Dynamic Response of Sweet Cherry Tree to Oscillating Excitations for Mechanical Harvest

Current mechanical cherry harvest uses either impact or vibration excitations to shake tree branches to create a detachment force on cherry stems to remove cherries from the tree. A comprehensive understanding of fruit detaching energy distribution on cherry trees is essential for designing such mechanical cherry harvesters. This research was conducted to quantify the dynamic response of cherry trees to forced oscillating excitation. In this study, a set of accelerometers were placed at different locations on a sweet cherry tree. Acceleration signals at those locations corresponding to a band of sinusoidal excitations ranging from 2 to 40 Hz, covering the most commonly used excitation frequencies in vibrating harvest, were recorded and then integrated into velocity and displacement responses. A peak excitation force of 28.9 N was used at all excitation frequencies. Results showed that the first two resonant frequencies of the test cherry tree were around 6 and 14 Hz, respectively. The responding displacement at monitoring locations ranged from 1 mm to 11 mm under the first two resonant frequencies. The third resonant frequency was found at around 25 Hz, under which the responding displacement were noticeably lower than the corresponding values at the first two resonant frequencies. Under the same force excitation amplitude, the displacement responses of most branches increased as the location is further away from the excitation point. Experimental results also indicated that the vibratory energy of each branch in the path from the excitation point to the terminal branch was transmitted and observably amplified along the straight transmission path without branch junctions at all resonant frequencies. It implies that cherry trees with simple structure, straight branch orientation and less branch junction will have higher harvest efficiency when in impact or vibration harvesting.