Research On Infrastructure Monitoring Method Based On Novel Overlapping Spectrum Demodulation Technology Of Fbgs

Fiber Bragg grating(FBG)technology boasts high accuracy,small size,corrosion resistance,and electromagnetic interference resistance,making it a popular choice for structural health monitoring in civil engineering,aerospace engineering,mechanical engineering and other fields.However,the single-fiber multiplexing capacity of traditional FBG wavelength division multiplexing sensing technology is limited,making it only suitable for quasi-distributed monitoring of small structures or local monitoring of large structures.For full-scale distributed monitoring of large and ultra-long structures,the monitoring performance is obviously insufficient.Overlapping spectrum demodulation methods for FBGs can improve the multiplexing capacity of FBG wavelength division multiplexing sensing networks,which is significant for the application of FBGs in large and ultra-long structure health monitoring.This paper aims to study novel strategies and methods for demodulating overlapping spectra of fiber Bragg gratings(FBGs).These strategies and methods can be used to improve the multiplexing capacity of FBGs with the same or similar wavelengths,and further enhance the monitoring distance and density of FBG sensing technology.In addition,novel infrastructure monitoring methods based on the aforementioned strategies and methods were investigated.The main contents of this paper are as follows:(1)Aiming to address the issues of the few existing overlapping spectrum demodulation models for series FBGs and the low demodulation accuracy of existing methods,a novel overlapping spectrum demodulation method for series FBGs was investigated.A novel overlapping spectrum demodulation model for series FBGs was established through theoretical analysis.Through numerical simulation and theoretical analysis,the effects of spectral attenuation,shadow crosstalk,and multiple reflection crosstalk on the demodulation accuracy of the proposed model were investigated.Additionally,a high-speed demodulation system for overlapping spectrum and independent spectrum of FBGs based on the FBGA demodulation module was proposed.A suitable algorithm for extracting the wavelength of the independent reflection spectrum of FBG was determined,and the feasibility of the demodulation system used for monitoring common structural parameters such as temperature,strain,and vibration was verified.Finally,experiments were conducted to verify the feasibility of the proposed model and system for demodulating the overlapping spectra of series FBGs.(2)To address the issues of low demodulation accuracy and slow demodulation speed in existing overlapping spectrum demodulation methods of series FBGs,an optimization method was proposed for solving the overlapping reflection spectrum of series FBGs.The weighted differential evolution algorithm(WDE),an improved version of the differential evolution algorithm,was utilized to enhance the speed and accuracy of model demodulation and increase the number of FBGs in the overlapping spectrum.Experimental results have validated the feasibility of the proposed method,which was also applied to demodulate the overlapping spectrum of series and parallel ultra-weak fiber Bragg gratings(UWFBG).(3)A high-speed demodulation method of the overlapping spectrum of FBGs based on bidirectional long short-term memory(Bi-LSTM)network was proposed to address the slow demodulation speed of the overlapping spectrum of FBGs.The principle of the proposed method and the acquisition method of training data were presented.The influence of the amount of training data and the number of network hidden units on the demodulation accuracy and training time of the model were studied through numerical simulations,and the demodulation time of the model was analyzed.Finally,the feasibility of the proposed method was verified through experiments.(4)A novel monitoring method of bridge scour based on the overlapping spectrum of UWFBGs was proposed.This approach aims to address the shortcomings of existing monitoring methods for bridge scour.The monitoring principle of the proposed method was described,and a new distributed sensor array suitable for bridge scour monitoring was designed.A laboratory simulation experiment was then designed and carried out to verify the feasibility of the proposed method.Finally,the difference of sensor signals before and after scouring was analyzed,and the applicable scope,advantages and limitations of the proposed method were discussed.(5)An anomaly signal monitoring method based on the overlapping spectral similarity of FBG was proposed.This method addresses the problems of sparse measurement points and easy-to-miss detection in existing FBG sensing technology for long-distance structural anomaly monitoring.And this method was applied to pipeline leakage monitoring.Firstly,the sensing system and abnormal signal monitoring principle were described,and the distributed abnormal signal evaluation method was proposed.Then,the principle of pipeline leakage monitoring method based on the proposed abnormal signal monitoring method was given,and a special sensing optical cable for leakage monitoring was designed.Finally,the feasibility of the proposed method was verified by laboratory simulation experiments.And the influence of temperature difference and the number of FBGs affected by temperature in the monitoring unit on the monitoring signal were explored.The calculation formula of the monitoring warning value under different temperature difference was obtained by fitting.In addition,to address the problem of low accuracy in leak location using the proposed method,the proposed method for optimizing the overlapping spectrum of series FBGs was used to achieve accurate location of small flow leaks.