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Accurate modal analysis
Electric vehicles are worlds
apart from ICE vehicles, but
they are still subject to many of the
same forces as their ICE
counterparts and benefit from
structural testing like any other
vehicle. Modal analysis is vital to
understanding and optimizing the
inherent dynamic properties and
behavior, leading to lighter, stronger
and safer constructions, greater
comfort and better performance,
regardless of propulsion system.
Modal analysis provides a
mathematical model of a structure’s
dynamic properties, consisting of a
set of modal parameters, each with
a mode shape and an associated
natural frequency and damping
ratio. The modal parameters can
be found by testing or simulation.
Integrating testing and simulation
in the development process helps to
cut development costs, reduces the
number of prototypes, and shortens
time from concept to production.
These benefits are all accomplished
by optimizing strategies for testing
structures, such as pre-test
planning, and improving the
development of simulation models
using model correlation and
updating based on test results.
Traditionally, testing has been
done using an impact hammer or
modal exciters (shakers) to artificially
excite the structure with a measured
input force, and the structural output
is simultaneously measured,
typically with accelerometers.
Frequency response functions
(FRFs) are calculated and then used
to derive the modal parameters
using various modal parameter
identification techniques also known
as curve-fitters. This testing method
is also referred to as modal testing,
classical modal analysis (CMA) or
experimental modal analysis.
In recent years, a complementary
method has become widely used:
Operational Modal Analysis (OMA).
In OMA, structural output is typically
measured using accelerometers as
in CMA, but the structure excitation
comes from the unmeasured natural
ambient and operating forces. This
makes it possible to do accurate
modal parameter identification
under actual operating conditions
(true boundary conditions, actual
force and response signals, or actual
environmental conditions), and
in situations where it is di icult to
artificially excite large or fragile
structures, and structures with
internal or ambient excitation that
is not possible to control or measure.
In the automotive industry, OMA
is used for a wide variety of tests
where CMA is impossible or would
produce erroneous results. This
includes engine and powertrain
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testing, suspension system testing
and testing of ride and handling.
Tests are performed under operating
conditions as close as possible to
real-life conditions.
OMA measurements are similar
to operating deflection shape (ODS)
measurements. However, ODS
analysis determines the vibration
pattern of a structure under given
conditions, as a function of time or
at specific frequency and order
components, while OMA determines
modal parameters. As such, they are
often used together.
An assumption with OMA is that
the input spectra to the structure
can be classified as shaped
stationary zero-mean gaussian
white noise. However, as
unmeasured natural input is used
in OMA, the input is typically neither
controllable nor well-defined as
when using hammer or modal
exciter excitation. For vehicle testing,
one challenge can be the content
of harmonic components originating
from rotating parts. If these
components that are superimposed
on the stochastic noise are close
to the natural frequencies of interest,
they must be removed prior to the
modal parameter identification
without a ecting the stochastic
noise content. There are highly
e ective techniques available today.
But, because the harmonic
components can be quite dominant,
a data acquisition system with a
high dynamic range is needed.
Brüel & Kjær o ers complete and
fully integrated solutions for OMA –
including accelerometers, data
acquisition hardware and software
for measurement, analysis and
test-simulation integration. Highly
e ective techniques for the removal
of harmonic components are
available as well as state-of-the-art
techniques for unbiased and
accurate modal parameter
identification. Automation ensures
that the job is done fast and with
accurate results.
New and complementary forms of simulation testing
methods are increasing efficiency in EV manufacturing
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OMA responses are
measured using
accelerometers, but
structure excitation comes
from unmeasured natural
ambient and operating
forces, making it possible for
accurate modal parameter
identifi cation under actual
operating conditions
Brüel & Kjær’s OMA software is a powerful tool for structural
dynamics tasks and provides a high degree of automation
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