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    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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    Inputs Outputs Program

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    Inputs and Outputs Devices • Push Buttons • Proximity switches • Photoelectric sensors • Temperature sensors • Pressure sensors • Motors • Solenoids • Indicator lamps • Resistive loads • Contactors Inputs Outputs Push button Photo Sensor Pressure Sensor Motor

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    Relays + + - -

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    Programmable Logic Controllers Box Type Modular or Rack Type

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    Typical PLC Applications • Coin-Operated Carwash • Conveyor Diverter Control • Greenhouse Irrigation • Lumber Mill Operation • Oil recovery systems

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    Three Phase Converter • A 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 button 3-Phase converter 220 VAC Power On/Off Switch Converter Operation • The power is switched on • To start the converter’s motor the push button is held for 1 to 2 seconds

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    Start Relay 1-Pole Contactor Timer Relay 2-Pole Contactor Stop Remote Stop Start Remote Start Remote Lamp 3-Phase Converter

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    3-Phase Converter

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    PLC 1-Pole Contactor 2-Pole Contactor Stop Remote Stop Start Remote Start Remote Lamp 3-Phase Converter

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    Example: Process Safety Pressure Emergency Release Solenoid Counter Pressure Sensor 10 Second Timer Pressure Release Valve

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    Example Process Sensors, Emergency Shut off Solenoids PLC (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 Devices All Output Devices PLC

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    Programmable Logic Controllers • PLCs (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 1980’s [4] • PLCs are rugged and designed to withstand the industrial environmen...

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    Components of a PLC • Inputs • CPU • Memory • Outputs 15,Inputs 15, 15,Outputs 15, 13,CPU 13, 14,Memory 14,

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    PLC Inputs and Outputs • Analog, Discrete or Digital • Protected by Optoisolators • Sourcing and Sinking • Communications Optoisolator

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    PLC Operation • Scanning is the process of running the PLC program • Cycle time is the total time for one loop. Check Input Status Execute Program Update Output Status

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    PLC Programming • Ladder logic is the main programming method used for PLCs [39] • It is a visual and symbolic programming language that resembles relays logic diagrams • Ladder logic has been developed to mimic relay logic to reduce amount of retraining needed for engineers and trades peop...

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    Ladder Logic • Compare a circuits diagram to a ladder logic diagram Relay Button Motor 24 V DC Input Button Power Rails Motor

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    Ladder Logic Rung 1 Rung 2 Rung 3 Rung 4 Power Rails

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    Ladder Logic • Ladder logic uses a variety of programming symbols • Power always flows from left to right • Output devices are in the ON state if power flows through them Normally Closed Contact Normally Open Contact Output Device or Coil Power rails

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    Contacts Contact programmed normally open Contact programmed normally closed Power flows when the input device is on Power flows when the input device is off

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    Ladder Logic Button Motor Motor Motor On lamp Off lamp Motor output Lamp for power applied Lamp for power not applied Contacts

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    Ladder Logic • Latching is the term for a self-maintaining circuit. Start button Motor Motor Stop button

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    PLC Pressure sensor Solenoid Counter + - - + Timer Solenoid Timer Pressure sensor PLC ladder logic program Wiring diagram for PLC Pressure emergency release

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    PLC Pressure Sensor Solenoid + - - + Safety Sensor - + Manual shut off - + Wiring diagram for PLC

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    Manual shut off Safety sensor Sensor Counter Timer Solenoid Timer PLC ladder logic program

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    Industrial Mixer • Filled with liquid • Heated • Mix for 10 minutes.

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    Sequential Function Chart State 2 State 1 State 3 Rotate Heat Fill with liquid Level full switch Timer Temperature Switch

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    Start button Pump Level full floater Heater Mixing Motor Temperature sensor 2 1 1 2 3 3 PLC

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    Start button Pump Pump Level full floater Level full floater Heater Temperature Sensor State 1 State 2

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    10 minute Timer 10 minute Timer Mixing motor Temperature Sensor State 3

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    Ladder Logic • Forcing • Time checks • Simulation • Software testing

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    Ladder Logic • Registers and bits • Data comparison • Arithmetic operations • Functions • PID control

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    PLCs solve problems • Flexible • Cost effective • Computational abilities • Trouble shooting • Reliable

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    Questions?

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