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What is a PLCPLC?

AAPProgrammablerogrammable LLogicogic CControllerontroller

is ais a microprocessormicroprocessor -- basedbased system,system,

used toused to controlcontrol machines and processes,machines and processes,in a wide variety ofin a wide variety ofindustriesindustries..

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Formal Definition ofFormal Definition ofPLCPLC

A digitally operating electronic apparatus,

which uses a programmable memory for the

internal storage of instructions, for implementing

specific functions, such as: logic, sequencing,

timing, counting, and arithmetic, to control,

through digital or analog input/output modules,

various types of machines or processes.*

AAdigitallydigitally operatingoperating electronicelectronic apparatus,apparatus,

which uses awhich uses a programmableprogrammable memorymemory for thefor the

internal storage of instructions, for implementinginternal storage of instructions, for implementing

specific functions, such as:specific functions, such as: logiclogic,

, sequencingsequencing,

,

timingtiming,

, countingcounting, and

, and arithmeticarithmetic, to, to controlcontrol,,

throughthrough digitaldigital oror analoganalog input/outputinput/output modules,modules,various types ofvarious types ofmachinesmachines oror processesprocesses..**

* According to NEMA (National Electrical Manufacturing Association)

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Historical Development ofHistorical Development ofPLCPLC

The first PLC was designed byThe first PLC was designed by GouldGould Corporation inCorporation in19681968 for General Motors (GM).for General Motors (GM).

The main reason for their design was to replaceThe main reason for their design was to replacehardwiredhardwired electricalelectrical relaysrelays used to control automaticused to control automatictransfer lines. Thus instead oftransfer lines. Thus instead ofrewiringrewiring relays,relays,reprogrammingreprogramming is employed to change the sequence ofis employed to change the sequence ofmachine operation.machine operation.

They are designed forThey are designed foreaseease of programming andof programming andmaintenance.maintenance.

The first controllers were capable ofThe first controllers were capable ofON/OFFON/OFF controlcontrolonly.only.

AnalogAnalog signal processing supported.signal processing supported.CommunicationCommunication capabilities introduced.capabilities introduced.

StateState--ofof--thethe--art PLCart PLCs are capable of advanceds are capable of advancedfunctions, and the trend in their development is towardsfunctions, and the trend in their development is towardscomputercomputercapabilities.capabilities.

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Hardware Components ofHardware Components of PLCPLC

1. Central Processing Unit (CPU)

Main Components:Main Components:

Sensors

2. Memory (ROM, RAM, EPROM, EEPROM,...)

3. Input modules (DI, AI)ProgramMemory

Processor

Bus System

CPU InputModule

Output

Module

PowerSupply

4. Output modules (DO, AO)

5. Power supply (PS)

6. Programming device (PG.)Optional Components:Optional Components:

7. Special Modules: Actuators

Communications Processor (CP): (PC, modem, Printer, PLC, Network, ..etc.)

Interface Modules (IM): Local Expansion, Remote/distributed I/O Simulators

Function Modules: HSC, Closed loop control, Weighing, positioning

8 . Operator panel (HMI).

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Why UseWhy Use PLCPLCss??

Programming FlexibilityProgramming Flexibility

Easier Trouble shooting reduces downtimeEasier Trouble shooting reduces downtime High Reliability ( High MTBF)High Reliability ( High MTBF)

Space savingSpace saving

Computational capabilitiesComputational capabilities

Cost effectiveCost effective

Ability to withstand harsh environmentAbility to withstand harsh environment

Expandability and ModularityExpandability and Modularity

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Principle of Operation of aPrinciple of Operation of a PLCPLC

Cyclic ScanCyclic Scan

1. Read All Inputs

2. Store Inputs in PII

3. Process Program

4. Store outputs in PIQ

5. Update Physical Outputs

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Criteria For Selecting aCriteria For Selecting a PLCPLC

Number of Inputs and OutputsNumber of Inputs and Outputs

Memory CapacityMemory Capacity

Scan TimeScan Time

Expansion CapabilitiesExpansion Capabilities

Special Expansion ModulesSpecial Expansion Modules Communication CapabilitiesCommunication Capabilities

SoftwareSoftware

Working EnvironmentWorking Environment

Vendor Support & ServicesVendor Support & Services

Cost

Cost