User`s guide

3 Linear Model Identification
After estimating model orders, use this valueasaninitialguessforestimating
other state-space models, as described in “How to Estimate State-Space
Models in the GUI” on page 3-84.
Estimating the Model Order at the Command Line
You can estimate the state-space model order using the n4si d command.
Use following syntax to specify the range of model orders to try for a specific
input delay.
m = n4sid(data,n1:n2,'nk',nk);
where data is the estimation data set, n1 and n2 specify the range of orders,
and
nk specifies the input delay. For multiple-input systems, nk is a vector
of input delays.
This com mand opens the Model Order Selection window. For info rmation
about using this plot, see “Using the Model Order Selection Window” on page
3-83.
Alternatively, you can use the
pem command to open the Model Order
Selecti on wind o w, as follows:
m = pem(Data,'nx',nn)
where nn = [n1,n2,...,nN] specifies the vector or range of orders you
want to try.
To omit opening the Model Order Selection window and automatically se lect
the best order, use the following syntax:
m = pem(Data,'best')
For a tutorial on estimating model orders for a multiple-input system, see
“Estimating a State-Space Model” in System Identification Toolbox Getting
Started G uide.
3-82