ej 501 shamsul anuar- chapter 3

8/10/2019 Ej 501 Shamsul Anuar- Chapter 3

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EJ501 PLC AND AUTOMATION SYSTEM


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small computers, dedicated to automation

tasks in an industrial environment"

Specialized computer used to control

machines and process.

PLC

Inputs Outputs

Program

A digitally operating electronic apparatus

which uses a programming memory for the

internal storage of instructions for

implementing specific functions such as logic,

sequencing, timing, counting and arithmetic tocontrol through digital or analog modules,

various types of machines or process.

* Nat ional Elect ri cal Manuf actu rers Associat ion of Ameri ca

Basic Characteristic of PLC


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The Hydramatic Division of the General Motors

(GM) Corporation specified the design criteria

for the first programmable controller in 1968.

Their primary goal:-

To eliminate the high costs associated with

inflexible, relay-controlled systems.

Programmable, reusable and reliable.


The controller had to be designed in modular

form, so that sub-assemblies could be removed

easily for replacement or repair.

The control system needed the capability to passdata collection to a central system.

The system had to be reusable.

The method used to program the controller had

to be simple, so that it could be easily

understood by plant personnel.



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1968 Programmable concept developed

1969 Hardware CPU controller, with logic instructions, 1 K ofmemory and 128 I/O points

1974 Use of several (multi) processors within a PLC - timers andcounters; arithmetic operations; 12 K of memory and 1024I/O points

1976 Remote input/output systems introduced Microprocessors -based PLC introduced

1980 Intelligent I/O modules developed

Enhanced communications facilities

Enhanced software features (e.g. documentation)Use of personal microcomputers as programming aids

1983 Low - cost small PLCs introduced

1985 on Networking of all levels of PLC, computerand machine using SCADA software.




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Less wiring.

Less expensive.

Wiring between devices and relay contacts

are done in the PLC program.

Easier and faster to make changes.

Trouble shooting aids make programming

easier and reduce downtime.

Reliable components make these likely tooperate for years before failure.

Handle much more complicated systems.




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(1)

(2)

Compact PLC

Modular PLC

Smallest and inexpensive Monoprocessor and fieldbus connection Fixed casing Fixed number of I/O (most of them binary) not expandable Typical product: Mitsubishi MELSEC F, ABB AC31, SIMATIC S7 Application Overhead Door control, etc.

Modular construction (backplane) One or multiprocessor system Fieldbus and LAN connection Connection to serial bus I/O Typical products: SIMATIC S5-115, Hitachi H-Serie, ABB AC110 Application Thermocouple input, etc.

3. Rack-PLC

Rack styles PLCs are usually more expensive, expandable, andpowerful than compact or modular PLCs.

The rack provides a power and communication backplane thatgreatly increases the communication rate between theprocessor and the modules as well as allowing some specialtymodules to communicate with each other without the

processor.

The number of available I/O points is also much higher in therack systems.

Racks also allow for redundant

processors for critical systems

such as waste water pumps

or fire control systems.



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Building structureof PLC

Typically a PLC system has

five basic components.Display/Indicator

Memory

Processor

Power

Supply

Input

Interface

Output

Interface

These are :-

Processor unit

Memory unit

Power supply

unit

Input interface

Output interface Display /indicator

CPUThe processor unit or central processing unit (CPU) is the unit

containing the microprocessor and this interprets the input

signals and carries out the control actions, according to the

program stored in its memory, communicating decisions as a

action signals to the output.

Memory UnitProgram is stored in memory unit, and is used for the

control actions to be exercised by the microprocessor.

1. Random Access Memory (RAM)

read / write

memory will be lost when the power supply disconnected.

2. Read Only Memory (ROM)

read-only memory type.

special equipment is used to delete a memory.



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Display/indicator unitDisplay and indicator unit refers to the internal relay

status display PLC. This can be seen in the Console

programming if the user using the mnemonic code and

programming computer screen if the user using software

programming.

Input InterfaceInputs carry signals from the process into the controller;

they can be input switches, pressure sensors, etc.

These are like the senses and sensors of the PLC.

Output InterfaceOutputs are the devices that the PLC uses to send

changes out to the world. These are the actuator the

PLC can change to adjust or control the process -

motors, lights, relays, pumps, etcBuilding structureof PLC



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PROCESSOR

POWER

SUPPLY

I M

N O

P D

U U

T L

E

O M

U O

T D

P U

U L

T E

PROGRAMMING

DEVICE

From

SENSORS

Pushbuttons,

contacts,

limit switches,etc.

To

OUTPUT

Solenoids,

contactors,

alarmsetc.

Four Steps in the PLC Operations

Input Scan

Scan the state of the Inputs

Program Scan

Processes the program logic Output Scan

Energize/de-energize the outputs

Housekeeping

This step includes communications,

Internal Diagnostics, etc.

The steps are continually repeated - processed in a loop


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Programming Unit (PC, Programming Console, Program LoaderUnit, VDU)

i) PC is the commonly used programming device. The softwareallows user to create, edit, document, store and troubleshootprograms. PC communicates with PLC via serial or paralleldata communications link.

ii)Programming Console

Programming Console usually used to program small PLC. Theyare compact, inexpensive and easy to use but are not able todisplay much logic on screen as computer monitor. They oftenused on the factory floor for troubleshooting, modify programand transferring program to multiple machines.

iii)Program Loader Unit

To load the program into the PLC system.

iv)VDU

To processing operations and system status information to bedisplayed by the PLC Building structureof PLC


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Housing Unit

To provide protection to the circuits and components

- the internal components of a PLC.

Secondary Storage Unit

This unit is related to the CPU where all the

programs and information stored.

Power Supply Unit

This unit provides power to the CPU, Input Unit and

Output Unit.Printer Unit

This unit is used to print the program a control

system that is controlled by the PLC, either graphics

or text.Building structureof PLC


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The input devices used with a PLC include

pushbuttons, limit switches, relay contacts,

photo sensors, proximity switches, temperature

sensors, etc. these input devices can be AC or

DC.

The input voltages can be high or low.

The input signals can be analog or digital.

Differing inputs require different input modules.

An input module provides an interface betweeninput devices and a PLCs CPU, which is only use

low DC voltage

The input modules function is to convert

the input signals to DC voltages that are

acceptable to the CPU.

Standard discrete input modules include 24V

AC, 48V AC, 120V AC, 220V AC, 24V DC, 48V

DC, 120V DC, 220V DC, and Transistor-

transistor Logic (TTL) level.

The devices controlled by a PLC include

relays, alarms, solenoids, fans, lights, and

motor starters. These devices may require

different levels of AC or DC voltages


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PLC operated by sending output signals to

external devices called output devices.

Example to output devices are alarm, light,

valve, motor starter etc.

Output modules are used to provide the

electrical connection between the internal

processing unit of PLC and the output devices to

which it provides a signal.

PLC manufactures produce various types ofoutput modules. The modules differ by the types

of electrical signal they produce such as variable

DC voltage, AC voltage and ON/OFF (discrete)

voltage.


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ej 501 shamsul anuar- chapter 3

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