Research On Optical Fiber Temperature And Humidity Sensing Technology For Grain Storage

The food security is related to the stability of the country and society,and the safety of food storage has a direct impact on food security.The current granary has higher and higher requirements for sensors.The traditional temperature and humidity sensors with large volume,cumbersome operation and low precision are gradually being phased out,while intrinsically safe,variable shape,anti-electromagnetic interference,large-capacity networking and Optical fiber temperature and humidity sensors for distributed measurement have been widely concerned.The main research contents and problems solved in this thesis are:1.In the actual application process of the tunable laser wavelength demodulation method,it will be affected by the ambient temperature,resulting in an overall drift of the demodulation wavelength.The demodulation errors of the demodulator under different operating temperatures are detected,the causes and results of the errors are analyzed,and the demodulation system is temperature compensated by polynomial fitting.2.Aiming at polyimide and polyvinyl alcohol,the two most commonly used humidity-sensitive materials for humidity sensor fabrication,fiber grating humidity sensors based on two different materials were fabricated by dip coating process.Through the humidity response test of the humidity sensor of two different materials,it is found that the sensor based on PI humidity sensitive material has better sensitivity and humidity response characteristics.The humidity sensor based on PVA material is more suitable for applications in the high humidity range.3.The influence of fiber cladding diameter on the sensor performance was studied.Fiber gratings with different cladding diameters were prepared by etching,and then PI was used as a humidity-sensitive material to coat a humidity sensor.Through the humidity response test of the sensor,it is found that the smaller the diameter of the fiber cladding,the better its humidity response characteristics.Based on this characteristic,several fiber grating humidity sensors with different cladding diameters were prepared,and their performance was nearly 30% higher than that of ordinary fiber grating sensors.4.A single grating semi-coated temperature and humidity sensor is proposed.The sensing mechanism of the single grating semi-coated temperature and humidity sensor and the fabrication process of the sensor are introduced in detail.The tests verified the sensor’s parameters such as temperature characteristics,humidity characteristics,repeatability,response time and measurement accuracy.5.Build a simple simulated granary environment to verify the practicability of the fiber grating temperature and humidity sensor.The sensors are arranged at different depths of the grain pile to monitor the temperature and humidity changes at different depths of the grain pile.The obtained temperature and humidity information was combined with the static equilibrium moisture theory to analyze and predict the grain moisture content in the grain pile,which was verified with the standard value of grain moisture.