3.1. Dig Assembly
Deep-rhizome Chinese herbal medicines typically grow at depths ranging from 40 to 50 cm, with some reaching depths of up to 60 cm, which presents significant challenges for digging. The dig unit, central to the herbal medicine digger, plays a critical role in the digging process and directly impacts yield quality. Commonly used dig units include vibratory shovels, disc cutters, and hybrid combinations. While vibratory shovels and disc cutters effectively penetrate and crush soil, they can damage herbal medicines and require high power consumption [
13,
14,
15]. In contrast, the inter-toothed dig unit, featuring 16 digging tools at the machine’s forefront, employs a progressive kneading method. This approach not only deepens the dig but also significantly reduces digging resistance. The inter-toothed design demonstrates strong adaptability to various soil types, improving medicinal material quality while minimizing resistance during digging between planting rows or among individual plants, thus reducing herbal medicine injury rates.
Manual digging of Chinese herbal medicines traditionally utilizes a sturdy iron fork, aligning with ergonomic principles to ensure efficient digging with minimal energy expenditure. In this study, the digging device mimics the manual digging motion akin to using an iron fork, aiming to achieve high-quality work with minimal power consumption for the inter-toothed self-propelled digger of deep-rhizome Chinese herbal medicines. To accurately replicate manual digging, the study filmed and analyzed the trajectory of these herbal medicine digging movements, as illustrated in
Figure 3.
The manual digging process involves three main stages: soil insertion, where the fork is placed into the ground nearly perpendicular to the surface, typically at an angle of approximately 85° to 90°; soil loosening, where the fork pivots at the soil contact point to rotate and loosen the earth; and separation of the medicinal materials from the soil, achieved by moving the soil forward, vibrating it for loosening, and then lifting and tossing the medicinal materials with the fork’s tines.
To replicate the trajectory of the iron fork tip during manual dig with the inter-toothed and self-propelled digger for deep-rhizome Chinese herbal medicines, it is crucial to analyze the fork tip’s movement through these three critical stages: insertion into the soil, loosening the soil by pressing backward, and separating the medicinal materials. Typically, the iron fork used in manual digging measures about 300 mm in length.
To determine the trajectory of the fork tip during the manual dig of medicinal herbs, a mathematical curve fitting method was employed to extract the path. Thirty data points were collected and imported into Origin 2021 software. A curve fitting tool was then applied to these data points, utilizing a ninth-degree polynomial to enhance the precision of the fit. This process resulted in the curve depicted in
Figure 4. The equation of the fitted curve is detailed below:
.
Following ergonomic dig principles, the structure of the medicinal material dig mechanism is meticulously designed to emulate the manual digging actions of insertion, shaking, flipping, lifting, and vibrating, as illustrated in
Figure 5a. This mechanism incorporates hydraulic cylinders, an opening and closing frame, lifting frames, a vibrator, and medicinal digging tools. In adherence to agricultural standards, the operational width is intentionally set wider than typical row spacing, specifically at 1000 mm, with 16 tools evenly distributed across this width. This arrangement is strategically optimized to maximize the spacing between tools, thereby minimizing soil resistance during penetration. Importantly, this optimization does not compromise the completeness or quality of the medicinal material dig, thereby ensuring efficient and thorough harvesting.
This design adopts a simulated manual dig approach for harvesting deep-rhizome Chinese herbal medicines. Upgraded from a shovel to an inter-toothed configuration, the machine is compact and user-friendly. This innovation addresses the challenges posed by large machinery in the small plots typical of deep-rhizome Chinese medicinal material cultivation in the hilly and mountainous regions of northwest China, as well as the inefficiencies of manual digging. The inter-toothed structure reduces soil penetration resistance and enhances soil-breaking capabilities, thereby improving dig efficiency. Additionally, compared to traditional shovel digging, this design is simpler and results in a lower damage rate.
Figure 5b presents a simplified diagram of the medicinal material dig mechanism, where point O serves as the origin. The Cartesian coordinate system (OXY) is defined with the horizontal direction aligned along the X-axis and the vertical direction along the Y-axis.
Based on the depth of growth, rhizomes are classified into shallow (less than 20 cm), medium (20 to 40 cm), and deep (greater than 40 cm) types [
16]. As a result, L
5 is designated with a value of 750 mm. Drawing from the analysis of manual operations, a design for a detachable and interchangeable medicinal material dig tool is derived. The initial step involves calculating the degrees of freedom F of the crank lever soil-digging mechanism [
17]:
where n is the total number of moving links, P
L is the number of lower pairs, and P
H is the number of higher pairs of the digging mechanism.
The vector equations of the digging mechanism are as follows:
The corresponding projection equations along the
X-axis and Y
-axis are as follows:
where L
1, L
3, and A
1A
2 are the lengths of the moving components and L
3 is the input link.
Given the initial overall machine dimensions of = 740 mm, to maximize the displacement of L4 along the x and y axes while keeping the sizes as compact as possible, Equation (4) was used for iterative calculations, resulting in the determination of link-lengths L2 = 360 mm, L4 = 300 mm, and = 230 mm.
Analysis of the movement of L3 reveals that at its maximum extension, θ3 reaches 185°, resulting in L3 measuring 785 mm. Conversely, when θ4 is at its minimum angle of 100°, L3 measures 451 mm. Consequently, hydraulic cylinder 3 is designed with a travel distance ranging from 0 to 334 mm and an overall length of 451 mm.
Considering that L5 is fixed at 750 mm, the stroke of hydraulic cylinder 2 is set to span from 0 to 750 mm. The optimal dig configuration for the inter-toothed self-propelled deep-rhizome Chinese medicinal material digger occurs when the swing frame is in close proximity to the machine frame. At this juncture, hydraulic cylinder 1 is at its shortest length of 310 mm. To facilitate the transportation of excavated medicinal materials to a distance for easy collection without hindering subsequent operations, the swing frame is designed to maintain a 45° angle with respect to the horizontal direction. When θ1 is 100°, hydraulic cylinder 1 reaches its maximum length of 450 mm.
The hydraulic cylinders are specified as follows: hydraulic cylinder 1 (∅60/310–450 mm), hydraulic cylinder 2 (∅30/0–750 mm), and hydraulic cylinder 3 (∅40/0–334 mm).
After careful calculation, the pertinent parameters and their descriptions are listed in
Table 2 below.
3.2. Hydraulic Transmission System
The hydraulic transmission system, illustrated in
Figure 6, comprises distinct circuits: the left track hydraulic motor circuit, right track hydraulic motor circuit, linear hydraulic cylinder circuit for the medicinal material dig mechanism, and hydraulic reduction gear rotation circuit. The engine drives the hydraulic pump, which supplies pressurized oil to these closed circuits. This oil flows through one-way valves and manual shift valves. Forward movement of both manual shift levers causes the hydraulic motors to rotate clockwise, propelling the track chassis forward. Conversely, pulling the levers backward initiates counterclockwise motor rotation for reverse movement. Changes in direction are managed by adjusting the operation of the left and right manual shift valves. A speed control valve allows adjustable speeds for both forward and reverse motions. Additionally, the hydraulic system includes an overflow brake valve. When the manual shift lever is centered, pressurized oil returns to the tank through the manual shift valve, preventing motor rotation and acting as a brake. This comprehensive system regulates track movement, speed adjustments, steering, lifting of the frame, forward motion of the swing frame, and dig operations, ensuring precise control over the digger’s functionalities.
The hydraulic system’s speed control circuit is driven by a diesel engine that powers the hydraulic pump, regulating the operating speed. The engine’s output torque ranges from 120 to 200 N-m, with a rotational speed of approximately 2200 r/min. The flow rate Q of the hydraulic pump is as follows:
where V is the displacement of the hydraulic pump, n is the engine output rotation speed, η
v is the efficiency of the hydraulic pump.
Based on Equation (5), the pump selected was a CBW-F316 type hydraulic pump, which has a maximum pressure of 25 MPa, a nominal displacement of 16 mL/r, a stable rotation speed of 2500 r/min, and a mass of 2.9 kg. The flow rate of the hydraulic motor is as follows:
where M
m is the torque of the hydraulic motor, P
m is the pressure of the hydraulic motor, and η
m is the mechanical power of the hydraulic motor, taken as 0.9.
Based on the calculation from Equation (6), the required displacement of this motor is approximately 210 mL/r. According to actual requirements, the design has selected a BM6-490-type gerotor hydraulic motor (Like hydraulic, Jining, China), with the main parameters shown in
Table 3.
3.3. Track Walking System
The optimal choice for the walking mechanism is the herringbone pattern rubber track system, comprising tracks, driving sprockets, idler wheels, carrier rollers, and a tensioning assembly [
18,
19], as illustrated in
Figure 7a. This entire track system is driven by a gerotor hydraulic motor. The drive sprocket engages with the track, and tension is maintained by idler wheels. Two sets of parallel idler wheels on either side allow for precise track tension adjustment, ensuring stable movement across various terrains. The hydraulic motor enables the system to perform a full 360-degree rotation in place. Key structural parameters for the track design include pitch, width, ground contact length, driving sprocket pitch diameter, and support roller pitch diameter.
The pitch of the crawler track is determined by the design of the overall machine quality [
20,
21,
22]. Based on Formula (7), the pitch t
0 (in mm) of the crawler track is obtained:
where m is the overall machine mass, equal to 1200 kg.
Based on Formula (8), the width b of the crawler plate is obtained:
According to Formula (9), the relationship between the ground contact length (L) of the crawler track and the width b of the crawler plate can be determined:
where
is a proportionality coefficient set to 0.185.
Using Formula (10), the track gauge B of the crawler is obtained:
Selecting a driving wheel with 13 teeth, the pitch circle diameter D
k of the driving wheel can be calculated with the following:
The tensioning wheel is installed at the front end of the crawler unit to prevent the crawler from deviating and falling off. By using Formula (12), the diameter Dt of the tensioning wheel can be determined.
The crawler features five road wheels that are supported by multiple points and evenly distributed at the bottom of the track. The pitch circle diameter of the road wheels is as follows:
The main parameters of the crawler travel system calculated using Formulas (7)–(13) are shown in
Table 4.
During operation, the crawler drive wheel engages and rotates with the crawler track, enabling the walking operation of the self-propelled deep-root Chinese herbal medicine digger with insertion teeth. The driving force F
k (in Newtons) delivered to the chassis of the machine can be calculated with the following:
where T
k is the torque of the gerotor hydraulic motor, equal to 1878 N-m, and R
k is the drive wheel radius in m. The calculated driving force is approximately 13,414 N.
On a horizontal surface, the rolling resistance of the crawler track can be estimated with the following:
where G is the overall weight of the machine (equal to 11,760 N) and f is the rolling resistance coefficient (equal to 0.08).
The climbing resistance F
p of the crawler track on a slope of angle θ is as follows:
The internal resistance F
n of the crawler track is as follows:
where f
n is the internal resistance coefficient, assumed to be equal to 0.7.
The total driving resistance is the sum:
The driving power P
q of the crawler track when moving at maximum speed V
max is as follows:
Using the above equations, depending on the amount of rolling resistance, the maximum climbing angle ranges between 21° and 27°.
The minimum turning radius is a crucial performance indicator for the self-propelled and deep-rhizome Chinese medicinal material digger with tooth insertion, allowing for quick turns both in the field and at the end of the plot during operation. During the turning process, reverse differential steering is used, with the gerotor hydraulic motors rotating forward and reverse separately, to achieve in-place turning.
Assuming that the crawler chassis’ minimum turning radius is satisfied under the condition of no slippage or skidding of the tracks, the minimum turning radius of the crawler chassis can be calculated with the following (see
Figure 7b):
where B is the track distance,
is the left track linear velocity, and
is the right track linear velocity.
The calculated minimum turning radius of the crawler chassis is 900 mm, and the theoretical ground contact length is 1386 mm. When using reverse differential steering for turning, the turning radius of the single track is 693 mm.
Figure 8 illustrates the actual turning radius of the single track.
3.4. Simulation Analysis of Driving Performance
RecurDyn2023 software was utilized to model and analyze the tracked walking system [
23]. Since the tracked walking chassis in this design is a low-speed vehicle, the Track-LM toolkit in RecurDyn2023 was used to construct the kinematic model of the fully hydraulic tracked chassis. Due to their geometric complexity, the engine, digging mechanism, hydraulic cylinders, hydraulic control valve blocks, hydraulic oil tank, air-cooled radiator, hydraulic motor, gear reducer, and hydraulic pump were modeled using SolidWorks2022. These models were then saved in x_t format and imported into RecurDyn2023 (see
Figure 9).
The condition of the walking surface, crucial for determining the driving characteristics of the fully hydraulic tracked chassis, was simulated using the Ground module within the software. Models were created to represent both soft and hard surfaces, each with a 20° slope, allowing for the simulation and testing of stability during straight-line travel and climbing maneuvers. The results of these simulations, depicting the vehicle’s states during these operations, are illustrated in
Figure 10.
A multi-body dynamic simulation was performed of the tracked walking system of the inter-toothed and self-propelled deep-rhizome Chinese medicinal material digger. The simulation aimed to analyze its movement on both hard and clay surfaces. Specifically, the walking drive wheel angular velocity was set at 3.7 rad/s, with a total driving time of 10 s. During this period, the vehicle operated on a flat surface for the initial 7 s, followed by a 20° slope for the remaining 3 s.
3.4.1. Driving Wheel Torque
Figure 11 depicts the torque variation over time for the driving wheels when the tracked walking system of the herb digger traverses both soft and hard surfaces. Initially, there is a higher torque demand as the tracked system starts moving, with peaks reaching approximately 1386 N·m on soft surfaces and 750 N·m on hard surfaces. This initial demand arises from the need to overcome static friction and accelerate the vehicle [
23]. On soft surfaces, where there is greater subsidence, the torque requirement is higher due to increased static friction, leading to elevated peak values.
As the system stabilizes during steady travel on level ground, the average torque decreases to approximately 111 N·m on hard surfaces and 162 N·m on soft surfaces. When the vehicle transitions to climbing, the average torque increases to around 416 N·m on hard surfaces and 273 N·m on soft surfaces. These findings highlight that, while torque variation remains modest during straight-line movement, climbing demands significantly higher torque on hard surfaces to propel the tracked chassis effectively.
3.4.2. Horizontal Velocity of the Center of Mass
The variation in the centroid’s horizontal speed over time when the crawler travel system of the digger operates on hard and soft surfaces is depicted in
Figure 12. Upon initiation, the speed rapidly reaches the set target within one second, showcasing its precise movement capabilities. Following this, there are slight fluctuations for a few seconds before the speed gradually stabilizes. According to the curve analysis, during consistent horizontal travel, the speed fluctuation on soft road surfaces is less pronounced compared to hard surfaces. Nonetheless, on both surfaces, the average speed remains stable, fluctuating within the expected range of 0.6 m/s.
After 7 s of operation on a horizontal surface, the digger begins ascending a slope, resulting in a decline in speed. By the 9 s mark of climbing, the digger reaches a stable phase, yet the speed fluctuations become notable, with an average speed varying within the range of 0.58 m/s. These variations are due to the physical properties of the soil and the tendency of the vehicle to slip [
24]. It is noteworthy that throughout these maneuvers, the overall average speed of the digger remains within the design specification of 0.6 m/s, indicating a relatively high level of operational stability.
3.4.3. Vertical Acceleration of the Centroid
The variation in the vertical acceleration of the center of mass over time when the herb digger’s tracked walking system operates on both soft and hard surfaces is depicted in
Figure 13. On hard surfaces, the vertical acceleration of the center of mass shows significant fluctuations at startup before stabilizing to a smoother state. In contrast, on soft surfaces, the curve of vertical acceleration fluctuates less at startup and maintains a more consistent pattern thereafter.
This indicates that the tracked chassis of the herb digger experiences poorer ride smoothness on hard surfaces, particularly during acceleration. Therefore, strict control of travel speed is necessary when operating on hard surfaces to ensure optimal smoothness and stability of the tracked chassis [
25].
3.4.4. Deviation
The variation curve of lateral deviation over time when the herb digger’s tracked walking system travels on both soft and hard surfaces is depicted in
Figure 14. The left and right drive wheels of the tracked chassis, operating at identical speeds, exhibit a certain degree of slip, causing the tracked chassis to deviate from its intended path. The curve shows that during straight-line travel on both surfaces, the lateral deviation on hard surfaces is greater compared to soft surfaces, albeit with relatively less vibration. As time progresses, the lateral deviation gradually increases.
The travel speed of the tracked chassis is approximately 0.61 m/s. After traveling for 7 s on a flat, hard surface, the final lateral deviation reaches 183 mm, which extrapolates to a 100 m deviation of 4.26 m, equating to a deviation rate of 4.26%. On a level, soft surface, the lateral deviation after 7 s measures 165 mm, resulting in a calculated 100 m lateral deviation of 3.84 m, with a deviation rate of 3.84%. As climbing begins on both soft and hard surfaces after 7 s, the rate of deviation slows down, with more pronounced variations observed on hard surfaces. This is due to the increased reaction force on the tracks during initial climbing on hard surfaces, leading to reduced stability. However, once the climb stabilizes, the average deviation tends towards zero.
The straightness assessment of the fully hydraulic tracked chassis conforms to the standards outlined in General Technical Conditions for Agricultural Tractors Part 4: Tracked Tractors, which stipulates that the 100 m deviation on dry, hard, level surfaces with slopes not exceeding 1% should not exceed 6 m [
26]. Based on the simulation results, the lateral stability of the tracked chassis meets these design requirements.
3.4.5. Pitch Angle
When the herb digger’s tracked walking system travels on both soft and hard surfaces, the variation curves of the center of mass pitch angle over time are illustrated in
Figure 15. Analysis of these curves reveals that the pitch angle of the tracked chassis is more pronounced on soft surfaces compared to hard surfaces. This indicates that traveling on soft surfaces induces greater undulations, leading to higher pitch angles. Consequently, it is essential to implement speed control measures that mitigate excessive pitch angles during operation on soft surfaces.
3.6. Field Test
Figure 19 illustrates the agronomic planting model for deep-rhizome herbal medicines. These herbs are cultivated in ridges with heights ranging from 100 to 200 mm. The planting configuration includes a minimum row spacing of 300 mm, ridge widths spanning from 1100 to 1800 mm, furrow widths between 400 and 500 mm, and a plant spacing of 30 to 40 mm. A minimum digging depth of 400 mm is required. To form the ridges, soil is dug to a depth of 50 to 90 mm and turned over onto the furrow, creating a flat-bottomed groove in the ridge cross-section. Organic fertilizer, spread evenly at a depth of 20 to 60 mm, is placed in this groove, followed by covering it with a 20 mm layer of fine soil. The soil surface is then compacted and leveled before planting the seedlings in the groove, ensuring their roots are covered with soil from the furrow.
The field performance validation test of the inter-toothed and self-propelled deep-rhizome herbal medicine digger was carried out at the Northwest Traditional Chinese Medicine Full Mechanization Research Base. The experimental setup consisted of single-ridge double rows, with a ridge spacing of 40 cm, ridge height of 15 cm, mulch width of 40 cm, plant spacing set at 30 mm, and row spacing at 30 cm. The test soil, characterized as loess with a moisture content of 20% and soil firmness of 2.04 MPa, was used. Post-operation, the performance of the inter-toothed and self-propelled deep-rhizome herbal medicine digger was evaluated according to the industry standard of the People’s Republic of China, NY/T 3481-2019 [
27] “Rhizome Chinese medicinal materials harvesters technical specification of quality evaluation”.
After completion of the task, the digger’s performance for harvesting deep-rhizome herbal medicines will undergo further testing in accordance with the industry standard NY/T 3481-2019 of the People’s Republic of China, titled “Rhizome Chinese medicinal materials harvesters technical specification of quality evaluation” of the People’s Republic of China”.
At the operating speed specified in the user manual and with the dig depth adjusted to the maximum, the machine should complete one round trip. In the two single trips, one should select four plots alternately, each plot being 3 m long and as wide as the working width. The dig method is shown in
Figure 20, the machine operation video is shown in
Video S1.
3.6.1. Parking Brake
On a 20% test slope for self-propelled wheeled units and a 25% test slope for self-propelled tracked units, the transmission should be in neutral and the engine turned off. The parking brake should reliably hold the machine in place in both uphill and downhill directions for at least 5 min. One should perform one parking brake test each for uphill and downhill. The specific test data are shown in
Table 5.
3.6.2. Noise Level at Ear Position
The testing site should be flat land or short grass, free from large noise-reflecting objects within a 25 m radius from the center of the measurement area. The average wind speed at a height of 1.2 m above the ground should not exceed 3 m/s. During testing, background noise levels should be at least 10 dB(A) lower than the measured noise.
The engine should operate at its rated speed with all moving parts running unloaded. If equipped, the cab doors and windows should be closed and the driver positioned centrally in the seat. The microphone should be placed 250 mm ± 20 mm from the vertical center plane of the driver’s head, with its axis horizontal and the diaphragm facing forward. The microphone’s center height and front-rear position should align with the driver’s eye level. A sound level meter should use the A-weighting slow response mode for measurement.
During stable operation, three measurements should be taken on each side of the machine, with at least a 5 s interval between each measurement. The difference between consecutive measurements on the same side should not exceed 3 dB(A). The arithmetic mean of the higher noise level side is recorded as the measurement result. Specific test data are detailed in
Table 6.
3.6.3. Rate of Root and Rhizome Damage
Separate the undamaged root and rhizome crops from those damaged (including whole root and rhizome crops with diameters of broken roots equal to or exceeding Φ2 mm) retrieved from the excavated soil in each plot. Weigh each category separately and compute the root and rhizome damage rate (M
z) for every plot using the following equation:
where W is the mass of undamaged root and rhizome crops within the plot (kg) and Z is the mass of damaged root and rhizome crops within the plot (kg).
Specific test data are detailed in
Table 7, showing the average of the computed rates.
3.6.4. Loss Rate
Harvest the root and rhizome crops from the excavated soil in each plot and manually extract the root and rhizome crops from the unexcavated soil in the same plots. Weigh them separately and compute the loss rate S for each plot with the following equation:
where Q is the mass of root and rhizome crops in the excavated soil within the plot and q is the mass of root and rhizome crops in the unexcavated soil within the plot.
Specific test data are detailed in
Table 7, showing the average of the computed rates.
3.6.5. Maximum Dig Depth
In the measurement area, take measurements at one point each on the left, center, and right sides every 5 m along the direction of the machine’s advancement, totaling 30 points. The arithmetic mean of these measurements is recorded. The measurement method is as follows: In flat fields, measure the digging depth as the vertical distance from the bottom of the dig trench to the unexcavated surface. In ridged fields, measure the digging depth from the bottom of the dig trench to the top of the ridge. Refer to
Figure 21 for specific measurement details, and consult
Table 8 for test data.
3.6.6. Maximum Gradient Capability
The ramp should measure at least 15 m in length, with an 8 to 10 m straight section leading up to it. The slope of the ramp should meet or exceed the gradient specified by the company. For slopes equal to or greater than 30%, the surface should be paved with cement; slopes less than 30% can be paved with asphalt. A 10 m speed measurement section should be placed in the middle of the ramp. Safety devices must be installed on slopes exceeding 40%.
To conduct the test, position the digging shovel in the transport position and set the gear to the lowest setting. Begin on a straight section of the approach road. Start the engine, fully open the throttle, and ascend the slope. Once on the ramp, bring the vehicle to a stop, shift the transmission to neutral, and turn off the engine for 2 min before attempting to climb again. Record the maximum slope that can be ascended under normal conditions. Refer to
Figure 22 for detailed measurement methods, and consult
Table 9 for specific test data.