| Citrus is one of the most important fruits in South China, but most of citrus harvesting is still done by hand. Harvest cost accounts for a very big portion of the citrus production costs. In order to improve harvesting efficiency and decrease harvesting costs,citrus shaking harvesting methods will be studied in this thesis.Firstly, the trunk shaking and the canopy shaking were compared by modeling and simulating. The vibration system of trunk shaker was divided into four parts, including a tractor, a vibrating mechanism, a trunk, and fruits. Then, in order to study the shaking characteristic of trunk shaking, a mathematic model was built and a numerical analysis was conducted. According to the theory of straight beam lateral forced vibration and the theory of pendulum vibrating under the acutating of harmonic displacement, a mathematic model of canopy shaker was built. According to the mathematic model we found that the main factors impact on vibrational effect are vibration frequency, loading position and the stem length, etc. Comparing results of those two methods indicated that the canopy shaking can harvest continuously and it is easy to tune working parameters. So the canopy shaking was selected as the main content in the following study.In order to study and simulate the process of citrus canopy shaking, Chapter Three measured and analyzed the relationship between fruit detachment force(FDF) and tension angle. It is demonstrated that tension angle has a significant influence on the FDF, and the FDF decreases as tension angle increases. The result showed that critical value of FDF is40 N. Besides, mechanical characteristic and some physical parameters, such as moisture content and destiny of citrus branch were measured.In order to do the kinetic analysis, a simplified canopy model was built in Chapter Four based on ANSYS Workbench. Then prestressed modal analysis was conducted to determine the inherent frequency. Finally, harmonic analysis was conducted to determine the optimum range of vibration frequency and the loading position. The simulation results demonstrated a positive correlation between vibration frequency and the stress of theconnecting node of fruit and stem. The stress will reach its maximum when the frequency is between 5 and 6Hz and then it will decrease. As the frequency is between 4 and 7Hz, the vibration effect is better. The closer the loading position is drawn to junction of limb and stem, the bigger the node stress is, the better vibrational effect will be. According to field test results, the vibrational effect is better while the frequency is between 4 and 5Hz, and the vibrational effect is better when the loading position is more closer to the junction of limb and stem. The simulation result and field test has a high fitting degree. The results can be referenced in selecting vibration frequency.A simple vibrational device was developed in Chapter Five to analyze the kinematic chain of canopy shaker. And a canopy shaker prototype was developed according to that analysis. A series of field canopy shaking tests were conducted in the orchard in Chapter Six, and the trees for test were selected randomly, the test results showed that:(1) The diameter of branch has some effects on its maximum acceleration, a obviously smaller branch will result in a bigger acceleration;(2) Removing of short stem fruit is much easier than long stem fruit and removed long stem fruit may have a short or intermediate length of stem or branch attached. From the shaking position to the trunk, the acceleration of branches decreases, and from shaking position to limb the acceleration changes little. But the acceleration of fruit is relative much smaller than the 2nd branch and the 3rd branches;(3) The stiffness of tine has a positive effect to acceleration. But it is not possible to establish well-defined rule to define that tendency. Those results showed some ideas in selecting parameters of citrus canopy shaking. |
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