In-Depth Notes on Programmable Logic Controllers (PLC)

History of PLC

  • 1968: Bill Stone of General Motors presented issues regarding machinery reliability and documentation at a Westinghouse Conference.

    • Goals:
    • Extend static circuit advantages to 90% of machines.
    • Minimize downtime related to control issues.
    • Ensure ease of maintenance and programming.
    • Facilitate future expansions.
    • Ensure functionality in industrial settings.
    • Include full logic capabilities except data reduction.

  • Key Contributors:

    • Companies involved in the development:
    • Allen-Bradley
    • Digital Equipment Corporation (DEC)
    • Century Detroit
    • Bedford Associates

  • 1960s: The use of manual relay panels with various issues:

    • Limited flexibility & expandability.
    • Troubleshooting challenges with dirty contacts and loose wires.

  • 1971: Allen-Bradley introduced the Bulletin 1774 PLC.

    • Terminology shifted from "Programmable Controller" to "Programmable Logic Controller" as PLC became the industrial standard.

  • 1973: Development of the Modicon 184.

    • Led to Modicon becoming a market leader.

Definition of PLC

  • A Programmable Logic Controller (PLC) is an industrial computer:
    • Controls and monitors industrial equipment.
    • Initially a relay replacement in the automotive industry.
    • Automates control activities in manufacturing.

Types of PLC

According to Structure

  1. Fixed/Integrated/Compact PLC:

    • Has a fixed number of inputs/outputs within a case.

  2. Modular PLC:

    • Allows for expansions using additional modules.

According to Output Type

  1. Relay Output Type:

    • Uses relay for output control (AC/DC).
    • Suitable for infrequent switching operations.

  2. Transistor Output Type:

    • Solid-state device provides faster switching but limited to DC.

  3. Triac Output Type:

    • Uses mirrored transistors for AC outputs.

  4. Analog Output Type:

    • Used for controlling devices like motors with ON/OFF switching capability.

According to Size

  1. Mini PLC:

    • 128 to 512 I/O points for smaller control systems.

  2. Micro PLC:

    • 15 to 128 I/O points, ideal for small-scale automation like amusement rides.

  3. Pico/Nano PLC:

    • 15 or fewer I/O points, suited for basic automation and educational purposes.

Components in a PLC System

  • Power Supply:

    • Converts power for the PLC system operations.

  • Central Processing Unit (CPU):

    • Handles data processing, instruction execution, and program memory.

  • I/O Modules:

    • Connects PLC to external devices, with input modules receiving signals and output modules sending signals.

  • Programming Device:

    • Device (PC or handheld) used for programming PLC and transferring the program via cables.

Applications of PLC

  • Domestic and Commercial:

    • Water level control, automatic washing systems, traffic control, elevators, etc.

  • Industrial:

    • Air compressor control, temperature regulation, production line control, etc.

  • Industries Using PLCs:

    • Petrochemical, oil and gas, steel manufacturing, food industry, etc.

Advantages of PLC

  • Easy installation and maintenance due to fewer wires compared to relay systems.
  • Programmable offline and online without needing wiring changes.
  • Minimal maintenance requirements, reducing operational costs.
  • Fast troubleshooting via programming software.
  • Quick operating times, generally in milliseconds.

Disadvantages of PLC

  • Not effective under high temperatures or excessive vibrations.
  • High initial cost not justifiable for systems not requiring wiring changes.

Conclusion

  • PLCs have revolutionized automation in various industries by enhancing control systems, offering easy configuration, and reducing downtime. Their evolution from relay-based systems to advanced programmable controllers represents significant advancements in industrial automation.