## 2 Design of Structure and Main Parameters
### Seed Feeding Component  
The seed feeding component is a key component that affects the stability of seed discharge [16; 17; 18]. As shown in Figure 2, the redesigned seed feeding component in this aerial seeding system consists of a seed filling chamber, a seed gear disc, and a seed delivery tube. Seeds drop from the seed box to the seed filling chamber via gravity, proceed to fill into the gaps between adjacent racks of the gear disc, and form a seed layer. The seed feeding motor rotates the gear disc and pushes the seeds into the vertical channel of the seed delivery tube. The seeds falling into the channel are carried upwards into the distributor by the airflow generated by the fan.  
Figure 1: An aerial seeding system for rapeseed based on a multi-rotor UAV and an air-assisted centralized metering device: (**a**) Overall structure; (**b**) Seed supply module and real device; (**c**) Seed distributing and guiding module.  
The process can be divided into four steps: seed filling, seed carrying, seed cleaning, and seed dropping. The seed gear disc not only transfers the seeds from the chamber to the vertical tube continuously and quantitatively, but also prevents the airflow from going back to the chamber and seed box.  
To guarantee the number of seeds per unit area, more seeds should be sown when the UAV works at a higher flying speed, and a higher rotational speed of the gear disc/seed feeding motor should be set. A formula was used to calculate the seeding quantity as follows:  
\[G_{Y}\geq\frac{GBV_{f}}{10} \tag{1}\]  
where \(G_{Y}\) is the seeding quantity per second (g/s); \(G\) is the quantity of seeds per unit area according to the agronomic requirements (kg/hm\({}^{2}\)); \(B\) is the working width of the aerial seeding system (m); and \(V_{f}\) is the flying speed of the UAV (m/s).  
According to the agronomic requirements introduced in Section 2.1, \(G\) is 6 kg/hm\({}^{2}\), since \(B\) is 2.1 m, and if we choose \(V_{f}\) to be 5 m/s, then we get \(G_{Y}\geq 6.35\) g/s. This means that the seed feeding component should deliver 6.35 g seeds per second or 381.00 g per minute at least.  
To calculate the seed feeding quantity according to the structure parameters of the gear disc and the rotational speed of the feeding motor, we proposed another formula as follows:  
\[G_{S}=\rho t\Big{[}\pi\big{(}R^{2}-r^{2}\big{)}-Zh(R-r)\Big{]}N/\big{(}6\times 1 0^{7}\big{)}\geq G_{Y} \tag{2}\]  
Figure 2: Seed feeding component. (**a**) Schematic diagram (t is the axial thickness of the seed gear disc); (**b**) Seed gear disc schematic (R is the outer diameter of the seed gear disc, r is the inner diameter of the seed gear disc, and h is the gear thickness). (**c**) Real seed gear disc.  
where \(G_{S}\) is the theoretical seed feeding quantity per second; \(N\) is the rotational speed of the gear disc; \(\rho\) is the density of the seed bulk; \(\pi\) is the mathematical constant \(Pi\); \(t\) is the thickness of the gear disc; \(Z\) is the number of racks or gaps; and \(R\), \(r\), and \(h\) are the structure parameters as shown in Figure 2.  
A small-size, lightweight but high-torque digital servo motor (model: LX224, Hiwonder Co. Ltd., Shenzhen, China) was chosen as the feeding motor. This is a serial bus servo motor that can work continuously and stably at 25 \(\sim\) 45 r/min with a torque range of 15 \(\sim\) 20 kg-cm. We chose \(N\) as 35 r/min for the average speed. The measured seed bulk density \(\rho\) was 674.5 kg/m\({}^{3}\)[19]. Considering the structure strength, installation space limitation, and air tightness requirements, we set \(h\) = 2 mm, \(Z\) = 15, \(R\) = 35 mm, and \(r\) = 20 mm, then we found \(t\) \(\geq\) 7.54 mm, which means that the gear disc thickness should be 7.54 mm at least and was rounded up to 8 mm at last.