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What is operational Modal Analysis |
You might wonder
what Operational Modal Analysis, OMA, is and
how it differs from the traditional experimental modal
analysis that has been around for the last
decades.
Operational Modal Analysis is also called output-only modal
analysis, ambient response analysis, ambient
modal analysis, in operation modal analysis, and
natural input modal analysis.
No matter which name that is used the idea is
the same: To do modal analisys without knowning
and/or controlling the input excitation.
This new modal
technology is capable of estimating the same
modal paramters as the traditional known
techniques. The modal parameters are the mode
shape, the natural frequency and the damping
ratio. Some thinks that Operational Modal
Analysis just is another name for Operating
Deflection Shapes, ODS. This is not the case.
Operational Modal Analysis
separates noise and
input and returns the modal information only.
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Benefits Compared to Traditional Modal
Analysis |
There are a
number of benifits in using the Operational
Modal Analysis compared to the more traditional
techniques.
Multiple Input
Multiple Output Modal Technology
The
Operational Modal Analysis are Multiple Input
Multiple Output, MIMO, techniques. This means
that the techniques are capable of estimating
closely space modes and even repeated modes with
a high degree of accuracy. Traditional modal
anlysis techniques are typically Single Input
Multiple Output, SIMO, or Multiple Input Single
Output, MISO, or in the worst case even Single
Input Single Output, SISO. Such testing
procedures will not be able to find repeated
poles due to the lack of mode seperation.
Easier
Laboratory Modal Testing
There is no need
for vibration shaker or impact hammer anymore.
If you are in your lab doing modal testing in a
test rig on some structural component, just do
some random tapping on the structure while you
are measuring the vibration response in muliple
locations. The tapping must be random in time
but also spatially. The excitation produced in
this way will be a good approximation of a
multivariate white noise stochastic process.
Winning Technology in In-situ Modal Testing
Vibration shakers
and impact hammers are impossible as excitation
sources when it comes to insitu testing of
structures, such as buildings or rotating
machinery. In cases like this the traditional
modalanalyse fails, because there are a number
of unknown input acting on the structures. What
is a problem for traditional modalanalysis is a
strength for Operational Modal Analysis. The
more random input sources there are the better
the modal results gets. Since the real strength
of the technology really lies
in the in-situ testing it is no wonder why the
technilogy is called Operational Modal Analysis.
Another important features that comes for free
are that the estimated modes are based on true
boundary conditions, and the actual ambient
excitation sources.
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Application of Operational Modal Analysis |
There are many
applications where Operational Modal Analysis
is the natural choice of technology for
supplying structural information.
Nondestructive testing
In non-destructive testing, NDT, the objective
typically is to monitor the health of a
structure over time. For this reason it is also
known as Structural Health Monitoring, SHM. Since the structure
is observed during service no other modal tool
can provide modal information in such a case.
Damage Detection
For many years
Operational Moda Analysis has been the
preferred tool in damage detection of large
structures. Mode shapes has been used to
identify local damages that caused curvature
changes and e.g. the modal frequencies has been
used to identify global damages. Research is
still extensive in this area combining
technologies such as modal analysis, neural
networks or response surfaces, and stochastic
decision theory.
Vibration Level
Documentation
If the vibration level needs to be documented in
locations where no measurements can be made,
Operational Modal Analysis can do it if you
have a Finte Element Model, FEM, available. From the
modal test you will obtain the modal coordinates
or modal response at some measurable locations.
These modal responses will then be extrapolated
to other unmeasured locations through the mode
shapes of the FE model, and by superposition the
actual responses at the location is estimated.
Even though the FE models only return normal
modes this extrapolation will be good most
structures.
Fatigue
Analysis
The above mentioned vibration level
documentation can also be extended to estimate
the accumulated damage at unmeasured location
such as underwater joints etc. In this way,
ordinary inspections at e.g. offshore facilities
can be optimized, since a few measurement points
can give the engineers valuable fatigue
estimates to help in the inspection planning.
Scaled Mode
Shapes
The drawback of Operational Modal Analysis has
been that the mode shape scaling has been
arbitrary causing incorrect modal participation
factors. This has caused problems in
applications such as response simulation and
structural modification. Recently, however,
techniques to obtain the right scaling has been
developed and they are now being tested on full
scale structures. The results of these tests
indicates that for even larger structures it is
possible to obtain an accurate Frequency
Response Function, FRF, from response
measurements only.
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