Design Of Wheat Straw Moisture Meter Based On AC Impedance Method

Straw is not only an important crop by-production, but also an important agriculturalproduction resource. Detection of straw moisture is of great significance for the efficientutilization of straw resources. Oven drying method is traditional method for detection ofmoisture of straw, although the method is of stable measurement and high accuracy, it haslimitations in the need of quick measurement of moisture of straw. At present, the existingtesting instruments for rice and wheat straw moisture is mainly the introduction of foreignproducts and the price is relatively high. So, it is quite necessary to develop a straw moisturemeter with low price, simple and convenient operation, quick and continuous measurement.Based on the electric properties of agricultural production measurement of moisture and ACimpedance method, a wheat straw moisture meter was designed with8-bit MCU AT89S52ascontrol core. The moisture prediction model was used to realize the detection of wheat strawmoisture. The main research contents and achievements are as follows:(1) The hardware system was designed in this paper. Taking AT89S52as the controlcore, complex impedance detection and processing module, pressure detection and processingmodule, temperature detection module, A/D conversion module, MCU control module, inputand output module and power supply module were designed.(2) The software system which was written by C language was developed, includingmain program, range switching subprogram, A/D conversion subprogram, temperatureacquisition subprogram, liquid crystal display subprogram, keyboard identificationsubprogram and moisture calculating subprogram.(3) The range and accuracy of complex impedance, pressure and temperature werevalidated. In the range of4~20kΩ、20~100kΩ, the accuracy of complex detection is±0.6kΩ、±3.2kΩ. In the range of0.7~69.3kPa, the accuracy of pressure detection is±5.2kPa.In the range of5~40℃, the accuracy of temperature detection is±0.2℃.(4) The straw of wheat was used as sample to study the influence of moisture content oncomplex impedance. The tests were at five levels (10.4%,12.5%,15.2%,17.6%and19.7%inwet basis), temperature range from5℃to40℃with5℃interval, and three bulk densitylevels (generally from75.3kg/m3to101.3kg/m3). The model describing complex impedanceand main factors was regressed. Newton iteration method was applied to program for predicting moisture content from obtained data. The model’s feasibility in predicting moisturecontent from10.0%~20.0%at5℃~40℃was verified. The result shows that the absoluteerror, and response time for predicting moisture content of wheat straw from obtainedcomplex impedance and temperature is-2.3%~0.9%and less3s, respectively.