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Analyzing Estimated Frequency Response

View Simulation Results

View Simulation Results Using Linear Analysis Tool

Use the Diagnostic Viewer to analyze the results of your frequency response estimation, obtained by performing the steps in Estimate Frequency Response Using Linear Analysis Tool.

To open the Diagnostic Viewer after performing frequency response estimation in the Linear Analysis Tool:

  1. In the Frequency Response Estimation tab, before performing the estimation task, select the Launch Diagnostic Viewer check box.

    This action sets the Diagnostic Viewer to open when the frequency response estimation is performed.

  2. Click to estimate the frequency response of the model.

    The Diagnostic Viewer appears in the plot pane.

To open the Diagnostic Viewer to view an estimated model in the Linear Analysis Tool:

  1. In the Linear Analysis Tab, select the estimated model using the Select Result list.

  2. Select Show diagnostic viewer in the Show Report list.

View Simulation Results (MATLAB Code)

Use the Simulation Results Viewer to analyze the results of your frequency response estimation, obtained by performing the steps in Estimate Frequency Response (MATLAB Code).

Open the Simulation Results Viewer using:

frest.simView(simout,input,sysest)

where simout is the simulated output, input is the input signal you created, and sysest is the estimated frequency response.

Interpret Frequency Response Estimation Results

By default, the Simulation Results Viewer shows these plots:

Select Plots Displayed in Simulation Results Viewer

  • To select the plots displayed in the Diagnostic Viewer using the Linear Analysis Tool:

    1. In the Figures tab, select the Diagnostic Viewer plot.

    2. In the Plot Visibilities section, select the check boxes for the plots that you want to view.

    To modify plot settings, such as axis frequency units, right-click on a plot, and select the corresponding option.

  • To select the plots displayed in the Simulation Results Viewer, choose the corresponding plot from the Edit > Plots menu. To modify plot settings, such as axis frequency units, right-click a plot, and select the corresponding option.

Frequency Response

Use the Bode plot to analyze the frequency response. If the frequency response does not match the dynamics of your system, see Troubleshooting Frequency Response Estimation for information about possible causes and solutions. While troubleshooting, you can use the Bode plot controls to view the time response at the problematic frequencies.

You can usually improve estimation results by either modifying your input signal or disabling the model blocks that drive your system away from the operating point, and repeating the estimation.

Time Response (Simulated Output)

Use this plot to check whether the simulated output is at steady state at specific frequencies. If the response has not reached steady state, see Time Response Not at Steady State for possible causes and solutions.

If you used the sinestream input for estimation, check both the filtered and the unfiltered time response. You can toggle the display of filtered and unfiltered output by right-clicking the plot and selecting Show filtered steady state output only. If both the filtered and unfiltered response appear at steady state, then your model must be at steady state. You can explore other possible causes in Troubleshooting Frequency Response Estimation.

    Note:   If you used the sinestream input for estimation, toggling the filtered and unfiltered display only updates the Time Response and FFT plots. This selection does not change estimation results. For more information about filtering during estimation, see How Frequency Response Estimation Treats Sinestream Inputs.

FFT of Time Response

Use this plot to analyze the spectrum of the simulated output.

For example, you can use the spectrum to identify strong nonlinearities. When the FFT plot shows large amplitudes at frequencies other than the input signal, your model is operating outside of linear range. If you are interested in analyzing the linear response of your system for small perturbations, explore possible solutions in FFT Contains Large Harmonics at Frequencies Other than the Input Signal Frequency.

Analyze Simulated Output and FFT at Specific Frequencies

Using the Diagnostic Viewer in Linear Analysis Tool

To analyze the results in the Diagnostic Viewer, use the controls in the Figures tab of the Linear Analysis Tool.

  1. In the Figures tab, select the Diagnostic Viewer plot.

  2. In the Frequency Selector section, specify the frequency range that you want to inspect. Use the frequency units used in the Bode plot in the Diagnostic Viewer.

Using the Simulation Results Viewer

In the Simulation Results Viewer, use Bode controls to display the simulated output and its spectrum at specific frequencies.

If you used the sinestream input signal in the estimation:

  • Drag arrows individually to display the time response and FFT at specific frequencies.

  • Drag the shaded region to shift the time response and FFT to a different frequency range.

If you used the chirp input signal in the estimation, drag the shaded region to increase or decrease the frequency range of the displayed time response and FFT.

Annotate Frequency Response Estimation Plots

You can display a data tip on the Time Response, FFT, and Bode plots in the Simulation Results Viewer by clicking the corresponding curve. Dragging the data tip updates the information.

Data tips are useful for correcting poor estimation results at a specific sinestream frequency, which requires you to modify the input at a specific frequency. You can use the data tip to identify the frequency index where the response does not match your system.

In the previous figure, the Time Response data tip shows that the frequency index is 11. You can use this frequency index to modify the corresponding portion of the input signal. For example, to modify the NumPeriods and SettlingPeriods properties of the sinestream signal, using MATLAB® code:

input.NumPeriods(11) = 80;
input.SettlingPeriods(11) = 75;

To modify the sinestream in the Linear Analysis Tool, see Modifying Sinestream Input Signal Using Linear Analysis Tool

Displaying Estimation Results for Multiple-Input Multiple-Output (MIMO) Systems

For MIMO systems, view frequency response information for specific input and output channels:

  1. In the Simulation Results Viewer, right-click any plot, and select I/O Selector.

  2. Choose the input channel in the From list. Choose the output channel in the To list.

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