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P L C

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Transcript

  • 1.
    • What is a PLC?
    Introduction to Programmable Logic Controllers (PLCs)
    • Hardware components of a PLC
    • Memory structure of a PLC
    • PLC Modes of Operation
    • Wiring Diagram
    • Digital Inputs/Outputs
  • 2. What is a PLC?
    • A Programmable Logic Controller (PLC) is
      • a microprocessor based control system,
      • designed for use in an industrial environment,
      • programmed to sense, activate and control industrial equipment.
  • 3. Hardware components of a PLC
  • 4. Memory structure of a PLC
  • 5. PLC Mode of Operation
    • A PLC operates in one of two modes:
      • Program Mode: Download the user’s program from the programming device to the memory of the PLC.
      • Run Mode: Execute the user’s program using the scan-based execution model, in three phases:
        • Read the state of the inputs and store them in the RAM referred to as the input image memory..
        • Execute the user’s program and update the state of the outputs in the RAM (output image memory).
        • Copy the states of the outputs from the RAM to the physical output devices.
      • The time needed to complete one scan cycle depends on the length and complexity of the user’s program.
      • PLC manufacturers specify the maximum number of program steps (say 1000) and the average scan time (few ms).
  • 6. Wiring Diagram
  • 7. DC Voltage Digital Input
    • The optoisolator isolates the internal components of the PLC and the external input devices.
    • Diode D1 provides protection during switching from inductive devices.
    • When the switch is closed, current flows through the status LED and the opto-isolator, giving a logic 1 to the input port.
  • 8. Connecting DC Proximity Switches
  • 9. AC Voltage Digital Input
    • AC input devices switch On and Off the mains voltage.
    • The mains voltage must be reduced using a voltage divider.
    • The mains voltage must be converted to DC using a rectifier and a smoothing circuit.
    • An AC proximity switch switches On and Off the mains voltage by triggering (firing) a triac. The proximity sensor controls the gate current of the triac.
  • 10. Digital Output (Relay and Transistor)
    • Most PLCs have relay outputs.
    • Relay outputs have the advantage that they can control DC as well as AC loads at relatively high currents.
    • The disadvantages of relays are due to their mechanical contacts.
    • Transistor outputs provide higher switching speeds with more switching life.
    • Transistor outputs can only be used with DC loads and are polarity sensitive. A diode is usually used for reverse polarity protection.
    • Another diode is usually used to protect the output transistor when switching inductive loads.
  • 11. Digital Output Using Triacs
    • Triac outputs are used to avoid the problems due to the mechanical contacts of relays.
    • An opto-diac is used to fire the triac.
    • An RC circuit is usually used to avoid problems due to the phase difference between the voltage and the current when driving inductive loads.