This article presents machinery fault diagnosis in electric motors through mechanical vibration monitoring. Using a commercial machinery fault simulator, two fiber Bragg grating (FBG) accelerometers are evaluated under one healthy condition and nine different fault conditions, including rotors with broken bars, faulted bearings, misalignment, and imbalance. The mechanical vibration spectrum for each fault condition is assessed by analyzing the Bragg wavelength shift modulated by the acceleration. Offline analysis is carried out to identify the mechanical vibration pattern characteristics. In this preliminary analysis, the high-order frequency characteristics of faulted-bearing conditions and the slip frequency characteristics of broken rotor bar conditions were well-identified by both accelerometers. Following this analysis, this article uses two concurrent approaches to indicate the feasibility of classifying and/or clustering the data from supervised or unsupervised approaches. To that extent, the sensor system is first analyzed using a supervised approach from the $k$ -nearest neighbors (kNN) approach for fault condition classification, where the proposed supervised algorithm resulted in an accuracy of 100% for the fault condition estimation. After the classification analysis, data clustering is also proposed. In the unsupervised approach, the highest score for seven clusters was observed. Besides in this work, nine fault conditions were studied, there are two fault conditions where the positions of the accelerometers were interchanged for a sensor’s acceleration sensitivity evaluation purpose (maintaining the same fault condition). Finally, the results obtained in this work have shown the feasibility of the use of FBG-based accelerometers in different fault conditions under supervised and unsupervised approaches.

This paper presents the development, analysis and application of a fiber Bragg grating (FBG) array for two-dimensional (2D) shape reconstruction in a cantilever beam. The structural elements made of Pinus wood and Nylon 6.0 were numerically analyzed using the finite element method for the strain distribution when constant loading is applied at the free end of the beam. In addition, the temperature compensation method is proposed to decouple the temperature cross-sensitivity in the deflection analysis. In this case, the temperature sensitivities of all sensing elements of the 5-FBG array were obtained. An additional FBG was encapsulated in a silicone mold for increased sensitivity and positioned in the clamping point in which deflection was negligible. Temperature compensation was achieved considering the temperature measured by the silicone-embedded FBG (sensitivity of 27.78 pm/°C) and the sensitivity of all five FBGs of the deflection-sensing array (9.14 pm/°C ± 0.33 pm/°C). In the deflection experiments, the sensors presented a high linearity, in which a determination coefficient (R²) higher than 0.995 was obtained in all of the analyzed cases. Furthermore, the 2D shape construction using the proposed sensor approach resulted in the elastic line estimation for all analyzed beams, where the experimental results were in agreement with the theoretical and numerical analysis with a R² higher than 0.99 in all of the analyzed cases. Therefore, the proposed sensor array is a feasible approach for real-time shape reconstruction of structural elements with the advantages related to the possibility of direct embedment in the measured structure.

This work presents high-resolution sensors based on Phase-Shifted Bragg Gratings (PSBG) for low-frequency vibrations monitoring and mass deposition sensing. We analyzed the reflected spectrum using a commercial interrogator for six values of deposited mass: 0, 0.22, 0.27, 0.32, 0.40, and 0.47 g. For the experimental data, a linear regression was calculated for PSBG 1 (R ² = 0.9379 with a sensitivity of 0.0491 nm/g) and for uniform FBG (R ² = 0.909 with a sensitivity of 0.05182 nm/g). Machinery Fault Simulator was used for the vibration monitoring setup for engine rotation frequency of 4 to 14 Hz in steps of 2 Hz. We used an alternative non-commercial interrogation system that consisted of a tunable laser, an optical circulator, an optical power acquisition board, and a prefixed Phase-Shifted Bragg. The proposed alternative interrogation system has the ability to center the tunable laser in the narrow valley region of the PSBG reflected spectrum, increasing the resolution for monitoring vibration at low frequencies. As a reference, the PSBG measurement results were compared with those of a piezoelectric accelerometer fixed at the same bearing. For 4, 6, 8, 10, and 14 Hz, the relative error was 15, 10, 0, 0, and 1.43%, respectively. According to these results, PSBGs are a suitable option for monitoring low-frequency vibrations and for mass deposition sensing.

This letter presents the development and application of a multimaterial polymer optical fiber (POF) for temperature and force sensing. The POF is fabricated from the well-known polymethyl methacrylate (PMMA) through a conventional extrusion method combined with the Polycaprolactone (PCL), a biodegradable material with low melting point, which is thermally molded into the PMMA. In this approach, the high customizability enables the development of two configurations: (i) one input, one output (configuration 1); (ii) one input, two outputs (configuration 2). An advantageous feature of the PCL is its transmittance increase as a function of the temperature, which is confirmed via spectrophotometry at different temperatures. In the experimental analysis, the configuration 1 is used in the temperature tests and demonstrates a high linearity (coefficient of determination of 0.981) and resolution below 1°C, whereas its temperature limit is around 55°C. Configuration 2 is tested in simultaneous variations of temperature and transverse force, resulting in relative force insensitivity in temperatures below 40°C, whereas an optical power attenuation as a function of the applied force is found in temperatures around 55°C. Thus, the proposed approach shows a novel, flexible and customized technique for POF multimaterial fabrication, which can be employed not only on sensors applications, but also in the development of optical components based in POFs.

This paper presents the development, analysis, and application of chirped fiber Bragg gratings (CFBGs) for dynamic and static measurements of beams of different materials in the single-cantilever configuration. In this case, the beams were numerically analyzed using the finite-element method (FEM) for the assessment of the natural frequencies and vibration modes of the beam for the dynamic analysis of the structural element. Furthermore, the static numerical analysis was performed using a load at the free end of the beam, where the maximum strain and its distribution along the beam were analyzed, especially in the region at which the FBG was positioned. The experimental evaluation of the proposed CFBG sensor was performed in static conditions for forces from 0 to 50 N (in 10 N steps) applied at the free end of the beam, whereas the dynamic evaluation was performed by means of positioning an unbalanced motor at the end of the beam, which was excited at 16 Hz, 65 Hz, 100 Hz, and 131 Hz. The results showed the feasibility of the proposed device for the simultaneous assessment of the force and strain distribution along the CFBG region using the wavelength shift and the full-width at half-maximum (FWHM), respectively. In these cases, the determination coefficients of the spectral features as a function of the force and strain distribution were higher than 0.99 in all analyzed cases, where a potential resolution of 0.25 N was obtained on the force assessment. In the dynamic tests, the frequency spectrum of the sensor responses indicated a frequency peak at the excited frequency in all analyzed cases. Therefore, the proposed sensor device is a suitable option to extend the performance of sensors for structural health assessment, since it is possible to simultaneously measure different parameters in dynamic and static conditions using only one sensor device, which, due to its multiplexing capabilities, can be integrated with additional optical fiber sensors for the complete shape reconstruction with millimeter-range spatial resolution.

We demonstrate of an off-centered tilted fiber Bragg grating (FBG) inscribed in cyclic transparent optical polymer (CYTOP) fiber as a curvature sensor. The gratings were inscribed using a femtosecond (fs) laser with the direct write, plane-by-plane method due to its high flexibility and suitability for customization, particularly for the development of a single-peak spectrum in multimode CYTOP fiber. The grating is applied to curvature sensing, where the spectral features, namely the wavelength shift and reflected optical power, are analyzed with respect to the curvature angle on the optical fiber. The off-center and tilted FBG response as a function of the angle is compared with a centered and uniform FBG inscribed in CYTOP fiber using the same inscription method and laser parameters. Results show a significant improvement for the off-center grating when the wavelength shift information is considered and lower optical power sensitivity as a function of the angle with respect to the centered FBG. In addition, the temperature sensitivity differences of the two gratings, enables the development of temperature-insensitive angle sensor using a single off-center tilted FBG.