Structural Design and Mechanistic Analysis of a Precision Variable-Rate Spoon-Type Millet Seed-Metering Device

Conventional spoon-type seed-metering devices have fixed spoon-cavity geometry, which makes it difficult to achieve both stable sowing of small-sized seeds and precise seeding-rate regulation. To address this problem, a coordinated variable-volume adjustment method based on an adjustment ring-seed-picking spoon structure was proposed, and a variable-rate spoon-type millet seed-metering device was designed. Through circumferential rotation of the adjustment ring, the effective volume of the scooping spoon and the length of the transition groove were synchronously adjusted, enabling precise regulation of the number of seeds per hill without replacing components. The zoning characteristics of seed-population flow in the seed-metering chamber and the effects of operating speed on seed-filling mechanical behaviour were investigated through theoretical analysis and discrete element simulation. The seed-filling and seed-clearing processes were clarified, and key parameter ranges were determined. Through response surface methodology (RSM) experiments, the optimal parameter combination was determined as an operating speed of 3.43 km∙h−1, a scooping-spoon inclination angle of 35.25° and a transition-groove inclination angle of 62.47°. Under these conditions, the qualified rate of seeds per hill was 93.11%, the average number of seeds per hill was 5.51 seeds and the coefficient of variation in seeds per hill was 23.71%. When the seeding-rate adjustment ring was adjusted within 0–10°, the average number of seeds per hill increased from 5.7 to 11.2 seeds, while the coefficient of variation decreased from 22.71% to 19.97%. The device can adapt to differences in grain shape among different millet varieties and to seeding-rate adjustment requirements under varying soil fertility conditions across different fields, achieving precise seeding-rate regulation and uniform seed metering. This study provides a reference for improving the variable-rate operating precision of precision hill-drop seeding devices for small-sized seeds.