The goal of this article is to experimentally study how the vibrational acceleration spreads along the branch shaken by PVC tine, steel tine, and nylon tine for citrus canopy shaking harvesting and to compare the difference. PVC tine and steel tine have potential to be used as shaking rod for citrus canopy shaking harvesting. Nylon tine is a commonly used shaking rod. A tractor-mounted canopy shaker was developed to do the trial. The shaking frequency was set at 2.5 and 5 Hz. Experimental results showed that the vibrational acceleration at the shaking spot is not the highest. Spreading from shaking spot to the stem, it increases evidently. When spreading from stems of the outside subbranch to stems of the nearest inside subbranch, its average decrease percentage is 42%. The overall vibrational acceleration of shaking at 5 Hz is 1.85 times as high as shaking at 2.5 Hz. The overall vibrational acceleration exerted by straight PVC tine and steel tine is 1.77 and 1.97 times as high as that exerted by straight nylon tine, respectively. It is indicated that replacing nylon tine with steel tine or PVC tine helps remove the fruits inside the canopy. Replacing with steel tine is more effective than with PVC tine.
Citrus commonly called orange was cultivated in China by 2500 BC [
The importance of citrus has motivated intensive research on mechanical citrus harvesting. Mechanized harvesting (shakers fruits) significantly reduces the period of harvesting of fruits per tree [
The overall detachment percentage being low is a main problem concerned about citrus canopy shaking harvesting. Harvesting trial using canopy shaker had indicated the fruit removal efficiency ranges between 80 and 90%. Analysis of the force and acceleration during the harvest in the tree canopy might give some insightful information for understanding and resolving the raised concerns [
(a) A picture of citrus grove, (b) a schematic drawing of canopy shaking harvester working along the row of citrus groves, (c) a schematic drawing of shaking harvesting, and (d) a schematic drawing of fruits and branch zones.
The large canopy shaker is unsuited to do shaking reaction and comparison trials because its adjust ability is very limited. Therefore, an experimental canopy shaking machine was designed and developed for conducting field trials, which is a tractor-mounted canopy shaker with adjustable shaking frequency, tine number, shaking height, and shaking pose and position (as shown in Figure
(a) The experimental canopy shaking machine and (b) the vibration mechanism.
Four micromachined capacitive accelerometers MMA7260Q (Freescale Semiconductor Inc., 2013) are used as acceleration sensors to detect vibrational acceleration generated by shaking canopy. Its internal sampling frequency is 11 kHz. Acceleration occurred in three axes
Next,
The magnitude of acceleration detected by an accelerometer. Because the sensors are set to sense any vibration in three axes up to acceleration of
These
The median of those acceleration peaks, which is expressed as
The MATLAB 2016a (The Mathworks, Inc.) is used in data process and calculating the median of those acceleration peaks. Figure
The accelerometer and signal conditioning system.
Accelerometer fixing methods on branches and stems.
The average of the median of the resultant acceleration peaks and their standard deviation, which can reflect the overall trend of acceleration more obviously, are referred to in result analysis. They are in turn calculated using equations (
In this paper, straight PVC tine, straight steel tine, and straight nylon tine are studied and compared in the shaking trials. The straight PVC tine and the straight steel tine are cut from round rods. Figure
Physical properties of shaking tines.
Tine number | Material | Diameter (mm) | Length (mm) | Quality (kg) | Bending stiffness (N⋅m2) |
---|---|---|---|---|---|
(1) | PVC | 18.3 | 1220 | 0.74 | 308731 |
(2) | Steel | 18.3 | 1220 | 2.72 | 769991 |
(3) | Nylon | 25.4 | 1220 | 0.72 | 19855 |
(a) Shaker installed with PVC tine and (b) shaker installed steel tine.
All experiments were conducted on Valencia trees in the grove located at Citrus Research and Education Center, Lake Alfred, Florida, USA. Two trials were designed for the research purpose, in which the first one mainly aims at studying how vibrational acceleration spreads along the same branch, and the second one mainly aims at studying how vibrational acceleration spreads in the canopy from outside to inside.
120 trees were selected to do this trial. One typical branch group was selected from each of those 120 trees as a shaking area, which includes a main branch, at least one 1-depth subbranch, at least one 2-depth subbranch and a stem. The shaking frequencies were set at 2.5 Hz and 5 Hz. The shaking spot is where the shaking tine touches the canopy and pushes the branch moving. In this test, the shaking spot is in the fork between a 2-depth subbranch and a stem. Before every shaking, the free end of the shaking tine stays horizontal and touches the canopy at the shaking spot. The shaking period is 10 s. But if any sensor falls down from the tree before 10 s, the shaking will be stopped. Those 120 trees are averagely divided into 6 groups. The grouping method of those 120 trees are listed in Table
The grouping method of experimental trees.
Group number | Tree quantity | Shaking tine | Shaking frequency (Hz) |
---|---|---|---|
(1) | 20 | Straight PVC tine | 2.5 |
(2) | 20 | Straight PVC tine | 5.0 |
(3) | 20 | Straight steel tine | 2.5 |
(4) | 20 | Straight steel tine | 5.0 |
(5) | 20 | Straight nylon tine | 2.5 |
(6) | 20 | Straight nylon tine | 5.0 |
The accelerometer placing method is shown in Figure
The shaking and detecting methods of trial 1.
The second experiment is designed to observe how the acceleration spreads in the canopy from the outside to the inside. There are also 120 trees used to do this experiment. The structure of branch group selected in this experiment is a little different from the first experiment, in which it includes a main branch and at least two 1-depth subbranches. The first of those two 1-depth subbranches is located at the outside of the canopy. Another one is next to the first one. The experimental tines, shaking frequency, tree grouping method, and shaking period are the same as those of the first experiment. But the shaking spot is located at the fork between an outside 2-depth subbranch and one of its stems.
The accelerometer placing method is shown in Figure
The shaking and detecting methods of trial 2.
The acceleration curves exerted by canopy shaker change dynamically and without any evident regularity, and the maximum of acceleration peaks varies in a large range. This is because, in the process of vibration, the shaking tine and the branch cannot contact together, and their movements are not synchronized. They may impact together anytime. Furthermore, axial rotation of branches and stems may happen anytime, which leads to drastic change in the three axes’ acceleration.
Table
The average and standard deviation of the median of the resultant acceleration peaks for trial 1 (m/s2).
Location 1 | Location 2 | Location 3 | Location 4 | ||||||
---|---|---|---|---|---|---|---|---|---|
Average | Standard deviation | Average | Standard deviation | Average | Standard deviation | Average | Standard deviation | ||
PVC tine | 2.5 Hz | 5.5 | 2.85 | 9.3 | 3.48 | 16.2 | 11.64 | 29.3 | 17.00 |
5 Hz | 11.0 | 6.24 | 19.1 | 8.60 | 34.5 | 13.19 | 54.7 | 13.85 | |
Steel tine | 2.5 Hz | 7.1 | 2.92 | 11.9 | 6.19 | 21.0 | 6.61 | 37.6 | 10.88 |
5 Hz | 11.8 | 5.38 | 22.2 | 6.95 | 34.3 | 8.70 | 50.8 | 16.72 | |
Contrast |
2.5 Hz | 4.7 | 1.02 | 5.8 | 1.35 | 8.6 | 6.08 | 15.1 | 7.94 |
5 Hz | 6.1 | 6.40 | 9.7 | 5.73 | 18.3 | 7.95 | 33.6 | 17.03 |
The overall average of the median of acceleration peaks for those three shaking tines in trail 1.
Table
The average of the median of the resultant acceleration peaks for trial 2 (m/s2).
Location 1 | Location 2 | Location 3 | Location 4 | ||||||
---|---|---|---|---|---|---|---|---|---|
Average | Standard deviation | Average | Standard deviation | Average | Standard deviation | Average | Standard deviation | ||
PVC tine | 2.5 Hz | 8.9 | 9.30 | 10.3 | 3.27 | 16.2 | 6.65 | 13.6 | 4.58 |
5 Hz | 16.5 | 12.95 | 19.6 | 7.32 | 33.1 | 13.96 | 26.8 | 11.97 | |
Steel tine | 2.5 Hz | 9.2 | 6.96 | 11.1 | 4.34 | 20.9 | 11.80 | 15.0 | 5.57 |
5 Hz | 20.3 | 11.75 | 23.2 | 6.31 | 33.7 | 13.62 | 31.4 | 9.82 | |
Contrast |
2.5 Hz | 4.9 | 2.83 | 6.4 | 2.30 | 7.2 | 3.77 | 9.3 | 2.57 |
5 Hz | 8.3 | 11.88 | 12.7 | 3.03 | 17.4 | 6.21 | 15.3 | 6.54 |
The overall average of the median of acceleration peaks for those three shaking tines in trial 2.
The resultant acceleration peaks of outside stems are listed in location 4 row of Table
The average decrease percentage of the median of acceleration peaks.
Outside (m/s2) | Inside (m/s2) | Decrease percentage (%) | ||
---|---|---|---|---|
PVC tine | 2.5 Hz | 29.3 | 16.2 | 45 |
5 Hz | 54.7 | 33.1 | 40 | |
Steel tine | 2.5 Hz | 37.6 | 20.9 | 44 |
5 Hz | 50.8 | 33.7 | 34 | |
Contrast |
2.5 Hz | 15.1 | 7.2 | 52 |
5 Hz | 33.6 | 17.4 | 48 |
The vibrational acceleration peaks of shaking using straight PVC tine and straight steel tine are much higher than shaking using straight nylon tine. For example, as the vibration frequency is 2.5 Hz, the average of the median of vibrational acceleration peaks shaking using straight steel tine at location 3 is 290.2% times as much as shaking using straight nylon tine. Figures
The rate of vibrational acceleration peaks exerted by straight PVC tine and straight steel tine compared to straight nylon tine for trial 1.
The rate of vibrational acceleration peaks exerted by straight PVC tine and straight steel tine compared to straight nylon tine for trial 2.
The above results and discussions lead to an understanding that replacing nylon tine with steel tine or PVC tine helps remove the fruits inside the canopy. Replacing with steel tine is more effective than with PVC tine. The second way to increase the removing rate of fruits in the thick branches density zone is to increase shaking frequency. However, we found in other experiments that if the shaking frequency is too high, the stability of the harvesting machine will become worse. According to the theory of mechanical vibrations, if the frequency is the same, acceleration peaks are in proportional to the amplitude. So, under the condition of the shaking frequency staying the same, increasing the shaking amplitude also helps remove the fruits inside the canopy. But the shaking amplitude of most citrus canopy shaker is unchangeable.
The acceleration curves exerted by canopy shaker change dynamically and without any regularity, and the maximum of acceleration peaks varies in a large range. From the first applied experiment, it was found that the vibrational acceleration at the shaking spot is not the highest. Spreading from shaking spot to the stem, it increases evidently. Spreading from shaking spot to the trunk, it decreases. The second trial indicated that when the vibrational acceleration of shaking spreads from the outside to the inside, it decreases evidently.
The overall vibrational acceleration of shaking at 5 Hz is 1.85 times as high as shaking at 2.5 Hz. The overall vibrational acceleration exerted by straight PVC tine and straight steel tine in turn is 1.77 and 1.97 times as high as that exerted by straight nylon tine. The overall decrease percentage of vibrational acceleration spread from a stem of the outside subbranch to a stem of the nearest inside subbranch is 42%.
It is indicated that replacing nylon tine with steel tine or PVC tine helps remove the fruits in the thick branches density zone. Replacing with steel tine is more effective than with PVC tine.
The authors declare that they have no conflicts of interest.
The authors greatly appreciate the assistance of Mrs. Sherrie M. Buchanon, Mr. Roy Sweeb, and Mr. Joseph Reichling Trotochaud for developing the experimental shaking machine which is used in this work. This research is cosupported by Guangdong Science and Technology Plan Project with Research Grant 2017A010102024, National Key R&D Plan of China with Research Grant 2017YFD0700100, and National Science Project of China with Research Grant 31571568. The authors also would like to acknowledge the China Scholarship Council for financial support.