*4.3. Experiment Scheme*

According to the previous literature search, field experimental research and harvest experience [25,26], taking the loss rate, breakage rate and impurity rate of seeds as the evaluation bases of the operation performance of the Chinese milk vetch seed combine harvester, machine forward speed, rotation speed of the roller, rotation speed of the cleaning fan and the scale sieve's opening were selected as the test variables to carry out the experimental research. Box–Behnken central composite design theory was used to carry out a four-factor three-level quadratic regression response surface test with a total of 29 groups. The test site was about 120 m long and 50 m wide, and the test length of each group was 30 m. Samples were taken for three times in each group after the harvester ran stably.

Experiment factors and codes are shown in Table 4. Among them, *X*1~*X*<sup>4</sup> represented the variables of each factor. According to the design parameters of the seed combine harvester, the relationship between the biological characteristics of Chinese milk vetch during harvest and the threshing and cleaning parameters of the harvester, the value range of each factor was determined, the median value of machine forward speed was set as 4 km·h−<sup>1</sup> and the low-speed value of 3 km·h−<sup>1</sup> and high-speed value of 5 km·h−<sup>1</sup> were selected accordingly. According to the threshing ability characteristics of Chinese milk vetch and referring to the rotation speed of the threshing drum for wheat, soybean and other crops, the middle value of rotation speed of the threshing drum was set as 675 r·min<sup>−</sup>1, the minimum rotation speed of the threshing drum was 550 r·min−<sup>1</sup> and the maximum rotation speed was 800 r·min−1. According to the simulation results of the centrifugal fan in the early stage, meanwhile, referring to the test method and standard of seed suspension speed of grain, oil, forage grass, etc., [27], the minimum rotation speed of the cleaning fan was set as 900 r·min−1, the maximum rotation speed was 1260 r·min−<sup>1</sup> and the intermediate value of 1080 r·min−<sup>1</sup> was determined. Based on external dimensions of pods and seeds of Chinese milk vetch varieties, the designed opening range of the scale

sieve was from 35 mm to 45 mm, the middle value was 40 mm and the corresponding limit values of the opening range were taken as high- and low-level values.

**Table 4.** Experiment factors and codes.


The adjustment methods of each parameter in the test were as follows: (1) Machine forward speed adjustment: The driver adjusted the infinitely variable gear to get different forward speeds. The forward speed was accurately detected by the speed sensor installed on the rear wheel and displayed on the instrument panel in the cab. (2) Rotation speed adjustments of the threshing drum and the cleaning fan: the V-belt with continuously variable transmission was adopted, and the moving plate of the belt wheel was adjusted by hydrostatic and manual operation so as to obtain different transmission ratios and different rotation speeds. (3) Scale sieve's opening adjustment: through the position rotation of the adjusting plate installed on the sieve frame, the scale's opening could be adjusted, and the specific scale's opening could be measured by ruler.

### *4.4. Experiment Results and Analysis*

The experimental design, data processing and statistical analysis were conducted by the software Design-Expert V8.0.6.1. The four-factor and three-level responsive section experiment was carried out according to the Box–Behnken design method, which is to select 29 points, including 24 analysis factors and 5 zero estimation errors. The experimental design and response results are shown in Table 5, and the variance analysis of the regression model is conducted in Table 6.

**Table 5.** Experiment design and response values.



**Table 6.** Variance analysis of regression model.

Note: \*\* indicates that the factors have a significant influence on the test index (0.01< *p* ≤0.05), \*\*\* indicates that the factors have a very significant influence on the test index (*p* ≤ 0.01).
