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Sébastien Triollet
Abstract
Development of an optical fiber sensor based on superimposed Fiber Bragg and Long Period Gratings: application to the discriminate measurement of temperature and strain
Sébastien Triollet - 3 December 2010
Optical Fibre Sensors present some interesting qualities when
considering its size and weight relatively light, which lead to
a low intrusivity of the sensor in a material (a composite
structure). These sensors are insensitive to electromagnetic
phenomenon, stable and long lasting with time, but sensitive to
several solicitations such as temperature, strain and pressure
hence a real need of discrimination. Among these sensors, we may
discern the Bragg gratings: the Long Period Grating one (LPG)
and the Fibre Bragg Grating (FBG) one.
This thesis work reports the development of an optical fibre
sensor based on two superimposed Bragg gratings: LPG and FBG for
measuring and discriminating temperature and strain. Several
studies are reported in literature without getting a real
condition use. That's why we propose a parameter E, which stand
for the discrimination efficiency leading to a possible
comparison of the existing techniques and highlight the quite
good potential of superimposed Bragg gratings. The settings of
such a structure are given in this thesis report and consist in
writing LPG first, then FBG over the entire length of the LPG,
which also gives multiplexing possibilities. Strain and
temperature calibration steps give sensitivities errors of 2%
for temperature and 3% for strain, which lead to estimated
errors on measured strain and temperature of 0.3°C and 3
microstrain respectively.
In an application point of view, the sensor has been used for
the instrumentation of a metallic structure subjected to a
variation of temperature and strain applied simultaneously. The
results exhibit a maximum error of 0.4°C and 3me for temperature
and strain respectively, which is a good validation of the
sensor for structural control and monitoring purpose. The second
studied application is about instrumentation of glass/epoxy
composite specimen for monitoring manufacturing processes: VARTM
(Vacuum Assisted Resin Transfer Moulding) and LRI (Liquid Resin
Infusion), for which temperature and strain have been monitored
with the superimposed Bragg gratings based sensor. Dielectric
analyses have also been performed during those processes in
order to compare and validate our results.
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