4.4.1. Model Establishment and Significance Verification

According to the sample data in Table 5, the regression models are established in Equations (5)–(7) as follows:

$$\begin{array}{l} Y\_1 &= 4.98 + 0.48X\_1 + 0.42X\_2 + 4.93X\_3 + 2.62X\_4 + 0.28X\_1X\_2\\ &- 0.46X\_1X\_3 + 0.13X\_1X\_4 + 0.90X\_2X\_3 + 0.82X\_2X\_4\\ &+ 1.69X\_3X\_4 + 0.018X\_1^2 + 0.14X\_2^2 + 1.67X\_3^2 + 0.28X\_4^2 \end{array} \tag{5}$$

$$\begin{array}{l} \text{Y}\_{2} = 1.74 + 0.093X\_{1} + 1.30X\_{2} - 0.20X\_{3} - 0.14X\_{4} + 0.040X\_{1}X\_{2} \\ + 0.072X\_{1}X\_{3} - 0.0075X\_{1}X\_{4} - 0.073X\_{2}X\_{3} - 0.13X\_{2}X\_{4} \\ - 0.04X\_{3}X\_{4} - 0.1X\_{1}^{2} - 0.14X\_{2}^{2} + 0.065X\_{3}^{2} - 0.11X\_{4}^{2} \end{array} \tag{6}$$

$$\begin{array}{l} \text{Y}\_3 = 1.38 + 0.21X\_1 - 0.20X\_2 - 0.29X\_3 + 0.81X\_4 + 0.05X\_1X\_2\\ - 0.085X\_1X\_3 - 0.03X\_1X\_4 - 0.093X\_2X\_3 - 0.17X\_2X\_4\\ - 0.16X\_3X\_4 + 0.025X\_1^2 - 0.074X\_2^2 + 0.18X\_3^2 + 0.10X\_4^2 \end{array} \tag{7}$$

Through the analysis of Table 6, the regression models of seed loss rate, *Y*1, breakage rate, *Y*2, and impurity rate, *Y*3, show that *p* < 0.0001. Moreover, the significance of each variable influencing the index in the regression equations is judged by the *F* value. In other words, the smaller the probability value *P* is, the higher the significance of the corresponding variable is. It can be seen from the *F* value of each factor that rotation speed of the cleaning fan and the scale sieve's opening have a significant effect on seed loss rate. The descending order of influencing factors is rotation speed of the cleaning fan > scale sieve's opening > machine forward speed > rotation speed of the threshing drum. Meanwhile, rotation speed of the cleaning fan and threshing drum has a significant effect on seed breakage rate, and the descending order is rotation speed of the threshing drum > rotation speed of the cleaning fan > scale sieve's opening > machine forward speed. In addition, all the four factors have a significant effect on seed loss rate, and the descending order is scale sieve's opening > rotation speed of the cleaning fan > machine forward speed > rotation speed of the threshing drum. The determination coefficients, *R*2, of *Y*1, *Y*<sup>2</sup> and *Y*<sup>3</sup> are 0.9580, 0.9723 and 0.975, respectively, showing that the model error is small, and it is reasonable to analyze and predict seed loss rate, breakage rate and impurity rate of the Chinese milk vetch seed combine harvester.

#### 4.4.2. Effect Analysis of Interaction Factors on Harvest Indexes

Based on the established optimization regression model, the influence of machine forward speed, rotation speed of the threshing drum, rotation speed of the cleaning fan and scale sieve's opening on the harvest quality index of the Chinese milk vetch seed combine harvester and the relationship among the factors were analyzed.

The response surfaces of seed loss rate are shown in Figure 10. In Figure 10a, the rotation speed of the threshing drum and scale sieve's opening were both zero, and the increase of machine forward speed had little influence on the seed loss rate when the rotation speed of the cleaning fan was fixed, and when machine forward speed was constant, seed loss rate gradually increased with the increase of the rotation speed of the cleaning fan. In Figure 10b, the machine forward speed was set at a low level, the rotation speed of the cleaning fan was set at a low level, when the value of threshing drum rotation speed was fixed, the seed loss rate increased with the increase of the scale sieve's opening, and when the scale sieve's opening was constant, the rotation speed of the threshing drum had little influence on the seed loss rate. Cause analysis: when the rotation speed of the cleaning fan and the scale sieve's opening increased, the air volume passing through the surface of the scale sieve in unit time would increase, and more seeds on the sieve surface would be blown away by the airflow; meanwhile, seeds that were prepared to pass the cleaning sieve would be blown out of the machine as well, resulting in the increase of seed loss rate.

**Figure 10.** Response surfaces of seed loss rate.

The response surfaces of seed breakage rate are shown in Figure 11. Machine forward speed and the rotation speed of the cleaning fan were both zero, as shown in Figure 11a, when the scale sieve's opening was fixed, and the faster the rotation speed of the threshing

drum was, the higher the seed breakage rate was; when the rotation speed of threshing drum was constant, the scale sieve's opening had little influence on the seed loss rate. In Figure 11b, the scale sieve's opening and the rotation speed of the threshing drum were both zero, and when machine forward speed was fixed, the seed breakage rate decreased slightly with the increase of cleaning fan rotation speed; when rotation speed of the cleaning fan was constant, machine forward speed had little influence on the seed breakage rate. Cause analysis: the faster the rotation speed of the threshing drum, the harder the threshing element struck Chinese milk vetch seed, and the higher the possibility of seed breakage. At the beginning, with the increase in the cleaning fan rotation speed, the fan mainly blew away most of the impurities, and the broken seeds flowed into the seed bin through the round-hole screen, and seed breakage rate changed little. However, with further increase of the cleaning fan rotation speed, part of the damaged seeds were blown away, while some complete seeds on the surface of the sieve were also blown away by the fan. According to calculation Formula (3), the breakage rate increased correspondingly.

**Figure 11.** Response surfaces of seed breakage rate.

The response surfaces of seed impurity rate are shown in Figure 12. Rotation speed of the threshing drum and rotation speed of the cleaning fan were both zero, as shown in Figure 12a, when machine forward speed was constant, and the larger the scale sieve's opening was, the higher the seed impurity rate was; when the scale sieve's opening was fixed, with the increase of machine forward speed, the impurity content of seed also increased slightly. In Figure 12b, the machine forward speed was set at a low level, and the scale sieve's opening was set at zero. When the rotation speed of the threshing drum was fixed, seed impurity rate decreased with the increase of cleaning fan rotation speed; when rotation speed of the cleaning fan was constant, the faster the rotation speed of threshing drum was, the smaller the seed impurity rate would be. Cause analysis: when the scale sieve's opening increased, most of the short stalks fell onto the round-hole screen from the gap of the sieve pieces, the number of stalks entering the round-hole sieve surface increased and the screening efficiency decreased, leading to the increase of impurity content. In addition, the lower the rotation speed of the cleaning fan was, the more surplus stalks would fall from the sieve pieces before they were blown away by the fan, resulting in incomplete cleaning separation and higher seed impurity content.

**Figure 12.** Response surfaces of seed impurity rate.

4.4.3. Parameter Optimization and Verification Test

In this paper, in order to meet requirements of lowest seed loss rate, lowest seed breakage rate and lowest seed impurity rate during the harvest of Chinese milk vetch, parameter optimization of the Chinese milk vetch green manure seed combine harvester

was carried out. Design-Expert data analysis software was used to optimize the established total factor quadratic regression model of the three indexes. Constraint conditions were established in Equation (8) as follows:

$$\begin{cases} \min Y\_1\\ \min Y\_2\\ \min Y\_3\\ \text{s.t.} \begin{cases} 3 \text{ km} \cdot \text{h}^{-1} \le X\_1 \le 5 \text{ km} \cdot \text{h}^{-1} \\ 550 \text{ r} \cdot \text{min}^{-1} \le X\_2 \le 800 \text{ r} \cdot \text{min}^{-1} \\ 900 \text{ r} \cdot \text{min}^{-1} \le X\_3 \le 1260 \text{ r} \cdot \text{min}^{-1} \\ 35 \text{ mm} \le X\_4 \le 45 \text{ mm} \end{cases} \end{cases} \tag{8}$$

The optimal parameter combination of the Chinese milk vetch seed combine harvester was obtained as follows: the machine forward speed was 3 km·h−1, the rotation speed of the threshing drum was 553.45 r·min−1, the rotation speed of the cleaning fan was 991.77 r·min−<sup>1</sup> and the scale sieve's opening was 35 mm, then, the seed loss rate, breakage rate and impurity rate predicted by the model were 2.40%, 0.19% and 0.48%, respectively.

In order to verify the accuracy of the above model, the validation test was carried out in Qingyijiang Town, Nanling County, Wuhu City, 17–18 May 2020. Considering the feasibility of the test parameters, the optimized parameters were adjusted so that machine forward speed was 3 km·h−1, the rotation speed of the threshing drum was 550 r·min−1, the rotation speed of the cleaning fan was 990 r·min−<sup>1</sup> and the scale sieve's opening was 35 mm. Three tests were carried out and the average value was taken as the test verification value. According to the mechanical industry standard of the People's Republic of China (JB/T 11912-2014), the loss rate and damage rate are required to be less than 5%, and the impurity rate must be less than 3%. The seed loss rate, breakage rate and impurity rate measured in the experiment were 2.35%, 0.22% and 0.51%, respectively, which were all lower than the standard.
