Research On Multi-core Fiber Strain Vector Sensing Structure For High-speed Rail Track Safety Detection

In recent years,with the increasing mileage of high-speed rail operations,the motivation for higher speed and shorter departure interval are the two main trends for high-speed rail development,and correspondingly,which leads to urgent request on the track safety detection technology.Tracks are the infrastructure in high-speed rail equipment,and their safety plays a pivotal role in high-speed rail operations.Tracks,in the wild environment for a long term,face the challenges of complicated environment issue,high load issue and unexpected sudden accidents.With the climate,tracks are continuously eroded by the harsh environment,such as large temperature changing,high humidity,wind erosion and water erosion.The vibrating shock between high-speed train and tracks frequently causes the tracks to deform timely.Meantime,tracks are also facing the threat of some nature or sudden change of geological structure?e.g.geological collapse,earthquake?.The interaction of the above three issues will affect the safety of high-speed rail operations.Therefore,it is quite important to find the suitable technology to obtain the real-time track strain vector information on the whole day,so as to better understand the geometrical change process of the track and improve the early warning capability.The traditional mechanical or electrical detection methods have poor waterproof and moisture resistance,easily zero shift of sensor.Besides,it requires more sensors to obtain the strain vector information of tracks.It is difficult for such method to meet the long-term need for strain vector information detection of tracks.While optical fiber sensing has the advantages of strong anti-interference ability,suitable for field environment,easy network and high sensitivity.Therefore,the combination of optical fiber sensing technology and track safety detection has caused extensive research.The scheme of respectively detecting the multi-parameter of tracks through several single-core fibers,has the advantages of high fault tolerance and mature technology,while leading to more complex sensing system,sensor redundancy,information islands and other phenomena.For those described problems above,this thesis proposes the multi-core fiber strain vector sensing structure with specific arrangement structure,to execute online safety detection of high-speed rail and obtain the strain vector information of the track.This scheme not only reduces the number of sensors,optimizes the structure of the sensing system,but also judges the strain vector information of the track with arrangement structure,so as to improve the safety of tracks.The main work of this thesis includes:Firstly,this thesis introduces the importance and significance of high-speed rail track safety detection,and describes the current status and schemes of high-speed rail track safety detection technology.Aiming at the shortcomings and challenges of current detection technology,we propose to use the multi-core fiber strain vector sensing structure to detect the track strain vector information online.Secondly,the structure and characteristics of multi-core fiber are introduced,and the strain vector sensing theory of multi-core fiber is analyzed.Through the local curvature vector ?_app,i,the curvature and direction of strain are determined.The sensing mechanism of multi-core fiber based on Bragg grating is studied,discussing the influences of strain vector information on the center wavelength ?_i of multi-core fiber Bragg grating.The theory shows that there is close relationship between the center wavelength shift?_?i of the core and its arrangement position.The strain curvature is linear with the central wavelength shift ?_?i.The strain direction affects the strain curvature sensitivity,in which the maximum strain curvature sensitivity is 0.5029nm/m^-1.Then,the optical fiber strain vector sensing cable has been simulated by the ANSYS finite element analysis method.The relationship between fiber spacing D and sensitivity is analyzed,in which the sensitivity fitting degrees were 0.94 and 0.96,respectively.The simulation results show that the sensitivity descends with the increase of the fiber spacing D.Meantime,the multi-core fiber strain vector sensing cable,which has smaller core pitch?than above cable,has been researched through the same method.The analysis shows that the sensitivity ascends with the core pitch?descending.According to the strain distribution cloud map of the high-speed rail,the strain-sensitive point of the track is the joint between the rail head and waist.In order to install the multi-core optical fiber strain vector sensing cable to the track strain sensitive point,a track fixture based on fiber strain vector sensing is designed.Through the lever principle,the multi-core optical fiber strain vector sensing cable can be fixed to the strain sensitive point.Besides,the feasibility of the structure is verified by ANSYS finite element simulation.Next,the experiment of strain vector sensing based on the multi-core fiber Bragg grating is performed.For the uniformity of Bragg gratings on multi-core fibers,a novel rotation writing method of fiber Bragg grating in multi-core fiber is proposed.By reducing the time of a single write and increasing the number of rotations,a reflected signal with a low 3 dB bandwidth,high reflection intensity,and good uniformity is obtained.The strain vector sensing characteristics of multi-core fiber Bragg gratings are experimentally studied.The experimental results show that the multi-core fiber can better judge the variable vector information,and its law is consistent with the theory.The relative error of the strain curvature?is less than 7%,and the strain direction ?_b error is less than 2°.Finally,this work is summarized and the future work is prospected.