Fiber Optic Sensor Technology
Short Summary
As use of composites in the automotive industry increases, as does the need to better understand the strength and performance of these materials. Luna’s sensing technology offers the advantage of using lightweight and low-cost fiber optic sensors to measure thousands of strain and temperature sensing points over an entire structure. Using this technology to better understand composites will help mitigate risk in designs, as well as offer a quantum leap in setup time.
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For engineers working
without design
guides, comprehensive
end-to-end testing and FEM
is all the more critical. This
is particularly true in the auto
industry where engineers
are exploring the use of
new lightweight materials
in the vehiclefs structural
components and drivetrain.
If nite element models
were always 100% accurate,
then model validation
through testing would not
be necessary. However, even
for traditional designs using
materials with well-
understood characteristics,
this is not the case and the
need to validate and calibrate
nite element models is not
uncommon in the design
process.
The use of polymer
composites adds a new level of
complexity, given their lack of
homogeneity compared with
the metals normally used in
automobile design. Traditional
point sensing using strain
gauges is inadequate and
may fail to accurately reect
the distribution of strain
throughout a structure
composed of these new
composite materials. This
is especially true of areas
with large strain gradients,
which strain gauges are not
capable of measuring.
High-resolution distributed
ber sensing addresses
the limitation of strain
gauges in testing structures
manufactured with
composites. Luna Innovations
of Roanoke, Virginia, has developed an advanced system
that uses ber optic cables as a
distributed sensor to measure
either strain or temperature
along a continuous length
of ber optic cable. When
illuminated, these cables
have the equivalent of an
optical ngerprint that will
change, in a predictable and
repeatable way, in response
to changes in temperature
and to ber elongation
when bonded to a structure
experiencing strain. This
ber optic cable replicates
a virtually continuous line
of strain gauges with just
millimeter spacing between
sensing points.
The ber optic cable
functioning as a sensor is
immune to EMI, inherently
dielectric and can be bonded
or embedded within a
structure under test. A single
cable can be laid out along the
structure in a grid pattern to
provide a full picture of the
distribution of strain. For
cylindrical objects, the ber
can be wrapped in a helical
fashion and can reect strain
resulting from either bending moments or torsion. Strain
or temperature data can
be displayed versus length,
or individual points can be
selected anywhere along the
ber and displayed versus
time. The location of these
measurement points can
be changed within minutes
versus the hours it would take
to reposition a strain gauge.
The application of the ber
optic distributed sensor is
similar to the traditional strain
gauge in that the surface is
sanded and cleaned prior to
the sensor being epoxied in
place. The difference is that
with a ber optic sensor, you
are able to install a virtually
continuous line or array of
sensors in one application.
Furthermore, this line of
sensors is connected to the
processing unit through the Fiber optic sensor
technology
For design validation of composite structures, Luna Innovations
offers its high-resolution distributed ber sensing system
Luna InnovationsABOVE: Luna has developed a new
system for strain measurement, which
uses distributed ber sensing
www.AutomotiveTestingTechnologyInternational.com
JUNE 2015158
the ber optic cable functions
as both the sensing elements
and the signal path for
these elements back to the
processing unit. This provides
an improvement by several
orders of magnitude over
traditional strain gauges in the
simplicity of the wiring and
size of the harness connecting
the strain sensors to the
processing unit. This not only
offers major advantages during
the initial test setup, but will
aid greatly in subsequent test
setups to accommodate any
design iterations.
High-resolution
measurement technology
using ber optics as sensors
helps mitigate risk in designs
using new polymer composites
by providing a full eld view
of the distribution of strain
within structures built with
these new materials. Even
for design and testing of
structures using traditional
metallic materials, distributed
ber optic technology offers a
quantum leap in setup time
and simplication of the
wiring and harness.