| Using Simulink | |
Triggered and Enabled Subsystems
A third kind of conditionally executed subsystem combines both types of conditional execution. The behavior of this type of subsystem, called a triggered and enabled subsystem, is a combination of the enabled subsystem and the triggered subsystem, as shown by this flow diagram.
A triggered and enabled subsystem contains both an enable input port and a trigger input port. When the trigger event occurs, Simulink checks the enable input port to evaluate the enable control signal. If its value is greater than zero, Simulink executes the subsystem. If both inputs are vectors, the subsystem executes if at least one element of each vector is nonzero.
The subsystem executes once at the time step at which the trigger event occurs.
Creating a Triggered and Enabled Subsystem
You create a triggered and enabled subsystem by dragging both the Enable and Trigger blocks from the Signals & Systems library into an existing subsystem. Simulink adds enable and trigger symbols and enable and trigger and enable control inputs to the Subsystem block icon.
You can set output values when a triggered and enabled subsystem is disabled as you would for an enabled subsystem. For more information, see Setting Output Values While the Subsystem Is Disabled. Also, you can specify what the values of the states are when the subsystem is reenabled. See Setting States When the Subsystem Becomes Reenabled.
Set the parameters for the Enable and Trigger blocks separately. The procedures are the same as those described for the individual blocks.
A Sample Triggered and Enabled Subsystem
A simple example of a triggered and enabled subsystem is illustrated in the model below.
Creating Alternately Executing Subsystems
You can use conditionally executed subsystems in combination with Merge blocks to create sets of subsystems that execute alternately, depending on the current state of the model. For example, the following figure shows a model that uses two enabled blocks and a Merge block to model an inverter, that is, a device that converts AC current to pulsating DC current.
In this example, the block labeled "pos" is enabled when the AC waveform is positive; it passes the waveform unchanged to its output. The block labeled "neg" is enabled when the waveform is negative; it inverts the waveform. The Merge block passes the output of the currently enabled block to the Mux block, which passes the output, along with the original waveform, to the Scope block.
The Scope creates the following display.
| | Triggered Subsystems | Control Flow Blocks | |
