Analysis on vibratory harvesting mechanism for trained fruit tree based on finite element method

Abstract: Currently, mass shake-and-catch tree fruit harvesting approach could cause fruit detachment efficiency in low level. Tree structure plays an important role in both harvest efficiency and machine performance for tree fruit. To obtain better harvest efficiency, it is important to optimize the machine parameters and provide proper excitation to the trees according to their growth morphology. This paper presented a finite element modeling method for the trained tree dynamic response study in relation to shaker type. Firstly, mathematic models of 3 types of shakers, including reciprocal shaker, orbital shaker and multidirectional shaker, were built, which were further used to analyze the basic working principles of these shakers. The basic principle of developed shaking models was analyzed according to their characteristics. In order to study the influence of different shakers on the dynamic response of trained tree, 3 training structures i.e. spindle, open center and vertical plane were selected. Their 3D (three-dimensional) physical models including trunk and secondary limbs were constructed in Pro/Engineer. Other elements, such as leaves and twigs, were removed from the model. Then, these models were imported into ANSYS software to analyze their modal shapes. The modal shape and resonance frequency were obtained in a frequency range of 1-50 Hz. Results showed that the resonance frequency and modal shape were influenced by the growth morphology of trees. Three types of trained tree model (spindle, open center and vertical plane) could obtain ideal response at the 10th, 14th, and 19th phase respectively. For the developed trained tree model, the natural frequency range was mainly contained from 7.0 to 20.0 Hz, corresponding to 420-1200 r/min of mechanical shaker. As having thick tree truck, the initial mode frequency of spindle-shape tree was greater than the other 2 types. To further study the dynamic response of the developed tree models, harmonic response simulation was conducted with 3 types of excitation patterns. For the spindle-shape tree, 3 excitations could all induce violent vibration at far-end of the limb (i.e. NODE4 and NODE5). Simulation results showed that multidirectional excitation could cause a relatively ideal response at 13.5 Hz (10th phase) for spindle-shape tree model. Multidirectional excitation could also induce the most violent response among the 3 methods and obtain the maximum displacement at featured location up to 1.844 m. Orbital excitation with 12.0 Hz (11st phase) could cause evident response with the maximum displacement of 2.485 m, and also provide relatively uniform response for open center tree model. However, a small part of area could only achieve a relatively low vibration response, as a portion of vibration energy was absorbed by the tree crotch. To obtain higher detachment efficiency, shaking from different directions would increase the machine performance for this type of trained tree. For vertical plane tree model, both reciprocal and orbital shaker could cause superior response considering mechanical harvest. Results obtained from field experiment validated the correctness of simulation results. The illustration method including modal analysis and harmonic response simulation, could be useful for new mechanical shaker design for trained fruit tree harvest.