In this article, we present a vibration measurement system based on low-frequency cantilever-based fiber Bragg grating accelerometers (CFAs) for a suspension bridge. Each accelerometer has an end-loaded cantilever beam, specifically tailored to achieve a uniform sensitivity for a frequency range of 0-4 Hz, a suitable detection range for the vibration analysis. In the field test, seven CFAs were installed at specific positions along the deck of a 110-m-long suspension bridge for synchronous multipoint vibration measurements. The reflection spectra of the CFA array were recorded and processed using the pseudo-high-resolution scheme to improve the signal quality and measurement accuracy. Three natural vibration frequencies: 1.15, 1.54, and 3.17 Hz have been identified from the measurement. Following that, the acquired time-domain signals were processed by a digital bandpass filter to retrieve the waveform at each natural frequency to determine the corresponding mode shapes. The results are in agreement with the phase difference between the frequency domain signal for each natural frequency. This investigation has shown the feasibility of the proposed measurement system for determining the mode shapes and dynamic frequency analysis of a suspension bridge. It is a potential method for structural health monitoring for other similar civil structures.

The design and performance of an innovative high-precision extensometer system, fabricated inexpensively using 3D printing technology, are discussed in this paper. In the development of the extensometer, an embedded Fiber Bragg Grating (FBG) strain sensor was 3D printed using a thermoplastic polyurethane (TPU) filament, which was used as the primary sensing element of the extensometer system, taking advantage of its excellent flexibility and high sensitivity to variations in localized strain. In the performance assessment carried out, the results obtained during the experimental test and validation have demonstrated that it could be used very effectively to measure strain variations, with an average wavelength responsivity of 0.0158 nm/cm (for displacement) and very high linearity (up to 99%). Furthermore, the protection integrated into the sensor systems design makes it well-suited for in-the-field applications, such as monitoring ground displacements which can lead to dangerous slippages of sloped earthworks. In addition, a field testing of the extensometer under simulated conditions has shown that a Fiber Bragg Grating (FBG)-based approach could be applied effectively to the measurement of strain, offering a wavelength responsivity of 0.0012 nm/ $\mu \varepsilon $ (for strain-sensitive FBGs) under both dry and wet soil conditions. Moreover, taking advantage of the high (~99%) linearity, the extensometer is a reliable instrument for use in different underground conditions, creating an easy-to-use ground movement monitoring system which then enables an excellent representation of the displacement profile of the earth to be made.

A Fiber Bragg Grating (FBG)-based inclinometer has been developed for field use, designed to incorporate biaxial 3-dimensional (3D) printed tilt sensors (in which four FBGs were used). The inclinometer was characterized by examining its response to a wide range of tilts, over the range from 0° to 90°, towards the inclination axes. An excellent linear correlation between the wavelength shifts and the inclination angle (up to the 90° used) was obtained, showing an average sensitivity of 0.01 nm per degree of inclination angle, for each of the FBGs used. In addition to the four FBGs that form the basis of the inclination measurement, a further FBG was included in the design to allow compensation for any temperature changes experienced during the measurements. The device was calibrated over the range from −25°C to 80°C (corresponding to the extremes of cold and hot weather conditions likely to be experienced in-the-field), and a sensitivity to temperature change of 0.011nm/°C was achieved, allowing an effective temperature correction to be applied. The data obtained from a full characterization of the performance of the sensor system, carried out in a stable, controlled environment, indicate that this inclinometer yields good sensitivity, making it highly applicable for use in monitoring rapid ground movements and deformations with its compact design allowing its wide use.

A surface-mounted tilt sensor was designed and fabricated to measure the inclination angle of engineered structures or slopes in two directions. The device utilizes two strain-sensitive FBGs for tilt angle measurement bi-directionally and one strain-free FBG to provide temperature compensation. In this work, a tilt sensor prototype was fabricated using a 3D printer, with a robust enclosure and a miniature actuator with dimensions of 115 x 65 x 30 mm and 45 x 20 x 3 mm, respectively. The device was first calibrated in the laboratory for tilt and temperature parameters. For tilt calibration, the device yields a sensitivity value of 0.0135 nm/° and 0.0123 nm/° for +x and -x directions. On the other hand, the device delivers a sensitivity value of 0.0105 nm/°C as the response to temperature changes. The tilt sensor was also tested for suitability in a real-field deployment where it was installed on a retaining wall and was left for 4 weeks. The field test data indicates no vertical displacement of the wall since the device exhibits zero inclination changes during the test period. This compact, robust, and easy-to-install tilt sensor has excellent potential for various geotechnical applications, mainly in landslide detections, ground movement, and engineered slope monitoring.

A 3D-printed tilt fiber sensor using a two-mode interferometer (TMFI) as the sensing mechanism has demonstrated. The TMFI was constructed by splicing single-mode fibers (SMF-28s) at both ends of a two-mode (TMF) to generate a comb-like interference spectrum sensitive to bending. A 3D-printed cantilever with a attached to one end was used to induce bending as the structure was tilted and by embedding the TMFI onto the 3D cantilever, different tilt angles can be measured, as a the bending of the fiber. The TMFI exhibited a linear response towards the tilt angles, θ, with a responsivity of nm/degat negative θ (0 to -90°) and 5 × 10 ^-3 nm/deg positive θ (0 to 90°), respectively. The sensor was able to small angle changes in increments as small as 1° and it performs better than embedded FBG sensors. The tilt proposed has a small form factor and simple design, as well as being cost-effective and light-weight, thus showing significant potential for a variety of civil engineering applications.