Fiber Bragg grating sensors for structural health monitoring of Tsing Ma bridge: Background and experimental observation

The rapid expansion of the optical fiber telecommunication industry due to the explosion of the Internet has substantially driven down the cost of optical components, making fiber optic sensors more economically viable. In addition, the rapid development of fiber-optic sensors, particularly the fiber Bragg grating (FBG) sensors offers many advantages and capability that could not be achieved otherwise. In the past few years, fiber Bragg grating sensors have attracted a lot of interest and they are being used in numerous applications. This paper describes the FBG sensors developed for structural health monitoring, and were installed on Hong Kong's landmark Tsing Ma bridge (TMB), which is the world longest (1377 m) suspension bridge that carried both railway and regular road traffic. Forty FBG sensors divided into three arrays were installed on the hanger cable, rocker bearing and truss girders of the TMB. The objectives of the study are to investigate the feasibility of using the developed FBG sensors for structural health monitoring, via monitoring the strain of different parts of the TMB under both the railway and highway loads as well as comparing the FBG sensors' performance with the conventional structural health monitoring system - Wind and Structural Health Monitoring System (WASHMS) that has been operating at TMB since the bridge's commissioning in May 1997. The experimental observations in this project show that the results using FBG sensors were in excellent agreement with those acquired by WASHMS.

Fiber Bragg grating sensors for structural health monitoring of Tsing Ma bridge : Background and experimental observation

In this article, approaches reported in the literature for multi-parameter measurement relying on fiber grating technology are reviewed. Due to its importance, the particular case of temperature/strain simultaneous measurement is addressed in further detail, as well as the techniques and sensing heads for temperature-independent strain measurement. Specific criteria are also proposed for sensing head classification in the context of multi-parameter measurement.

Applications of Fiber Optic Grating Technology to Multi-Parameter Measurement

An embedded fiber Bragg grating (FBG) sensor for simultaneous measurement of stress and temperature is proposed, where a uniform FBG is capsulated in a tapered polymer so that the grating is linearly chirped when the sensor is stressed The value of applied stress and temperature can be obtained by measuring the chirp and central wavelength via a coefficient matrix Experimental results demonstrate that the sensor has a stress sensitivity of 262/spl times/10/sup -3/nm/N and a temperature sensitivity of 0108 nm//spl deg/C Since the FBG is well protected and no sudden strain change is imposed on it, the sensor is expected to have an excellent reliability

An embedded FBG sensor for simultaneous measurement of stress and temperature

A new approach to measuring displacement and temperature simultaneously by use of a specially designed isosceles triangular cantilevered beam as a strain agent is demonstrated. A fiber Bragg grating epoxied onto the beam surface is experimentally demonstrated to have a temperature sensitivity of ~0.113 nm/ degrees C below 60 degrees C and a displacement sensitivity of 9.24x10(-2) nm/mm .

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Fiber Bragg grating sensor for simultaneous measurement of displacement and temperature

A new approach for tuning the wavelength of a fiber Bragg grating without chirping is demonstrated. Tuning is achieved by bending a triangular cantilever beam with the grating was epoxied on its surface. The isosceles triangle was firmly fixed at its base and bending was realised by exerting a force to its vertex. Chirp-free tuning of a fiber Bragg grating over a wavelength range of 4.52 nm is demonstrated. Using a correction index of 0.7, the experimental results are in good agreement with the theoretical values.

Chirp-free tuning of fiber Bragg grating using a cantilever beam

Fiber Bragg grating sensor system with interferometric demodulation technique

An active interrogation technique for a 16-cascaded wavelength-division multiplexing fiber Bragg grating sensor system is proposed. The system has been demonstrated using a combination of a feedback control technique, which makes the transmission of an employed tunable F-P filter track the Bragg wavelength of the interrogated sensor automatically, and a technique for choosing the output wavelength of a ring- compounded-cavity fiber laser by tuning the applied voltage of the filter. Demodulated by using an unbalanced scanning Michelson interferometer, a sensitivity of 1.682 Deg/(mu) (epsilon) has been achieved.

Youlong Yu

Papers

Fiber Bragg grating sensor for simultaneous measurement of displacement and temperature

Chirp-free tuning of fiber Bragg grating using a cantilever beam

Fiber Bragg grating sensor system with interferometric demodulation technique

Active interrogation technique for wavelength-division-multiplexed fiber Bragg grating sensor system