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Article

Optimized Design and Experiment of a Self-Covering Furrow Opener for an Automatic Sweet Potato Seedling Transplanting Machine

by
Guangwei Wu
1,2,3,4,
Shoujiang Wang
1,2,3,
Anqi Zhang
1,3,
Yuejin Xiao
1,3,
Liwei Li
1,3,
Yanxin Yin
1,3,
Hanqing Li
1,3,
Changkai Wen
3,4,* and
Bingxin Yan
1,*
1
Intelligent Equipment Research Center, Beijing Academy of Agriculture and Forestry Sciences, Beijing 100097, China
2
College of Mechanical and Electrical Engineering, Xinjiang Agricultural University, Urumqi 830052, China
3
State Key Laboratory of Intelligent Agricultural Power Equipment, Beijing 100097, China
4
National Engineering Research Center for Information Technology in Agricultural, Beijing 100097, China
*
Authors to whom correspondence should be addressed.
Sustainability 2023, 15(17), 13091; https://doi.org/10.3390/su151713091
Submission received: 14 July 2023 / Revised: 21 August 2023 / Accepted: 28 August 2023 / Published: 30 August 2023
(This article belongs to the Special Issue New Technological Applications in Agriculture for the Development of the Circular Bioeconomy)
Browse Figures
Versions Notes

Abstract

:
The yield and quality of sweet potatoes are significantly influenced by the transplantation posture of sweet potato seedlings. The performance of the sweet potato seedling transplanting opener directly affects the transplantation posture of sweet potato seedlings. In order to improve the yield and quality of sweet potatoes, this study proposes a joint simulation method based on discrete element and flexible multi body dynamics (DEM-FMBD), which optimizes the structure of a self-covering soil opener. By exploring the influence of self-covering soil trenchers on the planting depth and posture of sweet potato seedlings during horizontal transplantation, it was determined that the influencing factors of the experiment were wing spacing, soil reflux height, and soil reflux length. Based on the DEM-FMBD coupling simulation platform, single factor, and quadratic rotation orthogonal experiments were carried out. According to the results of the simulation test, the effect of the interaction of test factors on planting depth and planting attitude was analyzed by the response surface method. Finally, the optimal structural parameter combination was obtained by a multi-objective optimization method: the spacing of the wings was 58 mm, the height of the soil backflow port was 71 mm, and the length of the soil backflow port was 163 mm; thus, the quality of transplanting is improved effectively. This study provides the method and theory reference for the study of sweet potato transplanting.

1. Introduction

Sweet potato (Ipomoea batatas) is a sprawling tuberous crop formed from tubers inserted into underground stem nodes [1]. Sweet potato is a widely grown crop, with 9 million hectares planted globally and an annual production of about 131 million tonnes. More than 115 nations cultivate sweet potatoes, the majority of which are developing nations [2]. China is the world’s largest sweet potato producer, with 4.7 million hectares planted and 52 percent of the production [3]. As one of the world’s main food crops, sweet potato has a profound impact on the global agricultural development pattern and human well-being.
Sweet potatoes can serve as an important emergency food crop and have strategic significance in addressing global food security issues [4,5]. With the increase in the world population and the reduction in arable land, the food security issues facing humanity are becoming increasingly serious. Sweet potato can produce a large amount of food per unit time, and has more yield advantages than other staple crops [6]. Sweet potatoes are rich in minerals (calcium, potassium, copper, and iron), vitamins (A, B, and C), dietary fiber, and a small amount of fat and protein [7]. Meanwhile, sweet potatoes contain different groups of anthocyanins, which are related to antioxidant and anti-inflammatory effects; sweet potatoes also have unique effects on inhibiting cardiovascular diseases and preventing cancer [8].
Furthermore, sweet potatoes are crucial for sustainable development [9]. Environmental pollution contributes to global climate change, which significantly affects crop production. Unlike other staple crops such as wheat, corn, and rice, sweet potatoes have strong adaptability and resistance to drought and poor soil conditions [10,11]. Meanwhile, sweet potato is the basic raw material for extracting industrial starch [12]. The starch and sugars in sweet potato can be converted into bioethanol [13], which can be directly used as fuel or mixed with traditional petroleum fuels, greatly reducing greenhouse gas emissions [14].
Based on the aforementioned research, sweet potatoes play a critical role in human survival and development [15]; therefore, there is an urgent need to improve the yield and quality of sweet potatoes.
The planting depth and planting posture of sweet potato during horizontal transplanting are crucial factors that directly impact the yield and quality of the crop [16]. Planting sweet potato at a depth of more than 70 mm allows the crop to effectively absorb water from the soil, thereby improving its drought resistance [17]. On the other hand, planting sweet potato at a depth less than 50 mm can weaken its drought resistance and increase seedling mortality. Moreover, this may affect the permeability and growth of the bottom tuber, leading to deformities and cracks in the crop. Furthermore, the length of the underground part of the sweet potato planting posture should be considered [18,19]. If it is less than 80 mm, which means that the number of sweet potato nodes is less than 3, it is not conducive to increasing yield. Conversely, if the underground length of the planting posture is more than 100 mm, it may result in a short aboveground part, which is not conducive to photosynthesis and nutrient supply [20,21]. The opener serves as a critical component of the sweet potato transplanting machine. When the sweet potato seedlings fall into the seedling channel, the soil can automatically return to the seedling trench in accordance with the agronomic requirements to open a suitable depth of the seedling trench, ensuring that the sweet potato seedlings have a suitable depth of planting and a good planting posture [22,23,24]. Therefore, conducting research on sweet potato transplanting openers is crucial.
In recent years, according to different crops and agronomic requirements, scholars have conducted in-depth research on trenching devices [25]. By researching the principle of variation of the traction force of the furrow opener in chernozem, V A Ovtov et al. created a furrow opener appropriate for planting onions, which enhanced the quality of the furrow for onion planting [26]. To meet the high standards of sugarcane transverse planting, Li Shangping created a combination sugarcane transverse planting furrow opener by studying the movement of seeds, the discipline of soil movement, and the resistance of the furrow plow surface [27]. In order to design a no-tillage sowing opener, Aliakbar Solhjou et al. [28] investigated the link between the performance of the opener and horizontal and vertical soil disturbances. The research methods of the above scholars mainly focus on theoretical analysis, which requires more time and resources. Furthermore, this theoretical analysis is conducted within a completely idealized environment This method is more likely to overlook the complexity and uncertainty of field operations.
With the development of computer technology, some scholars use computer-aided digital simulation methods to design the structure of key components, and then conduct field experiments based on the optimized design results. Compared with the traditional test and trial and error method, the simulation method can design the required products more efficiently and improves the reliability and adaptability of the products [29,30,31,32]. Zhou et al. [33] established the corresponding EDEM soil particle model based on the soil characteristics of the black soil in the northeast cold region. Based on the soil model, the mechanism of the deep ditcher is designed by using the discrete element method. Eventually, the optimal structural parameters of the deep furrow opener are obtained: the width of the furrow opener is 37.52 mm, and the sliding cutting Angle is 42.27°. James Barr et al. [34] used EDEM software to analyze the rule of soil movement under different operation depths and speeds, providing a reference for the design target depth and operation speed of a no-till ditcher. In sandy orchards, the soil is very fluid, which makes ditches less stable. To solve this problem, Li Liangliang et al. [35] used both discrete element simulation experiments and soil bin experiments to design ditch-opening devices. For the above simulation research, the main focus is on the structure or soil state of the trencher, without fully considering the impact of soil movement on crop transplanting posture after the trencher operation.
This study has different research in the following two main aspects, in order to provide methodological and theoretical references for the research of the sweet potato bare seedling transplanter.
(1) The research on the ditcher in the above literature mainly focused on the study of soil disturbance patterns by furrow openers, and the effect of soil flow on the quality of furrowing after the furrowing operation. Based on previous studies, this study further investigates the influence of soil backflow on the planting posture of sweet potato seedlings after the operation of a self-covering trencher.
(2) The method of using discrete element simulation has become the main method to explore the ditcher. Most scholars’ research focuses on the effect of the ditcher on soil particles, or the interaction between soil particles. It is rare to consider the interaction between soil particles and crops, especially in the simulation research of sweet potato naked seedlings with strong flexibility using the discrete element method. Meanwhile, the simulation studies of flexible crops are very difficult.
Therefore, in this study, the discrete element method and flexible multi-body dynamic coupling simulation (DEM-FMBD) were used to simulate the operation process of “trenching—sending seedlings—self-covering soil”. The influence of soil reflux on the posture of naked sweet potato seedlings was analyzed by simulation, which provided a reference for the structural optimization design of self-covering soil ditcher. Finally, the results of the field experiment show that the method of DEM-FMBD coupling simulation is reliable, which provides a method and theoretical reference for the research on the sweet potato transplanter.

2. Materials and Methods

2.1. Structure and Working Principle of Automatic Transplanting Machine for Sweet Potato Bare Seedling

Combined with the agronomic requirements of the horizontal transplanting of sweet potato, the research group developed an automatic transplanting machine for bare sweet potato seedlings, as shown in Figure 1. The bare sweet potato seedling transplanting machine is hung at the tail of the tractor by three-point suspension. The machine is mainly composed of a ridging device, a seedling feeding device, a planting device, a furrow opener, a pressing wheel, and a ridge repair device.
The working principle of the sweet potato horizontal transplanting machine studied in this article is divided into the following two parts:
(1) Control part of the sweet potato naked seedling transplanting machine: The tractor pulls the sweet potato naked seedling transplanting machine forward through a three-point suspension, while the power transmission device transmits power to the rotary tillage device. Soil ridges were prepared for the transplanting of bare sweet potato seedlings by a rotary tillage and ridging device. The automatic transplanting controller can obtain real-time vehicle speed information. The automatic transplanting device combined vehicle speed information and plant distance information to calculate the motor speed of the seedling feeding device and planting device. The speed of the two motors matches each other, so that the seedling feeding device can feed the naked sweet potato seedlings into the planting device smoothly. The planting device adds the bare seedlings of sweet potato to start the movement.
(2) The planting part of naked sweet potato seedlings in the soil: the blade of the opener breaks the soil, and the wing plate of the opener squeezes the soil into both sides to form a seedling ditch. The planting device clamps the bare seedling movement, and the root of the bare seedling contacts the bottom of the ditch to bend. While the opener advances, the soil on both sides of the opener quickly returns and supports the sweet potato bare seedling. When the planting device moves to the release point, the naked sweet potato seedlings are released. After the suppression device completes the suppression, the watering device waters the planted sweet potato bare seedlings. Finally, the ridge shape is modified and compacted by the ridging side plow and ridging device.

2.2. Agronomic Requirements for Horizontal Transplanting of Sweet Potato

Sweet potato horizontal transplanting agronomy requires that the seedling ditch is flat, the soil moisture is consistent, and the planting depth of sweet potato seedlings is consistent. Appropriate shallow planting of sweet potato is beneficial to tuberization if implemented as soon as possible. The planting depth HD is generally less than 70 mm, but not less than 50 mm. Due to the physical characteristics of the bare seedlings of sweet potato, the bare seedlings at the bottom of the seedling ditch cannot fully present the horizontal state, but can only reach the class level planting state, and the effect is as shown in Figure 2.
Therefore, this study defines the lowest point of bare seedlings in the seedling ditch as the lower boundary of the class level. The upper boundary of the class level is 20 mm higher than the lower boundary of the class level. The vertical area between the upper and lower boundaries is the class level planting area (Hs). According to the growth characteristics of sweet potato, the horizontal planting length of sweet potato bare seedlings is 3~4 knots, and the soil layer is exposed to 2~3 knots. The length LS of sweet potato bare seedlings in the horizontal planting area is greater than 80 mm and less than 100 mm. The adoption of the horizontal planting method increases the yield of each large and medium-sized potato plant, thereby increasing the yield and commodity rate of sweet potatoes [36,37].

2.3. Structure and Working Principle of Trencher

The furrow opener is a crucial component of the automatic transplanting machine for sweet potato bare seedlings. The performance of the trencher directly affects the quality of naked sweet potato seedlings for horizontal transplantation, thereby affecting the yield and commodity rate of sweet potato products. Therefore, based on the agronomy of sweet potato horizontal transplanting, this study improved the sliding knife opener [38] and designed a self-covering trencher opener suitable for sweet potato horizontal transplanting. The overall structure is shown in Figure 3.
During operation, the furrow opener functions by breaking the soil with its blade, while the wing plate compresses the soil to create seedling ditches on both sides. Simultaneously, the planting disc securely grips the bare seedling as it rotates through the furrow opener. As a result, the roots of the bare seedlings make initial contact with the soil at the bottom of the seedling ditch and begin to bend. As the planting disc continued to rotate, the horizontal length of the bare seedlings in the seedling ditch increased. The soil originally blocked by the wing plates on both sides flowed into the ditch from the soil reflux outlet to support the bare seedlings in the ditch, so that the bare seedlings maintained a certain posture. The planting disc is then rotated to the release point to release the bare seedlings. Finally, the wheel is pressed to compact the loose soil, completing the horizontal planting operation in accordance with agronomic requirements [39]. The working principle is shown in Figure 4.

2.4. Analysis of the Effect of Soil Reflux on the Morphology of Bare Sweet Potato Seedlings into Soil

After the opener opens the seedling ditch, the soil on both sides of the wing plate flows back through the soil return port. The returning soil maintains the posture of naked sweet potato seedlings entering the soil. The effect of soil return directly affects the planting depth and planting posture of the sweet potato. According to the reference [38], there is a relationship between the depth of soil return and the distance between the two wings of the furrow opener and the depth of the furrow, which is shown in Equation (1). The relationship is shown in Figure 5.
H m = H − 7.245 W 0.367
where
Hm represents the soil backflow depth.
H is the ditching depth.
W is the width of the ditch opener.
Sustainability 15 13091 g005
Figure 5. Schematic diagram of soil backflow.
Figure 5. Schematic diagram of soil backflow.
Sustainability 15 13091 g005
It can be obtained from Equation (1) that when the depth of the trench is a fixed value, and as the spacing between the wing plates increases, the soil reflux depth becomes shallower. On the contrary, as the spacing between wing plates decreases, the depth of soil reflux increases. According to the agronomic requirements of the horizontal transplanting of sweet potatoes, either too deep or too shallow soil depth is not conducive to the growth and development of sweet potato seedlings [40].
After the trencher operation, the natural return of soil mainly includes soil at the edge of the soil layer and the fall of soil within the angle of repose, and the time of the return of both types of soil to the seedling furrow has an effect on the mulching of bare sweet potato seedlings [41]. When the height of the soil return port is too low, the soil at the edge of the soil layer is blocked by the wing plate, and there is not enough soil to return to the soil in time. In the above situation, the naked sweet potato seedlings will freely dump after reaching the release point, which directly affects the length of the planting posture.
Before the bare sweet potato seedlings reach the release point, some bare seedlings lying flat in the seedling furrow should be covered with soil of sufficient depth. When the length of the soil return opening is short, the soil is not returned in time, and the bare sweet potato seedlings will fall in any direction after being released. When the naked seedlings of sweet potatoes do not reach the release point, the horizontal seedling length in the seedling ditch is shorter. However, the length of the soil reflux port is longer, causing the soil to fall into the seedling ditch before the naked sweet potato seedlings. In the above situation, the curved part of the sweet potato seedlings in the ditch will be placed on top of the soil flow, which can easily cause the planting depth to become shallower [42].
According to the previous research of the research group, when the interval distance of the opener fender is from 40 to 80 mm, the height of the soil return port is from 50 to 100 mm, and the length of the soil return port is from 100 to 180 mm, the quality of sweet potato bare seedling transplanting has obvious changes. This study will further explore and obtain the optimal structural parameters of the opener to obtain better quality sweet potato transplanting.

2.5. EDEM and Recurdyn Co-Simulation

To determine the optimal combination parameters of the sweet potato transplanting opener, combined with the agronomic requirements of sweet potato transplanting, a simulation test was carried out with wing plate spacing, soil return port height, and soil return port length as influencing factors. At present, scholars use discrete element simulation technology (DEM) to simulate the motion state of trenching components and soil particles, and optimize the design of trenching components [43]. In the process of horizontal transplantation of sweet potato bare seedlings, in addition to studying the movement of the opener and soil particles, it is also necessary to consider the contact between sweet potato seedlings and soil and the dynamics of flexible bodies. Therefore, the discrete element and multi-body dynamics coupling simulation methods are used to simulate and analyze the operation process of “ditching-seedling-covering soil”.
During the horizontal transplanting process of sweet potato, the bare seedlings of sweet potato were clamped by the planting disc for rotating motion. The roots of the seedlings first contacted the seedling furrows and bent. As the opener continued to move forward, the seedlings in the furrows were partially covered by the backflow soil and maintained a certain shape. When the bare seedlings of sweet potato are released at the release point, the soil is returned to complete the soil covering. Among them, the bare sweet potato seedlings are bent and deformed in contact with the soil during transplanting. This large-deformation flexible body simulation is different from the small deformation of the rigid body, which is more complicated and cumbersome [44,45]. Using Recurdyn multi-body dynamics software and MFBD technology, the flexible model of sweet potato seedlings can be established through the coupling calculation of finite element flexible body and multi-body dynamics [46]. To make the flexible sweet potato bare seedling model have similar physical characteristics to the actual sweet potato seedlings, the physical parameters of the sweet potato bare seedlings were set by consulting the relevant literature [47]. The shear modulus was 18.68 MPa, the Poisson’s ratio was 0.3, and the density was 1132.2 kg/m3.
In the process of simulation analysis, the structure that does not affect the simulation results can be simplified or omitted to improve the simulation efficiency and facilitate the analysis of the main research objects. The branches and leaves of sweet potato seedlings did not influence on the simulation results; thus, the sweet potato seedlings were simplified into a slender cylinder with a length of 250 mm and a radius of 2 mm. In order to reduce the computational complexity of simulation and observe the burial posture of sweet potato seedlings, the part of the planting disc holding the bare seedlings was replaced by a cylindrical rigid body with a length of 100 mm, diameter of 4 mm, and flexible body. A rotating pair is added between the cylindrical rigid body and the ground. The distance between the rotating center and the end of the rigid body is equal to the distance between the center of the actual planting disc and the clamped bare seedling point.
In the actual operation process, the relative position of the furrow opener and the planting disc is shown in Figure 2. This study only focuses on the influence of the structure of the furrow opener on the transplanting effect. Therefore, the horizontal distance between the planting disc’s rotation and the furrow opener’s relative motion is calculated first. The position between the furrow opener and the bare seedlings of the flexible body is adjusted. A translation pair is established between the furrow opener and the ground. The dynamic simulation model is shown in Figure 6.
In the transplanting operation, there are mutual collisions, sliding, and interactions of soil particle flow between soil particles and the opener and bare sweet potato seedlings. The process of their interaction requires an accurate discrete element model for analysis. The ‘Hertz-Mindlin with JKR’ model in the EDEM discrete element software is a cohesive contact model. Based on the Hertz theory, the influence of the cohesive force between particles on the particle motion rule is considered. It is suitable for simulating cohesive particles and agglomerated soil, which is consistent with the motion characteristics of soil particles during the sweet potato transplanting operation in this study [48,49]. The preliminary test of the research group was carried out in the National Precision Agriculture Research Demonstration Base in Xiaotangshan Town, Beijing. The soil was clay with a water content of 12%. The soil parameters were calibrated by referring to the relevant literature [50], as shown in Table 1. To simulate the transplanting soil of sweet potato, a cuboid soil tank with length × width × height of 500 × 300 × 200 mm was established in EDEM software, and 2.2 × 105 soil particles were set in the particle factory. The built soil model is saved as Material Block, which is convenient to call directly later.
In EDEM software, the coupling interface with Recuedyn is opened, and the Wall established in RecurDyn is imported. When the soil model is selected, the previously established soil model can be directly imported. Then, it is returned to the RecurDyn interface where it performs Dyn/Kin simulation. The joint simulation model is shown in Figure 7.

2.6. Simulation Test Design

2.6.1. Single-Factor Experiment Design

The spacing of the opener wings, the height of the soil return port, and the length of the soil return port directly impact on the planting depth and planting posture of the sweet potato. Therefore, a single-factor test was carried out to explore the value of each factor further. In the previous study of the research group [39], the forward speed of the opener was set to 0.25 m/s, the top of the opener was 50 mm higher than the soil model, and the rotation speed of the bare sweet potato seedlings was 1.4 rad/s. The EDEM post-processing module ruler function was used for measurement. As shown in Figure 8, the planting depth was from the upper line of the class level to the soil surface, and the length of the seedling in the class level area was the planting posture. In addition, the results of the backflow of soil particles during the operation of the opener and the interaction between the bare seedlings of sweet potato and the soil are shown in Figure 9.
Firstly, the effect of the spacing of the wings on the planting depth and planting posture of the sweet potato bare seedlings was explored. In the test, the height of the soil backflow port was 75 mm, and the length of the soil backflow port was 140 mm. Five levels (50, 55, 60, 65, and 70 mm) of the spacing of the wings were selected for the test. As shown in Figure 10a, with the increase in wing spacing, the planting depth increased first and then decreased, and began to decrease after reaching the peak when the wing spacing was 60 mm. When the distance between the wing plates is less than 55 mm and more than 65 mm, the planting depth is less than 50 mm, and the water content near the surface of the soil layer is less, the sweet potato seedlings cannot absorb enough water for growth, which is not conducive to the survival of the seedlings.
As shown in Figure 10b, with the increase in wing spacing, the length of planting posture showed a decreasing trend. When the wing spacing was less than 55 mm, the transplanting posture of the sweet potato was greater than 100 mm, resulting in shorter sweet potato seedlings in the soil layer, which was not conducive to photosynthesis and nutrition supply. When the wing spacing is greater than 65 mm, the posture length of the sweet potato is less than 80 mm, and the number of nodes of bare sweet potato seedlings buried in the soil is small, which is not conducive to increasing the number of sweet potato tubers. Therefore, further research shows that the spacing between the wing plates of the opener should be controlled within the range of 55~65 mm.
Secondly, the influence of the height of the soil backflow port of the opener on the planting depth and planting posture of the sweet potato bare seedlings was explored. In the experiment, the spacing of the opener’s wing plate was 60 mm, the length of the soil backflow port was 140 mm, and five levels of soil backflow port height (50, 60, 70, 80, and 90 mm) were selected for the experiment. The test results are shown in Figure 11a. When the height of the soil backflow increases, the planting depth of the bare sweet potato seedlings increases rapidly and then decreases. There is an obvious peak when the soil return port is 70 mm. When the height of the soil backflow port is about 50~60 mm or 80~90 mm, the planting depth is less than 50 mm, which will lead to the thin soil layer covering the bare seedlings of sweet potato and less water content, which will affect the growth and development of sweet potato seedlings. At the same time, the height of soil backflow port has the same influence trend on the planting posture.
As shown in Figure 11b, when the height of the soil backflow port is about 50~60 mm or 80~90 mm, the length of the planting posture is less than 80 mm, resulting in not enough sweet potato nodules under the soil to bear fruit, affecting the yield of sweet potato. Based on the above analysis, this study decided to study further the influence of the opener in the range of a 60~80 mm height of the soil backflow port.
Finally, the influence of the length of the soil backflow port of the opener on the planting depth and planting posture of the sweet potato bare seedlings was explored. In the experiment, the spacing of the wing was 60 mm, the height of the soil backflow port was 75 mm, and five levels of the length of the soil backflow port (110, 125, 140, 155, and 170 mm) were selected for the test. As shown in Figure 12a, with the increase in the length of the soil backflow port, the planting depth first increased and then decreased, and began to decline after reaching the peak at 140 mm. When the height of the soil backflow port is 110~140 mm, the transplanting depth of sweet potato is less than 50 mm; that is, the bare seedlings of sweet potato are shallow and cannot absorb enough water, thus affecting the growth of sweet potato.
In addition, as shown in Figure 12b, the planting posture of bare sweet potato seedlings increased with the increase in the length of the soil backflow port. When the length of the soil backflow port was 155 mm, it reached the maximum value and then decreased. When the length of soil backflow port is between 110~140 mm, the planting posture of the sweet potato bare seedling is less than 80 mm, and the number of nodules is small, which affects the yield of sweet potato. According to the above analysis, the length of the soil backflow port in the range of 140~170 mm has a more appropriate choice.

2.6.2. Orthogonal Experimental Design

Through the results of the single factor test, it can be observed that under the action of various factors, the level of factors corresponding to the optimal planting depth and planting posture is different. In this study, to determine the optimal combination of these three factors (the spacing of the wing, the height of the soil backflow port, and the length of the soil backflow port), a quadratic rotation orthogonal test was carried out, and the test factors were coded as shown in Table 2.
The rotation orthogonal combination test design was completed by Design-Expert 12 software, as shown in Table 3, where X1, X2, and X3 are the factor coding values of x1, x2, and x3, respectively, and the planting depth Y1 and planting posture Y2 are test indicators.

3. Results and Discussion

3.1. Key Parameters’ Fitting and Analysis

The experimental data in Table 3 were analyzed and fitted by Design-Expert 12 software, and the mathematical models of planting depth Y1 and planting posture Y2 were established.
Through the analysis of the test results in Table 3, the regression model analysis of variance and significance test results are shown in Table 4. For the regression model of planting depth Y1, the fitting degree was extremely significant (p < 0.01). The p values of the spacing of the wing (X1), the height of soil backflow port (X2), the length of soil backflow port (X3), and the quadratic of the spacing of the wing (X12) were all less than 0.01, indicating that it had an extremely significant effect on the planting depth. The p values of the interaction term (X1X2) between the spacing of the wing and the height of the soil backflow port and the interaction term (X1X3) between the spacing of the wing and the length of soil backflow port were less than 0.01, indicating that their interaction had an extremely significant effect on the planting depth. However, the p values of the square of the height of soil backflow port (X22), the square of the length of soil backflow port (X32), and the interaction term of the height of the soil backflow port and the length of the soil backflow port (X2X3) were all greater than 0.05, indicating that they did not influence on the planting depth. Excluding the insignificant items in the regression model, the regression equation is Equation (2).
Y 1 = 54.41 + 2.48 X 1 + 2.54 X 2 − 2.27 X 3 − 7.80 X 1 X 2 + 4.87 X 1 X 3 − 6.09 X 1 2
The fit was extremely significant (p < 0.01) for the regression model of planting posture Y2. p-values for the spacing of the wing (X1) and the quadratic of the spacing of the wing (X12) were less than 0.05, indicating a significant effect on planting posture; p-values for the height of soil backflow port (X2), the length of soil backflow port (X3), the quadratic of the height of soil backflow port (X22), and length of soil backflow port (X32) were all less than 0.01, indicating a highly significant effect on planting posture. Furthermore, the p-values for the interaction of the spacing of the wing and the height of the soil backflow port (X1X2) were less than 0.01, showing that it had an extremely significant influence on planting posture. However, the interactions between the spacing of the wing and the length of the soil backflow port (X1X3) and the height of the soil backflow port and the length of the soil backflow port (X2X3) exhibited p-values greater than 0.05, suggesting that their interactions did not influence planting posture. After removing the non-significant terms from the regression model, the regression Equation (3) is obtained.
Y 2 = 89.36 + 1.48 X 1 + 3.13 X 2 + 3.35 X 3 − 3.15 X 1 X 2 − 1.88 X 1 2 − 9.21 X 2 2 − 3.70 X 3 2
According to the analysis of the regression model, the interaction between the spacing of the wing, and the height of soil backflow port, the interaction between the spacing of the wing and the length of the soil return port has a significant effect on the planting depth. At the same time, the interaction between the spacing of the wing and the height of the soil backflow port has a very significant influence on the planting posture. The origin software was used to process the response surface, as shown in Figure 13.
Figure 13a is the response surface diagram of the interaction between the spacing of the wing and the height of the soil backflow port on the planting depth when the length of the soil backflow port is 155 mm. When the height of the soil backflow port is kept constant, the increase in the spacing of the wing will gradually increase the planting depth. The same trend is: when the spacing of the wing remains unchanged, with the increase in the height of soil backflow port, the planting depth will also increase.
Figure 13b is the response surface diagram of the interaction between the spacing of the wing and the length of the soil backflow port on the planting depth when the height of the soil backflow port is 70 mm. When the length of the soil backflow port remains unchanged, the planting depth increases and then decreases slowly with the increase in the spacing of the wing. When the spacing of the wing remains unchanged, the planting depth decreases with the increase in the length of the soil backflow port.
Figure 13c is the response surface diagram of the interaction between the spacing of the wing and the height of the soil backflow port on the planting posture when the length of the soil backflow port is 155 mm. When the height of the soil backflow port is a certain value, the planting attitude increases with the increase in the spacing of the wing. When the distance between the spacing of the wing is a certain value, the planting posture increases first and then decreases with the increase in the height of the soil backflow port.
To obtain a better horizontal transplanting quality of sweet potato, the planting depth and planting posture were taken as the optimization objectives, and the Design-Expert 12 software was used to optimize the spacing of the wing, the height of the soil backflow port, and the length of the soil backflow port. The planting depth and planting posture of the sweet potato horizontal transplants were 50~70 mm and 80~100 mm, respectively, which met the agronomic requirements of sweet potato horizontal transplanting. When the planting depth and planting posture of sweet potato horizontal transplanting are close to 50 mm and 100 mm, respectively, the sweet potato will obtain better yield and quality. The optimization objective and constraint function are given in the following Equation (4).
min Y 1 ( X 1 , X 2 , X 3 ) max Y 2 ( X 1 , X 2 , X 3 ) − 1 ≤ X 1 ≤ 1 − 1 ≤ X 2 ≤ 1 − 1 ≤ X 3 ≤ 1
The optimal parameter combination obtained by calculation is: the spacing of the wing—58.05 mm, the height of the soil backflow port—71.21 mm, and the length of the soil backflow port—163.85 mm. After rounding, the final result is: the spacing of the wing—58 mm, the height of soil backflow port—71 mm, and the length of the soil backflow port—163 mm.

3.2. Field Verification Test

Through the results of a single factor test, it can be observed that under the action of various factors, the level of factors corresponding to the optimal planting depth and planting posture is different. In this study, to determine the optimal combination of these three factors (the spacing of the wing, the height of the soil backflow port, and the length of the soil backflow port), a quadratic rotation orthogonal test was carried out, and the test factors were coded as shown in Table 2.
To verify the effect of the operation performance of the optimized sweet potato opener on the transplanting quality of sweet potato, as shown in Figure 14, the group performed field verification tests in May 2022 on the National Precision Agricultural Demonstration Base’s experimental field in Xiaotangshan, Changping District, Beijing, China (longitude—116°26′34.13105″, latitude—40°10′59.53673″, and altitude—36 m), to validate the operating performance of the optimized sweet potato trencher. The test field consisted of dark soil with a moisture level of 12%. The test area had been plowed and tilled, and the soil was fine and satisfied the test requirements. The test equipment was a sweet potato bare seedling automated transplanting machine created previously by the subject group, which was driven by a Dongfeng 554 tractor with a forward speed of 0.25 m/s. The sweet potato variety was watermelon red, and 90 naked seedlings with an average length of 250 mm and a diameter of 4 mm were chosen. Upon regulating the machine’s speed, the experiment was divided into three groups, each containing 30 naked seedlings.
The soil on the side of the naked sweet potato seedlings was dug out and measured at the conclusion of the test. The planting depth criteria are satisfied when the planting depth reaches 50~70 cm and the planting posture reaches 80~100 cm. As indicated in Table 5, the number of qualifying planting depths N1, number of qualified planting postures N2, and number of plantings per group N0 were measured. W1 and W2 are the planting depth and planting posture qualifying rates, as derived from Equations (5) and (6), respectively.
W 1 = N 1 N 0 × 100 %
W 2 = N 2 N 0 × 100 %
The average certified rate of planting depth is 94.4%, while the average qualified rate of the planting posture is 88.9%. When the trial data before and after optimization were compared, it was clear that the transplanting quality of naked sweet potato seedlings had improved.

4. Conclusions

To increase sweet potato production and quality, this study utilized planting depth and planting posture as experimental indices. As experimental elements, the spacing of the wing, the height of the soil backflow port, and the length of the soil backflow port were calculated by researching the regular soil disturbance by the opener. This work suggested a structural optimization approach based on DEM and FMBD coupling simulation. The ideal structural parameters were established by multi-objective optimization using single-factor and quadratic rotation orthogonal experiments. Field studies were conducted to assess the performance of the optimized opener. The main conclusions of this study are as follows:
(1) In this study, a parameter optimization design method for the opener of a sweet potato transplanter based on DEM-FMBD coupling simulation was proposed. This method has been verified by single factor and quadratic rotation orthogonal and field test methods. The performance of the optimized opener is significantly improved.
(2) The results of the single-factor test showed that the spacing of the wing, the height of the soil backflow port, and the length of the soil backflow port were the main influencing factors of the planting depth and planting posture. The spacing of the wing was 55~65 mm, the height of the soil backflow port was 60~80 mm, and the length of the soil backflow port was 140~170 mm. Based on the range of factor level, the quadratic rotation orthogonal test scheme was determined and the test was carried out.
(3) The results of the quadratic rotation orthogonal test showed that the interaction between the spacing of the wing and the height of the soil backflow port, as well as the interaction between the spacing of the wing and the length of the soil backflow port, had a very significant effect on the planting depth. The interaction between the spacing of the wing and the height of the soil backflow port had a significant effect on the planting posture. Through the multi-objective optimization design method, the optimal parameters of the opener are obtained; the spacing of the wing is 58 mm, the height of the soil backflow port is 71 mm, and the length of the soil backflow port is 163 mm. Based on the optimized opener, the field test was carried out. The test results showed that the qualified rate of the planting depth was 94.4%, and the qualified rate of the planting posture was 88.9%. Compared with the opener before optimization, its working performance is significantly improved.
(4) In this work, the structural optimization design approach based on DEM-FMBD coupling simulation is used to swiftly carry out the simulation test of the opener with different parameter combinations, which saves a lot of time and resources and effectively completes the optimization design. However, this study only analyzed the process of transplanting single naked sweet potato seedlings. The next objective of this research will be to conduct a simulation analysis of the continuous transplanting of numerous sweet potato bare seedlings. At the same time, this study will focus on the effect of sweet potato transplanting and feeding speed on sweet potato transplanting.
(5) Since the intrinsic properties of the soil have some influence on the mulching performance of the self-covering furrow openers, the soil model for this study was built around the test field soil, which had a moisture content of 12% and a clay soil texture. Therefore, we will carry out relevant research for soil with different moisture content, and develop a self-covering soil trencher with better performance.

Author Contributions

Conceptualization, G.W., B.Y. and C.W.; methodology, G.W., A.Z. and S.W.; software, B.Y. and S.W.; validation, C.W.; formal analysis, G.W. and H.L.; investigation, S.W.; resources, G.W., L.L., Y.X. and Y.Y.; data curation, S.W.; writing—original draft preparation, G.W., C.W. and S.W.; writing—review and editing, B.Y. and H.L.; visualization, G.W. and S.W.; supervision, C.W.; project administration, G.W.; funding acquisition, B.Y. and C.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Beijing Academy of Agriculture and Forestry Sciences Postdoctoral Foundation (2022-ZZ-012), Automatic transplanting machine for sweet potato cutting seedlings (NK2022160301) and Key R & D projects in Shandong Province (2022CXGC010608).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

We would like to thank “Intelligent Equipment Research Center, Beijing Academy of Agriculture and Forestry Sciences” and “College of Mechanical and Electrical Engineering, Xinjiang Agricultural University”.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Automatic transplanter for naked sweet potato seedlings. 1—tractor; 2—automatic transplanting controller; 3—three-point suspension device; 4—power transmission device; 5—watering device; 6—seedling feeding device; 7—planting device; 8—ridging flank plow; 9—ridging device; 10—pressing device; 11—furrow opener; 12—ridging device.
Figure 1. Automatic transplanter for naked sweet potato seedlings. 1—tractor; 2—automatic transplanting controller; 3—three-point suspension device; 4—power transmission device; 5—watering device; 6—seedling feeding device; 7—planting device; 8—ridging flank plow; 9—ridging device; 10—pressing device; 11—furrow opener; 12—ridging device.
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Figure 2. Class horizontal planting effect diagram. 1—Sweet potato bare seedlings; 2—Soil ridge top; 3—Class horizontal upper line; 4—Class horizontal lower line.
Figure 2. Class horizontal planting effect diagram. 1—Sweet potato bare seedlings; 2—Soil ridge top; 3—Class horizontal upper line; 4—Class horizontal lower line.
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Figure 3. Schematic diagram of ditch opener. HK—the height of ditch opener; LU—distance from the top opening of ditch opener to soil backflow port; LX—distance from the bottom of the ditch opener to soil backflow port; LT—length of soil backflow port; α—entry angle of ditch opener; HT—height of soil backflow port; BK—spacing between wings.
Figure 3. Schematic diagram of ditch opener. HK—the height of ditch opener; LU—distance from the top opening of ditch opener to soil backflow port; LX—distance from the bottom of the ditch opener to soil backflow port; LT—length of soil backflow port; α—entry angle of ditch opener; HT—height of soil backflow port; BK—spacing between wings.
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Figure 4. Schematic diagram of ditch opener operation. 1—Soil ridge top; 2—Furrow opener; 3—Sweet potato bare seedlings; 4—Planting disc; 5—Press wheel; 6—Soil backflow port.
Figure 4. Schematic diagram of ditch opener operation. 1—Soil ridge top; 2—Furrow opener; 3—Sweet potato bare seedlings; 4—Planting disc; 5—Press wheel; 6—Soil backflow port.
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Figure 6. Multi-body dynamic model of furrow opener. 1—Sweet potato naked seedling; 2—Fixed pair; 3—Rotating pair; 4—Furrow opener; 5—Moving pair.
Figure 6. Multi-body dynamic model of furrow opener. 1—Sweet potato naked seedling; 2—Fixed pair; 3—Rotating pair; 4—Furrow opener; 5—Moving pair.
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Figure 7. Results of RecurDyn introduced into EDEM. 1—Soil Discrete Element Model; 2—Sweet Potato Bare Seedlings; 3—Fixed Pair; 4—Rotating Pair; 5—Furrow Opener; 6—Moving Pair.
Figure 7. Results of RecurDyn introduced into EDEM. 1—Soil Discrete Element Model; 2—Sweet Potato Bare Seedlings; 3—Fixed Pair; 4—Rotating Pair; 5—Furrow Opener; 6—Moving Pair.
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Figure 8. Results measurements. 1—Sweet potato naked seedlings; 2—Soil particles; 3—Class level up line; 4—Class level down line.
Figure 8. Results measurements. 1—Sweet potato naked seedlings; 2—Soil particles; 3—Class level up line; 4—Class level down line.
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Figure 9. Posture diagram of naked sweet potato seedlings buried in soil.
Figure 9. Posture diagram of naked sweet potato seedlings buried in soil.
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Figure 10. Simulation test results of wing plate spacing. (a) Diagram demonstrating the influence of the spacing of the wing on planting depth; (b) diagram demonstrating the influence of the spacing of the wing on planting posture.
Figure 10. Simulation test results of wing plate spacing. (a) Diagram demonstrating the influence of the spacing of the wing on planting depth; (b) diagram demonstrating the influence of the spacing of the wing on planting posture.
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Figure 11. Simulation test results of soil return port height. (a) Diagram demonstrating the influence of the height of soil backflow port on planting depth; (b) diagram demonstrating the influence of the height of soil backflow port on planting posture.
Figure 11. Simulation test results of soil return port height. (a) Diagram demonstrating the influence of the height of soil backflow port on planting depth; (b) diagram demonstrating the influence of the height of soil backflow port on planting posture.
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Figure 12. Simulation test results of soil return port length. (a) Diagram demonstrating the influence of the length of soil backflow port on planting depth; (b) diagram demonstrating the influence of the length of soil backflow port on planting posture.
Figure 12. Simulation test results of soil return port length. (a) Diagram demonstrating the influence of the length of soil backflow port on planting depth; (b) diagram demonstrating the influence of the length of soil backflow port on planting posture.
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Figure 13. Response surface of the influence of experimental factors on the index. (a) Diagram demonstrating the influence of the spacing of the wing and the height of soil backflow port on planting depth; (b) diagram demonstrating the influence of the spacing of the wing and the length of soil backflow port on planting depth; (c) diagram demonstrating the influence of the spacing of the wing and the height of soil backflow port on planting posture.
Figure 13. Response surface of the influence of experimental factors on the index. (a) Diagram demonstrating the influence of the spacing of the wing and the height of soil backflow port on planting depth; (b) diagram demonstrating the influence of the spacing of the wing and the length of soil backflow port on planting depth; (c) diagram demonstrating the influence of the spacing of the wing and the height of soil backflow port on planting posture.
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Figure 14. Field trial. 1—DF544 tractor; 2—Watering device; 3—Seedling feeding device; 4—Planting device; 5—Ridging flank plow; 6—Ridging device; 7—Furrow opener;.
Figure 14. Field trial. 1—DF544 tractor; 2—Watering device; 3—Seedling feeding device; 4—Planting device; 5—Ridging flank plow; 6—Ridging device; 7—Furrow opener;.
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Table 1. List of soil parameters.
Table 1. List of soil parameters.
Project NameNumerical Value
Particle radius/mm3
Density/(kg/m3)1850
Poisson’s ratio0.38
Shear modulus/pa106
Collision recovery coefficient0.5
Dynamic friction coefficient0.107
Static friction coefficient0.313
Table 2. Test factor coding.
Table 2. Test factor coding.
CodeFactor
The Spacing of the Wings
x1/mm		The Height of Soil Backflow Port
x2/mm		The Length of Soil Backflow Port
x3/mm
16580170
06070155
−15560140
Table 3. Experimental design scheme and results.
Table 3. Experimental design scheme and results.
Serial
Number		FactorPlanting Depth Y1/mmPlanting Posture Y2/mm
X1X2X3
1−1−1035.2170.05
21−1055.3779.36
3−11056.3983.49
411045.3780.21
5−10−152.6979.38
610−148.3281.52
7−10137.2885.29
810152.3888.96
90−1−151.2469.28
1001−159.2176.91
110−1151.2578.25
1201152.3981.39
1300054.8587.84
1400053.7191.28
1500055.2887.54
1600056.9489.57
1700051.2890.58
Table 4. Model variance analysis.
Table 4. Model variance analysis.
Variance
Source		Planting DepthPlanting Posture
Sum of SquaresDegree of FreedomF Valuep ValueSum of SquaresDegree of FreedomF Valuep Value
Model653. 37924.120.0002690.34943.31<0.0001
X149.35116.400.004917.5219.890.0163
X251.46117.100.004478.50144.320.0003
X341.22113.700.007689.78150.690.0002
X1X2243.05180.76<0.000139.63122.370.0021
X1X394.77131.490.00080.5910.330.5834
X2X311.6613.880.08975.0412.850.1355
X12156.21151.910.000214.8418.380.0232
X220.2310.080.7882356.941201.53<0.0001
X321.8010.600.464857.56132.500.0007
Residual error21.077 12.407
Misfit3.4330.260.85191.5630.19140.8972
Error17.644 10.844
Summation674.3816 702.7316
Table 5. Field test results.
Table 5. Field test results.
Test NumberPlanting Number N0Qualified Number of Planting Depth N1Qualified Number of Planting Posture N2Qualified Rate of Planting Depth W1/%Qualified Rate of Planting Posture W2/%
130282793.390.0
230292796.790.0
330282693.386.7
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Wu, G.; Wang, S.; Zhang, A.; Xiao, Y.; Li, L.; Yin, Y.; Li, H.; Wen, C.; Yan, B. Optimized Design and Experiment of a Self-Covering Furrow Opener for an Automatic Sweet Potato Seedling Transplanting Machine. Sustainability 2023, 15, 13091. https://doi.org/10.3390/su151713091

AMA Style

Wu G, Wang S, Zhang A, Xiao Y, Li L, Yin Y, Li H, Wen C, Yan B. Optimized Design and Experiment of a Self-Covering Furrow Opener for an Automatic Sweet Potato Seedling Transplanting Machine. Sustainability. 2023; 15(17):13091. https://doi.org/10.3390/su151713091

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Wu, Guangwei, Shoujiang Wang, Anqi Zhang, Yuejin Xiao, Liwei Li, Yanxin Yin, Hanqing Li, Changkai Wen, and Bingxin Yan. 2023. "Optimized Design and Experiment of a Self-Covering Furrow Opener for an Automatic Sweet Potato Seedling Transplanting Machine" Sustainability 15, no. 17: 13091. https://doi.org/10.3390/su151713091

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