Research cluster on vibration detachment characteristics of litchi population
Foreword: Lychee is a subtropical fruit that is widely cultivated in India, Vietnam, Australia, South Africa, southern China and other places. China is the country with the largest litchi planting area and the highest production in the world.
In 2020, China's total production reached 2,553,500 tonnes of lychees, with a short picking period of one week to avoid overripeness. Almost all lychees in China are picked by hand, so a lot of labor is required during the picking season.
Efficient harvesting machinery is urgently needed to replace manpower. The key issue of research is the development of harvesters that can improve harvesting efficiency and reduce fruit damage. Vibrating harvesting is one of the main methods of mechanical harvesting of fruits.
In the early stage, the lychee vibration harvester was developed, and the separation mode and mechanism in the vibration separation process were explored, which showed that the method was feasible for lychee harvesting. There are few systematic studies on fruit shedding and its influencing properties.
Lychees are fragile fruits that are easily damaged, and studies have shown that 7.6% of the fruit is lost due to mechanical damage during manual picking. Bryant found that the impact caused the peel of the lychee to browning or cracking. Recent studies have shown that the number of impacts and the speed of impact affect the degree of damage to lychee fruits.
Studies on lychees and other fruits show that low vibration frequencies and amplitudes can lead to low removal rates. Xu et al. found that mechanical vibration had a great influence on the quality of blueberries, increased the rot rate of blueberries, and reduced the activity of enzymes. Lu et al. found that vibration significantly increased the ethylene release and respiration rate of apple fruit, resulting in faster damage and shorter storage time for apples.
Select the appropriate vibration technology based on tree characteristics and fruit characteristics, including vibrator type, amplitude, frequency, and gripper position, to improve nut harvesting efficiency. Sergio et al. obtained a high harvest quality by selecting a suitable frequency range, and the harvesting efficiency was close to 85%.16 The goji berry harvester was optimized by finite element method and response surface method.
Optimum rotational speed and amplitude range is obtained, increasing harvesting efficiency and reducing damage. Therefore, choosing the appropriate vibration parameters is essential to improve fruit removal and reduce the degree of damage.
Vibrating harvesting is widely used in fruits such as sweet cherries, citrus, olives, etc., and many studies have focused on the influence of vibration parameters on vibration efficiency. Under the appropriate vibration parameters, the canopy vibrating screen system can achieve more than 90% fruit removal rate. Using the right vibration frequency and vibration time can improve the removal rate and removal efficiency of sweet cherries.
Shaking the citrus tree with the proper parameters and duration can also make fruit detachment more effective. For different ripening stages, using the appropriate vibration frequency and amplitude can improve the harvesting efficiency of the olive shaker. Blanco et al. studied the effects of the shaker's duration and number of repetitions on olive harvesting efficiency.
The study by Sola Girado et al. showed that when the vibration parameters are selected appropriately, fruit harvesting efficiency can be improved without damaging the fruit tree. Corals et al. reduce the frequency and increase the amplitude to achieve optimal mechanical harvesting of citrus. Carlos et al. determined in their experiments the optimal frequency range for separating highly ripe coffee cherries. The shaking position also has an important effect on fruit shedding.
Apply the handheld shaker to the branches of the sweet cherry tree. The results showed that the distance between the excitation location and the fruiting area affected the fruit detachment efficiency. How a fruit position index containing the geometric dimensions of fruiting and excited branches was introduced.
There is a strong correlation between these indicators and the acceleration of the fruit. The detachment time of the fruit during the shaking process is very important, which is one of the main limitations to increase the travel speed of the shaking machine, and determining the effective shaking position of the lychee ear is an urgent problem to be solved.
Surface impact is the main cause of fruit damage. For fruits grown in bunches, such as lychees and sweet cherries, collisions between fruits are inevitable during vibrating harvesting. Zhou et al. found that this effect may also occur between the fruit and the branch, and that the number of these effects is less than the effect of the fruit on the fruit.
Since the resulting branches of the litchi string are long and thin, the shaking position must be close to the fruit to obtain sufficient detachment energy and collisions between the fruit and the vibrating screen can also occur, which has not been studied.
Previous studies have focused on the effects of vibration parameters on harvest efficiency, removal rate, and fruit damage. Some studies have also observed the influence of morphological characteristics on harvesting effects.
For fruits grown in clusters, the effect of the group effect on fruit shedding remains unclear. In this paper, the following topics are studied by harvesting lychee ears by using a linear reciprocating vibration mechanism.
The influence of fruit position on separation time and collision between fruits at different vibration frequencies and amplitudes. The litchi clustering index considering the total fruit mass and shaking position information was introduced to study its influence on the time of separation. The effect of the shaking position on the fruit-shaker impact was also observed.
The horizontal throwing distance of ex vivo lychee fruits was calculated. This paper combines machine vision and target vibration picking to pick lychee fruits. Machine vision technology can analyze the morphological characteristics of lychee fruit bunches and guide vibrating screens to stimulate litchi fruit bunches with appropriate vibration parameters in the right position.
Conclusion: The general trend shows that the detachment time increases with the distance from the fruit to the shaker and the aggregation index. The aggregation index has a stronger correlation with the time of disengagement. It is explained that the shaking position and the total mass of the fruit affect the detachment speed. The separation time can be predicted using the cluster index.
On the basis of greater than the distance from fruit to shaker, the optimal vibration position of litchi cluster can be determined by further studying the cluster index and the number of fruits in the cluster.
