Function Block Diagrams | PLC Programming

Introduction

This tutorial will guide you through the basics of Function Block Diagrams (FBDs) in PLC programming. FBDs are a powerful tool used to design control systems and are especially useful in industrial automation. We'll explore how to create FBDs, their relationship with ladder logic, and the standard symbols defined by the IEC.

Step 1: Understanding Function Block Diagrams

• Definition: FBDs are graphical representations of functions and their inputs/outputs. They allow for easy visualization of complex processes.
• Comparison with Ladder Logic:
  • Ladder logic uses rungs to represent operations.
  • FBDs represent the same operations using blocks, making them intuitive for certain applications.

Practical Tips

• Familiarize yourself with basic PLC concepts before diving into FBDs.
• Consider how FBDs can simplify visualizing logic that may be complex in ladder form.

Step 2: Familiarizing with IEC Symbols

• Standard Symbols: Learn the standard symbols used in FBDs, as defined by the IEC. Common symbols include:
  • Input blocks (like switches)
  • Output blocks (like motors)
  • Function blocks (like AND, OR, NOT)

Practical Advice

• Create a reference sheet of these symbols for quick access while designing.
• Practice drawing these symbols to get comfortable with their shapes and meanings.

Step 3: Creating FBDs from Scratch

• Design Process:
  1. Identify the control process you want to implement.
  2. Break down the process into functions that can be represented as blocks.
  3. Arrange the blocks logically, connecting them to show the flow of information.

Example

• If controlling a motor based on a sensor input:
  • Create an input block for the sensor.
  • Create an output block for the motor.
  • Use a function block (e.g., AND) to link them.

Step 4: Generating FBDs from Ladder Diagrams

• Conversion Steps:
  1. Analyze the ladder diagram and identify the main components.
  2. Map each rung of the ladder to corresponding function blocks in the FBD.
  3. Connect blocks as per the logic represented in the ladder.

Common Pitfalls

• Ensure that the logical flow remains consistent between the two formats.
• Double-check connections to avoid misrepresenting the control logic.

Step 5: Using OpenPLC for FBD Programming

• Installation: Follow the installation guide for OpenPLC as outlined in the linked video.
• Programming:
  • Open the software and select the FBD programming option.
  • Use the symbols and blocks you’ve learned to create your FBD.
  • Test your program to ensure it behaves as expected.

Real-World Application

• Use OpenPLC to simulate your FBDs before implementing them in a physical PLC. This helps catch errors early.

Conclusion

Function Block Diagrams are an essential part of PLC programming that can enhance your ability to visualize and implement control logic. By understanding the symbols, creating diagrams from scratch, and converting from ladder logic, you can effectively utilize FBDs in your projects. As a next step, consider installing OpenPLC and practicing creating both FBDs and ladder diagrams to solidify your understanding.