Sequential Function Charts for Process Control

Understanding Sequential Function Charts in Modern Process Control

In the complex world of industrial automation, process control engineers face the ongoing challenge of designing systems that are both powerful and maintainable. Sequential Function Charts (SFCs) have emerged as one of the most effective programming methodologies for managing complex, step-based processes in manufacturing, water treatment, food processing, and countless other industries across Atlantic Canada and beyond.

As defined by the IEC 61131-3 standard, Sequential Function Charts provide a graphical programming language that organises control logic into discrete steps and transitions. This structured approach mirrors how engineers and operators naturally think about sequential processes, making SFC-based systems inherently more intuitive to design, troubleshoot, and maintain than traditional ladder logic or structured text alternatives.

For industries throughout Nova Scotia and the Maritime provinces—from seafood processing facilities in Lunenburg to pulp and paper operations in northern New Brunswick—understanding and implementing SFCs can significantly improve operational efficiency, reduce downtime, and enhance overall process reliability.

The Fundamental Architecture of Sequential Function Charts

Sequential Function Charts consist of several key elements that work together to create a comprehensive control structure. Understanding these components is essential for engineers looking to implement SFC-based solutions effectively.

Steps and Actions

Steps represent the fundamental building blocks of any SFC programme. Each step corresponds to a specific state or phase of the controlled process. Within each step, engineers define one or more actions that execute while the step is active. These actions can be categorised into several types:

• Non-stored actions (N): Execute continuously while the step is active and cease immediately upon step deactivation

• Set actions (S): Activate an output that remains on even after leaving the step

• Reset actions (R): Deactivate an output that was previously set

• Delayed actions (D): Begin execution after a specified time delay following step activation

• Pulse actions (P): Execute once upon step entry or exit

• Time-limited actions (L): Execute for a specified duration, then cease even if the step remains active

Transitions and Conditions

Transitions define the conditions that must be satisfied for the process to advance from one step to the next. These conditions can range from simple Boolean signals—such as a limit switch activation—to complex logical expressions involving multiple inputs, timers, counters, and process variables. A well-designed transition ensures that all prerequisites for the next process phase are genuinely met before advancement occurs.

Divergences and Convergences

Real-world processes rarely follow purely linear sequences. SFCs accommodate this reality through divergence and convergence structures. Single divergences allow the sequence to branch based on transition conditions, enabling different process paths depending on product type, operating mode, or other factors. Simultaneous divergences initiate multiple parallel sequences that execute concurrently—essential for processes where independent operations must occur simultaneously, such as filling multiple vessels or running parallel quality checks.

Practical Applications in Maritime Industrial Settings

The versatility of Sequential Function Charts makes them applicable across virtually every industry sector. Here in Atlantic Canada, several applications demonstrate particular relevance to our regional economy.

Seafood Processing Automation

Nova Scotia's seafood processing industry represents an ideal application for SFC-based control systems. Consider a typical lobster processing line where the sequence might include:

• Step 1: Receive product at intake station (actions: activate conveyor, monitor weight sensors)

• Step 2: Grade by size using vision system (actions: analyse dimensions, route to appropriate lane)

• Step 3: Cook in controlled steam chambers at precisely 100°C for species-specific durations ranging from 8-14 minutes

• Step 4: Rapid chill to below 4°C within 30 minutes to meet food safety requirements

• Step 5: Package according to customer specifications with automated labelling

Each step contains specific actions with defined parameters, while transitions ensure proper completion before advancement. The SFC structure allows operators to immediately identify the current process state and any conditions preventing advancement—critical for maintaining production schedules and product quality.

Water and Wastewater Treatment

Municipal water treatment facilities throughout the Maritimes rely heavily on sequential processes. A typical filter backwash sequence demonstrates SFC effectiveness:

The process begins with taking the filter offline, closing inlet and outlet valves over a 15-second window to prevent water hammer. The sequence then progresses through air scour (typically 3-5 minutes at 50-80 m³/m²/h), low-rate backwash, high-rate backwash at flows reaching 40-60 m³/m²/h, and finally filter-to-waste before returning to service. Each phase requires specific conditions—valve positions confirmed, flow rates achieved, turbidity levels acceptable—before transitioning to the next step.

Batch Chemical Processing

Chemical manufacturing and blending operations benefit tremendously from SFC implementation. A paint manufacturing facility, for example, might employ SFCs to control the precise sequencing of raw material additions, mixing times at specific agitator speeds, temperature control during reaction phases, and quality sampling intervals. The ISA-88 batch control standard specifically recommends SFC-based approaches for such applications, providing a framework that integrates seamlessly with Sequential Function Chart programming.

Design Best Practices for Robust SFC Implementation

Creating effective Sequential Function Charts requires more than simply translating process steps into graphical form. Engineers must consider numerous factors to ensure reliable, maintainable systems.

State Machine Principles

Every SFC should adhere to fundamental state machine principles. The system must always reside in exactly one well-defined state (or combination of states in parallel branches). Transitions must be mutually exclusive where branches diverge, preventing ambiguous or contradictory advancement conditions. Engineers should explicitly handle all possible scenarios, including fault conditions and operator interventions.

Timeout and Watchdog Implementation

Robust SFC designs incorporate timeout transitions for every step where indefinite waiting could indicate a problem. For instance, if a valve should open within 5 seconds of receiving the command, a timeout transition at 10 seconds can redirect the sequence to a fault-handling branch rather than allowing the system to wait indefinitely. This approach significantly reduces troubleshooting time and prevents minor issues from cascading into major problems.

Mode Management Integration

Industrial processes typically operate in multiple modes—automatic, semi-automatic, manual, maintenance, and various production modes. SFC designs should clearly define how mode changes affect active sequences. Common approaches include:

• Pause and resume: The sequence halts at its current step and continues when automatic mode resumes

• Controlled shutdown: The sequence advances to a safe parking state before surrendering control

• Immediate transfer: Control transfers immediately, with the SFC remembering its state for later resumption

Documentation and Commenting Standards

SFCs inherently provide better documentation than traditional programming methods, but engineers should enhance this advantage through consistent naming conventions, detailed step and transition descriptions, and clear indication of related interlocks and permissives. A maintenance technician responding to a 3:00 AM alarm should be able to identify the current state, understand what condition is preventing advancement, and determine appropriate corrective actions within minutes rather than hours.

Integration with Modern Control Platforms

Contemporary programmable logic controllers (PLCs) and distributed control systems (DCSs) provide robust support for Sequential Function Chart programming. Understanding platform-specific considerations helps engineers leverage these capabilities effectively.

Allen-Bradley and Rockwell Platforms

Rockwell Automation's Studio 5000 Logix Designer supports SFCs through the Phase Manager option for batch applications and native SFC programming for general sequential control. The platform allows embedding ladder logic, function block diagrams, or structured text within SFC actions, providing flexibility to use the most appropriate language for each specific function. Maximum step counts vary by processor, with ControlLogix supporting up to 400 steps per programme.

Siemens TIA Portal

Siemens implements sequential control through GRAPH programming in the TIA Portal environment. GRAPH provides full IEC 61131-3 SFC compliance with additional features including integrated supervision, automatic sequence documentation generation, and seamless integration with the Siemens HMI platforms common throughout Canadian industry. S7-1500 processors support complex sequences with minimal impact on scan time performance.

Schneider Electric and Other Platforms

Schneider Electric's EcoStruxure Control Expert offers comprehensive SFC capabilities across the Modicon M340 and M580 processor families. The implementation includes particularly strong support for ISA-88 batch control concepts, making it popular in pharmaceutical and food processing applications where regulatory compliance demands detailed batch records and sequence documentation.

Troubleshooting and Maintenance Considerations

One of the most significant advantages of SFC-based control systems emerges during troubleshooting scenarios. The structured nature of Sequential Function Charts enables systematic problem diagnosis that reduces downtime and maintenance costs.

Online Monitoring Capabilities

Modern programming environments provide real-time visualisation of SFC execution, clearly indicating the active step or steps, elapsed time in the current step, and the status of transition conditions. Maintenance personnel can immediately identify whether a stuck sequence results from an unsatisfied transition condition, a fault state, or an upstream process delay.

Common Fault Patterns

Experience across numerous installations reveals several recurring issues that engineers should anticipate:

• Sensor failures: Transition conditions that depend on single sensors should include timeout alternatives or redundant sensing

• Race conditions: Parallel branches that must synchronise require careful design to prevent one branch from waiting indefinitely for another

• Manual intervention recovery: Systems must gracefully handle situations where operators intervene during automatic sequences

• Power failure recovery: Non-volatile storage of step states and critical variables enables appropriate restart following unexpected shutdowns

Performance Optimisation

While SFCs provide excellent clarity, inefficient implementations can impact system performance. Engineers should limit the number of simultaneously active parallel branches, use appropriate action qualifiers to minimise unnecessary execution, and consider consolidating closely related sequences where practical. Scan time impacts typically remain minimal—often under 1 millisecond per active sequence—but complex systems with dozens of parallel operations warrant specific performance analysis.

Advancing Your Process Control Capabilities

Sequential Function Charts represent a mature, proven approach to process control that delivers tangible benefits in system clarity, maintainability, and operational efficiency. For industries across Nova Scotia and Atlantic Canada—from traditional manufacturing to emerging sectors—adopting SFC-based methodologies can significantly enhance competitive positioning.

However, successful implementation requires expertise in both the programming methodology and the specific process being controlled. Poorly designed SFCs can create as many problems as they solve, while well-designed systems pay dividends for years through reduced downtime, faster troubleshooting, and easier operator training.

Sangster Engineering Ltd. brings extensive experience in industrial automation and process control to clients throughout the Maritime provinces and beyond. Our team understands the unique requirements of regional industries and the importance of creating robust, maintainable control systems. Whether you're modernising an existing facility, designing a new process line, or seeking to optimise current operations through improved control strategies, we're ready to help you achieve your operational goals. Contact us today to discuss how Sequential Function Charts and other advanced control methodologies can benefit your operations.

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