Introduction To Programmable Logic Controllers

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The concept: a machine that can be started from two remote places

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InputsOutputsProgram

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Inputs and Outputs DevicesPush ButtonsProximity switchesPhotoelectric sensorsTemperature sensorsPressure sensors

MotorsSolenoidsIndicator lampsResistive loadsContactors

InputsOutputsPush buttonPhoto SensorPressure SensorMotor

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Relays++--

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Programmable Logic ControllersBox Type Modular or Rack Type

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Typical PLC ApplicationsCoin-Operated Carwash Conveyor Diverter Control Greenhouse IrrigationLumber Mill OperationOil recovery systems

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Three Phase ConverterA machine requires three phase power to operate but only two phase power is available.A power converter must be built using a three phase converter.

Three phase converter

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Starting capacitor

Momentary push button3-Phase converter220 VACPower On/Off Switch

Converter OperationThe power is switched onTo start the converters motor the push button is held for 1 to 2 seconds

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StartRelay1-PoleContactorTimerRelay2-Pole ContactorStopRemote StopStartRemote StartRemoteLamp3-Phase Converter

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3-Phase Converter

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PLC1-PoleContactor2-Pole Contactor

StopRemote StopStartRemote StartRemoteLamp3-Phase Converter

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Example: Process SafetyPressure Emergency ReleaseSolenoidCounterPressure Sensor10 SecondTimerPressureReleaseValve

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Example ProcessSensors,Emergency Shut offSolenoidsPLC(programmable logic controller)

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A PLC replaces the wiring between input and output devices. Instead of being wired together, all equipment is wired to the PLC.The logic implementd through wiring is now a program inside the PLC All Input DevicesAll OutputDevicesPLC

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Programmable Logic ControllersPLCs (Programmable Logic Controllers) is a miniature industrial computer performs control functions [4] The first PLCs can be traced back to 1968 and became popular in the 1980s [4]PLCs are rugged and designed to withstand the industrial environment.

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Components of a PLCInputsCPUMemoryOutputs

InputsOutputsCPUMemory

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PLC Inputs and OutputsAnalog, Discrete or DigitalProtected by OptoisolatorsSourcing and SinkingCommunications

Optoisolator

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PLC OperationScanning is the process of running the PLC programCycle time is the total time for one loop.

Check Input StatusExecute ProgramUpdate Output Status

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PLC ProgrammingLadder logic is the main programming method used for PLCs [39]It is a visual and symbolic programming language that resembles relays logic diagramsLadder logic has been developed to mimic relay logic to reduce amount of retraining needed for engineers and trades people [39]

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Ladder LogicCompare a circuits diagram to a ladder logic diagram

RelayButtonMotor24 V DC InputButtonPower Rails

Motor

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Ladder Logic

Rung 1Rung 2Rung 3Rung 4Power Rails

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Ladder LogicLadder logic uses a variety of programming symbols Power always flows from left to rightOutput devices are in the ON state if power flows through them

Normally Closed ContactNormally Open ContactOutput Device or CoilPower rails

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ContactsContact programmed normally openContact programmed normally closedPower flows when the input device is onPower flows when the input device is off

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Ladder Logic

ButtonMotorMotorMotor

On lampOff lampMotor outputLamp for power appliedLamp for power not appliedContacts

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Ladder LogicLatching is the term for a self-maintaining circuit.

Start buttonMotorMotor

Stop button

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PLC

Pressure sensorSolenoidCounter+--+

Timer

SolenoidTimerPressure sensorPLC ladder logic programWiring diagram for PLC Pressure emergency release

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PLC

Pressure SensorSolenoid+--+

Safety Sensor-+

Manual shut off-+

Wiring diagram for PLC

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Manual shut offSafety sensorSensorCounter

Timer

SolenoidTimerPLC ladder logic program

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Industrial MixerFilled with liquidHeatedMix for 10 minutes.

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Sequential Function ChartState 2State 1State 3RotateHeatFill with liquidLevel full switchTimerTemperature Switch

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Start buttonPump

Level full floaterHeaterMixing Motor

Temperature sensor

211233PLC

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Start buttonPumpPump

Level full floaterLevel full floaterHeater

Temperature SensorState 1State 2

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10 minute Timer10 minute Timer

Mixing motor

Temperature SensorState 3

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Ladder LogicForcingTime checksSimulationSoftware testing

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Ladder LogicRegisters and bitsData comparisonArithmetic operationsFunctionsPID control

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PLCs solve problemsFlexibleCost effectiveComputational abilitiesTrouble shootingReliable

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