Dynamic response of sweet cherry tree to the vibration of a limb shaker

To identify optimal vibration frequencies of a limb shaker for harvesting sweet cherry and the responding accelerations distributed on different limbs of a tree, a series of dynamic response tests were conducted on sweet cherry trees in orchard environment. Three varieties of cherry trees with different structures, including Bing with Y-trellis (Y-Bing), Skeena with Y-trellis (Y-Skeena) and Selah with upright fruiting offshoot (UFO-Selah), were tested in this investigation. Four to six limbs, including one being vibrated and the rests adjacent to the vibration limb, on each testing tree were selected as sample limbs. Three accelerometers were mounted at different locations on each limb to measure the acceleration responses to the vibration. The resonant frequencies were identified using frequency response function method. Obtained results showed that the resonant frequencies of Y-Skeena and UFO-Selah cherry trees had similar average values of 10.4, 15.7 and 22.6 Hz, while that of Y-Bing tree were higher at 12.4, 18.1 and 23.0 Hz. Analysis of acceleration generated under different vibration frequencies showed that the acceleration of shaker was only dependent on the input frequency. The ratios of average acceleration responses on the excited limbs and on its adjacent limbs were averaged at 6.6, 2.9 and 3.7 from Y-Bing, Y-Skeena and UFO-Selah cherry trees, respectively. This indicated that Y-Skeena trees with big diameter of limb were easier to get a whole tree vibration, while only little vibration could be obtained on the adjacent limbs of small size Y-Bing trees. The analysis of displacement showed that the peaks occurred near the resonant frequencies identified above. It also could be observed that small size tree generated large displacement.