Using Simulink

Enabled Subsystems

Enabled subsystems are subsystems that execute at each simulation step where the control signal has a positive value.

An enabled subsystem has a single control input, which can be scalar or vector valued:

• If the input is a scalar, the subsystem executes if the input value is greater than zero.
• If the input is a vector, the subsystem executes if any of the vector elements is greater than zero.

For example, if the control input signal is a sine wave, the subsystem is alternately enabled and disabled, as shown in this figure. An up arrow signifies enable, a down arrow disable.

Simulink uses the zero-crossing slope method to determine whether an enable is to occur. If the signal crosses zero and the slope is positive, the subsystem is enabled. If the slope is negative at the zero crossing, the subsystem is disabled.

Creating an Enabled Subsystem

You create an enabled subsystem by copying an Enable block from the Signals & Systems library into a subsystem. Simulink adds an enable symbol and an enable control input port to the Subsystem block icon.

Setting Output Values While the Subsystem Is Disabled.   Although an enabled subsystem does not execute while it is disabled, the output signal is still available to other blocks. While an enabled subsystem is disabled, you can choose to hold the subsystem outputs at their previous values or reset them to their initial conditions.

Open each Outport block's dialog box and select one of the choices for the Output when disabled parameter, as shown in the dialog box following:

• Choose held to cause the output to maintain its most recent value.
• Choose reset to cause the output to revert to its initial condition. Set the Initial output to the initial value of the output.

  

Setting States When the Subsystem Becomes Reenabled.   When an enabled subsystem executes, you can choose whether to hold the subsystem states at their previous values or reset them to their initial conditions.

To do this, open the Enable block dialog box and select one of the choices for the States when enabling parameter, as shown in the dialog box following:

• Choose held to cause the states to maintain their most recent values.
• Choose reset to cause the states to revert to their initial conditions.

  

Outputting the Enable Control Signal.   An option on the Enable block dialog box lets you output the enable control signal. To output the control signal, select the Show output port check box.

This feature allows you to pass the control signal down into the enabled subsystem, which can be useful where logic within the enabled subsystem is dependent on the value or values contained in the control signal.

Blocks an Enabled Subsystem Can Contain

An enabled subsystem can contain any block, whether continuous or discrete. Discrete blocks in an enabled subsystem execute only when the subsystem executes, and only when their sample times are synchronized with the simulation sample time. Enabled subsystems and the model use a common clock.

Note    Enabled subsystems can contain Goto blocks. However, only state ports can connect to Goto blocks in an enabled subsystem. See the Simulink demo model, clutch, for an example of how to use Goto blocks in an enabled subsystem.

For example, this system contains four discrete blocks and a control signal. The discrete blocks are

• Block A, which has a sample time of 0.25 second
• Block B, which has a sample time of 0.5 second
• Block C, within the enabled subsystem, which has a sample time of 0.125 second
• Block D, also within the enabled subsystem, which has a sample time of 0.25 second

The enable control signal is generated by a Pulse Generator block, labeled Signal E, which changes from 0 to 1 at 0.375 second and returns to 0 at 0.875 second.

The chart below indicates when the discrete blocks execute.

Blocks A and B execute independently of the enable control signal because they are not part of the enabled subsystem. When the enable control signal becomes positive, blocks C and D execute at their assigned sample rates until the enable control signal becomes zero again. Note that block C does not execute at 0.875 second when the enable control signal changes to zero.

Creating Conditionally Executed Subsystems

Triggered Subsystems