*2.5. Box–Behnken Experimental Design*

The experiment was conducted in October 2022 in a hazelnut orchard in the Heishan region of Liaoning Province, China, as shown in Figure 7.

The experiment was designed according to GB/T5667-2008 "Agricultural Machinery Production Test Methods" and with reference to the GB/T5262-2008 "General Provisions for the Determination of Test Conditions of Agricultural Machinery" standard. Each group of tests collected 1 m2 of fallen hazelnut surface, and each group was repeated three times, setting the picking mouth 20 mm above the ground, taking the average value as the test results recorded and analyzed, with the net fruit rate of hazelnut as the evaluation index. After the first harvest the overall weight in the hazelnut collection box was weighed as *Mtotal*, the weight of hazelnuts was manually selected as *MHazel*. Then, the total mass of the hazelnut collection box and the miscellaneous collection box in the hazelnut picking and sorting machine was weighed as *Mpick*, the surface of the plot was harvested manually again using manual harvesting, and the mass of the manual harvesting was *Mmiss*. The hazelnut net fruit rate *Y* was calculated by the following equation.

$$Y = M\_{H
u zcl} / M\_{total} \times 100\%$$

The factors influencing the hazelnut harvester were selected and combined with the previous CFD-DEM simulation analysis of the sieve plate angle, the distance of the sieve plate, and the air flow velocity factors. The sieve plate angle was set at 51–59◦, the distance of the sieve plate was 17–46 mm, and the air flow velocity was 9–18 m·s−1, and a threefactor, three-level Box–Behnken test was conducted. The experimental factor coding table is shown in Table 3.

**Figure 7.** Machine field trials: (**a**) hazelnut picking and sorting test; (**b**) harvester operating site. **Table 3.** Experimental factor and level table.

