Detect Data Changes

To detect changes in data values between time steps in Stateflow charts, use change detection operators and edge detection operators. These operators track when data values change or when expressions cross thresholds. Change detection operators compare current and previous values of data, while edge detection operators monitor when expressions rise above or fall below specified thresholds.

Change Detection Operators

To determine if data changes between two time steps, use the hasChanged operator. This operator returns true when a value differs from the previous time step and returns false otherwise.

Use this operator to detect changes in scalar values, matrices, or structures. When checking matrices, the operator returns true if any element changes. For structures, it returns true if any field changes.

To check if data changes from a specific value, use the hasChangedFrom operator. The hasChangedFrom operator compares the previous value against the specified value, then checks if the current value differs from that specified value. It returns true when the data previously matched the specified value but is now different.

To detect when data changes to a specific value, use the hasChangedTo operator. This operator verifies that the previous value differs from the specified value, then checks if the current value matches the specified value. It returns true when the data previously differed from the specified value but now matches.

For example, in this chart, a temperature monitoring system uses changes in temperature readings to manage an industrial process.

Stateflow chart that uses the change detection operators.

The system moves between the Normal, Warning, Critical, and Emergency states based on specific temperature thresholds. The system detects the changes by using the hasChanged, hasChangedTo, and hasChangedFrom operators. Each state implements control actions based on the current temperature, such as activating alarms, enabling cooling systems, or sending emergency notifications.

Change Detection Requirements

For charts that use MATLAB as the action language, you can use change detection operators with input data only. For charts that use C as the action language, change detection works with input, output, local, and data store memory.

You can use change detection for these data types:

  • Scalars

  • Matrices or elements of matrices

  • Structures or fields in structures

  • Combinations of structure fields and matrix elements

To use change detection operators with index matrix elements with constant integers and structure fields with dot notation, use this syntax:

// Matrix element indexing
hasChanged(matrix(2,3))  // MATLAB syntax
hasChanged(matrix[1][2]) // C syntax

// Structure field referencing
hasChanged(struct.field)

Edge Detection Operators

To identify when an expression crosses zero in either direction, use the crossing operator. This operator detects three types of transitions:

  • From a positive value to a zero or a negative value

  • From a zero value to a nonzero value

  • From a negative value to a zero or a positive value

crossing returns true when the expression changes sign. This operator behaves like a Simulink® Trigger block with the Trigger type parameter set to either.

To detect when an expression transitions from negative to zero or positive, apply the rising operator. The operator detects two types of transitions:

  • From a negative value to zero

  • From a negative value or zero to a positive value

This operator behaves like a Simulink Trigger block with the Trigger type parameter set to rising.

To detect when an expression transitions from positive to zero or negative, implement the falling operator. The operator detects two types of transitions:

  • From a positive value to zero

  • From a positive value or zero to a negative value

This operator behaves like a Simulink Trigger block with the Trigger type parameter set to falling.

For example, in this chart, a water level control system monitors and manages water levels using edge detection operators to detect when signals cross specific thresholds.

Stateflow chart that uses edge detection operators.

The WaterLevelControl parent state contains the substates Monitor, Evaluate, Fill, Drain, and EmergencyFill. These substates transition between each other based on water level changes relative to predefined thresholds. Each state handles different operating conditions. The Monitor state observes normal conditions. The Evaluate state analyzes level changes. TheFill state adds water when levels are low. The Drain state removes excess water. The EmergencyFill state activates when water reaches critically low levels.

Edge Detection Requirements

When you use edge detection operators:

  • The expression argument must be scalar-valued.

  • The expression can combine chart input data, constants, parameters, and continuous-time local data.

  • The expression can include addition, subtraction, and multiplication operations.

  • In atomic subcharts, you must map all input data that you use in edge detection operators to input data or nontunable parameters.

Performance Considerations

Follow these guidelines when using change detection and edge detection operators.

Code Generation Limitations

When generating code from charts that use change detection operations, enable column-major layout.

Transient Value Handling

Because change detection operators evaluate values only at time boundaries, they do not detect multiple changes that occur in a single time step. Consequently, change and edge detection operators only detect changes based on the data values at the end of a time step.

For example, if a local variable changes from its previous value but then reverts to the original value within the same time step, hasChanged returns false for the next time step.

See Also

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