Devices And Key Technologies For Whole Hydroponic Lettuce Harvesting With Low Damage

Hydroponic lettuce(Lactuca sativa L.)is one of typical greenhouse leaf vegetables,and mainly manually harvested and packaged in China with low efficiency and high labor cost.This paper aimed to improve the completely mechanical production level of hydroponic lettuce,and the study mainly included:(i)shearing test of lettuce stem,(ii)the whole scheme design of automatic harvester,(iii)harvesting method for whole hydroponic lettuce with low damage and the device optimization,(iv)position and posture characters of lettuce during putting lettuce down lengthways,(v)system integration and verification test.The main research contents and results are listed as follow:(1)Shearing test of lettuce stem.To investigate the optimal parameters combination of cutting factors,a shear fixture was designed to work on a universal materials tester to cut lettuce stems.The effects of blade distance,sliding cutting angle,skew cutting angle,and shearing angle on the shearing stress was analyzed using response surface methodology.A scanning electron microscope was used to observe the micro-structure of lettuce stem to analyze the shearing characteristics at the microscopic level.The result of universal materials tester showed that the shearing force-displacement curve presented two peaks,and this kind of shearing characteristics had relationship with the cell tissue arrangement characteristics of lettuce stem.The response surface methodology results showed that the order of significance for single factors was(i)sliding cutting angle,(ii)shearing angle,(iii)skew cutting angle,and(iv)blade distance.The sliding cutting angle had a highly significant influence on the shearing stress.The order of significance for double factors was(i)blade distance and shearing angle,(ii)sliding cutting angle and skew cutting angle,and(iii)the sliding cutting angle and shearing angle.The optimal combination of the factors was blade distance 1.66 mm,sliding cutting angle 39.88°,shearing angle 11.15°,and skew cutting angle 12.99°,which reduced 69.9%shearing stress of the maximum shearing stress.(2)The whole scheme design of automatic harvester.In order to improve the completely mechanical production level of hydroponic lettuce,the lettuce harvester(which contains the multilayer conveying device,device of gathering-cutting-putting lettuce down and packaging device)was designed.We investigated and surveyed the agronomy properties of hydroponic lettuce,and ascertained the functionality and performance of the harvester.The main size parameters of multilayer conveying device,device of gathering-cutting-putting lettuce down,and packaging device were calculated.(3)Harvesting method for whole hydroponic lettuce with low damage and the device optimization.In order to solve the issues of hydroponic lettuce manual harvesting with low quality but high cost,we proposed a new harvesting method.The leaves lying on the hydroponics planting board are gathered by the holding rods before cutting the stem.The factors those will influence the leaf damage is generalized.Image processing method(extracting the damaged parts artificially)was utilized to calculate the damage area of leaves.The influence of gathering angle and gathering height on damage area is analyzed though the mechanical analysis and high-speed photography.Orthogonal test is utilized to study the influence of gathering velocity,gathering angle,gathering height and velocity of putting lettuce down on damage area.The results show that the order of significance is(i)gathering angle,(ii)gathering height,(iii)velocity of putting lettuce down,and(iv)gathering velocity.The optimized combination of factors that gives the minimum damage area is obtained,and the value of gathering velocity,gathering angle,gathering height and velocity of putting lettuce down are 100 mm/s,15°,70 mm and 100 mm/s,respectively.The average value of damage area is 4.32×10~2 mm~2,and the average value of damage degree is 0.13%,which means the optimization is obvious.(4)Position and posture characters of lettuce during putting lettuce down lengthways.To investigate the excursion and reversal of the lettuce,an automatic packaging machine was designed and manufactured.The lettuce was put into the box by cylinders,and a guiding annulus was utilized to decrease lettuce excursion and reversal.The influence of guiding annulus on lettuce position and posture characters,for instance the horizontal excursion and attitude angle,was analyzed based on high-speed photograph.The position and posture characters were solved by 3 points.The horizontal excursion was solved by the middle point,and the attitude angle was solved by the two endpoints.The packaging test results showed that the average value of horizontal excursion with guiding annulus is 5.8mm when lettuce entering the box,which reduced 75.0%than that without guiding annulus.The average value of attitude angle with guiding annulus is 17.9°,which reduced 74.2%than that without guiding annulus.The guiding annulus could ensure the success rate of putting lettuce into the packaging box.(5)System integration and verification test.In order to realize the automation of the harvester,and reduce the manual operation of transitional link during multilayer conveying,gathering-cutting-putting lettuce down,and packaging,the multilayer conveying device,gathering-cutting-putting lettuce down device,and packaging device were integrated in this study.The verification tests for the complete machine were studied.The tests results showed that the operating time,the mean operating time and working efficiency of the harvester were 44–81 s,61 s,and 59 lettuces per hour,respectively.The damage degree of the harvester was 0.23%.The operation of the complete machine satisfied the performance requirements...