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8/3/2019 3_A Programmable Logic Controller
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Exc el lence i nA utomat ion & Drives:
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F. A. KETTANEH & CO. LTD.
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