Conditionally Executed Subsystems and Models

Control when subsystem or model executes

A conditionally executed subsystem, also known as a conditional subsystem, is a nonvirtual subsystem that allows you to control its execution with an external signal. The external signal, called the action signal or control signal, is attached to the action input or control input port. Conditional subsystems are useful when you create complex models that contain components whose execution depends on other components.

Simulink^® supports these types of conditional subsystems:

Action Subsystem — Executes at each time step when a logical condition is true in an If Action Subsystem or when a signal has one of a specified set of values in a Switch Case Action Subsystem. See Select Subsystem Execution.
Enabled Subsystem — Executes at each time step while the control signal is positive. Execution starts at the time step when the control signal crosses zero from the negative to the positive direction. See Using Enabled Subsystems.
Triggered Subsystem — Executes at each time step when the control signal rises or falls to zero or crosses zero. See Using Triggered Subsystems.
Enabled and Triggered Subsystem — Executes at the time step when the enable control signal has a positive value and the trigger control signal rises or falls to zero. See Using Enabled and Triggered Subsystems.
Message Triggered Subsystem and Message Polling Subsystem — Uses messages as a control signal, and executes based on whether a message is available at the control port. See Using Message Polling and Message Triggered Subsystems.
Function-Call Subsystem — Executes when the control port receives a function-call event. Events can occur one or more time during a time step. See Using Function-Call Subsystems.

Simulink also supports the Resettable Subsystem, which executes at each time step but resets all blocks within the subsystem to their initial condition when the control signal rises or falls to zero or crosses zero. See Using Resettable Subsystems.

You can configure a referenced model for conditional execution like a subsystem. See Conditionally Execute Referenced Models.

Simulink canvas with Conditionally Executed Subsystem block. The Subsystem block has a control input port at the top and a data input port at the left, both connected to Inport blocks. The Subsystem block has a data output port at the right, connected to an Outport block.

Blocks

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Action Subsystems

If	Select subsystem execution using logic similar to `if-else` statement
If Action Subsystem	Subsystem whose execution is enabled by If block
Switch Case	Select subsystem execution using logic similar to `switch` statement
Switch Case Action Subsystem	Subsystem whose execution is enabled by Switch Case block

Enabled and Triggered Subsystems

Enabled Subsystem	Subsystem whose execution is enabled by external input
Triggered Subsystem	Subsystem whose execution is triggered by external input
Enabled and Triggered Subsystem	Subsystem whose execution is enabled and triggered by external inputs
Message Triggered Subsystem	Subsystem whose execution is controlled by message input (Since R2022a)
Enable	Add enable port to subsystem or model
Trigger	Add trigger or function port to subsystem or model

Function-Call Subsystems

Function-Call Subsystem	Subsystem whose execution is controlled by external function-call input
Function-Call Generator	Provide function-call events to control execution of subsystem or model
Function-Call Split	Provide junction for splitting function-call line
Function-Call Feedback Latch	Break feedback loop involving data signals between function-call blocks
Trigger	Add trigger or function port to subsystem or model

Resettable Subsystems

Resettable Subsystem

Subsystem whose block states reset with external trigger

Functions

Simulink.getOutportInheritsInitialValue Determine if conditional subsystem Outport block inherits initial output value (Since R2021a)

Model Settings

Context-dependent inputs

Diagnostic action to take when function-call subsystem can change its inputs

Topics

Basics of Conditionally Executed Subsystems

Conditionally Executed Subsystems Overview
Overview of conditionally executed subsystems.
Conditionally Execute Referenced Models
Execute referenced models conditionally, similar to conditionally executed subsystems.
Ensure Output Port Is Virtual
Prevent buffering of conditional subsystem or model output.
Use Control Flow Logic
How to use control flow blocks to model control logic.

Types of Conditionally Executed Subsystems

Select Subsystem Execution
Execute subsystem once during a time step in response to an action signal.
Using Enabled Subsystems
Create a subsystem that executes when a control signal has a positive value.
Using Triggered Subsystems
Create a subsystem that executes with a trigger signal.
Using Enabled and Triggered Subsystems
Create a subsystem that executes with a trigger signal or control signal with a positive value.
Using Message Polling and Message Triggered Subsystems
Demonstrate and compare Message Triggered Subsystems and Message Polling Subsystems.
Using Function-Call Subsystems
Create a subsystem that executes with a function call.
Using Resettable Subsystems
Create a subsystem that resets all block states with a trigger signal.
Fixed-Point Data Type Support for If Block
This topic explains how to use a Compare To Constant block for If block fixed-point data type support.

Subsystem Output Initialization

Conditional Subsystem Initial Output Values
Specify initial output values for a conditional subsystem.
Conditional Subsystem Output Values When Disabled
Specify output behavior when conditional subsystem is disabled.
Simplified Initialization Mode
Control initial values for conditional subsystems.

Featured Examples

If-Then-Else Blocks

The effect of feeding a sine wave into If Action Subsystem blocks. It is designed to illustrate the similarity between the If Action Subsystem block and the Enabled Subsystem block.

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Modeling Clutch Lock-Up Using If Blocks

Use If/Else subsystems to build a clutch model. An 'If' subsystem models the clutch dynamics in the locked position while an 'Else' subsystem models the unlocked position. One or the other is enabled using the 'If' block. The dot-dashed lines from the 'If' block denote control signals, which are used to enable If/Else (or other conditional) subsystems. Checking any of the boxes on the GUI produces a plot of any of the selected variables (versus time).

Open Model

Implement Control Algorithm Using Enabled Subsystem

Use an enabled subsystems to implement a control algorithm based on a binary logic signal. The enabled subsystems execute when the binary signal has a positive value and implements the control algorithm. In this example, the super-twisting control algorithm is used to stabilize an unstable dynamic system. The super-twisting algorithm is one of the well known sliding mode control techniques for its robust performance.

Open Live Script

Control Block States and Output of Enabled Subsystem

Reset or hold states of blocks inside an Enabled Subsystem block when the subsystem is disabled and how to control the subsystem output. In this example, you see four different combinations of state and output value configurations for both discrete and continuous control signals. When the enabled subsystem is disabled, the combinations work as follows:

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Triggered Subsystems

Model showing triggered subsystems and different types of triggers.

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Counters Using Conditionally Executed Subsystems

Implement counters using Enabled and Triggered subsystems. In this example, the model sldemo_counters controls flow of water into a tank and uses a counter to count the number of times overflow occurs, where overflow occurs when the water level in the tank is 8 meters or more for 30 seconds or more.

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Resettable Subsystems

Use two resettable subsystems to implement a ramp signal.

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Discrete and Continuous Resettable Subsystems

Compares the behavior of discrete and continuous resettable subsystems. After you run the simulation, one scope shows the results for the discrete resettable subsystem, and the other scope shows the results for the continuous resettable subsystem.

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Simulink Subsystem Semantics

This set of examples shows different types of Simulink® Subsystems and what semantics are used when simulating these subsystems. Each example provides a description of the model and the subtleties governing how the model is executed.

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