• Smart Structures and Systems
• /
• v.19 no.3
• /
• pp.269-277
• /
• 2017

Recently, there has been significant interest in structural health monitoring for civil engineering applications. In this research, a specially designed tendon, proposed by embedding FBG sensors into the center king cable of a 7-wire strand tendon, was applied for long-term health monitoring of tensile forces on a ground anchor. To make temperature independent sensors, the effective temperature compensation of FBG sensors must be considered. The temperature sensitivity coefficient ${\beta}^{\prime}$ of the FBG sensors embedded tendon was successfully determined to be $2.0{\times}10^{-5}^{\circ}C^{-1}$ through calibrated tests in both a model rock body and a laboratory heat chamber. Furthermore, the obtained result for ${\beta}^{\prime}$ was formally verified through the ground temperature measurement test, expectedly. As a result, the ground temperature measured by a thermometer showed good agreement compared to that measured by the proposed FBG sensor, which was calibrated considering to the temperature sensitivity coefficient ${\beta}^{\prime}$. Finally, four prototype ground anchors including two tension ground anchors and two compression ground anchors made by replacing a tendon with the proposed smart tendon were installed into an actual slope at the Yeosu site. Tensile forces, after temperature compensation was taken into account using the verified temperature sensitivity coefficient ${\beta}^{\prime}$ and ground temperature obtained from the Korean Meteorological Administration (KMA) have been monitored for over one year, and the results were very consistent to those measured from the load cell, interestingly.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
• /
• v.22 no.3
• /
• pp.218-221
• /
• 2009

This paper reports on measurement method for the fiber optic strain monitoring of overhead contact line systems of trains, We used FBG (Fiber Bragg Grating) sensors to measure the strain variation of overhead contact line. FBG sensors can sensitively measure the variation of strain and! or temperature by the shift wavelength of reflected wave according to the lattice variation during the measurement. FBG sensor were attached on the contact line and connected to the monitoring system with optical fibers. The monitering system with FBG sensors showed very good sensitivity to measuring strain variation and this system could be applied to the overhead contact line of KTX (Korea Train eXpress).

• Journal of the Korean Society for Nondestructive Testing
• /
• v.29 no.5
• /
• pp.436-441
• /
• 2009

A fiber optic sensor is prospective to be applied to structural health monitoring. Especially, a fiber Bragg grating(FBG) sensor is one of the most popular sensors for the structural health monitoring. The FBG sensor has several demodulation systems for tracking the shift of the Bragg wavelength. The dynamic bandwidth is dependent on the demodulation system. In this paper, the sensing mechanism is that the slope of the optical spectrum of FBG could be used as its sensitivity when the tunable laser shot the monochromatic laser wavelength at the highest slope point. In this technique, the high sensitivity is guaranteed even though the sensing range is limited. In an example of the application, the composite plate embedding a FBG sensor was manufactured by using an autoclave method and the above sensing mechanism was applied to the composite plate. Firstly, the natural frequencies of the plate were successfully measured by the FBG sensor during the impact hammer test. Secondly, a high-power speaker was used to force the plate to be vibrated at the specific frequency that was one of the natural frequencies. During the shaking, the FBG sensor measures the dynamic characteristics and ESPI was also used to measure the mode shape. From the two dynamic tests, the availability of the FBG sensor system and the ESPI was proven as a technique for measuring the dynamic characteristics of composite structure.

• Proceedings of the Korean Geotechical Society Conference
• /
• 2009.03a
• /
• pp.341-348
• /
• 2009

Various types of micro cone penetrometers have been developed by using strain gages for the layered soil detection. Strain gages, however, are affected by several factors such as temperature, self heating and lead wire length. In this study, micro cone penetrometers with 3~7mm in diameter, are developed by using FBG sensor to overcome the defects of the strain gage, and compensate the effect of temperature during penetration. In order to verifiy the accuracy and reliability of the developed FBG cone, the cone penetration test is performed on the layered soil. The tip resistance of FBG snesor shows excellent sensitivity, and can detect the interface of the layered soils with higher resolution. In addition, the 3mm micro cone penetrometer which is impossible cone diameter by using strain gages presents much higher sensitivity than the 7mm cone penetrometer. This study suggests that FBG sensor is a useful sensor for manufaturing the ultra small sized cone, and effectively detects the interface of the layered soil.

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

• Korean Journal of Optics and Photonics
• /
• v.26 no.2
• /
• pp.83-87
• /
• 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 Korean Institute of Illuminating and Electrical Installation Engineers
• /
• v.26 no.3
• /
• pp.40-45
• /
• 2012

We substantially increased the temperature sensitivity of a fiber Bragg grating temperature sensor by gluing it onto a piece of bi-metal strip. The temperature-induced Bragg wavelength shift is increased upto 5 times more than that of the intrinsic FBG sensor by the strain effect from the deflected bi-metal. We showed the feasibility of the proposed sensor by comparing the temperature measuring experiments with those of the intrinsic FBG sensors. Also we measured temperature varying the length of the bi-metal pieces and analyzed the result effect of it.

• Korean Journal of Optics and Photonics
• /
• v.8 no.5
• /
• pp.415-419
• /
• 1997

A novel fiber optic sensor is demonstrated using a FBG in PM(Polarization-Maintaining) fiber. Gratings have been written in a Bow-Tie type fiber using the phase mask. The operation of the sensor simply involves monitoring back-reflected Bragg wavelengths from the grating. Since PM fiber has two principal semi-axes with two indices of refraction, two Bragg wavelengths were observed. We have observed the position of Bragg wavelengths for PM FBG shifted simultaneously by either applying the longitudinal strain or temperature change. The wavelength sensitivity of 1.2pm/$\mu$$\varepsilon$ about a longitudinal strain and the wavelength sensitivity of 11.4pm/$^{\circ}C$ about a temperature have been experimentally achieved. The wavelength sensitivity of both longitudinal strain and temperature are approximately same with the reported values for the single mode FBG. On the other hand, the change of separation between Bragg wavelengths was observed by the applying transverse stress. We observed that the separation between two Bragg wavelengths is proportional to the applied transverse stress. The wavelength sensitivity of 14.6 pm/N about a transverse stress has been achieved. We have demonstrated PM FBG sensors can measure the transverse stress independently from the effects of temperature.

• Proceedings of the Korean Society For Composite Materials Conference
• /
• 2000.04a
• /
• pp.184-189
• /
• 2000

Two types of fiber-optic sensors, EFPI(extrinsic Fabry-Perot interferometer) and FBG(fiber Bragg grating), have been investigated for measurement of thermal strain and temperature. The EFPI sensor is only for measurement of thermal strain and the FBG sensor is for simultaneous measurement of thermal strain and temperature. FBG temperature sensor was developed to measure strain-independent temperature. This sensor configuration consists of a single-fiber Bragg grating and capillary tube which makes it isolated from external strain. This sensor can then be used to compensate for the temperature cross sensitivity of a FBG strain sensor. These sensors are demonstrated by embedding them into a graphite/epoxy composite plate and by attaching them on aluminum rod and unsymmetric graphitelepoxy composite plate. All the tests were conducted in a thermal chamber with the temperature range $20-100^{\circ}C$. Results of strain measurements by fiber-optic sensors are compared with that from conventional resistive foil gauge attached on the surface.

• Proceedings of the KSR Conference
• /
• 2008.11b
• /
• pp.1227-1230
• /
• 2008

This paper reports on measurement method for the fiber optic strain monitoring of overhead contact line systems of trains. We used fiber Bragg grating (FBG) sensors to measure the strain variation of overhead contact line. FBG sensors were attached on the contact line and connected to the monitering system with optical fibers. The monitering system with FBG sensors showed very good sensitivity to measuring strain variation and this system could be applied to the overhead contact line of KTX.