 Good
morning. This is Wayne Tustin. Sometimes I'm called "Mr.
Random Vibration," about which I'll teach here at Santa
Barbara next month and at other cities in the coming months.
You might want me to teach at your facility. I also teach
via CD-ROM, with lessons coming to me via e-mail. We thank
Bruel & Kjaer for this web time and the Chicago Chapter
of the IEST for arranging these presentations and for
inviting me today to discuss "Vibration Aspects of Reliability
Enhancement via HALT, ESS and HASS". What are those acronyms
all about? |
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 I
think we'll all agree that we would be safe with this
hardware situation ... strength (red) greater than loads
(blue). We're 100% certain our hardware won't fail. Probability
of success = 1. 100%. But Figure 1 does not describe the
"real world", does it? In the "real world" ... |
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both the loads (blue) and the strengths (red) are distributed
in some manner. We're OK, of course, if our weakest product
is stronger than the most severe load we encounter. |
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Problems arise over time. Product
strength can deteriorate, so that it is possible for
an occasional severe load to exceed the strength of
an unfortunately weak product. In that situation, failure
can occur.
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We can't wait years to find out if
that's going to happen. Our bosses and our customers
want to know now that we have a reliable product.
So in our test laboratory we increase loads until we
get a failure. What kind of loads? Some labs use thousands
of on/off cycles. Most labs use rapid thermal ramping
+ random vibration. The latter is my subject today.
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 Much
of what I'm saying traces back to 1979. Here I want to
acknowledge one of my heroes, Willis J. Willoughby and
his NavMat document P9492, shown here. First the Navy,
then the Air Force and the Army called for environmental
stress screening (ESS), utilizing thermal ramping + 6g
RMS random vibration. They required ESS of military electronics
contractors. Reliability improved greatly. Many commercial
manufacturers adopted ESS. |
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 Here
in Figure 6 you see an example of a very large and a
very small ED or electrodynamic shaker. These
were developed back in the 'fifties. To this day, all
ED shakers operate ...
(courtesy MB Dynamics)
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on the same principle as electrodynamic loudspeakers.
Here, however we have direct current flowing in a field
winding. This creates a strong magnetic field across
this gap. That's where vibratory force is generated
by alternating current flowing in the driver winding.
Force travels up through the armature to the table surface
where test articles are mounted. The armature and test
articles vibrate together.
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 Often
the ED shaker is combined with a thermal chamber for thermal
ramp stressing combined with random vibration. Thus far,
we've only discussed single-axis-at-a-time shaking. For
years, we have settled for vibrating our products in their
X axis, stopped the test, turned our product into its
Y axis, stopped the test and finally turned our product
onto its Z axis. That is not very good simulation,
is it? Will you agree with me that "real world" shaking
is multiple-axis-simultaneous? |
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Assuming that we want to simulate the real world,
in our test labs, as at here at Hill AFB in Utah, we
should use multiple shakers to simultaneously vibrate
loads up-down, north-south and east-west ...
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as well as torsionally in roll, pitch and yaw. Another
reason is that multi-axis stimulation is more
effective and saves screening time. While the Air Force
can perhaps fund such an installation, not many companies
can.
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 They
might consider three ED shakers coupled to a common
platform, as in this videoclip from Anco, set just now
for maximum displacement. Anco also offers a six-shaker
version that adds roll, pitch and yaw. Realize that
each shaker requires ...
Click on the image to see this
video clip.
You will need Real Player to watch the video. Just click
here to download it for free.
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 ...
alternating current for the armature winding and direct
current for the field winding. These come from a power
amplifier. Into this cabinet ...
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the final power amplifier module is being installed.
(courtesy LDS)
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 Each
ED shaker also requires a channel of digital control.
Here the operator preadjusts intensity of random vibration
in units of g2/Hz over the desired frequency spectrum
for each shaker.
(courtesy Data Physics)
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 You
will probably hear some claiming that you only need
one shaker, that a "tilt" fixture such as this, skewing
the test article relative to the shaker's single axis,
provides multi-axis vibration. Don't believe it. Yes,
there is a component of motion in each of the test article's
axes, but those motion components are highly correlated.
In the "real world" motions in the several axes are
uncorrelated. Thus several independent shakers
are needed.
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 Much
less expense (and much less control) is possible with
a number of hammers - repetitive shock devices driven
by plant air. Note that they drive a platform in X,
Y and Z directions, also in roll, pitch and yaw. Articles
to be tested or screened are attached to the top of
the platform.
(courtesy Qualmark)
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 Note
that the vibrating platform is softly sprung. Note here
and on the next slide that the platform ...
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forms the bottom of a thermal chamber, so that test
or screening articles receive thermal stressing + repetitive
shock. Conditioned air, sometimes hot and sometimes
cold) is ducted for high air velocity through and across
the articles being screened.
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CRIQ in Montreal developed another
approach for multi-axis random excitation of printed circuit
cards. They use several loudspeakers to create vibratory
responses in cards. This approach offers spectral control,
as with ED shakers, but at much less cost. |
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 Here
are some upcoming "open" courses which you might attend.
Santa Barbara in February is not considered to be punishment.
I'm lucky to live here. While you are at www.vibrationand
shock.com or www.equipment-
reliability.com, please see the details of these
courses. Or consider having us "tailor"
training to meet your needs, for presentation at
your site. Details about distance
learning training are also shown.
Are there any final questions?
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 Thank
you for participating. If your computer had difficulty
with the video clips, or you didn't hear me clearly,
the presentation has been posted at Vibration and Shock
website.
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 Visit
our websites and find many free resources to enhance
your vibration and shock knowledge. Check out our message
board, our technical glossary
and our quarterly newsletter.
Thanks again to B&K and to IEST!
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