Parameter optimization and experiment of negative pressure precision seed-metering device for wheat

Abstract: Different from traditional cultivation techniques of sowing in lines for wheat, precision seeding technology ensures the most reasonable distribution of wheat seeds in the field through precisely controlling seeding quantity and uniformity. It makes each grain of wheat seed get enough nutrition area and space, absorb water at the most and possess effective tillering. So it ensures stable and high yield in seed-saving, fertilizer-saving and water-saving conditions. In order to meet the agronomic requirements of precision seeding technology, a negative pressure precision seed-metering device for wheat was designed. The seed-metering device was not a traditional single structure, but multiple seed-metering devices shared a hollow shaft. By the CFD (computational fluid dynamics) software STAR-CCM+, the influences of structural parameters of the seed-metering device on vacuum chamber fluid field were simulated and analyzed. The structural parameters included the diameter of seed-metering disc, the width of suction slit, the cross-section shape of ring groove, and the axial depth of gas chamber in the seed-metering disc. The analysis of pressure nephogram, velocity vectogram and streamline graph showed that more ideal structural parameters of seed-metering device are 0.5 mm width of suction slit, 150-200 mm diameter of seed-metering disc, 2.0 mm axial depth of gas chamber in the seed-metering disc, and arc-shaped cross-section of ring groove. Seeding uniformity test of the seed-metering device was done on the JPS-12 test-bed. It tested and analyzed the influence of cross-section shape of ring groove, width of suction slit, diameter of seed-metering disc and axial depth of gas chamber in the seed-metering disc on the qualified index, multiple index, missing index and coefficient of variation of qualified seed spacing. The qualified index of the seed-metering device with arc-shaped cross-section of ring groove is about 10% higher than the v-shaped and inverted-trapezoid-shaped structure, and about 17% higher than the straight-groove-shaped structure. Meanwhile, its multiple index (8.29%) and missing index (5.29%) are the minimum. The influence of the suction slit width on the seeding uniformity is very visible. In certain vacuum conditions, the decrease of suction slit width causes insufficient suction, which affects the uniformity of suction seed. For example, the missing index and the qualified index of seed-metering device with 0.3 mm suction slit respectively are 21.88% and 69.79%. The seeding uniformity of the seed-metering device with less than 200 mm disc diameter meets precision seeding technology agronomic requirements. The influence of seed-metering disc diameter on the seeding uniformity is very little. But when the disc diameter is larger than 250 mm, the missing index (20.94%) clearly increases. The main reason is that the outer edge of the seed-metering disc with large disc diameter has higher linear speed, which causes the filling seed performance of seed-metering device to reduce. The axial depth of gas chamber in the seed-metering disc is not less than 2.0 mm, or its missing index (18.32%) and multiple index (10.26%) significantly increase and its qualified index (71.43%) decreases. The better structural parameters of the seed-metering device were determined as arc-shaped cross-section of ring groove, 0.5-0.8 mm width of suction slit, 170-200 mm diameter of seed-metering disc and 2.0 mm axial depth of gas chamber in the seed-metering disc. Based on these, structural parameters of the seed-metering device were optimized: The diameter of seed-metering disc is 180 mm, the width of suction slit is 0.7 mm, the axial depth of gas chamber in the seed-metering disc is 2.5 mm, and the cross-section shape of ring groove is arc-shaped. The optimization seed-metering device was tested on the JPS-12 test-bed. The qualified index is 86.66%, the missing index is 5.09%, the multiple index is 8.25%, and the coefficient of variation of qualified seed spacing is 24.50%. These testing results fully coincide with the standard JB/T 10293-2013 Specifications of single seed drill (precision drill). The seed-metering device meets fully the requirements of wheat precision drilling.