Evaluation Engineering - 23

AUTOMOTIVE TEST

THE EVOLUTION OF
AUTOMOTIVE VIBRATION
FIXTURING
by Tony Araujo
The principal vibration testing capabilities in common use today largely
evolved from the early requirements of
the aerospace industry. Standard test
protocols, random vibration, single axis
testing, and combined temperature and
vibration testing were developed in response to the reliability testing needs of
the early Gemini and Apollo space programs. The use of electronics in mobile
applications was still in its infancy and
the need to deploy highly reliable systems
in space applications drove the testing
industry to develop better tools with
which to simulate the new end-use
environment.
While vibration testing using shakers
has been around since World War II, these
early shakers were limited to 85 Hz and
were more suited to examining the fatigue
properties of structural components.
Engineers knew that significant vibration energy could be present at higher frequencies and that electronics were more
susceptible to these frequencies, but they
couldn't test them until electrodynamic
shakers became more common. To ensure
that their device under test (DUT) experienced the energy present at these higher
frequencies, aerospace testing methods
stressed the necessity of vibration fixturing, which delivered high transmissibility
throughout the test frequency range.
The common use of electronics in autos
only started to take hold in the 1970s, and
back then it was largely isolated to engine control units as a response to meeting more stringent pollution controls.
Consequently, nearly all automotive vibration testing in this era was performed
below 100 Hz. With the increasing use of
electronics in vehicles to control various
body systems with individual ECUs-especially since the 1990s and particularly

today with electric vehicle and autonomous vehicle applications-the automotive industry has come to recognize the
benefits of vibration testing at higher frequencies using electrodynamic shakers.
While the automotive industry, like
aerospace, recognizes the need to test
electronics at higher frequencies, many
automotive engineers-trained in the
era of low-frequency testing or still using specifications based on these early
applications-fail to understand the
impact which the vibration fixture can
have in magnifying or attenuating higher
frequency vibration energy in their vibration tests.
This article describes how the industry got to this point and how segments
of the industry are leading the adoption
of qualified vibration fixturing as a necessary requirement of high-frequency
vibration testing.

The early days of vibration testing
Vibration is a relatively young field of
study. The first college course devoted to
the study of vibration was only introduced
in 1928. While vibration has been recognized for nearly a century as a critical environmental factor for which the designer
has to accommodate, it's only since the
1960s and the advent of electrodynamic
shakers that the modern practice of vibration testing really evolved. Before this
period, the mechanical vibration machine
technology that was available limited the
frequency range to 85 Hz. Indeed, the variety of machines and the very specific
vibration environments each machine
was attempting to simulate made it very
difficult to compare test results between
laboratories.
The WWII period introduced the concept of qualification testing on equipment

Figure 1. Early high-force electrodynamic
vibration machine.
Paragon Systems Testing

prior to use, and the period after the war
saw the rapid development of new vehicle
and weapon technologies. Vibration fixtures for the early low-frequency vibration test machines could be counted on
to be resonance-free, even though they
were only designed for strength. Unless
they were very large fixtures, the dynamic
response of the fixture was not a major
test consideration. After the war, the
lower (<100 Hz) and usually periodic frequency domains of propeller-driven aircraft began to be replaced by the higher
frequency (100 Hz to 2000 Hz) random
vibration environment produced by jet
and rocket engines.
The advent of the space programs of
the 1960s coincided with the introduction
of high-powered electrodynamic shakers
which were capable of performing vibration tests up to 2000 Hz and higher.
With the newfound ability to replicate
a high frequency spectrum, this higher
frequency vibration testing capability
quickly became the norm for qualification
testing in the aerospace industry. These
new machines made it easier to promote
inter-laboratory test repeatability. A lot
of this testing was also being driven by
necessity to increase the reliability of all
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