States and transitions in Simulink - Mini Project: Build & Apply

States and transitions

📖 Scenario: You are modeling a simple traffic light system using states and transitions. The traffic light has three states: Green, Yellow, and Red. It changes states in a fixed order: Green → Yellow → Red → Green, and so on.

🎯 Goal: Build a state machine in Simulink that models the traffic light states and transitions. You will create the states, define the transitions, and output the current state name.

📋 What You'll Learn

Create three states named Green, Yellow, and Red in a Stateflow chart.

Define transitions from Green to Yellow, Yellow to Red, and Red to Green.

Use a timer or counter to trigger transitions every fixed time interval.

Output the current state name as a string.

💡 Why This Matters

🌍 Real World

Traffic light controllers use state machines to manage light changes safely and predictably.

💼 Career

Understanding states and transitions is essential for designing control systems in automotive, robotics, and automation industries.

1

Create the states

In your Stateflow chart, create three states named Green, Yellow, and Red. Do not add any transitions yet.

Simulink

% Create three states named Green, Yellow, and Red in the Stateflow chart
% Your code here

Hint

Use the Stateflow editor to add three states. Name them exactly Green, Yellow, and Red.

2

Add a timer variable

Add a local data variable named timer of type int in the Stateflow chart. Initialize it to zero.

Simulink

% Create three states named Green, Yellow, and Red in the Stateflow chart
% This is done visually in Simulink Stateflow by adding states with these exact names

% Add a local data variable named timer of type int and initialize it to 0
% Your code here

Hint

In the Stateflow chart properties, add a local data variable called timer with type int and initial value 0.

3

Define transitions with timer increment

Add transitions from Green to Yellow, Yellow to Red, and Red to Green. Use the condition timer >= 5 to trigger each transition. Reset timer to zero on each transition. Increment timer by 1 every time step in each state.

Simulink

% Create three states named Green, Yellow, and Red in the Stateflow chart
% This is done visually in Simulink Stateflow by adding states with these exact names

% Add a local data variable named timer of type int and initialize it to 0
% In Stateflow, add local data: name = timer, scope = Local, type = int, initial value = 0

% Add transitions with condition timer >= 5 and reset timer to 0 on each transition
% Increment timer by 1 in each state's during action
% Your code here

Hint

Use the Stateflow editor to add transitions with the condition timer >= 5. Reset timer to zero in the transition action. In each state's during action, increment timer by 1.

4

Output the current state name

Create an output data named currentState of type string in the Stateflow chart. In each state, set currentState to the state's name: "Green", "Yellow", or "Red". Display the currentState output in the Simulink model.

Simulink

% Create three states named Green, Yellow, and Red in the Stateflow chart
% This is done visually in Simulink Stateflow by adding states with these exact names

% Add a local data variable named timer of type int and initialize it to 0
% In Stateflow, add local data: name = timer, scope = Local, type = int, initial value = 0

% Add transitions:
% Green --> Yellow with condition [timer >= 5] and transition action {timer = 0;}
% Yellow --> Red with condition [timer >= 5] and transition action {timer = 0;}
% Red --> Green with condition [timer >= 5] and transition action {timer = 0;}

% In each state, add during action: timer = timer + 1;

% Add output data currentState of type string and set it to the state's name in each state
% Your code here

Hint

Add an output data variable called currentState of type string. In each state's during action, assign the state's name as a string to currentState. Connect this output to a display block in Simulink.

Practice

1. In Simulink Stateflow, what does a state represent?

easy

A. A mathematical operation

B. A graphical icon only

C. A data storage variable

D. A mode or condition of the system

Solution

1. Step 1: Understand the role of states in Stateflow

   States represent different modes or conditions that a system can be in during execution.

2. Step 2: Differentiate states from other elements

   States are not variables or operations; they define behavior and system modes.

3. Final Answer:

   A mode or condition of the system -> Option D

4. Quick Check:

   State = system mode [OK]

Hint: States = system modes or conditions [OK]

Common Mistakes:

• Confusing states with variables
• Thinking states are just icons
• Mixing states with mathematical operations

2. Which of the following is the correct syntax to define a transition condition in Simulink Stateflow?

easy

A. state1 => state2 (condition)

B. state1 -> state2 {condition}

C. state1 --> state2 [condition]

D. state1 -- state2 [condition]

Solution

1. Step 1: Recall Stateflow transition syntax

   Transitions use the arrow --> followed by a condition in square brackets.

2. Step 2: Match syntax options

   state1 --> state2 [condition] uses --> and square brackets correctly, others use incorrect symbols or brackets.

3. Final Answer:

   state1 --> state2 [condition] -> Option C

4. Quick Check:

   Transition syntax = --> [condition] [OK]

Hint: Transitions use --> with [condition] brackets [OK]

Common Mistakes:

• Using wrong arrow symbols
• Using curly or round brackets instead of square
• Omitting the condition brackets

3. Given this Stateflow snippet:

state A
  entry: x = 0;
  during: x = x + 1;
end

transition from A to B [x >= 3]

What is the value of x when the system transitions from state A to B?

medium

A. 3

B. 4

C. 2

D. 0

Solution

1. Step 1: Analyze state A behavior

   On entry, x is set to 0. During state A, x increments by 1 each cycle.

2. Step 2: Determine when transition triggers

   Transition triggers when x >= 3. So after 3 increments, x equals 3 and transition occurs.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   Transition at x >= 3 means x = 3 [OK]

Hint: Transition triggers when condition first becomes true [OK]

Common Mistakes:

• Counting increments before entry action
• Assuming transition triggers at x > 3 instead of >= 3
• Forgetting initial value of x

4. Identify the error in this transition definition:

state A
end
state B
end

A --> B (x > 5)

medium

A. Missing condition brackets [] around (x > 5)

B. Incorrect arrow symbol used

C. States must have entry actions

D. Transition must be inside state definitions

Solution

1. Step 1: Check transition condition syntax

   Transition conditions must be enclosed in square brackets [], not parentheses ().

2. Step 2: Verify other syntax elements

   The arrow --> is correct, states can be empty, and transitions are defined outside states.

3. Final Answer:

   Missing condition brackets [] around (x > 5) -> Option A

4. Quick Check:

   Conditions use [] not () [OK]

Hint: Use [] for conditions, not () [OK]

Common Mistakes:

• Using parentheses instead of square brackets
• Thinking arrow is wrong
• Believing states must have actions

5. You want to model a traffic light system with states Red, Green, and Yellow. The system should transition from Green to Yellow only if a timer reaches 60 seconds. Which transition condition correctly models this?

hard

A. Green --> Yellow [timer > 60]

B. Green --> Yellow [timer == 60]

C. Green --> Yellow [timer < 60]

D. Green --> Yellow [timer != 60]

Solution

1. Step 1: Understand the timing condition

   The transition should happen exactly when the timer reaches 60 seconds, so the condition must check equality.

2. Step 2: Evaluate options

   Green --> Yellow [timer == 60] uses timer == 60, which triggers transition at 60 seconds. Others check greater, less, or not equal, which do not match the requirement.

3. Final Answer:

   Green --> Yellow [timer == 60] -> Option B

4. Quick Check:

   Exact timer match triggers transition [OK]

Hint: Use == for exact timer match in transitions [OK]

Common Mistakes:

• Using > or < instead of ==
• Using != which triggers at wrong times
• Forgetting square brackets around condition