Damage caused by multiple impacts of litchi fruits during vibration harvesting

Mechanical damage caused by fruit-to-fruit impact is a major challenge for mechanical harvesting of litchi (Litchi chinensis Sonn) fruits. Guiwei litchi fruits were used to study the impact behavior and damage degree due to multiple fruit-to-fruit impacts. The effect of impact times and impact energy absorbed by fruits on fruits damage degree were studied by using a pendulum device. The damage degree of fruits was determined with the modulus of elasticity of fruits. A comb-shaped shaker was used for mechanical detaching tests while the detaching time, impact times and impact energy absorbed of litchi fruits during vibration process at three frequencies of 18 Hz, 25 Hz and 32 Hz were investigated based on a high-speed camera. The results of the pendulum tests showed that the damage degree of litchi fruits increased with the increase of the string length from 0.1 m to 0.35 m at all impact times and most of the damage was caused before 10 times of impacts. Furthermore, the damage degree increased according to a power function (R-2 = 0.9433) with the total impact energy absorbed of all impacts, which can be used to estimate the damage degree of a fruit after multiple impacts. The mechanical detaching tests showed that 90% fruits were detached in 1.2 s, 0.9 s and 0.5 s at 18 Hz, 25 Hz and 32 Hz respectively and the average impact times decreased from 131 times to 6.6 times with the increase of frequency. The calculation results showed that a vibration frequency of 32 Hz induced the lowest average estimated damage degree, which was 5.69%. Therefore, harvesting litchi fruits mechanically at high rate and low damage might be feasible with a proper shaking frequency.