# Current Status and Future Directions for Combined Tillage and Sowing Technology and Equipment
## 4 Research on Precision Sowing Technology
### Pneumatic Precision Seed Metering Device  
Pneumatic precision seed metering devices were created by scholars combining mechanical architecture and fluid dynamics, and began to be widely used and popularized in the 1960s. The devices are driven by a fan to form air pressure in the seed filling chamber or seed filling box, pushing the seeds to be adsorbed onto the seed suction plate filled with holes, and then clearing the seeds through mechanical or pneumatic forces to realize the sowing operation. Pneumatic seed metering devices have become the main development direction of seeding technology, and have a wide range of in-depth applications in combined machines, where technology development is more advanced [90]. Existing pneumatic precision seed metering devices are categorized into air suction types, air blowing types, air pressure types, and air feeding types according to their working principle, and the typical structure of a pneumatic precision seed metering device is shown in Table 4.  
At this stage, the pneumatic precision seed metering device is more adaptable than the mechanical precision seed metering device, does not require a large seed shape and size, does not easily to injure the seed, the sowing volume is stable and uniform, the operational efficiency is high, and it is easy to realize single seed sowing. It is mainly used in large-scale combined tillage and sowing machines, and has been applied in a variety of crops such as wheat, corn, potato, and so on. [PERSON] et al. designed a seed metering device for pneumatic seeders to improve the quality of sandalwood seed sowing, and explored the effects of different air pressures and different seed discharging disks on detection accuracy. The internal structure diagram of the precision seed discharger and the diagram of the pairing of the type of hole and brush are depicted in Figure 13[91]. [PERSON] et al. developed a wheat precision seed meter with a combination of pneumatic and disk hole methods in response to the problems of small wheat kernels, irregular shapes, and large amounts of seeds;  
Figure 12: Finger clip precision seed metering device. (**a**) Overall structure. (**b**) The seed guiding blade. (**c**) Operational status. they also explored the influence of the size of the type holes on the change in the airflow, designed the key parameters of the seed discharger, and adopted the combined airflow-type hole working principle to enhance the performance of the single-grain seed filling of wheat [92]. [PERSON] et al. designed a pneumatic seed metering device for vegetable seedling with a combination of vibratory seed cleaning and pinhole adsorption method, which effectively increased crop yield and reduced labor cost [93]. [PERSON] designed a positive and negative pneumatic pressure combined seed discharge system and key components in a precision multiple planter for pak choi; it changes the number of holes and the rotational speed of the seed-displacement disk type to adapt to the seeding requirements of pak choi [94]. Relevant research has further enhanced the performance of pneumatic seeders used for different crops by means of changing the parameter structure of seed-filling holes, combining different mechanisms, and analyzing the mechanism laws. In the follow-up, the pneumatic precision seed metering device should be further simplified and integrated to reduce its use cost, improve its operation stability, expand the scope and scenarios of its use, promote its application in small and medium-sized combined tillage and sowing machines, and achieve coverage of more crop production processes. In order to improve the performance of the pneumatic precision seed metering device, exploring the factors affecting its working performance is the focus of current research. [PERSON] et al. analyzed and studied the effect of the particle spacing of the seeder, and effect of the pore shape and size on seed suction performance by means of mathematical models and other means.