## 2 Precision Seeding Technology
### The Seed-Spacing Consistency-Regulation Technology  
As a critical component of precision seeding [17], the seed metering device has high requirements for seed planting performance to ensure that there will be no re-sowing and no leakage and that the seeds are discharged smoothly and evenly during the seeding operation. There are mainly two mechanical and pneumatic seed metering devices for precision seeding [18]. Mechanical seed metering devices have a simple structure. Nevertheless, compared to pneumatic seed metering devices and seeding accuracy in general, the requirements for seeds are high; it is easy to cause damage to the seeds, and these devices are primarily used in low-speed sowing operations. The commonly used mechanical precision seed metering device mainly includes the tilting-disc type [19; 20; 21], spoon-wheel type [22], and finger-clip type [23] (Figure 2). \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Application Scenario** & **Major Technology** & **Each and Every Link** & **Influence Results** \\ \hline The precision seed-discharge technology & Seed metering device & \begin{tabular}{c} Mechanical seed metering devices, \\ pneumatic seed \\ metering devices \\ \end{tabular} & Discharge of seeds & \\ \hline The smooth seed-dropping technology & Seed-guiding device & \begin{tabular}{c} Conveyor-belt-type \\ seed guide, \\ brush-belt-type \\ seed guide \\ \end{tabular} & Seed path orientation & The consistency of seed spacing \\ \hline The seeds falling into & Seed bed & Seed-pressing wheel, seed-pressing tongue & The fall into the seed bed & \\ \hline The consistent & The devices in contact & \
\begin{tabular}{c} Terrain-adapting \\ furrow opener, soil \\ covering and \\ compacting \\ \end{tabular} & Covering seeds &
\begin{tabular}{c} The sowing-depth- \\ consistency \\ regulation \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Precision seeding: operational technology process. Among them, the structure of pneumatic seed metering devices is more complex, requiring a pneumatic system and transmission system to complete the operation. They have a high sowing accuracy and are primarily used in medium and large sowing machines; and the foreign developed model is more perfect, and mainly used in the high-speed precision seeder [24]. The commonly used pneumatic precision seed metering devices mainly include the pneumatic-suction type, the pneumatic-pressure type, the air-blown type, and the central set of row type [25; 26; 27; 28] (Figure 3). For example, the 'vacuum meter' pneumatic-suction type seed metering device developed by the Kinze Company has a seed metering disc with multiple seed-absorbing nests to reduce the rotational speed of the seed metering disc and extend the seed filling time. At the same time, an electric motor drives the seed-metering disc to ensure stable rotation to improve the seed-filling and cleaning effect. The operational speed can reach 12.8 km/h while providing quality seed discharge [29]. Heilongjiang BeiKeRuSi Modern Agricultural Technology Co (Manufacturer: Heilongjiang Backace Modern Agro-Tech Co., Ltd.  
City: Nangang District, Harbin City, Heilongjiang Province.China.). researches a pneumatic-suction type seed metering device, which is driven by a motor, with a rotating speed of 0-150 r/min, an air pressure of 6-8 kPa, a seed-discharge disc and a rotating hub which is airtight and solid and has a synchronous rotation; the rotating hub and the fan produce a negative pressure through the pneumatic suction to achieve seed discharge using gas suction. It has the advantages of a compact structure and stable operation for solving problems such as the seeds falling off, poor air tightness, and varying seed-discharge-disc rotation speed, caused by mechanical vibration in a high-speed no-tillage operation [30].