| Seedling transplantion in China, which is a labor-intensive task, is still performed manually. It is the key for China to develop the fully automatic transplanter which is structurally simple, functionally accurate, and economically feasible for local transplanting production. At present, it is difficult to develop the fully automatic transplanter used in production. The main reasons are that the interaction between the gripper and the root lump is not clear, biological and mechanical properties of seedlings are in shortage, and the relation between mechanism design and seedling technology is not matching. These reasons severely restrict the development of automatic transplanting machine. Therefore, in order to finish automatic and efficient pipeline transplanting of plug seedlings, the main research works are listed as follows:(1) With cucumber plug seedlings as the research objectives, tests of pulling seedling, compression, loading-unloading, creep and force relaxation were conducted. Results showed that operation of picking up seedlings was mainly to overcome the adhesion forces between roots and the cell walls. The gripping force of the root lumps along the vertical direction of the tray cells was1.94N with128-cell seedlings for transplanting. The capacity of resistance for compression with deformation was increasing slowly at first and increasing significantly at last. In compression process, there was no obvious yield failure point for plug seedlings. Compression failure for plug seedlings was mainly from less packing area than others, and then it was gradually expanding degree of fragmentation. In the elastic-plastic features, With the increase of compression, the plastic strain energy Ep and peak force Fmax were increasing, and the degree of elasticity rc were decreasing. The greater compressive deformation of bowls was, the stronger plastic capacity of bowls possessed. Capacity of plastic deformation for plug seedlings is strong, which is with a certain degree of plasticity, so it is feasible to clamp bowl from both sides or around the body. Compressing creep and force relaxation characteristics of bowl for plug seedlings are effectively expressed by Burgers model and2-unit Maxwell model, respectively, and corresponding viscoelastic parameters are obtained. Creep and force relaxation degree of different compression for plug seedlings is relatively smaller, which there is no significant impact on fast clamping the bowl.(2) Based on the analysis of the force of pulling seedlings from the trays, the friction resistance between the pins and the root lumps, and the compression force, technical design of the auto-picking seedling was carried out. It was obtained the theoretical relationship between pulling force FL, compressive strength F, compressing area AY, penetration angle a, gripping deformation angle Aa, gripping area AJ and gripping deformation x for the picking seedling end-effector of two-needle gripping type. The end-effector adapting to mechanical properties of plug seedlings was designed according to the mechanical characteristics of plug seedling by using gripping parameters relationship. Gripping forces were studied with the physical prototype testing. Results show that there is no significant difference in gripping force between testing data and calculated data, which demonstrates the reliability of the theoretical design.(3) In view of the root lumps with poor structural strength, such as tomato plug seedlings, effects on the crush resistance with different seedling parameters were studied using the orthogonal test method. It resulted in optimizing the seedling production process suitable for automatic transplanting. The best quality of tomato plug transplants production was achieved as the mixture of herbaceous peat, perlite and vermiculite in the volume proportion of2:0:1, filling volume of28.6ml and the moisture content of65%±2. In order to ensure the production of quality cucumber plug seedlings suitable for automatic transplanting, the growth and root characteristics were investigated with the use of five different peat-based substrates treatments. Comprehensive quality of cucumber plug seedlings was analyzed and ranked by TOPSIS. The results showed that the best the best result for quality cucumber seedlings was achieved with T1having a mixture of herbaceous peat, dry sphagnum, perlite and vermiculite were applied in the volume proportion of40%:20%:20%:20for general seedling index, optimal mechanical properties of root lumps and low production cost.(4) Engineering properties of the root lumps, such as elastic modulus, were studied, and the finite element grasping models were constructed for picking up seedlings from the tray cell. Comparing with the actual grasping tests, it was confirmed the correctness of the models. The effects of grasping root lumps using single pin and two pins were simulated. Viewed from the largest Von Mises stress, the grasping action with two pins was far superior to the single pin at the maximum stress of1.31to1.70times. It indicated that the extraction of plug seedlings using the single pin required the better seedling quality, and the extraction of plug seedlings using the two pins was good for succession in picking up seedlings from the tray cells.(5) According to seedling transplant production, a pick-up device was developed, which is a gantry-structure robotic mechanism consisting of a manipulator, an end-effector and two conveyors. The manipulator consists of a motor, a linear modules and a rodless cylinder. It moves the end-effector to the desired working position. The end-effector is a pincette-type gripper consisting of two cylinder fingers, a rubber bag and a limiting groove. Each cylinder finger rotating around a shaft within the limiting groove had double-acting cylinder, two pick-up pins and a stop block. The cylinder finger pushes the pick-up pins out to penetrate into the root lump, and pulls them back to deliver the seedling with the help of the inhibition of the stop block. The rubber bag is designed against the two cylinder finger with a cushion block increasing contact area. As a result, when the rubber bag is inflated, it makes the two cylinder fingers open. However, when the rubber bag is deflated with the help of spring force, it makes the two cylinder fingers close. The conveyors are designed to move the plug tray/pot to the end-effector’s working space. A plant divider was developed to bend the seedling in the manipulator’s working space with step transmission of plug seedlings. As the plant divider blocked off the seedlings aside, it could help to reduce damages to the foliages as a result of the end-effector’s vertical movement.(6) According to the operation design of the end-effector for seedling extraction using the robotic transplanter, the linear motion of the end-effector along the X axis and the conveyors were driven by the stepper motor system. The photo sensors detect the front-edge of a plug-tray in the end-effector’s working space. After the entire row of seedlings is transplanted one by one, the conveyors are advanced forward to place the next seedling-row into the end-effector’s working space. In addition, point-to-point control was used to control the movement of the end-effector. In order to eliminate motion errors efficiently and ensure the end-effector security run at a high speed, S Shape Acceleration-Deceleration method was adopted for the speed control process.(7) The robotic transplanter was test-operated to examine whether its functional requirements of step transmission and grasping force for seedling extraction were satisfied. Viewed from working cycle, it took23.579s for8plants to transplant seedlings from the128-cell tray into the greater pots. Thereby, the transplanting rate was20.36seedlings per minute for the128-cell tray. It took21.065s for6plants to transplant seedlings from the72-cell tray into the greater pots. Thereby, the transplanting rate was17.09seedlings per minute for the128-cell tray. In view of the128-cell and72-cell trays, the average step transmissions were32.1086mm and42.5866mm with standard deviation at0.5026and0.4047, respectively. The statistical analysis showed that there is no significant difference in the step transmissions between the testing value and the designed value. In the tests of grasping force for seedling extraction, the average grasp force was3.23N for successfully picking up seedlings from the tray cells with standard deviation at0.009. It was found that there were3times with grasping force loose to overcome the adhesion forces between roots and the cell walls as the end-effector lifted the seedlings from the tray. It indicated that grasping the root lumps of the seedlings for extraction was a process of gradual loose adhesion. When the grasping force was increased by30%on the basis of the theoretical design, the end-effector can effectively pick up seedlings from the tray cells. Being the first prototype, its optimal machine parameters and performance were examined under various conditions. In the optimum tests, it was found that the root lump moisture content had a highly significant effect (p<0.05) on the success ratio in picking up seedlings, the penetration angle had a significant effect (0.01<p<0.05), and the other factors had no significant effects (p value>>0.05). The performance tests were further conducted. For the four locally produced vegetable seedlings, when the transplanting rate was22seedlings per minute, the maximum success ratio in picking up seedlings was93.87%. The performance of transplanting was satisfactory. |
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