Study Of The Soil Moisture Measurement Based On Fiber Bragg Grating

The monitoring and control of soil moisture is the priority among priorities of agricultural production,soil moisture is essential to the survival of crop nutrients.Traditional methods of soil moisture measurement are mostly electronic,with resistance,capacitance,thermocouple,TDR,FDR and so on,but they all have their limitations.They can only be measured by location,which is not conducive to large area distributed real-time measurement.The fiber grating has its own unique advantages in the sensing measurement,and optical fiber grating sensing technology has been widely used in modern intelligent monitoring field,so the soil moisture measurement based on fiber Bragg grating is studied in this paper.Firstly,the research progress of FBG sensing technique and traditional soil moisture measurement method at home and abroad is introduced,the theory of temperature and strain sensing of fiber Bragg grating is expounded.On this basis,the cross sensitivity of fiber Bragg gratings is systematically analyzed.The sensing mechanism of fiber Bragg grating for soil moisture sensing is studied,and the spectral simulation of soil moisture sensing property of fiber Bragg grating is carried out.The simulation results of the relationship between temperature and humidity and the center wavelength of fiber grating are obtained.At the same time,the reflection spectrum of fiber grating with or without cross sensitivity is simulated.Secondly,adsorption mechanism and performance of water sensitive material requirements are studied in this paper based on the theory of polymer adsorption,then determine the polymer materials of polyimide(PI)as a water sensitive material,and the water sensitivity of PI is studied as well.Then,the synthesis mechanism of polyimide and the coating process of water sensitive material are studied.The polyimide of water sensitive material is successfully coated on the fiber Bragg grating.In addition,the encapsulation structure of the fiber Bragg grating soil moisture sensor is designed,and the concrete implementation scheme and temperature compensation scheme of packaging design are determined according to the basic application requirements of the sensor environment.Finally,an experimental system of fiber Bragg grating soil moisture measurement is set up,as well,the temperature and humidity characteristics and response time,reproducibility and other performance indicators of bare fiber grating with PI coating and the fiber Bragg grating soil moisture sensor designed in this paper are tested by experiment under different conditions.Through data analysis,we can see that the moisture sensor based on fiber Bragg grating is sensitive in the range of 5%RH-75%RH of soil moisture.After the temperature compensation,the humidity sensitivity coefficient is 8.75pm/%RH.The results show that,compared with the traditional probe type soil moisture sensor,the soil moisture measurement based on fiber Bragg grating studied in this paper can achieve real-time monitoring of soil moisture and have good application prospect.The fiber Bragg grating soil moisture sensor has the advantages of corrosion resistance,fast response,high precision,small volume,excellent characteristics of passivity,and can be applied to water conservancy test system,realizing the implementation of distributed real-time monitoring of soil moisture.