Unit‑5: Basics of PLC

 

📘 Unit‑5: Basics of PLC

5.1 Industrial Automation

  • Definition: Use of control systems (computers, PLCs, robots) and information technologies to handle industrial processes with minimal human intervention.

  • Block Diagram & Working:

    • Sensors/Transducers → detect process variables (temperature, pressure, speed).

    • Controller (PLC/DCS) → processes signals, applies logic/control algorithms.

    • Actuators → execute commands (motors, valves, relays).

    • Human–Machine Interface (HMI) → operator monitoring and control.

    • Communication Network → links devices for data exchange.

5.2 Programmable Logic Controller (PLC)

  • Definition: A digital computer used for automation of electromechanical processes.

  • Block Diagram (PLC):

    • Power Supply → provides DC power to PLC circuits.

    • Processor (CPU) → executes control program, manages memory.

    • Memory → stores user program and data.

    • Input Module → receives signals from sensors/switches.

    • Output Module → sends signals to actuators.

    • Programming Device → used to write and load logic.

  • Principle of Operation: PLC scans inputs, executes program, updates outputs in a continuous cycle.

  • Functions of Blocks:

    • Input conditioning, logic execution, communication, diagnostics, output driving.

5.3 Input & Output Modules

  • Digital I/O: ON/OFF signals (switches, relays, LEDs).

  • Analog I/O: Continuous signals (temperature, pressure, voltage).

  • I/O Specifications: Voltage/current rating, isolation, response time, number of channels.

5.4 Classification of PLCs

  • Fixed PLCs: Compact, limited I/O, economical.

  • Modular PLCs: Expandable, flexible, multiple modules for large systems.

5.5 PLC Scan Cycle

  • Steps:

    1. Read Inputs

    2. Execute Program

    3. Update Outputs

    4. Perform Diagnostics/Communication

  • Speed of Execution: Depends on CPU speed, program length, and I/O count (typically milliseconds).

5.6 Advantages & Applications

  • Advantages:

    • Flexibility (easy reprogramming)

    • Reliability (rugged, industrial grade)

    • Compact size

    • Faster response

    • Easy troubleshooting

  • Applications:

    • Assembly lines

    • Packaging machines

    • Process industries (chemical, cement, steel)

    • Robotics and material handling

    • Power system automation

5.7 PLC Installation (Detailed)

  • Steps:

    1. Site Preparation: Ensure dust‑free, vibration‑free, temperature‑controlled environment.

    2. Mounting: Install PLC in control panel/rack with proper grounding.

    3. Wiring: Connect power supply, input devices (sensors, switches), output devices (motors, relays).

    4. I/O Addressing: Assign addresses to each input/output channel.

    5. Programming: Load control logic via programming device.

    6. Testing: Verify input/output response, safety interlocks.

    7. Commissioning: Run system under supervision, check performance.

  • Precautions: Proper earthing, surge protection, shielding of cables, ventilation.

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