Design and Testing of an Offset Straw-Returning Machine for Green Manures in Orchards

Abstract

In order to solve the problems of traditional orchard-specific green manure crushing and returning machines, such as the single operation effect, root system damage, unsustainable green manure growth, and low utilization rate, an offset crushing–furrowing–burying–straw-returning machine was designed for green manures in orchards. Based on quadratic regression combination experiments, the Discrete Element Method (EDEM) was used to construct a discrete element model simulating the deep furrowing and burying processes of the furrowing and soil-covering device, where the advance speed, plow-shaped furrowing blade rotation speed, and furrowing depth were considered as experimental factors and the coverage rate was taken as an evaluation index, and then simulation analyses were carried out to obtain experimental data; Design-Expert was used to perform ANOVA and RSM analyses, thus finding that its optimal working parameter portfolio consists of the advance speed of 42 m/min, the furrowing blade rotation speed of 300 r/min, and the furrowing depth of 190 mm, and that the coverage rate is 95.82% when this parameter portfolio is applied. Field experiments were conducted to validate the optimal parameter portfolio. The experimental results show that with an average coverage rate of 90.87% (4.95% away from the optimal value based on the simulation experiments on average), an average crushing length qualification rate of 91.24%, and an average root system damage rate of 5.6%, this device is applicable for its operation conditions. The development of this machine and the construction of its parameter model can provide a certain reference value for developing and optimizing related machines including green manure-returning machines.

1. Introduction

According to the National Bureau of Statistics of China, the orchards in China covered a total area of 13.0095 million hectares in 2021 (up by 1.57% on the previous year) and a total area of 13.5106 million hectares in 2023, while the total area of the orchards in Xinjiang is kept at 1.4067 million hectares, accounting for about 13% of the total area of the orchards nationwide [1,2]. At the Sustainable Development Summit held at the United Nations Headquarters in New York, the 2030 Agenda for Sustainable Development, which clarifies the significance of sustainable development, was approved [3]. With the implementation of China’s strategy of promoting agricultural development through quality improvement and green technology, green manure planting has become a new mode of managing the soils in modern green orchards [4,5]. Green manure refers to a green plant that is crushed and returned in situ, is applied in another field, or is collectively fermented and used as a bio-fertilizer. Green manures have merits including soil physical and chemical property improvement [6], soil microecological environment improvement [7], soil fertility improvement [8], and environment purification [9], as well as pest infestation mitigation, weed growth inhibition [10], and provision of good living conditions for fruit plants in orchards [11]. In addition, orchards have gradually adopted the management mode of overturning and returning planted green manures. Due to the low green manure utilization rate, however, there are still problems with most green manures in terms of returning and fertilizer application, including single operation effect, unsatisfactory returning effect, and damage to root systems of multi-cropped plants. Therefore, it is of great significance to design a crushing and returning machine that can be used for multiple cropping.

Orchard green manure returning is a direct and effective way to utilize green manure stalks. Long-term fixed-place studies have confirmed that stalk returning can improve the fertility and physical behavior of the soil [12]. Research shows that in comparison with overturning, mulch returning has disadvantages as follows: insufficient contact with soil due to exposure to the air, slower decomposition rate due to the low activity of soil enzymes and microorganisms, and slower effect on the soil [13,14,15]. Ning Dongfeng et al. [16] found that an over-turned gramineous plant, such as ryegrass, decomposes relatively rapidly where nutrients are released during the grass-growing matter decomposition process in a 100~200 mm soil layer; Lv Lixia et al. [17] found that the green manure crops of white clover and ryegrass decompose most rapidly when the over-turn depth is 100 mm and relatively slowly when laid on the surface. Wang Zhenyuan et al. [18] found that the green manure crop of rape releases the maximum amounts of nutrients most rapidly when the returning depth is 100 mm and the crushing length is 60 mm. To sum up, green manures decompose most rapidly and their nutrient release patterns are the best when they are buried in the soil. Therefore, many returning machine studies have been carried out at home and abroad so as to solve the above problems.

Norremark et al. [19] developed a GPS-based interplant weeder that provides a low green manure decomposition rate due to mulch returning; Yang Wang et al. [20] designed a separating and burying device and Liang Fang et al. [21] developed a two-point double-screw burying and returning device, but both of them require green manures to be crushed and over-turned after a certain period and have the disadvantages of root system damage, unsustainable growth, and low green manure utilization efficiency. In addition, PERFECT Van Wamel BV (6658AE Beneden-Leeuwen Netherlands) and FENDT (a brand of AGCO Corporation, Beijing China) also developed a multifunctional operation machine with a front green manure crusher and a rear five- or eight-plow over-turn device. However, a tractor working with such a machine is required to have a power rating of 220 kW or above as well as a power output in the front. In general, such green manure crushing and overturning machines are large- or medium-duty. They are expensive and match poorly with small tractors used in orchards in terms of power [22,23]. To sum up, these crushing and returning machines designed by the above researchers meet the interrow green manure crushing and returning requirements of orchards, but in terms of how to return green manures (mainly including mulch returning and overturning), there are still problems such as low decomposition and post-returning root system damage. Moreover, there are still shortcomings in growth environment, management mode, and best crushing and returning timing of interrow green manures in orchards.

In the study presented in this paper, an offset straw-returning machine was designed according to the interrow green manure management mode of orchards in Xinjiang in order to solve the above problems. The green manure crushing and returning mechanism was analyzed; the design and calculations of its key parts were completed; and the main parameters affecting its working performance and their ranges were defined. A piece of simulation software was used to carry out simulation experiments of the furrowing and burying device. In addition, quadratic regression combination experiments were conducted to determine its optimal parameter portfolio. Field experiments were performed to validate this optimal parameter portfolio. Thus, a reference can be provided for designing new offset straw-returning machines for green manures in orchards.

2. Materials and Methods

2.1. Overview of Experimental Field

2.1.1. Green Manure Planting Pattern

The experimental field is located in a fragrant pear orchard in Alar City, Xinjiang Uygur Autonomous Region, that is under the Xinjiang Production and Construction Corps 12th Regiment (40°28’ N, 81°26’ E). On the edge of Taklimakan Desert, this place has a typical warm temperate zone continental dry climate. Its annual average temperature is 10.7 °C; its extreme maximum temperature is 40.6 °C; its annual average precipitation ranges from 40.1 mm to 82.5 mm; its annual average evaporation capacity ranges from 1876.6 mm to 2558.9 mm; its elevation is 1.011 × 10⁶ mm.

The soil in the experimental field is a sandy loam. This is an experimental field where green manures have been planted for three years (see Figure 1). In the orchard, the plant spacings are 4500 mm × 600 mm and the green manure width is 1500 m.

2.1.2. Mechanical Properties of Green Manure Stalks

Two green manures are mainly planted in orchards in Xinjiang: rape and alfalfa. Field measurements in the fragrant pear orchard (see Figure 2) give the following data: when the rape is in full bloom, its stem height ranges from 18 mm to 1534 mm, its stem diameter ranges from 3.46 mm to 19.68 mm, and its shear strength ranges from 100.9 N to 165.3 N; when the rape is in the capsule period, its stem height ranges from 25 mm to 1540 mm, its stem diameter ranges from 11.03 mm to 21.05 mm, and its shear strength ranges from 105.9 N to 195.1 N; when the alfalfa is in full bloom, its stem height ranges from 150 mm to 1100 mm, its stem diameter ranges from 2.55 mm to 4.37 mm, and its shear strength ranges from 156.8 N to 548.3 N; when the alfalfa is in the capsule period, its stem height ranges from 180 mm to 1080 mm, its stem diameter ranges from 2.56 mm to 4.63 mm, and its shear strength ranges from 185.8 N to 591.8 N.

2.1.3. Requirements for Green Manure-Returning Experiments

In order to improve the green manure-returning quality, a new agronomic technology for green manure crushing and returning needs to be developed to introduce a new returning mode for multi-stubble plants. This new mode should allow green manures to be returned to the field at the best times. Within a year, multiple crushing and returning operations can be performed to improve the green manure utilization efficiency. Its main agronomic process is shown in Figure 3. First, the interrow green manure is cut and crushed; then, a 200 mm deep furrow is dug on the right side; finally, green manure stalks are sent into the furrow on the right side and covered with soil. According to the agronomic measures for orchard green manure returning, soil conditions, climate, etc., the adaptive operation requirements for the offset crushing, furrowing, and burying machine are as follows: a 1500 mm wide piece of green manure should be selected as the operation object and an offset operation machine integrating crushing, furrowing, and soil covering should be designed. The machine should be able to fully crush green manure stalks and send them into the furrow on the right side. Its furrowing depth should be up to 200 mm for effective green manure stalk returning.

2.2. Structure and Working Principle of Offset Green Manure Straw Returning Machine

2.2.1. Structure of Offset Straw-Returning Machine

The overall structure of the offset straw-returning machine is shown in Figure 4. It is mainly composed of a chassis, a transmission system, grass crushers, and a furrowing and soil-covering device. Its overall structure is linked with the suspension of the tractor through three suspension points on its chassis. The grass crushers are under the chassis. The tractor transmits power to the grass crushers via the transmission system. The furrowing and soil-covering device, which is located on the right of the chassis, is mainly composed of a furrowing blade roller, plow-shaped furrowing blades, a soil-covering pan, and a drainage plate. It can ensure that the crushed green manure is well buried.

2.2.2. Working Principle

When the offset straw-returning machine works, the following processes are involved: the tractor pulls the crushing and returning machine forward so that power is transmitted via the universal coupling to the transmission system of this machine; the gearbox carries out power direction change and speed increase by changing gears, and this gear change drives the corresponding drive shaft to transmit power to the grass crushers and the furrowing and soil-covering device on the right side. Then, the shredding blades evenly arranged on the shredding blade disks cut the green manure plants, and the cut green manure stalks are internally cut and crushed several times to shreds. Subsequently, the plow-shaped furrowing blades rotating at high speed shear and crush the soil; the crushed soil is thrown backward for the green manure to be effectively dropped into the furrow; the shredded green manure stalks are thrown by the shredding blades into the furrow; the soil is dropped to cover the shredded stalks; and the soil barrier puts the soil particles in the rear on the green manure stalks to complete burying or covering. Finally, the soil-covering pan implements further covering with the soil particles on both sides, so as to achieve the returning sequence as follows: furrowing → stalk dropping → soil dropping.

2.3. Design and Parameter Determination of Key Parts

2.3.1. Design of Grass Crushers

• Structural Design of Grass Crushers

In order to allow the grass crushers to effectively cut and crush the green manure stalks during operation and convey some of the green manure to the right side, a disk-type crushing mechanism was designed. It is mainly composed of a shredding blade disk, shredding blades, and spacers. During operation, the transmission system drives the shredding blade disks to rotate, and the kinetic energy of the rotating shredding blades is used to cut the green manure for high crushing quality and deliver the green manure stalks to the right side for their returning.

In order to meet the agronomic requirements in terms of orchard green manure planting, the total width of the crushing devices is set to 1600 mm; the stubble heights of the green manures should not be lower than 50 mm in order to protect the roots and stems of the green manures to allow them to regrow and achieve their multiple crushing and utilizations upon seeding, so the distances from the shredding blade disks to the ground are set to 50~100 mm; in order to ensure grass cutting stability and maximize the cutting trajectory coverage ratio, three grass crushers are arranged. On each grass crusher, the shredding blade spindles are 135 mm away from the shredding blade disk, the total height of each shredding blade spindle is 170 mm, and a slotted nut and a split pin are used to prevent the blades from becoming loose, thus avoiding problems such as blade come-off. The blade arrangement is shown in Figure 5. Their dimensions are 90 mm × 40 mm × 2 mm (L × W × H). All the arc angles of their cutting edges are 160°. All the angles of their cutting edges are 45°. The four shredding blades on each side are arranged at intervals of 50 mm.

2.

Kinematic Analysis of Shredding Blade

In order to achieve better cutting, crushing, and conveyance effects, a kinematic analysis of the blade disk and shredding blades was carried out. For simplification, the kinetic characteristics of a single shredding blade were examined. A kinematic model of the shredding blade (Figure 6) representing the cutting process was constructed [24]. According to the Denavit–Hartenberg (D-H) principle of the connecting rod coordinate system, the blade disk and the shredding blade were simplified into two connecting rods. A speed–displacement relation was established for the shredding blade during movement. The following definitions were made: {0} was defined as the base coordinate system at the center of the shredding blade rotation shaft; {1} was defined as a moving coordinate system where the shredding blade spindle rotates around the center of the shredding blade rotation shaft; {2} was defined as a moving coordinate system where the shredding blade rotates around the center of the shredding blade rotation shaft; both the origins of {1} and {2} are the center of the shredding blade rotation shaft (O); {3} was defined as a coordinate system where the cutting edge of the shredding blade collides with the stalks.

The state of the shredding blade failing to cut green manure stalks is shown in Figure 6a. Under the centrifugal force generated when the blade disk rotates at high speed, the shredding blade has no motion in relation to the blade disk and it is in a stable force balance state. When the cutting edge of the shredding blade collides with green manure stalks, as shown in Figure 6b, the shredding blade swings back by an angle w_2t around the hinge center of the shredding blade spindle under the collision force from the green manure stalks until the weed stalks are cut off.

When the machine works, the absolute motion of the shredding blade is a resultant motion composed of its circular motion and its straightforward motion. That is, the trajectory of any point on the blade is a regular curve (cycloid). The trajectory of any point on the cutting edge of the shredding blade satisfies the following equation:

$$\left\{\begin{array}{c}x=\left(R+r\right){sin\omega }_{1}t+v_{m}t \\ y=\left(R+r\right){con\omega }_{1}t\end{array}\right.$$

(1)

where ω₁—angular velocity (rad/min); v_m—operation machine’s advance speed (m/min); t—time (min).

As shown in Figure 7, the horizontal distance between any two adjacent blade trajectories in the same cutting area is defined as cutting pitch (S). Seeing that two sets of shredding blades are evenly installed on each blade disk here, the cutting interval between the two sets of blades is as follows:

The time interval is $\mathrm{T}=2\mathsf{\pi }/\mathrm{z}{\omega }_1$, Thus, the cutting pitch formula is as follows:

$$\mathrm{S}={\mathrm{v}}_{\mathrm{m}}\mathrm{T}$$

(2)

where z—number of shredding blades; ω₁—angular velocity (rad/min); v_m—operation machine’s advance speed (m/min).

S directly affects the cutting area as well as the crushing quality. Because the green manure grew densely, the cutting pitch was set to a low value of 80 mm. It can be seen from the above that a higher advance speed means a higher rotational speed. According to the operation efficiency requirement and the common tractor advance speed range of 33.3~100 m/min (v_m), the rotational speed range was calculated at 1307~3925 rad/min.

Whether the shredding blades perform grass cutting operations normally depends on the ratio of the machine’s advance speed to the shredding blade spindles’ rotational speed. The cycloidal trajectory of each shredding blade’s cutting edge varies with the ratio of the machine’s advance speed to the corresponding shredding blade spindle’s rotational speed. On the trajectory of a shredding blade, the tangential component of the blade tip velocity is v. According to the tangential velocity formula (v = Rω₁), the ratio of the blade’s tangential speed to the advance speed is

$$\mathsf{\lambda }={\displaystyle \frac{v}{v_m}}={\displaystyle \frac{R{\omega }_1}{v_m}}$$

(3)

where v is the tangential component of the blade tip (m/min). When λ changes, the trajectory of the blade tip changes accordingly.

When the advance speed is 33.3 m/min, the minimum rotational speed is 1307 r/min, and the distance between the shredding blade spindle and the center of the blade disk (R) is 135 mm; the speed ratio λ > 1. In this case, the grass crusher can effectively perform cutting and crushing operations.

2.3.2. Design of Furrowing and Soil-Covering Device

This furrowing and soil-covering device was meant to bury green manures for fruit trees to accelerate their decomposition and provide nutrients so that green manures can be effectively utilized and the quality of the fruits can be improved. It consists of a furrowing blade roller, plow-shaped furrowing blades, and a soil-covering device. During operation, the rotating plow-shaped furrowing blades crush the soil and form a furrow by throwing the crushed soil and work together with the soil-covering device to throw the soil particles into the rear so that they cover the crushed green manure stalks.

• Furrowing Blade Roller

The design of the furrowing blade roller affects not only the furrow width but also the machine’s resistance during operation. The furrow width was set to 200 mm. In order to reduce the soil transport and throwing resistances when the furrowing device rotates, the plow-shaped furrowing blades were symmetrically arranged. The furrowing blade roller was 160 mm in radius. The angle of the two blades in any blade pair thereon was 150~180°. Its width was 220 mm. There were six sets of blades on the roller. See Figure 8. The cutting areas were independent of each other, so each furrowing blade roller has an independent soil cutting pitch (L), which can be expressed as

$$\mathrm{L}={\displaystyle \frac{2\mathsf{\pi }{v}_m}{\mathrm{Z}\mathrm{n}}}$$

(4)

where v_m—operation machine’s advance speed (m/min); n—furrowing blade roller’s rotation speed (rad/min); z—number of blades.

The soil in the green manure growth area in the orchard has high moisture content. In order to ensure high soil crushing quality, the soil cutting pitch should be shorter than 80 mm. The advance speed is known to be 33.3~100 m/min. A calculation based on Formula (4) showed that the cutting pitch meets the design requirement when Z = 6 and the rotational speed of the furrowing blade roller is 218–654 rad/min.

2.

Plow-shaped Furrowing Blades

Plow-shaped furrowing blades are key components that affect the soil transport and throwing resistances. When the furrowing depth is 10~200 mm, the green manure decomposition performance is the best. In addition, they throw soil particles into the rear. See Figure 8. During operation, the process of the rotating furrowing blades cutting the soil is just the process of them separating the upturned soil from the subsoil, changing its position, and making it gain speed. When the upturned soil has been thrown behind, the energy conversion is finally complete. Among the total power consumption values of the furrowing device, the power consumption values required for driving the furrowing blades into the soil and for soil throwing are dominant (accounting for 70~80% collectively) [25]. Therefore, in order to prevent the furrowing resistance from being excessive, the furrowing blades are symmetrically installed pair by pair, with the two blades in each pair at a certain angle with each other. The heads of the blades bend forward. The tool faces of the blades are curved surfaces. In the tool face of each of the blades, there are square holes, by which large soil particles are crushed due to extrusion. On the outside of each furrowing blade’s tool face, there is a wing plate that is low at the front and high at the back. These wing plates are meant to avoid the soil being thrown out of the housing, damaging fruits, and bringing losses.

The curved surface parameters of a furrowing blade determine its plowing surface’s working performance such as stress state and soil motion state. The curve of the cutting edge is designed to be a guide curve. In order to reduce the resistance during operation, the furrowing resistance was regarded as increasing as the advance speed increases. Research shows that exponential curves are better than straight lines and parabolas in resistance reduction [26,27]. Thus, the curve of the tool face’s portion into the soil is designed to be an exponential curve. A rectangular coordinate system as shown in Figure 9 is created. It was assumed that the soil entry point was A (x₁, y₁) and the end point of the guide curve was B (x₂, y₂).

The equation of the guide curve is assumed to be

$$\mathrm{y}={\mathrm{a}}^{\mathrm{x}}$$

(5)

where a—the base of the equation of the derivative curve.

$$\left\{\begin{array}{c}{\mathrm{y}}_{\mathrm{A}}^{,}=\tan \mathrm{\delta }={\mathrm{a}}^{{\mathrm{x}}_1}\mathrm{l}\mathrm{n}\mathrm{a}\\ {\mathrm{y}}_{\mathrm{B}}^{,}=\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{\beta }={\mathrm{a}}^{{\mathrm{x}}_2}\mathrm{l}\mathrm{n}\mathrm{a}\\ {\mathrm{y}}_2-{\mathrm{y}}_1=C\end{array}\right.$$

(6)

where β is the tangent angle (°); C is the height of the guide curve (mm).

The following equation is obtained through calculations based on Equations (5) and (6):

$$\mathrm{a}={\mathrm{e}}^{{\displaystyle \frac{\mathrm{t}\mathrm{a}\mathrm{n}\mathsf{\beta }-\mathrm{t}\mathrm{a}\mathrm{n}\mathsf{\delta }}{\mathrm{C}}}}$$

(7)

Thus, the equation of the tool face’s guide curve is

$$\mathrm{y}={\left({\mathrm{e}}^{{\displaystyle \frac{\mathrm{t}\mathrm{a}\mathrm{n}\mathsf{\beta }-\mathrm{t}\mathrm{a}\mathrm{n}\mathsf{\delta }}{\mathrm{C}}}}\right)}^{\mathrm{x}}$$

(8)

According to Formula (8), the shape of the tool face’s guide curve is determined by the soil entry angle (δ), the guide curve’s tangent angle (β), and C. β is set to 80°. According to the subsoiling shovel entry angle design given in the Agricultural Machinery Design Manual, δ is set to 25°. According to the agronomic requirements for green manure decomposition, C is set to 200 mm; therefore, a is equal to 1.03.

According to the plow-shaped furrowing blade design parameters shown in Figure 9, the parameters of the tool face are determined by the guide curve, height (h), soil entry angle (δ), soil entry fissure angle (β), and furrowing cutting edge angle. The movements of a soil particle along the soil trace on the furrowing plow’s curved surface were analyzed. The analyses are shown in Figure 10.

The stress analyses show that the soil particles undergo a vertically downward force of gravity (mg), friction force due to the curved plowing surface (f), and an extrusion force (F_N). Since the friction force and the extrusion force are spatial forces, a space rectangular coordinate system is established by setting the direction of the advance as the positive direction of the x-axis, the direction of the vertically downward gravity force as the positive direction of the z-axis, and the sideward direction of the friction force as the positive direction of the y-axis. The extrusion force is decomposed into three component forces along the three coordinate axes.

Thus, the following kinematic equations are obtained:

$$\left\{\begin{array}{c}{F}_{Nx}+f_{xz}cos\delta -f_{xy}sin\varnothing ={ma}_x\\ F_{Ny}+f_{xy}cos\varnothing ={ma}_y\\ \mathrm{m}\mathrm{g}+f_{xz}sin\delta -F_{Nz}={ma}_z\end{array}\right.$$

(9)

where

$$f_{xz}=tan\phi \sqrt{F_{Nx}^2+F_{Nz}^2}$$

(10)

$$f_{xy}=tan\phi \sqrt{F_{Nx}^2+F_{Ny}^2}$$

(11)

$${\displaystyle \frac{F_{Nx}}{F_{Ny}}}=tan\theta$$

(12)

$${\displaystyle \frac{F_{Nz}}{F_{Nx}}}=tan\delta$$

(13)

where F_Nx—extrusion force’s component along the x-axis (N); F_Ny—extrusion force’s component along the y-axis (N); F_Nz—extrusion force’s component along the z-axis (N); f_xz—friction force on solid particles along the xz plane (N); f_xy—friction force on soil particles along the xy plane (N); $\mathbf{\varnothing }$—dihedral angle (°); m—mass of soil particles (kg); δ—soil entry angle of furrowing blade’s tool face (°); φ—internal friction angle of soil (°).

The following equations can be obtained according to Equations (9)–(13):

$$a_x={\displaystyle \frac{F_N[1+tan\phi (1-tan\varnothing \left)\right]}{m\sqrt{1+{tan}^2\varnothing +{tan}^2\delta }}}$$

(14)

$$a_y={\displaystyle \frac{F_N(tan\phi +tan\varnothing )}{m\sqrt{1+{tan}^2\varnothing +{tan}^2\delta }}}$$

(15)

$$a_z={\displaystyle \frac{F_{N}tan\delta (tan\phi -1)}{m\sqrt{1+{tan}^2\varnothing +{tan}^2\delta }}}$$

(16)

where a_x—acceleration of soil particles along the x-axis (m/s²); a_y—acceleration of soil particles along the y-axis (m/s²); a_z—acceleration of soil particles along the z-axis (m/s²).

During operation, the soil particles are extruded on three sides as well as by the square hole so as to produce a slip line. Equations (14)–(16) show that the movements of the solid particles are related to the friction angle, the equivalent soil entry angle, its pressure on the curved plowing surface, etc. According to the soil types of south Xinjiang, φ is set to 30°. When $\mathbf{\varnothing }$ < 45° < (90° −φ) and δ > 0°, both a_y and a_z are higher if a_x is higher. This indicates that the solid particles have a tendency to move forward, sideward, and upward.

3.

Soil-Covering Device

After being thrown out, the solid particles collide with the soil-covering device and fall and cover the green manure. This device is used to cover the green manure with soil to prevent the soil from splashing and damaging the fruits. The height of the soil-covering device and the angle of the soil barrier mainly affect the throwing distance of soil particles, while its shape mainly affects the soil-covering effect.

In order to assign appropriate values to the related parameters to ensure that most of the thrown soil is put above the green manure, the 2D trajectory of the thrown soil on the zOx plane was analyzed, as shown in Figure 11.

The average throwing velocity of the soil thrown by the blade on the zOx plane is decomposed as follows:

$$\left\{\begin{array}{c}{v}_z=vcos{\theta }_t\\ v_x=vsin{\theta }_t\end{array}\right.$$

(17)

where v_z—vertical velocity component of thrown soil (m/s); v_x—horizontal velocity component of thrown soil (m/s).

On the premise that air resistance and inter-particle collisions impact the soil’s trajectory, the soil moves along an oblique upthrow trajectory and then along an oblique downthrow trajectory after collision with the housing. Its vertical displacement during its upthrow is approximately equal to the height of the soil-covering device (Ha), i.e.,

$$0.5(2v_z-g{t}_s)t_s\approx H_a$$

(18)

where t_s—soil movement time during upthrow (s); g—acceleration of gravity (9.8 m/s²).

According to Equation (18), the movement time of the crushed soil during its upthrow (t_s) is as follows:

$$t_s={\displaystyle \frac{v_z-\sqrt{v_z^2-2g{H}_a}}{g}}$$

(19)

Thus, the fall displacement of the crushed soil after collision satisfies the following equation:

$$0.5(2{Ev}_z+g{t}_1)t_1\approx Ha$$

(20)

where E—soil–iron plate collision recovery coefficient (equal to 0.6 [20], according to a test measurement); t₁—falling time of crushed soil (s).

According to Equation (20), the falling time of the crushed soil (t₁) is as follows:

$$t_1={\displaystyle \frac{{-Ev}_z+\sqrt{\left({Ev}_z^2\right)+2g{H}_a}}{g}}$$

(21)

In order to allow most of the soil to collide with the housing near the height of Ha and limit its lengthway displacement in the x-axis direction within the specified width, the following equation must be satisfied:

$$\left\{\begin{array}{c}{\displaystyle \frac{v_z^2}{2g}}>H_a\\ 0.8<(t_s+t_1)\left|v_x\right|<R+0.8\end{array}\right.$$

(22)

In order to reduce the traction resistance of the drainage plate, the angle of the soil barrier with the horizontal plane (α) is set to 160° and its height (H_b) is set to 20 mm. By substituting Equations (19) and (21) into Equation (22), the range of Ha can be identified to be 50 mm for better soil furrowing.

2.4. Simulation Experiments of Furrowing and Soil-Covering Device

2.4.1. Creation of Simulation Model

In order to determine optimal structural parameters for the furrowing and soil-covering device, the Discrete Element Method was used to carry out simulation experiments [28]. UG12.0 was used to create a simulation model representing the furrowing and soil-covering device. The model file was saved as an STL file and imported into the Discrete Element Method (EDEM 2018) software, as shown in Figure 12. The soil type in the experiment orchard and its vicinity is sandy loam. Thus, the soil particle contact model applied herein is a Hertz–Mindlin model with bonding because a model considering bond breaking can better describe the loose state of the sandy loam after it is crushed by the furrowing device [29]. The soil particles to be simulated are assumed to be 8 mm in diameter [30]. A long linear rigid model consisting of 15 spherical particles, 5 mm in radius, was created to simulate the green manure stalks in view of the following facts: the green manure stalk model needs to consider multiple states such as bending and breaking; its properties are considerably affected by the moisture content; and the model construction process is complex [31,32,33,34]. A 3000 mm × 500 mm× 300 mm soil groove was created to simulate the working area. The selected green manure is alfalfa and the selected rigid material is carbon steel 45. By referring to related data of green manure stalks [35], the physical properties and contact mechanical properties of the materials and the rigid material were identified, as shown in

Table 1. Model parameters.

Parameter	Value
Poisson’s ratio of soil	0.38
Soil density	1250
Soil shear modulus	1 × 106
Density of carbon 45 steel	7800
Poisson’s ratio of carbon 45 steel	0.3
Shear modulus of carbon 45 steel	7.8 × 1010
Density of alfalfa stalk	256
Poisson’s ratio of alfalfa stalk	0.4
Shear modulus of alfalfa stalk	5 × 107
Inter-stalk collision recovery coefficient of alfalfa	0.11
Inter-stalk static friction coefficient of alfalfa	0.45
Inter-stalk rolling friction coefficient of alfalfa	0.08
Inter-particle collision recovery coefficient of soil	0.2
Inter-particle static friction coefficient of soil	0.4
Inter-particle rolling friction coefficient of soil	0.3
Steel–particle collision recovery coefficient of stalks	0.16
Steel–stalk particle static friction coefficient	0.54
Steel–stalk particle rolling friction coefficient	0.24
Steel–soil static friction coefficient	0.65
Steel–soil rolling friction coefficient	0.05
Steel–soil collision recovery coefficient of soil	0.60

.

2.4.2. Experimental Factors and Indicators

In order to determine the optimal operation parameters, the machine’s advance speed was set to 33.3~100 m/min, the rotation speed of the plow-shaped furrowing blades was set to 200~600 r/min, and the furrowing depth was set to 50~200 mm. With the advance speed, furrowing blade rotation speed, and furrowing depth as experimental factors and the coverage rate as the evaluation index, the Box–Behnken test was carried out. For the values of these experimental factors, see

Table 2. Codes and values of experimental factors.

Code	Advance Speed (m/min)	Plow-Shaped Blade Rotation Speed (r/min)	Furrowing Depth (mm)
−1	33.3	200	100
0	66.65	400	150
1	100	600	200

.

During simulation data acquisition, the EDEM post-processing Analyst module was used to set three data acquisition points in the middle of the soil groove: A₁, A₂, and A₃. Each area had a size of 200 mm × 200 mm × 200 mm. Both the mass data of the stalks above the soil and the mass data of the stalks under the soil surface were collected for three timepoints at three locations. Equation (23) was used to calculate the green manure stalk coverage rate values and their average was found. The coverage rate after the furrowing and burying operations were carried out was calculated. The data acquisition method is shown in Figure 13.

$$F_1={\displaystyle \frac{M_d}{M_n+M_d}}\times 100\%$$

(23)

where F₁—stalk coverage rate; Mn—mass of the stalks above the soil (kg); Md—mass of the stalks under the soil surface (kg).

2.5. Field Test

2.5.1. Test Conditions

In order to verify the operation performance of the offset straw-returning machine for green manures, a field comparison test was carried out in the modern organic fragrant pear garden in Alar, Xinjiang, which is a demonstration base under the Xinjiang Production and Construction Corps (XPCC) 12th Regiment. The study object was alfalfa with an average height of 720 mm and an average stalk diameter of 4.44 mm. The soil here is a sandy loam. Within the thickness range of 0~250 mm, the soil has the average hardness of 37.82 N and a moisture content of 86.5%. The machine was powered by a Dongfeng 404 tractor (Changzhou Dongfeng Agricultural Machinery Group Co., Ltd., Changzhou, China) with a rated power of 29.4 kW and an operation speed of 33.3~100 m/min. The instruments and tools used during the test include an electronic balance, a digital-display soil hardness tester, a moisture analyzer, a photoelectric velocimeter, a soil sampling drill, a tape measure, a steel ruler, etc.

2.5.2. Test Procedures

According to the NY/T 500-2015; standard of Operation Quality for Straw-smashing Machine. China Agriculture Press: Beijing, China, 2015. The operation performance of the crushing and burying machine was tested. The evaluation indexes include crushing length qualification rate, green manure root system damage rate, coverage rate, etc. The field operation performance of the offset crushing, furrowing, and burying machine was comprehensively evaluated.

(1)

Procedure of determining the stalk crushing length qualification rate: select five test points (100 mm × 20 mm each) within the completed operation distance; collect the stalks out of the furrow; remove the other things mixed therein such as dirt and tree branches using a vibrating screen; weigh these stalks; take out the stalks with an unqualified crushing length (>80 mm) and weigh them; calculate the stalk crushing length qualification rate.

$$H={\displaystyle \frac{M_x-M_a}{M_x}}\times 100\%$$

(24)

where H—stalk crushing length qualification rate (%); M_x—total mass of all the stalks (kg); M_a—total mass of unqualified stalks (kg).

(2)

Procedure of determining the root system damage rate: select five test points (100 mm × 100 mm each) within the completed operation distance; wait ten days for the alfalfa stubbles in all the five test areas to germinate; record the total number of alfalfa stubbles as well as the number of alfalfa stubbles failing to germinate; calculate the root system damage rate using the following equation:

$$W={\displaystyle \frac{M_y}{M_y+M_b}}\times 100\%$$

(25)

where W—root system damage rate (%); M_y—number of stubbles failing to germinate; M_b—number of germinating stubbles.

(3)

Procedure of determining the coverage rate: select five test points (100 mm × 20 mm each) within the completed operation distance; collect and calculate the masses of the stalks above the soil and the stalks under the soil surface; calculate the stalk coverage ratio using the following equation:

$$F={\displaystyle \frac{M_c}{M_z+M_c}}\times 100\%$$

(26)

where F—stalk coverage rate; M_z—mass of the stalks above the soil (kg); M_c—mass of the stalks under the soil surface (kg).

3. Results and Analyses

3.1. Simulation Experiment Results and Analyses

3.1.1. Experiment Result Analyses

The Box–Behnken test design program and its results are shown in

Table 3. Simulation test results.

No.	Factor			Index
	Advance Speed (X1, m/min)	Plow-Shaped Blade Rotation Speed (X2, r/min)	Furrowing Depth (X3, mm)	Coverage Rate (F1, %)
1	−1	−1	0	90.5
2	1	−1	0	87.6
3	−1	1	0	93.5
4	1	1	0	91.3
5	−1	0	−1	86.3
6	1	0	−1	85.2
7	−1	0	1	95.8
8	1	0	1	92.1
9	0	−1	−1	84.8
10	0	1	−1	87.2
11	0	−1	1	92.3
12	0	1	1	95.8
13	0	0	0	92.4
14	0	0	0	92.6
15	0	0	0	92.5
16	0	0	0	92.6
17	0	0	0	92.3

, in which X₁, X₂, and X₃ are, respectively, the factor codes of advance speed (m/min), furrowing device rotation speed (r/min), and furrowing depth (mm). Design-Expert was used to carry out variance analyses of coverage rate variation coefficients.

The results of the coverage rate variance analysis are shown in

Table 4. Variance analysis of regression model.

Source	Sum of Squares	Freedom	Mean Square	F-Value	p-Value
Model	186.30	9	20.70	849.88	<0.0001
X1	12.25	1	12.25	502.98	<0.0001
X2	19.85	1	19.85	814.75	<0.0001
X3	132.03	1	132.03	5420.64	<0.0001
X1X2	0.1225	1	0.1225	5.03	0.0598
X1X3	1.69	1	1.69	69.38	0.0001
X2X3	0.3025	1	0.3025	12.42	0.0097
X12	3.92	1	3.92	160.98	0.0001
X22	2.63	1	2.63	107.89	0.0001
X32	11.67	1	11.67	479.22	0.0001
Residual	0.1705	7	0.0244
Lack of fit	0.1025	3	0.0342	2.01	0.2550
Pure error	0.0680	4	0.0170
Total correlation	186.48	16

Note: p < 0.01 (highly significant); p < 0.05 (significant).

. The p-value of the quadratic regression model for coverage rate (F₁) is lower than 0.001, indicating that the regression model is highly significant; the lack-of-fit p-value is higher than 0.05, indicating that lack of fit is not significant and the regression model provides good fitting. By removing the nonsignificant terms, the following coverage rate regression model equation is obtained:

F₁ = 92.48 − 1.24X₁ + 1.58X₂ + 4.06X₃ − 0.65X₁X₃ + 0.275X₂X₃ − 0.965X₁² − 0.79X₂² − 1.66X₃²

(27)

3.1.2. Response Surface Analysis

Design-Expert was used to obtain the response surface reflecting the impacts of the interaction factors among advance speed, furrower rotation speed, and furrowing depth on the coverage rate, as shown in Figure 14.

Figure 14a shows that when the furrowing depth is zero, an increase in plow-shaped furrowing blade rotation speed causes both the soil particle throwing speed and distance to increase, thus improving the coverage rate, and that an increase in advance speed causes the machine to advance faster and soil particles to be unevenly thrown, thus making the coverage rate gradually decrease as the advance speed increases. It can be seen that both the plow-shaped furrowing blade rotation speed and the machine’s advance speed each have a significant effect on the coverage rate. Figure 14b shows that when the plow-shaped furrowing blade rotation speed is zero, an increase in furrowing depth causes both the stalk overturn depth and the size of the soil particle fall space to increase, thus improving the coverage rate, and that when the advance speed increases, an increase in furrowing depth causes the machine to output higher power and the furrowing depth to be inconsistent, thus making the coverage rate gradually decrease as the advance speed increases. It can be seen that the interaction between grooving depth and advancing speed has a significant effect on the coverage rate. Figure 14c shows that when the advance speed is zero, an increase in furrowing depth causes more soil to cover the green manure, thus making the soil coverage rate increase; in addition, the coverage rate also increases as the plow-shaped furrowing blade rotation speed increases. It can be seen that the interaction between furrowing depth and plow-shaped furrowing blade rotation speed has a significant effect on the coverage rate.

3.2. Parameter Optimization and Experimental Verification

3.2.1. Parameter Optimization

The Optimization module on the Design-Expert 13.0 software platform was used to optimize the regression model and seek the optimal solution. The optimization goal is as follows:

$$\left\{\begin{array}{l}\max y_1(x_1,x_2,x_3)\\ \min y_2(x_1,x_2,x_3)\\ \min y_3(x_1,x_2,x_3)\\ s.t.\left\{\begin{array}{l}33.3 \mathrm{m}/\min \le x_1\le 100 \mathrm{m}/3 \min \\ 100 \mathrm{r}/\min \le x_2\le 300 \mathrm{r}/\min \\ 100 \mathrm{m}\mathrm{m}\le x_3\le 200 \mathrm{m}\mathrm{m}\end{array}\right.\end{array}\right.$$

(28)

The optimal parameter portfolio as follows was obtained: advance speed of 42 m/min, furrowing blade rotation speed of 300 r/min, and furrowing depth of 190 mm. When this parameter portfolio is applied, the operation performance of the offset straw-returning machine is the best and the coverage rate is 95.82%. The above optimal solution was used to carry out three identical tests. The tests show that the relative coverage rate error is 3.12%. The verification experiment results show that the experiment results are generally consistent with the predicted values, i.e., the optimized parameters meet the design requirements.

3.2.2. Experimental Verification

In order to verify the accuracy of the discrete element model and the reliability of the simulation optimization results, the optimal simulation parameters were selected to carry out field comparison experiments. During operation, the three-point suspension configuration was adjusted to ensure that the furrowing depth was 190 mm and the gear shift and acceleration pedal of the tractor together with the photoelectric velocimeter were used to ensure that the advance speed was 42 m/min and that the furrowing blade rotation speed was 300 r/min. The experimental site is shown in Figure 15.

Figure 15 shows what the field looked like when a field operation was complete. The stalk crushing length qualification rate, root system damage rate, and coverage rate were determined according to their determination procedures mentioned above. Their determination results are shown in

Table 5. Field experimental results.

No.	Crushing Length Qualification Rate (%)	Root System Damage Rate (%)	Coverage Rate (F, %)
1	89.54	5.32	89.53
2	92.05	7.98	91.47
3	91.93	4.58	87.58
4	90.18	6.26	93.86
5	92.49	3.85	91.91

. The experiment results show that the average stalk crushing length qualification rate is 91.24%, the average root system damage rate is 5.6%, and the average coverage rate is 90.87% (4.95% away from the optimal value based on the simulation experiments). Thus, the discrete element model is accurate and the simulation optimization results are reliable. The offset straw-returning machine performs well when the optimized parameters are applied.

4. Discussion

At present, the crushing and returning machines designed by the previously mentioned researchers meet the interrow green manure crushing and returning requirements of orchards, but there are still shortcomings in terms of growth environment, management mode, and best crushing and returning timing of interrow green manures in orchards. Although the existing crushing and returning machines meet the interrow green manure crushing and returning requirements of orchards, the factors of growth environment, management mode, and best crushing and returning timing were not considered when they were designed. No crushing and returning machines capable of improving the green manure utilization effect have been developed yet. In the study presented by this paper, therefore, an offset crushing, furrowing, and burying machine was designed for high-efficiency orchard green manure utilization involving crushing and burying. Its overall structure design is innovative and scientific. Liang Fang et al. [21] developed a rape stalk burying and returning device with double horizontally opposed screws. During operation, this device crushes stalks above the soil, then cuts and crushes the stalks, stubbles, and soil, and finally buries the stalks, stubbles, and soil evenly by means of its burying roller, thus achieving the uniform distribution of stalks in soil. Yang Wang et al. [20] developed a separating and burying device that places stalks before soil. It can improve both the green manure utilization efficiency and the operation efficiency as well as the green manure crushing and returning effect. The device developed in this study has both crushing and returning functions like the above devices. However, each of the two other devices above allow for only a single crushing and returning operation for a given piece of green manure and accordingly damage green manure root systems. Their designs fail to consider the growth cycles of green manures. The difference between this device and all the other devices is that it crushes and buries the interrow green manure near fruit trees at the best time according to its growth environment and management mode. When this device is used, a furrow is first dug, the crushed green stalks are delivered into the furrow, and finally, the placed stalks are covered with soil particles thrown by the plow-shaped furrowing blades. These experiments of the offset crushing, furrowing, and burying machine show that under the action of the plow-shaped furrowing blades, the soil particles can cover the green manure in a satisfactory way. The experimental results are consistent with the theoretical analysis results. The design of this device is reasonable.

Some structural design innovations are incorporated into this device. However, there are also shortcomings. The simulation and field experiments were only carried out for a fragrant pear orchard in Alar City, Xinjiang, in which there is only one soil type, i.e., sandy loam, and there are too few green manure kinds available. So, it is possible to examine the structural performances of the offset straw-returning machine with different kinds of green manure under different soil conditions.

5. Conclusions

In order to realize the multi-crop utilization of orchard green manures, an innovative offset straw-returning machine was presented in this paper. The machine consists of a chassis, a transmission system, grass crushers, and a furrowing and soil-covering device. First, the grass crusher cuts and crushes green manure stalks and delivers them to the right side; then, the plow-shaped furrowing blades break the soil and throw it up and backward; finally, the crushed green manure stalks are covered with soil particles.

(1)

According to the analyses of the grass crusher and the furrowing and soil-covering device, the structural parameters of the machine’s key parts were determined, and accordingly, a parameter model was created for this machine. The analysis of the three-factor and three-level response surfaces, along with the EDEM simulation experiments, has verified the feasibility of the model. The simulation experiment results show that when the advance speed is 42 m/min, the furrower rotation speed is 300 r/min, and the furrowing depth is 190 mm, the machine can perform the best, and the coverage rate in this case is 95.82%.

(2)

Field experiments were conducted to validate the optimal parameter portfolio. The experimental results show that its average soil coverage rate is 90.87% (4.95% away from the optimal value based on the simulation experiments on average), its average crushing length qualification rate is 91.24%, and its average root system damage rate is 5.6%. Thus, it can be seen that the simulation model is accurate, the simulation optimization results are reliable, and the offset straw-returning machine has good performance when the optimal parameter portfolio is adopted.