Transition conditions and actions in Simulink - Step-by-Step Execution

Concept Flow - Transition conditions and actions

Start State

↓

Check Transition Condition

↓

Perform Transition Action

↓

Move to Next State

This flow shows how Simulink checks if a transition condition is true, then executes the transition action and moves to the next state; otherwise, it stays in the current state.

Execution Sample

Simulink

if (input > 5) {
  action();
  state = nextState;
} else {
  state = currentState;
}

This code checks if input is greater than 5, performs an action and moves to the next state if true; otherwise, it stays in the current state.

Execution Table

Step	Input Value	Condition (input > 5)	Action Executed	State After Step
1	3	False	No	Current State
2	7	True	Yes	Next State
3	5	False	No	Current State
4	10	True	Yes	Next State
5	4	False	No	Current State

💡 Execution stops after processing all input values.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5
input	N/A	3	7	5	10	4
condition	N/A	False	True	False	True	False
action_executed	N/A	No	Yes	No	Yes	No
state	Current State	Current State	Next State	Current State	Next State	Current State

Key Moments - 2 Insights

Why does the state not change when the input is exactly 5?

Because the condition checks if input is greater than 5, not greater or equal. At step 3 in the execution_table, input is 5, condition is False, so no action or state change occurs.

What happens if the condition is false?

When the condition is false, the action is not executed and the state remains the current state, as shown in steps 1, 3, and 5 in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table at step 2. What is the state after this step?

A Current State

B Undefined

C Next State

D Previous State

💡 Hint

Check the 'State After Step' column for step 2 in the execution_table.

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

A Step 1

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Condition' column in the execution_table to find when it switches from True to False.

If the condition was changed to input >= 5, what would be the action at step 3?

A Action executed

B No action executed

C State becomes undefined

D State remains unchanged

💡 Hint

Refer to the condition logic and step 3 input value in the execution_table.

Concept Snapshot

Transition conditions check if a state change should happen.
If true, transition actions run and state updates.
If false, stay in current state.
Conditions often compare inputs.
Actions execute side effects during transition.

Full Transcript

In Simulink state machines, transitions happen when conditions are true. The system checks the condition, like if an input is greater than a threshold. If true, it runs the transition action and moves to the next state. If false, it stays in the current state. This example shows input values checked against a condition. When input is above 5, action runs and state changes. When input is 5 or less, no action runs and state stays the same. This helps control flow in models clearly and predictably.

Practice

1. In Simulink Stateflow, what does a transition condition specify?

easy

A. The initial value of a variable

B. The color of the state in the diagram

C. How long the system stays in a state

D. When the system should move from one state to another

Solution

1. Step 1: Understand transition conditions

   Transition conditions define the criteria that trigger moving from one state to another in Stateflow.

2. Step 2: Differentiate from other elements

   Colors and variable initial values do not control state changes; only conditions do.

3. Final Answer:

   When the system should move from one state to another -> Option D

4. Quick Check:

   Transition condition = state change trigger [OK]

Hint: Transition conditions control state changes timing [OK]

Common Mistakes:

• Confusing transition conditions with state colors
• Thinking actions control timing instead of conditions
• Assuming initial values trigger transitions

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

easy

A. [x > 5]

B. (x > 5)

C. {x > 5}

D. 5>

Solution

1. Step 1: Recall Stateflow transition syntax

   Transition conditions are enclosed in square brackets, e.g., [condition].

2. Step 2: Compare options

   Only [x > 5] uses square brackets correctly; others use parentheses, braces, or angle brackets which are invalid.

3. Final Answer:

   [x > 5] -> Option A

4. Quick Check:

   Transition condition syntax = square brackets [OK]

Hint: Use square brackets for transition conditions [OK]

Common Mistakes:

• Using parentheses instead of square brackets
• Confusing braces or angle brackets with correct syntax
• Omitting brackets entirely

3. Consider this Stateflow transition: [speed > 10] / count = count + 1; What happens when speed is 12 and count is 3?

medium

A. Transition does not occur and count stays 3

B. Transition occurs and count becomes 4

C. Transition occurs but count stays 3

D. Syntax error due to missing semicolon

Solution

1. Step 1: Check transition condition

   Condition is speed > 10. Given speed = 12, condition is true, so transition occurs.

2. Step 2: Apply transition action

   Action increments count by 1: count = 3 + 1 = 4.

3. Final Answer:

   Transition occurs and count becomes 4 -> Option B

4. Quick Check:

   Condition true + action increments count = 4 [OK]

Hint: Check condition first, then apply action on transition [OK]

Common Mistakes:

• Ignoring the action and leaving count unchanged
• Assuming transition does not occur when condition is true
• Thinking semicolon causes syntax error in Stateflow

4. You wrote this transition in Stateflow: [temp > 50] count = count + 1; but the action does not execute. What is the error?

medium

A. Missing slash '/' before the action

B. Condition syntax is wrong

C. Count variable is not initialized

D. Semicolon should be outside brackets

Solution

1. Step 1: Review transition syntax

   Transition actions must follow a slash '/' after the condition, e.g., [condition] / action.

2. Step 2: Identify missing slash

   The given transition lacks '/', so action is not recognized or executed.

3. Final Answer:

   Missing slash '/' before the action -> Option A

4. Quick Check:

   Action requires '/' separator [OK]

Hint: Always put '/' before actions in transitions [OK]

Common Mistakes:

• Forgetting the slash '/' before actions
• Assuming condition syntax error without checking slash
• Thinking variable initialization affects action execution here

5. You want a transition that only happens when pressure > 100 and increments alarmCount only once per transition. Which transition syntax is correct?

hard

A. [pressure > 100] alarmCount = alarmCount + 1;

B. / [pressure > 100] alarmCount = alarmCount + 1;

C. [pressure > 100] / alarmCount = alarmCount + 1;

D. [pressure > 100] / alarmCount += 1

Solution

1. Step 1: Confirm correct transition condition and action format

   Condition must be in square brackets, followed by a slash '/', then the action ending with a semicolon.

2. Step 2: Evaluate each option

   [pressure > 100] / alarmCount = alarmCount + 1; matches correct syntax: [condition] / action;. [pressure > 100] alarmCount = alarmCount + 1; misses '/', C has '/' before condition, D uses '+=' which is not standard in Stateflow.

3. Final Answer:

   [pressure > 100] / alarmCount = alarmCount + 1; -> Option C

4. Quick Check:

   Condition in [] + '/' + action with ';' = correct [OK]

Hint: Use [condition] / action; format for transitions [OK]

Common Mistakes:

• Omitting slash '/' before action
• Placing slash before condition
• Using '+=' instead of '=' for assignment