User`s guide

Importing Data into the MATLAB
®
Workspace
For a continuous-time system, the transfer function relates the Laplace
transforms of the input U(s) and output Y(s):
Ys GsUs() () ()=
In this case, the frequency function G(iw) is the transfer f unction evaluated
on the imaginary axis s=iw.
For a discrete-time system sampled with a tim e interval T,thetransfer
function relates the Z-transforms of the input U(z) and output Y(z):
Yz GzUz() () ()=
In this case, the frequency function G(e
iwT
) is the transfer function G(z)
evaluated on the unit circle. The argument of the frequency function G(e
iwT
)
is scaled by the sampling interval T to make the frequency function periodic
with the sampling frequency
2π
T
.
For a sinusoidal input to the system, the output is also a sinusoid w ith the
same frequency. The frequency-response data magnifies the amplitude of
the input by
G
and shifts its phase by
ϕ=argG
. Because the frequency
function is evaluated at the sinusoid frequency, the values of the frequency
function at a specific frequency describe the response of the linear system to
aninputatthatfrequency.
Frequency-response data represents a (nonparametric) model of the
relationship between the input and the outputs as a function of frequency.
You might use such a model, which consists of a table of values, to study
the sy stem f requency respo nse. H ow ever, you cannot use this model for
simulation and prediction and must create a parametric model from the
frequency-response data.
How to Import Frequency-Response Data into the Software
Therearetwowaystorepresentfrequency-responsedataforsystem
identification. The first approach lets you manipulate the data using b oth
System Identification Tool GUI a nd the command line. The second approach
is only used for w orking with data in the System Identification Tool GUI.
1-11