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Step Through Simulation

This example shows how to step through major time steps of a simulation while viewing signal values in the block diagram. In this example, you start the simulation by clicking Step Forward so the simulation pauses after the first major time step. You can also start stepping through a simulation after you pause by clicking Pause or after the simulation pauses on a breakpoint. When the simulation pauses on a breakpoint, additional stepping options are available that allow you to step through a time step block by block. For more information, see Debug Simulation Using Signal Breakpoints.

Open and Configure Model

Open the model vdp, which implements the Van der Pol equation. For more information about the model, see Van der Pol Oscillator.

openExample("simulink_general/VanDerPolOscillatorExample",...
    supportingFile="vdp")

The model vdp.

You might run several simulations while analyzing and debugging your model. To save time by compiling the model only before the first simulation, enable fast restart. In the Simulink® Toolstrip, on the Simulation tab, click Fast Restart.

Add port value labels so you can see the signal values as you step through the simulation. Click an empty area in the upper left of the canvas and drag to select everything in the block diagram. Then, on the Debug tab, in the Tools section, in the Port Values button group, click Show port value label on selected signal . The port value labels show no data yet until the first forward step in simulation after you add the port value labels.

The model vdp with port value labels on all signals.

Step Forward One Step at a Time

When you want to run a simulation one step at a time from the beginning, you can start the simulation by clicking Step Forward. In the Simulink Toolstrip, on the Simulation tab, click Step Forward to start a simulation of the model vdp. The simulation starts and pauses just after calculating the output values for the first simulation time and before stepping to the next simulation time.

The model vdp with port value labels on all signals. The port value labels display the current signal values.

To monitor the simulation status and simulation time, use the status bar at the bottom of the Simulink Editor. In the middle of the status bar, the software displays the time that corresponds to the most recent output values and a progress bar that indicates the simulation status. Some solvers perform calculations to determine the time of the next major time hit before the point in the simulation loop where the simulation pauses when you click Step Forward. In this case, the solver is ahead in simulation time relative to the block diagram. The asterisk on the simulation time indicates that the solver has moved ahead in simulation time.

The status bar shows the simulation time is 0 with an asterisk and the simulation is paused.

Each time you click Step Forward, the simulation advances one major time step. The time in the status bar updates, and the port value labels update to display the new signal values.

Step Forward Multiple Time Steps at a Time

To advance the simulation by multiple time steps each time you click Step Forward, configure the simulation stepping options. To open the Simulation Stepping Options dialog box, click the Step Back button arrow then select Configure Simulation Stepping. In the Simulation Stepping Options dialog box, specify Move back/forward by as 5. Then, click OK.

Now, when you click Step Forward again, the simulation advances through five major time steps. The time changes by a larger increment, and you can see the overall simulation progress indicated in the status bar.

Double-click the Scope block. The plot updates to display the data for the signals x1 and x2 as the simulation advances. Click Step Forward to see the Scope block update to display data from five more time steps.

The Scope block displays approximately 7 seconds worth of simulation data for the signals x1 and x2.

Step Back in Simulation

Before you can step back in simulation, you have to enable stepping back. To support stepping back in simulation, the software captures snapshots of the model operating point while advancing forward in simulation. Capturing these snapshots requires memory and can affect simulation performance. For more information, see How Stepping Through Simulation Works.

Open the Simulation Stepping Options dialog box. Click the Step Back button arrow then select Configure Simulation Stepping. In the Simulation Stepping Options dialog box, select Enable stepping back then click OK.

Because stepping back was not enabled at the start of simulation, the software does not have a snapshot from a prior time in the simulation. Before you can step back, you must step forward. You cannot step back to a simulation time before the time you enabled stepping back.

Click Step Forward twice. Then, click Step Back. In the status bar, the simulation time updates to reflect the step back. The port value labels also update to display the value from the time to which you stepped back.

The model vdp with port value labels on each signal and the status bar.

When you click Step Back a second time, the software disables the Step Back button because you cannot step back farther in this simulation.

Finish or Stop Simulation

When you are done stepping through the simulation, you can continue the simulation until the end or stop the simulation at the current time step.

To run the simulation through the simulation stop time, in the Simulation tab, click Continue.

To stop the simulation at the current time step, in the Simulation tab, click Stop.

See Also

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