Evaluation Engineering - 25

methods are still partly based upon the
legacy 85 Hz era of electromechanical
vibration machines.
These latter methods are completely
silent on the issue of vibration fixture
performance because it was largely a
non-issue with low frequency tests. It's
not uncommon for a testing laboratory to
receive an automotive component from
a client for vibration testing to a 5 Hz to
2,000 Hz spectrum, where the vibration
fixture consists of a simple steel tube
space frame which must be clamped to
the shaker table surface. Steel tube fixtures are economical to fabricate and
readily configured for three-axis vibration
test deployment, but they are not known
for their rigidity over 85 Hz.
When a vibration test method that
has a 5 Hz to 2,000 Hz spectrum doesn't
define the dynamic performance requirements of the fixture or the necessity of
monitoring test levels at specific product
mounting locations in the way a 5 Hz to
85 Hz test would in the past, the vibration
engineer can't do much more than expose
the fixtured sample to the vibration test
spectrum using a single accelerometer
mounted to the shaker surface. An experienced vibration test engineer will do this
with the knowledge that the test sample
may be both over-tested and under-tested
over significant portions of the test spectrum. But because fixture performance
and monitoring locations are not defined
in the test method, the engineer is simply
controlling the test level at the interface
between the fixture and the vibration
table surface.
Vibration is an area of study not covered
in detail by many engineering programs.
The knowledge required to design vibration fixtures is an even rarer skillset. So,
it stands to reason that most engineers
who will end up reading a vibration test
report produced under these conditions
will not recognize that the test sample may
not have experienced the full effect of the
vibration test described in the method.
If the test sample failed the vibration
test, there will likely be further investigations which may then pinpoint the fixture
as the reason for the failure. However, a
more likely outcome is that the product
designer will respond to a sample failure

Figure 3. Detail of state-of-the-art vibration
fixture.
Paragon Systems Testing

by spending time and resources redesigning a product that was overtested because
of fixture resonances.
And if the product passed the vibration test? At first glance, this this might
seem to be a more benign outcome. After
all, the product engineer can now get on
with the next phase of product deployment. But, without any knowledge of the
fixture's effect on the test, an undertested
product may be released to production
with the potential for a future field failure
and recall.

The future of automotive
vibration testing
During the first decades of electronics deployment into automobiles, the
consequences of a sample failure were
not that consequential. If your radio or

power windows stopped working, an automotive manufacturer might face a bill
for the repair costs. In the coming era of
autonomous vehicles, where electronic
systems will be in complete control of a
vehicle, the consequences of a failure are
more serious. Automotive safety recalls
have cost manufactures billions of dollars. To avoid this potential in the future,
automotive engineers need to recognize
what aerospace engineers have known
for decades-namely that understanding
vibration fixture performance is a critical requirement of any high-frequency
vibration test.
Tony Araujo is vice president - testing at Paragon
Systems Testing. He has
worked in the vibration
testing and vibration fixture
field since 1995. He can be contacted at
tonya@paragonsystems.ca
OCTOBER 2019 EVALUATIONENGINEERING.COM

25
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Evaluation Engineering

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