Plug damage detection and parameter optimization of picking up cucumber seedlings from tray cells based on Micro-CT

Abstract: The root cause for transplanting failure is the damage of plug which clamped by clamper when transplanter automatically transplants vegetable plug seedlings. To reduce the damage of plug, in this study, we prepared 128-hole plate cultivation of cucumber seedlings using the peat, perlite and vermiculite mixed in a volume ratio of 3:1:1. We used X-ray Micro-CT to test non-destructively three states of cucumber seedlings plug: pin: not inserted into the plug, pin into the plug, and pin contraction to maximum displacement. In the tomogram, root threshold was obtained as 93-102, and pore threshold was obtained as 0-16. Roots and pores were segmentation extracted and 3D reconstructed from tomogram by using threshold segmentation method. The plug was divided into six equal parts vertically and horizontally. Root distribution density was defined as the ratio of every part in the area where the root volume and volume of plug. The pore density was defined as the ratio of every part of the pore volume and volume of plug. The volume and distribution density of root and pore were calculated by SPSS18 software. It was concluded that there was no significant (P>0.05) displacement for root during the clipping, and its role was wrapping around the substrate to prevent the plug scattered. The change for pore volume and distribution density was very significant (P<0.05), new pores and cracks mainly appeared around the pin and between the pins. Pores accumulation and cracks formation were the main cause for plug damage. We then treated the increments of pore volume around the pin as the evaluation of broken seedlings indicators. For the two kinds of pins: Circular and flat, scanning tests and analyzes were carried out. When pin diameters were 2, 2.5 and 3 mm, and the initial angles of clamp were 7°, 9° and 11°, the results showed that increments of pore volume increased as the pin diameter increased, and decreased as the initial angles of clamp increased. When shrinkage of pin was 5 mm; increment of pore volume of circular pin was less than flat pin in the same condition. When clamp force equaled to the anti-stress of yield point for seeding plug as 7.31 N, shrinkage of circular and flat pins were 4.75 and 4.26, respectively; increments of pore volume were 843.7and 786.1 mm3, the damage of plug which clamped by flat pin was less than circular pin. We set the shape, diameter, initial angle and shrinkage amount of pin as test factor, and set damage rate of seeding plug as assessment indicators. Then the test was carried out by two fingers and four pins structure seedling clawing. Plug was damaged severely when root lump weight lost more than 25%. Test result showed that damage rate of seeding lump was 6.3% which was less than test results of other parameters when the diameter of flat pin was 2 mm; clamping force and clamping initial angle were 7.31 N and 11°, respectively. The results of the test were in agreement with the results of Micro-CT scanning. Overall, this research provides a new approach to the design for automotive transplanter, the resulting optimization parameters could be used as theoretical basis for the selection of seedling clamper design and take seedling parameters.