Rail Strain Monitoring Technology Based On Fiber Bragg Gratings

The variations of rail strain take very important information about the operating railway lines. Fiber-Bragg-grating (FBG) is usually used in railway lines to detect the strain changes of rail track. The information of rail including train loads and temperature can be obtained by demodulating the strain information, and the status of railway lines can be mastered in real time. Consequently, the operation safety of railway lines can be ensured according to the obtained information. For the demand of rail strain monitoring, this paper explores a reliable and practical strain detection technology based on FBGFirstly, the principle of FBG for strain and temperature sensing are briefly introduced. Moreover, the simultaneous sensitivity (crossed sensitivity) between strain and temperature to FBG is discussed by analyzing the merits and shortages of several novel and typical solutions. Then, a set of packaging and fixing methods for FBG sensors in rail strain sensing system are investigated.Secondly, the strain distribution on a section of rail is studied when the concentrated load is increased at the vertical direction. Furthermore, an improved rail sensing scheme utilizing matched fiber-Bragg-gratings under bi-directional strains is proposed, which can increase the reliability and sensitivity of strain sensing detection and keep the advantages of traditional matching grating demodulation method. The rationality of scheme is verified by the detection of rail strain by using the resistance strain gauges and the analysis of fiber grating reflection spectrum measured by an optical spectrum analyzer when the rail is loaded. The advantages of this scheme are evident compared with the traditional matching grating demodulation method according to the experiments. In addition, a dynamic loading experiment is carried out, which imitates the periodic dynamic load from the running train’s wheels and verifies the feasibility of the proposed solution in practical application.Finally, a set of quasi distributed and instantaneous detection scheme to rail strain is proposed based on the principle of match grating strain demodulation, the foundation of optical time division multiplexing (TDM) and wavelength division multiplexing (WDM) technology. Experiments for detecting rail strain caused by vertical and axial loads are carried out via the quasi distributed real-time strain detection scheme with the armored FBG sensors. Compared with the results obtained from the resistance strain gauges, the experiments prove that the system can detect multipoint rail strain accurately, and has a higher response rate, which verifies its feasibility for the quasi distributed and instantaneous detection of rail strain.