arxstruc (System Identification Toolbox)

Compute loss functions for a set of different model structures of single-output ARX type.

Syntax

V = arxstruc(ze,zv,NN)
V = arxstruc(ze,zv,NN,maxsize)

Description

NN is a matrix that defines a number of different structures of the ARX type. Each row of NN is of the form

```
nn = [na nb nk]
```

with the same interpretation as described for arx. See struc for easy generation of typical NN matrices for single-input systems.

Each of ze and zv are iddata objects containing output-input data. Models for each of the model structures defined by NN are estimated using the data set ze. The loss functions (normalized sum of squared prediction errors) are then computed for these models when applied to the validation data set zv. The data sets, ze and zv, need not be of equal size. They could, however, be the same sets, in which case the computation is faster.

Note that arxstruc is intended for single-output systems only.

The output argument V is best analyzed using selstruc. It contains the loss functions in its first row. The remaining rows of V contain the transpose of NN, so that the orders and delays are given just below the corresponding loss functions. The last column of V contains the number of data points in ze. The selection of a suitable model structure based on the information in v is normally done using selstruc. See Model Structure Selection and Validation in the "Tutorial" for advice on model structure selection and cross-validation.

See Algorithm Properties for an explanation of maxsize.

Examples

Compare first to fifth order models with one delay using cross-validation on the second half of the data set. Then select the order that gives the best fit to the validation data set.

NN = struc(1:5,1:5,1);
V = arxstruc(z(1:200),z(201:400),NN);
nn = selstruc(V,0);
m = arx(z,nn);

arxdata bj