Design and test of a three-row Salvia Miltiorrhiza transplanter for single ridges

Salvia miltiorrhiza is one of the most significant medicinal crops in China. Its transplantation quality can directly dominate the root development, the accumulation of effective medicinal components, and the final yield. The one-ridge-three-row staggered planting pattern has been widely practiced in Shaanxi Province due to the soil-moisture conservation and field ventilation. However, conventional manual transplanting cannot fully meet the large-scale production in recent years, due to the high labor intensity and low efficiency. It is often required for the uniform planting quality under the configuration. Existing transplanting machinery can also suffer from several limitations, including low compatibility between machine structure and staggered agronomic configuration, unstable seedling posture during insertion, oversized machine dimensions that reduce field maneuverability, and insufficient adjustability of planting depth. Salvia miltiorrhiza cultivation is often required for the planting uniformity and production efficiency. In this study, a micro self-propelled transplanter was designed and developed, suitable for one-ridge-three-row staggered planting. The machine consisted of a lightweight frame, a rotary staggered seedling-feeding mechanism, a large-stroke eccentric-wheel-cam-linkage planting mechanism, a walking mechanism, and a compact transmission. According to the spatial characteristics of the staggered three-row configuration, the rotary feeding structure was developed to continuously deliver the seedlings in a coordinated and phase-matched manner. A large-stroke planting mechanism was incorporated with an eccentric-wheel-cam-linkage system, according to the agronomic requirements for deep insertion, stable anchoring, and upright seedling posture, together with the biomechanical properties of Salvia miltiorrhiza seedlings. The vertical displacement was extended for the optimal insertion angle. The better kinematic performance was achieved to reduce lateral disturbance during soil entry and enhance seedling uprightness after transplanting. A complete kinematic model of the planting mechanism was established to evaluate and optimize its mechanical behavior. Optimization objectives were defined from the perspectives of planting stability, insertion-angle control, vertical-depth accuracy, and adequacy of planting stroke. A parameter optimization framework was constructed using a multi-objective genetic algorithm . The optimal combination of member dimensions was determined using MATLAB App Designer. Fifteen structural parameters were determined for optimization, including critical linkage lengths and pivot coordinates. Sensitivity analysis identified L₁, L₂, L₄, L₆, and L₇ as the most influential parameters on the planting stroke and seedling uprightness. The mechanism achieved a theoretical planting stroke of 324 mm after optimization, providing for sufficient vertical motion during deep insertion. The minimum insertion depth reached 191.44 mm, thus meeting the agronomic requirements of the effective seedling anchorage. Furthermore, the near-vertical soil penetration significantly enhanced the verticality of the plug seedlings for the minimum disturbance from lateral displacement. The optimal mechanism was also suitable for the agronomic requirements of the deep, upright, and stable planting. A prototype micro self-propelled transplanter was fabricated using the optimal parameters. A typical field test was conducted to validate the optimization in Shaanxi Province. Field tests showed that the high operational reliability was achieved in a qualified planting rate of 94.75% at a theoretical plant spacing of 304 mm and an average seedling length of approximately 150 mm. The coefficient of variation of plant spacing was 8.71%, indicating acceptable planting uniformity for medicinal-crop production. The planting-depth qualification rate reached 89.90% for the effective depth control during operation. Seedlings also shared high uprightness and stable anchorage after transplanting, further validating the effectiveness of the optimized mechanism. Overall, the micro self-propelled transplanter effectively integrated a rotary staggered feeding mechanism with an optimal large-stroke planting mechanism. Field performance demonstrated that the mechanized transplanting also met the agronomic requirements for Salvia miltiorrhiza cultivation. This finding can offer an efficient and reliable solution to transplanting equipment for medicinal crops in precision planting.