| Seedling nursery has been popularized and applied widely in China,because of high efficiency,low cost,continuous production and other advantages.Along with the application of seedling nursery,automatic transplanter is becoming very important.But most domestic plug seedling trasplanters in greenhouse have low efficiency and poor reliability.Based on our transplanter prototype in Zhejiang University,we tried a lot to improve the efficiency and reliability.Our works include making a new strategy of replugging seedlings,designing a seedling tray transfering device,re-designing the seedling tray separating device,designing a new end-effector and testing the efficiency,building a force measure system to measure the forces during transplanting process,analyzing the impact of agronomic factors to extrusion force and adhesive force,finding out the relationship between these two forces and transplanting performance,and planning the replugging tour based on intelligence algorithm.The main and research contents and conclusions are listed as follows:(1)A new strategy of replugging seedlings was proposed and a seedling tray transfering device was designed.Seedling trays were classified accordding the number of bad and missing cells in the trays.After detection of machine vision system,the transfering device would transfer and deliver seedling tray with multi bad and missing cells to the origin tray conveyor and deliver seedling tray with few bad and missing cells to the target tray conveyor.A seedling tray separating device was designed for the machine vision system.The device had a row of curly curtains and the curtains could separate seedling from the bottom to the top,thus would not hurt the seedling leafs.(2)Combined the advantages of slide form and clamp form,a new efficient end-effector with only one drive cylinder was designed.This end-effector could clamp the plug the same time with the four needles penetrating into the nutritional substrate.The depth into media and the verticality of needles could be adjusted conveniently for different seedling trays.The structure parameters were optimized based on Visual Basic according to the motion trail of needle cusp.To verify the parameters,a virtual prototype model was established based on the obtained optimal parameters and simulated in ADAMS 2012.The simulated motion trail of needle cusp was compared with that in Visual Basic.A full-test with three factors(the proportion of ingredients,the moisture content,the mass of substrate)and three levels was conducted to verify the efficiency of end-effector.All the conditions meet the requirement of seedling nursery with substrate water potential bigger than-10kpa and substrate porosity bigger than 85%.For the total 270 plug seedlings,the designed end-effector has a 100%transplanting success rate,and the seedling plug average damage rate was 17%.Under eight combinations of the full-test,there are no harmed root plug.Through this end-effector,the transplanting success rate was greatly increased and the root plug damage rate was reduced without increasing time consumption(averagely 2.8 s per seedling).The transplanting success rate for tomato and pepper seedlings in Q72 trays were also 100%,and the seedling plug seriously damage rate were 2.5%and 4.5%seperately.For cucumber seedlings in Q128 trays,the transplanting success rate was also 100%,and the plug damage rate was 23.8%.(3)A force measure system with tension and pressure transducers was installed on the designed end-effector.The adhesive force FL between the root plug and the cell of seedling trays and the extrusion force FK on the root plug could be measured during transplanting process.The relationship between these two forces and stress on the transducers were established after force analysis.The transducers were calibrated on a standard electric universal testing machine(CMT4204).To find out the effect of substrate moisture content on these two forces,an experiment was conducted.Six levels of moisture content(79%,76%,74%,67%,63%and 56%respectively)were got by controlling the time of evaporation.The results showed that the adhesive forces increased first and then decreased when the moisture content of growth substrate decreased.The extrusion forces increased when the moisture content of growth substrate decreased.When the moisture content was high,the standard deviation of adhesive forces in the same group was small.When the moisture content was moderate(74%),the standard deviation of extrusion forces in the same group was small.Moisture content of root plug also had significant effect on the performance of transplanting.Moderate moisture content(74%or 67%)was good for automatic transplanting.(4)To analyze the impact of main agronomic nursery factors to the extrusion force and adhesive force,an interacting effect trial on the moisture content of the substrate,substrate bulk density and volume proportion of substrate ingredients was designed.9 seedling trays were prepared and each tray was divided into three regions.Each region was a experiment group,and totally there were 27 groups.10 seedlings of each region were selected randomly and transplanted.The results showed that the transplanting performance was not related with the extrusion force and adhesive force directly.Additionally,it was found that the end-effector did not perform very well when the value of FK/FL was beyond the range of 5.99?8.67.This could provide a scientific basis for end-effector application in transplanting.After analysis based on ANOVA,the results showed that all three variable factors and their interactions had significant effects on the extrusion force(P<0.05).The most significant effect on extrusion force was from the variation of moisture content,secondly was the variation of substrate bulk density,last was the variation of substrate volume proportion.All three variable factors also had significant effects on the adhesive force(P<0.05).But only the interaction of substrate bulk density and volume proportion had significant effect.The most significant effect on adhesive force was from the variation of substrate bulk density,the second was from the variation of moisture content,the last from was the variation of substrate volume proportion.When the moisture content decreased gradually within 82.5%to 78.5%,the extrusion force and adhesive force increased linearly.When the volume proportion was 7:2:1,the average value of extrusion forces was maximum because of the increase of peat.When the volume proportion was 6:2:2,the average value of adhesive forces was maximum because of the increase of perlite.The average extrusion force increased first and then decreased when the bulk density increased,since the variation of bulk density would changed the compressive property of seedling plug.The adhesive forces increased with the increase of bulk density,but the trend of increase slowed down.The interspace of plug cell was invariable,thus the squeezing action between substrate particles would increase with the increase of bulk density.Then the expansion stress of seedling plug would increases and the interaction between seedling plug and plug cell enhanced.(5)Replugging bad or missing cells with healthy seedlings is an important task of automatic transplanters in greenhouses.The gripper consumes a considerable amount of time when shuttling between the source and target trays during replugging.Fortunately,work efficiency could be improved by tour planning.The ant colony algorithm(ACA),genetic algorithm(GA),and common sequence method(CSM)were implemented in the replugging tour planning and were subsequently compared.The optimal values of the parameters used in the GA and ACA were selected based on tour lengths.The best parameter set of GA was identified as pc= 0.8 and pm = 0.3.And the best parameter set is identified as ?=1,?=20,?=0.1.Two types of seedling trays with 50 and 200 cells were used in this test.Both the GA and ACA implemented in replugging tour planning have obvious advantages over the CSM when used in production.However,the GA cannot work with one empty cell.Compared with the GA,the performance of the ACA meets real-time requirements,although the latter is more time-consuming than the former one.The average elapsed time of the GA and ACA are 0.32 and 0.94 s.All the computation could be completed before the gripper complete all empty cells in the front tray.The standard deviation of the ACA(?A)is usually smaller?than that of the GA(?G).The re.lative advantages of the ACA and GA are affected by the sum(p+q)of empty cells in the target tray and available healthy seedlings in the source tray.With the increase of the sum,the relative advantage of the ACA becomes increasingly satisfactory.When a 20×10 tray is used,the average length of the ACA after finishing 40 empty cells in a tray is shorter than GA and CSM,by 6000.0mm and 13058.4mm,respectively.Overall,the ACA is more efficient than the GA in the replugging tour planning for seedling transplanters. |
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