• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.32 no.5
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• pp.50-57
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• 2004

For the application of structural health monitoring such as AE detection, a stabilized FBG sensor system with wide dynamic range was proposed. A tunable Fabry-Perot filter with narrow free spectral range(FSR) was used to simplify the multiplexing demodulator for FBG vibration sensors. Stabilization controlling system was also developed for the maintenance of maximum sensitivity of the sensors. In order to verify the performance of the proposed FBG vibration sensor system, we measured sensitivity, and the system showed the average sensitivity of 256 $n{\in}_{mas}/{\sqrt{Hz}}$. Finally, multi-points vibration tests using in-line FBG sensors were conducted to validate the multiplexing performance of the FBG system.

• Smart Structures and Systems
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• v.7 no.4
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• pp.303-317
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• 2011

A specially designed tendon, which is proposed by embedding an FBG sensor into the center king cable of a 7-wire strand tendon, was applied to monitor the prestress force and load transfer of ground anchor. A series of tensile tests and a model pullout test were performed to verify the feasibility of the proposed smart tendon as a measuring sensor of tension force and load transfer along the tendon. The smart tendon has proven to be very effective for monitoring prestress force and load transfer by measuring the strain change of the tendon at the free part and the fixed part of ground anchor, respectively. Two 11.5 m long proto-type ground anchors were made simply by replacing a tendon with the proposed smart tendon and prestress forces of each anchor were monitored during the loading-unloading step using both FBG sensor embedded in the smart tendon and the conventional load cell. By comparing the prestress forces measured by the smart tendon and load cell, it was found that the prestress force monitored from the FBG sensor located at the free part is comparable to that measured from the conventional load cell. Furthermore, the load transfer of prestressing force at the tendon-grout interface was clearly measured from the FBGs distributed along the fixed part. From these pullout tests, the proposed smart tendon is not only expected to be an alternative monitoring tool for measuring prestress force from the introducing stage to the long-term period for health monitoring of the ground anchor but also can be used to improve design practice through determining the economic fixed length by practically measuring the load transfer depth.

• Korean Journal of Optics and Photonics
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• v.26 no.2
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• pp.83-87
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• 2015

This study was focused on developing an optical sensor that monitors ultraviolet (UV) light. Recently, we proposed and demonstrated a novel, highly sensitive UV sensor based on a fiber Bragg grating (FBG). To ensure that the incident UV light is focused on the FBG surface, the sensor was coated with an azobenzene polymer material that acts as a UV-induced stretchable functional material, in combination with a cylindrical focal lens. In this study we have improved the sensitivity of the sensor by employing a cylindrical focal mirror as a curved reflector, to refocus the UV light passing through the FBG. We considered the performance of several different types of reflectors and chose the optimal radius of curvature for the reflector. Compared to the UV sensor without an auxiliary device, the sensitivity of the FBG sensor with a focal lens and a curved reflector was 15 times as high.

• Journal of the Society of Disaster Information
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• v.18 no.4
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• pp.847-860
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• 2022

Purpose: The purpose of this paper is to present a basic study on the development of a self-generation infrastructure for monitoring the health of harbour structures. Method: By developing a self-generation system and fiber optic sensors for seawater, the study provides basic research data on port structure health monitoring. Result: Through sunlight simulation analysis, 4-5 hours of sunlight can be secure in the domestic environment. Through this, the optical splitter (Introgate) that collects the raw data from the FBG sensor applicable to seawater, the MCU that calculates it, the IoT module with wireless communication functionality, the monitoring server and the supply system are set up. Conclusion: Monitoring port structures directly with fiber optic probes (FBG) and the possibility of using selfpowered systems were confirmed.

• Journal of the Korean Geotechnical Society
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• v.28 no.5
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• pp.13-25
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• 2012

Ground anchor method is one of the most popular reinforcing technology for slope in Korea. For the health monitoring of slope which is reinforced by permanent anchor for a long period, monitoring of the tension force of ground anchor is very important. However, since electromechanical sensors such as strain gauge and V/W type load cell are also subject to long-term risk as well as suffering from noise during long distance transmission and immunity to electromagnetic interference (EMI), optical FBG sensors embedded tendon was developed to measure strain of 7-wire strand by embedding FBG sensor into the center king cable of 7-wire strand. This FBG sensors embedded tendon has been successfully applied to measuring the short-term anchor force. But to adopt this tendon to long-term monitoring, temperature compensation of the FBG sensors embedded tendon should be done. In this paper, we described how to compensate the effect in compliance with the change of underground temperature during long-term tension force monitoring of ground anchors by using optical fiber sensors (FBG: Fiber Bragg Grating). The model test was carried out to determine the temperature sensitivity coefficient (${\beta}^{\prime}$) of FBG sensors embedded tendon. The determined temperature sensitivity coefficient ${\beta}^{\prime}=2.0{\times}10^{-5}/^{\circ}C$ was verified by comparing the ground temperatures predicted from the proposed sensor using ${\beta}^{\prime}$ with ground temperatures measured from ground thermometer. Finally, temperature compensations were carried out based on ${\beta}^{\prime}$ value and ground temperature measurement from KMA for the tension force monitoring results of tension type and compression type anchors, which had been installed more than 1 year before at the test site. Temperature compensated tension forces are compared with those measured from conventional load cell during the same measuring time. Test results show that determined temperature sensitivity coefficient (${\beta}^{\prime}$) of FBG sensors embedded tendon is valid and proposed temperature compensation method is also appropriate from the fact that the temperature compensated tension forces are not dependent on the change of ground temperature and are consistent with the tension forces measured from the conventional load cell.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.26 no.2
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• pp.44-50
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• 2012

When the fiber Bragg gratings are embedded in a fiber-optic Sagnac loop for measuring temperature or strain, it is difficult to separate the Bragg wavelengths. The transmitted light is mixed with the reflected Bragg wavelengths in the photo-detector, working as noises. To suppress the noises, we placed the FBG sensors and a fiber-optic attenuator at asymmetric positions in the loop. With the arrangement the reflected light became much bigger than the transmitted light, enabling the separation of the reflected Bragg wavelengths with almost the same signal-to-noise ratio of the FBG sensors outside the loop.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2008.04a
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• pp.133-138
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• 2008

This study proposes a novel method for in-service evaluation of force in an external prestressing 7-wire tendon which is employed for retrofitting bridge superstructure. For this propose, a smart strand 7.0m long whose king wire is replaced by a steel tube and the FBG sensor, is developed. Performance of the strand is demonstrated through loading-unloading tests for a RC T-shaped beam 6.4m long. Finally, a couple of test results are presented to discuss effect of temperature change in the FBG sensor.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2000.04a
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• pp.163-166
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• 2000

Fiber Bra99 grating (FBG) sensor, one of the fiber optic sensor (FOS) offers lots of advantages for structural health monitoring due to its multiplexing capability. Also, it is proper to measure the structural vibration with no mass concentration effect. In this paper, we constructed two sensor arrays composed of 9 FBG sensors for the vibration and mode sensing of a composites beam. For an accurate measurement of wavelength shift, a signal processing board with an electric circuit based on time-interval counting was developed. This sensor system showed a good resolution of dynamic strain (<10${\mu}{\varepsilon}$). Using this sensor system, dynamic strains at 9 points of composite beam was measured and strain measured mode shape of the beam was calculated from the acquired strains and compared with numerical results by ABAQUS.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2002.05a
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• pp.159-162
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• 2002

We introduce a simple optically passive detection scheme for Bragg grating sensors. This detection scheme is based on two cavity lengths in Fabry-Perot read-out interferometers to produce two quadrature phase shifted signals from the Bragg grating sensor. The passive detection technique is demonstrated by the use of Bragg grating sensors in measuring the dynamic vibrations of the composites.

• Proceedings of the Korea Concrete Institute Conference
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• 2001.05a
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• pp.423-428
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• 2001

Fiber Bragg Grating sensors currently attract a great deal of attentions, mainly due to their potentials in health monitoring for civil structures and composite materials. In this experimental study, the strains of reinforced concrete beams were measured to failure In order to verify the applicability of FBG(Fiber Bragg Grating) sensors. The FBG sensors were directly buried in concrete and attached to re-bars at the time of fabrication. In this experiment, the changes of strains in concrete and re-bars were successfully measured as the movement in wavelength of light signals. The FBG sensors may be a very effective tool to investigate the behavior inside of reinforced concrete structures.