KEYWORDS: Signal to noise ratio, Spatial resolution, Fiber Bragg gratings, Data acquisition, Signal processing, Composites, Reflectometry, Signal detection, Optical fibers, Structural health monitoring
We demonstrate a distributed localization of an impact source in the composite cylinder using a home-made phase sensitive optical time domain reflectometer (Φ-OTDR). The system shows the advantages of simple configuration using semiconductor optical amplifier to improve the spectral response of the pump pulses, high spatial resolution and high signal-to-noise ratio (SNR). With a spatial resolution of 10 m, the SNR could be obtained as high as 6.5 dB. A composite cylinder is longitudinally divided in multiple sections of equal lengths by bending the sensing fibers around it at different locations, and the section of a source impact is localized using the time-delay of the signal detected by the sensing fibers.
KEYWORDS: Acoustics, Single mode fibers, Signal processing, Signal to noise ratio, Data acquisition, Signal detection, Reflectometry, Semiconductor lasers, Semiconductor optical amplifiers, Fiber Bragg gratings
We analyze the relations of parameters in moving average method to enhance the event detectability of phase sensitive optical time domain reflectometer (OTDR). If the external events have unique frequency of vibration, then the control parameters of moving average method should be optimized in order to detect these events efficiently. A phase sensitive OTDR was implemented by a pulsed light source, which is composed of a laser diode, a semiconductor optical amplifier, an erbium-doped fiber amplifier, a fiber Bragg grating filter, and a light receiving part, which has a photo-detector and high speed data acquisition system. The moving average method is operated with the control parameters: total number of raw traces, M, number of averaged traces, N, and step size of moving, n. The raw traces are obtained by the phase sensitive OTDR with sound signals generated by a speaker. Using these trace data, the relation of the control parameters is analyzed. In the result, if the event signal has one frequency, then the optimal values of N, n are existed to detect the event efficiently.
A time-division Brillouin optical correlation domain analysis system was successfully achieved using simplified laser diode (LD) modulation and pump lightwave optimization. A complicated transfer function for a precise output waveform of a LD was required for the conventional system. However, a very simple modulation function gave a power output very close to a required ideal rectangle waveform without sacrificing optical output spectrum. An electrical input waveform applied into a gate in the pump lightwave path was also optimized for eliminating a probe lightwave included in a pump lightwave and for passing consecutive pump pulses alternatively. So the stimulated Brillouin scattering gain was attained without seriously distorting FM modulation, and the targeted spatial resolution was clearly accomplished. Additionally, using high speed response of a semiconductor optical amplifier (SOA), unlike an erbium-doped fiber amplifier (EDFA), the possibility was investigated that an SOA was going to replace an EDFA and a modulator used as a gate in the same time.
A fiber optic quasi-distributed sensing technique was suggested to use for detecting the location and
severity of water leakage. A novel fiber optic sensor probe was devised with a vessel of water absorption
material called as water combination soil (WCS) located between two highly reflected connectors. The
largest vessel probe provides the highest sensitivity, 0.267 dB/ml, while the small one shows relatively
low sensitivity, 0.067 dB/ml, and unstable response. The sensor probe with a high output value provides a
high sensitivity with various detection levels while the number of total installable sensor probes
decreases.
Fiber optic sensors can be used to measure some physical quantities through long optical fibers applied on structures.
Three types of fiber optic sensors, fiber Bragg grating sensor, Brillouin OTDR(optical time domain reflectometry) and
Raman OTDR(optical time domain reflectometry), are mostly useful to sense structural health status. In my works,
multiplexing of FBG sensors, Raman OTDR and fiber optic BOTDA(Brillouin Optical Time Domain Analysis) sensor
will be introduced in this presentation. Novel auto correction Raman OTDR can only sense temperature without any
influence, such as bending losses. Double pulse technique is introduced to enhance spatial resolution of BOTDA. An
error compensation of distributed strain measurement is also developed to get precise strain measurement using the
BOTDA sensor. Finally, the applications of fiber optic sensors are shown in several fields including aerospace, civil
structures and wind turbine blades in South Korea.
Structural health monitoring (SHM) techniques based on guided waves have been of great interests to many researchers.
Among various SHM devices used for guided wave generation and sensing, lead zirconate titanate (PZT) transducers and
fiber Bragg grating (FBG) sensors have been widely used because of their light weight, non-intrusive nature and
compactness. To best take advantage of their merits, combination of PZT-based guided wave excitation and FBG-based
sensing has been attempted by a few researchers. However, the PZT-based actuation and the FBG-based sensing are
basically two independent systems in the past studies. This study proposes an integrated PZT/FBG system using a single
laser source. Since power and data delivery is based on optical fibers, it may alleviate problems associated with
conventional wire cables such as electromagnetic interference (EMI) and power/data attenuation. The experimental
procedure for the proposed system is as follows. First, a tunable laser is used as the common power source for guided
wave generation and sensing. The tunable laser beam is modulated and amplified to contain an arbitrary waveform. Then,
it is transmitted to the PZT transducer node through an optical fiber for guided wave actuation. The transmitted laser
beam is also used with the FBG sensor to measure high-speed strain changes induced by guided waves. Feasibility of the
proposed technique has been experimentally demonstrated using aluminum plates. The results show that the proposed
system could properly generate and sense the guided waves compared to the conventional methods.
The oil leakage of off-shore pipelines will cause ocean contamination and economic losses. These accidents may happen
by the failures of offshore pipelines due to corrosion, impulse and free-spanning. So, it is very urgent on pipeline health
monitoring. Fiber optic distributed sensors should be used to know when and where failures may occur. In this study, a
feasibility of BOTDA (Brillouin Optical Time Domain Analysis) system is studied on off-shore pipeline distributed
strain monitoring influenced by free spanning. Strain distribution of an off-shore pipeline is calculated by numerical
analysis and strain measurement experiments are carried on a beam bending test using BOTDA system. BOTDA could
be an excellent tool to monitor the long-distance pipeline.
In recent years, nondestructive testing (NDT) has gained popularity for structural health monitoring and damage
detection applications. Among the NDT methods, guided wave based NDT techniques have attracted the attention of
many researchers due to their relatively long sensing range. These guided waves can be generated in a structure and
sensed by a variety of techniques. The present study proposes a new scheme for PZT excitation and sensing based on
laser and optoelectronic technologies, where power as well as data can be transmitted via laser. This paper mainly
focuses on the excitation aspect. An arbitrary waveform is generated using a light source and transmitted to the PZT. A
photodiode connected to the PZT then converts the light into an electrical signal and excites the PZT. The technique can
be configured either for wired or wireless PZT excitations. Finally, the feasibility of the proposed power transmission
scheme has been experimentally demonstrated in a laboratory setup.
A fiber optic Bragg grating based acoustic emission sensor system is developed to provide on-line monitoring of cracks
or leaks in reactor vessel head penetration of nuclear power plants. Various type of fiber Bragg grating sensor including
the variable length of sensing part was fabricated and prototype sensor system was tested by using PZT pulser and pencil
lead break sources. In this study, we developed a cantilever type fiber sensor to enhance the sensitivity and to resonant
frequency control. Two types of sensor attachment were used. First, the fiber Bragg grating sensor was fully bonded to
the surface using bonding agent. Second one is that one part of fiber was partially bonded to surface and the other part of
fiber will be remained freely. The resonant frequency of the fiber Bragg grating sensor will depend on the length of
sensing part. Various kinds of resonant type fiber Bragg grating acoustic emission sensors were developed. Also several
efforts were done to enhance the sensitivity of FBG AE sensor, which include FBG spectrum optimization and electrical
and optical noise reduction. Finally, based on the self-developed acquisition system, a series of tests demonstrate the
ability of the developed fiber sensor system to detect a pencil lead break event and continuous leak signal.
There are to be some cracks on the material degradation part or the stress concentration parts of the main members,
which carry on over-loads, of structures. Because these cracks can be used to evaluate the structural health status, it is
important to monitor the crack growth for maintaining the structural safety. In this study, the fiber Bragg grating sensor
with a drop ball was developed as a sensor for crack growth detection of an existing crack. The crack growth detection
sensor was constructed with three parts: a probe part, a wavelength control light source and receiver part, and an impact
part. The probe part was just formed with a fiber Bragg grating optical fiber. The wavelength control light source part
was composed of a current supplying circuit, a DFB laser diode, and a TEC control circuit for wavelength control. Also,
the impact part was just implemented by dropping a steel ball. The performance of this sensor was confirmed by the
experiments of the crack detection with an aluminum plate having one existing crack. According to these experiments,
the difference of the sensor signal outputs was correlated with the crack length. So, it was confirmed that this sensor
could be applied to monitor the crack growth.
Optical fiber mesh can be applied on some fences to secure some protective facilities. It is necessary to give the fiber
macro bending signal in order to detect an intruder at the initial intrusion stage with the off signal from fiber cutting.
Therefore, in this work, we made two step signal processing algorithm of the optical fiber mesh for detecting the intruder
effectively. First step is composed of the discrimination processing whether the fiber has some macro bending or not. If
the signal tells the macro bending, then it means that some intruder tries to invade. Second step is the signal destruction
by cutting the optical fiber. This step means that the intruder invaded by cutting the optical fiber mesh.
Continuous monitoring of soil movements is important to maintain civil structures. Fiber optic sensors can give the sensing solution for continuous displacement monitoring because they can be multiplexed with various sensors on one line of optical fiber, have the durability to harsh environments, and also are not affected by electro-magnetic effects. In this study, we work on the development of a multiplexed optical loss based fiber optic sensor for measuring displacements using the signal difference between the two reflected signals from a pair of optical connectors with various bending losses between them through an optical time domain reflectometer. We fabricate a multiplexed optical loss based fiber-optic sensor detecting linear displacements of 5 measuring positions of an object by setting these new 5 fiber-optic sensor probes on a single mode fiber simultaneously. We find that these sensors have a good capability to measure the displacements.
In order to do continuous health monitoring of large structures, it is necessary that the distributed sensing of strain and temperature of the structures are to be measured. So, we present the strain and temperature measurement distributed on a beam using fiber optic BOTDA(Brillouin Optical Time Domain Analysis) sensor. Fiber optic BOTDA sensor has good performance of strain measurement. However, the signal of fiber optic BOTDA sensor is influenced by strain and temperature. Therefore, we applied an optical fiber on the beam as follows: one part of the fiber, which is sensitive the strain and the temperature, is bonded on the surface of the beam and another part of the fiber, which is only sensitive to the temperature, is located at the same position of the strain sensing fiber. Therefore, the strains can be determined from the strain sensing fiber with compensating the temperature from the temperature sensing fiber. These measured strains were compared with the strains from electrical strain gages. After temperature compensation, it was concluded that the strains from fiber optic BOTDA sensor had good agreements with those values of the conventional strain gages.
We have focused on the development of a fiber optic BOTDA (Brillouin Optical Time Domain Analysis) sensor system in order to measure temperature distributed on large structures. Also, we present a feasibility study of the fiber optic sensor to monitor the distributed temperature on a building construction. A fiber optic BOTDA sensor system, which has a capability of measuring the temperature distribution, attempted over several kilometers of long fiber paths. This simple fiber optic sensor system employs a laser diode and two electro-optic modulators. The optical fiber of the length of 1400 m was installed on the surfaces of the building. The change of the distributed temperature on the building construction was well measured by this fiber optic sensor. The temperature changed normally up to 4 degrees C through one day.
Distributed optical fiber sensors are effective in the monitoring of large structures because of their large sensing range. In this research, the methods of strain and deflection measurement for bending beams using Brillouin distributed optical fiber sensors were presented. The deflection monitoring of beams makes possible the damage detection of large structures such as bridges. The measured strain with Brillouin distributed optical fiber sensors is expressed as the averaged value within its spatial resolution. Due to the characteristic of the spatial resolution, strain near the ends of a measurement range cannot be correctly measured with respect to the measuring position. Therefore, its compensation is required. The strain measurement method suitable to the sensor characteristic was proposed, and it was verified by the analysis of aluminum-tube beams. The beam deflection could be monitored using the distributed measurement more simply and effectively than using general point-measurement.
Comparing with general optical fiber sensors performing localized measurement, the distributed optical fiber sensors measure along the length of an optical fiber and thus they have a large measuring range. This implies that the distributed optical fiber sensors are very effective in the health monitoring of large structures. The Brillouin scattering is one of the inelastic scatterings in optical fibers, and it can be used in the measurement of strain and temperature with the Brillouin frequency shift. The surface- mounting with epoxy is generally used in the application of optical fiber sensors to monitoring structures. This is also used in the integrity monitoring of structures with the Brillouin-scattering distributed optical fiber sensors. In this research, the Brillouin-scattering distributed optical fiber sensors, which are mounted on the surface of a structure with epoxy, was evaluated with the finite element method. From the analysis results of the strain transfer through the structure, epoxy adhesive, optical fiber coating, cladding and core, the strain transfer rate was calculated. By using the strain distribution, the influence of the epoxy free-end was also studied.
Optical frequency modulated fiber optic interferometric sensor is developed to sense the mechanical quantities, such as displacement, strain, force etc. It has been difficult to distinguish the increase and decrease of the mechanical quantities measured by the conventional fiber optic interferometric sensors because their signals only have a sinusoidal wave pattern related to the change of the quantities. In this study, in order to measure the mechanical quantities with the distinction of the changing direction of the quantities, the fiber optic Michelson interferometric sensor is simply constructed by the laser light modulated with saw tooth wave pattern.
Fiber optic TR-EFPI (total reflected extrinsic Fabry-Perot interferometric) sensor is developed to measure the strain of structures, such as building, bridge, aircraft, etc. It has been difficult to distinguish the increase and decrease of the strain from the conventional fiber optic EFPI sensor because their signals only have a sinusoidal wave pattern related to the change of strain. In this study, in order to measure the magnitude and the direction of strain, the fiber optic sensor is simply constructed with the total reflected EFPI sensor probe and the digital signal processing.
The newly developed TEFPI (transmission-type extrinsic Fabry- Perot interferometric) optical fiber sensor can distinguish the direction of measurement more simply and effectively than the conventional reflection-type EFPI optical fiber sensors. The output signal of the TEFPI optical fiber sensor has the characteristics that the signal level of fringes shows a negative slope for a tensile direction and a positive slope for a compressive direction. Based on these characteristics, the direction of measurement of the TEFPI optical fiber sensor can be distinguished with ease. In this paper, the signal processing algorithm adequate to the TEFPI optical fiber sensor was developed. This algorithm can process signal with recognition of the positions of peaks, valleys and signal levels of fringes. Thus this can determine a measurement direction and the positions of direction changes by using the change trend of signal levels. The developed algorithm makes the post-process and real-time process of the signal of the TEFPI optical fiber sensor possible.
The damage detection system of a real steel truss bridge was developed to identify the location and severity of the damaged members. At first, the loading test was performed to characterize the real bridge. The real steel truss bridge was measured by electrical strain gages and accelerometers when the train passed. The measured strains and acceleration were used to refine the stiffness and the mass of the finite element model. The damage scenario, that can be happened in the real situation, was simulated by the refined finite element model. The damage localization was implemented to classify the damaged part in the bridge by the neural networks. The neural network was constructed as two steps: at 1st step, the half-span, which had some damages occurred, was found, and at 2nd step, the severest abnormal part in the total 8 parts of the real bridge was detected. The learned neural network was verified by the used data.
The need to monitor and undertake remedial works on large structures has greatly increased in recent years due to the appearance of widespread faults in large structures such as bridges and buildings, etc, of 20 or more years of age. The health condition of structures must be monitored continuously to maintenance the structures. In order to do in-situ monitoring, the sensor is necessary to be embedded in the structures. Fiber optic sensors can be embedded in the structures to get the health information in the structures. The fiber sensor was constructed with 3 X 3 fiber couplers to sense the multi-point strains and failure instants. The 4 RC (reinforced concrete) beams were made to 2 of A type, 2 of B type beams. These beams were reinforced by the reinforcing bars, and were tested under the flexural loading. The behavior of the beams was simultaneously measured by the fiber optic sensors, electrical strain gages, and LVDT. The states of the beams were interpreted by these all signals. By these experiments, there were verified that the fiber optic sensors could measure the structural strains and failure instants of the RC beams. The fiber sensors were well operated until the failure of the beams. It was shown that the strains of the reinforcing steel bar can be used to monitor the health condition of the beams through the flexural test of RC beams. On the other words, the results were arrived that the two strains in the reinforcing bar measured at the same point can give the information of the structural health status. Also, the failure instants of beams were well detected from the fiber optic filtered signals.
A digital signal processing algorithm, Kwon's method, was developed for strain measurement by a passive quadrature demodulated fiber optic Michelson interferometric sensor. The fiber optic Michelson sensor, which is constructed of a 3 by 3 fiber optic coupler, can give the information about the magnitude and direction of the strain of structures. The beating, drifting, and noise, which are caused by the longitudinal strain and the lateral strain of the fiber, bring about the counting error of the phase differences. Kwon's algorithm is based on the reference line crossing count method and resets the reference line during the presence of the signal drifting. The accuracy of the strain calculation was confirmed by the various simulated fiber optic signals with signal beating, drifting and noise. A passive quadrature demodulated 3 by 3 fiber optic Michelson interferometric sensor was bonded on the cantilevered aluminum beam to experiment the strain sensing. The capability of the real-time processing was verified by the real fiber optic signals.
The quadrature phase-shifted 3x3 fiber optic Michelson sensors were tried to monitor the health status of a steel truss bridge which was scaled down to 1/15 of the real bridge for the laboratory experiments. The fiber optic sensors and electrical strain gages were bonded on the surface of some frames to sense the strain pattern induced by the abnormal structural behavior. The fiber optic signals were immediately processed by personal computer for the strain determination. In order to confirm the strain sensitivity of the fiber optic sensors, these fiber optic strains were compared with the strains of the strain gages. The static behavior of the bridge was analyzed by finite element analysis with SAP2000. These finite element analysis results were compared with the structural strain pattern obtained by the electrical strain gages and were arranged with the database for the determination of the bridge health condition. It was shown that the breakage of some frames could be detected from the changes in strain pattern.
The fiber optic extrinsic Fabry-Perot sensor was embedded in composite beam to sense the strain and failure of composite structures. A tensile test was performed to confirm the strain sensitivity of the fiber optic sensor embedded in composite specimens. The strain sensitivity of the extrinsic Fabry-Perot sensor showed very good agreement with the theoretical value. The bending deformation and matrix cracking were investigated through four-point bending tests of cross-ply composite beams with embedded fiber optic extrinsic Fabry-Perot sensor. The failure due to matrix cracks in the composite beam was confirmed by an edge replica method. The strain and failure signals were separated by digital filtering from the signal of fiber optic sensor. The failure instants were obviously noticeable from the failure signal obtained from the fiber optic signal by high pass filtering. The dominant failure strain of the composite beam was determined by strain signal obtained by low pass filtering.
The bending deformation and matrix cracking were investigated by conducting a four-point bending test for a cross-ply composite beam with an embedded fiber optic Michelson sensor. The fiber optic Michelson interferometric sensor was constructed and embedded in the composite beam. The failure of composite beam, due to the matrix cracking, was successfully detected by the fiber optic sensor and the matrix crack in the composite beam was confirmed by an edge replica method. The characteristics of the failure signals from the fiber optic sensor were studied. The strain and failure signals of the composite beam were separated by digital filtering of the signal from the fiber optic sensor. The failure was obviously detectable by the failure signal filtered from the optical signal.
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