Study On Measurement Technology Of Grain Moisture And Temperature Based On Combining Microwave And Acoustic Method

Moisture and temperature are important parameters for grain storage.Ensuring grain moisture and temperature in an appropriate range is an important way to keep the stored grain in a safe condition.Currently,the measurement of grain moisture content is time-consuming and laborious.Although new methods,such as infrared method,resonant cavity perturbation method,and capacitance method,that can achieve online measurement,most of them have limited sensor region and the results are lack of representativeness.The microwave has good permeability,and it can be applied to a wide region of grain moisture measurement.However,the measured results are affected by bulk density and temperature.Furthermore,the large number of temperature sensors used in the grain pile will seriously interfere with the microwave signal,which limits the microwave method being adopted.As a result,the microwave method for measuring grain moisture needs to be combined with a non-invasive temperature measurement method.The acoustic method is a non-invasive temperature measurement method,and the acoustic wave has good penetrability as well.Therefore,it is reasonable to combine the microwave method and the acoustic method to measure grain moisture and temperature.The main research work and innovations are as follows:1.Study on the grain complex permittivity measuring method.Based on simulation analysis,the constraints of designing the open-ended coaxial probe for grain were studied.It was found that the radius of the sensing region was needed to be at least 2.5 times the length of the grain kernel.In addition,the radius of the sensing region increased linearly with the increase of the radius of the inner conductor in the frequence range of 1 to 5 GHz.Secondly,a complex permittivity probe for wheat,which length was of less than 10mm,was designed.Considering the influence of the sample holder on the electromagnetic field,an analytical model of the probe was established based on the mode matching method.Finally,calculating the reflection coefficient of the probe from the complex permittivity can be solved by the least square method,and the inverse problem can be solved by sweeping narrow-band method and the convolutional neural network.2.Establish the model to describe the relationship among grain moisture content,temperature,bulk density,and complex permittivity.Firstly,twelve white winter wheat samples in the moisture content(wet basis)range of 1.1%to 25.4%were taken as examples.Their complex permittivities were measured in the temperature range of 15～35℃ and the frequency range of 1～5 GHz.The results showed that in the frequency range of 1～5 GHz,the dielectric constant decreased with the increase of the frequency,and the changes of loss factor with frequency was related to the moisture content(wet basis).For wheat with a moisture content(wet basis)under 7.9%,the loss factor decreased with the increase of the frequency.However,the loss factor of wheat with a moisture content(wet basis)greater than 7.9%increased with the increase of the frequency.When the frequency was fixed,the dielectric constant of wheat increased with the increase of water content(wet basis),and the loss factor tended to firstly increase slowly and then increase rapidly,and finally increase slowly with the increase of water content(wet basis).Both the dielectric constant and the loss factor of wheat increased with the increase of temperature.Secondly,wheat was regarded as a mixture of dry matter,bound water,free water,and air.The wheat complex permittivity model was established,and it fitted the experimental data well(R2>0.99).The effect of temperature on the complex permittivity model of wheat can be corrected by a set of linear functions.Finally,the moisture content(wet basis)model and the bulk density model were obtained by decoupling the complex permittivity model,and they fitted the experimental data well.The R2 of the moisuture content model was greater than 0.99,and the R2 of the bulk density model was greater than 0.97.3.Establish the model to describe the relationship among the moisture content,temperature,bulk density,and acoustic wave velocity.Firstly,the acoustic wave velocity of 12 groups of wheat samples with the moisture content between 1.1%and 25.4%were measured in three different bulk density,in the frequency range from 500 Hz to 1500 Hz,and in the temperature range from 15℃ to 25℃.It was found that the acoustic velocity and attenuation coefficient increased with the increase of the frequency.The higher the frequency,the faster the acoustic wave,and the greater the attenuation.This trend can be fitted with quadratic functions.At the same frequency,the velocity decreased first and then increased with the increase of the moisture content(wet basis).This trend can be fitted by a quadratic function.At the same moisture content(wet basis),the velocity increased with the increase of the temperature,and the velocity decreased with the increase of the bulk density.Secondly,A model describing the relationship among acoustic wave velocity,moisture content,temperature,and bulk density was proposed,and the model fitted the experimental data well(R2→0.97).Finally,the average pores’ size of wheat was estimated by the JCA(Johnson-Champoux-Allard)model and particle swarm algorithm.The effect of the moisture content on the average pores’ size was analyzed.The results showed that the average pores’size trended to slightly decrease first and then increase dramatically with the increase of the moisture content.4.A novel method for measuring grain moisture content,temperature,and bulk density simultaneously based on combining the microwave and acoustic method was proposed.In order to solve the problem that the grain moisture,temperature and bulk density have an interactive effect on electronic sensors,the time delay and the attenuation of microwave signals,and the time delay of the acoustic wave were measured by using microwave and acoustic methods.Combined with the complex permittivity model of grain and the velocity model of the acoustic wave in grain,the moisture content,temperature,and bulk density can be calculated from the complex permittivity and the velocity.On this basis,the experimental device with a micro-grain pile was designed to verify the performance of the proposed method.The methods of estimating the time delay of the acoustic wave,measuring the attenuation and the time delay of the microwave signals,calibrating experimental device,and calibrating models were discussed.Finally,considering the common moisture content range of stored wheat,12 groups of wheat samples with different moisture content(11.8%～17.2%,wet bases),temperature(25～35℃),and bulk density(737～790 mg/cm3)were taken to test the performance of the proposed method.The experimental results showed that the absolute error of moisture content was less than 0.7%,the absolute error of temperature was less than 0.9℃,and the absolute error of bulk density was less than 10 mg/cm.