Design and Experiment of In Situ Soil-Lifting Shovel for Direct-Injection Straw Deep-Burial Machine

Abstract

A direct-injection straw deep-burial method was proposed to address the issues of insufficient depth and compaction during the process of straw deep burial. Based on the working principle of oblique cutting, an in situ soil-lifting shovel is designed for oblique cutting of soil and in situ lifting of soil, forming a deep burial space for straw. Elaborating on the working principle of the in situ soil-lifting shovel, we analyzed the stress situation of each stage during the operation process and determined the structural parameter values of the in situ soil-lifting shovel. Using the DEM simulation analysis method, the regression orthogonal simulation test is carried out with the soil-opening angle, the soil-lifting angle, and the camber angle of the in situ soil-lifting shovel as the test indicators, and the number of straws deeply buried and the operational resistance as the evaluation indicators. The regression equation and the response surface mathematical model were established to analyze the influence of the interaction of various factors on the operational performance of the in situ soil-lifting shovel. The simulation results showed that the significant order of effect on the number of straws buried deeply and the operational resistance was camber angle > soil-opening angle > soil-lifting angle. After optimization, the structural parameters were soil-opening angle of 17°, soil-lifting angle of 37°, camber angle of 30°, the corresponding number of straw buried was 228.29, and the operating resistance was 2840.45 N. The average value of operational resistance obtained from the field validation test was 3145.95 N, and the error with the simulation results was only 11%. The quantity of straw buried deeply was 90.21%. The straw deep burial experiment further indicates that the operation effect meets the agronomic requirements of straw deep burial.

1. Introduction

Crop straw is the most economical and practical organic nutrient resource in agricultural production, and returning crop straw to the field is a critical way to make full use of straw resources [1,2]. At the same time, returning straw to the area can effectively optimize the farmland’s ecological environment, improve the soil’s physical and chemical properties, and increase crop yield [3,4,5].

Maize straw returning to the field mainly includes straw crushing returning to the field and stubble crushing returning to the field [6,7]. According to the location of straw distribution after an operation, there is straw mulching returning to the field, straw mixed burying returning to the field, and straw ditch burying returning to the field [8,9,10]. Straw deep burial is a kind of straw ditch burial. Appropriate straw-returning methods increase soil porosity, improve soil physical properties of the plow layer, and improve sowing process passability. In recent years, researchers have conducted extensive research on the way of straw burying and returning to the field. Given the insufficient depth of straw returning to the field, Wang et al. proposed the use of the reverse rotary tillage device to complete the functions of cutting soil, crushing soil, burying straw, pressing straw, and covering soil to realize deep burying and returning of straw to the field [11]. To improve the soil plow layer structure and realize mixed burying and returning of straw, Lin et al. developed a strip-combined burying and returning machine for maize straw, which uses a mixing device to realize mixed burying of straw [12]. Tian et al. designed a pneumatic straw deep-burying returning machine. The field machine-conveying device realizes the functions of straw breaking, picking and crushing, ditching and crushing soil, and deep straw burial, and the straw is deeply buried below 20 cm in the soil depth between rows [13]. However, it has some shortcomings: the machining process of the variable helix trenching device is complex, and the wear resistance needs to be improved; the width of the trench is 400 mm. When the amount of buried straw is small, an excessive width of the trench can cause higher power consumption. Aiming at the problems of large amounts of straw in Northeast China, easy blocking of sowing, and the slow rise of ground temperature after sowing, Chen et al. proposed a method of picking up and crushing straw strips for deep burial using ditch shovels: convey deep burial [14]. At the same time, there are also some problems, such as insufficient trench depth, which causes the soil not to cover the straw well, which affects subsequent sowing operations. Based on the working principle of the rotary shovel, Song et al. designed a straw deep-burying and returning operation machine suitable for two rows of prominent ridges to realize straw collection, ditching, and burial [15].

In summary, the existing methods of deep burrowing straw are mostly a mixed burial of rotary tiller blades and deep burial of ditching knives. However, the cutters suitable for deep burial of straw, constant plow layer, and reduced soil disturbance need further research [16].

To solve the problems of insufficient burial depth of surface straw, unreal suppression, and influence on subsequent sowing operations, this paper designs a direct-injection straw deep-burial machine in situ digging shovel. It analyzes its force by establishing a mathematical model of critical parameters. The orthogonal test determined the optimal structural combination parameters of the in situ soil-lifting shovel through three-factor three-level quadratic regression [17,18]. It verified the operating performance of the in situ soil-lifting shovel through field experiments, providing a reference for the research and development of straw deep burial and returning to the field.

2. Materials and Methods

2.1. The Structure and Working Mechanism

2.1.1. The Overall Structure

The direct-injection straw deep-burial machine is shown in Figure 1. It is mainly composed of a frame, a picking device, a crushing device, a conveying device, a ridging device, an in situ soil-lifting device, a shaping and pressing device, a depth-limiting device, and a transmission system. Mechanical–pneumatic-conveying devices include mechanical- conveying devices and pneumatic-conveying devices. The depth-limit height of the depth-limiting device, the depth of the ridging device, and the in situ soil-lifting device can be adjusted by coordinating different bolt holes.

2.1.2. Working Mechanism

The working principle of the in situ soil-lifting shovel for direct-injection straw deep- burial machine is shown in Figure 2. The direct-injection straw deep-burying machine is a rear three-point suspension traction system. The tractor PTO output provides the driving force for the entire machine through the gearbox, and the pulley drive drives the crushing and conveying devices, while the sprocket drive drives the picking device. When the machine moves forward, the ridging device performs the ridging operation, and the in situ soil-lifting device completes the in situ soil-lifting operation in the ridge. The picking device picks up the straw and then transports it to the crushing machine. The crushed straw thrown back is sent to the mechanical-conveying device under the airflow action and transported to the pneumatic-conveying device under the movement of the spiral auger and straw-conveying blades. The straw is transported to the space behind the in situ soil-lifting device under the direction of the high-speed fan throwing blades, and the soil falls back to complete the deep burial operation of the straw. The shaping and pressing device compresses and reshapes the ridge shape forming a clean seed bed.

During the operation, the tip of the in situ soil-lifting shovel breaks the soil, and the lifting plate lifts the soil without flipping. After the in situ soil-lifting shovel, a particular space is formed, and the mechanical–pneumatic-conveying device guides the straw into this space through the straw-conveying duct. The in situ soil-lifting shovel lifts the soil and naturally falls back, completing the deep burying of straw, reducing the amount of soil movement, and ensuring the structure of the plow layer.

2.2. Design of Key Components

2.2.1. Structural Design of In Situ Soil-Lifting Shovel

When designing the structure of the in situ soil-lifting shovel, it is necessary to consider both the penetration capacity of the shovel tip and the soil-lifting plate and to reduce the operational resistance.

The in situ soil-lifting device is shown in Figure 3, mainly composed of a fixed plate, U-shaped bolts, and an in situ soil-lifting shovel. The in situ soil-lifting shovel mainly includes a shovel handle, shovel tip, and soil-lifting plate.

The angle between the tip of the in situ soil-lifting shovel and the horizontal plane is the angle of penetration α; the size ranges from 20° to 30°. The angle between the front section of the in situ soil-lifting shovel and the horizontal plane is the soil-opening angle β, which ranges from 10° to 20°. The angle between the rear end of the in situ soil scraper and the horizontal plane is the angle of soil-lifting angle γ; the size ranges from 30° to 50°. The outward bending angle of the rear end plane of the original lifting shovel is camber angle δ, and the size ranges from 20° to 40°.

According to the camber angle δ, three types of soil-lifting plates were designed: outward inclined, non-inclined, and inward inclined, as shown in Figure 4. Three different in situ soil-lifting shovels were designed based on the different soil-lifting plates.

2.2.2. Force Analysis of In Situ Soil-Lifting Shovel

The force analysis of the in situ soil-lifting shovel is mainly subjected to the force of the shovel tip and the soil-lifting plate during soil lifting and straw deep-burying operations. The following is an analysis of the force exerted by the inclination of the in situ soil-lifting shovel when the soil plate is tilted outward.

• Analysis of the force on the tip of the in situ soil-lifting shovel

During the operation process of the in situ soil-lifting shovel, the shovel tip is mainly subjected to the horizontal traction force of the in situ soil-lifting shovel, the pressure of the soil at the bottom of the ditch on the shovel tip, and its gravity [19,20,21]. The stress situation is shown in Figure 5.

Through force analysis, the force balance equation in the horizontal direction of the shovel tip of the in situ soil-lifting shovel is:

$$F_{x1}=N_1\sin \alpha +f_1\cos \alpha +k{b}_1$$

(1)

where F_x1 is the horizontal traction force exerted on the tip of the in situ soil-lifting shovel (N); N₁ is the positive pressure applied to the normal direction of the shovel tip of the in situ soil-lifting shovel (N); F₁ is the frictional force acting on the tip of the in situ soil-lifting shovel (N); b₁ is the width of the shovel tip of the in situ soil-lifting shovel (mm); α is the angle at which the tip of the in situ soil-lifting shovel enters the soil (°); k is the pure cutting resistance of soil per unit width (N/mm);

The force balance equation for the tip of the in situ soil-lifting shovel in the vertical direction is:

$$f_1\sin \alpha =N_1\cos \alpha +G_1$$

(2)

$$f_1={\mu }_1{N}_1$$

(3)

where μ₁ is the friction coefficient between the soil and the tip of the in situ soil-lifting shovel; G₁ is the self-gravity of the tip of the in situ soil-lifting shovel (N).

The pure cutting resistance of soil is relatively small and can be ignored. Substituting Formulas (2) and (3) into Formula (1) yields:

$$F_{x1}=\frac{G_1}{{\mu }_1\sin \alpha -\cos \alpha }\left(\sin \alpha +{\mu }_1\cos \alpha \right)$$

(4)

From Equation (4), it can be seen that the angle of penetration α there significantly impacts the resistance of the in situ soil-lifting shovel, mainly affecting its penetration performance and forward resistance. As the angle of penetration α increases, the penetration ability deteriorates, and the resistance increases. The angle of penetration α decreases, and the effect of loose soil lifting is poor. Based on the physical characteristics of the soil when returning straw to the field, and in combination with the design of the deep loosening shovel angle in the agricultural machinery design manual, determines the in situ soil-lifting shovel tip angle into the soil α 25° [22].

2.

Analysis of the force on the lifting plate of the in situ soil-lifting shovel

The lifting plate of the in situ soil-lifting shovel is an essential factor affecting the deep burial effect of straw and the operation resistance. To explore the optimal parameters of the lifting plate structure of the in situ soil-lifting shovel, a mechanical contact model between the lifting plate and the soil is established, and the stress analysis of the lifting plate is carried out. As shown in Figure 6, the process of the soil-lifting shovel’s lifting plate is mainly divided into three stages, namely the (I) stage of oblique cutting of soil in the front section of the soil plate and lifting of soil; in the (II) stage, the middle section of the soil-lifting plate further lifts the soil and separates the soil; in the (III) stage, the soil is lifted and separated from the soil after the soil plate is lifted, completing the natural fall of the soil.

Establish a Cartesian coordinate system o₂x₂y₂ at any point on the surface of the front and middle sections of the lifting plate, with the x₂ axis indicating the forward direction of the lifting plate during operation and the y₂ axis indicating the vertical direction. Analyze the stress situation of the lifting plate when it comes into contact with soil particles in the (I) stage, including the soil support force N₂, soil friction force f₂, and its gravity G₂. The combined force acting on the lifting plate is the forward resistance F_x2 of the lifting plate [23]. As shown in Figure 7, the force balance equation for the (I) stage is:

$$F_{x2}=f_2\cos \beta +N_2\sin \beta$$

(5)

$$f_2\sin \beta =N_2\cos \beta +G_2$$

(6)

$$f_2={\mu }_2{N}_2$$

(7)

where F_x2 is the horizontal traction force applied to the lifting plate in the (I) stage (N); N₂ is the positive pressure applied to the normal direction of the lifting plate (N); f₂ is the frictional force acting on the lifting plate (N); β is the lifting angle of the soil plate (°); μ₂ is the friction coefficient between the soil and the lifting plate; G₂ is the gravity of the lifting plate itself (N).

Substituting Equations (6) and (7) into Equation (5) yields:

$$F_{x2}=\left({\mu }_2\cos \beta +\sin \beta \right)\frac{G_2}{{\mu }_2\sin \beta -\cos \beta }$$

(8)

The force acting on the soil plate in the (II) stage is similar to that in the (I) stage. Similarly, it can be concluded that the force equation in the (II) stage is:

$$F_{x3}=\left({\mu }_2\cos \gamma +\sin \gamma \right)\frac{G_2}{{\mu }_2\sin \gamma -\cos \gamma }$$

(9)

where F_x3 is the horizontal traction force applied to the lifting plate in the (II) stage (N); γ is the soil-lifting angle of the soil plate (°).

From Equations (8) and (9), it can be seen that the forward resistance F_x2 and F_x3 of the soil-lifting plate in stages (I) and (II) are related to the soil-opening angle β, soil-lifting angle γ. When increasing, the traction force increases regarding soil-opening angle β and soil-lifting angle γ. After too little, the effect of raising the soil cannot be achieved. Based on the relevant knowledge of the Agricultural Machinery Design Manual, determine the soil-opening angle β where the range of values is 10° to 20°, determining the soil-lifting angle γ where the value range is 30° to 50°. The following text will explore its optimal parameter combination through EDEM simulation experiments.

The force on the lifting plate of the in situ soil-lifting shovel in the (III) stage is shown in Figure 8. At any point on the surface of the lifting plate in the (III) stage, a spatial Cartesian coordinate system o₄x₄y₄z₄ is established, with the direction perpendicular to the forward direction as the x₄ axis, the direction tangential to the (II) stage working surface as the y₄ axis, and the direction perpendicular to the (II) stage working surface as the z₄ axis. Analyze the stress situation of the soil-lifting plate when it comes into contact with soil particles in the third stage, including the positive pressure N₄ on the lifting plate, which is perpendicular to the inclined plane. The horizontal traction force F₄ in the forward direction is along the direction of the implement’s forward movement. The direction of friction force f₄ at o₄ is the flow direction of soil particles at o₄. The self-gravity G₂ is perpendicular to the forward direction and downward. According to the analysis, the equilibrium equations for the x₄, y₄, and z₄ axes are:

$$f_4\cos \delta \sin {\theta }_1-N_4\sin \delta \sin {\theta }_2=0$$

(10)

$$f_4\cos \delta \cos {\theta }_1-N_4\sin \delta \cos {\theta }_2-G_2\sin \gamma -F_4\cos \gamma =0$$

(11)

$$F_4\sin \gamma -f_4\sin \delta -N_4\cos \delta -G_2\cos \gamma =0$$

(12)

$$f_4={\mu }_2{N}_4$$

(13)

where F₄ is the horizontal traction force on the soil-lifting plate in the (III) stage (N); N₄ is the positive pressure applied to the normal direction of the lifting plate (N); f₄ is the frictional force acting on the lifting plate (N); δ is the camber angle of the soil-lifting plate (°); θ₁ is the angle between the projection of frictional force f₄ in the o₄x₄y₄ plane and the y₄ axis (°); θ₂ is the angle between the projection of positive pressure N₄ in the o₄x₄y₄ plane and the y₄ axis (°).

Angle θ₁ is related to the direction of soil flow at point o₄. The direction of positive pressure N₄ is fixed, so θ₂ the angle is fixed and constant. By combining Formulas (10)–(13), it can be concluded that the forward direction traction force F₄ is:

$$F_4=\frac{t{G}_2\cos \gamma +{\mu }_2{G}_2\sin \delta \sin \gamma +G_2\sin \gamma \cos \delta }{t\sin \gamma -{\mu }_2\sin \delta \cos \gamma -\cos \gamma \cos \delta }$$

(14)

$$t={\mu }_2\cos \delta \cos {\theta }_1-\sin \delta \cos {\theta }_2$$

(15)

According to Formulas (14) and (15), it can be seen that the horizontal traction force in the forward direction is related to the soil-lifting angle of the in situ soil-lifting shovel’s lifting plate γ, regarding camber angle δ.

To determine the camber angle δ value range for different camber angle angles δ, three types of in situ soil-lifting shovels were designed for single-factor simulation pre-testing, as shown in Figure 9.

Figure 9 shows that under the three types of in situ soil-lifting shovels, the soil plow layer did not change. Regarding the depth of straw burial, the outward and normal conditions were better than the inward inclination conditions. Statistical analysis of the amount of deeply buried straw and operational resistance data under different shovel types are shown in Figure 10 and Figure 11.

The analysis shows that the depth of straw burial under outward inclination and the non-inclined shovel type is stable over time. After 1.1 s, the straw cannot be deeply buried under the inward inclined shovel type. Under the three shovel conditions, the pressure exerted by the in situ soil-lifting shovel is not significantly different. In contrast, the operating resistance is more significant under inward-inclined conditions than that under outward-inclined and non-inclined conditions. Analysis shows that under inward-inclined conditions, the soil aggregation effect is better, which hinders the transportation of straw and affects the downward transport of straw in the straw-conveying duct. The inward incline of the soil plate is compressed by the soil, resulting in more excellent resistance. Based on the above analysis, under the condition of inward inclination, the effect of straw deep burial is not good. Under non-inclined conditions, the effect of straw burial is not much different from that of outward inclination, so the camber angle δ is set and the range is 0° to 40°.

2.3. EDEM Simulation

2.3.1. Purpose and Method of Simulation Test

To determine the optimal combination parameters of each angle of the soil-lifting plate of the in situ soil-lifting shovel and to obtain trenches that meet the requirements of deep straw burial and small operating resistance, combined with the above analysis, the simulation regression orthogonal test was conducted, with the soil-opening angle, soil-lifting angle, and camber angle as the main factors affecting the straw deep-burial effect, and the amount of deeply buried straw and operating resistance as the evaluation indicators to determine the optimal parameter combination [24,25,26].

2.3.2. Construction of a Soil Model and a Geometric Simulation Model for an In Situ Soil-Lifting Shovel

• Model parameters of in situ soil-lifting shovel

To simplify the computational complexity of the computer, the in situ soil-lifting shovel is simplified by hiding and removing other components except for the shovel handle, shovel tip, soil-lifting plate, and straw-conveying duct. Use the 3D drawing software SolidWorks to model the implementation and import it into the EDEM software in .step file format. According to the characteristics of the physical prototype trial production, the material properties of the shovel handle and straw transport conduit were set to 45 steel, and the material properties of the shovel tip and lifting plate were set to 16Mn steel. The simulation model parameters are shown in

Table 1. Simulation model parameters.

Parameters		Data
45 Steel	Poisson’s Ratio	0.31
	Shear Modulus/Pa	7.0 × 1010
	Density/(kg/m3)	7800
16Mn Steel	Poisson´s Ratio	0.25
	Shear Modulus / Pa	7.9 × 1010
	Density/(kg/m3)	7865

. The gravitational acceleration was set to 9.81 m/s².

2.

Soil and straw model parameters;

The physical properties of soil were closely related to the operation effect of the in situ shovel, and the soil moisture content, solidity, and soil type have an essential impact on the simulation. To ensure that the simulation results are similar to the actual work, the physical parameters of the soil in the experimental field in Xiawo Town, Shangjie District, Zhengzhou City, Henan Province were measured, and the stacking angle was calibrated in the discrete element simulation software EDEM. According to the test and combined with the simulation efficiency, the soil particle radius of 5 mm was selected as the particle model radius. The Poisson’s ratio of the soil measured in the test was 0.35, and the Shear modulus was 1.0 × 10⁶ Pa, with a soil bulk density of 2560 kg/m³. Hertz–Mindlin with bonding contact model was selected as soil particles’ indirect contact mechanics model [27]. Due to differences in soil firmness at different depths, different layer soil models were set up. In combination with the actual situation, after the straw was crushed in the field, the multi-ball model was selected, and the extended linear model with a diameter of 16 mm and a length of 80 mm was set as the straw particle model. The Hertz–Mindlin non-sliding contact model was selected as the straw model, and the straw density was 240 kg/m³, the Poisson’s ratio’s ratio was 0.4, and Shear modulus was 1 0 × 10⁶ Pa. According to previous publications [28,29,30,31], the dynamic friction coefficient, static friction coefficient, and recovery factor between 45 steel–soil particles, 16Mn steel–soil particles, soil particles–soil particles, and 45 steel–straw particles were set up. The relevant parameters are shown in

Table 2. Material contact parameters of the simulation.

Parameters	45 Steel–Soil	45 Steel–Straw	16Mn Steel–Soil	Soil–Soil
Coefficient of rolling friction	0.04	0.01	0. 03	0.20
Coefficient of static friction	0.50	0.30	0.50	0.54
Coefficient of restitution	0.28	0.30	0.28	0.20

.

Set up four layers of polygon virtual planes in EDEM to generate soil particles. The filled soil model has a length L of 2200 mm and a width B of 700 mm. The first layer has a height h₁ of 110 mm, the second layer has a height h₂ of 100 mm, the third layer has a height h₃ of 100 mm, and the fourth layer has a height h₄ of 100 mm. The simulation model is shown in Figure 12.

2.4. Orthogonal Test

2.4.1. Experimental Design

According to the theoretical analysis and single-factor pre-test verification results, the soil-opening angle, soil-lifting angle, and camber angle were selected as the test factors to optimize the parameters better and determine the interaction between the test factors. The amount of deeply buried straw and operating resistance were used as the three-factor three-level quadratic regression orthogonal test indicators. The test factors and coding are shown in

Table 3. Test factors and coding.

Code	Factor
	Soil Corner β/(°)	Soil-Lifting Angle γ/(°)	Camber Angle δ/(°)
−1	10	30	0
0	15	40	20
1	20	50	40

.

2.4.2. Measurement of the Quantity of Deeply Buried Straw

Extract changes in the amount of straw in the soil tank before and after the stable operation period. Use the analysis option in the EDEM software to export the quantity of deeply buried straw, as shown in Figure 13.

2.4.3. Measurement of Operation Resistance

By using EDEM software, extract the resistance of the in situ soil-lifting shovel operation during the stable operation period during the simulation process. At the same time, export the operation resistance at each time node in the software analyst option, and calculate the average value of the operation resistance throughout the entire process, as shown in Figure 14.

2.5. Field Experiment

To further verify the operation effect of the designed in situ soil-lifting shovel, field tests of the whole machine were conducted in the experimental field of Xiawo Town, Shangjie District, Zhengzhou City, Henan Province. The average annual temperature there was 14.5 °C and the annual precipitation was 638 mm. The average soil moisture content of the experimental field was 14.63%, the soil density was 2560 kg/m³, the average soil compaction at a depth of 10 cm was 365 kPa, and at 30 mm was 496 kPa. The diameter range of the maize straw was 11.31~19.47 mm and the average water content was 16.9% ± 5%. The coverage of maize straw was 1 kg/m² and the on-site test is shown in Figure 15.

3. Results and Discussion

3.1. Simulation Operation Process

During the simulation process of the in situ soil-lifting shovel, the in situ soil-lifting shovel model was set to be located at one end of the soil groove for initial operation, as shown in Figure 16. Referring to the forward speed of the straw-returning machine during the process, the initial set of the machine’s forward speed v_m is 0.83 m/s, and the depth of the in situ soil-lifting shovel entering the soil is 30 mm. To ensure simulation stability, set its fixed time step to 5.10 × 10⁻⁵ s (i.e., 10% of the Rayleigh time step), with a total simulation time of 3.4 s and an adequate operation time of 3.0 s, of which 3.0–3.4 s were when the machine leaves the soil tank and waits for the particles to stabilize.

The simulation analysis shows that during the in situ soil-lifting shovel operation, the movement undergoes four processes: I: soil being lifted → II: soil twisting → III: for-ward falling → V: backward falling. Through the soil particle movement of processes I, II, and III, a particular space is formed at IV. The straw conveyed downwards by the straw transport pipe fills the space behind the in situ soil-lifting shovel, completing the straw burial.

3.2. Analysis of Simulation Results

The simulation test results of the in situ soil-lifting shovel are shown in

Table 4. Experimental design and results.

Test Serial Number	Factor			Straw Quantity Y1/pcs	Operation Resistance Y2/N
	Soil-Opening Angle A (°)	Soil-Lifting Angle B (°)	Camber Angle C (°)
1	20	40	0	196	2966.59
2	15	30	0	191	2741.61
3	15	30	40	213	2747.44
4	15	40	20	241	2896.45
5	15	50	40	209	3013.03
6	10	30	20	210	2731.47
7	10	40	40	220	2741.68
8	20	40	40	218	2938.41
9	10	50	20	198	2972.25
10	10	40	0	214	2847.47
11	15	40	20	232	2752.72
12	15	40	20	228	2894.05
13	20	50	20	215	3042.85
14	20	30	20	202	2816.36
15	15	40	20	227	2873.12
16	15	40	20	224	2883.09
17	15	50	0	203	3023.18

, and data processing and statistical analysis were conducted using Design Expert software. A, B, and C are the factor coding values in the table.

3.2.1. Regression Analysis and Significance Testing

Perform secondary regression analysis on the experimental results and perform multiple regression fitting to obtain the regression equation between the experimental indicator of deep buried straw quantity Y₁ and the operational resistance Y_2, as shown in Formula (16), and test its significance.

Table 5. Variance analysis of experimental results.

Source of Variance	Straw Quantity					Operation Resistance
	Sum of Squares	Freedom	Mean Square	F	p	Sum of Squares	Freedom	Mean Square	F	p
Model	2702.02	9	300.22	7.39	0.0076 **	1.63 × 105	9	18,079.17	6.08	0.0133 *
A	15.12	1	15.12	0.3722	0.5611	27,770.17	1	27,770.17	9.34	0.0184 *
B	10.12	1	10.12	0.2492	0.6330	1.29 × 105	1	1.29 × 105	43.26	0.0003 **
C	392.00	1	392.00	9.65	0.0172 *	2390.52	1	2390.52	0.8039	0.3997
AB	156.25	1	156.25	3.85	0.0907	51.05	1	51.05	0.0172	0.8994
AC	64.00	1	64.00	1.57	0.2498	1505.83	1	1505.83	0.5064	0.4997
BC	64.00	1	64.00	1.57	0.2498	63.84	1	63.84	0.0215	0.8876
A2	274.55	1	274.55	6.76	0.0355 *	560.19	1	560.19	0.1884	0.6773
B2	1088.02	1	1088.02	26.78	0.0013 **	1570.33	1	1570.33	0.5281	0.4910
C2	448.87	1	448.87	11.05	0.0127 *	18.87	1	18.87	0.0063	0.9387
Residual	284.45	7	40.64			20,815.58	7	2973.65
Spurious term	111.25	3	37.08	0.8564	0.5320	6113.36	3	2037.79	0.5544	0.6722
Error	173.20	4	43.30			14,702.22	4	3675.56
Total	2986.47	16				1.84 × 105	16

Note: ** means highly significant (p < 0.01), and * means significant (0.01 ≤ p < 0.05).

shows the results of analysis of variance. The quadratic regression model for the quantity of deeply buried straw (Y₁) had a p < 0.01, indicating that the regression model was extremely significant; C and B² had a significant impact; A² and C² had a significant impact. The misfit term p > 0.1, and the misfit was not significant, indicating that the quadratic regression equation fitted by the model was consistent with the reality, and can correctly reflect the relationship between the quantity of deeply buried straw (Y₁) and the soil-opening angle A, soil-lifting angle B, and camber angle C. The regression model can better predict different test results in the optimization test. The analysis of variance showed that the significance of each factor on the quantity of deeply buried straw (Y₁) was in the order of camber angle C, soil-opening angle A, and soil-lifting angle B.

The quadratic regression model of operation resistance (Y₂) (p < 0.05) indicates that the regression model was significant. The impact of B was extremely significant; A had a significant effect. With the misfit term p > 0.1, the misfit was not significant, which indicates that the quadratic regression equation fitted by the model was consistent with the reality. The variance analysis showed that the significance of each factor on the operation resistance (Y₂) is in the order of significance to small: soil-lifting angle B, soil-opening angle A, and camber angle C.

$$\left\{\begin{array}{l}{Y}_1=230.4-1.37A+1.13B+7C+6.25AB+4AC-4BC-8.08A^2-16.08B^2-10.32C^2\\ Y_2=2859.89+58.92A+126.8B-17.29C-3.57AB+19.4AC-3.99BC+11.53A^2+19.31B^2+2.12C^2\end{array}\right.$$

(16)

where A, B, and C are the factor coding values.

3.2.2. Response Surface Analysis

Within the range of experimental parameters, there were differences in the significant order of the experimental factors that affected the two major performance evaluation indicators of straw deep burial and operational resistance. To more clearly and intuitively describe the impact of each experimental factor and its interaction on the experimental evaluation indicators, a response surface diagram of the interaction effect of each factor was drawn based on the regression model analysis results. The impact of experimental factors on straw deep burial is shown in Figure 17; the influence of experimental factors on job resistance is shown in Figure 18.

When the camber angle was at the central level (20°), and the soil-opening angle was fixed, the quantity of deeply buried straw increases first and then decreases with the rise of the soil-lifting angle. When the soil-lifting angle was constant, the amount of deeply buried straw first increases and then decreases with the increase of the soil-opening angle. When the soil-lifting angle changes, the variation range of the quantity of deeply buried straw was relatively large, and the soil-lifting angle has a more significant impact on the amount of deeply buried straw index (Figure 17a). When the soil-lifting angle was at the central level (40°) and the soil-opening angle was constant, the quantity of deeply buried straw increases first and then decreases with the rise of the camber angle. When the camber angle was constant, the amount of deeply buried straw increases first and then decreases with the rise of the soil-opening angle. When the camber angle changes, the change range of the quantity of deeply buried straw is extensive and the camber angle has a more significant impact on the amount of deeply buried straw indicators (Figure 17b). When the soil-opening angle was at the central level (15°) and the soil-lifting angle was constant, the quantity of deeply buried straw increases first and then decreases with the rise of the camber angle. When the camber angle was constant, the amount of deeply buried straw increases first and then decreases with the rise of the soil-lifting angle. When the camber angle changes, the change range of quantity of deeply buried straw was extensive, and the camber angle had a more significant impact on the amount of deeply buried straw indicators (Figure 17c).

When the camber angle was at the center level (20°) and the opening angle was fixed, the operating resistance increases with the increase of the soil-lifting angle. When the soil-lifting angle was constant, the working resistance increases with the rise of the soil-opening angle. When the soil-lifting angle changes, the range of variation in operational resistance was relatively large and the soil-lifting angle had a more significant impact on operational resistance indicators (Figure 18a). When the soil-lifting angle was at the center level (40°) and the soil-opening angle was fixed, the operating resistance decreases with the increase of the camber angle. When the camber angle was constant, the working resistance increases with the rise of the soil-opening angle. When the soil-opening angle changes, the range of variation in working resistance is relatively large and the soil-opening angle had a more significant impact on the working resistance index (Figure 18b). When the soil-opening angle was at the central level (15°) and the soil-lifting angle was constant, the operating resistance has little change with the increase of the camber angle. When the camber angle was constant, the working resistance increases with the rise of the soil-lifting angle. When the soil-lifting angle changes, the range of variation in operational resistance was relatively large and the soil-lifting angle had a more significant impact on operational resistance indicators (Figure 18c).

The operational resistance of the in situ soil-lifting shovel increases with the increase of the soil-opening angle and soil-lifting angle, and the soil-lifting effect increases with the increase of the soil-opening angle and soil-lifting angle [32]. When the soil-opening angle was 10° and the soil-lifting angle was 20°, the operational resistance of the in situ soil-lifting shovel is minor, but it is not easy to form an ample space, reducing the effect of straw deep burial. When the opening angle was 20° and the soil-lifting angle was 50°, the lifting effect of the in situ soil-lifting shovel is more substantial. Although it enhances the lifting effect of the in situ soil-lifting shovel on the soil, it is easy to cause the soil to accumulate in front of the shovel tip, forming a soil compaction area. The camber angle mainly affects the soil’s falling effect. Suitable soil-opening, soil-lifting, and camber angles can reduce soil accumulation and compression, reduce operational resistance, and achieve better straw burial.

3.2.3. Optimization and Verification

The regression model for the quantity of deeply buried straw burial and operational resistance was optimized and solved to obtain the optimal combination of structural parameters for various angles of the in situ soil-lifting shovel. Using the optimization module, the optimization objective was to maximize the quantity of deeply buried straw and minimize operational resistance. The objective function was established as follows:

$$\left\{\begin{array}{l}\max Y_1\left(A,B,C\right)\\ \min Y_2\left(A,B,C\right)\\ s.t.\left\{\begin{array}{l}10°\le A\le 20°\\ 30°\le B\le 50°\\ 0°\le C\le 40°\end{array}\right.\end{array}\right.$$

(17)

The optimal solution can be obtained according to formula (17), and the optimal operation parameters were as follows: the soil-opening angle was 16.84°, the soil-lifting angle was 36.74°, the camber angle was 29.60°, the quantity of deeply buried straw was 228.29, and the operation resistance was 2840.45 N. Considering the operability of manufacturing, the optimized adjustment were as follows: the soil-opening angle was 17°, the soil-lifting angle was 37°, and the camber angle was 30°. The optimal structure of the in situ soil-lifting shovel optimized by simulation experiments provides a basis for subsequent field experiments.

Based on the principles of low soil disturbance and low operational resistance, as well as agronomic design requirements, the height a of the in situ soil-lifting shovel was designed to be 140 mm, the length c was 350 mm, b₁ was 50 mm, b₂ was 130 mm, b₃ was 260 mm, e₁ was 30 mm, and e₂ was 150 mm.

3.3. Field Experiment Analysis

According to the theoretical analysis and simulation results, when the soil-opening angle is 17°, the soil-lifting angle is 37°, and the camber angle is 30°. The field experiment was conducted. The effect of straw deep burial is shown in Figure 19 and the field test results are shown in

Table 6. Field test results of operational resistance.

No.	Operational Resistance/N
1	2876.69
2	3192.46
3	3425.24
4	2796.37
5	3143.71
6	3386.83
7	3081.97
8	3264.36
Average	3145.95

.

Figure 19 and Table 6 showed that the quantity of deeply buried straw was 90.21%, and the average operating resistance value was 3145.95 N. In the study by Tian Yang et al., the deep burial rate of straw was 94%, slightly higher than the study in this article. However, there were problems such as poor compaction of straw covering the soil [33]. The error between the operational resistance value and the simulation experiment was only 11%. Our analysis suggests that during field experiments, soil characteristics such as different moisture content and external factors such as wind power and machine vibration significantly impact the actual operation results, leading to deviations in simulation test values. At the same time, the maximum and average depth of straw burial were measured. The maximum depth was 33.58 cm, and the average was 29.73 cm.

4. Conclusions

(1)

A direct-injection method for the deep burial of straw was proposed, and an in situ soil-lifting shovel was designed based on the principle of oblique cutting. While obliquely cutting the soil, the soil was lifted in situ to form a deep burial space for straw and complete the deep burial of straw.

(2)

Theoretical analysis was conducted on the force acting on the tip of the in situ soil-lifting shovel, and the angle of penetration of the shovel tip into the soil was determined. Based on the analysis of the three stages of the operation of the soil-lifting plate part, three soil-lifting plates were designed according to the different camber angles, and then three in situ soil-lifting shovel structures were designed. The value range of the camber angle was determined by analyzing three kinds of in situ soil-lifting shovels.

(3)

The DEM method was used to analyze the operational resistance and the amount of deeply buried straw. The results showed that as the soil-opening angle increased, the amount of deeply buried straw first increased and then decreased, increasing operational resistance. As the soil-lifting angle increased, the amount of deeply buried straw first increased and then reduced, leading to an increase in operational resistance. With the rise of camber angle, the amount of deeply buried straw first increases and then decreases, and the operational resistance decreases. Aiming at the maximum amount of deeply buried straw and the minimum operational resistance, the soil-opening angle of the in situ soil-lifting shovel was 17°, the soil-lifting angle was 37°, and the camber angle was 30°.

(4)

The field test results showed that when the angle of penetration was 25°, the soil-opening angle was 17°, and the soil-lifting angle was 37°. The camber angle was 30°, the amount of deeply buried straw was 90.21%, the operating resistance was 3145.95 N, and the error with the simulation experiment was only 11%.

The designed direct-injection straw deep-burying machine has the potential to make an essential contribution to improving the straw deep burial quality of maize straw in China. However, further research is needed on several aspects, including long-term experiments, energy consumption research, and device vibration research. Furthermore, in the future, the operating parameters of straw deep-burying machines should be researched to improve the deep burial rate of maize straw.