Design and Testing of a Self-Propelled Fork-Tooth Harvester for Medicinal Plant Rhizomes

Abstract

To address the challenges of manually excavating deep-rooted medicinal herbs in the cold and arid regions of northwest China, such as low efficiency, high costs, and difficulties with mechanized methods, a self-propelled fork-tooth digger was developed for use in hilly and mountainous terrains. Key components, including the fork-tooth excavation device, hydraulic control system, and reverse trapezoidal crawler chassis, were designed and analyzed. A multi-body dynamics model (MBD) and discrete element model (DEM) for Astragalus and soil were developed, employing a DEM-MBD coupling method to simulate the harvesting process. Field trials demonstrated an excavation efficiency of 98.2%, a stem damage rate of 1.8%, a loss rate of 3.0%, and a maximum digging depth of 600 mm, all meeting existing industry standards. The results confirmed the design’s effectiveness in meeting the mechanization needs for harvesting rhizome medicinal herbs.

1. Introduction

Gansu Province is one of the key regions for the cultivation of Chinese medicinal herbs. With the continuous development of the industry, the planting area has expanded annually, leading to increased performance requirements for herb harvesting machinery [1]. In 2020, the planting area in Gansu exceeded 3.0 × 10⁵ hm², with rhizome medicinal materials, such as Astragalus, Codonopsis, and Licorice, making up a significant portion. Astragalus, one of the most representative medicinal herbs in Gansu, is widely used in traditional Chinese medicine and modern drug research. However, its deep-growing roots and the complex terrain of the high-altitude hilly regions where it is mainly cultivated pose significant challenges to mechanized harvesting. Traditional manual harvesting is labor-intensive, inefficient, and can damage the roots, affecting the quality of the medicinal material and severely limiting the sustainable development of the Astragalus industry [2].

In recent years, several countries have developed advanced harvesting technologies for shallow-rooted crops like potatoes and yams [3]. Japan has developed a yam harvester [4] with remarkable success in harvesting yam tubers. However, due to the shallow root system, this technology cannot be applied to the harvesting of deep-rooted medicinal crops. Khurana et al. [5] from India, designed a root crop harvester suitable for multiple crops, with high separation efficiency, but it is complex in design and has low operational efficiency. N.G. Bayboboev et al. [6] from Uzbekistan developed a small single-row vibrating-screen potato digger that improves harvesting efficiency, though it may cause damage when working in harder soils. Sunil Shirwal et al. [7] from India developed a tractor-drawn carrot harvester that operates efficiently, but the longer soil separator increases the damage rate of the carrots. Since the 1960s, China has introduced foreign harvesting machinery for rhizome crops [8] and has developed various medicinal material harvesters since the 1990s, such as the 4GJ-880 [9] multifunctional rhizome harvester. Currently, numerous research teams are dedicated to improving harvesting technologies for medicinal materials. For example, Professor Zhang Zhaoguo’s team [10,11] at Kunming University of Science and Technology developed several types of Ginseng harvesting machines suitable for the hilly and mountainous regions of Yunnan. Li Tao et al. [12] designed an automatic row alignment system, significantly enhancing the operational performance of root crop harvesters. Xie Xinya et al. [13] at Xinjiang Agricultural University optimized the structure of digging shovels in rhizome medicinal material harvesters, improving separation efficiency and reducing damage to the materials. Song Jiang et al. [14] at Heilongjiang Bayi Agricultural Reclamation University have made improvements to the 4B-1200 medicinal material harvester.

However, current research on the mechanized harvesting of root crops primarily focuses on cereal crops and shallow-rooted medicinal herbs, with limited attention given to the mechanized harvesting of deep-rooted medicinal herbs. Existing equipment generally suffers from issues such as insufficient digging depth and high digging resistance, making it inadequate for meeting the harvesting needs of deep-rooted medicinal herbs. Optimal harvesting of deep-rooted medicinal herbs requires a digging depth of at least 60 cm, along with processes such as soil crushing and separation. Therefore, considering the unique terrain and soil conditions of the hilly regions of northwest China, the development of mechanized equipment suitable for harvesting deep-rooted medicinal herbs, especially specialized machinery for herbs like Astragalus, has become an urgent technical challenge that must be addressed.

To address the issues of low efficiency and high digging resistance in medicinal material harvesting, this paper designs a fork-tooth self-propelled root crop medicinal material digger based on the planting model and agronomic requirements of Astragalus in the cold and arid hilly areas of northwest China. This equipment can simultaneously perform forced soil entry, soil turning, and soil separation. First, the structural design of the fork-tooth digger is theoretically developed [15]. Then, the DEM-MBD coupled method is used to simulate the harvesting process, analyzing the turning and separation effects of the digging shovel on the soil-material mixture. Finally, field experiments are conducted to validate the simulation results.

2. Materials and Methods

2.1. Overall Structure

Figure 1 illustrates the self-propelled rhizome medicinal material harvester, which is composed primarily of a fork-tooth excavation device, a hydraulic control system, a reverse trapezoidal crawler-based self-propelled chassis, a rear suspension support device, and other components (Table 1).

2.2. Principles of Operation

The fork-tooth self-propelled rhizome medicinal material harvester is powered by a Shifeng ZS1115-G4 diesel engine (Shandong Shifeng Group Co., Ltd., Liaocheng, China). The diesel engine drives a gear pump through one belt, supplying power to the hydraulic system, while another belt powers an AC generator that operates the air-cooled radiator, ensuring stable cooling performance. Hydraulic oil is drawn from the tank, pressurized by the gear pump, and distributed to hydraulic cylinders and walking hydraulic motors.

The harvester features a crawler-based self-propelled chassis. The gear pump supplies hydraulic oil to the hydraulic motors, converting hydraulic energy into mechanical motion to propel the harvester. The operator controls movement (forward, backward, and operation of the fork-tooth excavation device) using manual control valves. This setup enables precise step-back excavation functions, such as forced soil entry, soil turnover, and herb–soil separation.

The chassis includes a transmission assembly with six gear ranges for high- and low-speed operations and steering, allowing the harvester to adapt to various road conditions and operational requirements. A rear suspension support device provides additional stability on hard ground, ensuring the machine can achieve the required excavation depth.

2.3. Fork-Tooth Excavation Device

As illustrated in Figure 2a, the fork-tooth excavation actuator primarily comprises the fork-tooth excavation shovel, a lift-type fixed frame, a hydraulic cylinder, a self-cast bearing support, and other components.

The operating principle of the fork-tooth excavation device mimics the traditional agricultural technique of using a fork to turn soil (plowing). This process involves three stages: soil penetration, soil cutting, and soil throwing. During the soil penetration stage, the excavation shovel enters the soil at an almost vertical angle, with the shovel-to-ground angle ranging from 85° to 90°. In the soil cutting stage, the shovel slices through soil lumps, while in the soil throwing stage, it propels the cut herb–soil mixture outward [16].

According to the harvesting requirements for medicinal herbs, the width of the fork-tooth excavation shovel must exceed the width of the crawler to prevent secondary compaction or damage to the excavated soil and herbs. Consequently, the width of the fork-tooth shovel is set at 120 cm. To ensure the thorough excavation of roots and seedlings, the spacing between the fork-teeth is designed to be 90 mm. The overall design incorporates 12 evenly distributed fork-teeth, made of threaded steel with a diameter of 32 mm and a tensile strength of HRB335. After processing, the fork-teeth have an average thickness of 16 mm.

The design of the fork-teeth considers the growth patterns of rhizome medicinal materials and the characteristics of the soil. The front ends of the fork-teeth are slightly curved to better conform to the shape of the rhizomes, minimizing damage during excavation. This design effectively preserves the integrity of the herbs and enhances excavation efficiency. The fork-teeth are uniformly welded to 10 mm thick L-shaped angle steel, ensuring the stability and durability of the entire structure.

Shovel Surface Force Analysis

The purpose of excavation is to break the adhesion between rhizomes and soil, which can result in issues such as separation, fracturing, or fragmentation of the adhesive bonds. Therefore, it is essential to develop a mechanical model of the shovel surface and analyze the factors contributing to excavation resistance—see Figure 3.

Based on Figure 3a, the lateral force equilibrium equation for the excavating shovel results is:

(1)$F=G\sin \phi +\left(\mu G+KS+f\right)\cos \phi$

where F is the excavation resistance (N), G is the weight of the root–soil mixture above the excavation shovel (N), $\phi$ is the soil entry angle (°), K is the soil adhesion (Pa), S is the working contact area of the excavation shovel (mm²), f is the pure cutting force (N), and μ is the soil internal friction coefficient.

Using Figure 3b, a force analysis of the root–soil mixture above the excavation shovel is performed in the horizontal and vertical directions. The horizontal equilibrium equation is:

(2)$G_n\cos \delta +\left(KS+f+\mu G_n\right)\cos \phi +KS\sin \delta -G_t\cos \phi -\mu G_n\cos \delta =0$

where G_n is the normal load on the excavation shovel (N), G_t is the lateral load on the excavation shovel (N), and δ is the front failure surface inclination angle (°).

The vertical equilibrium equation is:

(3)$\mathrm{G}+\left(\mu G_n+KS+f\right)\sin \phi +KS\cos \delta +\mu G_n\sin \phi {-G}_n\sin \delta =0$

By combining these equations, the resistance during the motion of the excavation shovel through the soil is obtained as:

(4)$F={\displaystyle \frac{\mathrm{G}}{\mathrm{Z}}}+{\displaystyle \frac{KS+f}{\mathrm{Z}\left(\sin \mathsf{\delta }+\mu \cos \mathsf{\delta }\right)}}+{\displaystyle \frac{KS}{\mathrm{Z}\left(\sin \mathsf{\phi }+\mu \cos \mathsf{\phi }\right)}}$

where Z is a constant given by:

(5)$\mathrm{Z}={\displaystyle \frac{\cos \mathsf{\phi }-\mu \sin \mathsf{\phi }}{\sin \mathsf{\phi }+\mu \cos \mathsf{\phi }}}+{\displaystyle \frac{\cos \mathsf{\delta }-{\mu }_1\sin \delta }{\sin \mathsf{\delta }+{\mu }_1\cos \mathsf{\delta }}}$

where μ₁ is the soil–metal friction coefficient.

These parameter values are summarized in Table 2 [17].

Through the mechanical model and force analysis of the excavation shovel and the root–soil mixture during excavation, it becomes evident that excavation resistance is influenced by factors such as the geometric parameters of the shovel, the physical properties of the soil, and the operational parameters. When the fork-tooth excavation shovel reaches its maximum digging depth, it begins to cut into the soil. The force exerted on the shovel occurs as the fork-tooth breaks apart the soil lump. This resistance represents the maximum resistance encountered throughout the excavation process.

2.4. Hydraulic Control System

The hydraulic system of the fork-tooth, self-propelled rhizome medicinal material harvester consists of an oil tank, gear pump, single-acting hydraulic cylinder, control handle, hydraulic drive motor, and radiator, forming a complete closed-loop circuit. The primary design goal of the hydraulic system is to support the walking and steering functions of the crawler-based mechanism while providing hydraulic power output. The fork-tooth excavation shovel, which performs tasks such as forced soil penetration, herb–soil overturning, and mixture separation, is intermittently actuated by hydraulic cylinders. The crawler-type chassis is powered by a hydraulic motor. As a result, the hydraulic control system utilizes an open hydraulic circuit [18], as depicted in the overall hydraulic layout in Figure 4.

2.4.1. Walking Hydraulic Motor

The hydraulic motor selected for the power chassis must meet the maximum torque and maximum travel speed requirements across various working conditions. In mountainous and hilly terrain, the greatest running resistance occurs on slopes, which includes running resistance, slope resistance, and inertial resistance [19], as described by the following equation:

(6)$F_t=m\left(gf+g\sin {\alpha }_0+\delta a\right)$

where F_t is the slope running resistance (N), m is the total machine mass, taken as 860 (kg), g is the gravitational acceleration (m/s²), f is the rolling resistance coefficient, generally between 0.08 and 0.12, taken as 0.12, α₀ is the maximum slope angle, taken as 30 (°), δ is the mass increase factor, generally between 1.14 and 1.3, taken as 1.3, and a is the travel acceleration, taken as 0.8 (m/s²).

The required torque M (N·m) for the walking hydraulic motor is calculated as:

(7)$\mathrm{M}={\displaystyle \frac{F_{t}R}{n{\eta }_1}}$

where R is the drive wheel radius, taken as 0.14 (m), n is the number of drive wheels, taken as 2, and η₁ is the travel acceleration, taken as 0.8 (m/s²).

The theoretical displacement V_m of the walking hydraulic motor in mL/r is:

(8)${\mathrm{V}}_m={\displaystyle \frac{2\pi M}{p{\eta }_2}}$

where p is the maximum working pressure of the hydraulic system, taken as 20 MPa, and η₂ is the mechanical efficiency of the hydraulic motor, taken as 0.9.

The hydraulic motor must meet the maximum travel speed requirements, so its maximum rotational speed n_max in rpm is:

(9)${\mathrm{n}}_{max}={\displaystyle \frac{1000v_{max}}{60\times 2\pi r}}$

where v_max is the maximum travel speed, taken as 7.2 km/h, and r is the radius of the drive wheel, taken as 14 cm.

Substituting the design parameters into Equations (5)–(8), the machine’s maximum running resistance is 6120 N, the maximum torque of a single drive wheel is 378 N·m, the theoretical displacement of the walking hydraulic motor is 36 mL/r, and the maximum rotational speed of the walking hydraulic motor is 130 rpm. A BMR-50 hydraulic motor (Shandong Yongcheng Hydraulic Technology Co., Ltd., Jining, China) is selected, with the following parameters: maximum displacement of 50 mL/r, output torque of 390 N·m, maximum output rotational speed of 130 rpm, maximum working pressure of 17.5 MPa, and maximum input flow rate of 40 L/min.

2.4.2. Hydraulic Pump

The output flow rate of the hydraulic pump in L/min is:

(10)$Q_p={\displaystyle \frac{K{V}_m{n}_{max}}{1000{\eta }_3}}$

where K is the leakage coefficient, taken as 1.1, and η₃ is the volumetric efficiency of the walking hydraulic motor, taken as 0.9.

The theoretical displacement of the hydraulic pump in mL/r is:

(11)$V_p={\displaystyle \frac{1000Q_P}{n_f{\eta }_4}}$

where n_f is the speed of the hydraulic pump, taken as 2000 r/min, and η₄ is the volumetric efficiency of the hydraulic pump, taken as 0.9.

One gear pump is used to provide hydraulic power to the walking motor. Substituting the walking hydraulic motor parameters into Equations (9) and (10), the output flow rate of the hydraulic pump is 16.06 L/min, and the theoretical displacement is 12 mL/r. A CBFC-25 gear pump (Xingtai Shengping Hydraulic Machinery Co., Ltd., Xingtai, China) is selected, with the following parameters: maximum displacement of 25 mL/r, maximum pressure of 20 MPa, and maximum speed of 2000 r/min.

2.5. Inverted Trapezoidal Crawler Self-Propelled Chassis

To navigate the undulating terrain of hilly and mountainous roads and adapt to a variable operating environment, the harvester must exhibit exceptional stability, obstacle-crossing capability, and flexibility [20]. The crawler chassis, with its low ground pressure and excellent maneuverability, is better suited for hilly and mountainous conditions than a wheeled chassis. The crawler mechanism primarily serves to support the machine’s weight while converting the power from the drive wheels into movement through the crawler [21]. As such, the drive wheels are positioned above the crawler support rollers, based on the hydraulic motor’s placement on the chassis, and the overall shape of the crawler is designed as an irregular reverse trapezoid. This design effectively increases ground clearance while reducing the machine’s overall size.

As illustrated in Figure 5, the crawler walking system comprises a transmission assembly, rubber crawler, drive wheel, support wheel, tensioning wheel, frame, and other components. The drive system is rear-wheel drive, with the drive wheel powered by the output shaft of the transmission assembly, enabling the machine to move forward, backward, and turn.

2.5.1. Crawler Size Parameter Design

Based on the planting conditions for rhizome medicinal plants in hilly and mountainous areas, field research shows that most walking surfaces are dirt roads, with only a small portion paved with concrete. The rubber crawler, with its large contact surface, helps reduce soil damage caused by the machine’s weight. It also provides stability, traction, and friction with the ground. As a result, rubber crawlers are chosen for the walking chassis, with the design incorporating parameters such as the width, contact length, and pitch of a single rubber crawler, as well as the diameter and width of the drive, support, and tensioning wheels [22]. The crawler size calculation formulas are as follows:

(12)$b=\left(0.9~1.1\right)·209\times \sqrt[3]{G_z}$

(13)$B=(3.5~4.5)\times b$

(14)$t_0=(15~17.5)\times \sqrt[4]{G_z}$

(15)$D_q={\displaystyle \frac{t_0}{sin\frac{180}{Z_q}}}$

(16)$L_j\approx 1.07\times \sqrt[3]{G_z}$

(17)$L_0\approx L_j+0.35h_0$

where b is the track width (mm), B is the track chassis gauge (mm), t₀ is the track pitch (mm), D_q is the pitch circle diameter of the drive wheel (mm), L_j is the track ground length (mm), L₀ is the track support length (mm), h₀ is the track height (mm), and Z_q is the number of teeth on the drive wheel.

The number of teeth on the drive wheel Z_q is taken as 12, thus:

(18)$\left\{\begin{array}{c}{D}_y=0.8D_q\\ D_z=0.8D_y\end{array}\right.$

where D_y is the diameter of the track tensioning wheel (mm) and D_z is the diameter of the track supporting wheel (mm).

Based on the machine’s weight of 860 kg, the crawler height is designed with an average value of 450 mm. The size parameters of the crawler are summarized in Table 3.

2.5.2. Crawler Mobility Analysis

The design of the crawler size parameters above is intended to enhance the machine’s passability and stability on hilly and mountainous terrain. The key parameters that reflect these characteristics include the crawler’s ground contact pressure E_a, which is the vertical load applied per unit area of the contact surface between the crawler and the ground in kPa:

(19)$E_a={\displaystyle \frac{g{G}_x}{2000b{L}_j\times {10}^{-6}}}$

where G_x is the mass of the machine (kg), g is the acceleration due to gravity, b is the track width (mm), and L_j is the track ground length (mm).

Based on the working conditions, the ground contact pressure when the machine is empty is 19.16 kPa, and when fully loaded, it is 22.27 kPa, which are less than the average range of ground contact pressure for construction machinery which is 30–70 kPa [23]. The smaller the ground contact pressure, the better the passability of the machine on soft ground.

The machine must also be able to turn in place at the end of the field, so the design must meet the crawler steering requirements:

(20)${\displaystyle \frac{L_0}{B}}\le {\displaystyle \frac{2(\phi -\rho )}{\mu }}$

where φ is the traction adhesion coefficient, taken as 0.7, μ is the rotational resistance coefficient, taken as 0.5, and ρ is the rolling resistance coefficient, taken as 0.02.

2.6. Coupled Simulation Experiment Based on EDEM-RecurDyn

2.6.1. Selection of Contact Model and Parameter Settings

The physical interaction between Astragalus roots and soil primarily results from the mechanical stress between the roots (including the lateral roots) and soil particles. The growth status of the roots directly impacts the strength of the physical connection with the soil, as well-developed roots can more effectively overcome the resistance of the soil. Soil moisture content significantly influences this interaction. Higher moisture levels can reduce soil compaction, promote root growth, and decrease resistance, while lower moisture levels increase soil compaction and limit root penetration. Additionally, water content affects the viscosity and fluidity of soil particles, indirectly altering the physical interaction between the roots and the soil [24]. Surface adhesion mainly arises from molecular forces (such as van der Waals forces), electromagnetic forces, and other interactions between particle surfaces. The adhesion model follows the JKR theory, where the normal contact force comprises the non-adhesive force from Hertz theory and the adhesive surface energy, while the tangential contact force results from the sliding and peeling between particles. By assigning appropriate adhesive surface energy and applying specific acceleration, particle aggregation and clumping can be induced [25].

Given the adhesion characteristics of the soil, the Hertz–Mindlin with JKR model was selected as the discrete element composite model for the interaction between Astragalus rhizomes and planting soil. In this model, the JKR adhesive parameters are used to represent the adhesive forces between the rhizomes and the soil. The JKR parameters for both the rhizomes and the soil, as well as those between the soil particles, can be found in the literature [26]. A summary of the discrete element simulation parameters is provided in Table 4 and Table 5 [27,28].

2.6.2. Coupled Model Development

The software used for the coupled simulation analysis includes EDEM 2022.3 and RecurDyn 2023.

Astragalus grows vertically in the soil. To enhance the accuracy of the simulation and better reflect the actual operating conditions of the fork-tooth digger during harvesting, the Astragalus simulation model consists of 220 particles, with a maximum radius of 7.5 mm, a minimum radius of 4.3 mm, and a model length of 486 mm. The soil particle model, designed to account for both realistic soil conditions and computational efficiency, uses spherical particles with a 7 mm diameter. The Astragalus model is shown in Figure 6.

In the Discrete Element Method (DEM), the soil properties are assumed to be ideal under specific conditions, with subsequent simulation analyses conducted based on this assumption. A soil tank with dimensions of 800 mm × 750 mm × 600 mm is created. Particle generation systems for both the Astragalus and soil models are established, ensuring that the orientation of the Astragalus model aligns with the Y-axis in EDEM. The row spacing for the Astragalus model is adjusted according to standard agronomic planting requirements. Finally, the generated Astragalus model is fully covered with soil particles to a depth of 600 mm, simulating its growth state and providing a basis for the subsequent harvesting simulation.

To reduce computational load, only the fork-tooth excavation device is included in the coupled dynamic simulation, which is imported into RecurDyn in x_t format. In RecurDyn, the material properties of the excavation device are defined, and the rotational pairs for the moving components are configured. A velocity-driven system (time-based) is used, with a step function applied for definition. To track the absolute motion of the shovel tip relative to the ground, a marker point is added to the tip in Trace. Finally, the fork-tooth excavation shovel model is exported as a WALL file and imported into EDEM for simulation.

In EDEM’s post-processing module, the soil tank model is exported as a Simulation Deck. After saving and opening the file, the generated WALL file is imported, and the relative position between the two models is adjusted (Figure 7).

2.7. Field Test

2.7.1. Astragalus Planting Requirements

In the cold, arid regions of northwest China, medicinal herbs are predominantly cultivated using plastic film mulching. As illustrated in Figure 8, the cultivation method for Astragalus under plastic film mulching with exposed tops adheres to agronomic standards. The ridge width measures 200 mm, the ridge body is 400 mm wide, and the ridge height ranges from 60 to 80 mm. The distance between the edge of the plastic film and the seedling head is maintained at 20–30 mm, ensuring that the top of the Astragalus plant is exposed above the plastic film while being covered by 40–50 mm of soil. The seedlings are planted at an angle of 15°, with a spacing of 58–60 mm between individual plants. A black plastic film, 400 mm wide and 0.01 mm thick, is used to cover the ridge body.

2.7.2. Experimental Conditions and Methods

The field performance validation test of the fork-tooth self-propelled rhizome medicinal material harvester was conducted in the medicinal herb planting experimental field located in Meichuan Town, Min County, Dingxi City, Gansu Province, in the cold and arid region of northwest China. The working conditions are shown in Figure 9. The test soil is black loess, and the planting variety is Astragalus. The test field measures 100 m in length and 60 m in width, with a level surface.

In the experiment, the fork-tooth self-propelled herb excavator achieved a stepwise excavation operation. After completing the operation, the performance of the herb excavator was evaluated based on the measurement requirements outlined in NY/T 3481-2019 “Quality Evaluation Technical Specifications for Rhizome Herb Harvesters” [29]. The primary performance indicators measured included excavation efficiency, stem damage rate, and loss rate during the machine’s operation.

A plot at least 50 m in length, with stable zones at both ends measuring at least 10 m, and a width at least 8 times the operational width of the harvester, was randomly selected for testing. The harvester was tested at an operational speed and excavation depth within its optimal operating range, performing two full passes. For each pass, three random sampling zones, each 3 m long and equal to the working width of the harvester, were selected.

The methods for determining excavation efficiency, stem damage rate, and loss rate are as follows: After completing two passes, rhizomes from the harvesting bin, all rhizomes in the measurement area, and any damaged rhizomes (those with more than 20% damage to the main root) are collected and weighed. Excavation efficiency, stem damage rate, and loss rate for each sample area are then calculated using Equations (21)–(23), and average values are determined.

(21)$W_1={\displaystyle \frac{M_1}{M}}\times 100\%$

(22)$W_2={\displaystyle \frac{M_2}{M}}\times 100\%$

(23)$S={\displaystyle \frac{M_2+q}{M+q}}\times 100\%$

where W₁ is the net excavation efficiency (%), W₂ is the root damage rate (%), S is the loss rate (%), M₁ is the undamaged root and tuber mass (kg), M₂ is the damaged root and tuber mass (kg), M is the root and tuber mass in excavated soil (kg), and q is the root and tuber mass in untouched soil (kg).

After completing the operation, the rhizome weight and damage data have been summarized in Table 6.

The excavation depth calculation method involves measuring the excavation depth at 11 points during each pass. At each point, the vertical distance from the excavation bottom to the ground surface is measured (Table 7).

(24)$\mathrm{H}={\displaystyle \frac{\sum _{i=1}^n{H}_i}{n}}$

where H is the average excavation depth (mm), n is the number of measurement points, and H_i is the excavation depth at the i-th point (mm).

3. Results and Discussion

3.1. Simulation Results and Analysis

3.1.1. Shovel Tip Marker Point Motion Trajectory

Based on the motion trajectory of the shovel tip marker point shown in Figure 10, the motion of the shovel tip marker point can be divided into three stages. The horizontal and vertical axes represent the displacements in the X and Y directions, respectively. In the first stage, the shovel is inserted almost vertically into the ground, and the displacement in the -Y direction gradually deepens. The lower part of the trajectory shows fluctuations, simulating the process of the shovel tip adjusting its depth through shaking. In the second stage, the shovel reaches its maximum depth and begins to turn the herb–soil mixture. At this point, the motion trajectory no longer shows significant vertical displacement, but instead, it moves horizontally, transporting the herb–soil mixture to the surface. In the third stage, the shovel tip starts to shake to optimize the separation of the herb–soil mixture. The motion trajectory exhibits several consecutive peaks, reflecting the process of the shovel tip performing multiple shakes.

3.1.2. Changes in Discrete Element Particle Interaction Speed

The operation of the fork-tooth harvester is divided into five stages: insertion, digging, pushing, shaking, and lifting. Based on the results from the coupled simulation experiment, a detailed analysis is as follows:

From 0 to 1.8 s, the shovel is inserted into the ground in preparation for the subsequent excavation of medicinal herbs. From 1.8 to 3 s, the shovel continues to push downward to dig, which is the core phase of the excavation, characterized by significant particle movement and accumulation. From 3 to 4.2 s, the digging shovel is hydraulically controlled to perform continuous shaking, reducing adhesion and friction between the particles, which facilitates the separation of the herb–soil mixture. From 4.2 to 5 s, the shovel returns to its initial position after completing the task, preparing for the next stroke.

Four adjacent Glycyrrhiza models were selected, and their simulation speed curves are shown in Figure 11. The speed changes of Astragalus during the excavation process are clearly visible in the simulation.

The speed cloud map for Astragalus during the simulation is shown in Figure 12. The shovel moves in the -Y direction, and at 0.3 s, it begins to make contact with the herb–soil mixture. Soil particles surrounding the Astragalus experience minor displacements, but the Astragalus itself remains relatively stationary. From 0.3 to 2 s, the shovel tip continues downward, pushing both the soil and the Astragalus. Under downward pressure, the Astragalus gradually detaches from the fixed soil layer. It begins to move in the direction of the shovel tip, accumulating with the surrounding soil particles. Between 2 and 3 s, the shovel’s motion transitions into a pushing action. Hydraulic control reduces friction and adhesion between the particles, allowing the Astragalus to separate from the soil and accelerate. From 3 to 4 s, continuous shaking of the shovel tip significantly reduces the adhesion between the Astragalus and soil particles, enabling the Astragalus to completely loosen and move upward or forward, forming a loose herb–soil mixture. This process demonstrates the dynamic changes of Astragalus, from initial contact to gradual detachment from the soil.

By extracting the X- and Y-axis coordinates of four adjacent Astragalus models at different time points and calculating their average values, the results show that during the entire excavation process, the Astragalus model experienced a horizontal displacement of 214 mm and a vertical displacement of 310 mm, demonstrating a clear separation effect. Throughout the excavation process, the Astragalus moved together with the herb–soil mixture under the action of the shovel, with minimal contact between the Astragalus and the shovel, effectively reducing collisions and abrasion on the Astragalus. Once the herb–soil mixture was brought to the surface, the continuous shaking of the shovel tip further accelerated the separation of the soil and herb. During the simulation, some of the Astragalus fell directly onto the surface, while others were covered by the soil, requiring manual collection later. The simulation results show that the fork-tooth excavator worked stably, providing a good herb–soil separation effect, significantly reducing the damage to the Astragalus roots, and meeting the design requirements.

In the post-processing module of EDEM, real-time changes in particle speed during the excavation process were obtained, allowing for the analysis of the maximum and minimum particle speeds under typical working conditions, as shown in Figure 13.

By analyzing the speeds of particle interactions, it is observed that at 0.4 s, when the fork-tooth just enters the material, the maximum particle speed is 1.28 m/s. The particles around the fork-tooth move in the direction of excavation. At 1 s, as the fork-tooth penetrates deeper into the material, the particle speed increases significantly, reaching 3.12 m/s. The particles primarily move in the direction of the shovel’s motion, forming a concentrated and orderly flow pattern. At 2 s, when the shovel reaches its maximum digging depth, the particles accumulate in front of the shovel. The microscopic dynamics of the herb–soil mixture and the changes in particle speed during interaction with the fork-tooth reveal the formation process of excavation resistance.

3.1.3. Fork-Tooth Excavation Shovel Force Analysis

During the material excavation operation of the fork-tooth harvester, the fork-tooth comes into direct contact with the material. As the excavation progresses, the fork-tooth is affected by the forces exerted by the material particles. In the simulation, contact between the fork-tooth and the material begins at 0.3 s. The force variation of the fork-tooth over the 0–5 s period is shown in Figure 14.

Figure 14 clearly shows that after 0.3 s, the fork-tooth begins to make contact with the soil particles, and the force on the fork-tooth starts to increase, primarily due to the contact between the soil particles and the shovel tip. Between 1 and 3 s, the fork-tooth continues to penetrate deeper into the soil, beginning to excavate the herb–soil mixture. During this process, mutual compression occurs between the mixture particles and between the particles and the fork-tooth, causing the relative positions of the mixture particles to continuously adjust and stabilize. As a result, the force on the shovel fluctuates during this period. Notably, between 1 and 2 s, the deeper soil is more compact and generates greater resistance. The shovel must overcome this resistance through continuous shaking and vibration to proceed with excavation. The frequent vibrations and feedback cause force fluctuations, which stem from changes in soil hardness, particle friction, and applied force. Consequently, during this phase, the force on the shovel experiences continuous fluctuations. At 2 s, the force on the shovel reaches approximately 7820 N. Due to the influence of human operation, the force fluctuations between 1 and 3 s vary with the shovel’s tip penetration into the soil, the applied force, and the soil hardness. Between 3 and 4.2 s, the excavation task nears completion. The shovel finishes excavating the herb–soil mixture and begins continuous shaking. As the gravitational force of the herb–soil mixture and the frictional resistance between the mixture and the fork-tooth decrease, the force on the shovel shows a clear downward trend. To provide a clearer illustration of the specific forces acting on the shovel, force cloud maps at different times are presented in Figure 15.

Figure 15 shows that at 0.3 s, the fork-tooth has not yet made contact with the soil particles, meaning that the excavation shovel is not subjected to any force from the soil particles. Between 1 and 2 s, the excavation shovel gradually penetrates the herb–soil mixture, with the force being mainly concentrated at the root of the shovel. At 2 s, the shovel reaches its maximum digging depth, and the force becomes further concentrated at the root, particularly at the weld joint between the fork-tooth and the L-shaped angle steel. During the period between 2 and 3 s, the shovel flips the herb–soil mixture to the surface, with the force concentrated on the contact surface and the root of the shovel. As the shovel continues to shake between 3 and 4 s, the herb–soil mixture is separated, and with most of the mixture falling off the shovel, the force gradually decreases until the stroke ends and the shovel returns to its initial position.

During the excavation of Astragalus, the excavation shovel is subjected to multiple forces from the herb–soil mixture, resulting in varying degrees of stress variation during its operation. By mapping the force distribution, the stress across different parts of the shovel can be visually demonstrated, especially at the root of the shovel where the fork-tooth and L-shaped angle steel are welded, which shows significant stress concentration. These high-stress areas are more vulnerable to excessive forces, leading to potential damage of the shovel. Therefore, the core objective of the optimization design is to effectively distribute the stress and prevent local stress concentration. To address this issue, the first step is to optimize the shovel’s design by adding reinforcement ribs to effectively distribute the local stress concentration, thereby improving the overall structural strength and stability of the shovel. Additionally, in high-stress areas, particularly at the junction between the fork-tooth and L-shaped angle steel, welding steel reinforcements longer than the angle steel can further strengthen the shovel’s tensile strength. This significantly enhances the shovel’s structural durability and effectively prevents fatigue damage caused by local stress concentrations.

The force distribution map not only reveals the variation of forces exerted by the herb–soil mixture during the excavation process but also provides valuable experimental data and theoretical support for the subsequent finite element analysis and optimization of the fork-tooth excavation shovel’s design. Through these optimization measures, the performance and reliability of the shovel can be improved, ensuring its stability under high-load working conditions and extending its service life, thus providing technical support for more efficient and durable Astragalus excavation.

3.2. Experimental Results

The field test results of the self-propelled rhizome medicinal material harvester and the Astragalus digger [30] are compared in the following table.

The experimental results in Table 8 show that the performance indicators of the fork-tooth self-propelled rhizome herb excavator in this design meet national and industry standards. It demonstrates good operational performance and is capable of performing harvesting tasks effectively.

According to the agronomic requirements for Astragalus cultivation, the design of the crawler self-propelled chassis ensures smooth passage between crop rows, staying as close to the centerline of the rows as possible to achieve efficient operation and reduce damage to the medicinal materials. The specific design includes a furrow width of 200 mm, a ridge body width of 400 mm, a track gauge of 630 mm, and a track width of 188 mm. These parameters ensure the stable movement of the chassis between crop rows, effectively reducing damage to the medicinal materials.

The initial manufacturing cost of the fork-tooth self-propelled herb excavator is RMB 25,000, which includes core components such as the fork-tooth digging device, hydraulic control system, and trapezoidal crawler chassis. Due to its simple mechanical structure and durable components, the maintenance cost is extremely low, with annual maintenance expenses of only RMB 500, demonstrating significant potential for cost reduction and efficiency improvement. When used for harvesting Astragalus, this excavator can replace three to five workers. Based on a daily wage of RMB 180 per worker, it can save at least RMB 48,600 in labor costs during the 3-month harvesting period each year. After deducting maintenance costs, the net annual profit is approximately RMB 48,100, with a payback period of only 0.52 years. Long-term use of this machine not only continues to save labor costs but also further enhances economic benefits due to its low maintenance costs.

This herb-digging machine is capable of operating in a variety of terrains, demonstrating strong maneuverability and good passing capability. Compared to traditional equipment, it is more suitable for small-scale operations in the cold, arid regions of the northwest. Under conditions requiring greater digging depth, the fork-tooth excavator shows high reliability, with no malfunctions during operation. The digging process proceeds smoothly without blockages.

4. Conclusions

Based on the agronomic requirements for Astragalus cultivation and the mechanized harvesting needs for rhizome medicinal materials, a fork-tooth self-propelled rhizome medicinal material harvester was designed.

Design analysis: The key components of the prototype were analyzed and designed, including the hydraulic-driven fork-tooth excavation device, the hydraulic control system, the reverse trapezoidal crawler-based self-propelled chassis structure, and the operational parameters. The designed hydraulic-driven fork-tooth excavation device effectively meets the mechanized harvesting needs for rhizome medicinal materials.

Simulation results: The harvesting process was simulated using RecurDyn and EDEM. The simulation results indicated that the harvester operated smoothly throughout the process, excavating the herb–soil mixture with a certain degree of separation and minimal damage to the Astragalus rhizomes.

Field test results: Field tests showed that the fork-tooth self-propelled harvester can achieve a digging depth of up to 600 mm. The excavation efficiency for Astragalus reached 98.2%, with a stem damage rate of 1.8% and a loss rate of 3.0%. The harvester demonstrated excellent working performance, and the field performance test results met national and industry standards.

Author Contributions

Methodology, L.W., W.S. and L.Z.; investigation, L.W., W.S., L.Z. and M.Z.; software, L.W. and W.S.; formal analysis, L.Z. and M.Z.; resources, W.S. and L.Z.; writing—original draft, L.W.; writing—review and editing, W.S., P.A.S., M.Z. and L.Z.; funding acquisition, W.S. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Data are reported within the article.

Conflicts of Interest

Author Ming Zhao was employed by the company Dingxi Sanniu Agricultural Machinery Manufacturing Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Overall structure of the machine. (a) Isometric view; (b) front view.

Figure 2. Fork-tooth excavation device. (a). The detailed structure of the fork-tooth excavation shovel; (b). fork-tooth excavation shovel.

Figure 3. Force analysis of the excavating shovel. (a) Lateral force analysis of the excavating shovel; (b) force analysis of the excavating shovel and root–soil mixture.

Figure 4. Schematic of the hydraulic control system.

Figure 5. Structure of the crawler self-propelled chassis.

Figure 6. Astragalus model.

Figure 7. Establishment process of the DEM-MBD coupling simulation model.

Figure 8. Diagram of Astragalus membranaceus agronomic cultivation mode.

Figure 9. Field experiment.

Figure 10. Shovel tip marker point motion trajectory.

Figure 11. Velocity curve of Astragalus movement.

Figure 12. Velocity contour map of Astragalus model during the simulation process. (a) t = 0.3 s; (b) t = 2 s; (c) t = 3 s; (d) t = 4 s.

Figure 13. Velocity contour map of soil particles during the simulation process. (a) t = 0.3 s; (b) t = 2 s; (c) t = 3 s; (d) t = 4 s.

Figure 14. Variation of force on the fork-tooth excavating shovel.

Figure 15. Force contour map of the excavation shovel during the simulation process. (a) t = 0.3 s; (b) t = 2 s; (c) t = 3 s; (d) t = 4 s.

Table 1

Explanation of related parameters.

Parameters	Symbol	Value
Overall Dimensions	L × W × H/mm × mm × mm	2000 × 1200 × 2200
Working Width	mm	1200
Diesel Engine Power	kW	14.7
Total Machine Weight	kg	860
Excavation Depth	mm	600

Table 2

Summary parameters.

Parameters	Symbol	Value
Soil Density	kg·m−3	1350
Internal Friction Angle	°	15.41
Soil Internal Friction Coefficient	Constants	0.276
Soil Adhesion	Pa	15855
Front Failure Angle	°	37.295
Soil Adhesion Coefficient	Constants	2.2
Friction Coefficient Between Soil and Metal	Constants	0.7

Table 3

Structural dimension parameters of the crawler self-propelled chassis.

Parameters	Symbol	Value
Track Width	mm	188
Track Pitch	mm	82
Track Gauge	mm	630
Track Ground Length	mm	1170
Track Support Length	mm	1330
Track Height	mm	450
Pitch Circle Diameter of the Track Drive Wheel	mm	276
Track Tensioning Wheel Diameter	mm	220
Track Supporting Wheel Diameter	mm	176

Table 4

Basic parameters of the discrete element simulation.

Material	Poisson’s Ratio	Shear Modulus (Pa)	Density (kg/m³)
Soil	0.3	2.1 × 1010	2600
Astragalus	0.25	1 × 108	1217
Steel	0.3	7.9 × 1010	7860

Table 5

Contact parameters of the discrete element simulation model.

Contact Type	Static Friction Coefficient	Rolling Friction Coefficient	Coefficient of Restitution	JKR Surface Energy (J·m−2)
Soil–Soil	0.68	0.27	0.21	14.52
Soil–Astragalus	0.453	0.17	0.587	5.34
Soil–Steel	0.31	0.13	0.54	/
Astragalus–Astragalus	0.453	0.086	0.587	/
Astragalus–Steel	0.349	0.074	0.509	/

Table 6

Data on root and tuber mass and damage.

Item		Root and Tuber Mass in Excavated Soil (kg)	Root and Tuber Mass in Untouched Soil (kg)	Undamaged Root and Tuber Mass (kg)	Damaged Root and Tuber Mass (kg)
First Pass	1	4.18	0.13	4.30	0.12
	2	3.87	0.14	4.00	0.13
	3	5.00	0.05	4.90	0.10
Second Pass	1	5.76	0.18	5.93	0.29
	2	6.03	0.25	6.27	0.38
	3	6.50	0.10	6.40	0.05
Average		5.31	0.16	5.25	0.16

Table 7

Excavation depth measurement data.

Item	1	2	3	4	5	6	7	8	9	10	11
First Pass Excavation Depth (mm)	595	612	598	584	603	589	616	602	600	595	595
Second Pass Excavation Depth (mm)	590	605	615	586	598	584	600	610	600	593	595

Table 8

Field trial results.

Measurement Index	Technical Standard	Fork-Tooth Self-Propelled Medicinal Herb Digger	Astragalus Digger
Net Excavation Rate	≥95(%)	98.2	97.42
Root Damage Rate	≤5(%)	1.8	2.44
Loss Rate	≤5(%)	3.0	2.44
Excavation Depth	≥500(mm)	600	200–400

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