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? (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.

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.

Early PLCs

• Only relay replacers

• Did not have timers or counters

• No sequencer instructions

• No math instructions

• No data manipulation instructions

Why a PLC? (1 of 2)

• Easily changeable

• Programmable

• Reliable

• Smaller

• Fast switching

Why a PLC? (2 of 2)

• 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 (1 of 2)

• 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 (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

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 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 PLC what to do, it is nothing more than a box full of electronic components.

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.

What Makes a PLC Work? (2 of 4)

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

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 program’s instructions, this is called solving the user program.

• The PLC is running or in RUN MODE.

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.

Common PLC Inputs

• Pushbuttons

• Selector switches

• Limit switches and level switches

• Proximity sensors

• Photo switches

• Relay contacts

• Motor starter contacts

An Overview of a PLC System (1 of 2)

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.

Conventional Circuit

Representation of a PLC Program

PLC Ladder Program Rung

Series 90-30 and 90-20 Hand-held 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

Image courtesy of Allen-Bradley, a Rockwell Automation business

SLC 500 Power Supply and a Four-slot Rack

Image courtesy of Allen-Bradley, a Rockwell Automation business

Installation of an I/O Module

Image courtesy of Allen-Bradley, a Rockwell Automation business

Allen-Bradley SLC 500 Modular Processor

Image courtesy of Allen-Bradley, a Rockwell Automation business

Limit Switch Interface

Output Module Wiring to a Motor Starter Coil