Open loop block diagram

Compute a linear model of the combined controller-plant system without the effects of the feedback signal. Use a Bode plot of the resulting linear model to see the open-loop response.

Open Simulink model.

sys = 'watertank'; open_system(sys)

The Water-Tank System block represents the plant in this control system and contains all of the system nonlinearities.

In the Simulink Editor, define the portion of the model to linearize:

1. Right-click the PID Controller block input signal (the output of the Sum block). Select Linear Analysis Points > Input Perturbation.
2. Right-click the Water-Tank System output signal, and select Linear Analysis Points > Open-loop Output.

Annotations appear in the model indicating which signals are designated as linearization I/O points.

Open the Linear Analysis Tool for the model.

In the Simulink Editor, select Analysis > Control Design > Linear Analysis.

By default, the I/O points you specified in the model are the selected Analysis I/Os for linearization, as displayed in the Analysis I/Os menu.

Linearize the model with the specified I/Os, and generate a Bode plot of the linearized model.

Click Bode. The Bode plot of the linearized plant appears.

• Tip Instead of a Bode plot, generate other response types by clicking the corresponding button in the plot gallery.

View the minimum stability margins for the model.

Right-click the plot and select Characteristics > Minimum Stability Margins.

The Bode plot displays the phase margin marker. Click the marker to show a data tip that contains the phase margin value.