Moisture content is the major parameter that affects the quality of grain storage, transportation, and processing. Bulk density and multiple reflections of electromagnetic waves are the two main interference factors that affect the accuracy of the microwave method of grain moisture content detection. In this work, a free space traveling-standing wave method is proposed for eliminating interference. A microwave radar measurement system was developed for the rapid, non-destructive, and high-precision detection of grain moisture content. The moisture content density-independence equations are established based on the phase shift and attenuation of the traveling-standing wave node. For soybean and wheat, the determination coefficients of moisture content calibration equations are 0.995 and 0.975, respectively, and the standard errors of prediction are 0.564% and 0.983%. The system can complete the measurement in a few seconds, and the portable structure enables the device to be flexibly applied to the field of industry and agriculture.

Moisture content is an important index to assess the grain quality and food processing conditions. A measurement system based on the traveling–standing microwave attenuation method is designed for a fast and nondestructive grain moisture content determination. The proposed system consists of a microwave cavity oscillator, microwave transmitting and receiving horn antennas, microwave detector, slide rail, sample container, weight sensor, temperature sensor, and controller. The traveling–standing wave caused by free space microwave multiple reflection is discussed. The moisture content calibration functions eliminated the interference of bulk density and temperature are proposed based on the attenuation of the maximum field strength of the transmission traveling–standing wave. The moisture content of rice, which ranges from 10.75% to 27.62%, is obtained with a standard error of prediction (SEP) of 0.586% and a coefficient of determination (R2) of 0.988, whereas the moisture content of corn, which ranges from 7.72% to 24.46%, is obtained with a SEP of 0.340% and R2 of 0.991. The main results might provide technical support for the development of accurate and intelligent grain quality detection equipment.

蔬菜含水率是影响蔬菜品质的重要指标之一.针对传统含水率检测方法测量精度低、费时费力等问题,该研究提出了一种基于微波空间驻波法的蔬菜含水率预测方法,研发了行驻波雷达测量系统.装置包括微波振荡器、微波发射及接收天线、检波器、样品夹持器、滑轨及控制器.以常温贮存的绿叶白菜、生菜为研究对象,采用X波段微波,对测量过程中形成的空间行驻波进行分析,通过线性回归分析建立蔬菜含水率预测模型.测量结果表明,白菜、生菜含水率预测方程决定系数R2分别为0.9920和0.9919,预测结果与直接干燥法相比,均方根误差RMSE分别为1.188％和0.803％,性能标准误差SEP分别为1.071％和1.179％.该研究方法不受微波在空间中的多重反射影响,装置结构简单,单次测量时间小于10 s,能够实现对蔬菜含水率的快速、无损、高精度检测.研究结果为农产品检测相关应用提供参考.