## 2 Materials and Methods  
### Overall Structure and Working Principle  
Figure 1 shows the structure of the tested pneumatic drum seed metering device. The lugs at the ends fasten the air intake plate to the frame. One end of the drum is fitted with this plate to enable relative rotation, with the other embedded in the end cover and a rubber sealing ring at each end. The end cover is connected to the drive shaft with a flat key and transfers the shaft's rotational motion. During the device's operation, the air intake plate remains stationary, and the drive shaft drives the end cover, which in turn drives the drum to rotate. We used a suction hole structure in the form of a seed suction nozzle with a conical flow channel inside, as shown in Figure 2. The seed suction nozzles were installed in the through holes processed on the surface of the drum. Figure 1: Pneumatic drum seed metering device: (**a**) explosion diagram of the drum; (**b**) overall structure and working principle; (**c**) working principle of the air intake plate; (**d**) bench; (1) drive shaft; (2) end cover; (3) drum; (4) rubber sealing ring; (5) air intake plate; (5-1) negative pressure port; (5-2) pressure isolation roller; (5-3) positive pressure port; (6) seed box; (7) seed cleaning device; (8) seed recycling bin; (9) plug tray; (10) electrical machinery; (11) fan; and (12) air compressor. Given the requirement for the drum's operational linear speed to be below 0.35 m/s, its diameter was set at 120 mm after considering rotational speed and seeding efficiency. A 128-hole plug tray (8 \(\times\) 16) was employed in the experiments of this study (as shown in Figure 3), so the number of axial suction hole rows of the drum was set to 8, which corresponds to the holes in the plug tray. The number of circumferential suction holes of the drum was set to 16 (evenly distributed at an interval of 22.5\({}^{\circ}\)), so that the 128-hole plug tray can be filled after one revolution of the drum. As shown in Figure 1c, the air intake plate consists of three parts: negative pressure port, pressure isolation roller, and positive pressure port. The negative pressure port is connected to the fan, enabling the fan to exhaust air to create a vacuum inside the drum. The pressure isolation roller clings to the inner wall of the drum to isolate the vacuum pressure. The air outlets set below the roller correspond to the suction holes. The positive-pressure air flow from the air compressor enters the inner cavity of the roller through the positive pressure port and then is ejected through the air outlets, blowing off the seeds. Figure 4 shows that the function unit of the seed cleaning device is equipped with three air inlets on its outer side. Eight rows of air outlets are evenly arranged on the inner side of the function unit, corresponding to the layout of the axial suction holes of the drum. The function unit is installed on the frame through the connecting piece to achieve fixation. As shown in Figure 1b, the positive-pressure air flow from the air compressor enters the inner cavity of the function unit through the air inlet and then is ejected from the air outlets, blowing off the extra seeds. The working principle of the pneumatic drum seed metering device (as shown in Figure 1b): The fan exhausts air through the negative pressure port, creating a vacuum inside the drum. In the seed filling area, the pressure difference pulls seeds into moving suction holes as they pass the seed box. After finishing the seed filling, the drum carries the seeds into the seed cleaning area, where the extra seeds are removed under the effect of  
Figure 3: The 128-hole plug tray. Figure 2: Seed suction nozzle: (a) structure; (b) internal conical flow channel. (\(D_{h}\) and \(D_{b}\) denote suction hole diameter and major diameter of the conical flow channel, respectively). gravity and positive-pressure air flow from the seed cleaning device. Completing the seed cleaning, the drum with seeds enters the seed dropping area.