During last decades, sensor elements based on the fiber Bragg grating (FBG) have been widely studied and developed due to the advantages of immunity to electromagnetic interference, compact size, high precision, and so on. The FBG itself is sensitive to axial strain and temperature variation directly and can indirectly measure these complex physical parameters, such as pressure, displacement, and vibration, by using some specially designed elastic structures to convert them into the axial strain of the FBG. Whether the FBG is fixed on the measured object to measure the strain directly or fixed on an elastic structure body to measure other physical quantities, these types of FBGs could be collectively called as strain sensing FBGs. The packaging of the FBG has important influence on FBG characteristics that directly affect the measurement accuracy, such as strain transfer, temperature characteristic, and spectral shape. This paper summarizes the packaging methods and corresponding temperature compensation methods of the currently reported strain sensing FBGs, focusing especially on fully pasted FBG, pre-stretched FBG with double-end fixed, and metallic packaging. Furthermore, the advantages and drawbacks of different packaging methods have been analyzed, which can provide a reference for future researches.

This paper presents a fiber Bragg grating (FBG) displacement sensor with high abrasion resistance for displacement monitoring of a steel spring floating slab damping track. A wedge-shaped sliding block and an equal-strength beam form a conversion mechanism to transfer displacement to the deflection of the beam, and the deflection-induced strain is exerted on two FBGs. A special linear guide rail-slider and a precision rolling bearing have been adopted onto the conversion mechanism, which turned sliding friction into rolling friction and thus significantly reduced the friction during frequent alternating displacement measuring. Sensing principle and the corresponding theoretical derivation have been demonstrated. Experiment results show that the sensor has a sensitivity of 34.32 pm/mm and a high resolution of 0.0029 mm within a measurement range of 0~90 mm. Besides, the sensor has also a good measurement capability for micro-displacement within a range of 0~3 mm. The repeatability error and hysteresis error are 1.416% and 0.323%, respectively. Good creep resistance and high abrasion resistance for alternating displacement measurement have also been presented by a performance test. These excellent performances satisfy the requirements of high precision and long-term stability in structural health monitoring for machinery equipment and civil engineering, especially in the displacement monitoring of a floating slab damping track.

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Purpose The purpose of this paper is to present the latest sensing structure designs and principles of information detection of fiber Bragg grating (FBG) displacement sensors. Research advance and the future work in this field have been described, with the background that displacement and deformation measurements are universal and crucial for structural health monitoring. Design/methodology/approach This paper analyzes and summarizes the existing FBG displacement sensing technologies from two aspects principle of information detection (wavelength detection, spectral bandwidth detection, light intensity detection, among others) and principle of the sensing elastomer structure design (cantilever beam type, spring type, elastic ring type and other composite structures). Findings The current research on developing FBG displacement sensors is mainly focused on the sensing method, the construction and design of the elastic structure and the design of new information detection method. The authors hypothesize that the following research trends will be strengthened in future: temperature compensation technology for FBG displacement sensors based on wavelength detection; a study of more diverse elastic structures; and fiber gratings manufactured with special fibers will greatly improve the performance of sensors. Originality/value The latest sensing structure designs and principles of information detection of FBG displacement sensors have been proposed, which could provide important reference for research group.

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