The film-covered sweet potato transplanting method requires ensuring the transplantation conditions of small planting holes and large lateral displacement. In the soil insertion phase, the transplantation machine requires a mechanism design with multiple timed poses, and the existing design methods are still imperfect. For this reason, this article proposes a multi-target trust region parameter-guided optimization algorithm. This algorithm aims to achieve multi-objective optimization design with more timed pose conditions starting from individual timed pose conditions. First, multi-target problems are decomposed into multiple subproblems, and the parameter arrays are kept with the minimum polymerization value of each subproblem. Then, the approximate function value reduction for each target is calculated using this parameter set, and the step size for the next iteration of each subproblem is determined by comparing this approximate reduction with the actual reduction. After many iteration calculations, the parameter arrays end the calculation when the parameter group is no longer updated. This paper uses the design of a film-covered sweet potato transplanting mechanism as a complex optimized application example. The algorithm is used to obtain the optimization results of the target values of eight groups of institutions. The smallest hole is 2.99 mm, and the horizontal transplanting distance is 108.40 mm. The maximum hole is 17.64 mm, and the horizontal transplanting distance is 124.97 mm. Considering the size of the hole and the horizontal transplanting distance of sweet potato transplanting, the mechanism’s target value of the horizontal transplanting distance at 119.92 mm and the hole size at 0.31 mm were selected to design the sweet potato transplanting machine. The correctness of the results is verified, which reflects the practicability of the algorithm.

In order to improve the adaptability of the planting mechanism for different plant spacings, a variable differential gear train planting mechanism based on precise pose and trajectory control was proposed by combining the open chain 2R rod group and the variable differential gear train. According to the pose requirements of receiving seedling point, transporting seedling point and planting point, three precise pose points of constrained planting trajectory were determined. Through the three-position motion generation structural synthesis method, combined with computer-aided optimization design software, a set of mechanism parameters that meet the planting requirements were optimized. Based on the optimized mechanism parameters, by only changing the coordinates of two trajectory shape control points, three planting trajectories with key point position information adapted to 300 mm, 400 mm and 500 mm plant spacing were obtained by interpolation, and three pairs of total transmission ratio of three groups of variable differential gear trains were calculated. When distributing the total transmission ratio of the mechanism, the fixed axis gear train and the differential gear train are combined. The fixed axis gear train included a pair of non-circular gear pairs and a pair of positive gear pairs, which were convenient for disassembly and assembly. The former drives the sun gear at variable speed, and the latter drives the planet carrier at uniform speed. Based on this structure, the transmission ratio of the positive gear pair is –1, and the transmission ratio of the differential gear train is 0.5. The sub-transmission ratio of the single-stage non-circular gear pair was calculated and the pitch curves of three pairs of non-circular gears were solved. Three pairs of non-circular gear pairs with different transmission ratios were replaced in turn and three sets of planting mechanisms were modeled in three dimensions. The virtual prototype motion simulation was completed by ADAMS software, and the physical prototype was built for vegetable pot seedling planting test. The theoretical solution was consistent with the attitude and trajectory of the actual test. When the test sample size was 100 plants, the actual average plant spacing was measured to be 303 mm, 402 mm, and 503 mm, with errors of 1.3%, 1.25%, and 1.88%. The width of the moving hole was 72 mm, 70 mm, and 71 mm, and the planting success rate were 94%, 96%, and 95%. The test results verified the correctness of the optimization design results of the mechanism, indicating that the variable differential gear train planting mechanism can adapt to a variety of plant spacing and has good planting effect.