| Automatic seedling pick-up device is the key to realize fully-automatic transplanting of vegetable plug seedlings,which can significantly reduce the labor cost and improve the efficiency of transplanting.At present,most of the research on automatic seedling picking technology in China is still in the laboratory stage.There are some crucial problems,such as untimely innovation of seedling raising technology,unclear interaction mechanism between root and substrate,and weak combination of mechanism design and seedling raising technology,and thus the above factors restrict the development of automatic seedling picking and dropping technology of vegetable plug seedlings.Therefore,aiming at these above key problems,this research mainly completed the following works:(1)The effects of biochar on mechanical and growth characteristics of vegetable plug seedlings were studied.A certain proportion of biochar was added to the original substrate which composed of peat,perlite and vermiculite according to 3:1:1.It was found that the mixing of biochar significantly improved the p H value,EC value and porosity of the original substrate,and also reduced the bulk density of the substrate.In addition,the mixing of biochar could significantly improve the water retention performance of the substrate and also promote the slow-release effect of nitrogen fertilizer.The growth parameters of cucumber plug seedlings were the best when the proportion of added biochar reached 10%.The sugar content and nitrogen content in the stems and leaves of plug seedlings also increased significantly in such condition,and the root system grew vigorously.However,when the proportion of biochar continued to increased by more than 20% or even 50%,the sugar content in stems and leaves and the transport capacity of nutrients in plug seedlings decreased significantly,so excessive biochar in the substrate significantly inhibited the growth of plug seedlings.The inherent excellent characteristics of biochar improved the physical and chemical properties of the original substrate,and also improved the nutrient transport and metabolism of plug seedlings in a certain proportion.By the way,the cost of biochar was lower than peat.To sum up,based on the analysis of root growth and substrate strength,the mixing of 10% and 20% biochar in the substrate can significantly promote the mechanical properties of seedling pot,and this trend is also applicable to tomato plug seedlings.(2)The anchorage and reinforcement mechanism of root-substrate complex was explored.According to the diameter range,the root system of plug seedling was divided into four categories: main root(0.86 mm ? 1.19 mm),primary lateral root(0.41 mm ?0.62 mm),secondary lateral root(0.23 mm ? 0.36 mm)and adventitious root(0.13 mm? 0.18 mm).The content of chemical components of roots in different diameter classes were determined,which indicated that the cellulose content would be higher with smaller root diameters,and the root diameter had no significant effect on the content of hemicellulose and lignin in the root.By analyzing the mechanical properties of single root,it was concluded that with the increase of root diameter,the ultimate tensile and shear resistance of single root increased significantly,but the tensile and shear strength decreased significantly,which suggested that the rigidity of root will be better with smaller root diameter.The results showed that the friction resistance increased linearly with the increase of root diameter,but the friction stress and friction coefficient decreased significantly with the increase of root diameter,so the finer the root was,the better the friction performance between root and substrate would be.Through the remolded direct shear test of root-substrate complex,it was found that the shear resist of the remold complex will be better with smaller diameter and larger number roots mixed into it.The growth trend and distribution characteristics of root system of plug seedling in different periods were studied,which found that the rapid growth period of root system of plug seedling was from day 7 to day 15.During the cultivation process,the main root generally grew vertically downward in the substrate,and other “fine roots”were largely distributed on the periphery of the substrate.Combined with Micro-CT reconstruction technology and the root growth distribution parameters,the 3D topological structure of root system of plug seedling was rebuilt in Solidworks,and it was imported into EDEM to establish discrete element models of different root combinations for mechanical properties analysis.The results showed that the adhesion between root and substrate particles and the mechanical properties of the formed rootsubstrate complex would be stronger with more “fine roots” in the substrate.To sum up,the main root mainly plays the role of anchoring in the root system,while the other“fine roots” play the role of reinforcing.A variety of diameter root systems intertwine and bite through the substrate,forming anchoring and reinforcing effect on the substrate,and significantly improving the mechanical properties of the root-substrate complex.(3)The effects of different biochar proportions combined with water and nitrogen on the growth of plug seedlings were discussed.In order to cultivate vegetable plug seedlings with excellent growth which could meet the needs of automatic seedling picking,on the basis of mixing a certain proportion of biochar in the substrate,and the moisture content and initial nitrogen content of the substrate were changed in order to design a three factor and three-level all factor experiment.Taking the growth parameters,root parameters,dry matter,root shoot ratio,specific root length,water and N utilization efficiency and crushing resistance of plug seedlings as evaluation indexes,the results showed that when the biochar proportion in substrate was 5%,the moisture content of substrate was 80% and the initial nitrogen content was 50%,the cultivated plug seedlings could get excellent growth,strong roots and better crushing resistance.After that,the plug seedlings under different treatments were transplanted into the flowerpot and cultivated until the flowering stage.The growth potential and photosynthetic data were measured to explore the impact of pre-treatment on the subsequent growth of plug seedlings.The results showed that the plug seedlings with good growth potential in the early stage grew more rapidly after transplanting,and the photosynthetic effect was very strong,while the plug seedlings with poor growth potential in the early stage grew more slowly,which highlighted the importance of promoting the growth of plug seedlings before transplanting.(4)A new air ejection seedling picking method was proposed.This chapter compared and analyzed the widely used clamp-type seedling picking method and ejection-type seedling picking method by using the Recur Dyn and EDEM coupling simulation combined with the actual seedling picking test.It was found that the clamptype seedling picking method which inserted into the seedling pot by using clamping needle,would cause serious root disturbance and damage to the seedling pot,while the ejection-type seedling picking method could not restrict the seedlings which were ejected out,and these two methods put forward high requirements for seedling raising.According to this,a new seedling picking method which used airflow to blow out the plug seedlings was proposed.This method was in flexible contact with the plug seedlings by airflow,which could realize low loss and high efficiency seedling picking,and put forward low requirements for seedling raising.Meanwhile,a new type of peripheral package clamping end effector was designed and optimized,with a coupling simulation carried out.The results showed that the effective clamping of plug seedlings could be realized when the clamping needle was 8.5 ° to the vertical direction,and the results of clamping test were consistent with the simulation.The air nozzle parameters and airflow parameters of the air ejection seedling picking method were explored and optimized,and the diameter of the air nozzle was established to be 3.5 mm.Combined with the law of compression deformation at the bottom of the plug tray,the structure of the air nozzle was proposed,and the soft suction cup was preferably installed in the front of the air nozzle to ensure the air tightness of the structure.Based on the theory of air jet dynamics,it was concluded that the pressure required for air jet to blow out the plug seedlings was 0.146 MPa ? 0.315 MPa,and the air blowing time was about 0.3 s.The fluid structure coupling model between the flexible seedling pot and air flow was established by COMSOL software to explore the air pressure,attitude,flow field distribution and stress of the plug seedling during the process of seedling blowing.The results showed that when the air pressure exceeded 0.4 MPa,the plug seedlings could be blown out from the plug tray,but considering the damage of the plug seedlings,the air pressure should not be too high.Besides,the Fluent-EDEM coupling simulation was carried out to explore the stress and damage of the discrete element model of the plug seedling under different air pressures.Totally speaking,when the air pressure is 0.4MPa and the air blowing time is 0.5s,the seedlings can be picked out with high efficiency and low loss,which are consistent with the simulation results.(5)According to the above parameters,a new type air ejection-package clamping fully-automatic seedling pick-up device was developed,and the key components of this device were designed in detail.The cooperation mode of each mechanism was planned and the working principle of the pick-up device was optimized.An automatic control system which could realize fully-automatic seedling picking and dropping was proposed according to the working principle,which was combined with the pneumatic system to realize the electromechanical automation.The prototype was designed and the key parameters were optimized by orthogonal test,and the seedling picking frequency was set to 120 plants/min during the process.The results showed that the success ratio of seedling picking reached more than 95% after parameter optimization of the seedling pick-up device,which could meet the needs of fully-automatic transplanting technology.The above research can provide theoretical basis for the development of highquality seedling raising technology and automatic seedling picking technology of vegetable plug seedlings,and can also bring guiding significance to promote the development of automatic transplanting technology. |
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