Basics of PLC

 

Part B: Programmable Logic Controllers (PLC)

1. Introduction to PLC

  • Definition: A PLC is an industrial digital computer designed for automation of electromechanical processes.

  • Need: Replaces relay-based logic, offers flexibility, reliability, and easy programming.

2. PLC Architecture

  • Main Components:

    • CPU (Central Processing Unit)

    • Input Module (sensors, switches)

    • Output Module (motors, lamps)

    • Memory (stores program)

    • Power Supply

3. PLC Programming Languages

  • Ladder Logic (most common): Resembles relay logic diagrams.

  • Function Block Diagram (FBD).

  • Structured Text (ST).

  • Instruction List (IL)

4. Basic PLC Instructions

  • Input/Output Instructions: Read sensors, activate actuators.

  • Timers: Delay ON/OFF operations.

  • Counters: Count events or objects.

  • Arithmetic Instructions: Addition, subtraction, multiplication.

  • Comparison Instructions: Greater than, equal to, less than.

5. PLC Applications

  • Industrial Automation: Conveyor belts, packaging machines.

  • Process Control: Temperature, pressure, flow.

  • Electrical Machines: Motor starting, speed control.

  • Building Automation: Lighting, elevators.

6. PLC Advantages

  • Flexibility in programming.

  • Easy troubleshooting.

  • Compact and reliable.

  • Handles complex logic with ease.

7. Sample Ladder Logic Problems

  • Problem 1: Design a ladder diagram to start a motor when a start button is pressed and stop it when a stop button is pressed.

  • Problem 2: Create a program to switch ON a lamp after 10 seconds delay.

  • Problem 3: Use a counter to count 5 objects passing on a conveyor and then stop the motor.

📑 Summary

  • Control Systems: Focus on block diagram reduction, stability analysis, time/frequency domain methods, and controllers.

  • PLC: Understand architecture, programming, instructions, and industrial applications.

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