Fiber Bragg grating temperature sensor with enhanced sensitivity

In this research, fiber Bragg grating (FBG) temperature sensors are embedded in composites in order to detect highly localized temperature gradients in the composite structures. The primary goal is to perform structural health monitoring on a composite structure. A secondary goal is to use the sensors as a diagnostic tool to determine the optimal composite materials, architectures, or structures that are the least susceptible to thermal damage. Initial results will be discussed for two composite materials using a single sensor to measure temperature variations. The tests include measurements of the temporal and spatial thermal response of the composite resulting either from an applied heat source or to high energy radiation incident on the surface. Additional tests demonstrate the response using a 3x2 array of sensors to simultaneously measure the temperature at three varying depths in the composite, using three FBGs aligned with the heat source, and three FBGs located a short lateral distance (3cm) away from the heat source. In addition, since FBGs respond to strain as well as to temperature, any strain in the composite is coupled into the embedded fiber and is also detected by the FBG sensors. Initial measurements demonstrate the simultaneous response of FBG sensors to both temperature and strain. The various components of strain that are observed in the composite will be discussed, and possible methods to isolate these components and mitigate their response will be considered.

This paper reports the design, characterization and implementation of a fiber Bragg grating (FBG) temperature sensor in a power electronic inverter which has a high electromagnetic interference (EMI) radiation. The performance and characteristics of the sensor are investigated by monitoring the temperature variation of the insulated gate bipolar transistor (IGBT) in photovoltaic (PV) arrays inverter. The thermal expansion of the optical fiber is enhanced by mounting the FBG sensor to a metal sheet. Due to this enhancement, the FBG temperature sensor recorded an improvement in terms of linearity and sensitivity. A comparison is made between the performance of the enhanced FBG sensor and a thermocouple in a high EMI environment, and the results show that the FBG sensor performs better.

We studied the application of the fiber Bragg grating (FBG) temperature sensing method based on support vector regression optimized by a genetic algorithm (GA-SVR) for constant and decreasing external ambient temperature cases by simulation. The external ambient temperature could be retrieved from both the transient FBG wavelength and its corresponding change rate using GA-SVR, before the FBG temperature sensor reached the thermal equilibrium state with the external ambient temperature. FBG wavelengths and their corresponding change rates in the cases of FBG sensor temperatures higher and lower than the external ambient temperature were studied and used to construct the training data set. We found that there exist singularity points in the curves of the wavelength change rate when the FBG sensor temperature is higher than the external ambient temperature in some cases, which is different from the case where the FBG sensor temperature is lower than the external ambient temperature. Its application for sensing the constant and decreasing external ambient temperature in real time was demonstrated with an accuracy of 0.32°C in those two cases. It also indicates that for real applications of this temperature sensing method where the external ambient temperature varies randomly, the FBG sensor temperature changes rather than the external ambient temperature changes play the dominant role. What is more, the demodulation time was decreased to 0.002s, which is approximately 0.05‱ of the time constant of the FBG temperature sensor. In other words, this method makes it possible to realize the external ambient temperature determination using a time smaller than the time constant of the FBG sensor. The high sensing accuracy and fast demodulation speed are crucial for future high-performance real-time FBG temperature sensing.

Surface temperature is an important parameter in clinical diagnosis, equipment state control, and environmental monitoring fields. The Fiber Bragg Grating (FBG) temperature sensor possesses numerous significant advantages over conventional electrical sensors, thus it is an ideal choice to achieve high-accuracy surface temperature measurements. However, the effects of the ambient temperature and installation types on the measurement of surface temperature are often overlooked. A theoretical analysis is implemented and a thermal transfer model of a surface FBG sensor is established. The theoretical and simulated analysis shows that both substrate strain and the temperature difference between the fiber core and hot surface are the most important factors which affect measurement accuracy. A surface-type temperature standard setup is proposed to study the measurement error of the FBG temperature sensor. Experimental results show that there are two effects influencing measurement results. One is the “gradient effect”. This results in a positive linear error with increasing surface temperature. Another is the “substrate effect”. This results in a negative non-linear error with increasing surface temperature. The measurement error of the FBG sensor with single-ended fixation are determined by the gradient effect and is a linear error. It is not influenced by substrate expansion. Thus, it can be compensated easily. The measurement errors of the FBG sensor with double-ended fixation are determined by the two effects and the substrate effect is dominant. The measurement error change trend of the FBG sensor with fully-adhered fixation is similar to that with double-ended fixation. The adhesive layer can reduce the two effects and measurement error. The fully-adhered fixation has lower error, however, it is easily affected by substrate strain. Due to its linear error and strain-resistant characteristics, the single-ended fixation will play an important role in the FBG sensor encapsulation design field in the near future.

A novel packaging method for FBG temperature sensors based on ultrasonic-assisted soldering technology

Thermal modeling of the power transformer to estimate the internal temperature only reflects the winding hot spot to a certain extent. The application of optical fiber sensors to measure transformer internal temperature directly can obtain the true temperature with high accuracy and stability and overcome the shortcomings of traditional electric sensors. This paper analyses the measuring principle of FBG (Fiber Bragg Grating) sensor, and constructs transformer internal temperature measurement platform with FBG temperature sensors, then adopt short-circuit method to simulate the process of winding heat under different load conditions. Finally the measurement of transformer internal temperature is conducted respectively with FBG temperature sensors and thermocouples. Comparison and analysis of the results shows that FBG sensors can effectively measure the internal temperature of the transformer.

Abstract. Optical temperature sensors offer unique features which make them indispensable for key industries such as the energy sector. However, commercially available systems are usually designed to perform either distributed or distinct hot spot temperature measurements since they are restricted to one measurement principle. We have combined two concepts, fiber Bragg grating (FBG) temperature sensors and Raman-based distributed temperature sensing (DTS), to overcome these limitations. Using a technique called incoherent optical frequency domain reflectometry (IOFDR), it is possible to cascade several FBGs with the same Bragg wavelength in one fiber and simultaneously perform truly distributed Raman temperature measurements. In our lab we have achieved a standard deviation of 2.5 K or better at a spatial resolution in the order of 1 m with the Raman DTS. We have also carried out a field test in a high-voltage environment with strong magnetic fields where we performed simultaneous Raman and FBG temperature measurements using a single sensor fiber only.

Fiber Bragg grating (FBG) temperature sensors are now regarded as a relatively mature sensing technology for temperature measurement in ambient conditions, and may offer potential as a cryogenic temperature sensor for quench detection. In this paper, the temperature response of three different coated FBG sensors from 77 to 293 K was studied; bare (silica) FBG, acrylic coated, and polyimide coated FBG sensors. We examine the effect of fiber coatings on the temperature sensitivity of an embedded FBG sensor down to cryogenic temperatures. Measurements were repeated with sensors from two FBG suppliers. The measured temperature response was then compared and contrasted with the predicted sensor response. In order to assess the sensor sensitivity, repeatability, and any hysteresis, these sensors were then subjected to repeated cool-down / warm-up cycles (77-293 K). The effect of epoxy resin impregnation on the temperature response characteristics was analyzed by comparing the sensor response characteristics before and after embedding into a 10 mm × 2 mm × 4 mm epoxy block. We compare this to the uncoated fiber for embedment in HTS coils. Finally, we recommend the optimum low-cost sensor construction for use in quench detection in coils.

In this paper, we summarize our recent research work on two types of optical fiber sensors: long distance sensor and biomedical sensor. For long distance sensing, we demonstrated our 150-km long distance fiber Bragg Grating (FBG) temperature and vibration sensor system and Brillouin optical time domain analysis distributed sensor system. For biomedical sensing, we demonstrated our microbend fiber sensors for measurements of breathing rate/heat rate and Ballistocardiogram waveform. A new sensor for patient's breathing measurement by using fiber loop ringdown spectroscopy is also introduced.

Review of fabrication and packaging of UV-induced FBGs for high temperature sensing

Fiber Bragg grating (FBG) temperature sensors are embedded in composites to detect localized temperature gradients resulting from high energy infrared laser radiation. The goal is to detect the presence of radiation on a composite structure as rapidly as possible and to identify its location, much the same way human skin senses heat. A secondary goal is to determine how a network of sensors can be optimized to detect thermal damage in laser-irradiated composite materials or structures. Initial tests are conducted on polymer matrix composites reinforced with either carbon or glass fiber with a single optical fiber embedded into each specimen. As many as three sensors in each optical fiber measure the temporal and spatial thermal response of the composite to high energy radiation incident on the surface. Additional tests use a 2 × 2 × 3 array of 12 sensors embedded in a carbon fiber/epoxy composite to simultaneously measure temperature variations at locations on the composite surface and through the thickness. Results indicate that FBGs can be used to rapidly detect temperature gradients in a composite and their location, even for a direct strike of laser radiation on a sensor, when high temperatures can cause a non-uniform thermal response and FBG decay.

The electro-mechanical impedance (EMI) technique has been applied successfully to detect minor damage in engineering structures including reinforced concrete (RC). However, in the presence of temperature variations, it can cause false alarms in structural health monitoring (SHM) applications. This paper has developed an innovative approach that integrates the EMI methodology with multilevel hierarchical machine learning techniques and the use of fiber Bragg grating (FBG) temperature and strain sensors to evaluate the mechanical performance of RC beams strengthened with near surface mounted (NSM)-fiber reinforced polymer (FRP) under sustained load and varied temperatures. This problem is a real challenge since the bond behavior at the concrete–FRP interface plays a key role in the performance of this type of structure, and additionally, its failure occurs in a brittle and sudden way. The method was validated in a specimen tested over a period of 1.5 years under different conditions of sustained load and temperature. The analysis of the experimental results in an especially complex problem with the proposed approach demonstrated its effectiveness as an SHM method in a combined EMI–FBG framework.

This paper presents an innovative calorimetric method for measurements of AC losses of high-temperature superconducting (HTS) tapes by using effect of optical fiber Bragg grating (FBG) wavelength shift resulting from temperature change caused by AC losses, where it is adopted as temperature sensor. The HTS specimen on which the FBG are attached on are adiabatically processed before measuring. Comparing with conventional electric and calorimetric methods, FBG temperature sensor has advantage of rapid response and anti-electromagnetic interference due to its thin geometrical sizes and optical characteristics, it is thus very suitable to AC loss measurement of HTS applications in much more complicated electromagnetic circumstance. By measuring the effect of temperature on wavelength shift in low temperature range, it is indicated that FBG has good enough repeatability of dependence of wavelength shift on temperature measurement around liquid nitrogen temperature of 77 K. After calibration, AC loss of BSCCO HTS tape in self-field is measured by the FBG sensor, and the measured loss is also measured by conventional electric method using lock-in amplifier (LIA). It is shown that the measured AC loss is agreement with the results measured by electric method and those based on Norris theory, so that the presented calorimetric method is available for measuring the AC loss of superconductors and expected to be generalized to AC loss or thermal measurements of other bulk superconductors.

In this paper, we review the role of Fiber Bragg Grating (FBG) sensors for temperature monitoring and their application in the smart factories. In the first instance, we present the historical context and fundamental principle of FBG based sensors. Then, we review of recent development in FBG temperature sensors. We explore the use of FBG sensors in industry 4.0, specifically in smart factories, and offer a few ideas that might possibly open up a lot of doors for the development of fiber Bragg grating sensors.

This paper is aimed at applying BP neural network and Dempster-Shafer (D-S) evidence theory to realize the real-time monitoring and the fault diagnosis by taking power transformer as the object of fault diagnosis. We make use of the neural network's ability of better fault tolerance, strong generalization capability, characteristics of self-organization, self-learning, and self-adaptation, and take advantage of multi-source information fusion technology to realize comprehensive processing for uncertainty information. Combining with BP neural network and D-S evidence theory, a characteristic layer fusing model of power transformer fault diagnosis has been established. As high-voltage electric equipment has complex structure and works in harsh environments, the fiber bragg grating (FBG) temperature sensors are used to monitor the real-time thermal characteristics of the power transformer hotspot. The simulation results of power transformer fault diagnosis shown that this method is effective.