Initialize Signals and Discrete States

About Initialization

Simulink® allows you to specify the initial values of signals and discrete states, i.e., the values of the signals and discrete states at the Start time of the simulation. You can use signal objects to specify the initial values of any signal or discrete state in a model. In addition, for some blocks, e.g., Outport, Data Store Memory, or Memory, you can use either a signal object or a block parameter or both to specify the initial value of a block state or output. In such cases, Simulink checks to ensure that the values specified by the signal object and the parameter are consistent.

When you specify a signal object for signal or discrete state initialization, or a variable as the value of a block parameter, Simulink resolves the name that you specify to an appropriate object or variable, as described in Symbol Resolution.

A given signal can be associated with at most one signal object under any circumstances. The signal can refer to the object more than once, but every reference must resolve to exactly the same object. A different signal object that has exactly the same properties will not meet the requirement for uniqueness. A compile-time error occurs if a model associates more than one signal object with any signal. For more information, see Simulink.Signal and the Merge block.

Using Block Parameters to Initialize Signals and Discrete States

For blocks that have an initial value or initial condition parameter, you can use that parameter to initialize a signal. For example, the following Block Parameters dialog box initializes the signal for a Unit Delay block with an initial condition of 0.

Use Signal Objects to Initialize Signals and Discrete States

You can use signal objects that have a storage class other than 'auto' or 'SimulinkGlobal' to initialize

  • Discrete states with an initial condition parameter

  • Signals in a model except bus signals and blocks that output constant value

To specify an initial value, use the Model Explorer or MATLAB® commands to do the following:

  1. Create the signal object.

    Model Explorer

    MATLAB Command

    S1 = Simulink.Signal;

    The name of the signal object must be the same as the name of the signal that the object is initializing. Although not required, consider setting the Signal name must resolve to Simulink signal object option in the Signal Properties dialog box. This setting makes signal objects in the MATLAB workspace consistent with signals that appear in your model.

    Consider using the Data Object Wizard to create signal objects. The Data Object Wizard searches a model for signals for which signal objects do not exist. You can then selectively create signal objects for multiple signals listed in the search results with a single operation. For more information about the Data Object Wizard, see Create Data Objects for a Model Using Data Object Wizard in the Simulink documentation.

  2. Set the signal object's storage class to a value other than 'auto'or 'SimulinkGlobal'.

    Model Explorer

    MATLAB Command

  3. Set the initial value. You can specify a MATLAB expression, including the name of a workspace variable, that evaluates to a numeric scalar value or array.

    The Simulink engine converts the initial value so the type, complexity, and dimension are consistent with the corresponding block parameter value. If you specify an invalid value or expression, an error message appears when you update the model.

    Model Explorer

    MATLAB Command

    S1.InitialValue = '0.5'

    If you can also use a block parameter to set the initial value of the signal or state, you should set the parameter either to empty ([]) or to the same value as the initial value of the signal object. If you set the parameter value to empty, Simulink uses the value specified by the signal object to initialize the signal or state. If you set the parameter to any other value, Simulink compares the parameter value to the signal object value and displays an error if they differ.

Some initial value settings may depend on the initialization mode. For more information, see Underspecified initialization detection.

Classic initialization mode: In this mode, initial value settings for signal objects that represent the following signals and states override the corresponding block parameter initial values if undefined (specified as []):

  • Output signals of conditionally executed subsystems and Merge blocks

  • Block states

Simplified initialization mode: In this mode, initial values of signal objects associated with the output of the following blocks are ignored. The initial values of the corresponding blocks are used instead.

  • Output signals of conditionally executed subsystems

  • Merge blocks

Using Signal Objects to Tune Initial Values

Simulink allows you to use signal objects as an alternative to parameter objects (see Simulink.Parameter) to tune the initial values of block outputs and states that can be specified via a tunable parameter. To use a signal object to tune an initial value, create a signal object with the same name as the signal or state and set the signal object's initial value to an expression that includes a variable defined in the MATLAB workspace. You can then tune the initial value by changing the value of the corresponding workspace variable during the simulation.

For example, suppose you want to tune the initial value of a Memory block state named M1. To do this, you might create a signal object named M1, set its storage class to 'ExportedGlobal', set its initial value to K (M1.InitialValue='K' ), where K is a workspace variable in the MATLAB workspace, and set the corresponding initial condition parameter of the Memory block to [] to avoid consistency errors. You could then change the initial value of the Memory block's state any time during the simulation by changing the value of K at the MATLAB command line and updating the block diagram (e.g., by typing Ctrl+D).

Example: Using a Signal Object to Initialize a Subsystem Output

The following example shows a signal object specifying the initial output of an enabled subsystem.

Signal s is initialized to 4.5. To avoid a consistency error, the initial value of the enabled subsystem's Outport block must be [] or 4.5.

If you need a signal object and its initial value setting to persist across Simulink sessions, see Create Persistent Data Objects.

Initialization Behavior Summary for Signal Objects

The following model and table show different types of signals and discrete states that you can initialize and the simulation behavior that results for each.

Signal or Discrete StateDescriptionBehavior
S1Root inport
  • Initialized to S1.InitialValue.

  • If you use the Data Import/Export pane of the Configuration Parameters dialog to specify values for the root inputs, the initial value is overwritten and may differ at each time step. Otherwise, the value remains constant.

X1Unit Delay block — Block with a discrete state that has an initial condition
  • Initialized to X1.InitialValue.

  • Simulink checks whether X1.InitialValue matches the initial condition specified for the block and displays an error if a mismatch occurs.

  • At first write, the output equals X1.InitialValue and the state equals S1.

  • At each time step after the first write, the output equals the state and the state is updated to equal S1.

  • If the block is inside an enabled subsystem, you can use the initial value as a reset value if the subsystem's Enable block parameter States when enabling is set to reset.

X2Data Store Memory block
  • Data type work (DWork) vector initialized to X2.InitialValue. For information on work vectors, see DWork Vector Basics.

  • Simulink checks whether X2.InitialValue matches the initial condition specified for the block, and displays an error if a mismatch occurs.

  • Data Store Write blocks overwrite the value.

S2Output of an enabled subsystem
  • Initialized to S2.InitialValue or the value of the Outport block. If multiple initial values are specified for the same signal, all initial values must be the same.

  • The first write occurs when the subsystem is enabled. The block feeding the subsystem output sets the value.

  • The initial value is also used as a reset value if the subsystem's Enable block parameter States when enabling or Outport block parameter Output when disabled is set to reset.

S3Persistent signals
  • Initialized to S3.InitialValue.

  • The output value is reset by the block at each time step.

  • Affects code generation only. For simulation, setting the initial value for S3 is irrelevant because the values are overwritten at the model's simulation start time.

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