User`s guide

1 Data Processing
Transforming Frequency-Response Data
In the System Identification Tool G UI, frequency-response data has an icon
with a yellow background. You can trans form frequency-res ponse data to
frequency-domain data (
iddata object) or to frequency-response data with a
different frequency resolution.
When you select to transform single-input/single-output (SISO)
frequency-response data to frequency-domain data, the toolbox creates
outputs that equal the frequency responses, and inputs equal to 1. Therefore,
the ratio between the Fourier transform of the output and the Fourier
transform of the input is equal to the system frequency response.
For the multiple-input case, the toolbox transforms the frequency-response
data to frequency-domain data as if each input contributes independently to
theentireoutputofthesystemandthen combines information. Fo r example,
if a system has three inputs,
u1, u2,andu3 and two frequency samples, the
input matrix is set to:
100
100
010
010
001
001
⎡
⎣
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
In general, for nu inputs and ns samples (the number of frequencies), the
input matrix has
nu columns and (ns
⋅
nu)rows.
Note To create a separate experiment for the response from each input,
see “Transforming Between Frequency-Domain and Frequency-Response
Data” on page 1-128.
When you transform frequency-resp onse data by changing its frequency
resolution, y ou can modify the number of frequency values by changing
between line ar o r logarithmic spacin g . You might specify variab le frequency
spacing to increase the number of data points near the system resonance
1-124