The preservation of heritage structures and buildings is a major concern to guarantee access of the cultural and architectural values to current and future generations. It also contributes significantly to the society in terms of identity, economy, education, and tourism. Due to their ancient nature, combined with the human activity (in and/or around these structures) and the environmental conditions, these structures are particularly susceptible to material deterioration and damage, that can compromise their integrity and the human safety. Therefore, it is crucial the implementation of an adequate monitoring system to analyse the static and dynamic responses of structures. In this paper, we discuss the development and installation of a sensing system for long-term monitoring of crack and joint movements, comprising eight displacement sensing devices and two temperature sensors, in the Monastery of Batalha, Portugal. The displacement sensing devices are formed by transparent small dimension holders and fiber Bragg gratings, with minimal intrusion and very low visual impact. They are relatively easy to fabricate, reproduce, and install, being a viable, efficient, and cost-effective alternative for Structural Health Monitoring sensing applications. Furthermore, they present simple design and principle of operation, which contributes to a straightforward installation and operation, and great versatility, as the alternative pre-strain mechanism of these devices allows to easily adjust the distance between the optical fiber holders. The operational performance of the sensing devices was analysed, and the monitoring results obtained during the following months after their installation are presented and discussed. The long-term data gathered by this sensing system can in the future be used to evaluate possible interventions.

In the present work, conducted as part of an internship at Siemens Gamesa Renewable Energy, optical fiber accelerometers based on Bragg gratings were developed with the aim of structural health monitoring of wind turbine blades. The dissertation begins with a description of the context of global and local energy consumption, as well as the motivation for this dissertation. In addition to a theoretical introduction to fiber optic Bragg gratings and the main methods used for their inscription, the state of the art regarding different geometries, sensitivities, and characteristics (such as the presence of metallic elements) of optical fiber sensor-based accelerometers is presented. An approach to optical accelerometers available on the market is also presented, highlighting some key parameters that should be considered in the process of selecting a device. The typical structure of wind turbine blades is described, as well as some fundamental concepts, such as the types of damage typically observed, which allow for the subsequent analysis of the tests carried out. Due to the wide versatility of geographical locations where a wind turbine can be installed and the consequent environmental variability, it is important to understand the response of the sensors used to these environmental parameters, so that these effects can be compensated. Thus, through the characterizations performed, a self-compensated temperature sensitivity of (3.6 ± 0.6) pm/ºC and a self-compensated relative humidity sensitivity of (1.6 ± 0.1) pm/%RH were obtained, confirming that these parameters can be neglected. For an operating frequency range of 0 to 30 Hz, an acceleration sensitivity of (77 ± 2) pm/G was achieved. To evaluate and verify the sensor's performance in a real context, tests were conducted in a controlled environment with a composite sample of identical characteristics to a blade, where its free-end length was varied to obtain different frequencies within the desired measurement range. Other parameters, such as the sensor's position or the excitation method, were also tested. In the final phase, tests were conducted on an SG170 blade where an intervention for repairs was carried out. As a result, a discrepancy was observed between the simulated results and the experimental results obtained, which was attributed to the damages and repairs present in the blade.

In this study, we developed an algorithm for identifying failures caused by broken bars in the rotors of three-phase induction motors through the analysis of their dynamic deformation using fibre optic Bragg gratings (FBGs) as sensing elements. The analysis of dynamic deformation enables the detection and diagnosis of various mechanical faults, such as misalignment, imbalance, bearing failures, and mechanical looseness. Furthermore, it allows for the identification of electrical faults, such as fractures or cracks in the rotor rings or bars To measure the dynamic deformation, we employed FBG-based sensors known for their key features, including high multiplexing capability, electromagnetic radiation immunity, and long-distance operation. Experimental tests were conducted on a small-scale induction motor (3 HP) to validate the method and explore its applicability to medium and large-scale machines. The motor was powered by two different supply sources: the utility power grid and a controled power sources, under load conditions of 75% and 100% of the rated load. During the tests, we used a rotor without any bar defects and subsequently a rotor with a broken bar. The presence of a broken bar was successfully identified under both load conditions and across all two power supply sources. The fault caused by the broken bar in the rotor was detected in two frequency regions obtained from the three sets of experiments. The first region was centred around the mechanical rotational frequency of the rotor, while the second region was approximately twice the electrical frequency of the power supply. The system demonstrated high sensitivity with a good signal-to-noise ratio and showcased advantages over conventional methods and sensors commonly used for identifying broken bar faults in induction motors.

In this paper we present results on the normalized temperature sensitivity of UV- and fs-induced fiber Bragg gratings in a singlemode fiber with ~4.7 mol% GeO2 and having an Ormocer coating. In the 1500-1600 nm wavelength range, the former shows an almost constant value of 6.165x10-6 K-1, whilst the fs-induced present some variation not related with the strength of the grating but probably due to induced birefringence. The average value obtained was 6.191x10-6 K-1 which is higher than the former. For the UV-induced gratings in the Corning SMF-28 fiber (3.67 mol% GeO2) the value obtained was 6.143x10-6 K-1. The achieved values are compatible with the use of Corning 7980 silica-based cladding fiber. Preliminary results also show no measurable impact of the hydrogenation process or the strength of the grating on the normalized temperature sensitivity.

Optical sensors have hit their maturity and a new kind of systems is being developed. This paper deals with the development of a new sensing structure based on polymeric foils and optic fiber sensors, namely the Fiber Bragg Grating sensors. Sensor integration in polymeric foils, using industrial process is the proposed goal. To achieve this goal, Finite Element Analysis was used for prototype modeling and simulation. The model was subjected to loads and restraints in order to retrieve information about stress distribution and displacement of specific points. From the simulation was possible to predict the sections where the sensor should be positioned. A prototype was then fabricated using industrial processes. Tests indicate that the polymeric foil influence on the sensor performance may exist. However, the prototype was able of transferring the full deformation to the optical sensor. Moreover, the optical sensor, which is incorporated in the polymeric foil, is fully functional with high sensitivity, 0.6 picometer by microstrain, allowing deformation measurements, up to 1.2 millimeter.

A structural integrity monitoring system based on optic fiber sensors is an important development at the smart structures level. However, direct sensors incorporation, without a substrate structure, creates few difficulties in eventual sensor maintenance or replacement. This paper presents an approach to overcome this issue. The fabrication, using industrial fabrication processes, and characterization of a polymeric foil able to sense and gather sensitive information, and send it for remote analysis is explored. The described example uses Fiber Bragg Grating sensors embedded in laminated polymeric sheets commonly used in different industries, as automotive, aeronautic, civil, among others. The fabricated foil is capable of transferring the full deformation to the optical sensor. Tests indicate that the polymeric foil influence on the sensor performance may exist. However, the presented optical sensor incorporated in the polymeric foil is fully functional with high sensitivity, 0,6 picometer by microstrain, measuring deformation, up to 1,2 millimeter

This paper presents the fabrication and characterization of a polymeric foil able to sense deformation, gather sensitive information and send it for further analysis. Fiber Bragg Grating sensors are embedded in laminated polymeric sheets commonly used in the car’s flooring. The fabrication of the Bragg sensors and the integration of the optic sensors in the foil is described, using industrial fabrication processes. The obtained foil is capable of transferring the full deformation to the optic sensor and returning to the initial position when no deformation exists, providing good sensibility. Application of this sensing foil includes automobile chassis monitoring.

Optical sensors have hit their maturity and a new kind of systems is being developed. This paper deals with the development of a new sensing structure based on polymeric foils and optic fiber sensors, namely the Fiber Bragg Grating sensors. Sensor integration in polymeric foils, using industrial process is the proposed goal. To achieve this goal, Finite Element Analysis was used for prototype modeling and simulation. The model was subjected to loads and restraints in order to retrieve information about stress distribution and displacement of specific points. From the simulation was possible to predict the sections where the sensor should be positioned. A prototype was then fabricated using industrial processes. Tests indicate that the polymeric foil influence on the sensor performance may exist. However, the prototype was able of transferring the full deformation to the optical sensor. Moreover, the optical sensor, which is incorporated in the polymeric foil, is fully functional with high sensitivity, 0,6 picometer by microstrain, allowing deformation measurements, up to 1,2 millimeter.

Monitoring systems can already be found in common applications, from automotive to aeronautics or biomedical. However, the application of these monitoring systems has not yet been a very easy task, especially, at the sensor application step. A smart structure with sensing capabilities would allow to overcome the existing issues in the installation of sensor networks. As the size of this type of systems and the market requirements are significant, an industrial manufacturing process needed to be considered. An integrated solution based in Fiber Bragg Grating sensors embedded in PVC laminates was manufactured by industrial spread-coating process. The resultant structure is a temperature and strain sensitive foil that was characterized in terms of surface structure, optical response and overall performance. The integrated solution behavior had a slope of 0.8 pm by microstrain with a measurement range of 1.2 mm when the fiber was straight in the foil. Different fiber layouts were also tested and defined the minimum curve radius in 10 mm. The foil also presented a dimensional stability above 99%, ensuring the capability to sustain molding process. A mechanical analysis to evaluate the elongation capabilities and optical response was conducted. Three samples with different sensor positioning were subjected to the test. In the end, a sample, with the sensor in a 45à ° angle regarding the displacement axis, achieved the best compromise between maximum elongation range (25%) and optical response linearity. For the temperature response, a wavelength deviation of 1.7 nm was obtained for a 100 ºC temperature change.