full of electronic components.


What Makes a PLC Work?

• 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.


What Makes a PLC Work? (cont’d.)

• The processor reads input signals and follows the instructions that the programmer has stored in the PLC’s memory.



• 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.



• 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.


Common PLC Inputs

• Push buttons• Selector switches• Limit switches and level switches• Proximity sensors• Photo switches• Relay contacts• Motor starter contacts


An Overview of a PLC System


An Overview of a PLC System (cont’d.)

• Incoming signals, or inputs, interact with instructions in the user program to help the PLC to determine when an input instruction is either true or false.


Conventional Circuit


Representation of a PLC Program


PLC Ladder Program Rung


Series 90-30 and 90-20 Handheld Programmer

Image courtesy of GE Fanuc Automation


Interfacing a PC to an Omron CQM 1 PLC

Image courtesy of Omron Electronics, Inc.


Correlating Ladder Program

Rung to Actual PLC Wiring


Programmable Controller Block Diagram


Product Sensed in Position Will Send an

Input Signal


Allen-Bradley SLC 500 Fixed PLC

Image courtesy of Allen-Bradley, a Rockwell Automation business


Allen-Bradley SLC 500 Modular PLC



SLC 500 Power Supply and a Four-slot Rack



Installation of an I/O Module



Allen-Bradley SLC 500 Modular

Processor



Limit Switch Interface


Output Module Wiring to a Motor Starter Coil

copyright © 2002 delmar thomson learning chapter 1 welcome to the world of programmable logic...

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