The article discusses the sensing
characteristics embedded in optical fibre
that are set to create a new trend in
monitoring the health of utility network,
civil infrastructure as major elements of
Smart cities…
Fibre sensing technologies mentioned in this
article are for Utility/Structural/Infra health
monitoring that STL can leverage being in
fibre business. This feature details on the
sensing characteristics embedded in optical
fibre that are set to become trendsetters in
monitoring the health of civil infrastructure,
especially for upcoming technologies like
bullet trains & smart cities for utility
pipelines, dams, tunnels, bridges, residential
& commercial complexes. One great add-on
value with such fibre is that it not only serves
as sensor but also acts as a communication
media to send down the signals to central
command centres.
Sensing via Optical fibre technology
Sensory fibres can be classified on the basis
of Light characteristics (such as intensity,
wavelength, phase or polarisation; whether
sensing segment is inside or outside of fibre,
and local, if it is quasi or fully distributed).
Fibre optic sensors are majorly of two types:
intension-metric and interfero-metric.
Intension metric sensors rely on variations of
the radiant power transmitted through an
optical fibre. For examples a micro-bend
sensor is a common intension-metric sensor,
while interfero-metric sensors rely on
measured induced phase change in light
propagating through the optical fibre such
as Fabry-Perot and Bragg grating.
The optical fibre sensing distribution can be
of two types: spatial and fully distributed
sensing techniques. For spatially distributed
way of monitoring use technique Fibre
Bragg Gratings (FBG) and for fully
distributed sensing technique Raman DTS
(Distributed temperature sensor) and
Brillouin Optical Time Domain
Reflectometry (BOTDR) can be used even for
infra more than 100 Km with the ability to
measure temperature & strain at thousands
of point in a single fibre.
Major measurable parameters of these
sensors include:
- Stress: When considering the safety of a
structure, the maximum stress in a
member due to live load, earthquake,
wind, or other unexpected loadings
must be checked not to exceed the
allowable stress of a member. In
allowable stress design of steel
structures, if the maximum stress in a
member reached the yield stress, the
member is considered to be analogous to
failure. Therefore, to guarantee the
safety of a structure and its users, the
maximum stress in a member must be
monitored. In this case, the reliability of
the evaluated safety depends on the
number and location of point sensors.
- Strain: It is noticeable that due to the
deformation measured is the average
value measured, the strain variation or
stress distribution of a beam can be
considered by using several fibre optic
sensors, and by means of these sensors
the maximum strain or stress in a beam
can be measured. The strain sensors
consist of a fibre Bragg grating
sandwiched between layers of carbon
composite material and are about 50
mm long and 0.5 mm thick. The accuracy
and sensitivity of the sensors are
dependent upon the optical system.
- Temperature: Thermal expansion is an
important factor in all types of structures
where differential heating may occur,
either from environmental effects, such
as the solar heating of pavements and
bridge decks, or from service conditions,
such as in nuclear-reactor, pressure
vessels or furnaces. The thermal
expansion coefficient will be a variable
quantity depending on materials.
- Crack Monitoring: The existing
condition of many important concrete
structures can be accessed through the
detection and monitoring of cracking.
Conventionally, crack detection and
monitoring have been carried out by
visual inspection. The procedure is time
consuming, expensive, and yet
unreliable. An optical fibre is embedded
in the concrete element in a “zigzag”
shape and before the formation of
cracks, the light intensity distribution
along the fibre is measured. When a
crack opens in the structure, fibre bend
to stay continuous and consequently
light intensity is changed.
- Cable and Fibreglass reinforced
plastic (FRP) monitoring: With the
development of the fibre optic sensing
techniques, the applications of fibre
optic sensors have been extended from
the laboratory test to in site
experiments. Some kinds of fibre optic
sensors have been applied to the health
monitoring of FRP and cable structures
in recent years. They employed fibre
optic sensors to monitor the strains of
steel, FRP (fibre reinforced polymer),
pre-stressing tendons, post-tensioning
cables and etc. They can measure strain
and temperature in arbitrary regions.
- Bridge Monitoring: The applications of
fibre optic sensors to bridge monitoring
are focused in measurement of short
and long-term parameters of bridges.
One of these bridges is the Mounted
Bridge. Up to 100 fibres optic sensors
were mounted inside the bridge and six
locations were monitored in order to
assess the strains at the centre of the
bridge and the shear strains close to one
of the supports of bridge. Another
monitoring is presently being used in
the construction of the bridge over River
Ave, in Portugal. In this bridge, besides
the Bragg grating sensors, conventional
resistive strain gauges in concrete,
reinforcing and pre-stressing steel and
etc. were added.
- Moisture Monitoring: Corrosion can
occur internally without this being
evident from the outside. This is often
due to the ingress of water corroding the
reinforcements, which is hastened by
the salts and chloride ions dissolved in it.
At the moment, fibre optic based
humidity sensor has been developed and
used for the measurement of moisture
absorption in concrete. The sensor was
fabricated using a fibre Bragg grating
coated with a moisture sensitive
polymer. The sensor itself exploits the
inherent characteristics of the fibre
Bragg grating.
- Welding Residual Stresses Monitoring:
This feature is combined with excellent
stability and also with the possibility of
having several optical fibres. As result of
their features, a kind of sensors
constitutes a very powerful tool for the
analysis of welding transient and
residual stresses. Under the assumption
of a perfect bonding between the plate
and the sensor, we are actually
measuring transient and residual strains
in the material being welded.
Other parameters may include
curvature/bend sensors, electric and
magnetic Field sensors, torsion/twist
sensors, transversal loading sensors,
refractive index sensors, vibration sensors,
and multi-parameter sensing.
Fibre at the right time
Many of the newly proposed smart cities in
India are Greenfield. In retrofit cities it is
difficult to work out on monitoring of
existing infra but in new cities it becomes
quite easy to deploy the fibre along with
deployment of existing infrastructure. In
some case wireless sensors are also used in
multiple applications of Smart cities but
continuously powering them is big
constraint, and putting these sensors across
100 Km of infra is not pragmatic.
Major implementation areas can be bridges,
buildings, tunnels, wind turbines, Railway
infrastructure, upcoming bullet trains
project and utility pipelines in India.
Applications
Fibre optic sensors are used in several areas
such as
- Measurement of physical properties
such as strain, displacement,
temperature, pressure, velocity, and
acceleration in structures of any shape
or size.
- Monitoring the physical health of
structures in real time.
- Buildings and Bridges: Concrete
monitoring during setting, crack
(length, propagation speed)
monitoring, pressurising monitoring,
spatial displacement measurement,
neutral axis evolution, long-term
deformation (creep and shrinkage)
monitoring, concrete-steel interaction,
and post-seismic damage evaluation.
- Tunnels: Multipoint optical
extensometers, convergence
monitoring, shot Crete / prefabricated
vaults evaluation, and joints monitoring
damage detection.
- Dams: Foundation monitoring, joint
expansion monitoring, spatial
displacement measurement, leakage
monitoring, and distributed
temperature monitoring.
- Heritage structures: Displacement
monitoring, crack opening analysis,
post-seismic damage evaluation,
restoration monitoring, and old-new
interaction.
(The article has been authored by Madhukar
Srivastava - Business Head, Vikram Singh Mains
-Business Analyst & Deepak Sharma, Head -
Solutions from Telecom Services Business)
. Optical Fibre sensors are majorly of two types: intension-metric and interfero-metric. Intension metric sensors rely on variations of the radiant power transmitted through an optical fibre to the home. For examples a micro-bend sensor is a common intension-metric sensor, while interfero-metric sensors rely on measured induced phase change in light propagating through the optical fibre such as Fabry-Perot and Bragg grating. The Smart City Project involves Sterlite’s Cat 5 cables which are suited for enhanced performance for transmission of high- speed data. \n\n The optical fibre sensing distribution can be of two types: spatial and fully distributed sensing techniques. For spatially distributed way of monitoring use technique Fibre Bragg Gratings (FBG) and for fully distributed sensing technique Raman DTS (Distributed temperature sensor) and Brillouin Optical Time Domain Reflectometry (BOTDR) can be used even for infra more than 100 Km with the ability to measure temperature & strain at thousands of point in a single fibre. The fibre broadband is paving the way for the smart cities with successful results. ', 'https://www.stl.tech/sterlite-live/blog_banners/11/medium/smart-city-innovation3f8d.jpg?1460020195', 'innovation-in-smart-cities-via-sensory-fibre.html')){kind=link}
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