Sequential Function Chart (SFC) in PLC Programming Explained

A Sequential Function Chart (SFC) is a graphical programming language used in PLC programming to design and control sequential processes. It breaks down complex automation tasks into steps and transitions, making it easier to visualize and manage the flow of operations.

How It Works

Think of an SFC like a flowchart that controls machines step-by-step. Each step represents a specific action or state, like turning on a motor or waiting for a sensor signal. The process moves from one step to the next through transitions, which are conditions that must be true to proceed.

This is similar to following a recipe: you complete one step, check if the conditions are right, then move to the next step. In PLCs, SFC helps organize these steps clearly, so the machine knows exactly what to do and when.

Example

This example shows a simple SFC controlling a conveyor belt that starts, runs, and stops based on sensor input.

structured-text

(* SFC example in Structured Text for PLC *)
PROGRAM ConveyorControl
VAR
  Step_Start : BOOL := TRUE; (* Initial step active *)
  Step_Run : BOOL := FALSE;
  Step_Stop : BOOL := FALSE;
  Sensor_Active : BOOL := FALSE; (* Simulated sensor input *)
  Conveyor_Motor : BOOL := FALSE; (* Output to control motor *)
END_VAR

(* Step transitions *)
IF Step_Start AND Sensor_Active THEN
  Step_Start := FALSE;
  Step_Run := TRUE;
END_IF

IF Step_Run AND NOT Sensor_Active THEN
  Step_Run := FALSE;
  Step_Stop := TRUE;
END_IF

(* Actions based on steps *)
IF Step_Start THEN
  (* Conveyor stopped *)
  Conveyor_Motor := FALSE;
ELSIF Step_Run THEN
  (* Conveyor running *)
  Conveyor_Motor := TRUE;
ELSIF Step_Stop THEN
  (* Conveyor stopped *)
  Conveyor_Motor := FALSE;
END_IF

Output

When Sensor_Active becomes TRUE, the conveyor motor starts running; when Sensor_Active becomes FALSE, the motor stops.

When to Use

Use SFC when you need to control processes that happen in clear, ordered steps, such as assembly lines, batch processing, or machine startup sequences. It is especially helpful when the process has multiple states and conditions that must be checked before moving on.

For example, in a bottling plant, SFC can manage filling, capping, and labeling steps in order, ensuring each step completes before the next begins.

Key Points

• SFC divides automation into steps and transitions for clear control flow.
• It is graphical but can be implemented in code like Structured Text.
• Ideal for sequential and conditional processes in industrial automation.
• Makes complex sequences easier to understand and maintain.

Key Takeaways

Sequential Function Chart (SFC) organizes automation into clear steps and transitions.

SFC simplifies complex sequential processes by visualizing each action and condition.

It is best used for processes requiring ordered, conditional control like manufacturing lines.

SFC can be programmed graphically or with code such as Structured Text in PLCs.