Programmable Logic Programmable Logic ControllerController

(PLC)(PLC)

ContentsContentsControl ProcessWhat is PLC?General Architecture of PLCPLC Operating CyclePLC Hardware TypePLC Architecture EvolutionProgramming of PLC Basic PLC AdvantagesApplications

Control ProcessControl ProcessPhysical Quantity is sensed in the form of

small current/voltage e.g. Temperature, Pressure, Flow, Level etc (Sensor)

This Electrical Signal is amplified to a certain level (Amplifier)

Then amplified analogue output is converted into digital form. (Analogue to Digital Converter i.e. A/D)

This digital output is fed to the controller of the system to control the various physical quantities with the help of different devices. (PLC, Microcontroller, DSP etc…)

Block DiagramBlock Diagram

Physical Quantity(Temperature, Pressure etc..)

Amplifier

Analogue output in mA/mV

A/D

Amplified Analogue output

Controller(PLC, Microcontroller

etc…)

Devices(Motors etc…)

Digital

Control signals Digital

/Analogue

What is PLC?What is PLC?A digitally operating electronic

apparatus which uses a programmable memory for internal storage of instructions

Implement specific functions, such as logic, sequencing, timing, counting and arithmetic

Control various types of machines or process through digital or analogue input/output.

Traditional concept of PLCTraditional concept of PLC

PLC performs relay equivalent functions

PLC performs ON/OFF control

Designed for industrial environment

General Architecture of General Architecture of PLC PLC

PLC Operating CyclePLC Operating Cycle

PLC Signal FlowPLC Signal Flow

Output

PLC Hardware TypePLC Hardware TypeA most basic PLC system is a self contained

PLC which has two terminal blocks, one for the Input and other for the Output, called “Micros”.

Typically they provide front panel LED status indication of I/O and processor status.

2. Modular Chassis Based 2. Modular Chassis Based PLCPLCThe vast majority of PLC’s installed

today are modular chassis based PLC consisting of:

3. Modular Chassis-less PLC 3. Modular Chassis-less PLC SystemsSystemsThe advanced PLC’s are chassis-

lessThese are modular PLC systems.These systems also have: -Processor - Power supply -I/O modules -Communication

card These components mount directly

on a panel to allow easy insertion and removal.

General PLC BlocksGeneral PLC Blocks

Power Supply

CPU

Communicat-ion Module

Analogue Input

Analogue Output

Digital Input

Digital Output

220V, 50/60Hz

AC to DC conversion (12V or 48V)

Personal Computer

RS 232

Contd…Contd…

Contd…Contd…

48V DC

Contd…Contd…

48V DC

Contd…Contd…

Contd…Contd…

Bus system is

PLC Architecture EvolutionPLC Architecture Evolution

PLC Architecture EvolutionPLC Architecture Evolution

PLC Architecture EvolutionPLC Architecture Evolution

PLC Architecture EvolutionPLC Architecture Evolution

PLC Architecture EvolutionPLC Architecture Evolution

PLC Architecture EvolutionPLC Architecture Evolution

Programming of PLCProgramming of PLC The purpose of a PLC program is to

control the state of outputs based on the current condition of the inputs.

Types of programming :Ladder Logic (Analogous to Relay

Technology)Function Block (FB’s based on AND-OR logic)Visual Basic (VB based on C language)

*DOS based programming (Old Technology)

1. Ladder Logic1. Ladder LogicMost popular and most commonly

used PLC language is known as “Ladder Logic” .

Ladder Logic purposely resembles relay logic.

In order to control a process we try to design the logical continuity from input to output of program.

Ladder Logic ConceptsLadder Logic Concepts

Line Neutral

Ladder Logic ConceptsLadder Logic Concepts

Ladder Logic ConceptsLadder Logic Concepts

Read and Write InstructionRead and Write Instruction

Read Instruction

Write Instruction

Some Important Some Important InstructionsInstructionsLatching/Unlatching Instruction:

Latch

UnlatchRetain the output energized until reset

Timer InstructionsTimer Instructions

Timer ON Delay(TON)

Timer OFF Delay(TOF)

Retentive timer-on delay(RTO)

Counter InstructionsCounter Instructions

Counter Up (CTU)

Counter Down (CTD)

Comparison InstructionsComparison Instructions

Equal (EQU)Not Equal (NEQ)Less Than (LES)Less Than or Equal (LEQ)Greater Than (GRT)Greater Than or Equal (GEQ)

Some other instructions are Sequencer Instruction, Shift Register Instruction etc…

Function BlockFunction BlockBased on pre-defined blocks of

various functions

ANDAB

Basic PLC AdvantagesBasic PLC AdvantagesEase of ProgrammingEase of MaintenanceDesigned for Industrial

EnvironmentSuitable for Extreme

Environmental ConditionsQuick InstallationAdaptable to Change

ApplicationsApplications