Mechanical harvesting of UFO Cherry: Investigation of tree plant dynamics

Abstract

A series of harvesting experiments was conducted in a high density sweet cherry (Prunus avium L.) orchard to advance our knowledge of energy requirements and harvest efficiency for mechanical harvest of fresh market quality fruit. Our primary goals were to better understand the dynamic response of fruiting wood to forced vibrations and the transmitting pattern of vibratory energy throughout the target and non-target trees. Experiments were conducted on trees trained to a novel architecture comprised of unbranched upright shoots. Field data collected from 31 trees revealed constant resonant frequencies within upright limbs at 8~10 Hz, which we attribute to the simple morphological structure. Single impact excitation was applied also to the same limbs and with the same peak force. Resulting relative kinetic energy ratio (RKER) showed that resonant oscillation could obtain high amplification of vibratory energy whereas the impact energy was attenuated. The supporting wire of trellis system has little effect on dynamic characteristics of the tree limbs. But it could help in propagating vibratory energy within the tree and the adjacent trees. In contrast, the foliage and fruit have a significant damping effect on both the dynamic response and vibratory energy transmission along limbs. A preliminary assessment on mechanical harvest efficiency revealed 77% fruit removal by vibratory force applied near the resonant frequency range of the trees (between 12 and 16 Hz).To develop an appropriate mechanical harvester suitable for a novel cherry tree architecture system of upright fruiting offshoot (UFO), a series of fruit harvesting dynamics investigation was performed on three-year-old UFO sweet cherry (Prunus avium L.) of ‘Selah’ cultivar in an intensively-managed orchard. Our goal was to obtain a comprehensive understanding of the tree responses to forced vibration as well as the transmittal pattern of vibratory energy which causes fruits to detach. Field data collected from 31 UFO cherry trees indicated that UFO trees featured a low morphological variability and resonant frequencies. Therefore, a representative UFO tree was chosen to test its dynamic behaviors under oscillation of 8~30 Hz and showed its resonant frequency at 10 Hz and 18 Hz. In contrast to oscillation, impact excitation also was applied to the representative tree at the same excited offshoot and the same peak force. Relative kinetic energy ratio (RKER) in the tree showed that resonant oscillation could obtain high amplification of vibratory energy while impact was attenuated. Furthermore, dynamic trials were conducted on UFO cherry tree to assess the effect of cordon and growth stage on the resonant frequency and vibratory energy transmission. The results provided important information on regulating mechanical harvest condition for optimizing yields. The supporting wires have little effect on dynamic behaviors of UFO tree while foliage and fruits have a significant damping effect on the dynamic response and vibratory energy transmission. Accompanying dynamic analysis, mechanical harvest yield of UFO cherry under oscillation was assessed by applying a range of frequency (6~20 Hz). It showed that the resonant frequency region (12~16 Hz) had noticeable efficiency on harvesting the tested UFO cherry trees, which at the highest frequency was above 77%.