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Mode-Selection Model Reduction

Model selection eliminates poles that fall outside a specific frequency range of interest. This method is useful when you want to focus your analysis on a particular subset of system dynamics. For instance, if you are working with a control system with bandwidth limited by actuator dynamics, you might discard higher-frequency dynamics in the plant. Eliminating dynamics outside the frequency range of interest reduces the numerical complexity of calculations with the model. There are two ways to compute a reduced-order model by mode selection:

  • At the command line, using the freqsep command.

  • In the Model Reducer, using the Mode Selection method.

For more general information about model reduction, see Model Reduction Basics.

Mode Selection in the Model Reducer App

Model Reducer provides an interactive tool for performing model reduction and examining and comparing the responses of the original and reduced-order models. To approximate a model by mode selection in Model Reducer:

  1. Open the app and import an LTI model to reduce. For instance, suppose that there is a model named Gms in the MATLAB® workspace. The following command opens Model Reducer and imports the model.

    modelReducer(Gms)
  2. In the Data Browser, select the model to reduce. Click Mode Selection.

    In the Mode Selection tab, Model Reducer displays a plot of the frequency response of the original model and a reduced version of the model. The app also displays a pole-zero map of both models.

    The pole-zero map marks pole locations with x and zero locations with o.

    Note

    The frequency response is a Bode plot for SISO models, and a singular-value plot for MIMO models.

  3. Model Reducer eliminates poles that lie outside the shaded region. Change the shaded region to capture only the dynamics you want to preserve in the reduced model. There are two ways to do so.

    • On either the response plot or the pole-zero map, drag the boundaries of the shaded region or the shaded region itself.

    • On the Mode Selection tab, enter lower and upper cutoff frequencies.

    When you change the shaded regions or cutoff frequencies, Model Reducer automatically computes a new reduced-order model. All poles retained in the reduced model fall within the shaded region on the pole-zero map. The reduced model might contain zeros that fall outside the shaded region.

  4. Optionally, examine absolute or relative error between the original and simplified model. Select the error-plot type using the buttons on the Mode Selection tab.

    For more information about using the analysis plots, see Visualize Reduced-Order Models in the Model Reducer App.

  5. When you have one or more reduced models that you want to store and analyze further, click . The new model appears in the Data Browser.

    After creating a reduced model in the Data Browser, you can continue adjusting the mode-selection region to create reduced models with different orders for analysis and comparison.

You can now perform further analysis with the reduced model. For example:

  • Examine other responses of the reduced system, such as the step response or Nichols plot. To do so, use the tools on the Plots tab. See Visualize Reduced-Order Models in the Model Reducer App for more information.

  • Export reduced models to the MATLAB workspace for further analysis or control design. On the Model Reducer tab, click Export.

Generate MATLAB Code for Mode Selection

To create a MATLAB script you can use for further model-reduction tasks at the command line, click Create Reduced Model, and select Generate MATLAB Script.

Model Reducer creates a script that uses the freqsep command to perform model reduction with the parameters you have set on the Mode Selection tab. The script opens in the MATLAB editor.

Mode Selection at the Command Line

To reduce the order of a model by mode selection at the command line, use freqsep. This command separates a dynamic system model into slow and fast components around a specified frequency.

For this example, load the model Gms and examine its frequency response.

load modeselect Gms
bodeplot(Gms)

Gms has two sets of resonances, one at relatively low frequency and the other at relatively high frequency. Suppose that you want to tune a controller for Gms, but the actuator in your system is limited to a bandwidth of about 3 rad/s, in between the two groups of resonances. To simplify calculation and tuning using Gms, you can use mode selection to eliminate the high-frequency dynamics.

[Gms_s,Gms_f] = freqsep(Gms,30);

freqsep decomposes Gms into slow and fast components such that Gms = Gms_s + Gms_f. All modes (poles) with natural frequency less than 30 are in Gms_s, and the higher-frequency poles are in Gms_f.

bodeplot(Gms,Gms_s,Gms_f)
legend('original','slow','fast')

The slow component, Gms_s, contains only the lower-frequency resonances and matches the DC gain of the original model. Examine the orders of both models.

order(Gms)
ans = 18
order(Gms_s)
ans = 10

When the high-frequency dynamics are unimportant for your application, you can use the 10th-order Gms_s instead of the original 18th-order model. If neglecting low-frequency dynamics is appropriate for your application, you can use Gms_f. To select modes that fall between a low-frequency and a high-frequency cutoff, use additional calls to freqsep.

See Also

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