Research On Soil Moisture Measurement Method Of Long Period Fiber Grating

Soil moisture monitoring is essential to the development of modern agriculture and is an indispensable part of achieving precision agriculture.The principles of traditional soil moisture monitoring methods are different,and have different limitations,such as the disadvantages of large destructiveness,strong radiation,poor real-time performance,and small range.The long-period fiber grating has more and more applications in the field of intelligent sensor monitoring due to its advantages such as corrosion resistance,small size,and good sensing performance.This paper first analyzes the basic sensing theory of long-period fiber grating,and further studies the cross-sensitivity characteristics of long-period fiber grating.On this basis,the measurement mechanism of soil moisture sensing based on long-period fiber grating is studied.The water-sensitive material converts the moisture information into the change of the coating refractive index,and transmits it to resonant wavelength through the refractive index characteristic,so as to achieve the soil moisture measuring.Simultaneously,the characteristics of long-period fiber grating soil moisture sensing were simulated and studied,which provided a theoretical basis for the subsequent design and manufacture of sensors.Secondly,based on the measurement mechanism of the soil moisture sensor and the moisture-sensing characteristics of the water-sensitive material,it was determined to use the polymer polyvinyl alcohol(PVA)as the water-sensitive material,and at the same time,the characteristics of the water-sensitive material were analyzed.Combined with the characteristics of water-sensitive materials,through the selection and optimization of different coating processes,an automatic lifting coating method was designed to realize the coating of long-period fiber grating water-sensitive materials.Then,according to the specific requirements of the use environment,the package structure of the soil moisture measurement sensor combined with desensitization packaging and protective packaging is proposed,and the temperature compensation method is studied,the temperature compensation scheme based on the reference fiber grating method is determined.Finally,build a bare fiber grating performance test experimental system,test the temperature and humidity characteristics of the bare fiber grating coated with PVA,and then build a long-period fiber grating soil moisture sensor test system.First,soil samples were prepared using soil water content by weight and FDR voltage type soil moisture measuring instrument to obtain the temperature and humidity characteristics of the sensor.Then perform temperature compensation according to the reference fiber grating method,compare the humidity characteristics before and after compensation,analyze the temperature compensation effect,and test and analyze the sensor repeatability and stability.It can be seen from the experimental data that the long-period fiber grating soil moisture sensor designed in this paper has a use range of 30%to 70%and a sensitivity of 41.51pm/%The experimental results show that,compared with the traditional soil moisture sensor,the long-period fiber grating soil moisture sensor designed in this paper not only has high sensitivity and wide measurement range,but also can form a soil moisture sensor monitoring network to achieve distributed real-time monitoring and has broad development space.