Reducing Force And Tillage Performance Of A Subsoiler Based On The Discrete Element Method(DEM)

Tillage is very important for agricultrual production. The moldboard system has been applied in China for a long time. It can augment the crop yields, but it can also cause some field problems like wind erotion, water erotion and plough pan. As a no tillage or minimum tillage technology, conservation tillage has been widely used around the world. Subsoiling is an important part of conservation tillage and the subsoiler is the tool mounted on the subsoiling machine used for loosening the soil. However, there are some problems in subsoiling like too large draft force, too much energy cost and unsatisfied tillage performance,which are the major constraint to the promotion of subsoiling technology. At this stage, no suitable optimizing methods can be applied on the design of subsoilers.In view of above problems, this study analyzed the working process of subsoilers based on the DEM. The main research contents and conclusions of this study are listed below.(1) Based on the soil characteristics in the field, established the model of soil using DEM by adding the parallel bond to the basic model. Parallel bond was added to the basic DEM model to simulate the liquid bridge between soil particles caused by the capillary water which has a large effect on soil resistance of subsoilers. Triaxial compression test using DEM was compared with the same experiment in the lab to calibrate the parameters of soil particles.Then the calibrated soil was used for simulating the subsoiler’s working process under the working depth of 180 mm to 260 mm and the speed of 1m·s-1. Field test was done under the same condition using this subsoiler, and the test result showed that the relative error of the DEM simulation was between 2.96% to 14.95% under this tillage condition, which showed that the DEM could reflect the sbsoiler’s working process accurately.(2) A bionic subsoiler with 300 mm maximum tillage depth was designed based on the structure of brown bear’s claw. The bionic subsoiler was compared with the standard arc-shaped subsoiler under the same working conditions and the force reducing effect of the bionic subsoiler was analyzed. What’s more, the reasons of bionic subsoiler’s reducing draft force and tillage performance were also determined. Based on the structure of a brown bear claw, the cruve of the claw was extracted and used for the curve of the subsoiler’s inside and outside edge, a bionic subsoiler was designed according to a standard arc-shaped subsoiler.The strength check was done using the finite element method for the designed subsoiler. To analyze the reducing force effect of the bionic subsoiler, the designed subsoiler was compared with the standard arc-shaped subsoiler in the soil bin test as well as the DEM simulation under the speed of 0.8m·s-1, depth of 220, 260 and 300 mm. The bionic subsoiler could reduce the draft force by 10.99% to 26.81% and the reducing force effect of subsoiler’s shank and point was analyzed separately. The drag reduction rate of the bionic point is 3.43% to 16.11% and the bionic shank is 7.82% to 12.75%. The soil movement, velocity field, contact force field and parallel-bond field of bionic subsoiler based on brown bear claw and standard arc-shaped subsoiler were compared, which showed that the tillage performance of two subsoilers is close to each other. There are two reasons that cause the large draft force of standard arc-shaped subsoiler: soil moving towards the centre of the circle leads to the soil piled up;more lifting effect than the bionic subsoiler.(3) The soil loosening process of a black bear claw was analyzed and the curve of the black bear claw at the rake angle of 30° was used for the design of bionic subsoilers. To obtain the structure of black bear claw, 3D scanning method was used in this study. The tillage performance of the black bear claw was studied under different rake angles(0°~45°)and working depths(3~18mm). This study showed that the draft force increased linearly with the rake angle while the vertical force increased in a quadratic function. The dislodged soil first increased slightly then decreased as the rake angle increasing. Tillage force, disloged soil and soil porosity change increased quickly as the working depth increased. By considering all factors mentioned above, the tillage performance index was used to evaluate the total tillage performance of the black bear claw and the results showed that best tillage point was when the rake angle equaled to 30°. Based on the above analysis, the curve of the black bear claw under 30° was extracted to design a bionic subsoiler according to the CASE subsoiler. The tillage performance of the bionic subsoiler and the CASE subsoiler was compared using DEM under the speed of 0.8m·s-1, depth of 350 mm and performance index showed that the bionic subsoiler’s tillage performance was 9.5 times of CASE subsoiler.(4) The wings’ effect on tillage performance was evaluated at the end of this study. The soil cutting efficiency was used for evaluating the tillage performance of winged and non-winged subsoiler. The winged subsoiler and the non-winged subsoiler were compared using DEM on the aspects of draft force and disturbing area under different rake angles(23°~40.5°) and working depths(225 mm~350 mm). Soil cutting efficiency was used here to evaluate the tillage performance of these two subsoilers. The non-winged subsoiler had the highest cutting efficiency when the rake angle was 33.5°, while the the rake angle had no significant effect on winged subsoiler. Adding wings to the subsoielr could increase thecutting efficiency after studying the two subsoilers under different depths. Both these two subsoilers had a higher cutting efficiency under the lower working depth.