Understanding Sequential Function Chart (SFC) Basics

Sequential Function Chart (SFC)

Definition

Sequential Function Chart (SFC) is a graphical programming language standardized in IEC 61131-3 (the international standard for programmable logic controllers, PLCs). It models complex sequential processes as a series of steps (representing process states) and transitions (conditions that trigger movement between steps). SFC is ideal for designing and documenting sequential control logic (e.g., machine operation cycles, batch processes) due to its visual clarity and ability to handle parallel/branching workflows.

Core Components

SFC uses a set of standardized symbols to represent process elements, making it universally understandable across industrial systems:

1. Step (Étape)

A step represents a stable state of the process (e.g., “Motor Running,” “Valve Open,” “Heating Active”). Each step can include:

Action: A task performed while the step is active (e.g., “Turn on Pump 1,” “Set Temperature to 80°C”). Actions are annotated with modifiers to define timing (e.g., L = latching, D = delayed start, P = pulsed).
Initial Step: A special step (marked with a double rectangle) that starts the sequence (e.g., “System Idle” at startup).
Inactive/Active State: Steps are inactive (empty rectangle) until a transition condition is met; active steps are filled (solid color) and execute their actions.

2. Transition (Transition)

A horizontal bar that separates steps and defines the condition to move from one step to the next (e.g., “Pressure ≥ 5 bar,” “Timer T1 Expired,” “Sensor S1 = ON”). Transitions are only evaluated when the preceding step is active—once the condition is true, the current step deactivates, and the next step activates.

3. Directed Links (Connectors)

Lines that connect steps and transitions, defining the flow of the sequence (left-to-right by default, but can branch or loop).

4. Branching & Merging

SFC supports two types of branching to handle parallel or alternative workflows:

Alternative Branch (Exclusive OR): A single path is taken from multiple options (only one transition condition is true at a time). Represented by a horizontal bar with multiple outgoing/incoming links (e.g., “If Tank Full → Drain; Else → Fill”).
Parallel Branch (AND): Multiple steps execute simultaneously (all paths are activated at once). Represented by a double horizontal bar (e.g., “Heat Oven AND Open Feed Valve” at the start of a batch process). Parallel branches merge back into a single sequence with a double bar once all parallel steps complete.

5. Jump & Reset

Jump: Skips to a specified step (labeled with a reference) to handle exceptions or loops (e.g., “Jump to Initial Step if Error Detected”).
Reset: Deactivates all active steps and returns to the initial step (e.g., emergency stop).

SFC Workflow Example: Batch Mixing Process

A simple batch mixing process illustrates how SFC components interact:

Initial Step: “System Ready” (double rectangle)
Transition: “Start Button Pressed” → activates next step.
Step: “Fill Tank” (action: Open Inlet Valve; monitor Level Sensor).
Transition: “Tank Full (Level Sensor = ON)” → deactivates “Fill Tank,” closes inlet valve.
Step: “Mix Contents” (action: Start Mixer; timer T1 = 5 minutes).
Transition: “Timer T1 Expired” → stops mixer.
Parallel Branch:
- Path 1: “Heat Mixture” (action: Turn on Heater; set to 70°C).
- Path 2: “Recirculate Mixture” (action: Start Recirculation Pump).
Parallel Merge: Both paths complete when “Temperature ≥ 70°C” AND “Recirculation Time ≥ 3 minutes.”
Step: “Drain Tank” (action: Open Outlet Valve).
Transition: “Tank Empty (Level Sensor = OFF)” → closes valve.
Jump: Return to “System Ready” step for next batch.

Key Features of SFC

1. Visual Clarity

SFC’s graphical format makes complex sequential logic easier to design, debug, and document than text-based languages (e.g., ladder logic for multi-step processes). Operators and engineers can quickly trace the process flow and identify bottlenecks or errors.

2. Support for Complex Sequences

SFC natively handles:

Sequential Flows: Linear step-by-step processes (e.g., assembly line stages).
Branching: Alternative paths (e.g., error handling, product variations).
Parallelism: Simultaneous tasks (e.g., heating and mixing in a batch process).
Loops: Repeating sequences (e.g., continuous production cycles).

3. Modularity

Steps and sequences can be grouped into subcharts (reusable blocks) to simplify large systems (e.g., a “Cleaning Subchart” that is called before each batch). This reduces code duplication and improves maintainability.

4. Compliance with IEC 61131-3

As a standardized language, SFC is supported by all major PLC manufacturers (e.g., Siemens, Allen-Bradley, Schneider Electric) and is compatible with PLC programming software (e.g., TIA Portal, RSLogix, EcoStruxure Machine Expert).

SFC vs. Other IEC 61131-3 Languages

SFC is often used alongside other IEC 61131-3 languages for hybrid control systems:

Language	Use Case	Relationship with SFC
Ladder Logic (LD)	Discrete control (e.g., relay-based logic)	Transitions or step actions can call LD routines.
Function Block Diagram (FBD)	Continuous control (e.g., PID loops)	Actions (e.g., temperature control) use FBD function blocks.
Structured Text (ST)	Complex mathematical logic (e.g., calculations)	Transitions or actions can execute ST code.
Instruction List (IL)	Low-level machine code (rarely used)	Legacy support for simple actions.

Applications of SFC

1. Manufacturing

Assembly Lines: Sequential control of robotic arms, conveyor belts, and station operations (e.g., car manufacturing: welding → painting → assembly).
Batch Processing: Pharmaceutical, food, and chemical production (e.g., mixing ingredients, heating, cooling, packaging).

2. Process Industries

Water/Wastewater Treatment: Sequential control of filtration, disinfection, and pumping cycles.
Oil & Gas: Wellhead control, pipeline valve sequencing, and storage tank filling/emptying.

3. Machinery

Packaging Machines: Carton forming, filling, sealing, and labeling sequences.
CNC Machines: Tool change cycles, workpiece loading/unloading, and machining steps.

4. Building Automation

HVAC Systems: Sequential startup/shutdown of boilers, chillers, and fans (e.g., pre-purge before ignition).
Elevators/Escalators: Floor selection, door control, and safety interlock sequences.

Advantages & Limitations

Advantages

Simplifies Complex Logic: Visual representation reduces cognitive load for multi-step/parallel processes.
Improves Troubleshooting: Operators can see which step is active and why a transition failed (e.g., a sensor not triggering).
Enhances Collaboration: Common graphical language aligns engineers, operators, and maintenance teams.
Supports Reusability: Subcharts enable modular design and faster project development.

Limitations

Performance: Complex parallel branches may introduce minor latency in high-speed processes (mitigated by optimized PLC firmware).rth Emperor Nail, and Heaven-Confusing Bell are also Heavenly Mystic Treasures, though their specific rankings on the Chaos Myriad Spirits Ranking are not explicitly stated in the novel.

Overhead for Simple Logic: Overkill for basic on/off control (ladder logic is more efficient).

Learning Curve: Requires understanding of sequential state modeling (vs. intuitive ladder logic).