| The research and design of seed-metering device mainly have two kinds of methods. One is the design method and theory based on the continuum mechanics method, but it considers granular particles as a continuous whole, as a result it can not analyze the movement process of every particle and interactions between particles, so it can not solve problems existing during designing seed-metering device well. Another method is according to the experience and trial, whereas usually it is not only time-consuming but also strenuous without ideal results.The discrete element method (DEM) proposed by Cundall in seventies of the 20th century is popular method to research particles dynamics problems. The interactions between granular materials (such as soybean seeds) and seed-metering and the flow process of seeds are studied by the DEM, which can be used to estimate the working performance of seed-metering, such as the rate of single seed, the rate of empty cave and the rate of destroyed seeds etc., and also to simulate and evaluate the dynamic working process of seed-metering device. Thus the seed-metering device of new theory can be found, however, it can not be realized by the existing design methods and technologies.In this paper, the research status of soybean seed-metering device and the apply of DEM in the design of the soybean seed-metering device were summarized; the physical and mechanical properties of soybean were determined by using apparatus and home-made devices; two kinds of soybean seed-metering device(combination concave cell corn precision seed- metering device, hole-wheel seed-metering device) were test by indoor test, and the seed performance and the movement of soybean were analyzed; the work process of seed-metering device was simulated and analyzed by soft of distinct element method based on the boundary element of CAD models which was developed by research group, and the results of simulation were compared with the results of test,in order to a new method of design soybean seed-metering device was explored.The focal points of the main work discussed in this paper are as follows:(1) According to the need of DEM analysis, the physical and mechanical properties of soybeans were determined and analysis, including moisture content, three-dimensional size, thousand grain mass, kernel density, friction coefficients, stiffness coefficients, rupture force, coefficients of restitution and elastic modulus of soybeans. The result are as follows:①The moisture content of the original species of JiKeDou, JiXinDou, JiDou are 9.2%, 9.1%, 9.4%.②Thousand grain mass of JiXinDou was largest, followed by that of JiKeDou, and JiDou's thousand grain mass of the smallest; The distribution of three-dimensional size of three kinds soybeans basically approximate normal distribution. Three soybeans approximate the shape of ellipsoid, the shape of JiKeDou is largest, followed by that of JiXinDou, and the shape of JiDou is smallest; The spherical rate of JiKeDou, JiXinDou and JiDou is high, indicating the three soybeans closer to sphere. Therefore, when DEM simulation used, in addition to soybeans can be assumed to be oval or elliptical, but also can be assumed to be circle or sphere.③The density of JiDou is largest, followed by that of JiXinDou, JiKeDou's density of the smallest.④The friction coefficients of Soybeans change with the moisture content of soybeans changes. The dynamic friction coefficients and static friction coefficients are influenced greatly by the moisture content of soybeans, with the moisture content increased, dynamic friction coefficients and static friction coefficients increase rapidly.⑤Under the same conditions, the stiffness coefficients and rupture force of the JiXinDou and JiDou are slightly larger than that of JiKeDou. This may be related to the structure and components of soybeans, grain size and shape, and the specific reasons for their further studies.At the time of extrusion, the placed manner of soybean have a great influence to the stiffness coefficients and rupture force of soybean. Stiffness coefficients and rupture force of the soybean was flated is larger than that of soybean be placed on side or be let stand, and soybean be placed is no less than soybean be let stand.Loading speed have a certain influence to the stiffness coefficient and rupture force, but the influence is little. With the loading speed increased, stiffness coefficient and rupture force increase slightly.The stiffness coefficient and rupture force of soybeans are changed with the moisture content of soybeans changes. The stiffness coefficient and rupture force are influenced greatly by the moisture content of soybeans, with the moisture content increased, stiffness coefficient and rupture force decrease rapidly. This phenomenon produceded is because with the moisture content increased, the soybean become "softer", which weaken the ability of resist deformation.⑥Different collision surfaces have different coefficients of restitution. Under the same conditions, coefficient of restitution of soybean and iron and coefficient of restitution of soybean and plexiglass were larger than that of soybean and plastic.Different kinds of soybeans have different coefficients of restitution. Under the same conditions, coefficients of restitution of JiDou is the largest, followed by that of JiXinDou, and that of JiKouDou of the smallest.Different drop heights have different coefficients of restitution. With the drop height increased, the coefficients of restitution of soybeans decrease. The reason is that the higher drop height, the greater speed of collision, the more energy lost in the process of collision.The coefficients of restitution of soybeans change with the moisture content of soybeans changes. As the moisture content of the soybeans increase, the coefficients of restitution of soybean decrease. ⑦The elastic modulus of JiXinDou and JiDou is larger than that of JiKouDou. As the moisture content of the soybean increase, the elastic modulus of soybeans decrease rapidly. The main reason is that as the moisture content increase, the soybean become "softer", which weaken the ability of resist deformation.(2) Through the bench test, two kinds of soybean seed-metering devices in the working proces was observed and analyzed by high-speed video technology, and the performance of seed-metering device and the movement of seeds were analyzed. The result are as follows:①As the combination concave cell corn precision seed- metering device, the initial angle and terminal angle of clear seeds are influenced greatly by seed-round speed, and the angle of filling seeds and cast seeds are influenced slightly by seed-round speed. With the seed-round speed increased, the initial angle and terminal angle of clear seeds and the angle of filling seeds increase, and the angle of cast seeds decrease.The stacking angle changes slightly with the seed-round speed changes. With the seed-round speed increased, moisture content increased, the stacking angle increased slightly, and under the same speed, there are no difference in the stacking angle of three soybeans.At the range of test speed, this seed-metering device has a better performance of work. When the rotational speed is less than 52r/min, the single-particle rate of the seed-metering is 100%, and two-particle rate is 0. When the speed reached 65.43r/min, the single-particle rate descrease and double-particles rate increase. This shows that as the speed increased, the performance of the clearance of the seed-metering device became worse. When the rotational speed is less than 65.43r/min, the hole rate of the seed-metering device is 0, indicating that the performace of the filling of the seed-metering device is very good.The speed of seed in filling seed point, protceting seed point and casting seed piont is influenced greatly by rotational speed, with the speed increased, the speed of seeds increase; the speed of seed in the group point is influednced slightly, at the range of the test seed-round speed, the speed of seed fluctuate in a certain range. And the kind of soybean have no difference to the speed of seed.The trajectory of the seed in filling seed point and the trajectory of the seed in group point is irregular curve, and the trajectory of casting is parabola. With the seed-round speed increased, the opening of parabola is larger.②For the hole-wheel seed-metering device, the passing rate of sowing , the repeat rate of sowing and the leakage rat of sowing are influenced significantly by the rotational speed. With the rotational speed increased, the passing rate of sowing decrease, and the leakage rat of sowing increase, and the repeat rate of sowing increased first and then decreased.Observed by the test, we can see that the movement of seeds inmetering device in cavity of seed-metering deviceis not intense, and have a layer seeds closed seed-round move back and forth in a certain range with the rotating of seed-round, this layer named towing seed layer. The movement of seeds in the layer is influenced greatly by seed-round speed, with the speed increased, the speed of seeds increase, and the speed of seeds in casting seed point is also increase.The track of seed in "towing seed layer" is irreguler curve,but its movement back and forth within a certain range.The track of casting seed is similar to straight-line, which is influenced greatly by seed-round speed. With the seed-round speed increased, the angle of track and the horizontal line decrease.(3) The work process of seed-metering device was simulated and analyzed by soft of distinct element method based on the boundary element of CAD models which was developed by research group, and the results of simulation were compared with the results of test, although there is a certain degree of error, but their trend of change is the same. Therefore DEM can validly simulate the movement of seeds in working seed-metering, and it provides a kind of new method for the design and performance evaluation of seed-metering machines. |
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