Study On Measuring Technique And Theory Based On Fiber Bragg Grating And Their Application In The Power Engineering

With the increasing demand of the power, power equipments are applied more and more widely, and thus the first is to guarantee the security and economical efficiency during the operating process. To realize the purpose, health status monitoring of power equipments on-line or intermittent is a very important component. Obviously, the acquiring data by monitoring can be useful to the optimum design and operation as a reference. As for the security of service of power equipments, advanced measuring method and measurement device are imperative to guarantee the maintenance and overhaul of power equipments timely, and thus avoid the hazardous accidents.Aim to the security monitoring and optimum operation, fiber Bragg grating (FBG) smart sensing component is utilized. In this paper, sensing mechanism and characteristics of the FBG were studied in detail. Temperature and strain sensing response have been measured. Many types of sensors and interrogation and test system based on the FBG were fabricated, including vibration modal, relative humidity, refractive index, liquid and gas concentration sensors and systems etc., which were realized to monitor vibration, etching, noise characteristics, temperature and stress, leakage, fuel gas components, concentration and moisture parameters etc. of power equipments.First, sensing theory and characteristics of the FBG were studied in detail. The reflectance spectrum of the FBG can be computed on the basis of the transmission matrix method, and the temperature and strain sensing mechanism were recognized fully. The theoretical results demonstrated that the reflectance spectrum is affected by modulation depth of the refractive index and grating length. The temperature and strain sensitivity of the FBG is 10.2 pm/℃and 0.9742 pm/με, respectively. Many interrogation methods for Bragg wavelength were investigated, such as optical spectrum analyzer, edge filtering method based on wavelength division multiplexing (WDM) and tunable narrow band laser scanning method and so forth.Second, modal frequencies of the cantilever by using FBG sensing unit were measured, and vibration characteristics of the cantilever was analyzed. Another measuring method based on modal analysis system CRAS was used to measure the modal frequencies of the cantilever, and finite element software ANSYS also was used to compute the theoretical modal of the cantilever. The results based on three types of methods demonstrated FBG measuring method for the modal frequencies measurement are feasible. Therefore, modal frequencies of the turbine blade with and without constraint condition were measured by using FBG. The experimental results illustrated that measuring method based on the FBG is reliable. Thus, this investigation is beneficial to carry out application of measuring the vibration modal characteristics in the power engineering field.Third, FBG relative humidity sensor was studied by the combination of the FBG with polyimide moisture sensitive material. Moisture sensitive material is coated uniformly on the fiber cladding of the FBG, which can transform the change of relative humidity into response of the strain. The volume expansion and shrinkage of the polyimide moisture sensitive material after water absorption and desorption have direct effects on the FBG and result in the shift of the Bragg wavelength. Moisture sensitive material, FBG relative humidity sensing head, test system and responding characteristics of the sensing head were studied. Experimental results showed that the applicable low-cost relative humidity sensor has a linear, reproducible and reversible response over a wide RH range from 11% to 98%. The thickness a of the thermoplastic polyimide coating considered as a moisture sensitive polymer material are 8μm, the relative humidity and temperature sensitivity of the sensor is -0.266 mV/%RH and -24.91 mV/℃, and the response time is approximately 5 s, which build solid foundation for measuring the fuel gas moisture, hydrogen moisture, coal moisture, etc..Fourth, according to evanescent field absorption theory, FBG can be used to measure the refractive index. By etching the fiber cladding of the FBG, the evanescent field of the transmission light in the fiber core is exposed into the surrounding medium. The change of surrounding refractive index result in the change of effective refractive index of the FBG, and thus the Bragg wavelength shifts. By detecting the shift of the Bragg wavelength, surrounding refractive index can be obtained. As the thickness of the residual fiber cladding is 17μm, refractive index sensitivity is 3.40 V/RIU. Thinned-FBG evanescent field refractive index sensor was used to measure the concentration of the four representative chemical solutions (acid, sugar, alcohol) with the wide range from the zero to saturated concentration. Furthermore, the sensor also was applied to measure the concentration of nanofuild. In order to improve the refractive index sensitivity, U-shaped FBG was employed. The result showed that, compared with the straight uniform thinned FBG, refractive index sensitivity approximately increases by 25%.Sixth, aim to measuring technique for the microstructure and microfuild, this paper studied the thinned and microstructure FBG (ThMs-FBG), which can be used to measure the temperature and refractive index. The numerical simulation for the reflectance spectrum of the ThMs-FBG was calculated, the phase shift down-peak could be observed from the reflectance spectrum. Many factors influencing the reflectance spectrum were considered in detail for simulation, including the etched depth, length and position. The simulation results would be considered as a reference. Coating and sandwich-solution etching methods were utilized to realize the microstructure of the ThMs-FBG, and the fabricating method and technology and refractive index sensing characteristics were studied by the experiments. Under the condition that the length and cladding diameter of the ThMs-FBG microstructure were 800μm and 14μm, respectively, and the position of the microstructure of the ThMs-FBG in the middle of grating region, down-peak wavelength of the ThMs-FBG increased with the increment of the surrounding refractive index, the refractive index sensitivity of the ThMs-FBG was 0.79 nm/RIU with the wide range of 1.33-1.457, and a high resolution of 1.2×10^-3was obtained if the detecting resolution of the Bragg wavelength is 1pm. Two microstructure FBG fabricated by the coating method can be realized to measure the temperature and refractive index simultaneously.Last, FBG can be used to measure the gas concentration due to its tunable filtering characteristics. According to the gas absorption attenuation theory, test system for gas concentration measurement was set up. CO and C₂H₂ gas concentrations were investigated experimentally, and absorption characteristics and absorption spectrum peak were obtained. The experimental results demonstrated that peak wavelength of CO and C₂H₂ gas absorption spectrum is 1546.6 nm and 1531.52 nm, respectively. Because the FBG possesses advantage of wide tunable wavelength and easy multiplexing, many kinds of gases can be testified and gas concentration was measurement.According to the above investigation, this paper hopes that FBG measuring technique as a prospective sensing one can be induced to monitor the security of power equipments and obtain the optimum operation in power engineering field.