| In the industries of grain storage and transportation, food processing,building, paper making, textile, tobacco, soil science, wood processing,the moisture content of materials have important effects on productquality and performance. The related methods and characteristics ofmicrowave measurement technology were analyzed in the paper. The fastnondestructive testing method and device of moisture content were studiedunder different circumstances using microwave free-space technology.The influence factors of dielectric property of matter in microwavefrequency were studied. Water is a polar molecule. Under the effect ofelectric field in high frequency, the polarization and orientation willoccur which can result in energy storage and attenuation. After thespatial interaction between microwave and material, microwave reflectioncoefficient, transmission coefficient, power attenuation, standing wavephase, standing wave ratio and other parameters will change. Based on themeasurement of the parameters and the design of the correspondingdetection device, the moisture content, bulk density, relative dielectricconstant, and other physical properties of materials can be detected.After microwave transmission through a certain thickness of material,the relationship between the moisture content of materials and energyattenuation is analyzed. A transmission type moisture detection deviceis designed. The device consists of microwave signal unit, isolator,attenuator, microwave transmitting antenna, the sample container,microwave antenna, microwave detector, the signal processing unit andoutput display control unit, etc. The function of each unit and thespecific design method were instructed. Through the selection andmatching of known moisture content of sand, rice and corn as a sample,experiments are conducted by microwave transmission device. Theexperimental results were analyzed and according to the different material and different thickness to get the best calibration curve. Forthe moisture content of10.1%to28.5%of the rice, the best fitting ofthe calibration equation is obtained when the sample thickness is5cm.The fitting degree is0.991and the calibration standard error is0.284%.For the moisture content of12.3%to27.3%of the corn, the best fittingof the calibration equation is obtained when the sample thickness is3cm. The fitting degree is0.990and the calibration standard error is0.204%.For the moisture content of0%to11.3%of the sand, the bestfitting of the calibration equation is obtained when the sample thicknessis3cm. The fitting degree is0.991and the calibration standard erroris0.285%.In the process of transmission type moisture content detection devicemeasuring, the sample thickness needs to be determined. The experimentcannot achieve accurate measurement when the sample moisture content ishigher which results in larger energy attenuation. According to the abovesituation, the microwave wave reflection coefficient and the change ofthe dielectric properties of medium ware studied in the paper. Areflection type moisture content detection device is designed. The deviceconsists of microwave signal unit, isolator, attenuator, mixer, microwavetransceiver reuse antenna, the signal processing unit, etc. The workingprinciple of microwave frequency mixing unit and the specific designscheme were analyzed in the paper. The mixing structure was introducedto reuse the microwave antenna transceiver, which makes the reflectiondevice has the advantages of simplified structure, cost reduction,improving the measurement precision and sensitivity. The structure designof device and the specific processing schemes are analyzed and illustrated.The sensor probe was designed by closed stainless steel cylindricalstructure. Probe port material was selection of alumina ceramic structure.Microwave transmitting and receiving unit is enclosed within the sensor. For the moisture content of0%to15%of sand, the fitting degree is0.993and the calibration standard error is0.39%.In order to get the best accuracy of measurement, when the moisturetest was conducted by the transmission or reflection way, the measuredmaterials need to pile up tight and uniform distribution. The samplestacking density and the temperature change is the direct factorsaffecting the measurement results. According to the above situation, theeffects of granular material density and temperature changes impacted onthe dielectric properties of samples were studied. A kind of moisturecontent and bulk density synchronous measurement device was designedbased on the reflection standing wave detection. The device consists ofmicrowave cavity oscillator, transceiver horn antenna, slippery course,sample container, microwave, mixing unit, and voltage detection units,etc. The microwave free-space reflection principle of standing wave andmeasuring method were studied. Based on the measurement of standing waveamplitude ratio and phase change, sample moisture content and bulk densitysynchronous detection model was established. For the sand and rice samples,the standard error of moisture content is0.506%and0.695%respectively,and the standard error of bulk density is0.0382g/cm3and0.0144g/cm3respectively. The fit of the moisture content measurement calibrationequation is greater than0.982, the fit of the bulk density measurementcalibration equation is greater than0.961. The sample materials need notdeliberately compaction, and the device is suitable for industrial onlinenon-destructive testing applications.The signal processing circuit and PC software were designed. Thesystem hardware circuit including power supply module, the zeroamplifying circuit, single-chip microcomputer, liquid crystal displayscreen, a serial port module, keys module, etc. The PC software wasdeveloped using the labview virtual instrument. For sample types, moisture content, measurement time and other information, the devicerealized the following functions as real-time monitoring, trend display,analog output, serial port communication, data storage and control, andother functions. |
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