Mechanism analysis and experiment on vibration harvesting of wolfberry

Abstract: Wolfberry acreage is growing increasingly year by year in China. However, artificial harvesting is labor intensive and inefficient, so harvest problem has become a bottleneck in the development of Chinese wolfberry industry. Literature shows that vibration device in fruit harvest has obvious advantages. In order to further study the mechanism of wolfberry vibration harvesting, fruit-pedicle separation conditions and dynamic transfer characteristics of wolfberry branch were analyzed, and then harvest conditions of wolfberry were obtained. Vibration harvesting usually uses vibration source to directly vibrate fruit branch (fourth branch) to achieve fruit separation, but when fruit branch is not directly vibrated, it needs to rely on vibration energy of third branch to achieve fruit-pedicle separation. Vibration separation test bench was set up, which included electric vibration test system, motion control system, vibration separation test rack, and so on. Vibratory separation test device was used to carry out fruit-pedicle vibration separation test of fruit branch, which was designed by quadratic regression universal rotary combination design method. The main factors affecting fruit-pedicle separation rate were as follows: excitation frequency, excitation amplitude and vibration time (replaced by branches travelling speed), with mature fruit separation rate and immature fruit separation rate as response index. Searching for the optimum vibration parameter combination of fruit-pedicle separation, high-speed camera system was used to analyze acceleration response of vibrated part and end part, and as a result vibration separation conditions of wolfberry were obtained. The experimental results showed that the optimal combination parameters were the excitation frequency of 18.22 Hz, the excitation amplitude of 7.87 mm and the branches traveling speed of 20.93 mm/s. And the results showed that the separation rate of mature fruit was 95.18% and the separation rate of immature fruit was 6.43%. Dynamic responses of the optimal vibration combination parameters were analyzed, and the results showed that the acceleration of vibration was 518.38-551.06 m/s2 when fruit branch was directly vibrated; the vibration energy was transferred to end of fruit branch by direct excitation of third branch when fruit branch was not vibrated, and fruit branch acceleration was required to reach 347.64-390.56 m/s2. In this experiment, a single random stimulus was used to wolfberry third branch, vibration transfer of third and fruit branch was tracked by high-speed camera system, and wolfberry branch dynamic response was analyzed by a high speed motion analysis software ProAnalyst. The Shapiro-Wilk's normal test was performed with acceleration data of third branch and fruit branch of wolfberry, corresponding confidence intervals were calculated, and then acceleration response relationship of third branch and fruit branch was obtained. The acceleration required for wolfberry vibration harvesting under different excitation conditions was obtained. Firstly, when all the fruit branches were directly vibrated, acceleration of vibration was required to reach 518.38-551.06 m/s2; secondly, when all third branches (parent third branch and adjacent third branch) were directly vibrated and part of fruit branches could not be vibrated, only the dynamic response of parent third branch and fruit branch needed to be considered, and the maximum acceleration from parent third branch to fruit branch was reduced by 5 times, with the lag time of 0.032 s, and the acceleration of parent third branches vibration needed to reach 1 738.20-1 952.80 m/s2; thirdly, when partial fruit branch and third branch (adjacent third branch) were not vibrated, the maximal acceleration value from adjacent third branch to fruit branch was attenuated by 6 times and the lag time was 0.048 s. Acceleration of adjacent third branch vibration needed to reach 2 085.84-2 343.36 m/s2. As large vibration acceleration can cause wolfberry branch injury, in the design of wolfberry harvesting device, the structure should ensure that all fruit branches are directly vibrated as far as possibly, and at least ensure that all third branches are directly vibrated; vibrating rods can use nylon and other flexible material with less damage to branches. The results of this study can provide theoretical basis for the mechanized harvest of wolfberry.