Air-assisted centralized metering devices cannot fully meet the large-scale seed supply during high-speed operations in recent years. It is high demand for better compatibility in the seeding rate and size of rice, wheat, and rapeseed, particularly on the high flowability of rice and wheat. In this study, a type-hole wheel was proposed to specifically design for the compatible mode with rice, wheat, and rapeseed. The number and rotational speed of the hole wheels were rapidly adjusted to allow for the flexible seeding rate in the seeding system, leading to the simultaneous use of rice, wheat, and rapeseed. The spiral arrangement of the hole wheels was used to increase the mobility of seeds within the seed-filling area. Thereby, the stability of seed supply was improved during high-speed seed feeding operations. Mechanical analysis was also carried out on the seed filling, seed carrying, and seed falling stages during the seed supply process. The type-hole structure parameters were determined using the physical characteristics of rice and wheat seeds. A hole wall curve was designed using brachistochrone. EDEM simulation experiments were conducted to analyze the population motion states under different rotational speed conditions at the filling, carrying, and falling stages. The variation curves were derived for the number of seeds that were supplied in the spiral and linear arrangement of the hole wheel with time. The peaks, valleys, and periods of the curves were statistically analyzed. The EDEM simulation experiments show that better performance was achieved at 40 r/min for the rice, wheat, and rapeseed supply stages. There was more outstanding behavior of the dragging and bouncing during the seed-filling stage at 80 r/min. Some seeds cannot fully enter the mold hole during the initial stage of seed carrying. But the seeds were moved down the side wall of the mold hole for the secondary seed filling, as the mold hole wheel rotated. All seeds were fed in a timely manner without any "seed sticking" phenomenon during the seed-feeding stage. Once the number of holes was determined, the pulsation period was inversely proportional to the speed of the hole wheel. The adjacent holes were sequentially misaligned on the hole wheel, when the hole wheel was spirally arranged. The continuous seed filling and feeding performance were improved to reduce the pulsation, indicating a more stable seed supply. Furthermore, the fluctuation range was reduced by more than 30 grains in the spiral arrangement of the shaped hole wheel for the rice and wheat seeds under the same rotational speed conditions, compared with the linear arrangement. The bench test was carried out to determine the rotation speed range with the better seed supply rate and stability. The regression model of seed supply rate was constructed for the rice, wheat, and rapeseed. Therefore, the coefficient of variation was less than 1% for the stability of the seed supply rate, and the seed supply rates were 1 050.62-1 535.87, and 4 171.82-5 073.76 g/min, respectively, for the hybrid rice and wheat seeds, when the number of type-hole wheels was 16, and the speed of seed supply ranges were 30-50, and 50-70 r/min, respectively. In rapeseed, the coefficient of variation was less than 0.5% for the seed supply rate stability, and the range of rapeseed supply rate was 160.42-227.45 g/min, when the number of type-hole wheels was 1, and the rotational speed of seed supply was 20-30 r/min. Moreover, there was a relative error of less than 2% between the regression model and the experimental value of the seed supply rate of rice, wheat, and rape. The field seeding experiment showed that the stability coefficients were 1.32%, 1.16%, and 1.07% for the variation of the total displacement in the collector for the rice, wheat, and rapeseed, respectively when the operating speed was 8-10 km/h. The seeding system fully met the high requirements for the multi-crop seeding operations with the rice, wheat, and rapeseed. This finding can provide a strong reference to optimize the structural parameters of the air-assisted centralized metering device for multiple crops.

Multiple compression and impact of the air-assisted centralized seed metering device on rice seeds during high-speed sowing (≥12km/h) can easily lead to seed damage and adversely affects the emergence rate. This study aims to explore the damage of rice seeds under single and multiple mechanical compression in the air-assisted centralized seed metering device, according to the damage accumulation theory. The fitting equations of seed impact damage and compression were clarified to determine the matching range of stirring speed and the conveying airflow parameters during high-speed seeding. A systematic analysis was also implemented to explore where the seed was damaged under the repeated compression by the seed mixing device during high-speed seeding. The multiple impacts of airflow were evaluated during transportation and distribution in the air-assisted centralized seed metering device. The single compression test was carried out on the hybrid rice and conventional rice varieties. An elastic-plastic model was constructed suitable for rice seeds, in order to determine the critical value of elastic-plastic deformation. The multiple equal displacement compression tests were then carried out to determine the process of damage to rice seeds under multiple compressions. The critical deformation of rice seeds was determined and verified to combine with the seed germination rate test under multiple compressions. According to the conservation of energy, the critical speed was constructed at which the rice seeds were compressed and damaged by the stirring device. The fitting equation of impact damage and compression was combined to analyze the speed of seed airflow impact damage. The results showed that the crushing forces of conventional and hybrid rice seeds under single compression were (118.4±35.6) N and (113.2±39.9) N, respectively, the deformation during crushing was (0.59±0.07) mm and (0.63±0.11) mm, respectively, and the average critical deformation of elastic plasticity was 0.4 and 0.44 mm, respectively. In multiple compression tests, the maximum compression force of the seeds remained consistent, when the compression distance was less than the elastic-plastic average critical deformation; Once the compression distance was greater than the critical elastoplastic deformation, the maximum compressive force on the seed decreased gradually. Verification tests were also performed on the rice seeds germination rate. There was the a 0.44 mm critical deformation of hybrid rice seeds that caused damage. When the deformation of compressed seeds was less than 0.44 mm, there was no significant difference in the germination rate between compressed and uncompressed seeds after multiple compressions; When the compression deformation of the seeds was greater than 0.44 mm, the germination rate of rice seeds decreased after multiple cycles of compression. Therefore, the deformation of rice seeds after being loaded should be avoided to be greater than 0.44 mm. After that, the rotation speed range of the stirring device should be lower than 125 r/min and the seed transport speed should be lower than 8.5 m/s, in order to reduce the compression and impact damage of the air-assisted centralized seed metering device on rice seeds. This finding can provide a theoretical basis and parameter range to match the operating parameters during rice high-speed sowing.

Air-assisted centralized metering can be operated at high speed for rice, wheat, and rapeseed. However, the seed stirring device is prone to damage the seeds, leading to the low stability of discharge quantity. In this study, a spiral-inclined flexible seed-stirring device was designed for high-speed, air-assisted, and centralized metering. The influencing factors on the seed damage were determined using Hertz contact theory, including the material properties of the seed stirring rod, the top structure, and the stirring speed. A comparison was made on the variation in the top structure of the seed stirring rod and the contact force of the population over time using EDEM simulation. A systematic investigation was also implemented to clarify the impact of the seed stirring rod on the seed supply. The test results show that the tangential and normal forces of the arc-shaped seed stirring rod in contact with the population were smaller than the trapezoidal seed stirring rod. There was better stability of seed supply when the inclination angle of the seed stirring rod was 45°. A single-factor test was conducted on a bench to determine the optimal range of seed stirring speed ratio. The results showed that the rice and wheat seeds performed better within the range of 1-2 seed stirring speed ratios, while the rapeseed seeds were in the range of 0.5-1.5 seed stirring speed ratios. A three-factor, three-level, and quadratic rotation orthogonal experiment was carried out to optimize the influencing factors. A regression model was then established for the damage rate, seed supply rate, and stability coefficient of variation of rice, wheat, and rapeseed. There was an optimal matching relationship between the stirring speed and the operating speed within the range of 10-14 km/h. Field experiments were also conducted to verify the simulation. It was found that the stability coefficients of the variation in the total displacement of rice, wheat, and rapeseed were 1.92%, 1.27%, and 1.14%, respectively, under the optimal combination of parameters, when the operating speed was 12 km/h. The coefficients of variation in the total number of seedlings within 1 m² were 15.47%, 12.98%, and 17.93%, respectively, which fully met the requirements of the standard for the high-speed seeding. The finding can provide a strong reference to achieve high-speed and low-loss seeding in the parameter selection of high-speed, air-assisted, and centralized metering devices for rice, wheat, and rapeseed.