Example 3: Frequency Plot
SIMEQ has a command to
find a frequency response of a signal.
If you in the Command Window type:
» freq60('Tagname', 'From', 'To') e.g.
» freq60('Z6MOSI', '7-jul-03 23:20:00','8-jul-03 02:00:00')
The below plot is made based on 60 seconds samples from the log files.
The signal is first shown as a function of time.
This is to check that there are no missing values or outliers
in the log
files. Then a Fast Fourier Transformation is made, and finally the power
spectrum is calculated and
plotted with semi logarithmic x-axes.
Frequency responses can be used to search for
oscillations in signals buried in so much noise that the original
signals
are not recognisable.
If 3 or 4 different frequencies plus noise make
up a signal, it’s very difficult to realize this without a frequency
analyse
of the signal.
Another version of freq60 exists which uses 10 sec log (called freq10).