Il-Bum Kwon

Novel auto-correction method in a fiber-optic distributed-temperature sensor using reflected anti-Stokes Raman scattering.

A novel method for auto-correction of fiber optic distributed temperature sensor using anti-Stokes Raman back-scattering and its reflected signal is presented. This method processes two parts of measured signal. One part is the normal back scattered anti-Stokes signal and the other part is the reflected signal which eliminate not only the effect of local losses due to the micro-bending or damages on fiber but also the differential attenuation. Because the beams of the same wavelength are used to cancel out the local variance in transmission medium there is no differential attenuation inherently. The auto correction concept was verified by the bending experiment on different bending points.

Monitoring of fatigue damage of composite structures by using embedded intensity-based optical fiber sensors

In this paper, the feasibility of real-time monitoring of the fatigue damage of composite laminates under fatigue loading using the embedded intensity-based optical fiber (IBOF) sensors was evaluated. The IBOF sensor system used in this study has a relatively simple sensing principle. Thus the system can be composed of inexpensive components compared with other optical fiber sensors such as Fabry-Perot and Bragg grating sensors. The performance of the fabricated IBOF sensors was evaluated by comparison with a surface-mounted extensometer. The process of conversion of the detected IBOF sensor signal into the stiffness was proposed. Experimental results showed that the sensing response of embedded IBOF sensors showed good agreement with that of the surface-mounted extensometer. The IBOF sensor system showed good potential as a health monitoring system which can give composite structures the capability of fatigue damage monitoring by detecting the stiffness change of the composite structures under fatigue loading. The IBOF sensors showed relatively good durability under fatigue loading compared with commercial electrical strain gages.

A study on the development of transmission-type extrinsic Fabry-Perot interferometric optical fiber sensor

The conventional reflection-type extrinsic Fabry-Perot interferometric (EFPI) optical fiber sensor has good sensitivity and resolution compared with other types of optical fiber sensors. However, they have the disadvantage that the distinction of strain direction of EFPI is difficult because of measurement method by only fringe counting. This paper presents the newly developed transmission-type EFPI (TEFPI) optical fiber sensor, which has been improved by additional functions, and whose measuring system differs from that of the reflection-type EFPI optical fiber sensors using a single-mode fiber (SMF) and multimode (MMF) fibers as light guides and reflectors, respectively. The output signal of the TEFPI optical fiber sensor was analyzed with the uniform plane wave-based model, the SMF power distribution-based model and the splice loss-based model; the analyzed signals were then verified experimentally. Based on the results of analysis, the TEFPI optical fiber sensor was fabricated using two single-mode fibers connected to the light source and optical receiver; this was then used in strain measurement. The strain measured by the TEFPI optical fiber sensor was compared with that measured by the electric strain gauge.

Deflection estimation of a wind turbine blade using FBG sensors embedded in the blade bonding line

Estimating the deflection of flexible composite wind turbine blades is very important to prevent the blades from hitting the tower. Several researchers have used fiber Bragg grating (FBG) sensors—a type of optical fiber sensor (OFS)—to monitor the structural behavior of the blades. They can be installed on the surface and/or embedded in the interior of composites. However, the typical installation positions of OFSs present several problems, including delamination of sensing probes and a higher risk of fiber breakage during installation. In this study, we proposed using the bonding line between the shear web and spar cap as a new installation position of embedded OFSs for estimating the deflection of the blades. Laboratory coupon tests were undertaken preliminarily to confirm the strain measuring capability of embedded FBG sensors in adhesive layers, and the obtained values were verified by comparison with results obtained by electrical strain gauges and finite element analysis. We performed static loading tests on a 100 kW composite wind turbine blade to evaluate its deflections using embedded FBG sensors positioned in the bonding line. The deflections were estimated by classical beam theory considering a rigid body rotation near the tip of the blade. The evaluated tip deflections closely matched those measured by a linear variable differential transformer. Therefore, we verified the capability of embedded FBG sensors for evaluating the deflections of wind turbine blades. In addition, we confirmed that the bonding line between the shear web and spar cap is a practical location to embed the FBG sensors. (Some figures may appear in colour only in the online journal)

Development of fiber optic BOTDA sensor for intrusion detection

We present a compact fiber optic Brillouin optical time domain analysis (BOTDA) sensor system, which has the capability of detecting and locating intrusion attempts over several tens of kilometers long paths. The system employs a laser diode and two electro-optic modulators. Simulation of an intrusion effect was achieved by use of a strain-inducing setup. Distance resolution of 3 m was obtained for a 4.81 km long optical fiber within 1.5 s. Actual intrusion detection experiment was also performed using a step-on stage setup and clearly discernable detection signals were obtained in less than 1.5 s. # 2002 Published by Elsevier Science B.V.

Strain event detection using a double-pulse technique of a Brillouin scattering-based distributed optical fiber sensor

Stimulated Brillouin scattering in optical fibers can be used to measure strain or temperature in a distributed manner. Brillouin optical time domain analysis (BOTDA) is the most common sensor system based on the Brillouin scattering. To improve the spatial resolution of these measurements, shorter pulses must be used, resulting in reduced signal powers causing a decrease of the dynamic range. In this paper, a doublepulse technique was proposed to enhance the spatial resolution of BOTDA. Experimental results showed that the ability to resolve two adjacent events could be enhanced, about twice, by using a double-pulsed pump light without decreases in the dynamic range.

Structural monitoring of a bending beam using Brillouin distributed optical fiber sensors

Due to the large measurement range, distributed optical fiber sensors are very effective in the monitoring of large structures. Brillouin distributed optical fiber sensors, which measure strain by using Brillouin frequency shift, show some measurement error for non-uniformly distributed strain because of their spatial resolution characteristics. They require compensation of measured values and positions in the end-parts of the measurement range because measured values are not distributed over all the positions of the spatial resolution range. In this paper, we present the strain measurement characteristics of Brillouin distributed optical fiber sensors and the compensation method in the end-parts of the measurement range. A deflection measurement method for a beam subjected to a bending load using Brillouin distributed optical fiber sensors is also presented. This is verified by a three-point bending experiment employing an 8 m aluminum beam. The experimental results showed that the measured value agrees with the one expected by Brillouin measurement theory.

A tip deflection calculation method for a wind turbine blade using temperature compensated FBG sensors

The tip deflections of wind turbine blades should be monitored continuously to prevent catastrophic failures of wind turbine power plants caused by blades hitting the tower. In this paper, a calculation method for wind turbine blade tip deflection is proposed using a finite difference method based on arbitrary beam bending and moment theory using measured strains. The blade strains were measured using fiber optic Bragg grating sensors. In order to confirm this method, a 100 kW composite wind turbine blade was manufactured with epoxy molded fiber optic Bragg grating (FBG) sensors installed in the shear web of the blade. A number of these sensors, normal FBG probes, were fabricated to only measure strains and the other sensors, temperature compensated FBG probes, were prepared to also measure strain and temperature. Because the output signals of FBG sensors are dependent on strains as well as temperatures, the sensor output signals should be compensated by the temperatures to obtain accurate strains. These FBG sensors were attached on the lower and upper parts of the web at one meter intervals throughout the entire length of the blade. To evaluate the measurement accuracy of the FBG sensors, conventional electrical strain gauges were also bonded onto the surface of the web beside each FBG sensor. By performing a static load test of the blade, the calculated tip deflection of the blade was well determined within an average error of 2.25%. (Some figures may appear in colour only in the online journal)

Structural Performance Tests of Down Scaled Composite Wind Turbine Blade using Embedded Fiber Bragg Grating Sensors

In this study, the structural performance tests, i.e., static tests and dynamic tests of the composite wind turbine blade, were carried out by using the embedded fiber Bragg grating (FBG) sensors. The composite wind turbine blade used in the test is the 1/23 scale of the 750 kW composite blade. In static tests, the deflections along the blade were evaluated. Evaluations were carried out with simple beam theory and quadratic fitting method by using the embedded FBG sensors to predict the structural behavior with respect to the load. The deflections were compared to those obtained from the laser displacement sensor and electric strain gauges. They showed good agreement. Modal tests were performed to investigate the dynamic characteristics using the embedded FBG sensors. The natural frequencies obtained from the FBG sensors corresponding to the nine mode shapes of the blade were compared to those from the laser Doppler vibrometer. They were found to be consistent with each other. Therefore, it is concluded that the embedded FBG sensors have a great capability for measuring the structural performances of the composite wind turbine blade when structural performance tests are carried out.

Sensitivity enhancement of fiber optic FBG sensor for acoustic emission

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.