How to Model Traffic Light Using Stateflow in Simulink

To model a traffic light using Stateflow in Simulink, create states for each light color (Red, Green, Yellow) and define transitions with timing conditions. Use temporal logic like after() to control state durations and simulate the traffic light cycle.

Syntax

In Stateflow, you define states and transitions to model behavior. For a traffic light:

• States: Represent light colors (Red, Green, Yellow).
• Transitions: Arrows between states with conditions like after(time, sec) to control timing.
• Actions: Use entry: or during: to set outputs (e.g., turning lights on/off).

stateflow

state Red {
    entry: light = 'Red';
    during: if after(10, sec) transition to Green;
}

state Green {
    entry: light = 'Green';
    during: if after(15, sec) transition to Yellow;
}

state Yellow {
    entry: light = 'Yellow';
    during: if after(3, sec) transition to Red;
}

Example

This example shows a complete Stateflow chart for a traffic light with three states cycling automatically using after() timing conditions. Each state sets the light color output.

stateflow

chart TrafficLight {
    output string light;

    state Red {
        entry: light = 'Red';
        during: if after(10, sec) transition to Green;
    }

    state Green {
        entry: light = 'Green';
        during: if after(15, sec) transition to Yellow;
    }

    state Yellow {
        entry: light = 'Yellow';
        during: if after(3, sec) transition to Red;
    }

    initial state Red;
}

Output

Simulink simulation cycles through states: Red (10s) → Green (15s) → Yellow (3s) → Red ...

Common Pitfalls

Common mistakes when modeling traffic lights in Stateflow include:

• Not setting an initial state, causing simulation errors.
• Using incorrect timing functions or units (e.g., forgetting sec in after()).
• Missing transitions or conditions, which can cause the model to get stuck.
• Not defining outputs properly in each state, so the light color does not update.

Always verify transitions and timing carefully.

stateflow

/* Wrong: Missing initial state and incorrect timing */
state Red {
    entry: light = 'Red';
    during: if after(10) transition to Green; /* Missing 'sec' */
}

/* Right: */
initial state Red;
state Red {
    entry: light = 'Red';
    during: if after(10, sec) transition to Green;
}

Quick Reference

Concept	Description	Example
State	Represents a traffic light color	state Red { entry: light = 'Red'; }
Transition	Moves between states based on condition	if after(10, sec) transition to Green;
Initial State	Starting point of the chart	initial state Red;
Temporal Logic	Controls timing in states	after(15, sec)
Output	Variable to show current light	output string light;

Key Takeaways

Define states for each traffic light color with entry actions to set outputs.

Use temporal logic like after(time, sec) to control how long each light stays on.

Always specify an initial state to start the traffic light cycle.

Check transitions carefully to avoid the model getting stuck.

Test the model in Simulink to verify the traffic light cycles correctly.