Bosch Rexroth Sequential Function Charts (SFC) for Traffic Light Control

Implementing Traffic Light Control with Sequential Function Charts (SFC)

Traffic signal control systems manage the safe and efficient flow of vehicles and pedestrians at intersections. PLCs implement signal timing plans, coordinate with adjacent intersections, respond to traffic demands, and interface with central traffic management systems.

This walkthrough demonstrates practical implementation using Bosch Rexroth ctrlX WORKS / IndraWorks and Sequential Function Charts (SFC) programming.

System Requirements:

A typical Traffic Light Control implementation includes:

Input Devices (Sensors):
1. Inductive loop detectors embedded in pavement for vehicle detection: Critical for monitoring system state
2. Video detection cameras with virtual detection zones: Critical for monitoring system state
3. Pedestrian push buttons with ADA-compliant features: Critical for monitoring system state
4. Preemption receivers for emergency vehicle detection (optical or radio): Critical for monitoring system state
5. Railroad crossing interconnect signals: Critical for monitoring system state

Output Devices (Actuators):
1. LED signal heads for vehicle indications (red, yellow, green, arrows): Primary control output
2. Pedestrian signal heads (walk, don't walk, countdown): Supporting control function
3. Flashing beacons for warning applications: Supporting control function
4. Advance warning flashers: Supporting control function
5. Cabinet cooling fans and environmental controls: Supporting control function

Control Equipment:

• NEMA TS2 or ATC traffic controller cabinets


• Conflict monitors for signal verification


• Malfunction management units (MMU)


• Uninterruptible power supplies (UPS)

1. Primary Control: Automated traffic signal control using PLCs for intersection management, timing optimization, and pedestrian safety.
2. Safety Interlocks: Preventing Timing optimization
3. Error Recovery: Handling Emergency vehicle priority

Implementation Steps:

Step 1: Survey intersection geometry and traffic patterns

In ctrlX WORKS / IndraWorks, survey intersection geometry and traffic patterns.

Step 2: Define phases and rings per NEMA/ATC standards

In ctrlX WORKS / IndraWorks, define phases and rings per nema/atc standards.

Step 3: Calculate minimum and maximum green times for each phase

In ctrlX WORKS / IndraWorks, calculate minimum and maximum green times for each phase.

Step 4: Implement detector logic with extending and presence modes

In ctrlX WORKS / IndraWorks, implement detector logic with extending and presence modes.

Step 5: Program phase sequencing with proper clearance intervals

In ctrlX WORKS / IndraWorks, program phase sequencing with proper clearance intervals.

Step 6: Add pedestrian phases with accessible pedestrian signals

In ctrlX WORKS / IndraWorks, add pedestrian phases with accessible pedestrian signals.

Bosch Rexroth Function Design:

Rexroth engineers lean heavily on reusable function blocks packaged as ctrlX libraries. The mapp-technology-equivalent SDK apps expose motion-profiled FBs, recipe-driven parameter handlers, and cockpit widgets as pre-built components. OEM machine builders maintain private app catalogues for their machine families, with versioned FBs that can be swapped between machine variants without rewiring upstream code. IEC 61131-3 OOP extensions (classes, interfaces, methods) are used in more advanced teams but are optional.

Common Challenges and Solutions:

1. Balancing main street progression with side street delay

• Solution: Sequential Function Charts (SFC) addresses this through Perfect for sequential processes.

• Solution: Sequential Function Charts (SFC) addresses this through Clear visualization of process flow.

• Solution: Sequential Function Charts (SFC) addresses this through Easy to understand process steps.

• Solution: Sequential Function Charts (SFC) addresses this through Good for batch operations.

• Conflict monitoring to detect improper signal states


• Yellow and all-red clearance intervals per engineering standards


• Flashing operation mode for controller failures


• Pedestrian minimum walk and clearance times per MUTCD


• Railroad preemption for track clearance

• Scan Time: Optimize for 5 inputs and 4 outputs


• Memory Usage: Efficient data structures for ctrlX CORE XM21 capabilities


• Response Time: Meeting Infrastructure requirements for Traffic Light Control

Bosch Rexroth Diagnostic Tools:

ctrlX WORKS Trace tool — multi-variable waveform logging at up to 1 ms sample rate,Data Layer Explorer — browse every runtime variable in a hierarchical tree with live values,Web-based diagnostics interface — device-level health, CPU and memory utilisation,IndraWorks MotionManager — axis commissioning, tuning plots, and envelope monitoring,ctrlX I/O Engineer — field-bus topology view with per-slave diagnostic status,Integrated Git history for project files with visual diff between versions,Wireshark integration for EtherCAT and Profinet frame capture and analysis,Linux journalctl access on ctrlX CORE for controller-side system log inspection,REST API query tools (Postman, curl) for runtime variable inspection during development,SSH access to the ctrlX controller for deep diagnostics when support escalation is required

Bosch Rexroth's ctrlX WORKS / IndraWorks provides tools for performance monitoring and optimization, essential for achieving the 1-2 weeks development timeline while maintaining code quality.