Soil Moisture Content Measurement Method Based On The Heated Fiber Bragg Grating

As an emerging sensing technology,optical fiber sensing has been widely used in the field of road and bridge monitoring.At present,quasi-distributed Fiber Bragg Grating(FBG)is mainly used for strain monitoring of civil bridge engineering.In engineering,it is often necessary to measure the change of soil moisture content to prevent the occurrence of disasters.However,the commonly used electrical moisture content sensing equipment is bulky,time-consuming and has low accuracy.Therefore,researchers began to use the temperature-sensitive characteristics of fiber gratings to make actively heating fiber grating sensors,which are used in a series of fields such as dam seepage and slope drainage that need to measure soil moisture content.The core idea of the actively heating optical fiber method(AHFO)of soil moisture content monitoring is to use the different thermal conductivity of soil with different moisture content,and to establish the relationship between the measured temperature and the soil moisture content according to the diffusion effect of heat in the soil.Calibrate the relationship to determine the moisture content of the soil.In order to obtain an accurate calibration relationship,it is necessary to measure the temperature change during the heat transfer process as much as possible,while avoiding the disturbance of the soil caused by heating.The calibration relationship is revised by changing the heating time.The work of this thesis is mainly divided into two parts:monitoring of soil moisture content above 0℃and fiber grating demodulation.The specific content is as follows:(1)A sensor based on internally heated FBG alumina ceramic tube is designed.The electric heating wire is energized by the DC power supply to realize the symmetrical internal heating function of the sensor,and combined with high temperature glue to seal capillary tubes of different materials between the grating and the ceramic tube,the thermal conductivity of the sensor in the soil volumetric water content test is changed,thereby improve the temperature response of FBG.The calibration relationship between the measured temperature and the volumetric water content is obtained by fitting.The correlation coefficient of the sensor with the capillary steel tube inside is 0.98,and that of the capillary plastic tube is 0.99.The continuous measurement of the volumetric water content of 2 cm~3/cm~3 to 30 cm~3/cm~3 in the soil environment above 0℃is realized.A method for testing the volumetric water content of soils heated FBG in a short time is proposed.This test method avoids the disturbance of the soil environment caused by the long-term heating of the original test method,and the correlation between the temperature rise and the volume water content in the obtained calibration relationship is better,and the measurement accuracy of the volume water content by the active heating fiber method is improved.(2)A wavelength demodulation method of fiber grating based on tunable Fabary-Perot(F-P)filter is designed.In this method,the STM32 single-chip microcomputer controls the tunable F-P filter to scan the reflected light of the fiber grating.After photoelectric conversion of the fiber grating Bragg wavelength light that meets the FP interference condition,the voltage signal is collected and processed by STM32 to obtain the center wavelength of the FBG.This demodulation method has established a certain foundation for the further engineering application of FBG soil moisture sensor.