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Dealing with Harmonics in EFDD |
If a structure
being tested has
rotating parts
the measurements
made will not
only include the
response to the
natural input
but also the
response coming
from the
sinusodial
forced
excitation. The
influence of
these harmonic
components can
be eliminated
from the modal
estimation.
The consequences
of having
harmonic
components
present in the
responses depend
on both the
nature of the
harmonic
components
(number,
frequency and
level) and the
modal parameter
extraction
method used. For
the EFDD
technique it is
important that
harmonic
components
inside the
desired SDOF are
identified and
their influence
eliminated
before
proceeding with
the modal
parameter
extraction
process.
Below an example
demonstrates
what happens
without
detection and
elimination of
harmonics, and
what happens
when detection
and elimination
is active.
Without
Detection and
Elimination of
Harmonics
In the figure
below a EFDD
Peak Picking has
been done on a
mode that is
located close to
a harmonic. The
harmonics
typically has
much higher
energy than the
nearby modes and
the resulting
SDOF model is
completely
dominated by the
harmonic.
SDOF model
strongly
affected by the
neighboring
harmonic.
This means that
the estimated
mode shape looks
like the
operating
deflection shape
at the frequency
of the harmonic.
The estimated
natural
frequency
corresponds to
the frequency of
the harmonic,
and the damping
is
underestimated
during to the
persistant
sinusodial
excitation. The
damping and
frequency
estimates are as
such extremely
inaccurate,
which can be
seen in the
validation
diagram of the
EFDD method,
shown below.
Free decay
estimation also
strongly
affected by the
sinusodial
excitation. The
decay is
unrealistic
small.
With Detection
and Elimination
of Harmonics
When detection
is enabled all
the harmonic
components
lights up with a
green background
color and the
EFDD estimator
is informed
about their
location in the
SVD diagram.
Since the
estimator knows
the position it
can reconstruct
the SDOF model
at those
positions by
interpolation.
The result is a
SDOF model that
is unaffected by
any nearby
harmonics as
shown below:
SDOF model now
unaffected by
the neighboring
harmonic
indicated by a
green vertical
line.
Since the SDOF
model peaks at
the correct
frequency and
with correct
shape the modal
parameters are
now estimated
correctly, which
can be validated
as seen below:
Free decay
estimation show
a much more
realistic decay
and the extreme
values used for
the damping
estimation looks
much better now.
Back to
Technical
Details
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