连续单粒式谷物在线水分测定仪的设计与试验

为了提高谷物干燥设备自动化水平和干燥后谷物品质,提出一种基于电阻法检测原理,测量稻谷、小麦和大麦的连续单粒式谷物在线水分测定仪。其主要由谷物取样机构、谷物采样机构和信号采集电路等部分组成。通过测量谷物单粒外形尺寸统计出谷物等效粒径。运用谷物等效粒径和谷物与金属表面的静滑动摩擦角,计算确定谷物取样机构中不锈钢制异向正弦螺旋杆的中径和螺距分别为16和9 mm。由螺旋杆与分粒拨刀组成的谷物取样机构,在剔除杂物和多余谷物的同时,使谷物以连续单粒的形式进入进料口。选定模数为0.4 mm斜纹表面滚花形式碾压辊作为碾压电极,测量10%~35%含水率范围内稻谷、小麦和大麦单粒电阻值。构建稻谷、小麦和大麦的单粒阻值-含水率对应关系曲线并回归出水分计算函数(稻谷R~2=0.998;小麦R~2=0.999;大麦R2=0.999)。设计多路复用比例检测电路、二阶压控有源低通滤波器和50Hz陷波等信号处理电路。采用基于ARM Cortex TM-M3核的低功耗32位微处理器硬件和软件平台完成谷物水分数据的采样、处理和计算。现场水分在线检测与烘干法对比试验表明,在循环式谷物烘干机烘干过程-5~55℃的谷物温度和10%~35%含水率范围内,单粒式在线水分测定仪的在线水分测量绝对误差≤±0.4%,一次100粒谷物测量平均时间≤55s,水分测量重复误差≤±0.3%,研究结果为实现谷物烘干过程水分在线检测提供参考。

Design and experiment of continuous single grain typed on-line grain moisture test apparatus

Grain drying operation is an important part of food safe storage. In recent years, many scholars and engineers have done a lot of work on grain drying technology aiming to reduce energy consumption and realize automatic control of drying devices, but online moisture accurate detection in the grain drying that is core technique is still application difficulty to be solved, which restricts the development of grain drying system. Based on the analysis of the advantages and disadvantages of the technical principle of current online grain moisture meters, this paper presented a continuous single grain online moisture meter based on the water detection principle of resistance method. Physical dimensions of main varieties in main paddy, wheat and barley producing areas in China were measured, statistics of grain equivalent diameter were made, and pitch diameter and helix angle of stainless-steel incongruous sine screw rod were calculated by applying static sliding friction angle between single grain and metal surface. Therefore, the grain sampling device of moisture tester was designed, which was composed of a double-screw rod and a split grain knife. The grain sampling device screened out sundries and excess grain, and ensured single grain and continuity at the same time when sampling. According to the results of the experiment and simulation, 3 kinds of knurling forms with 0.4 mm modulus were chosen, including straight lines, oblique lines and net-mesh lines, and net-mesh lines had the optimal rolling smoothness among them, followed by straight lines knurling and oblique lines knurling. Considering the removal effect of adhesion on roller, the brushing effect of oblique lines knurling was the best, so the roller surface chose oblique lines form. Yangjing 239, Huaimai 33, and Yangnongpi 4 were chosen respectively as the test samples of paddy, wheat and barley, prepared as multiple samples with different moisture content. Meanwhile, when 105 ℃ drying method was adopted to test moisture content of each sample in the laboratory, the direct-current (DC) resistance of single grain with different moisture content compacted by the roller was measured by using universal electronic testing machine, programmable DC power supply with high precision, digital multimeter and other equipment; the steady resistance value was taken when the distance between the 2 rollers was 0.4 mm, and average resistance value was obtained after testing 100 samples for the samples with the same moisture content. Regression curves between the grain moisture content and single grain resistance of paddy, wheat and barley were built according to the experimental data. The calculation function expression of grain moisture content was obtained through polynomial regression on curve. Multiplex proportional circuit was designed, providing possibility to solve precise detection of extremely wide resistance of specific volume ranging from 10-2 to 1010 ?·cm. Second order voltage-controlled active low-pass filter based on four-channel amplifier AD8643, and 50 Hz band-stop filter circuit consisting of operational amplifier LM358 were designed to meet the electromagnetic compatibility requirements for on-line grain moisture tester detection. Hardware and software platforms of ARM CortexTM-M3 core's low power 32-bit microprocessor STM32L151CBQ6 were applied to finish both the sampling of grain moisture and the design of processing program. Online detection comparison tests on site showed that, when the drying temperature ranged from -5 to 55 ℃ and the moisture content ranged from 10% to 35% for paddy, wheat and barley, the online moisture measurement accuracy of the tester was less than ±0.5%, which satisfied online moisture detection requirements for the drying of the 3 kinds of grains. The design method can provide a reference for the online detection of water in the drying process of grain.