Chemical Engineering December 2022 - 31

terpret the data for successful diagnosis
and problem resolution. If your
organization has experience collecting
and interpreting ODS data, you
have a head start, but this head start
shouldn't be confused with the finish
line. There is a lot to learn about
how to best use this new technology,
and in the words of American
author and motivational speaker Zig
Ziglar, " Some of us learn from other
people's mistakes, and the rest of us
have to be other people. "
Capex versus service. Given its
novelty, and the commitments required
to truly make video vibration
analysis technology successful in
your organization, each company
should evaluate whether it makes
sense to acquire, staff and maintain
the technology in-house, or if
it is preferable to rely on third-party
service providers. Sometimes organizations
opt for an incremental
approach, trialing video vibration
analysis via a third party, before
committing to purchasing the technology
outright. If your organization
does engage with third parties, either
for evaluation purposes or for
permanent supply of the capability
as a service, do not assume that
any organization with a high-speed
camera and some software knows
what they are doing. As discussed
previously, this is a tool that requires
training and experience to utilize
properly, and if your organization
prefers to outsource, make certain
that the training and experience is indeed
present in the third party. As an
additional consideration, proficiency
with a diagnostic tool does not automatically
make one a skilled problem
solver. In order to properly diagnose
and solve problems using video vibration
analysis, people are still required
to understand and interpret
the data, and conceive of solutions,
so make sure you are hiring the right
people, not just time with a widget.
If you do conclude the technology
belongs in your organization,
or even if you opt to rely on service
providers, you will need to do some
homework to understand the various
capabilities available.
Detection threshold
One of the most important concepts,
and possibly the least explained in
the industry, is the detection threshold.
Perhaps you
have heard complaints
about the
technology not
working except
for very high vibration
levels, or
have experienced
this issue yourself.
This is likely
due to the detection
threshold. In
short, video vibration
analysis has
a minimum level
of displacement
required before
it can detect the
signal (Figure 2).
FIGURE 2. Any velocity values above the detection threshold lines will be accurately
detected and magnified. Understanding the detection threshold as it
relates to frequency is very important for diagnosing lower-displacement issues
Unlike the piezo-electric accelerometer,
which can effectively measure
nearly zero vibration, the pixel
of the camera sensor has a noise
baseline that makes a measurement
of near-zero impractical (more on
that later). Thus, is becomes critical
to understand what that minimum
level of displacement required is,
and equally important, what the displacement
depends on, because it's
not absolute - it's relative.
Given that this is an optical measurement,
and that a pixel has specific
dimensions of its own, each
pixel
is subsequently
covering
a
given amount of space on whatever
the camera is viewing. As a basic example,
consider a four-pixel camera
(two pixels wide by two pixels high).
If this four-pixel camera was observing
a 1-in. × 1-in. square, then
each pixel is observing a 0.5-in. ×
0.5-in. area for motion, and its detection
threshold scales accordingly
for this area. Now if the same fourpixel
camera then observed a 1-ft ×
1-ft square, each pixel would then
be observing a 6-in. × 6-in. area for
motion, and the detection threshold
would be scaled higher to accommodate
the larger area. Since there
are typically millions of pixels in play,
the detection threshold is most effectively
considered as a function of
the pixels in aggregate, or the " field
of view. " Rather than determining
what each pixel is measuring to figure
out the detection threshold, their
combined dimensions can be easily
determined. For example, a 10-ftwide
field of view (meaning the camera
is viewing something 10-ft wide
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from horizontal edge to edge) has
a given detection threshold when
considering all the horizontal pixels
together (for example, 1,920 pixels
wide). In this example, each pixel is
observing a 0.0625-in. × 0.0625-in.
square, with the detection threshold
being a function of that area.
The detection threshold is considerably
less than the pixel dimension,
but is not zero. The technical reason
this detection threshold exists is due
to the noise in the sensor. Recall that
the fundamental measurement is variation
in light intensity. The noise in the
sensor (one of the millions of camera
light detectors) effectively produces
a relatively constant baseline level of
light intensity variation. Consequently,
actual change in light intensity due
to movement within the level of the
noise floor is not distinguishable from
the noise itself, and therefore is not
accurately measurable, or magnifiable
for visual motion interpretation.
Keep in mind that this detection
threshold is a displacement value,
not a direct velocity or acceleration
measurement. While there is no
frequency dependency to the displacement
value, there is a practical
frequency relationship. As the
frequency of vibration increases, the
velocity required to reach a given displacement
also increases (as the frequency
of oscillation increases, there
is less time for the pump to move
from point A to B). Typically, vibration
velocity is the measurement of
interest when determining machinery
health, so a fixed-displacement
" detection threshold " will equate to
an increasing level of velocity " de31
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Chemical Engineering December 2022

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