User`s guide

8 Model Analysis
Using Nonlinear ARX Plots to Validate Models
In this section...
“About Nonlinear ARX Plots” on page 8-52
“How to Plot Nonlinear ARX Plots Using the GUI” on page 8-52
“Configuring the Nonlinear ARX Plot” on page 8-53
“Axis Limits, Lege nd , and 3-D Rotation” on page 8-54
“How to Plot Nonlinear ARX Plots at the Command Line” on page 8-55
About Nonlin ear ARX Plots
The Nonlinear ARX plot displays the characteristics of model nonlinearities
as a function of one or two regress ors. For more information about estimating
nonlinear ARX models, see “Identifying Nonlinear A RX Models” on page 4-4.
Examini ng a n onlinear ARX plot can hel p you gain insight into which
regressors have the strongest effect on the m odel output. Understanding the
relative importance of the regressors on the output can help you decide which
regressors should be included in the nonlinear function.
Furthermore, you can create several nonlinear models for the same data
set using different nonlinearity estimators, such a wavelet network and
tree partition, and then compare the nonlinear surfaces of these models .
Agreement betw een nonlinear surfaces increases the confidence that these
nonlinear models capture the true dynamics o f the system.
How to Plot Nonlinear ARX Plots Using the GUI
To create a nonlinear ARX plot in the System Identification Tool GU I, select
the Nonlinear ARX check box in the Model Views area. For general
information about creating and working w ith plots, see “Working with Plots
in the System Identification Tool GUI” on page 12-15.
Note The Nonlinear ARX check box is unavailable if you do not have a
nonlinear ARX model in the M od el Board .
8-52