Allen-Bradley Sequential Function Charts (SFC) for Safety Systems

Implementing Safety Systems with Sequential Function Charts (SFC)

Safety Systems systems in Universal require careful consideration of advanced control requirements, real-time responsiveness, and robust error handling. This walkthrough demonstrates practical implementation using Allen-Bradley Studio 5000 (formerly RSLogix 5000) and Sequential Function Charts (SFC) programming.

System Requirements:

A typical Safety Systems implementation includes:

Input Devices (5 types):
1. Safety light curtains: Critical for monitoring system state
2. Emergency stop buttons: Critical for monitoring system state
3. Safety door switches: Critical for monitoring system state
4. Safety mats: Critical for monitoring system state
5. Two-hand control stations: Critical for monitoring system state

Output Devices (4 types):
1. Safety relays: Controls the physical process
2. Safety contactors: Controls the physical process
3. Safety PLCs: Controls the physical process
4. Safety I/O modules: Controls the physical process

Control Logic Requirements:

1. Primary Control: Safety-rated PLC programming for personnel protection, emergency stops, and safety interlocks per IEC 61508/61511.
2. Safety Interlocks: Preventing Safety integrity level (SIL) compliance
3. Error Recovery: Handling Redundancy requirements
4. Performance: Meeting advanced timing requirements
5. Advanced Features: Managing Safety circuit design

Implementation Steps:

Step 1: Program Structure Setup

In Studio 5000 (formerly RSLogix 5000), organize your Sequential Function Charts (SFC) program with clear separation of concerns:

• Input Processing: Scale and filter 5 sensor signals


• Main Control Logic: Implement Safety Systems control strategy


• Output Control: Safe actuation of 4 outputs


• Error Handling: Robust fault detection and recovery

Step 2: Input Signal Conditioning

Safety light curtains requires proper scaling and filtering. Sequential Function Charts (SFC) handles this through perfect for sequential processes. Key considerations include:

• Signal range validation


• Noise filtering


• Fault detection (sensor open/short)


• Engineering unit conversion

Step 3: Main Control Implementation

The core Safety Systems control logic addresses:

• Sequencing: Managing emergency stop systems


• Timing: Using timers for 4-8 weeks operation cycles


• Coordination: Synchronizing 4 actuators


• Interlocks: Preventing Safety integrity level (SIL) compliance

Step 4: Output Control and Safety

Safe actuator control in Sequential Function Charts (SFC) requires:

• Pre-condition Verification: Checking all safety interlocks before activation


• Gradual Transitions: Ramping Safety relays to prevent shock loads


• Failure Detection: Monitoring actuator feedback for failures


• Emergency Shutdown: Rapid safe-state transitions

Step 5: Error Handling and Diagnostics

Robust Safety Systems systems include:

• Fault Detection: Identifying Redundancy requirements early


• Alarm Generation: Alerting operators to advanced conditions


• Graceful Degradation: Maintaining partial functionality during faults


• Diagnostic Logging: Recording events for troubleshooting

Machine guarding implementations face practical challenges:

1. Safety integrity level (SIL) compliance
Solution: Sequential Function Charts (SFC) addresses this through Perfect for sequential processes. In Studio 5000 (formerly RSLogix 5000), implement using Ladder Logic features combined with proper program organization.

2. Redundancy requirements
Solution: Sequential Function Charts (SFC) addresses this through Clear visualization of process flow. In Studio 5000 (formerly RSLogix 5000), implement using Ladder Logic features combined with proper program organization.

3. Safety circuit design
Solution: Sequential Function Charts (SFC) addresses this through Easy to understand process steps. In Studio 5000 (formerly RSLogix 5000), implement using Ladder Logic features combined with proper program organization.

4. Validation and testing
Solution: Sequential Function Charts (SFC) addresses this through Good for batch operations. In Studio 5000 (formerly RSLogix 5000), implement using Ladder Logic features combined with proper program organization.

Performance Optimization:

For advanced Safety Systems applications:

• Scan Time: Optimize for 5 inputs and 4 outputs


• Memory Usage: Efficient data structures for ControlLogix capabilities


• Response Time: Meeting Universal requirements for Safety Systems