| Automatic transplant can improve the work efficiency and reduce production costs,and thus is the development direction of vegetable seedling plug transplanting techniques.So far,numerous relevant studies in China have brought about diverse automatic transplanting machine.However,the problems of working unreliability and low transplanting quality restrict the application of automatic transplanting machine into agricultural production and severely inhibit the development of vegetable seedling raising and transplanting techniques.Given these problems,here we conducted some research.(1)The structures and distribution of vegetable seedling plug roots in the seedling raising substrate were characterized.The vegetable seedling plugs were scanned and the roots were three-dimensionally reconstructed by using an X-ray micro-computed tomography(micro-CT),and the root segmentation and extraction effect was optimized at the scanning voltage,current and resolution of 55 kVp,72 ?A and 45 ?m respectively.The ratio of root volume to root lump volume in the roots was defined as the root distribution density,the minimum to maximum ratio of root distribution density at the vertical direction was named as the root uniformity coefficient,and the root distribution density around the outmost zone of root lumps at the horizontal direction was called the peripheral root distribution density.The root uniformity coefficient and the peripheral root distribution density were adopted to evaluate the compactness of roots wrapping the seedling raising substrate.The roots of tomatoes,cucumbers and lettuces grown under the same conditions were statistically analyzed,and their root uniformity coefficients were 0.532,0.587 and 0.504 respectively and their peripheral root distribution densities were 0.0294,0.0346 and 0.0263 respectively.The root growing quality of seedling plugs ranked as cucumber > tomato > lettuce.(2)To find out the tomato seedling plugs suitable for mechanical transplantation and with high growing quality,we conducted orthogonal tests with the substrate composition,substrate disking compactness and nutritive medium concentration as the breeding factors.The seedling raising quality was assessed by using the compressive resistance,plug abscission force,sound seedling index,root uniformity coefficient,and peripheral root density.The optimal scheme was determined through a comprehensive scoring method.When the substrate composition,substrate compactness,and medium concentration were 3:1:1,1.2,and 1.2 ms/cm,respectively,the final tomato seedling plugs were assigned with both high mechanical transplant performance and growing quality.(3)To decrease the pick-up damage to root lumps by the gripper,we scanned the tomato root lumps under the pick-up status by using Micro-CT,extracted the roots and holes from the root lumps,and thereby quantitatively analyzed the volumes and distribution densities of both roots and holes.No significant variation was found in the volume or distribution density for either roots or holes during the picking-up.The peripheral lateral roots functioned to wind and wrap the root lumps,avoiding them from loosening.The lateral roots that diverge radially from the main roots can inhibit crack expansion,since the new hole growth and crack formation around the pick-up pins are the major causes for root lump destruction.The vertical and horizontal directions of root lumps were the fracturable zone and the breakable zone respectively.The hole distribution densities at these two zones when the pick-up pin contracted to the largest displacement can be used to evaluate the root lump damage degree.The root lump was considered as "damaged" when the hole distribution density at these two zones was 0.168.Also the effects of pin shape,pin diameter and initial pick-up angle on the root lump damage degree were studied.The root lump damage can be decreased by reducing the pin diameter,enlarging the initial pick-up angle,or by using round pins instead of flat pins.The damage to root lumps was minimized by using 2-mm-diameter round pick-up pins at the initial pick-up angle of 24°.(4)A mechanical model underlying the collision between tomato seedling plugs and the dibble-type planting device was built.Mechanical equations expressing the collision between root lumps and the planting device were built using the Hertz theory and Newton theorems.The motion equations showing the collision between different parts of tomato seedling plugs and the planting device were constructed using the momentum and momentum moment theorems.Also the effects of the planting device and seedling connecting angle and the rotational speed on the forces and movement velocity of tomato seedling plugs were investigated.A multi-objective optimization function was established via the objective programming approach,and the variation at the transplanting frequency of 45 to 50 plants/min was determined.It was found decreasing the plant height of tomato seedling plugs contributed to decreasing the seedling plug movement velocity after collision.(5)Regarding the large film surface holes during the membrane fracturing hole-forming transplantation by the dibble transplanter,we studied the relationships of planting device structure,soil-penetrating angle,and movement velocity with the film hole sizes.It was found these three parameters were all quadratically correlated with both the longitudinal and transverse dimensions of film holes.The movement of transplanting structures was simulated on ADAMS.When the trajectory coefficient ? increased from 0.95 to 1.203,the longitudinal dimensions of the trajectory envelopes at the films first decreased and then increased.At longitudinal dimensions of the trajectory envelopes minimized at ?=1.093.The planting-device film fracturing and soil penetration model was built on the finite element software HyperMesh.The film hole-forming process was simulated at different angular velocities of the planting device.At the same trajectory conditions,as the angular velocity of the planting device rose,the longitudinal and transverse dimensions of film holes both were enlarged.At ?=1.093,when the angular velocity of the planting device was 1.178,1.309 and 1.440 rad/s,the longitudinal dimensions of the film holes were 9.7,10.2 and 10.6 cm respectively,and the transverse dimensions were 4.2,4.4 and 4.5 cm respectively,which were all the smallest.(6)The existing automatic transplanter test beds were improved,and the optimized gripper and dibble-like transplanting structures were installed on each test bed prior to pick-up film transplanting tests.It was found the breakage to root lumps was minimized to 4.41% with the use of 2-mm-diameter round pin at the pick-up frequency,initial pick-up angle,and deviceseedling connection angle of 50 plants/min,24° and 72° respectively.At ?=1.093,the longitudinal and transverse dimensions of film holes minimized,which validated the correctness of the optimized working parameters.The film hole-forming transplanting tests of tomato and cucumber seedling plugs were conducted at the transplanting frequency of 50,55 and 60 plants/min.The transplanting qualified rates of both types of seedling plugs decreased with the increase of transplanting frequency.Under the same transplanting conditions,the transplanting qualified rate of cucumber seedling plugs was higher than that of tomato plugs.The transplanting frequency significantly affected the hole and root lump damages,but not the occurrence of lodging.When the transplanting frequency was below 55 plants/min,the transplanting qualified rates of tomatoes and cucumbers were both higher than or close to 90%.Thus,the automatic transplanters designed here satisfy the hole-forming and transplanting requirements. |
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