Ruby,
There is significant interest in using random vibration stresses
on repetitive shock (RS) and electrodynamic (ED) shakers, for test-
time compression and better reliability assessment. The primary
differences in random excitation on the two shakers are as briefly
stated below.
1) The number of axes excited:
RS shaker provides excitation along six directions (3
translational and 3 rotational), while the ED shaker provides
excitation along a single axis, usually translational motion normal
to the plane of the shaker table. Some preliminary studies suggest
simultaneous triaxial vibration induced stresses to be more
effective for testing than the random vibration stresses induced in
single axis shakers (Larson et. al., 1997, Mercado, 1994,
Henderson, 1993), while other suggest the use of a single axis
shaker (Romanchik, 1994).
2) Distributions of energy:
The amplitude distribution on the RS shaker is typically non-
Gaussian while that on the ED shaker is Gaussian.
3) Frequency spectrum and its tailorability:
The operating bandwidth of the ED shaker is 0-2500 Hz while
that on the RS shaker is typically between 0-25000 Hz. Moreover
the spectral response of the ED shaker is tailorable, while it is
inherent to the setup on the RS shaker. Although some studies
suggest the high frequency content in RS shakers being more
advantageous for life testing (Henderson, 1993), there are many
unanswered questions regarding the advantages of the low
frequency excitation on the ED shaker vs the high frequency
excitation on the RS shaker.
There is significant confusion in the literature, in the standards, and
in industry, regarding the most costeffective and scientific way to
select a platform and conduct accelerated stress testing for
electronic packages exposed to vibration loading accompanied by
an absence of scientific comparisons between the two types of
vibration (Howe, 1994, Caruso, 1994). The other critical issue is the
need for a rational method to relate the test results quantitatively to
inservice reliability, using a scientific acceleration transform. In
other words, the amount of testtime compression achieved in the
accelerated test must be determined quantitatively, based on the
physics of the relevant failure mechanisms. This issue is equally
important on both shakers.
CALCE EPSC did conduct a study on the Comparison of Repetitive
Shock and Electrodynamic Vibration Test Machines for
Accelerated Stress Application, with the deliverable being
acceleration transforms to predict in-service life. Unfortunately, this
information is currently only available to CALCE members. It may
be published sometime later this year.
Best Regards
Craig
On 18 Jul 00, at 11:08, Ruby Hazen wrote:
> Hello all,
>
> Could anyone tell me about the advantages and disadvantages between
> electrodynamic (ED)and repetitive shock (RS) vibration table? What's
> the guideline for choosing between them?
>
> Thanks in advance!
>
> Ruby
>
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Dr. Craig Hillman
CALCE Electronic Products and Systems Consortium
University of Maryland
College Park, MD 20742
(301)-405-5316
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