Software for Operational
Modal Analysis
ARTeMIS 


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
outputonly 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 knowing
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.
There are a number of benefits 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 Insitu 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 insitu 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.
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 nondestructive 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.
