Vibration shedding characteristics of the grapes under the excitation of broken stems and experimental research

Abstract: Fruit particle shedding can be often caused by the vibration excitation of broken stems in the clip-cut grape picking. In this study, a dynamic model of the fruit-stem-dividing plane pendulum in the vertical plane was first established, according to the actual growth of grapes. The theoretical angular velocity of fruit shedding was then deduced under the excitation force of the broken stem. After that, the critical condition was determined for the separation of the fruit-stalk swing and falling off, where the swing trajectory was actually a part of the circular motion. In research objects, the freshly picked round grapes were collected to survey with a similar appearance, maturity, and no damage on the surface. The main physical characteristics of the grapes were also measured at the ripening stage in turn. Specifically, the average length of the main stalk of grapes was 214.44 mm, the average longitudinal diameter of the grape berries was 28.99 mm and the equatorial diameter was 26.45 mm; the average weight was 20.01 g; the average hardness was 9.44 kg/cm2; the weight of the stalk was 7.53 g. In a simulation, a SolikWorks2020 three-dimensional modeling software was utilized to model the grapes. Then the ABAQUS software was used to analyze the dynamic response and swing trend of a single grape under the excitation of broken stems. As such, the swing and deformation trends of a single grape were determined under the excitation of broken stems without squeezing. Since there was much squeezing between the grape particles, the model was simplified to 14 independent, isotropic, and uniform linear elastic spherical particles. The main shearing stem was also applied for the simplified clamping model, thereby collecting the input information, such as the shear force, and the clamping force under the excitation of the broken stem. The simulation test showed that the shear force was 6.454 N when cutting the main stem, and the clamping force changed from the initial 3.3 N to the maximum 6.4 N. The finite element analysis was then performed on the vibration using the simplified model of cluster grapes. A dataset was thus obtained, including the displacement, velocity, acceleration, as well as the stress and strain of the grape fruit relative to the junction of the fruit stalk at the moment before falling off. An optimal combination of parameters was achieved to determine the critical vibration shedding of the grapes. In the 18-order modal analysis, the vibration excitation was transmitted to the vibrated fruit via the main stem-sub-stem-fruit three levels, where the fruit particles appeared the indefinite anisotropic torsional swing. Specifically, the 0-25 Hz sweep frequency was selected for the whole bunch of grapes under modal superposition. Consequently, the frequency of the vibrated fruit suddenly changed to a peak frequency of 4 Hz, and then rapidly attenuated to 0 under the excitation. The maximum swing amplitude of the vibrated fruit was 49.88 mm, the peak velocity was 0.92 mm/s, and the acceleration peak was 39.08 mm/s2, ranging from the shedding and then gradually decay to 0 in a picking cycle. The vibration frequency of the fruit particles presented a great mutation over their own natural frequency. At this time, the tertiary fruit-stalk junction was broken to cause the particles to fall off. The trend of vibration was the same under the same excitation, although the position of each fruit was different. The finding can provide a theoretical basis for the parameter design of anti-shedding picking.