This example shows how to estimate the frequency response of a portion of a Simulink® model using the Linear Analysis Tool. To estimate the frequency response, you specify the portion of the model you want to estimate, the operating point for estimation, and the input signal to use for estimation.
Open the Simulink model.
sys = 'scdDCMotor'; open_system(sys)
In the Simulink Editor, select Analysis > Control Design > Linear Analysis. The Linear Analysis Tool opens.
By default, Linear Analysis Tool uses the linearization analysis points defined in the
model (the model I/Os) to determine where to inject the test signal
and where to measure the frequency response. The model
contains predefined linear analysis points: an input point at the compensator output, and
an open-loop output after the unit gain block. For this example, use these predefined
model I/Os to obtain the frequency response of the inner loop of the model with the outer
If you want to obtain the frequency response of a different portion of the model, in the Estimation tab of Linear Analysis Tool, use the Analysis I/Os drop-down list. (Analysis points for estimation work the same way as analysis points for linearization. For more information about linear analysis points, see Specify Portion of Model to Linearize.)
You perform frequency response estimation at a steady-state operating point of the model. You can compute or specify an operating point in Linear Analysis Tool using the Operating Point drop-down list. By default, Linear Analysis Tool uses the operating point defined by the model initial conditions. For this example, use that operating point. (For more information about operating points, see About Operating Points.)
Frequency response estimation injects an input signal into the input analysis point you specify for estimation. For this example, configure a sinestream signal, which is a series of sinusoidal perturbations at frequencies you specify. (For more information about input signals, see Estimation Input Signals.)
In the Estimation tab, in the Input
Signal drop-down list, select
Create sinestream input dialog box opens.
To specify frequency points for the input signal, Click . In the Add frequencies dialog box, specify the frequency range and number of points for the input signal. The frequency points you specify are the frequencies at which Linear Analysis Tool computes the estimated response.
For this example, specify a range from 0.1 to 100 rad/s. Also, specify 100 logarithmically spaced frequencies.
Click OK.The added points are visible in the frequency content viewer of the Create sinestream input dialog box.
Select all these frequency points for the estimation input signal.
Specify the amplitude of the input signal. Enter
1 in the
Amplitude field. When you specify a scalar value, Linear
Analysis Tool uses the same amplitude for all frequencies.
Click OK to create the sinestream input signal. The new
in_sine1, appears in the Linear Analysis
You can now estimate the frequency response and generate a frequency-domain plot of
the result. To do so, click
Bode. The estimated frequency response, appears in the
Linear Analysis Workspace as the
To export the estimated frequency response model to the MATLAB® workspace for further analysis, drag it from the Linear Analysis Workspace section of the Data Browser to the MATLAB Workspace section.
The simulation results viewer in Linear Analysis Tool lets you examine further details of the frequency response estimation. For more information, see Analyze Estimated Frequency Response.