Structural Design and Analysis of Bionic Shovel Based on the Geometry of Mole Cricket Forefoot

Abstract

In the mechanized harvesting of root vegetables, loosening is a key factor that restricts harvesting efficiency. Existing mechanical loosening methods have poor loosening effect and high operational resistance. Therefore, more efficient agricultural machinery is needed to reduce energy consumption and improve harvesting efficiency. To this end, based on the efficient excavation mechanism of the first claw toe structure of the mole cricket forefoot, this paper designs the shovel tip structure of the bionic loosening shovel by extracting its contour curve and analyzing the excavation process, constructs the working resistance model and dynamic balance equation of the bionic loosening shovel, determines the optimal working parameters through two-factor and three-level orthogonal simulation experiments, and carries out comparative simulation experiments with the common loosening shovels. The results show that the optimal combination of operating parameters for the bionic loosening shovel is the rotational speed $\omega$ = 5 r/s and the traveling speed of the whole machine v = 0.5 m/s. The disturbance performance of the 31 bionic loosening shovel on the soil is improved by 51.59% compared with that of the common loosening shovel, and the working resistance is reduced by 12.17%. The results of this study proved that the bionic structure of the first claw toe of the mole cricket can significantly improve the working performance of the loosening shovel, which can effectively improve the cutting effect of the soil and reduce the energy loss during the working process.

1. Introduction

In agricultural production, mechanized harvesting in the world’s agricultural production occupies a pivotal position, loosening the soil is an important part of the harvesting process, its effectiveness and efficiency directly affect the subsequent harvesting process and quality [1,2,3,4,5,6,7,8,9,10]. Conventional loosening shovels may have some limitations in their design, resulting in greater soil resistance during loosening, which seriously restricts the development of mechanized harvesting of root vegetables. In nature, mole crickets exhibit exceptional digging ability, and their forefoot claw toe structure has unique advantages [11,12,13,14]. Therefore, in order to improve the efficiency of the mechanized harvesting of root vegetables and reduce the loss rate, it is necessary to optimize the structure and working parameters of the common loosening spade [15,16,17,18,19,20,21]. The study and application of the bionic structure of the first claw toe of the mole cricket forefoot can bring innovation and improvement to the design of loosening shovels.

In the actual operation process of agricultural machinery, in order to overcome the working resistance and soil adhesion, the touching parts will produce a large amount of energy consumption, and its energy consumption occupies 30%~50% of the overall agricultural machinery, which greatly increases the production cost. Excessive work resistance and soil adhesion also directly affect the quality of the work; therefore, researchers have used the idea of bionics, by learning to imitate the good characteristics of the animal structure and its application in the design of agricultural machinery optimization, and have achieved good results [22,23]. Hu, J. et al. [24] combined bionic engineering techniques and biological properties to design a bionic blade based on the structure of locust upper jaw cutting teeth, which improved the cutting performance of existing harvesters. Guan, C. et al. [25] designed profiled rotary cutter teeth based on badger claw toe geometry, which effectively reduced the soil-cutting effect during rotary tillage operation. Tian, K. et al. [26] designed a rotary bionic blade for kenaf stalk cutting using the upper and lower jaws of a bat as a bionic prototype. Wang, Z. et al. [27] designed a bionic loosening shovel based on the claws of grassland zokors to improve the loosening effect of degraded grasslands. Soni et al. [28,29] designed the surface structures of loosening shovels with different types of features obtained by mimicking the convex envelope structure of dung beetle body surface. Akter and Basak [30] designed bionic fighting teeth based on the forepaw structure of anteaters and American badgers using different ways of incorporation with the claws of American badgers. Zhou, W. et al. [31] designed a bionic drag-reducing shovel based on the principle of bionics using a badger claw toe as a bionic prototype to improve the operational performance of a carrot combine operation. Su, F. et al. [32] designed a new tool to minimize defects in CFRP machining based on the unique structure of the lower incisors of bamboo rats. Luo, Y. et al. [33] used the principle of bionics to optimize the trapezoidal tooth structure of a tea picker using the mandible of Aeolesthes induta Newman as a bionic object. Qin, K. et al. [34] designed a crank-rocker furrowing device to reduce the operational energy consumption of a furrowing and loosening machine by using the manual shoveling action as a bionic object. At present, there have been more mature research results for the bionic optimization of structures, such as cutters, plows, and robots, but there is still a lack of research on the problems related to the reduction of resistance and the increase of disturbance of loosening shovels applied to the harvesting process of root and tuber vegetables. Therefore, this study aims to improve the performance of loosening shovels in agricultural machinery applications through biomimetic design principles to fill this gap in the existing research.

The reasonable setting of the working parameters of the loosening shovel is also a key factor affecting its working resistance and loosening performance, while the discrete element method can accurately simulate the interaction between the loosening shovel and soil particles. Therefore, it is equally crucial to investigate the performance of loosening shovels under different operating parameters based on the discrete element approach [35,36,37,38,39]. Yuan, H. et al. [40] carried out s numerical simulation based on the EDEM discrete element method on an automatic device for glutinous rice filling of lotus root to solve the optimal combination of operating parameters and to improve the filling efficiency. Yang, Q. et al. [41] used the discrete element simulation method (EDEM) combined with experimental methods to study the soil contact characteristic parameters in East Asia, which provided more comprehensive soil characteristic parameters for the design and optimization of various soil contact machinery. Wang, M. et al. [42] analyzed the contact between billets, related mechanical components, and billet motion laws in a grain metrology device based on the discrete element method, and proposed a simulation method for the crystal filling process based on EDM, which proved the validity of the EDEM simulation of the seed filling process. Liu, H. et al. [43] constructed the shoveling process of a V-shaped root-cutting knife and the root–soil complex based on the discrete element method to optimize the structure and parameters of a root-cutting knife for cotton stalk harvester. Huang, W. et al. [44] optimized the parameters of a garlic digging shovel based on the discrete element method using quadratic regression orthogonal rotating combinatorial simulation tests. Luo, Q. et al. [45] optimized the structure and parameters of a melon kernel separation unit based on the discrete element method. Jiang, D. et al. [46] constructed a coupled model of a cotton rhizome–soil mixture based on the discrete element method to optimize the structure and parameters of the key working parts of a cotton harvester.

Although many results have been achieved in the previous studies, there are still limitations in optimizing the loosening shovel for the harvesting process of root vegetables. In order to further improve the harvesting efficiency and to reduce the energy consumption, this study optimized the tip structure of the loosening shovel by extracting the contour curve and analyzing the digging process based on the efficient digging mechanism of the first claw toe structure of the mole cricket forefoot, and constructed the working resistance model and the dynamic balance equations of the bionic loosening shovel. Secondly, a two-factor and three-level orthogonal simulation experiment was set up for the bionic loosening shovel to determine the optimal working parameters, and a comparative simulation experiment was carried out to compare the loosening performance with that of the common loosening shovel. Finally, this study will improve the working performance of the loosening shovel, increase the disturbance range and loosening efficiency, reduce the traveling resistance during the working process, and reduce the energy consumption.

2. Materials and Methods

2.1. Analysis and Characterization of the Morphology of the Forefoot of Mole Crickets

In nature, mole crickets exhibit exceptional digging ability, with the unique structure of the clawed toes of their forefeet providing a strong digging advantage. Through an analysis of the morphological characteristics of the mole crickets’ forefoot claws and toes, it can be found that the mole crickets’ forefoot is broad and stout, shovel-shaped or sickle-shaped, with serrated protrusions on the outer edge, which is a kind of highly specialized “digging foot” with well-developed muscles, which can generate strong digging force and help to cut and push the soil in the loosening process. The structural design of the loosening shovel is based on the first clawed toe of the mole cricket as a bionic object because the mole cricket’s forefoot is not subject to friction damage when it frequently interacts with the soil.

The forefoot structure of mole crickets was magnified 50 times for observation, as shown in Figure 1. The forefoot of mole crickets mainly consists of claw toes, tarsal claw toes, feathered spines, and palms; the overall shape is similar to that of a sickle-type shovel, and this special structure can make it highly adapted to a digging behavior in the soil. In order to further study the morphological characteristics of the first claw toe of mayflies and their role in the excavation process, the first claw toe of mayflies were removed and placed on slides using forceps to avoid damaging the surface of the claw toes. The sample of the removed first claw toe was placed under a microscope (VHX-900F, KEYENCE, Osaka, Japan) and, through magnified observation, it can be seen that the edge structure of the first claw toe is sharp, with fine serrated protrusions on the surface, similar to a wedge-shaped structure. The toe tip angle α is about 30°, which can better balance the cutting resistance and the cutting depth, and this structure is more suitable for shallow excavation of loose soil, which helps to enhance its ability to cut the soil. In addition, the surface of the first claw toe has a degree of curvature and hardness, a morphological feature that contributes to the efficiency of digging and reduces the resistance to digging in hard soils.

In order to extract the contour of the first claw toe of mayflies, the collected microscopic pictures of the first claw toe of the mayflies’ forefoot were intercepted and enlarged, and the image processing module in MATLAB (2022) software was utilized to convert the contour image into a parametric mathematical model according to the theory of mathematical morphology through the function commands in the software, and ultimately, the accurate two-valued contour function coordinates (x, y) and a clear contour curve were obtained. The specific method and process are shown in Figure 2.

According to the processing results, the boundary contour curve of the first claw toe of mole cricket is obtained, as shown in Figure 3, and the curve is located within the pixel array of 856 × 550. Since the curve of the first claw toe of mole crickets presents nonlinear characteristics, in order to ensure the accuracy of curve fitting, the segmented fitting method is adopted, and the contour curve of the first claw toe of mole crickets is divided into three independent sub-curves 1, 2, and 3 for analysis.

Due to the special characteristics of the curve waveform, in order to be able to better describe the complex boundary curve morphology, the method of gradually increasing the polynomial order was used to make the fitting results highly coincide with t he boundary contour curve of the first paw toe of the mole cricket. After several tests and verifications, the fitting method was finally determined to be the least squares ninth degree polynomial. The final fitted equation is shown in Equation (1).

$$p(x)=w_0+w_{1}x+w_2{x}^2+w_3{x}^3+w_4{x}^4+w_5{x}^5+w_6{x}^6+w_7{x}^7+w_8{x}^8+w_9{x}^9$$

(1)

The fitting results of the parameters of the three curve equations are shown in

Table 1. Results of curve fitting.

	Curve 1		Curve 2		Curve 3
	Numerical Value	Standard Error	Numerical Value	Standard Error	Numerical Value	Standard Error
Intercept	−176,719.85597	41,238.84514	153.48664	17.11392	562.90381	1.49474
B1	24,356.23756	5877.10854	10.09504	0.49056	−4.22914	0.06302
B2	−1470.71051	367.64159	−0.12334	0.00582	0.04162	0.00101
B3	51.19512	13.25019	8.01458 × 10−4	3.77633 × 10−5	−3.04654 × 10−4	8.30783 × 10−6
B4	−1.13229	0.30326	−3.13673 × 10−6	1.48216 × 10−7	1.35471 × 10−6	3.93509 × 10−8
B5	0.01651	0.00457	7.67921 × 10−9	3.6726 × 10−10	−3.66429 × 10−9	1.13467 × 10−10
B6	−1.5862 × 10−4	4.54059 × 10−5	−1.18002 × 10−11	5.77675 × 10−13	6.05682 × 10−12	2.02054 × 10−13
B7	9.69497 × 10−7	2.86583 × 10−7	1.10412 × 10−14	5.59005 × 10−16	−5.96405 × 10−15	2.16735 × 10−16
B8	−3.42103 × 10−9	1.04335 × 10−9	−5.74668 × 10−18	3.03342 × 10−19	3.21045 × 10−18	1.28248 × 10−19
B9	5.31204 × 10−12	1.66991 × 10−12	1.27575 × 10−21	7.06104 × 10−23	−7.26981 × 10−22	3.21344 × 10−23
R2	0.99234		0.99904		0.99995

, in which the coefficients of determination R² of all the fits are greater than 0.992, and the higher R² values reflect the good fitness of the fitted equations to the experimental data, indicating that the fitted equations have a high degree of precision and credibility. This result indicates that the chosen least-squares ninth-degree polynomial fitting method can adequately capture the key features of the curve and accurately describe the trend of the boundary contour of the first claw toe of mayflies. It provides a solid theoretical foundation for the subsequent quantitative analysis and the construction of the three-dimensional model.

The curve 1 obtained through Origin (2021) software fitting is shown in Figure 4a, the curve has 543 pixel points, showing the trend of first rising sharply and then tending to flatten gradually; the peak of the curve is 485. The curve shape clearly reflects the change rule of the top of the first claw toe of the mole cricket; The equation for curve 1 is shown in Equation (2). The curve 2 obtained from the fitting is shown in Figure 5a, which has 880 pixel points and shows a sharp decline followed by a gradual flattening trend, with the peak of the curve at 485. The equation for curve 2 is shown in Equation (3). The curve 3 obtained from the fitting is shown in Figure 6a, which has 1217 pixel points and shows a sharp decline followed by a gradual leveling off trend, with the peak of the curve at 444. The equation for curve 3 is shown in Equation (4).

$$\begin{array}{ll}{p}_1(x)& =-176719.85597+24356.23756x-1470.71051x^2+51.19512x^3\\ & -1.13229x^4+0.01651x^5-1.5862\times {10}^{-4}{x}^6+9.69497\times {10}^{-7}{x}^7\\ & -3.42103\times {10}^{-9}{x}^8+5.31204\times {10}^{-12}{x}^9\end{array}$$

(2)

$$\begin{array}{ll}{p}_2(x)& =153.48664+10.09504x-0.12334x^2+8.01458\times {10}^{-4}{x}^3\\ & -3.13673\times {10}^{-6}{x}^4+7.67921\times {10}^{(-9)}{x}^5-1.18002\times {10}^{-11}{x}^6\\ & +1.10412\times {10}^{-14}{x}^7-5.74668\times {10}^{-18}{x}^8+1.27575\times {10}^{-21}{x}^9\end{array}$$

(3)

$$\begin{array}{ll}{p}_3(x)& =562.90381-4.22914x+0.04161x^2-3.04654\times {10}^{-4}{x}^3\\ & +1.35471\times {10}^{-6}{x}^4-3.66429\times {10}^{(-9)}{x}^5+6.05682\times {10}^{-12}{x}^6\\ & -5.96405\times {10}^{-15}{x}^7+3.21045\times {10}^{-18}{x}^8-7.26981\times {10}^{-22}{x}^9\end{array}$$

(4)

The residual distributions of the three fitted curves are shown in Figure 4b, Figure 5b and Figure 6b, and all the residuals are distributed within the range of ±3, showing no obvious deviation, and the fitting effect is good, which further verifies the reliability and accuracy of the fitting.

In order to better reveal the efficient digging characteristics of mole crickets, and to deeply investigate the whole digging action process of mole crickets in the soil, a strong mole cricket in good condition was selected and placed in an acrylic container filled with sodium alginate (a food additive with similar hardness and friability as the soil) gel, with the inner dimensions of 300 mm × 10 mm × 200 mm (length × width × height), and recorded by a high-speed camera (i-SPEED DF, Olympus Corporation, Tokyo, Japan) to record the free digging behavior of mayflies.

The digging process of the collected mole cricket forefeet is shown in Figure 7, which demonstrates the complete action of the mole cricket forefeet during a single digging cycle. It can be seen from the figure that when mole crickets are digging, both forefeet will firstly extend forward and embed into the “soil”, and then the two forefeet will synchronously extend outward in the horizontal plane, which effectively pushes the “soil” in front of the body away; in the process of pushing away the “soil”, the forefeet will also push the “soil” in front of the body. This process effectively pushes the “soil” in front of the body away, and compacts the “soil” in the process of pushing away, and finally, the two forefeet return to the initial position, thus completing a complete excavation movement cycle, which is about 0.3 s. The excavation and extension time of mole criket’s forefeet accounts for about 84.2% of the whole cycle in one cycle.

Through the dynamic observation of the soil excavation behavior of mole crickets, it can be clearly found that the single excavation cycle of mole crickets could not completely remove all the “soil” within the small area of the front of the body, which means that the space created by a single excavation cycle is not enough to support the advance of mole crickets, and it needs to repeat the excavation action 3–4 times to complete a body advance. The first clawed toe of the mole cricketplays a crucial role in the whole digging process, taking on the main task of soil-cutting and transferring. At the same time, the digging action of mole crickets is not a simple linear movement, and the digging action of its forefoot will produce a certain angle with the horizontal plane, which is not a complete translational movement, showing the digging adaptability of mole crickets in complex environments.

2.2. Structural Design and Mechanical Modeling of Bionic Loosening Shovels

Through the previous detailed analysis of the movement trajectory of the mole cricket’s forefoot and the contour curve of the first claw toe, this paper takes the movement behavior of the mole cricket’s digging soil and its contour curve characteristics of the first claw toe of its forefoot as the bionic basis, extracts its key structural parameters and functional characteristics. Based on the overall dimensions of the widely used household loosening shovels on the market, constructs a bionic shovel tip structure with high efficiency of loosening performance, as shown in Figure 8. The overall profile of the shovel tip draws on the curved shape of the first claw toe of the mole cricket’s forefoot, which effectively improves the soil-cutting effect and reduces the operating resistance at the same time. Meanwhile, the shovel tip extends a guide piece with a 30° bend, which enables the shovel tip to simulate the digging action of mayflies in the process of loosening soil; in the process of loosening soil, the design can form a more uniform loosening effect, avoiding soil crusting or compaction phenomenon that may be caused by the traditional shovel tip [47].

In order to further investigate the working mechanism of the loosening shovel, the bionic loosening shovel is taken as an example to analyze its force during the loosening process, as shown in Figure 9. In the process of cutting soil, v is the forward speed of the whole machine and $\omega$ is the angular velocity of the bionic loosening shovel. Taking the force at blade 4 as an example, it can be seen that the forward speed of the whole machine v can be decomposed into tangential speed v₁ and normal speed v₂, and the normal speed v₂ is all converted into the normal resistance F_n1 to impede the movement of the shovel tip, while the tangential speed F_t1 is the cutting force of the bionic loosening shovel when it is in contact with the soil. Meanwhile, the geometric characteristics of the shovel tip directly determine the forces during operation.

The cutting force F_t1 generated by a single blade in a bionic loosening shovel when cutting soil is shown in Equation (5).

$$F_{\mathrm{t}1}\left(\theta ,t\right)=k\left(\theta ,t\right)\cdot A_1\left(\theta \right)\cdot f_1\left(\theta \right)$$

(5)

where: $F_{\mathrm{t}1}\left(\theta ,t\right)$ is the bionic loosening shovel cutting soil-cutting force, N; $k\left(\theta ,t\right)$ for the soil-cutting resistance coefficient, take 0.35; $A_1\left(\theta \right)$ is the cutting area of the bionic loosening shovel, m²; $f_1\left(\theta \right)$ is the bionic loosening shovel rotated to a certain angle when the cutting force is N; $\theta$ is the loosening shovel’s rotational angle, °; $t$ is the working time, s.

Suppose a certain single blade of a bionic loosening shovel at different angles and different positions has different cutting forces on the soil, the bionic loosening shovel rotates to an angle that produces a cutting force $f_1\left(\theta \right)$, as shown in Equation (6).

$$f_1\left(\theta \right)=\sin \theta$$

(6)

The cutting moment $M_{r1}(t)$ of the bionic loosening shovel during operation is shown in Equation (7).

$$M_{r1}(t)=2{\displaystyle \sum _{\mathrm{i}=1}^4{\displaystyle {\int }_0^{2\pi }R\cdot F_{\mathrm{t}1}}}\left(\theta ,t\right)d\theta$$

(7)

where: $M_{r1}(t)$ is the forward resistance of the bionic loosening shovel in contact with the soil, N; R is the radius of rotation of the bionic loosening shovel, m. (i = 1, 2, 3, 4).

Substituting Equation (5) into Equation (7) results in the following equation:

$$M_{r1}\left(t\right)=2{\displaystyle \sum _{\mathrm{i}=1}^4{\displaystyle {\int }_0^{2\pi }R\cdot k\left(\theta ,t\right)}}\cdot A_1\left(\theta \right)\cdot f_1\left(\theta \right)d\theta$$

(8)

And the forward resistance $F_{r1}\left(t\right)$ of the bionic loosening shovel is shown in Equation (9).

$$F_{r1}\left(t\right)=2{\displaystyle \sum _{\mathrm{i}=1}^4{\displaystyle {\int }_0^{2\pi }{F}_{t1}\left(\theta ,t\right)}}\cdot \cos \phi \left(\theta \right)d\theta$$

(9)

where: $F_{r1}\left(t\right)$ is the component of the cutting force in the forward direction of the whole machine during the contact between the bionic loosening shovel and the soil, N; $\phi \left(\theta \right)$ is the angle between the cutting direction of the bionic loosening shovel and the forward direction, °. (i = 1, 2, 3, 4).

Substituting Equation (5) into Equation (9) results in the following equation:

$$F_{r1}\left(t\right)=2{\displaystyle \sum _{\mathrm{i}=1}^4{\displaystyle {\int }_0^{2\pi }k\left(\theta ,t\right)}}\cdot A_1\left(\theta \right)\cdot f_1\left(\theta \right)\cdot \cos \phi \left(\theta \right)d\theta$$

(10)

Dynamic equilibrium equations are established for the bionic loosening shovel by combining the theory of the rotational inertia, as shown in Equation (11).

$$I_1\cdot {\displaystyle \frac{d\omega }{dt}}=M_1$$

(11)

where: $I_1$ is the rotational inertia of the bionic loosening shovel, kg/m²; $\omega$ is the angular velocity of the bionic loosening shovel, rad/s; $M_1$ is the external torque generated during the working process of the bionic loosening shovel, N·m.

Among them, the external moment mainly includes the soil-cutting moment during the working process of the loosening shovel and the mechanical resistance moment of the loosening shovel, which is directly proportional to its angular velocity, so the mechanical resistance moment, $M_{f1}$, of a unilateral bionic loosening shovel is shown in Equation (12).

$$M_{f1}=c_f\cdot \omega \left(t\right)$$

(12)

where: $M_{f1}$ is the mechanical resisting moment of the bionic loosening shovel, N·m; $c_f$ is the scale factor; $\omega \left(t\right)$ is the instantaneous angular velocity of the bionic loosening shovel, rad/s.

Substituting Equations (8) and (12) into Equation (11) results in the following equation:

$$I_1\cdot {\displaystyle \frac{d\omega }{dt}}=2{\displaystyle \sum _{\mathrm{i}=1}^4{\displaystyle {\int }_0^{2\pi }R\cdot k\left(\theta ,t\right)}}\cdot A_1\left(\theta \right)\cdot f_1\left(\theta \right)d\theta -c_f\cdot \omega \left(t\right)$$

(13)

Also, since $\omega \left(t\right)=d\theta /dt$, Equation (13) can be rewritten as follows:

$$I_1\cdot {\displaystyle \frac{d^2\theta }{d{t}^2}}+c_f\cdot {\displaystyle \frac{d\theta }{dt}}-2{\displaystyle \sum _{\mathrm{i}=1}^4{\displaystyle {\int }_0^{2\pi }R\cdot k\left(\theta ,t\right)}}\cdot A_1\left(\theta \right)\cdot f_1\left(\theta \right)d\theta =0$$

(14)

The final obtained equations of resistance to travel and kinetic equilibrium during the operation of the bionic loosening shovel are shown in Equation (15).

$$\left\{\begin{array}{c}{F}_{r1}\left(t\right)=2{\displaystyle \sum _{\mathrm{i}=1}^4{\displaystyle {\int }_0^{2\pi }k\left(\theta ,t\right)}}\cdot A_1\left(\theta \right)\cdot f_1\left(\theta \right)\cdot \cos \phi \left(\theta \right)d\theta \\ I_1\cdot {\displaystyle \frac{d^2\theta }{d{t}^2}}+c_f\cdot {\displaystyle \frac{d\theta }{dt}}=2{\displaystyle \sum _{\mathrm{i}=1}^4{\displaystyle {\int }_0^{2\pi }R\cdot k\left(\theta ,t\right)}}\cdot A_1\left(\theta \right)\cdot f_1\left(\theta \right)d\theta \end{array}\right.$$

(15)

Similarly, the resistance to travel of a common loosening shovel tip can be expressed as Equation (16).

$$F_{r2}\left(t\right)=2{\displaystyle \sum _{\mathrm{i}=1}^4{\displaystyle {\int }_0^{2\pi }k\left(\theta ,t\right)}}\cdot A_2\left(\theta \right)\cdot f_2\left(\theta \right)\cdot \cos \gamma \left(\theta \right)d\theta$$

(16)

where: $F_{r2}\left(t\right)$ is the component of the cutting force in the forward direction of the machine during the contact between the common loosening shovel and the soil, N; $A_2\left(\theta \right)$ is the cutting area of the common loosening shovel, m²; $f_2\left(\theta \right)$ is the cutting force generated when the common loosening shovel is rotated to a certain angle, N; $\gamma \left(\theta \right)$ is the angle between the cutting direction of the common loosening shovel and the forward direction, °. (i = 1, 2, 3, 4).

In the working process of the loosening shovel, due to the flat structure of the common shovel tip, the angle $\gamma$ of the soil particles in the sliding process on the surface of the shovel tip maintains a constant value or there is a situation that the soil particles deviate from the shovel tip, which results in the increase of $\gamma$ with the increase of the sliding distance; the curved surface of the bionic shovel tip is designed to guide the soil particles along the surface of the shovel tip, which slows down the change of the angle. In the loosening shovel work engineering, when the soil particles first contact the shovel tip, the cutting direction of the loosening shovel and the forward direction of the angle is equal, that is, $\phi =\gamma$, and with the soil particles in the sliding of the shovel tip surface, the bionic loosening shovel cutting direction and the forward direction of the angle $\phi$ is gradually greater than the cutting direction of the common loosening shovel and the forward direction of the angle $\gamma$. Thus, $\cos \phi \left(\theta \right)<\cos \gamma \left(\theta \right)$, at the same time, the cutting area of a bionic loosening shovel of the same length is significantly smaller than the surface area of a common loosening shovel tip. Thus, $A_1\left(\theta \right)<A_2\left(\theta \right)$, then $F_{r1}\left(t\right)<F_{r2}\left(t\right)$.

2.3. Numerical Simulation of Loosening Shovel Based on Discrete Element Method for Simulation Test

The method of rapid filling of the particle model is used in EDEM2020 software for the establishment of the soil particle bed model. Firstly, a 500 mm × 500 mm × 200 mm (length × width × height) box is established for dynamic particle filling, the filled Block box is converted to a Material Block and, at the same time, the actual required soil particle bed is established (the size of the Block is an integer multiple of the Block), the size of which is 2000 mm × 1000 mm × 200 mm (length × width × height). Finally add the Block Factory to complete the rapid filling of the soil particle bed, as shown in Figure 10.

Because the model of the loose soil shovel is more complex, it will produce unnecessary setup steps in the EDEM simulation process and affect the length of the simulation, so in this paper, the model of the loose soil shovel is simplified, and the material of the loose soil shovel is set to 65 Mn. The simplified model of the loose soil shovel, its rotational direction, and the setup of the direction of the traveling motion are shown in Figure 11. Based on the basic condition that the planting soil for root vegetables is mainly sandy loam, we focused on the operational performance under sandy loam conditions. Soil particles and loose soil shovel parameters (Poisson’s ratio, density, shear modulus), contact parameters between particles and contact parameters between materials are shown in

Table 2. Simulation parameterization of the loosening process [48,49].

Serial Number	Parameter	Parameter Value	Unit
1	Density of soil particles $\rho$	kg/m3	1698
2	Poisson’s ratio of soil particles $\nu$	-	0.3
3	Shear modulus of soil particles G	Pa	107
4	Collision recovery coefficient for soil particles	-	0.42
5	Static friction coefficient of soil particles		0.176
6	Rolling friction coefficient of soil particles	-	0.22
7	Poisson’s ratio of 65 Mn $\nu$	-	0.282
8	Density of 65 Mn $\rho$	kg/m3	7850
9	Shear modulus of 65 Mn G	Pa	8 × 1010
10	Coefficient of recovery for collision between 65 Mn and soil	-	0.4
11	Coefficient of static friction between 65 Mn and soil	-	0.67
12	Coefficient of rolling friction between 65 Mn and soil	-	0.05
13	Soil particle radius	mm	4
14	Contact radius of soil particles	mm	2
15	Normal stiffness per unit area for Bonding V2 Contact Models	Pa	5 × 106
16	Shear Stiffness per Unit Area for Bonding V2 Contact Models	Pa	5 × 106
17	Critical Positive Stresses for Bonding V2 Contact Models	Pa	45,000
18	Critical Shear Stresses for Bonding V2 Contact Models	Pa	5000
19	Bonding Radius for Bonding V2 Contact Models	mm	1

.

In order to deeply investigate the operational performance of the loosening shovel in the actual working process and to optimize its structure and motion parameters, a two-factor, three-level orthogonal simulation experiment was designed for the loosening process based on the discrete element method. Combined with the actual working conditions of the loosening operation, under the basic conditions of determining the loosening depth of 100 mm, and in order to ensure that the simulation test has a high degree of representativeness and consistency, the main selection of the loosening shovel rotational speed and the whole machine speed, as the main influencing factors of the loosening process for comparative analysis, was tested to determine the optimal combination of the loosening shovel rotational speed and the forward speed of the machine. The experimental factors are shown in

Table 3. Coding table of test factors.

Coding Level	Factor
	$\mathbf{Rotational} \mathbf{Speed} \mathbf{of} \mathbf{Loosening} \mathbf{Shovel} \mathit{\omega }$ (r/s)	Traveling Speed of the Whole Machine v/(m/s)
−1	5	0.3
0	6	0.4
1	7	0.5

[50].

The rotational speed of the loosening shovel affects the cutting efficiency and the amount of the cutting force exerted on the soil particles, while the travel speed determines the area covered by the loosening shovel’s work process, affecting the volume of the soil cut and the loosening shovel–soil interaction. Based on this two-factor, three-level orthogonal test method, different parameter settings were sequentially applied to nine combinations of test protocols, and the interactions between the loosening shovel and soil particles were monitored during the simulation process and analyzed by taking the force of the loosening shovel, the motion state of the soil particles, the area of soil disturbance, and the distribution of the force as the key evaluation indexes.

3. Results and Discussion

Since the soil is disturbed differently by different rotational speeds of the loosening shovel and the traveling speed, the post-processing module based on the EDEM derives the disturbance cloud diagrams of the soil particles under different rotational speeds and traveling speeds and the same position of the loosening shovel. At the same time, in order to facilitate the comparative analysis, the upper and lower limits of the color scales in the velocity cloud diagrams and the intervals are set to the same parameter, so that the soil disturbance cloud diagrams under the final conditions of different parameters are as shown in Figure 12.

As can be seen from the soil disturbance cloud diagrams in Figure 12a–i, the soil disturbance areas in the three cross-sections of YOZ, XOZ, and XOY vary under different rotational speeds and different traveling speeds. In order to more accurately analyze the soil disturbance area under each parameter condition, the scale was marked on the soil disturbance cloud map and exported in the form of a picture. MATLAB was used to extract the soil disturbance area. Through color segmentation, the red, green, and yellow colors in the soil disturbance cloud map were separated from the area where they were obviously disturbed, the image was binarized, and the soil disturbance area was finally calculated under the nine parameters of the bionic loosening shovel. The soil disturbance parameters of YOZ, XOZ, and XOY cross sections are shown in

Table 4. Table of parameters for soil disturbance.

Test Number	$\mathbf{Rotational} \mathbf{Speed} \mathbf{of} \mathbf{Bionic} \mathbf{Loosening} \mathbf{Shovel} \mathit{\omega }$/(r/s)	Traveling Speed of the Whole Machine v/(m/s)	Area of Soil Disturbance in YOZ Section (m2)	Area of Soil Disturbance in XOZ Section (m2)	Area of Soil Disturbance in XOY Section (m2)	Volume of Soil Disturbance (m3)
1	5	0.3	6.24 × 10−3	1.17 × 10−2	1.73 × 10−2	8.47 × 10−4
2	5	0.4	7.82 × 10−3	1.40 × 10−2	2.23 × 10−2	1.18 × 10−3
3	5	0.5	1.21 × 10−2	1.63 × 10−2	3.27 × 10−2	1.91 × 10−3
4	6	0.3	7.59 × 10−3	1.52 × 10−2	2.54 × 10−2	1.29 × 10−3
5	6	0.4	8.04 × 10−3	1.38 × 10−2	2.90 × 10−2	1.35 × 10−3
6	6	0.5	1.23 × 10−2	1.55 × 10−2	3.07 × 10−2	1.82 × 10−3
7	7	0.3	1.29 × 10−2	2.22 × 10−2	3.08 × 10−2	2.24 × 10−3
8	7	0.4	8.55 × 10−3	1.79 × 10−2	3.49 × 10−2	1.74 × 10−3
9	7	0.5	9.99 × 10−3	1.15 × 10−2	3.54 × 10−2	1.52 × 10−3

.

The soil disturbance area on the three cross sections can clearly show the soil disturbance of each cross section under nine groups of different parameter combinations, but it cannot intuitively reflect the disturbed volume of the entire soil particle bed. In order to more intuitively and accurately analyze the effect of the bionic loosening shovel on the soil disturbance in the process of the operation, in this paper, the soil disturbance area is equivalent to the soil disturbance area that can accurately reflect the non-homogeneous characteristics of the soil disturbance region of the ellipsoid, as shown in Figure 13. The disturbed volume of the soil particle bed is approximated based on the disturbed area of the soil in the three orthogonal planes YOZ, XOZ, and XOY. Therefore, based on the volume equation of ellipsoid $V=4\pi abc/3$, assuming that the length of the ellipsoid’s semiaxis in the direction of the X-axis is a, the length of the semiaxis in the direction of the Y-axis is b, and the length of the semiaxis in the direction of the Z-axis is c, the projected areas of soil disturbed particles on YOZ, XOZ, and XOY planes are given by the formulas of $S_{yz}=\pi bc$, $S_{xz}=\pi ac$, and $S_{xy}=\pi ab$, respectively. This is obtained by multiplying the areas of these three cross sections as follows:

$$abc=\sqrt{{\displaystyle \frac{S_{yz}\cdot S_{xz}\cdot S_{xy}}{{\pi }^3}}}$$

(17)

Then, the volume of the disturbed area of the soil particle bed can be expressed as Equation (18).

$$V={\displaystyle \frac{4}{3}}\sqrt{{\displaystyle \frac{S_{yz}\cdot S_{xz}\cdot S_{xy}}{\pi }}}$$

(18)

Based on this assumption method, the formula calculated the soil disturbance volume under nine groups of different parameters, as shown in Table 4. At the same time, in order to analyze the disturbance of the soil by each parameter more intuitively, the bar-dot line combination of soil disturbance area and disturbance volume was plotted, as shown in Figure 14.

The force of the loosening shovel is a complex multi-factor coupling process. As can be seen from Figure 12j, the maximum force point of the bionic loosening shovel occurs at the end of the tool, which is mainly derived from the cutting resistance generated by the tip of the loosening shovel when it rotates and cuts into the soil, the extrusion resistance generated by the soil on the tool, the collision force generated between the loosening shovel and the soil particles, and the friction resistance generated between the loosening shovel and the soil particles in the process of rotating the loosening soil.

In order to analyze and select the optimal parameter combinations more accurately, the total force and soil particle velocity distribution of the loosening shovels under nine parameter combinations in the course of time changes were statistically calculated in this paper. The statistical results are shown in Figure 15 and Figure 16.

From the loosening shovel working process force analysis graph can be seen that the 5 r/s–0.5 m/s parameter combination of the loosening shovel force is more stable, with no obvious force fluctuations, and the overall force compared to other parameter combinations are at a lower level, while at the 7 r/s–0.3 m/s parameter combination of the loosening shovel force there is a small section of obvious fluctuations, although the 1.2–2.7 s loosening shovel force is at a lower level, but the overall performance of the force is unstable.

As can be seen from the soil particle velocity distribution graph, the soil particle velocity distribution under the parameter combination of 5 r/s–0.5 m/s is more uniform, and there is no obvious velocity fluctuation phenomenon. The average particle disturbance velocity is 0.61 m/s, which is not significantly different from the soil particle disturbance velocity under other parameter conditions, but there are obvious fluctuations and peaks in the soil particle disturbance velocity under the other parameter combinations, which may be due to the presence of soil particle splashing during the working process of the loosening shovel. Thus, synthesizing the simulation analysis and the practical operational requirements, a loosening shovel rotation speed of 5 r/s and a whole machine travel speed of 0.5 m/s result in the best loosening effect. So, the selection of $\omega$ = 5 r/s, v = 0.5 m/s is the optimal parameter combination.

In order to more intuitively compare the difference in the soil loosening performance between the bionic loosening shovel and the common loosening shovel, combined with the optimal combination of parameters of the loosening shovel and the traveling speed of the whole machine analyzed in the previous subsection, the loosening performance of the common loosening shovel is simulated with the rotational speed $\omega$ = 5 r/s and the traveling speed of the whole machine v = 0.5 m/s. The pre-processing of the test and the parameter settings are the same as that of the bionic loosening shovel, and the final soil disturbance obtained is shown in Figure 17.

By MATLAB, the disturbed areas of the common loosening shovel in the three cross sections of YOZ, XOZ, and XOY are calculated as 6.69 × 10⁻³ m², 1.36 × 10⁻² m², and 3.08 × 10⁻² m², respectively. Based on the area of soil disturbance in the three orthogonal planes of YOZ, XOZ, and XOY, the volume of soil disturbance for a common loosening shovel calculated by Equation (18) is 1.26 × 10⁻³ m³. From Figure 18, it can be seen that the maximum force point of the common loosening shovel is similar to that of the bionic loosening shovel, which also mainly appears at the tail end of the tool, and mainly originates from the cutting resistance generated by the tip of the loosening shovel when it rotates and cuts into the soil, the extrusion resistance generated by the soil to the tool, the collision force generated between the loosening shovel and the soil particles, and the friction resistance generated between the loosening shovel and the soil particles in the process of rotating and loosening the soil. In order to compare the performance difference between the bionic loosening shovel and the common loosening shovel more significantly, the force curves and soil particle disturbance speeds of the common loosening shovel and the bionic loosening shovel were compared and analyzed, and the plotted curves are shown in Figure 19 and Figure 20.

In the loosening shovel loosening operation process, by comparing the loosening shovel force curve comparison diagram in Figure 19, it can be clearly observed that there are significant differences in the force characteristics of the common loosening shovel and the bionic loosening shovel; although, the time change of the force curve shows a similar trend of change between the two, but the specific performance is very different. The force curve of the common loosening shovel shows more violent fluctuations, which means that the resistance it is subjected to varies greatly during the loosening process, showing obvious peaks and valleys, which may be due to the impact and instability of its loosening shovel structure when interacting with the soil, and the maximum peak value is 71.756 N. This indicates that, at a certain moment, the loosening shovel needs to be subjected to a higher resistance, which may cause a greater shock to the drive system and loss of tool life, as well as increasing the energy consumption of the whole machine.

By comparison, the force of the bionic loosening shovel shows a small range of fluctuations over time during the simulation process, which indicates that the resistance changes in the loosening process are relatively gentle. This is mainly due to the design of the bionic structure, which effectively reduces the impact and vibration in the process of loosening; at the same time, the peak force of the bionic loosening shovel is relatively low, with a maximum peak value of 63.026 N. The simulation results are completely consistent with the previous theoretical analysis, and the peak force of the bionic loosening shovel is obviously smaller than that of the common loosening shovel, which verifies the accuracy of the theoretical analysis. This result shows that the bionic loosening shovel is subjected to less resistance during the loosening process, which enables more efficient utilization of the overall machine power, reduces energy consumption, and extends the service life.

By analyzing the soil particle disturbance velocity comparison graph shown in Figure 20, it can be clearly seen that the disturbance characteristics of the two to the soil particles show significant differences. In the simulation of the operation process of the soil particles, the disturbance velocity of the common loosening shovels presents a low-frequency fluctuation characteristics. This implies that the velocity of movement of the soil particles varies more slowly, with relatively long time intervals between peak and valley transitions, but with a significant peak mutation in the interval 1.5–2.0 s, with a peak velocity of 1.566 m/s. This suggests that, at this moment in time, the common loosening shovel produced a large impact on the soil, resulting in momentary acceleration of the soil particles, but did not produce a sustained high velocity disturbance during the overall operation.

By comparison, the disturbance velocity of the soil particles of the bionic loosening shovel showed higher frequency fluctuation characteristics during the simulation operation, which indicated that its disturbance velocity changes were more frequent. Multiple velocity fluctuations occurred in a shorter time interval, but the range of fluctuations was smaller, which implied that the change of disturbance velocity of soil particles was relatively smooth. Similarly, the disturbance velocity of the bionic loosening shovel in the time interval of 1.5–2.0 s also showed large fluctuations, but the peak velocity was 1.264. This indicates that the bionic loosening shovel effectively avoids excessive impact of soil particles while ensuring soil disturbance, and presents a better soil disturbance phenomenon compared to common loosening shovels.

In summary, the volume of soil disturbed during the simulated work of the common loosening shovel is 1.26 × 10⁻³ m³, while the volume of soil disturbed by the bionic loosening shovel is 1.91 × 10⁻³ m³, which is an improvement of 51.59% compared to the common loosening shovel. This result shows that, under the same working conditions, the bionic loosening shovel is able to change the structural properties between soil particles more effectively and make them loose; in terms of working resistance, the maximum value of working resistance produced by the bionic loosening shovel, 63.026 N, is 12.17% lower compared to that of the common loosening shovel, 71.756 N.

It can be seen that the bionic loosening shovel design based on the first claw toe of mole crickets significantly outperforms the ordinary loosening shovels in work, which not only can disturb the soil in a wider range and improve the loosening efficiency, but significantly reduces the working resistance and energy consumption. This study is the first time that the geometry of the first claw toe of the mole cricket has been applied to a rotary loosening shovel for the mechanized harvesting of root crops. Its performance is consistent with bionics-based studies by other scholars. Although our research work has made some breakthroughs, there are some limitations. Our study focuses more on the performance test in a sandy loam environment, and has not been studied for other types of soil. In future research, the working performance under different properties of soil should be studied in depth, to potentially adapt its design for optimal operation across a broader spectrum of soil conditions.

4. Conclusions

Aiming at the current excavation and loosening operation process with high resistance and high damage rate, in this paper, a mathematical model of the cutting resistance of a bionic loosening shovel was established based on the structure of the first claw and toe of the mole cricket by extracting its contour curve and analyzing the digging process of the forefoot. A high perturbation rate bionic loosening shovel structure was designed, and the loosening performance of common loosening shovels was comparatively analyzed. The specific conclusions can be summarized as follows.

First, based on the efficient digging mechanism of the first claw toe structure of the mole cricket’s forefoot, the shovel tip structure of the bionic loosening shovel was designed by extracting its contour curve and analyzing the digging process. At the same time, the working resistance model and the kinetic equilibrium equation of the bionic loosening shovel were constructed. By comparing the working resistance of the bionic loosening shovel with that of the common loosening shovel, it was determined that the traveling resistance of the bionic loosening shovel was much smaller than that of the common loosening shovel.

Second, the bionic loosening shovel’s disturbance of the soil particle bed was analyzed by a two-factor, three-level orthogonal simulation test for nine sets of different parameter combinations. The optimal parameter combination of the bionic loosening shovel was determined as $\omega$ = 5 r/s and v = 0.5 m/s by combining the statistical analysis of the total force of the bionic loosening shovel over time and the velocity distribution of soil particles under nine groups of parameters. At the same time, the soil disturbance and force conditions of the bionic loosening shovel and the common loosening shovel under the optimal parameter combinations were compared, and it was determined that the disturbance performance of the bionic loosening shovel on the soil was improved by 51.59% compared with the common loosening shovel, and the working resistance was reduced by 12.17%. This result suggests that the bionic structure of the first claw toe of the mole cricket can significantly improve the working performance of the loosening shovel, which can effectively improve the cutting effect of the soil and reduce the energy loss during the working process.