Mechanism Study On Vibration Harvesting Of Ziziphus Jujuba Mill.CV Dongzao

Dongzao is a kind of fresh jujube,which is rich in nutrition and vitamins.Its harvesting is a labor-intensive process.The goal of this research is to investigate the suitable vibratory harvesting technologies for Dongzao,which aimed to achieve a high harvesting efficiency and low fruit damage.The main results are as follows:(1)To understand the harvesting mechanism of winter jujube vibration,force gague was applied to determine the detachment force of Dongzao fruit,and high-speed camera was used to study the inertia force and detaching patterns of fruits under vibration.The results showed that under the same maturity,the fruit detachment force between fruit and stalk was significantly less than that between branch and stalk,and both detachment force decreased as the fruit ripening.There was a significant difference in the detachment force of fruit and stalk between the crisp ripe fruits and white ripe fruits or underripe fruits,which indicated a possibility of selective mechanical harvesting.The accumulated number of tilting motion accounts for 81%,66% and 78% of all the motion patterns at 15,20 and 25 Hz;therefore indicated that continuous shaker should be used in mechanical harvesting of Dongzao.(2)To reduce fruit damage in collecting,sorting and packing,a compression test and finite element simulation were carried out to analyze the compression properties of different ripening fruit under different compression directions.It is found that the rupture force of white ripe fruit was larger than that of crisp ripe fruit,and the deformation of white ripe fruit was less than the deformation of crisp ripe fruit with the same loading force.The calculated elastic modulus values of white ripe and crisp ripe fruit under transverse compression were 3.53 MPa and 3.13 MPa,and that were 4.22 MPa and 3.95 MPa under vertical compression.The compressive stress nephogram showed that the maximum stress point in upper contact surface was larger than that in lower contact surface,and the contact stress distribution of upper and lower surface was symmetrical under transverse compression.When the fruits were ruptured,the maximum stress of white ripe and crisp ripe fruit under transverse loading were 0.603 MPa and 0.523 MPa,and that were 0.977 MPa and 0.733 MPa under vertical loading.The dropping mechanical characteristics were investigated using a dropping test system consisted of impact force sensor and data collector.The results indicated that height had significant effect on the dropping mechanical properties of jujube fruit.The maximum value of impact acceleration,force and deformation of fruit growed;the residual deformation and its corresponding damage also aggravated as the increase of dropping height.At the same maturity and the same drop height,the peak stress of fruit was the largest when the the contact material was the steel plate;the peak stress was the second at the contact material of rubber plate,and the peak stress was the least at the contact material of paperboard.When the contact materials were steel plate,rubber plate and paperboard,the safe hight of white ripe fruit were 0.41 m,0.65 m and 0.68 m,and the safe hight of crisp ripe fruit were 0.37 m,0.50 m and 0.54 m.(3)Aimed to seek the optimum frequency range for vibration harvesting of Dongzao,the vibration experiment on Dongzao trees were carried out.The low order natural frequencies of trees were obtained by spectral analysis,and the frequency range of first three natural frequencies was 5~15 Hz.Then in this range,a shaking experiment on trees was carried out and the range of branches' resonance frequency was determined in 10~20 Hz.Finally,in the resonance frequency range a fruit removing experiment was conducted and reached a conclusion that the removing rate of crisp ripe fruits was the largest in the range of 12.5~17.5 Hz.(4)The vibration responses of trees were analyzed using the method of field experiment and simulation.Acceleration transducers were applied to monitor the responses of trunk and branches,and the ANSYS software was used to similate the shaking process of tress.Then a comparasion was made between experimental and simulation data aimed to obtain the relationship between excitation and responses as well as experiment and simulation.The results showed that the acceleration of trees growed as the increase of shaking frequency.The experiment acceleration values were larger than simulation values.The experimental and simulation values of average acceleration had similar changing tendency and good correlation,and the mean correlation coefficient was 0.62.However,there is poor correlation between experimental and simulation values of maximm acceleration.The acceleration increased gradually from the crotch to the top of branch.