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

Abstract: 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 Ω·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.