Design and experiment of the pneumatic needle suction device for the precision seeding of small particle size sprout seeds

Microgreen cultivation is often required for the precise sowing due to the small size and sensitivity of the seeds. However, it is still lacking in the equipment for the multi-row parallel mechanized seeding. Manual sowing is also confined to the uniform distribution during operations, due to the high risk of seed damage. In this study, a pneumatic needle suction device was developed for precision seed metering. A kinetic model was established to analyze the seed adsorption via the suction needles in the seeding device. Computational Fluid Dynamics simulations were conducted to optimize the airflow patterns and pressure distribution around the needles, in order to realize the efficient seed pickup and release. The key mechanical components were determined with their operational parameters, including a vibratory seed-feeding device, a pivoting air cylinder for needle movement, a seed tray conveyance, and a tray stacking mechanism. A systematic experiment was employed to identify the optimal combination of the operational parameters. The critical factors were determined, such as the vacuum pressure level, the orifice diameter of the suction needles, and the vibration frequency of the pneumatic vibrator in the feeding system. Firstly, single-factor experiments were performed to determine the approximate effective ranges for each factor. Two representative microgreen seeds were selected: the Toona sinensis and Medicago sativa. The results indicated that for Toona sinensis seeds, the superior seeding performance was achieved within the following ranges: the suction orifice diameters between 0.6 and 1.2 mm, vibration frequencies from 10.5 to 11.5 Hz, and vacuum pressures ranging from 6 to 11 kPa. In the smaller Alfalfa seeds, the effective parameters were a slightly broader orifice diameter range of 0.5 to 0.7 mm, but a narrower vibration frequency band of 9 to 10.5 Hz and a lower vacuum pressure requirement of 5 to 9 kPa. Subsequently, a quadratic orthogonal rotation combination was implemented to determine the optimal parameter and factor interactions. This advanced statistical analysis revealed that the optimal seeding performance for the Toona sinensis was obtained at a vibration frequency of 12 Hz, a suction needle orifice diameter of 0.9 mm, and a vacuum pressure of 7.5 kPa. In the Alfalfa seeds, the optimal combination of the parameters was a vibration frequency of 10 Hz, an orifice diameter of 0.6 mm, and a vacuum pressure of 5 kPa. Bench tests were finally conducted to validate the performance under these optimal parameters. The results demonstrated that there was a highly qualified seed index of 95.48% and a very low miss-seeding index of merely 1.70% for the Toona sinensis. In Alfalfa, the excellent performance was also obtained with a qualified index of 92.10% and a miss-seeding index of 2.30%. The pneumatic seeding device fully met the agronomic requirements for the precision sowing of the microgreen seeds. In conclusion, a pneumatic seed metering device was successfully optimized for the microgreen seed sowing. The theoretical modeling, CFD simulation, and experiment were combined to determine the optimal parameters for the different seed types. The crucial guidance was provided for the operational adjustment of such equipment. The findings can offer valuable insights and a solid reference for future structural optimization of the high-performance seeding devices in the emerging microgreen industry. A technological gap can be effectively bridged for the high efficiency of microgreen production in small-scale precision agriculture.