Chemical Engineering December 2022 - 32

tection threshold " as the frequency
increases. When this concept is applied,
as the frequency increases,
displacement levels are lower for a
given velocity, and it becomes all
the more significant to understand
the displacement-based
detection
threshold to ensure adequate
measurement and magnification at
higher frequencies.
If you have followed the logic thus
far, you may be thinking, " why not just
reduce the field of view to see lower
levels of vibration, even at higher frequencies? "
This is indeed possible,
to the point of detecting nanometer
levels of displacement, but there is
a trade-off. As discussed earlier, the
primary advantage of this technology
is the comprehensive nature of
the measurement, whereby the user
gets to " see " and measure all the
components of the system together
at once. By narrowing the focus to
achieve lower levels of displacement
detection, the user has resorted
back to a small measurement area,
which behaves similarly to the accelerometer,
and thus forfeits the
underlying value of the technology.
With this fundamental understanding
of detection threshold in
hand, note that there are two types
of data realized, and potentially two
different detection thresholds. Depending
on the vendor, the vibration
measurement may or may not
have a different threshold than the
magnification threshold. It is important
to do your homework, and ask
for data to support the claims, since
your mileage may vary if the quoted
measurements are not provided in
an appropriate context (for example,
what is the field of view for a given
detection threshold?).
Also, with regard to the magnified
motion capability, there are differing
approaches to enhance the signal,
producing the magnified motion
videos. The two major methods are
optical flow and sub-pixel magnification.
Primarily in an effort to speed
up the processing of all the pixellevel
data, optical flow takes the
millions of pixels collected and averages
them together into regions,
and applies significant gain, along
with interpolation across regions, to
effectively recreate the wire-frame
model of ODS. If the averaged region
size is sufficiently small (and
32
the user sufficiently patient), reasonable
enhanced video results can be
achieved, but the user must appreciate
that the interpolation combined
with noise can leave much to the
imagination, and finer details can
be lost in the process. An analogy
would be drawing with a paint brush
versus a mechanical pencil.
On the other hand,
sub-pixel
magnification maintains a pixel-level
analysis to keep the hard-won resolution
in the diagnostic assessment.
This makes detecting issues requiring
detail possible that might be
missed or misrepresented by optical
flow, such as foundation cracking,
loose fasteners, piping supports,
and similar issues, while still
being able to magnify overall motion
issues, such as misalignment, that
may be also displayed with optical
flow. The motion is much more precise
than optical flow, and effective
at lower levels of displacement.
Accuracy
Regarding the accuracy of the technology,
if all you need is a single-point
vibration measurement, it is still going
to be faster, easier and more accurate
to obtain that single point of data with
an accelerometer for the foreseeable
future. The advantage of using video
for vibration
measurement is
the
comprehensive nature of the measurement,
effectively taking millions of
measurements all at once. But how
accurate are the vibration data that
the video vibration analysis provides?
Is it just a visualization tool, or can you
measure the vibration as well?
With
the above discussion
of detection threshold
in mind (since accuracy will
not be applicable if the signal
is below the noise floor),
there are several
practical
elements to consider to improve
the accuracy. Since
the measurement is one of
changing light intensity, it is
important to make sure there
is a " Goldilocks " level of light
- not too bright, not too
dark. Either end of the spectrum
ends up clipping the
vibration signal, resulting in
inaccurate
measurements.
Relatedly, since the technology
is looking for a change
in light, contrast is required
FIGURE 3. A moderate level of light and ligh-dark contrast are
required for accurate displacement measurements
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DECEMBER 2022
to make a measurement. Given a
featureless,
smooth, single-color
surface, there will be no accurate
measurement possible. In situations
where targets can be applied to provide
a point of contrast, the camera
and software can accurately derive
displacement measurements (Figure
3). Once there is sufficient lighting
and contrast, it is still up to the user
to provide a dimensional reference
of some sort, either the distance
between the camera and target, or
a reference length within the image
itself. Either way, the system needs
a way to correlate the pixel size back
to real-world size. Any lack of accuracy
in this step will directly correspond
to a lack of accuracy with
the calculated measurement.
And again, any vendor claims
should be substantiated with data
to support them, and their relevance
should be clearly understandable
by the user. For example,
if the system has a certain accuracy,
but it is for 0.1 in. peak-to-peak of
displacement motion with a 1-ft
field of view, it is not very relevant
to the real-world pumping system
conditions for which the user will
likely be seeking accurate data. Understanding
the accuracy, particularly
as the system approaches the
noise floor, and over the entirety of
the frequency span, will help ensure
users know what to expect when
deploying the technology.
Additional capabilities
In addition to magnifying the motion
present as a result of operating
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Chemical Engineering December 2022

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