Relationship between frequency spectrum characteristics and vibration responses of Ginkgo biloba trees during mechanical harvesting operation

Abstract: Mechanical harvesting is the most effective method for the forest fruit harvesting. Under forced vibration, the removal of fruits is affected by not only the excitation amplitude, frequency, and duration, but also the morphology and inherent frequency spectrum characteristics of the fruit tree itself. In order to discover the relationship between the frequency spectrum characteristics and the vibration responses and attain the difference among the vibration responses in different directions, the frequency spectrum of a small Y-shaped Ginkgo biloba tree in different directions was tested under the impact excitation in the laboratory. Then the spatial acceleration response under the harmonic excitation was detected at the frequency of the peak and trough points in the frequency spectrum curves. Results of this study indicate that there was a corresponding relation between the frequency spectrum characteristics and vibration responses. Strong vibration response couldn't be induced by the fundamental frequency and the frequency lower than 10.00 Hz. The maximal amplitudes could be tested at the resonant frequency but the values were small at low frequency and increased as the excitation frequency increased. When the excitation frequency was higher than 25.00 Hz, stronger acceleration response couldn't be induced again. The optimal excitation frequency was 23.75 Hz for the small Y-shaped Ginkgo biloba tree. At the same testing position, vibration responses in three directions exhibited the similar characteristics but different amplitudes. The acceleration amplitude in the direction parallel to the growing orientation of trees increased significantly as the testing position was located away from the excitation point. At the same time, this direction was the dominant direction during the process of the vibration response transmission. This suggests that during the mechanical harvesting of forest fruits, the frequency spectrum of trees could be firstly tested to obtain the resonant frequency. Then trees could be excited under the certain resonant frequency to achieve strong vibration response. Higher excitation frequency didn't necessarily induce stronger vibration response. There was difference among different directions at the same testing position and fruits on different positions could be removed by the inertial force in different directions.