How to Model a Vending Machine Using Stateflow in Simulink

To model a vending machine using Stateflow in Simulink, create states representing machine conditions like Idle, WaitingForMoney, and DispenseItem. Use transitions triggered by events such as coin insertion or selection, and define actions to update variables or outputs accordingly.

Syntax

A Stateflow chart models behavior using states, transitions, and actions. Each state can have entry, during, and exit actions. Transitions connect states and have conditions or events that trigger them.

• state StateName: Defines a state.
• entry: action;: Action when entering a state.
• during: action;: Action while in a state.
• exit: action;: Action when leaving a state.
• transition [condition] / action;: Moves between states when condition is true, optionally performing an action.

stateflow

chart VendingMachine {
  state Idle {
    entry: displayMessage = "Insert Coin";
  }
  state WaitingForMoney {
    entry: displayMessage = "Select Item";
  }
  state DispenseItem {
    entry: dispense = true;
  }

  Idle -> WaitingForMoney [coinInserted] / credit = credit + coinValue;
  WaitingForMoney -> DispenseItem [itemSelected && credit >= itemPrice] / credit = credit - itemPrice;
  DispenseItem -> Idle / dispense = false;
}

Example

This example shows a simple vending machine with three states: Idle, WaitingForMoney, and DispenseItem. It accepts coins, allows item selection, and dispenses items if enough credit is available.

stateflow

chart VendingMachine {
  int credit = 0;
  int coinValue = 1;
  int itemPrice = 2;
  bool coinInserted = false;
  bool itemSelected = false;
  bool dispense = false;
  string displayMessage = "";

  state Idle {
    entry: displayMessage = "Insert Coin";
    during: if (coinInserted) {
      credit = credit + coinValue;
      coinInserted = false;
      transition to WaitingForMoney;
    }
  }

  state WaitingForMoney {
    entry: displayMessage = "Select Item";
    during: if (itemSelected && credit >= itemPrice) {
      credit = credit - itemPrice;
      itemSelected = false;
      transition to DispenseItem;
    }
  }

  state DispenseItem {
    entry: dispense = true;
    during: dispense = false;
    exit: displayMessage = "Thank You!";
    transition to Idle;
  }
}

Common Pitfalls

Common mistakes when modeling vending machines in Stateflow include:

• Not initializing variables like credit or flags, causing unexpected behavior.
• Missing transition conditions, which can cause the machine to get stuck in a state.
• Forgetting to reset event flags like coinInserted or itemSelected after handling them.
• Not defining clear entry and exit actions, leading to unclear outputs or states.

Always test transitions with different inputs to ensure smooth state changes.

stateflow

/* Wrong approach: Missing reset of coinInserted flag */
state Idle {
  during: if (coinInserted) {
    credit = credit + coinValue;
    transition to WaitingForMoney;
  }
}

/* Right approach: Reset coinInserted after use */
state Idle {
  during: if (coinInserted) {
    credit = credit + coinValue;
    coinInserted = false;
    transition to WaitingForMoney;
  }
}

Quick Reference

Key points for modeling vending machines in Stateflow:

• States: Represent machine modes like idle, waiting, dispensing.
• Transitions: Triggered by events or conditions like coin insertion or selection.
• Actions: Update variables, display messages, or control outputs.
• Events: Use boolean flags or events to signal user inputs.
• Initialization: Set variables to default values to avoid errors.

Key Takeaways

Use states to represent different vending machine modes clearly.

Define transitions with proper conditions and reset event flags after use.

Initialize all variables to avoid unexpected behavior.

Use entry, during, and exit actions to control outputs and messages.

Test the model with various inputs to ensure smooth operation.