Design of pneumatic maize precision seed-metering device with synchronous rotating seed plate and vacuum chamber

Abstract: Aiming at the problems of airtightness reduction, vacuum pressure demand increase and planting precision decline, which are caused by the friction between seed plate and sealing gasket of vacuum chamber in traditional vacuum precision seed-metering device, a new type of vacuum precision seed-metering device with synchronously rotating seed plate and vacuum chamber was designed in this study. Different from the traditional vacuum seed-metering device with relatively rotating seed plate and vacuum chamber, in the new seed-metering device, the seed plate and the vacuum chamber were fixed together so that they rotated synchronously during the process of operation with no friction between each other, resulting in no pressure loss and high planting accuracy. A circular frustum plate with rectangular embossment was designed based on mechanical disturbance and gravity-assisted filling principle. Different from traditional metering plates, the cells of the newly designed plate were distributed on a tapered profile, instead of on a sub face. Working area was divided into five zones: filling zone, clearing zone, carrying zone, unloading zone and transition zone, based on the working principle and force analysis. The force of filling process and clearing process, the trajectory and speed of seeds were also analyzed. And the parameters of each metering-device working zone was analyzed and determined. Specially, the filling zone was in the first quadrant，the angle ranged from 0°to 45° (horizontal direction of first quadrant is set to 0°, and the angle circulated counterclockwise). The component force of seeds gravity to the holes in this area reduced the pressure requirements for filling process. The scope of the other four zones were 45° to 70°, 70° to 205°, 205° to 235°and 235°to 360°, respectively. The main structural parameters of the plate were defined based on mechanics and kinematics analysis. Plate was found to be 26 holes with the diameter of 5.4 mm. A 64°angle between radial direction and cone bus was determined for the falling back of extra seeds in clearing zone. Four kinds of plates with different disturbance structures were designed, and the disturbance structures helped to improve the activeness of seeds. The influence of four different mechanical disturbance structures on seed kinetic energy was simulated by EDEM (higher kinetic energy refers to higher seeds activity, and higher seed activity refers to easier filling process). Average particle kinetic energy changing with time step of the four different plates was recorded. Simulation results showed that the plate with rectangular embossment along the direction of generating line brought the highest kinetic energy and the plate with no disturbance structure indicated the minimum kinetic energy. To measure the performance of this newly designed seed-metering device, a two-factor contrast test was carried out in different velocity and pressure. The operation effects were examined by a testing instrument for maize (Zea mays) precision seed meter's performance detection developed by China Agricultural University. Qualified index, missing index and multiple index were recorded. Experiment results showed that the qualified index of seed spacing exceeded 91.6%, the missing index below 5.2% and the multiple index below 5.4% for the newly designed metering device, when the operation velocity was under 10 km/h and the negative pressure value ranged from -3.5 to -5.5 kPa. The results above could completely meet the requirement of the technical specifications of quality evaluation for drills in China. To achieve the same qualified index, the pressure demand of the newly designed metering device was lower compared with the Kverneland metering device. The standard deviation of qualified index within various velocities under same pressure was calculated. The result showed that the standard deviation of qualified index qualified index of Kverneland metering device was greater than the newly designed one, indicating that the newly designed one had a wider range of velocity adaptation and more stable working status. The gravity helped to reduce the air pressure requirements for vacuum precision seed meters.