Evaluation Engineering - 24

AUTOMOTIVE TEST

the new electronic systems that were being introduced.
Early on, aerospace engineers recognized that these new higher-frequency
domains would require a thorough understanding of the vibration fixture-
the interface between the shaker table
surface and the individual attachment
points on the DUT. Without any pre-existing standards for fixture performance,
many questions about vibration fixture
design needed to be answered. Should
we replicate the impedance of the actual
in-service mounting? Where should we
monitor the vibration input into the DUT?
What is acceptable performance?
The evolution of MIL-STD-810 paints
an accurate picture of the changes in vibration fixture design philosophy. In 1962,
the standard indirectly defined fixture
performance was, "The test item shall be
installed... by its normal mounting means
directly to the vibration exciter table, or
by means of a rigid fixture capable of
transmitting the vibration conditions
specified herein," and, "The vibratory acceleration levels or DAs of the specified
test curve shall be maintained at the test
item mounting points. For large items
where there is a variation of the vibration level between mounting points, the
minimum input vibration shall be that
of the specified test curve." Crosstalk
(transverse) motion was limited to 100%,
regardless of frequency.

Today's aerospace testing laboratories find fixture performance has been
much better defined in the current version MIL-STD-810H and fixture modal
survey or resonance search of the vibration fixture is required before any testing
is performed.

Vibration testing in the
automotive industry
Adoption of electronics in the aerospace
industry in the 1950s and 1960s, and the
new high-frequency vibration environment that these systems were required
to operate in, drove the industry approach
to vibration fixture performance. In contrast, during the same time period, the
automobiles of the same era used very
little, if any, electronics. Vibration tests-
when they were performed on automotive
components-were also limited to the
lower frequencies provided by electromechanical vibration machines.
As in aerospace, the automotive industry developed special-purpose machines
to simulate the automotive vibration environment. One of the earliest of these
vibration machines was developed to
perform testing of automotive lamps.
The test machine and method were

originally defined in 1940 in SAE J577 and
performed fixed displacement testing up
to 60 Hz. However, unlike the aerospace
industry, which moved quickly to more
modern vibration machines, the automotive industry was much slower in making
the switch. In fact, the mechanical headlamp vibration test machine is still specified in FMVSS 108 as the only recognized
machine for this purpose.
The original slow pace of electronics
adoption in automobiles has been replaced today by an explosion in electronic
applications for road vehicles. While ABS
brakes and airbags are considered "mission-critical" applications for electronic
controls-where the consequences of
failure of an ECU are serious-they don't
compare to the consequences of an autonomous transport truck losing control
on a congested highway.
While electronics in critical applications are now in common use throughout
a modern vehicle, the adoption of modern vibration testing methods have not
kept up the same pace. Many automotive
manufacturers have embraced the current state-of-the-art vibration test methods to tailor test methods for individual
applications. But many automotive test

Figure 2. SAE J577 Mechanical headlamp
vibration machine.
Paragon Systems Testing

EVALUATION ENGINEERING OCTOBER 2019

Evaluation Engineering

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