3.1. Box–Behnken Experimental Results and Analysis
Using the Design-Expert 8.0.6 software, 17 sets of experiments were designed, and the experimental scheme and results are shown in
Table 3.
x1,
x2, and
x3 represent the coded values for the width of the pressing wheel
B1, the spring deformation
, and the installation angle
, respectively. Meanwhile,
Y1 and
Y2 represent the SW and SC after the seeding operation, respectively.
The experimental results were subjected to multivariate regression analysis using the Design-Expert 8.0.6 software, resulting in the regression model for the SW and its variance analysis, as shown in
Table 4.
From
Table 4, it is evident that the regression model for the SW is highly significant (
p < 0.01), with a non-significant lack of fit (
p > 0.05), a coefficient of variation of 8.34%, a determination coefficient
R2 = 0.9703, and an adjusted determination coefficient
R2adj = 0.9321, indicating that the model fits well and is reliable. The data in
Table 4 were fitted through quadratic multiple regression, and the quadratic regression equation between each factor and the SW was obtained as follows:
The analysis of variance shows that x1, x2, and x3 had highly significant effects on the SW, the interaction term had no significant effect on the SW, and x12, x22, and x32 had highly significant effects on the SW. By comparing the F values, it can be seen that the influence of each factor on SW, from great to small, was as follows: x2 > x3 > x1.
The SC regression model’s analysis of variance results are shown in
Table 5.
From
Table 5, it is evident that the regression model for the SC is highly significant (
p < 0.01), with a non-significant lack of fit (
p > 0.05), a coefficient of variation of 2.14%, a determination coefficient
R2 = 0.972, and an adjusted determination coefficient
R2adj = 0.9361, indicating that the model fits well and is reliable. The data in
Table 5 were fitted through quadratic multiple regression, and the quadratic regression equation between each factor and the SC was obtained as follows:
The analysis of variance showed that x1 and x2 had highly significant effects on the SC, x3 had a significant effect on the SC, x1x2 and x2x3 had highly significant effects on the SC, x1x3 had no significant effect on the SC, x12 and x22 had significant effects on the SC, and x32 had no significant effect on the SC. By comparing the F values, it can be seen that the influence of each factor on the SC, from great to small, was as follows: x2 > x1 > x3.
In order to analyze the influence of the interaction term of each factor on the SC, the influence of the interaction between the width of the pressing wheel and the deformation of the spring on the SC was obtained under the condition that the installation angle was 15°, as shown in
Figure 9a. When the width of the pressing wheel was constant, the SC increased with the increase in the spring deformation. When the spring deformation was constant, the SC decreased with the increase in the width of the pressing wheel.
Under the condition that the width of the pressing wheel is 65 mm, the influence of the interaction between the spring deformation and the installation angle on the SC was obtained, as shown in
Figure 9b. When the spring deformation was constant, the SC increased with the increase in the installation angle. When the installation angle was constant, the SC increased with the increase in spring deformation.
The reason for this analysis is that, according to the calculation Formula (3) of the pressing wheel compression force, it can be inferred that adjusting the effective working length of the spring, that is, changing the spring deformation, can change the compression force. The greater the deformation of the spring, the greater the compression force of the pressing wheel on the soil, the better the compaction effect on the seed furrow soil, and the greater the SC. Therefore, within the test range, when the width of the pressing wheel and the installation angle were fixed, the SC increased with the increase in spring deformation. When the spring deformation and installation angle were constant, according to the calculation Formula (6) of the compression strength, the compression strength was inversely proportional to the width of the pressing wheel. The greater the width of the pressing wheel, the greater the contact area between the pressing wheel and the soil, the lower the compression strength, the weaker the compaction effect on the soil, and the lower the SC. Therefore, within the test range, the SC decreased with the increase in the width of the pressing wheel. When the width of the pressing wheel and the deformation of the spring were constant, with the increase in the installation angle, the squeezing effect of the pressing wheel on the soil was enhanced, the compaction effect was increased, and the SC was increased. Therefore, within the test range, SC increased with the increase in the installation angle.
3.2. Optimization of Parameters
To explore the best combination of experimental factors, with the minimum SW and the maximum SC as the optimization objectives within the target range, and taking into account the boundary conditions of each experimental factor, the established regression model was used for both optimization and solution. The objective function and constraints were as follows:
Using the Design-Expert 8.0.6 software for optimization and solution, the optimal results were obtained when the pressing wheel width was 60.57 mm, the spring deformation was 55.19 mm, and the installation angle was 10.70°, resulting in an SW of 18.75 g and an SC of 42.17 kPa.
In order to verify and analyze the operation performance of STCP, the operation effects of STCP and VCP were compared and analyzed. The soil-covering performance test, soil-covering thickness verification test, anti-adhesive performance test of pressing wheel, and seedling emergence verification test were carried out.
3.4. Comparative Test of Soil-Covering Thickness
3.4.1. Test Methods
After the soil-covering operation of the planter, if the weight of the covered soil is too large, the thickness of the covering soil will be too large after being pressed, and the sowing depth will be too deep. If the weight of the covered soil is too small, the thickness of the covering soil will be small after being pressed, and the sowing depth will be too shallow. Excessively deep or shallow sowing depths will affect the soil environment for seed growth and affect germination and emergence. According to the local corn planting agronomy, the sowing depth should be 40~50 mm. In order to verify whether the weight of the covered soil under the optimal parameter combination meets the requirements of agronomic production after pressing, field comparative tests were carried out. During the test, a star-tooth concave disc soil-covering device was installed in front of the VCP on the left side of the corn no-tillage precision planter, and the pressure of the two rows of VCP was adjusted to be consistent. After the sowing operation was completed, five seeds each in the left and right rows were randomly selected. The cross sections of the seeds in the furrow were gouged, and the sowing depth of the seeds was measured.
3.4.2. Test Results
The measurement results of the soil-covering thickness are shown in
Table 7.
Hs represents the measurement results of the soil-covering thickness after the operation of the “star-tooth concave disc soil-covering device + VCP”.
HV represents the measurement results of the soil-covering thickness after the operation of the VCP.
According to the test results, the measurement results of the soil-covering thickness after the operation of the “star-tooth concave disc soil covering device + VCP” and VCP were 46.8 mm and 38.9 mm, respectively, and the coefficients of variation were 5.82% and 20.08%, respectively.
Under clay conditions, after the installation of the star-tooth concave disc soil-covering device, the thickness of the covering soil meets the agricultural requirements, while the thickness of the covering soil of VCP is shallow. By comparing the coefficient of variation, it can be seen that the uniformity of the thickness of the covering soil can be obviously improved after the installation of the star-tooth concave disc soil-covering device.
Combined with the test results of soil covering performance, the reasons for the shallow soil covering thickness and poor consistency of VCP were analyzed.
In some places, the phenomenon of lax closure of seed furrows and soil adhesion of VCP’s pressing wheels appeared, resulting in the small weight of covered soil above the seeds, the shallow soil-covering thickness, and the poor uniformity of soil covering operation.
3.6. Validation Test of Seedling Emergence
In order to compare the performance of STCP and VCP, the field verification tests were carried out with the emergence rate of corn seeds as the evaluation index. The test conditions and equipment are the same as in
Section 3.5.1. The corn seed variety Tianyu 108 was selected, and the seeds were coated. When measuring the seedling emergence rate, from the first day of seedling emergence in the test field, a 50 m counting area was randomly selected in the test area to count the number of seedlings. The measurement time was 5:00 pm every day. The end time node of the test is that the emergence rate in the test area does not change. Three counting areas were randomly selected in the test field to calculate the average value of the emergence rate.
The measurement results of the emergence rate are shown in
Table 9.
Es represents the measurement results of seed emergence rate after STCP operation.
EV represents the measurement results of seed emergence rate after VCP operation.
According to the measurement results of seedling emergence rate, the seedling emergence rate of corn after STCP operation was 96.7%, the seedling emergence rate of corn after VCP operation was 92.8%, and the seedling emergence rate increased by 3.9%. The operation effect of STCP was better. The reason for the increase in seedling emergence rate was analyzed. Under the same operating conditions, compared with VCP, STCP solved the phenomenon of seed “hanging” and ”drying”, effectively improved the contact between soil and seeds, and provided a good growth environment for seed germination and seedling emergence.