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4 Nonlinear Black-Box Model Identification
For more information abo ut re gress ors, see “Using Regre ssors ” on page 4-6.
For a list of nonlinearity estim ators supported by nonlinear ARX models, see
“Nonlinearity Estimators for Nonlinear ARX Models” on page 4-9.
Using Regressors
You can use the follow ing types of regressors for no nline ar ARX models:
Standard regressors—Pastinputu(t) and output signals y(t),computed
automatically as delay transformations f or specied model orders.
Custom regressors Products, powers, and ot her MATLAB expressions
of input and output variables that you specify.
Specifying Model Order and Delays
You must specify the following model orders for computing standard
regressors:
n
a
The number of past output terms used to predict the current output.
n
b
The number of past input terms used to pre dict the current output.
n
k
The delay f rom input to the output in terms of the n umber of samples.
This value denes the least delay ed input regressor.
The meaning of n
a
and n
b
is simila r to the linear-ARX model parameters in
the sense that n
a
represents the number of output terms and n
b
represents the
number of input terms. n
k
represents the minimum input delay from an input
to an output. For more information about the linear AR X m odel structure, see
“What Are Black-Box Polynomial Models?” on page 3-41.
Note The total number of regressors in the model must be greater than zero.
If you o nly need to use custom regressors, set n
a
=n
b
=n
k
=0 to omit crea ting
standard regressors.
4-6
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