Understanding Function Block Diagrams for Industrial Automation

Function Block Diagram (FBD)

Definition

A Function Block Diagram (FBD) is a graphical programming language used in industrial automation (e.g., PLCs, DCS, SCADA systems) to design and visualize control logic. It represents a system’s behavior using pre-defined “function blocks”—self-contained units that perform specific operations (e.g., logic gates, timers, mathematical calculations)—connected by wires that signal data flow between blocks. FBD is standardized by IEC 61131-3 (the international standard for programmable controller programming languages) and is widely used for its intuitive, flowchart-like structure, which simplifies complex control logic design and troubleshooting.

Core Concepts of FBD

1. Function Blocks

A function block is the fundamental building block of FBD, defined by:

Inputs: Data values required for the block to execute (e.g., a boolean Input A and Input B for an AND gate).
Outputs: Results generated by the block after execution (e.g., a boolean Output Q for an AND gate).
Parameters: Configurable settings that modify the block’s behavior (e.g., the time interval for a timer block).
Internal Logic: Pre-programmed operations (e.g., arithmetic, logic, or timing) that transform inputs into outputs.

Common Function Block Types (per IEC 61131-3):

Logic Blocks: AND, OR, NOT, XOR, NAND, NOR (for boolean logic operations).
Timers/Counters:
- TON (Timer On-Delay): Activates output after a set time when input is ON.
- TOF (Timer Off-Delay): Deactivates output after a set time when input is OFF.
- CTU (Counter Up): Increments count until a preset value is reached.
- CTD (Counter Down): Decrements count from a preset value to zero.
Mathematical Blocks: ADD, SUB, MUL, DIV, SIN, COS (for numeric calculations).
Comparators: GT (Greater Than), LT (Less Than), EQ (Equal), NEQ (Not Equal) (for comparing values).
Memory Blocks: SR (Set-Reset), RS (Reset-Set) flip-flops (for storing boolean states).
Communication Blocks: For data exchange between devices (e.g., MODBUS read/write blocks).

2. Wires & Data Flow

Wires in FBD represent the flow of data (boolean, integer, float, or custom data types) between function blocks:

A wire connects an output of one block to an input of another (one-way flow only).
Multiple wires can branch from a single output to feed data to multiple inputs (parallel data flow).
Wires cannot connect two outputs or two inputs—data always flows from output to input.

3. Execution Order

FBD executes logic in a top-to-bottom, left-to-right sequence (unless overridden by explicit control flow blocks):

Blocks with no incoming wires execute first (e.g., a sensor input block).
A block executes only when all its required inputs have valid data.
Outputs are updated immediately after a block executes, triggering downstream blocks.

FBD Structure & Example

Basic FBD for a Motor Start/Stop Circuit

A simple example of FBD logic for controlling a motor with a start button (NO contact), stop button (NC contact), and overload relay (NC contact):

Input Blocks:
- Start_Button: Boolean input (TRUE when pressed).
- Stop_Button: Boolean input (FALSE when pressed, since it’s NC).
- Overload_Relay: Boolean input (FALSE when tripped, NC).
Logic Blocks:
- AND1: AND gate with inputs Start_Button and Motor_Run (latching signal).
- AND2: AND gate with inputs from AND1 and Stop_Button (stop interlock).
- AND3: AND gate with inputs from AND2 and Overload_Relay (overload interlock).
- SR_FlipFlop: Set-Reset flip-flop where AND3 sets the output (Motor_Run = TRUE) and Stop_Button/Overload_Relay resets it (Motor_Run = FALSE).
Output Block:
- Motor_Output: Boolean output that activates the motor contactor (TRUE = motor runs).

Data Flow:

Start_Button → AND1 (with feedback from SR_FlipFlop output) → AND2 (with Stop_Button) → AND3 (with Overload_Relay) → SR_FlipFlop (Set) → Motor_Output (TRUE) + feedback to AND1 (latching).
Pressing Stop_Button or tripping Overload_Relay resets the SR_FlipFlop, turning off Motor_Output.

Key Advantages of FBD

Visual Clarity: FBD’s graphical format makes complex logic easy to understand at a glance—ideal for teams (engineers, technicians, operators) to collaborate and troubleshoot.
Reusability: Function blocks can be saved as libraries and reused across multiple projects (e.g., a custom “pump control” block for a water treatment plant).
Modularity: Logic is divided into independent blocks, so changes to one part of the system (e.g., a timer setting) do not affect unrelated blocks.
Standardization: IEC 61131-3 compliance ensures compatibility across different PLC/DCS platforms (e.g., Siemens, Allen-Bradley, Schneider Electric).
Scalability: FBD easily scales for large systems (e.g., a factory production line) by adding blocks and expanding data flow.

FBD vs. Other IEC 61131-3 Languages

FBD is one of five programming languages standardized by IEC 61131-3; it is often compared to:

Feature	Function Block Diagram (FBD)	Ladder Diagram (LD)	Structured Text (ST)	Instruction List (IL)	Sequential Function Chart (SFC)
Format	Graphical (blocks + wires)	Graphical (rungs)	Textual (high-level)	Textual (low-level)	Graphical (steps + transitions)
Best For	Continuous control, math, logic	Discrete control (relay logic)	Complex algorithms, math	Simple logic, legacy systems	Sequential processes (batch operations)
Learning Curve	Low (intuitive visuals)	Low (familiar to electricians)	Moderate (programming knowledge)	Low (assembly-like)	Moderate (sequential logic)
Example Use Case	PID control loops, analog signal processing	Motor start/stop, conveyor control	Recipe management, data logging	Simple relay logic	Pharmaceutical batch production

Applications of FBD

1. Industrial Automation

Process Control: Designing PID loops for regulating temperature, pressure, or flow in chemical plants, refineries, or water treatment facilities.
Discrete Manufacturing: Controlling assembly lines, robotic cells, or packaging machines (e.g., coordinating sensors, actuators, and timers).
Building Automation: Managing HVAC systems, lighting, and security systems (e.g., adjusting temperature based on occupancy sensors).

2. Energy & Utilities

Controlling power generation equipment (turbines, boilers) in power plants using FBD logic for load balancing and safety interlocks.
Managing renewable energy systems (solar inverters, wind turbine controllers) with mathematical blocks for power optimization.

3. Transportation

Designing control logic for railway signaling systems (e.g., track occupancy detection, signal timing) or automotive manufacturing lines.

Best Practices for FBD Design

Test Blocks Individually: Validate each block’s behavior before integrating it into the full system (e.g., test a timer block with a simulated input).

Name Blocks Clearly: Use descriptive names (e.g., Tank_Level_PID instead of FB1) for readability.

Limit Wire Crossings: Minimize overlapping wires to avoid confusion (use “jumpers” if needed).

Group Related Blocks: Cluster blocks by function (e.g., all timer blocks for a single process) to simplify navigation.

Use Comments: Add text comments to explain complex logic (e.g., “Overload trip delay = 5s”).