Study On Microstrip Antenna Sensors And Analysis Model For Grain Moisture Determination

Microwave detection technology has been widely used in the field of grain moisture determination by its unique advantages such as strong penetration,wide detection range,and multiple derived parameters.However,some key technical issues,such as the cooperative working mechanism between microwave devices and flexible electronic technology,the cooperative working mechanism between microwave sensors and measured grains,and the measurement system based on multiple microwave parameters,still need to be further studied.This paper focuses on the research of microwave moisture measurement technology and designs a microwave detection structure for grain moisture measurement,namely a slot microstrip patch antenna sensor.The dielectric property changes of grain under different moisture under microwave were tested and analyzed.A microwave electromagnetic perturbation model was established.Multiple derived parameters were selected to accurately characterize the moisture model combined with a multivariate linear regression algorithm.The main research contents of this paper are as follows:（1）The microstrip antenna sensor is designed,and the heterogeneous integration technology of microwave detection structure and flexible substrate is studied,including electrode screen printing and heat treatment.Firstly,a microstrip patch antenna sensor with a slot structure is proposed.Secondly,the conventional microstrip patch antenna sensor was prepared by the wet etching process.In order to realize the cross-fusion of microwave devices and flexible electronic technology,the flexible microstrip patch antenna sensor was prepared by the screen printing process.Then,the compatible technology of microwave sensor and flexible substrate is studied,the preparation scheme of silver conductive ink is improved,and the influence of four sintering methods of the heating plate,reflow welding,muffle furnace,and vacuum tube furnace on the surface morphology and electrical characteristics of the silver conductive film is explored.The test results show that the ratio of high-frequency power to low-frequency power of the sintered silver conductive film after the heating plate is 0.0205,the square resistance value is 1.080Ω/sq,the silver conductor conductivity is high and the variance of the finished product resistance value is small.Finally,a flexible antenna was printed on the polyimide substrate and tested for its flexibility.The test results show that the flexible antenna has good flexibility based on the resonant frequency f_res,echo loss S₁₁,and the standard deviationσof??being 0.84 MHz,0.1d B,and 0.023°,respectively.It provides reliable guidance for improving the performance of microwave moisture measurement sensors.（2）The microwave antenna sensor was applied to the determination of grain moisture,and the characterization mechanism of microwave parameters such as resonant frequency,return loss,and phase was studied.Ten rice and wheat samples with moisture content ranging from 9%to 18%were prepared by water conditioning.The grain moisture content was determined by conventional and flexible microstrip antennas.The test results show that with the increase of water content,the resonant frequency f_res and the operating frequency at the phase lowest point f_φmin of the microstrip antenna move to low frequency and the return loss S₁₁ increases.The best characterization parameter of rice based on conventional microstrip antenna is f_φmin,the fitting linearity R² is 0.97,and the minimum Mean Relative Error（MRE）is 0.004%.The best characterization parameter of wheat based on a conventional microstrip antenna is S₁₁,whose fitting linearity R² is 0.97,and the minimum MRE is 0.004%.The best characterization parameter of rice based on the flexible microstrip antenna is f_φmin,the fitting linearity R² is 0.97,and the minimum MRE is 2×10^-7.The best characterization parameter of wheat based on a flexible microstrip antenna is f_res,whose fitting linearity is R²≈0.99,and the minimum MRE is 9.106×10^-8.The results above confirm that conventional and flexible microstrip antennas can be used for the determination of grain moisture.（3）The effects of grain density,impurity,and detection height on microwave moisture measurement were studied,and a multivariable linear regression model was established to achieve a high-accuracy grain moisture prediction model.The results showed that the sensitivity of low body density rice was higher,f_φmin is the best characterization parameter of rice,its fitting linearity R² is 0.98,and the minimum MRE is 0.012%.S₁₁ is the best characterization parameter of wheat with fitting linearity R²≈0.99 and minimum MRE 0.003%.The detection sensitivity of the antenna is higher for undoped grain samples.f_φmin and f_res are the best characterization parameters to evaluate the doping of rice and wheat,respectively.The fitting linearity of rice is R²≈0.97 and the minimum MRE is 0.004%,and the fitting linearity of wheat is R²≈0.99 and the minimum MRE is 0.006%.The antenna has the highest detection sensitivity at the detection height of 30 mm,S₁₁ is the best characterization parameter,rice R²is 0.77,and the minimum MRE is 0.016%.Wheat R² is 0.93,and the minimum MRE is 0.03%.In addition,in view of the fact that currently,most studies on microwave moisture measurement adopt a single-parameter method with limited characterization ability and poor robustness,in order to show the advantages of the diversity of microwave detection parameters,this paper combined with multivariable linear regression algorithm extracted resonant frequency,return loss and phase lowest point frequency,and constructed a moisture analysis model by linear fitting combination.The results showed that the mean square error of the rice and wheat water content models was 0.0052%and 0.0094%,respectively.It was proved that the multivariate linear regression analysis model was a forecasting model with higher accuracy and more universality than the univariate analysis model.