Design and Simulation of Chinese Cabbage Harvester

Abstract

In view of the problems of low work efficiency and high operating costs caused by manual harvesting of Chinese cabbage in China, in this study, a Chinese cabbage harvester with agronomic integrity was designed. The harvester is mainly composed of a crawler chassis, a drawing device, a flexible clamping device, a cutting device, and a horizontal delivery device. Firstly, physical properties of Chinese cabbage such as diameter, plant height, weight, and drawing rate of Chinese cabbage were measured and analyzed to provide necessary basic data for the design of the harvester. Secondly, simulation tests were conducted on the Chinese cabbage harvesting process; a 3D model of Chinese cabbage using SolidWorks 2022 was established and filled with particles using the three-layer stacking method. At the same time, SolidWorks was applied to simplify the model of the Chinese cabbage harvester. The belt of the machine model was set as a flexible body through RecurDyn 2023 software and coupled with EDEM 2022 for simulation analysis. Based on single factor tests, the BBD model was applied to conduct multi-factor response surface analysis on the above factor levels. The optimal working conditions of the harvester were obtained as follows: the rotating speed of the cutting device was 207.85 r/min, the rotating speed of the flexible clamping conveyor belt was 165.51 r/min, the rotating speed of the drawing device was 102.38 r/min, and the machine walking speed was 1.37 km/h. The qualified rate of Chinese cabbage harvesting was the highest, achieving a maximum theoretical value of 97.91%. Field validation tests were conducted on the designed Chinese cabbage harvester. Based on the actual operating conditions of the Chinese cabbage harvester and the simulated operating parameters, the optimal parameter combination was finally determined as follows: rotating speed of the root cutting device was 200 r/min, rotating speed of the flexible clamping conveyor belt was 160 r/min, rotating speed of the drawing device was 100 r/min, and machine walking speed was 1.4 km/h, respectively. Through field verification tests, the highest qualified rate of Chinese cabbage harvesting reached 93.19%, showing a good harvesting effect, which approximates the simulated optimal qualified rate of 97.91%, meeting the mechanized harvesting demand of Chinese cabbage. This study provides reference to the further design and development of Chinese cabbage harvesters in the future.

1. Introduction

Chinese cabbage is planted throughout China, since it is cold resistant, storage-proof, and transportation tolerant with a year-round supply [1]. According to data in 2019, the planting area of Chinese cabbage in China reached 2.6412 million hectares, ranking first in vegetable cultivation in the country in terms of sales volume. At present, the harvesting process of Chinese cabbage in China is still mainly completed manually [2]. During harvesting, the root system of Chinese cabbage needs to be completely cut off using tools such as machetes and sickles, followed by picking, peeling off surface leaves, stacking, and packing and container handling. Such a way of harvesting consumes a lot of resources and has extremely low production efficiency, making it unable to meet the requirements of large-scale agricultural production [3]. Therefore, achieving the mechanized harvesting of Chinese cabbage is of key significance [4].

As early as the end of the last century, extensive research has been conducted abroad on the operation mode and performance of Chinese cabbage harvesters. In recent years, in order to meet the needs of agricultural modernization, Japan and South Korea have continuously improved and innovated Chinese cabbage harvesting technology, aiming to improve the harvesting efficiency and quality of Chinese cabbage. The main Chinese cabbage harvesting equipment include the following: tractor side suspended Chinese cabbage harvester [5], which can be used in conjunction with a 15 kW wheeled tractor to achieve functions such as harvesting, sorting, containerization, and handling in the field. Kim and Yeongsoo designed a crawler self-propelled Chinese cabbage harvester [6], which consists of a drawing device, a conveying device, a cutting device, and a crawler. Mechanized harvesting of vegetables in China started and developed relatively late, but the industry grows rapidly. In recent years, some mechanized harvesting equipment has emerged in China, such as the design of a knapsack cabbage harvester, which is mainly composed of a profiling mechanism, a cutting device, a clamping mechanism, a lifting device, a horizontal transmission device, and a loading mechanism [7]. A prototype was designed and made, and a performance test was conducted in the field. The harvester has full functionality and can adapt to various terrains and cabbage growth states, but the tractor power consumption is large, affecting the operation efficiency. Wu et al. designed a kind of self-propelled cabbage harvester, which uses full hydraulic steering, and verification tests were conducted on its main components [8]. The machine features smooth transmission and easy operation, which is conducive to improving the harvest quality, but the hydraulic system failure maintenance is difficult, and the maintenance cost is high. Zhang designed a crawler self-propelled cabbage harvester [9], whose asymmetric cutting device would easily get blocked or damaged in the face of complicated landscapes and irregular planting conditions.

Foreign technology is superior in operation efficiency, but the equipment is expensive and difficult to maintain. The country has made achievements in adapting to local conditions, but the reliability and operational efficiency stability need to be improved, and the technology needs to be continuously optimized to catch up with the international advanced level.

In response to the above situation and considering the relevant physical characteristics of Chinese cabbage, in this study, a Chinese cabbage harvester that adopts the method of “flexible clamping + low damage harvesting” was designed to replace traditional manual operation. It can reduce labor intensity, improve the harvesting efficiency and quality of Chinese cabbage, and provide equipment support for Chinese cabbage industry in China.

2. Physical Characteristic Analysis of Chinese Cabbage

2.1. Collection of Basic Physical Characteristics of Chinese Cabbage

By taking the cabbage variety Xiayangbai that is mature and ready for harvest from Xiayang County, Hebei Province, as the test material and removing immature and rotten cabbage in the field, 100 heads of Chinese cabbage were randomly selected. The following individual parameters of Chinese cabbage were mainly measured: diameter, plant height, root diameter, root length, divergence, and weight. The measured parameters are shown in

Table 1. Physical and geometric characteristics of cabbage plants in Xiayangbai.

Parameters	Diameter/mm	Plant Height/mm	Weight/kg	Rhizome Length/mm	Rhizome Diameter/mm
Mean value	155	164	2.77	60.55	28.36
Maximum value	162	190	3.18	74.32	32.63
Minimum value	148	138	2.35	50.68	42.57
Standard deviation	7	3.41	0.42	11.87	7.29
Coefficient of Variation	0.05	0.07	0.15	0.20	0.26

:

Statistical analysis of measurement data shows that the diameter, plant height, weight, rhizome length, and mean rhizome diameter of Chinese cabbage are 155 mm, 164 mm, 2.77 kg, 60.55 mm, and 28.36 mm, respectively. For the convenience of subsequent tests, the Chinese cabbage was regarded as an approximate cylinder or sphere.

2.2. Measurement of Mechanical Properties in Chinese Cabbage Harvesting

2.2.1. Measurement of the Cabbage Drawing Force

The drawing force in Chinese cabbage harvesting can provide theoretical reference to the design of the drawing device of harvesters [10]. A total of 100 plants were randomly selected in the field. A rope loop was tied to the root stem junction of a cabbage at one end and to the digital tension meter at the other end. Then, the cabbage were pulled out, and the maximum tensile force was measured with the digital tension meter, and the data of every test were recorded. Then, 10 samples of cabbage were measured, and their average was taken. The results of the drawing force measurement of Chinese cabbage are shown in

Table 2. Measurement results of pulling force.

Parameters	Drawing Force (N)
Mean value	288
Maximum value	354
Minimum value	222
Standard deviation	45.7
Variable coefficient	0.16

: the maximum pulling force on Chinese cabbage is 354 N, the minimum is 222 N, and the average pulling force is 288 N.

2.2.2. Measurement of the Shear Properties of Chinese Cabbage

To ensure the root-cutting quality, the appropriate cutter and corresponding shear force should be selected for the cutting performance test of the cabbage rhizome [11]. Cutting a cabbage 10~15 mm from the root can easily remove the leaves around the rhizome and avoid second root-cutting later. The test was conducted on the UTM6503 texture analyzer made in the USA, as shown in Figure 1. By adopting the method of static measurement, the stress in the test was set to 5000 N as the maximum value; the speed was set to 100~500 mm/min. The initial stress was 0.01 N. After the cabbage was cut off, the maximum shear stress was measured. Ten samples were selected in each group of the experiment.

The results of the shear characteristic test on the Chinese cabbage and the fitted curves are shown in Figure 1.

It can be seen from Figure 2 that the shear stress on cabbage increases rapidly with displacement, then decreases and gradually stabilizes, and then continues to increase. The reason for this is that the root epidermis of the Chinese cabbage is composed of fibers. As the stress gradually increases, the epidermis is cut off, and the stress decreases and tends to stabilize. Then, when approaching the lower epidermis, the stress continues to increase until all parts are completely cut off.

According to the graph of fitted values, when the displacement is 8 mm, the shear force reaches its maximum, with a maximum shear force of 275.53 N.

2.2.3. Measurement of Compression Characteristics of Chinese Cabbage

In order to prevent damage to the Chinese cabbage by clamping and squeezing on the conveyor belt, which affects its quality, a test on the radial compression mechanical properties was conducted on the upper, middle, and lower parts of the Chinese cabbage. The static measurement method was adopted to exert stress on the cabbage at a speed of 10 mm/min. As soon as the cabbage is damaged, the action is stopped immediately, and the compression force on the cabbage at the same time is measured to determine the minimum compression force after damage, as shown in Figure 3.

The critical values of internal and external damage can be obtained through compression tests, providing a basis for subsequent field tests. The working parameters of each part of the harvesting process of Chinese cabbage can be adjusted according to the damage situation, thereby optimizing the harvesting efficiency. Figure 4 shows the fitted curve in compression force and damage [12].

From the fitting results, it can be seen that in the elastic stage, during compression loading, the bonding gap between cells decreases, the volume of the cabbage decreases, and the apparent density increases. This is manifested as a low slope rising OA segment on the compression characteristic curve. In the yield stage, the compression load increases with the increase in deformation until the first peak point B, which is the stress yield point, emerges. At this stage, the tissue cells of the cabbage undergo elastic deformation, and the cabbage begins to crack stably. However, even before reaching the stress yield point, which is point A, the interior of the cabbage may have been damaged at different levels. The Chinese cabbage ruptured at a compression displacement of 27.86 mm, with a maximum crushing force of 820.34 N at this time. The damage generally occurs when the internal structure and tissues of the cabbage are subjected to pressure beyond their ability to withstand external forces. Its organizational structure may undergo deformation, leading to internal damage. The damages may show in the form of cell rupture, tissue tearing, or moisture loss, affecting the quality of the cabbage.

3. Overall Structural Design and Working Principle

3.1. Overall Structural Design of the Chinese Cabbage Harvester

Based on the physical characteristics of the Chinese cabbage, the harvester is mainly composed of a crawler chassis, a drawing device, a flexible clamping device, a cutting device, and a horizontal transmission device. Its structural diagram is shown in Figure 5.

During operation of the machine, it is necessary to adjust the level of the workbench to the proper height to ensure the drawing device is close to the ground. The drive shaft of the chassis engine provides the necessary power for key mechanical components such as the drawing device, lower clamping belt, flexible clamping belt, and cutting blade, enabling them to work together. In actual practice, the pulling of Chinese cabbage is achieved through the internal rotation of the drawing device combined with the entrainment. The flexible clamping device fixes and clamps the transported Chinese cabbage during the transmission process and reaches the cutting blade. After the root-cutting device cuts off the root of the Chinese cabbage, it is transported by the lifting transmission system to the transport vehicle at one side for collection. The 3D model diagram of the Chinese cabbage harvester is shown in Figure 6.

3.2. Technical Parameters

According to the demands of harvesting operations, the technical parameters of the machine are shown in

Table 3. Technical parameters of whole machine.

Technical Parameters	Values
Matched power/kW	2.8
Length × width × height/mm	2700 × 1100 × 1000
Line number	1
Working width/mm	700
Suitable row spacing/mm	500–750
Plant distance/mm	400
Gear position	Four-speed gear shift: 3-forward + 1 Reverse
Operating speed/km·h−1	0–1.8

.

The basic parameters of the self-propelled chassis are shown in

Table 4. Technical parameters of the crawler chassis.

Technical Parameters	Values
Track grounding length/mm	860
Track drive wheel diameter/mm	180
Track model/mm	100 × 60 × 45
Track width/mm	100
Minimum ground clearance/mm	170
Minimum ground clearance part	Lower supporting crossbar in front of the crawler chassis
Drive mode of walking	Mechanical speed change

.

4. Design and Check of Key Components

4.1. Design of the Drawing Device

The drawing device is mainly composed of the drawing discs and the lower clamping belt. The drawing discs are in the front of the machine. As the machine goes forward, the drawing discs move the bottom of the cabbage and rotate inward; the lower clamping belt clamps the root of the cabbage and pulls it backward. By exerting drawing force upward and tractive force backward, the cabbage is pulled into the flexible clamping device. Moreover, the rotation of the drawing disc can prevent cabbage blocking in the drawing process.

The drawing disc designed based on the requirements above is shown in Figure 7. The drawing discs are symmetrical on both sides and installed at the front end of the machine. They are connected to the body frame in the lower part through a connecting rod. There is a driving wheel between the drawing discs and the connecting rod, and it provides power to the drawing disc through the lower clamping belt, enabling it to pull out the cabbage against the rotation. The drawing disc adopts a tooth-shaped structure [13]. Since the two drawing discs are symmetrical, the teeth between them can clamp the root of the Chinese cabbage and hold the bottom of the Chinese cabbage to complete the pulling operation.

The lower clamping belt is located at the bottom of the working part of the machine. As shown in Figure 8, when the drawing discs extend into the bottom of the Chinese cabbage, the lower clamping belt holds the root of the Chinese cabbage and pulls it backwards. The lower clamping belt not only pulls out the cabbage by working with the drawing disc but also supports the cabbage to prevent it from falling down.

Figure 9 shows the speed analysis of the cabbage in drawing process, and the relation between the speed and angle is as follows:

$${\mathrm{V}}_{\mathrm{m}}\cos \mathsf{\alpha }>{\mathrm{V}}_{\mathrm{p}}$$

(1)

After analysis on the motion status of the cabbage, the speed composition relationship is as follows:

$${\overrightarrow{\mathrm{V}}}_{\mathrm{b}}={\overrightarrow{\mathrm{V}}}_{\mathrm{m}}+{\overrightarrow{\mathrm{V}}}_{\mathrm{p}}$$

(2)

According to trigonometric function relation, the following relationship can be obtained:

$${\displaystyle \frac{{\mathrm{V}}_{\mathrm{m}}}{{\mathrm{V}}_{\mathrm{p}}}}={\displaystyle \frac{\sin \mathsf{\beta }}{\sin (\mathsf{\beta }-\mathsf{\alpha })}}=\mathrm{K}$$

(3)

where ${\mathrm{V}}_{\mathrm{P}}$ is the forward speed of the Chinese cabbage harvester, m/s; ${\mathrm{V}}_{\mathrm{m}}$ is the clamping speed of the lower clamping belt, m/s; ${\mathrm{V}}_{\mathrm{b}}$ is the moving speed of the cabbage, m/s; $\mathsf{\alpha }$ is the angle between the working body of the harvester and the ground, ($°$); $\mathsf{\beta }$ is the angle between the absolute speed of the cabbage and the ground, ($°$).

According to Equation (3), when K is less than 1, the β angle is greater than 90°, which will cause the Chinese cabbage to tilt forward in the absolute speed direction, making the cabbage move forward and moving the cutting position, leading to a lower qualified rate of harvesting of the Chinese cabbage harvester. When K is greater than 1 and β is less than 90°, this will cause the absolute velocity direction of the Chinese cabbage plant to tilt backwards at a certain angle, making the axis of the Chinese cabbage perpendicular to the straight line of the header, thereby improving the harvesting quality. In summary, it is necessary to ensure that the K value is greater than 1, so that the axis of the Chinese cabbage is perpendicular to the direction of the straight line along with the header during the pulling process. Therefore, it is advisable to set the angle α between the header and the ground to 15°.

4.2. Design of the Flexible Clamping and Transmission Device

The flexible clamping device [14] was installed at the center of the working part of the Chinese cabbage harvester. The mechanism can adapt to the diameter changes of different varieties of Chinese cabbage, achieve low damage clamping, and improve the harvesting quality. The designed flexible clamping and transmission device is shown in Figure 10.

The clamping device is composed of a pair of belts with a spacing of 200 mm between them, which can adapt to the diameter of different varieties of Chinese cabbage. The maximum spacing is 300 mm. Adjusting the screw can control the maximum extension of the spring and adjust the tension of the tensioning wheel. When the Chinese cabbage passes through, the tensioning wheel tightens and stretches, and the rear spring achieves flexible clamping throughout the entire process, reducing damage to the Chinese cabbage [15].

Based on the analysis of the motion process of the cabbage in the clamping and transmission device, the ability to clamp the cabbage by the flexible clamping device in the no-load condition and the maximum extrusion force on the cabbage in the full-load condition were explored, and the mechanical analysis diagram is shown in Figure 11.

The force condition of cabbage feeding is shown in Figure 12 and Figure 13.

In holding and conveying the cabbage, the compression force from the tensioning wheel and friction from the belt on the cabbage is the key to keeping the cabbage from falling. Among them, the extrusion force is prominent; thereby, the extrusion force was analyzed.

The following equation can be obtained based on mechanical analysis diagram [16]:

$${\mathrm{F}}_{\mathrm{p}}={\mathrm{F}}_{2\mathrm{y}}={\mathrm{F}}_2\sin {\mathsf{\theta }}_2={\displaystyle \frac{{\mathrm{L}}_1\mathrm{k}\left({\mathrm{x}}_0+\mathsf{\delta }\right)\cos {\mathsf{\theta }}_1}{{\mathrm{L}}_2}}$$

(4)

where ${\mathrm{F}}_2$ is the tensioning force of the tension wheel, N; ${\mathrm{F}}_{\mathrm{p}}$ is the compression force on the cabbage, N; ${\mathrm{L}}_1$ is the arm of force of the tensioning wheel, mm; ${\mathrm{L}}_2$ is the arm of force at the joint between spring and tensioning wheel, mm; ${\mathrm{x}}_0$ is the length of the spring, mm; $\mathsf{\delta }$ is the stretched length of the spring; k is the spring constant; $\mathsf{\theta }$ is the angle between the arm of force L₁ of the tensioning wheel and the cabbage, ($°$).

The Chinese cabbage is subjected to the compression force from the two belts on both sides. In order to ensure that the Chinese cabbage does not fall off during the clamping process, the frictional force ${\mathrm{F}}_{\mathrm{f}}$ on the Chinese cabbage should meet the following requirements:

$${\mathrm{F}}_{\mathrm{f}}=2\mathsf{\mu }{\mathrm{F}}_{\mathrm{p}}\ge \mathrm{G}=\mathrm{m}\mathrm{g}$$

(5)

According to the measured parameters of Chinese cabbage, the weight of a single mature cabbage is 3. Based on the measured parameters of Chinese cabbage, the weight of a single mature cabbage is 3.64–5.0 kg, the diameter of it is between 303 and 361 mm, and the ${\mathsf{\theta }}_1$ is 30° under an empty load. The minimum adjustable spacing between the feeding ports of the flexible clamping device designed in this paper is 200 mm. The tension wheel L₁ designed in this paper is 120 mm, ${\mathrm{L}}_2$ is 40 mm, the spring elasticity coefficient k is 0.47, the initial length x₀ of the spring is 30 mm, and the length δ of the spring elongation is 20 mm. Therefore, according to the formula, the friction coefficient μ of the belt should meet the following requirements:

$$\mathsf{\mu }\ge {\displaystyle \frac{\mathrm{m}\mathrm{g}{\mathrm{L}}_2}{2{\mathrm{L}}_1\mathrm{K}\left({\mathrm{x}}_0+\mathsf{\delta }\right)\cos {\mathsf{\theta }}_1}}=0.9$$

(6)

Based on the above conditions, it is determined that when the load is full, the angle ${\mathsf{\theta }}_1$ is 60°, and the length after spring elongation δ is 70 mm. According to Equation (5), the squeezing force on Chinese cabbage is 141 N, which is much smaller than the maximum crushing force of Chinese cabbage, thus meeting the requirements.

In order to reduce the damage on the cabbage as much as possible, in this project, the CR high density elastic sponge belt clamps and transmits the cabbage to increase the friction between the cabbage and the transmission belt. At the same time, by designing the tensioning device, some deformation of the cabbage is further conveyed to the transmission belt. In the process of clamping and transmission, the transmission belt itself has deformation, which may alleviate the deformation of the cabbage to some extent, preventing the cabbage from slipping and invalid root cutting.

4.3. Design of the Root Cutting Device

A root cutting device was designed. The root cutting device was regarded as two discs, and the rhizome of the cabbage was taken as a whole circle. The diameter of the rhizome is ${\mathrm{D}}_1$, and the force analysis of the rhizome is shown in Figure 14.

$${\mathrm{R}}_{\mathrm{x}}={\mathrm{N}}_{\mathrm{x}}+{\mathrm{F}}_{\mathrm{x}}$$

(7)

$${\mathrm{T}}_{\mathrm{y}}={\mathrm{F}}_{\mathrm{y}}+{\mathrm{N}}_{\mathrm{y}}$$

(8)

where N represents the normal reaction on the cabbage from the cutter; its horizontal component is ${\mathrm{N}}_{\mathrm{x}}$, and its vertical component is ${\mathrm{N}}_{\mathrm{y}}$, N. F is the friction of the cutter on cabbage rhizome, and its horizontal component is ${\mathrm{F}}_{\mathrm{x}}$, and its vertical component is ${\mathrm{F}}_{\mathrm{y}}$, N.

In order to let the cabbage be clamped by the disc type cutter, the following condition should be satisfied:

$${\mathrm{T}}_{\mathrm{y}}>0$$

(9)

${\mathrm{F}}_{\mathrm{y}}$ > ${\mathrm{N}}_{\mathrm{y}}$, and $\mathrm{F}=\mathrm{N}\mathrm{f}$, that is

$$\mathrm{N}·\mathrm{f}\cos \mathsf{\alpha }>\mathrm{N}·\sin \mathsf{\alpha }$$

(10)

Therefore, when $\mathrm{f}>\tan \mathsf{\alpha }$, the disc type cutter has good clamping performance.

$$\mathsf{\alpha }={\cos }^{-1}{\displaystyle \frac{\mathrm{L}/2}{\left({\mathrm{D}}_1+{\mathrm{D}}_2\right)/2}}={\cos }^{-1}{\displaystyle \frac{\mathrm{L}}{\left({\mathrm{D}}_1+{\mathrm{D}}_2\right)}}$$

(11)

where f is the friction coefficient between the disc cutter and Chinese cabbage rhizome; $\mathsf{\alpha }$ is the angle between the normal reaction force of the disc cutter on the rhizome of Chinese cabbage and the x-axis, ($\mathrm{d}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{e}\mathrm{e}$); $\mathrm{L}$ is the spacing between the two disc cutters, mm; ${\mathrm{D}}_1$ is the diameter of Chinese cabbage rhizome at the cutting position, mm; ${\mathrm{D}}_2$ is the diameter of the disc cutters, mm.

In order to avoid incomplete cutting of the cabbage rhizome, let the two serrated disk cutters overlap with each other partially to balance the horizontal force in root cutting and ensure the neat and complete root cutting; on this basis, a pair of disc cutters with a center spacing of 198 mm and a disc diameter of 198 mm were designed, with the cutter angle $\mathsf{\alpha }$ ≈ 31.02°, to achieve an ideal clamping effect.

5. Simulation Test

5.1. Model Establishment

5.1.1. Establishment of the Cabbage Model

The simulation model of cabbage was established [17,18,19]. A 3D model was built based on the physical parameters of cabbage. After simplifying the surface features with the SolidWorks software, the solid model of the cabbage was re-built, and the head of the cabbage was simplified by the solid extraction method.

In the process of re-building the solid model, SolidWorks can effectively handle the surface feature set, for example, the concavities, convexities, and irregularities of the cabbage head. Through this process, we can precisely re-build the whole structure of the cabbage, including the curvature of the leaves and the geometrical shape of the root.

The simplification of the cabbage head with the solid extraction method is meant to optimize the complexity and calculation efficiency of the model. In this method, the part inside the surface that is not actually touched is removed, and its shape is preserved, and the physical properties of the model are ensured. This not only reduces the dimension and file size of the model but also improves the efficiency of subsequent engineering analysis and simulation.

The cabbage model was imported into the EDEM software, and a three-layer stacking model was applied to fill the cabbage with particles [20,21,22]. In the design, the root, the middle part, and the upper part of the cabbage were stacked with three types of particles, respectively. Based on the collection description of the physical characteristics of the cabbage, the parameters of the material structure for each part were set, as shown in

Table 5. Parameter setting of the Chinese cabbage model [9].

Model Structure	Three-Layer Stacking Model
	Upper Part	Middle Part	Root
Elasticity modulus/Mpa	0.135	0.284	0.469
Density/kg·m−3	385.2	534.6	794.3
Poisson’s ratio	0.30	0.30	0.30

. The contact parameters between particles of the Chinese cabbage are shown in

Table 6. Contact coefficient between Chinese cabbage particles.

Contact Coefficient
Recovery coefficient	0.29
Static friction coefficient	0.58
Dynamic friction coefficient	0.032

.

The bonding relation between particles is an important factor that decides the first process. Therefore, the Hertz–Mindlin with Bonding model was selected, and the particles were set to bond together to resist tangential motion until the maximum value for the breakage of the bonding key is reached.

In this study, SolidWorks was used for the modeling of the cabbage, as shown in Figure 15. The cabbage was divided into two parts, the plant and the rhizome. The model was established based on test data to simulate real cabbage. After the model was established, it was imported into EDEM for particle filling.

5.1.2. Simulation Model Verification

Simulated cutting and compression tests are an important method for verifying the discrete element model of Chinese cabbage. The simulated cutting test can simulate the mechanical action of particle materials in the cutting process and provide detailed data for particle separation and breakage. It is of key significance to understand the mechanical response of particles in the actual cutting process. After comparing the results of simulation test and actual tests, the accuracy and consistency of the EDEM model in predicting particle cutting action was evaluated. The comparison results show that the simulation test data and actual test data fit each other; among them, the relative error of the maximum shear force was lower than 5%, as shown in Figure 16.

The compression test can simulate the condensation and deformation process of particle material under compression and provide detailed information on particle stacking and compression characteristics. After comparing the results of the simulation test and actual tests, the accuracy and consistency of the EDEM model in predicting the particle cutting action was evaluated. The results show that the simulation data fit well with the test data, in terms of the particle stacking density and mechanical properties of compression, as shown in Figure 17. The results have proved the reliability of the cabbage model.

5.1.3. Establishment of the Simulation Model of the Cabbage Harvester

In the simulation calculation process by the EDEM software, the geometry that is not in contact with particles will produce excessive computation [23], so the machine geometry was simplified during simulation, and a simplified 3D model was established, as shown in Figure 18.

In the actual harvesting process, the action of clamping will cause deformation of the cabbage; however, the model in the EDEM software as a rigid body cannot show such deformation. Therefore, the RecurDyn software is required for coupling simulation. In this study, RecurDyn and EDEM were combined [24,25,26] to simulate the harvesting process of Chinese cabbage. The simplified model was imported into RecurDyn, and the Young’s modulus of the belt, density, and Poisson’s ratio were set to 200 Gpa, 0.941 g/cm³, and 0.38; then, the belt was set as a flexible body in the software. At last, the simulation calculation was finished through the coupling interface.

The discrete element method was used to simulate the process of pulling and cutting roots, simplifying the device into two pairs of pulling disks and cutting disks with a certain angle. The density of drawing disc material, elasticity modulus, and Poisson’s ratio were set to 7.85 g/cm³, 80 GPa, and 0.32, respectively. Figure 19 shows the simulated pulling process diagram and the stress nephogram. The basic material of the cutting disc was defined as 65 Mn, and its material density, elasticity modulus, and Poisson’s ratio were set to 7.85 g/cm³, 206 GPa, and 0.3, respectively. Figure 20 shows the simulated cutting process diagram and the stress nephogram.

5.2. Test Factors and Indexes

By taking the qualified rate of cabbage harvesting as the evaluation index, the mechanized harvesting of Chinese cabbage should be started when the cabbage becomes compact, to ensure that 2–3 outer leaves are preserved to protect the cabbage; at the same time, the skin should be kept clean and complete to avoid damage [27].

By taking the rotating speed of the root cutting device (x₁), rotating speed of the flexible clamping and conveying belt (x₂), rotating speed of the drawing device (x₃), and forward speed of the machine (x₄) as test factors, these test factors were selected based on their key influence on the overall performance of the Chinese cabbage harvester. Too fast of a speed for the root cutting mechanism will easily damage the roots; if too slow, the cutting of the root is not complete; an appropriate speed is necessary to ensure the quality of the root cutting and a smooth harvest. Too slow of a speed for the flexible clamping conveyor belt will result in low efficiency; if too fast, it is easy to cause cabbage slip or damage; a reasonable speed is very important for efficient and safe conveying. The speed of the pulling mechanism is inconsistent with that of the pulling mechanism, which leads to difficulty in the pulling out process and the injury of vegetables. If the machine walking speed is too fast, it affects the accuracy of the operation; if too slow, it reduces efficiency; a reasonable speed can result in efficient connection of all the links efficient, achieving the best harvest effect. The factor level of the simulation test is shown in Table 6 [26]. There were 29 groups of tests, and the test scheme and results are shown in

Table 7. Factor level of the simulation test.

	Rotating Speed of the Root-Cutting Device ${\mathbf{x}}_1$/r·min−1	Rotating Speed of the Flexible Clamping and Conveyor Belt ${\mathbf{x}}_2$/r·min−1	Rotating Speed of the Drawing Device ${\mathbf{x}}_3$/r·min−1	Walking Speed of the Machine ${\mathbf{x}}_4$/km·h−1
+1	300	200	120	1.8
0	200	160	100	1.4
−1	100	120	80	1.0

.

Coupling RecurDyn and EDEM [27], by changing the working parameters such as the forward speed of the implement, the speed of the traction mechanism, the speed of the flexible clamping device, and the speed of the root cutting device, simulates the movement of Chinese cabbage in a real working environment and enables the obtainment of the final parameters such as the force distribution and cutting position of the internal particles of Chinese cabbage. In the simulation process, the reaction between the cabbage root particles and soil particles reflects the actual drawing force. Therefore, JKR was selected as the contact model for the soil, to accurately reflect the actual cabbage pulling process.

The qualified rate of harvesting is defined based on the production and technical specifications for common head cabbage and the harvesting quality of stem and leaf vegetables [28]. There should be no obvious breakage, crushing damage, cutting damage, or other situations caused by mechanical harvesting operations. The calculation formula for the qualified rate of harvesting is as follows:

$$\mathrm{N}={\displaystyle \frac{{\mathrm{N}}_1}{{\mathrm{N}}_0}}\times 100\mathrm{\%}$$

(12)

where N is qualified rate of harvesting, %; ${\mathrm{N}}_1$ is the number of qualified harvested cabbages in one single test; ${\mathrm{N}}_0$ is the total number of cabbages in one single test.

5.3. Results and Analysis

In the simulation test, the internal force situation of cabbage particles was analyzed, and the simulation test results and the actual test results of mechanized cabbage harvesting were compared and analyzed to find out if the internal force of the cabbage would damage the cabbage and reduce the qualified rate of harvesting. Four groups of tests among the four levels of tests were selected for display, as shown in Figure 21.

The regression analysis and results are shown in

Table 8. Design scheme and results of regression analysis.

No.	${\mathbf{x}}_1$/r·min−1	${\mathbf{x}}_2$/r·min−1	${\mathbf{x}}_3$/r·min−1	${\mathbf{x}}_4$/km·h−1	Qualified Rate of Harvesting y1/%
1	200	160	100	1.4	96.88
2	200	160	80	1	91.97
3	300	160	80	1.4	88.52
4	200	160	100	1.4	96.75
5	100	200	100	1.4	90.45
6	200	200	120	1.4	89.9
7	300	160	100	1	90.37
8	100	160	100	1.8	83.54
9	300	120	100	1.4	91.12
10	200	160	100	1.4	97.98
11	300	200	100	1.4	92.37
12	200	200	100	1	89.4
13	200	160	120	1.8	95.38
14	200	160	120	1	89.2
15	200	200	100	1.8	90.53
16	300	160	100	1.8	91.4
17	100	160	100	1	92.74
18	200	120	100	1.8	82.69
19	200	120	80	1.4	83.75
20	100	120	100	1.4	87.16
21	200	160	100	1.4	98.71
22	300	160	120	1.4	90.86
23	100	160	80	1.4	86.19
24	200	200	80	1.4	88.58
25	100	160	120	1.4	92.65
26	200	120	100	1	91.49
27	200	160	80	1.8	80.02
28	200	160	100	1.4	97.92
29	200	120	120	1.4	88.42

, and the variance analysis results are shown in

Table 9. Significance analysis of the regression equation.

Source of Variation	Quadratic Sum	Degree of Freedom	Mean Square	F	p	Significance
Regression model	456.47	14	32.61	30.32	<0.0001	Extremely significant
x1	8.42	1	8.42	7.83	0.0142	Significant
x2	32.18	1	32.18	29.92	<0.0001	Extremely significant
x3	24.03	1	24.03	22.34	0.0003	Extremely significant
x4	9.94	1	9.94	9.24	0.0088	Extremely significant
x1 x2	4.08	1	4.08	3.79	0.0718	Insignificant
x1 x3	1.12	1	1.12	1.04	0.324	Insignificant
x1 x4	1.4	1	1.4	1.31	0.2723	Insignificant
x2 x3	0.0306	1	0.0306	0.0285	0.8684	Insignificant
x2 x4	3.76	1	3.76	3.5	0.0824	Insignificant
x3 x4	19.05	1	19.05	17.72	0.0009	Extremely significant
x12	94.77	1	94.77	88.13	<0.0001	Extremely significant
x22	188.96	1	188.96	175.72	<0.0001	Extremely significant
x32	170.78	1	170.78	158.81	<0.0001	Extremely significant
x42	82.72	1	82.72	76.92	<0.0001	Extremely significant
Residue	15.05	14	1.08
Lack-of-fit	12.35	10	1.23	1.82	0.2953	Insignificant
Errors	2.71	4	0.6771
Sum	471.53	28

.

Table 9 shows the regression equation model has a p value < 0.0001 (<0.01), and the lack-of-fit p = 0.2953 (>0.05), showing the credibility of the regression equation model. Therefore, this table can be used to predict the optimization value of the qualified rate of harvesting, and the maximum value of predicted qualified rate of harvesting of the Chinese cabbage is 97.91% by the regression equation, and the corresponding rotating speed of the root cutting device (${\mathrm{x}}_1)$ is 207.85 r/min, the rotating speed of flexible clamping and transmission belt (${\mathrm{x}}_2$) is 165.51 r/min, the rotating speed of the drawing device (${\mathrm{x}}_3$) is 102.38 r/min, and the walking speed of the machine (${\mathrm{x}}_4$) is 1.37 km/h.

According to the analysis and Table 9, all the single factors had a significant effect on the results. Moreover, it was observed that only the interaction of ${\mathrm{x}}_3{\mathrm{x}}_4$ was significant, at the same time, ${\mathrm{x}}_{12}$, ${\mathrm{x}}_{22}$, ${\mathrm{x}}_{32}$, and ${\mathrm{x}}_{42}$ had significant effects. However, other than the factors, the other factors failed in achieving significance. It shows that in the tests, these factors played an important role in the qualified rate of harvesting of the Chinese cabbage, but the detailed influencing mechanism should be further explored.

The results of the regression variance significance analysis are shown in Table 9.

The Design-Expert13 software was used to obtain the regression equation for the qualified rate of harvesting of Chinese cabbage as follows:

$$\begin{gathered} \mathrm{y}=97.65+0.8375{\mathrm{x}}_1+1.64{\mathrm{x}}_2+1.42{\mathrm{x}}_3-0.91{\mathrm{x}}_4-1.01{\mathrm{x}}_1{\mathrm{x}}_2-0.5{3\mathrm{x}}_1{\mathrm{x}}_3+ \\ 0.5925{\mathrm{x}}_1{\mathrm{x}}_4-0.875{\mathrm{x}}_2{\mathrm{x}}_3+0.97{\mathrm{x}}_2{\mathrm{x}}_4+2.18{\mathrm{x}}_3{\mathrm{x}}_4-3.82{\mathrm{x}}_1^2-5.4{\mathrm{x}}_2^2-5.13{\mathrm{x}}_3^2-3.57{\mathrm{x}}_4^2 \end{gathered}$$

(13)

After eliminating insignificant factors, the regression variance was obtained as follows:

$$\mathrm{y}=97.65+0.8375{\mathrm{x}}_1+1.64{\mathrm{x}}_2+1.42{\mathrm{x}}_3-0.91{\mathrm{x}}_4+2.18{\mathrm{x}}_3{\mathrm{x}}_4-3.82{\mathrm{x}}_1^2-5.4{\mathrm{x}}_2^2-5.13{\mathrm{x}}_3^2-3.57{\mathrm{x}}_4^2$$

(14)

According to the data above, it was found that there was a significant interaction between the rotating speed of the drawing device (x₃) and the walking speed of the machine (x₄), which influenced the qualified rate of harvesting of the Chinese cabbage. Figure 22 shows the interaction effect between the rotating speed of the drawing device and the walking speed of the machine on the qualified rate of harvesting of Chinese cabbage. The figures help better understand the influence of different factors on the qualified rate of harvesting of Chinese cabbage. Through observation of the figures, the test results can be further analyzed to offer reference and guidance to test design and machine optimization.

According to observation results of Figure 22a, the 0 level was set when the rotating speed of the root-cutting device (x₁) was 200 r/min and the rotating speed of the flexible clamping and conveyor belt was 160 r/min; when the rotating speed of the drawing device was a fixed value, with the increase in the walking speed of the machine, the qualified rate of harvesting of Chinese cabbage rose first and then decreased. When the walking speed of the machine was a fixed value, with the increase in the rotating speed of the drawing device, the qualified rate of harvesting shows a trend of decreasing after an increase. When the walking speed of the machine is fixed and the rotating speed of the drawing device is low, the drawing device cannot remove the outer leaves of the cabbage in collision, and the rotation of the lower clamping belt and the clamping effect is affected. When the rotating speed of the drawing device is too high, the drawing device cannot work properly with the walking speed, and the cabbage will easily get smashed by its rotation. When the walking speed of the machine is too slow, the drawing time will increase, and the contact time of the cabbage and the drawing device will also increase, and the friction between them will affect the drawing effect. When the walking speed is too high, the machine cannot work properly with the drawing device, and the machine will go forward without successfully drawing the cabbage and will cause losses. Subsequent studies will pay attention to the direct coordination between the drawing device and the walking speed of the machine, to improve the qualified rate of harvesting of Chinese cabbage.

6. Field Test

6.1. Test Conditions and Method

6.1.1. Test Conditions

The field harvest performance test was conducted on 15 July 2024 in Laidong Village, Yancheng City, Jiangsu Province, China. The cultivation mode of single row planting was adopted in the test site, with a row spacing of about 600 mm and a plant spacing of 400 mm. The on-site harvest test is shown in Figure 23.

The test subject is the Chinese cabbage variety “Xiayangbai” from Jiangsu and Zhejiang provinces in China. It takes about 90 days from planting to harvesting. The diameter of mature Chinese cabbage is (155 ± 7.32) mm, the plant height is (302 ± 17.84) mm, the total mass is (2.77 ± 0.41) kg, the diameter of the rhizome is (28.36 ± 4.27) mm, the length of the rhizome is (60.55 ± 13.77) mm, and the spread length is (543 ± 104.34) mm.

6.1.2. Test Method

Based on the results of the Chinese cabbage harvesting test in Section 5, a field test was conducted. The Chinese cabbage harvester designed in this paper uses a tracked chassis to provide power, and its walking speed is in four gears, one reverse gear and three forward gears, with forward gears of 1.0 km/h, 1.4 km/h, and 1.8 km/h, respectively. Based on the results of multiple factor tests, the rotating speed of the root-cutting device, the flexible clamping device, and the drawing device was set to 200 r/min, 160 r/min, and 100 r/min, respectively. Three forward gears of the machine’s walking speed were selected, and three groups of tests were conducted under the same conditions. Each group of tests was repeated five times, and the qualified rate of harvesting of Chinese cabbage was finally calculated.

6.2. Test Results and Analysis

According to the

Table 10. Statistics of harvesting test results.

Walking Speed /km·h−1	No.	Sum/Particles	Damaged Number	Qualified Rate of Harvesting/%	Damage Rate/%
1.0	1	215	19	91.16	8.84
	2	234	21	91.03	8.97
	3	219	13	94.06	5.94
	4	223	18	91.93	8.07
	5	217	15	93.09	6.91
	Mean value			92.25	7.75
1.4	1	275	23	91.64	8.36
	2	285	26	90.88	9.12
	3	254	25	90.16	9.84
	4	265	25	90.57	9.43
	5	261	28	89.27	10.73
	Mean value			90.50	9.50
1.8	1	259	43	83.40	16.60
	2	278	42	84.89	15.11
	3	263	47	82.13	17.87
	4	294	44	85.03	14.97
	5	281	53	81.14	18.86
	Mean value			83.32	16.68

, it can be seen that the qualified rate of harvesting of Chinese cabbage is greater than 90% when the walking speed of the harvester is 1.0 km/h and 1.4 km/h, with a qualified rate of harvesting of 92.25% and 90.5%, respectively. When the walking speed of the Chinese cabbage harvester reaches 1.8 km/h, the qualified harvesting rate is the lowest, with a maximum value of 85.03% and a minimum value of 81.14%.

After analyzing the reasons, it was found that when the walking speed of the cabbage harvester is slow, the key components coordinate well with each other, so the harvesting efficiency of the cabbage harvester is high at this time. When the operating speed of the Chinese cabbage harvester is increased to 1.4 km/h, the qualified rate of harvesting decreases, the walking speed of the Chinese cabbage harvester increases, the operating speed of the machine increases, the contact time between each component and the cabbage decreases, and collisions between the cabbage and the harvester can easily cause incomplete harvesting, resulting in increased damage and a reduction in the qualified rate of harvesting. Therefore, after considering all factors, the walking speed of the Chinese cabbage harvester was set to 1.4 km/h. When the walking speed of the Chinese cabbage harvester was elevated to 1.8 km/h, it was necessary to adjust the parameters of each component to satisfy the demand of operation.

7. Conclusions

(1)

By taking Chinese cabbage as the test object, the basic physical characteristics and mechanical properties of Chinese cabbage in harvesting were studied, to provide reference to its structural design and simulation analysis;

(2)

A hand-held Chinese cabbage harvester with agronomic integration was designed. It is mainly composed of the drawing device, the flexible clamping and transmission device, and the root-cutting device, and force analysis was conducted, showing that the structural design was reasonable and could satisfy the working demand of the Chinese cabbage harvester;

(3)

Coupling simulation of the harvesting process for Chinese cabbage was conducted by using the EDEM and RecurDyn software. The optimal parameters combination for the qualified rate of harvesting for the Chinese cabbage harvester were determined as follows: the rotating speed of the root cutting device was 200 r/min, the rotating speed of the flexible clamping device was 160 r/min, the rotating speed of the drawing device was 100 r/min, and the machine walking speed was 1.4 km/h. At this time, the qualified rate of harvesting of Chinese cabbage was 97.91%;

(4)

Field verification tests were carried out on the Chinese cabbage harvester based on simulation test results. By considering the qualified rate of harvesting and harvesting efficiency of Chinese cabbage, 1.4 km/h was finally selected as the final walking speed. At this time, the mean value of the qualified rate of harvesting reached 90.5%, which satisfied the demand of Chinese cabbage harvesting and proved to have a good effect in the field verification tests.