Programmable LogicController (PLC)

Mat Nor Mohamad

Brief PLC History

• The PLC was invented in response to the needs of theAmerican automotive industry.

• Before the PLC, control, sequencing, and safetyinterlock logic for manufacturing automobiles wasaccomplished using relays, timers and dedicatedclosed-loop controllers.

• The process for updating such facilities for the yearlymodel change-over was very time consuming andexpensive, as the relay systems needed to be rewiredby skilled electricians.

• In 1968 GM Hydramatic (the automatic transmissiondivision of General Motors) issued a request forproposal for an electronic replacement for hard-wiredrelay systems.

Brief PLC History

• The winning proposal came from Bedford Associatesof Boston, Massachusetts.

• The first PLC, designated the 084 because it wasBedford Associates eighty-fourth project, was theresult. Bedford Associates started a new companydedicated to developing, manufacturing, selling, andservicing this new product: Modicon, which stood forMOdular DIgital CONtroller.

• One of the people who worked on that project wasDick Morley, who is considered to be the "father" ofthe PLC.

• The Modicon brand was sold in 1977 to GouldElectronics, and later acquired by German CompanyAEG and then by Schneider Electric, the currentowner.

Disadvantages of Classic controlpanel

1. Large amount of work requiredconnecting wires

2. Difficulty with changes orreplacements

3. Difficulty in finding errors;requiring skillful/experienced workforce

4. When a problem occurs, hold-uptime is indefinite, usually long

Introduction

What is PLC?

PLC is an industrial computer controlsystem that continuously monitors thestate of input devices and makesdecisions based upon a custom programto control the state of output devices

Definition

A formal definition of a PLC comes from the NationalElectrical Manufacturers Association (NEMA):

A digitally operating electronic apparatus which uses aprogrammable memory for the internal storage ofinstructions for implementing specific functions such aslogic, sequencing, timing, counting, and arithmetic tocontrol, through digital or analog input/output modules,various types of machines or processes.

PLC Overview

• A PLC has many "input" terminals, through which itinterprets "high" and "low" logical states from sensorsand switches.

• It also has many output terminals, through which itoutputs "high" and "low" signals to power lights,solenoids, contactors, small motors, and other deviceslending themselves to on/off control.

• In an effort to make PLCs easy to program, theirprogramming language was designed to resemble ladderlogic diagrams.

• Thus, an industrial electrician or electrical engineeraccustomed to reading ladder logic schematics wouldfeel comfortable programming a PLC to perform thesame control functions.

Input Output

Input Output

Advantages of a PLC Control System

Eliminates much of the hard wiring thatwas associated with conventional relaycontrol circuits.

The program takesthe place of much ofthe external wiringthat would be requiredfor control of a process.

Example

• Lets say that two push buttons, PB1 and PB2, are connectedto a PLC.

• Two pilot lights, PL1 and PL2, are also connected to the PLC.• The way these devices are connected now pressing push

button PB1 turns on pilot light PL1 and pressing push buttonPB2 turns on pilot light PL2.

• Lets say that you want to change this around so that PB1controls PL2 and PB2 controls PL1.

• In a traditional system, you would have to rewire the circuit sothat the wiring from the first push button goes to the secondpilot light and vice versa.

• However, because these devices are connected to a PLC,making this change is as simple as making a small change inthe control program.

Advantages of a PLC Control System

Increased Reliability:Once a program has been written and testedit can be downloaded to other PLCs.

Since all thelogic is containedin the PLC’smemory, thereis no chanceof making alogic wiringerror.

Advantages of a PLC Control System

More Flexibility:Original equipment manufacturers (OEMs) canprovide system updates for a process by simplysending out a new program.

It is easier tocreate and changea program in a PLCthan to wire andrewire a circuit.End-users can modifythe program in thefield.

Advantages of a PLC Control System

Lower Costs:Originally PLCs were designed to replace relaycontrol logic. The cost savings using PLCs havebeen so significant that relay control is becomingobsolete, except for power applications.

Generally, if an applicationrequires more than about6 control relays, it willusually be less expensiveto install a PLC.

Advantages of a PLC Control System

Communications Capability:A PLC can communicate with other controllersor computer equipment.

They can be networkedto perform such functionsas: supervisory control, datagathering, monitoringdevices and processparameters, and downloadingand uploading of programs.

Advantages of a PLC Control System

Faster Response Time:PLCs operate in real-time which means that anevent taking place in the field will result in anoperation or output taking place.

Machines that processthousands of items persecond and objects thatspend only a fraction ofa second in front of asensor require the PLC’squick response capability.

Advantages of a PLC Control System

Easier To Troubleshoot:PLCs have resident diagnostic and overridefunctions that allows users to easily trace andcorrect software and hardware problems.

The control programcan be watched inreal-time as itexecutes to findand fix problems

PLC Size Classification

2048 and aboveVery Large

1024 to 4096Large

128 to 2048Medium

32 up to 128Small

Up to 32Micro

I/O CountSize

CPM1AOmron

VersaMaxGeneral Electric

MicroLogixRockwell

FX0Mitsubishi

SimaticSiemens

PLCManufacturer

Others PLC Brands

Others well known PLC brands are ABBLtd., Koyo, Honeywell, Modicon, Allen-Bradley, Tesco Controls and Panasonic

(Matsushita)

PLC Vs. PC

This is where the PC excels becauseof it's hard drive. Any long term datastorage, history and trending is bestdone on a PC.

Memory is limited in its ability to store a lot ofdata.

Datamanagement

A PC is very flexible and powerful inwhat to use for programming

Languages are typically fixed to ladder logic,function block or structured text

Programminglanguages

A PC locking up and crashing isfrequent

never crashes over long periods of time. ("Never"may not be the right word but its close enough tobe true.)

Reliability

by design, are meant to handlesimultaneous tasks. They havedifficulty handling real time events

execute a single program in sequential order. Thehave better ability to handle events in real time

Speed

Typical PCs are limited by thenumber of cards they canaccommodate and are not easilyexpandable.

in rack form are easy to exchange and add parts.They are designed for modularity and expansion

Flexibility

Operating systems like Windows arecommon. Connecting I/O to the PC isnot always as easy

friendlier to technicians since they are in ladderlogic and have easy connections.

Ease of Use

not designed for harsh environments.Industrial PCs are available but costmore

designed for harsh conditions with electrical noise,magnetic fields, vibration, extreme temperaturesor humidity

Environment

PCPLC

PLC Hardware

PLC hardware falls into two physicalconfigurations:

1) Fixed- has all of its components (input section,

CPU, memory, power supply, output section) built intoone self-contained unit, and are not removable.

2) Modular- comes as separate pieces of power

supplies, CPU, input and output cards or I/O modules.

* flexibility

Fixed I/O PLCs

Modular I/O PLCs

PLC System

PLC System

• A PLC system consists of a Central Processing Unit(CPU) containing an application program and Input andOutput Interface modules, which is directly connected tothe field I/O devices. The program controls the PLC sothat when an input signal from an input device turns ON,the appropriate response is made. The responsenormally involves turning ON an output signal to somesort of output devices.

• The Central Processing Unit (CPU) is a microprocessorthat co-ordinates the activities of the PLC system. Itexecutes the program, processes I/O signals &communicates with external devices.

There are various types of memory unit. It is the area that holds theoperating system and user memory. The operating system is actually asystem software that co-ordinates the PLC. Ladder program. Timer andCounter Values are stored in the user memory. Depending on user’s need,various types of memory are available for choice:

(a) Read —Only Memory (ROM)ROM is a non-volatile memory that can be programmed only once. It istherefore unsuitable. It is least popular as compared with others memorytype.

(b) Random Access Memory (RAM)RAM is commonly used memory type for storing the user program and data.The data in the volatile RAM would normally be lost if the power source isremoved. However, backing up the RAM with a battery solves this problem.

(c) Erasable Programmable Read Only Memory (EPROM)EPROM holds data permanently just like ROM. It dose not require batterybackup. However, exposing it to ultraviolet light can erase its content. Aprom writer is required to reprogram the memory.

(d) Electrically Erasable Programmable Read Only Memory (EEPROM)EEPROM combines the access flexibility of RAM and the non-volatility ofEPROM in one. Its contents can be erased and reprogrammed electrically,however, to a limit number of times.

Input Devices

• Intelligence of an automated system is greatly depending on theability of a PLC to read in the signal from various types of automaticsensing and manual input field devices.

• Push buttons, keypad and toggle switches, which form the basicman-machine interface, are types of manual input device.

• On the other hand, for detection of work piece, monitoring of movingmechanism, checking on pressure and or liquid level and manyothers, the PLC will have to tap the signal from the specificautomatic sensing devices like proximity switch, limit switch,photoelectric sensor, level sensor and so on.

• Types of input signal to the PLC would be of ON/OFF logic oranalogue. These input signals are interfaced to PLC through varioustypes of PLC input module.

Output Devices

• An automatic system is incomplete and the PLC system is virtuallyparalyzed without means of interface to the field output devices.

• Some of the most commonly controlled devices are motors,solenoids, relays indicators, buzzers and etc.

• Through activation of motors and solenoids the PLC can controlfrom a simple pick and place system to a much complex servopositioning system. These type of output devices are the mechanismof an automated system and so its direct effect on the systemperformance.

• However, other output devices such as the pilot lamp, buzzers andalarms are merely meant for notifying purpose. Like input signalinterfacing, signal from output devices are interfaced to the PLCthrough the wide range of PLC output module.

Programming Devices

The programming device is used to input the desiredinstructions. These instructions determine what thePLC will do for a specific input

The PLC can be programmed by;

1) Personal Computer (PC programming device)

2) Programming Console (Hand held programming device)

PC Programming Device

• Manufacturers software required.

• Large amounts of logic can be displayed.

• Circuit elements can be highlighted in color to indicate status.

• More than one program can be stored on the computers hard drive

• The computer can be used to document the program.

• PC software provides cut-and-paste features for program developingand editing.

• Allows easy monitoring of data tables.

• Easy to make copies of the program on floppy disk,CD-ROM, orhard drive

MELSOFT

SIMATIC

RSLogix

CX-Programmer

ProWorx 32

Mitsubishi

Siemens

RockwellAutomation

OMRON

Telemecanique

Programmingsoftware

Manufacturer

Hand Held Programming Device

• Has a connecting cable so thatit can be plugged into a PLCprogramming port.

• Are compact, inexpensive, andeasy to use.

• Contains keys for instructionentering and editing, andnavigation keys for movingaround the program