States and transitions in Simulink - Step-by-Step Execution

Concept Flow - States and transitions

Start State

↓

Check Transition Condition

↓

Change State

↓

Execute State Actions

↓

Loop Back to Check Transition

The system starts in a state, checks if conditions to move to another state are met, transitions if yes, otherwise stays, then performs actions and repeats.

Execution Sample

Simulink

state = 'Idle'
if input_signal > 5:
  state = 'Active'
output = state

This code checks an input signal and changes the state from 'Idle' to 'Active' if the signal is greater than 5.

Execution Table

Step	Input Signal	Current State	Condition (input_signal > 5)	Transition Taken	New State	Output
1	3	Idle	False	No	Idle	Idle
2	7	Idle	True	Yes	Active	Active
3	4	Active	False	No	Active	Active
4	10	Active	True	Yes	Active	Active
5	2	Active	False	No	Active	Active

💡 Execution stops after 5 steps of checking input and updating state accordingly.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5
state	Idle	Idle	Active	Active	Active	Active
input_signal	N/A	3	7	4	10	2
output	N/A	Idle	Active	Active	Active	Active

Key Moments - 2 Insights

Why does the state remain 'Active' at step 3 even though input_signal is 4 (less than 5)?

Because the transition condition is only checked to move from 'Idle' to 'Active'. Once in 'Active', the example code does not specify a transition back to 'Idle', so the state stays 'Active' as shown in execution_table row 3.

What happens if the input_signal is exactly 5?

The condition input_signal > 5 is False when input_signal is 5, so no transition occurs. The state remains the same, as the condition strictly requires greater than 5.

Visual Quiz - 3 Questions

Look at the execution table, what is the state after step 2?

A Active

B Undefined

C Idle

D Error

💡 Hint

Check the 'New State' column at step 2 in the execution_table.

At which step does the condition input_signal > 5 become false after being true?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look at the 'Condition' column in execution_table rows 2 and 3.

If input_signal was always 6, what would the state be after step 5?

A Idle

B Active

C Alternating

D Unknown

💡 Hint

Refer to how the state changes when condition is true in the execution_table and variable_tracker.

Concept Snapshot

States and transitions:
- System starts in a state.
- Checks conditions to move to another state.
- If condition true, transition happens.
- Otherwise, stay in current state.
- Actions execute in current state.
- Repeat checking continuously.

Full Transcript

This example shows how a system in Simulink can be in different states and change states based on input conditions. We start in the 'Idle' state. When the input signal is greater than 5, the system transitions to the 'Active' state. If the input is not greater than 5, it stays in the current state. The execution table shows step-by-step how the input signal affects the state and output. The variable tracker records the state and input values after each step. Key moments clarify why the state remains the same when conditions are not met and what happens at boundary values. The quiz tests understanding of state changes and conditions. This helps beginners see how states and transitions work visually and practically.

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