Feature Structural integrity monitoring with fibre Bragg grating sensors Robert Bogue Associate Editor, Sensor ReviewAbstract Purpose – This paper describes a recent collaborative project involving the development of a multiplexed fibre Bragg grating (FBG) sensor system for structural integrity monitoring. Design/methodology/approach – The system is described and field trials on both conventional and novel composite bridges are discussed. A FBG sensor-based structural monitoring system was developed, based on a fluorescent fibre as the optical source. It used a tuneable, fibre-coupled, Fabry- Perot filter, actuated by piezoelectric transducers and operated over the bandwidth of the source at up to 250 scans/second. Light from the source was filtered and reflected back from the Bragg gratings, through optical couplers, to eight photodiode detectors. These detected the resulting time-domain spectra of the sensors in each of the serially connected sensor arrays. The system was tested at City University and then subjected to trials on the Mjosund road bridge in Norway and on West Mill bridge in Oxfordshire, UK, which is the first bridge to be fabricated from a new type of composite material. Findings – During the Norwegian trials the system was arranged with four or five FBG sensors per channel giving a total of 32 measurement points with eight parallel channels. Twelve conventional foil strain gauges and a number of thermocouples were also installed. Different static and dynamic loads were applied over a period of 18 months and the results showed that the thermally compensated strain data obtained optically matched those from the resistive gauges to within ,5 m1. During the construction stage of the Oxfordshire bridge , sections of the decking and longitudinal composite support beams were instrumented with 40 FBG sensors with temperature compensation, placed at pre-selected sites of maximum strain. These exhibited a resolution of^5 m1and an operating range of over ^2,000 m1. Originality/value – This research has shown that multiplexed, multi-point FBG sensor systems can accurately and reliably monitor both static and dynamic strains in large structures over a range of temperatures and for extended periods of time. Keywords Condition monitoring, Fibre optic sensors, Sensors Paper type Technical paper Integrit y monitoring is an essential tool for ensuring the safety and ass ess ing the condit ion of crit ica l concre te, stee l and other structures such as bridges, roads, offshore rigs, railways and process pla nt. Traditionally , it has uti lis ed a dis par ate range of condition monitoring and NDT techniques such as ultrasonics, acoustic emissi on, strain gauge measu rement s and various optical inspect ion methods. However , to monitor the strain in a large structure at several points simultaneously and over an exten ded perio d of ti me is pro ble mati c, as existi ng techni ques suf fer dra wbacks such as ele ctrical interf erence or the inability to make multiple measurements in real time. In recogn ition of these limitati ons, a collab orati ve, three- year, £0.8 million research project was started in 2001. This was funded by the UK’s Department of Trade and Industry (DTI) and the Engine ering and Physi cal Science Research Counc il (EPSRC) under the Far aday/ Inters ect partnersh ip. Intersect and Faraday Partnerships The UK government’s Intersect scheme provides expert advice based on the experience of a network of academic and industrial partners in the sensor, me asure me nt and data anal ysis fie lds. It is supporte d by the DTI (Dep artmen t of Tr ade and Indu stry) and EPSRC (the Enginee ring and Physical Sciences Research Council) and managed by SIRA and the NPL(National Physical Laboratory). A Farad ay Partnersh ip is an allia nce which can inclu de researc h and technol ogy organisa tions, univers ities, profess ional institute s, trade asso ciatio ns and compa nies , whose aim is to impro ve the compe titiveness of UK Industry through the research, development, transfer and exploitation of new and improved science and technology. It involved a combination of UK universities, potential users and technology providers and aimed to exploit state-of-the-art opt oelectronic, communications and sensor tec hnology to develop a system for the real time measurement of strain and temperature in structures. Project partners City University Cranfield University UK Highways Agency Corus QinetiQ National Physical Laboratory (NPL) EM Technology BNFLThe Emerald Research Register for this journal is available at www.emeraldinsight.com/researchregister The current issue and full text archive of this journal is available at www.emeraldinsight.com/0260-2288.htm Sensor Review 25/2 (2005 ) 109–113 q Emerald Group Publishing Limited [ISSN 0260-2288] [DOI 10.1108/02602280510585682] 109
6
Embed
Structural Integrity Monitoring With Fibre Bragg Grating Sensors
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
8/4/2019 Structural Integrity Monitoring With Fibre Bragg Grating Sensors
AbstractPurpose – This paper describes a recent collaborative project involving the development of a multiplexed fibre Bragg grating (FBG) sensor system forstructural integrity monitoring.Design/methodology/approach – The system is described and field trials on both conventional and novel composite bridges are discussed. A FBGsensor-based structural monitoring system was developed, based on a fluorescent fibre as the optical source. It used a tuneable, fibre-coupled, Fabry-Perot filter, actuated by piezoelectric transducers and operated over the bandwidth of the source at up to 250 scans/second. Light from the source wasfiltered and reflected back from the Bragg gratings, through optical couplers, to eight photodiode detectors. These detected the resulting time-domainspectra of the sensors in each of the serially connected sensor arrays. The system was tested at City University and then subjected to trials on the
Mjosund road bridge in Norway and on West Mill bridge in Oxfordshire, UK, which is the first bridge to be fabricated from a new type of compositematerial.Findings – During the Norwegian trials the system was arranged with four or five FBG sensors per channel giving a total of 32 measurement pointswith eight parallel channels. Twelve conventional foil strain gauges and a number of thermocouples were also installed. Different static and dynamicloads were applied over a period of 18 months and the results showed that the thermally compensated strain data obtained optically matched thosefrom the resistive gauges to within,5 m 1 . During the construction stage of the Oxfordshire bridge, sections of the decking and longitudinal compositesupport beams were instrumented with 40 FBG sensors with temperature compensation, placed at pre-selected sites of maximum strain. Theseexhibited a resolution of ^5 m 1 and an operating range of over ^2,000m 1 .Originality/value – This research has shown that multiplexed, multi-point FBG sensor systems can accurately and reliably monitor both static anddynamic strains in large structures over a range of temperatures and for extended periods of time.
The system was based on multiplexed fibre Bragg grating
( FB G) sensors and needed to m eet certain critical
requirements, i.e.. multi-point sensing capability;. robust, for use in harsh environments, with effective
sensor protection;. repeatable, stable and accurate;. compensated for temperature effects;. ability to measure static and dynamic strain;. flexible, for periodic or continuous measurements;. remote instrument control and data management, via a
web interface and database system.
A FBG sensor is a section of an optical fibre with a Bragg
grating written into it, that is, a periodic perturbation of the
refractive index of the core of the fibre. The combined strain
and temperature sensing response of the grating is given by a
single combined Bragg wavelength shift which can be
represented by a linear relationship. This contains a numberof thermo-optic and strain-optic coefficients which, when
known for the fibre type used, yields a sensing technique that
is self-calibrating and which allows drift-free, long-term strain
and temperature measurements. Being independent of any
fluctuations in the power of the source, this type of sensor is
ideally suited to long-term monitoring applications. However,
the need to decouple the temperature and strain signals poses
something of a challenge and numerous different approaches
have been tried and described in the literature in recent years.
These include the use of two superimposed FBGs at two
different wavelengths from which the temperature and strain
components of the wavelength shift can be discriminated
simultaneously using two linear equations and using a fibre
Fabry-Perot cavity and an FBG grating in which the FP cavitysenses strain and the Bragg grating measures both the strain
and temperature. In this system a differential technique was
used, whereby an FBG sensor that is subject to the thermal
variations but positioned so as to be free from strain acted as a
compensation element.
A fluorescent fibre was used as the optical source with a
bandwidth of some 20-40 nm from 1,520 to 1,560 nm. The
tuneable filter is a fibre-coupled, free space, Fabry-Perot filter,
actuated by piezoelectric transducers and operated over the
bandwidth of the source at up to 250 scans per second. Light
from the source is filtered by the FP filter and reflected back
from the Bragg gratings in the array, through the optical
couplers, to eight photodiode detectors. These detect the
resulting time-domain spectra of the grating sensors in each of
the serially connected arrays. The overall measurement
scheme is shown in Figure 1 and comprises three sections.
The first is a PC-based computer system with graphical user
interface (GUI) software for real time data visualisation and a
fast serial interface connected to the digital sampling
processor (DSP) system. The synchronous DSP, in turn,
controls and takes data from the optoelectronic system which
is typically connected to the FBG sensor arrays in a
multiplexed sensor system with wavelength division
multiplexing (WDM) architecture.
Prior to field trials, prototype systems were tested
extensively at City University (Plate 1). Critical issues
addressed during this phase were testing the strain response
under both static and dynamic loadings, evaluating various
means of attaching the sensors to the test structures, sensor
protection and evaluating a range of thermal compensation
techniques. Plate 2 shows a strain-isolated temperature sensor
on a steel bridge box section.
A prototype system was tested on the Mjosund road bridge
in Norway (Plate 3). In addition to providing an opportunityto test the sensor system under harsh operating conditions,
the work aimed to assist the Norwegian Roads Authority in
monitoring the many bridges joining the island coastline of
Norway to the mainland. The bridge was a 346 m-long steel
box section structure with a concrete platform carrying the
road access to the bridge. The system was arranged with four
or five FBG sensors per channel giving a total of 32
measurement points with eight parallel channels. Twelve
c onve nt io nal f oi l s tr ai n g au ge s an d a n umb er o f
thermocouples were also installed. The sensor configuration
was such that for each foil gauge, there were two FBG sensors
placed at the same strain point for data verification. Different
static and dynamic loading conditions were applied at
different times over a test period of 18 months and the
results showed that the thermally compensated strain data
obtained optically closely matched the readings from the
resistive gauges to within ,5 m 1 (equivalent to the system
noise). Figure 2 shows the data from the FBG sensors and the
strain gauges when a 50T lorry was driven across the bridge at
30 km/h. The temperature variation on the location of this
bridge ranged from 2408C in the winter to þ258C in
summer, which demonstrated well both the importance and
effectiveness of the temperature compensation technique
used.
One of the most important tests for the system was
monitoring Europe’s first public highway bridge to be
constructed entirely from advanced composites. This is West
Mill bridge in Oxfordshire, UK, which was fabricated from a
new type of composite material of glass and carbon fibre-reinforced polymer. This is a very strong and light material,
which can replace reinforced concrete or steel bridge decks.
The bridge was developed under the four-year, £2.9 million
ASSET (Advanced Structural Systems for Tomorrow’s
Infrastructure) project and was part-funded by the EU and
seven European partners, led by Mouchel, the bridge’s
designers. It was constructed on four longitudinal polymer
beam supports with the composite transverse decking being
fabricated in large extruded profile sections and delivered to
the site for bonding. During the construction stage, prior to
bonding of the composite profiles sections, several sections of
the composite decking and longitudinal composite support
beams were instrumented with 40 FBG sensors, both as
single-axis and rosette gauges w ith tem perature
compensation, strategically placed at pre-selected sites of
maximum strain. These exhibited a strain resolution of ^5 m 1
and an operating range of .^2,000 m 1. As with the
Norwegian trials, a number of resistive strain gauges were
also attached for comparative measurements. The sensors and
the fibre cabling were protected with composite stripes from
transverse strain effects as well as construction site hazards
and silicon compound was applied for moisture protection.
The system was web-enabled so as to provide real time data
over the internet, as shown in Figure 3.
Prior to the bridge being opened to the public in late 2002,
madatary commissioning tests were carried out. These
Structural integrity monitoring with fibre Bragg grating sensors
Robert Bogue
Sensor Review
Volume 25 · Number 2 · 2005 · 109–113
110
8/4/2019 Structural Integrity Monitoring With Fibre Bragg Grating Sensors
Figure 2 Dynamic loading data when a lorry was driven across the Mjosund bridge at 30 km/h (Top trace: FBG sensor data; Lower trace: foil straingauge data)
Figure 3 Schematic of the web-based instrument control and data acquisition system
Structural integrity monitoring with fibre Bragg grating sensors
Robert Bogue
Sensor Review
Volume 25 · Number 2 · 2005 · 109–113
112
8/4/2019 Structural Integrity Monitoring With Fibre Bragg Grating Sensors