1. Introduction

2. Structure and working principle of the machine

3. Design of main working parts

3.1. Seedbed preparation device

Seedbed preparation device is one of the core working parts of the combined machine for transplanting outcrop of Codonopsis with micro ridge covered with film. The design and spatial location of the shaper directly affect the quality of seedbed preparation and subsequent seedling casting, thus affecting the overall outcrop effect of Codonopsis seedlings after mulching. Therefore, according to its agronomic requirements, this paper has conducted the relevant analysis and calculation of the shaper to achieve the function of the seedbed preparation. At the same time, the soil-covering belt of seedlings is formed, which ensures the amount of soil thrown by rotary tillage. A seedbed preparation device is specifically designed, which is connected with the frame. The seedbed preparation device is composed of two shapers, each has a crushing roller and the distance between the two shapers is 200mm.

3.1.1. Structure and operation principle

As shown in Figure 3, the seedbed preparation device is mainly composed of shaper, crushing roller, crushing roller regulating rod, connecting rod of shaper, adjusting rod of shaper, etc. The structure size and spatial position of shaper directly affect the molding of ridge bed and the quality of collected soil in ridge and furrow. The shaper is made of 65Mn steel plate with a thickness of 10mm. The working surface is designed into a right angle trapezoid structure. The angle between the hypotenuse and the long right angle is 75°, which can ensure that the included angle between the seedlings and the horizontal plane is 15°. In order to ensure that the soil will not exceed the top of the shaper when scraping, the width of the shaper is set to be 330mm, and the length of the short side is 200mm. The shaper is welded on the connecting rod of the shaper. In order to meet the requirements of different ground conditions, the operator can adjust the shaper up and down through the adjusting rod of the shaper and the adjusting bolt. The crushing roller is installed on the shaper through the adjusting rod of the crushing roller, and its axis is parallel to the bevel of the shaper. The height of the crushing roller can be adjusted through the crushing roller adjusting rod and adjusting nut. The upper and lower height adjustment of the shaper and the crushing roller and the adjustment of the ground wheel of the operation machine jointly determine the depth of the shaper into the soil and the position of the crushing roller. The adjustable range of the shaper and the crushing roller is 0~150mm and 0~250mm respectively.

Figure 4 shows the operation process of seedbed preparation device. In the process of seedbed preparation, while the shaper and crushing roller move forward, the crushing roller would crush the soil and suppress soil moisture. The seedbed and seedlings soil-covering belt are formed after the soil is turned through the shaper. When the crushing roller contacts with the soil on the seedbed, it would roll and crush the soil to overcome the friction force and cohesion between seedbed and soil particles, compress the gravity water, attached water and the space between soil particles in the soil layer, and discharge the air in the soil layer[13] so as to ensure the soil particles are closely arranged and the seedbed is flat, thus avoiding the instability of plant spacing, the overall deviation of the seedlings and poor outcrop.

3.1.2. Theoretical aggregate amount of soil

As shown in Figure 5, while moving forward, the shaper needs to push soil in front, transport the soil to the lateral side and extrude the soil so as to form the soil- covering belt of seedlings. In order to make sure the amount of soil collected on the soil-covering belt of seedlings and the thickness of the covering soil on the seedlings in the subsequent rotating tillage process, the amount of soil collected by the shaper needs to be calculated theoretically. The area of soil collection is calculated through the effective cross-sectional area of the shaper multiplies a certain length, and then the advance distance of the whole machine equals to the amount of collected soil Y

$$Q_1=\gamma {\mathit{YS}}_1$$

(1)

Where: $S_1=h_z{L}_1+\frac{L_1^2\tan 15°}{2}+L_2\left(L_1\tan 15°+h_z\right)$

Where Y is the advancing distance of the whole machine which is 1m

$\gamma$ is the bulk weight of soil (yellow soil) (1300kg/m) [32].

S₁ is the cross-section area (m²)

h_z is the depth of the short side of the shaper into the soil (0.07m)

L₁ is the cross-sectional length of the shaper (0.25m)

L₂ is the distance between the two shapers (0.2m)

It can be calculated from Formula (1) that $Q_1=$69.2kg

When the advance distance of the whole machine is Y, the amount of covering soil required by the seedlings

$$Q_2=\gamma {\mathrm{YS}}_2$$

(2)

Where: $S_2=\frac{2H{L}_1}{\cos {15}^{\circ }}$

Where, the thickness of covering soil required by seedlings H is 0.07m.

It can be calculated from Formula (2) that $Q_2=$37.6kg

Because $Q_1$ ˃$Q_2$, the soil collection operation of shaper meets the theoretical requirement.

Figure 5. Schematic diagram of operation process of the effective cross-section of the shaper. 1. Bulldozing surface 2 Scraper plane 3. The action line of the rotary blade tip 4 The soil-covering belt of seedlings.

Figure 5. Schematic diagram of operation process of the effective cross-section of the shaper. 1. Bulldozing surface 2 Scraper plane 3. The action line of the rotary blade tip 4 The soil-covering belt of seedlings.

3.1.3. Mechanical analysis of shaper

In order to study the operation rule of the shaper for collecting soil in ridge and analyze the main influencing factors on the quality of collected soil in ridge, it is found that the quality of collected soil in ridge is not only related to the structural parameters and soil parameters of the shaper, but also to the installation angle of the shaper. Suppose the angle between the working surface of the shaper and the moving direction of the machine is α (that is, the installation angle α), then the larger the installation angle α, the larger the operation width, the greater the resistance of the scraper, which is prone to cause the soil be piled up[14,15,16]. If the installation angle α is too small, it can not meet the required width of seedbed, that is, the installation angle α should be reduced when the width of seedbed is met. In order to determine the mechanical conditions of soil scraping along ridges and furrows, it is assumed that there is no relative force between soil particles at the moment of sliding. There is no relative displacement between the soil particles and the working face of the shaper. Figure 6a shows the spatial mechanical analysis of the moment of slip of a single soil particle. Taking the soil particle center as the origin O, the direction parallel to the seedbed as x, the forward direction of the operating machine as y, and the direction perpendicular to the seedbed as z, the space rectangular coordinate system is established. Setting the direction of motion of a single particle at the moment of slip be A_z, it is seen that the operation surface slip of the shaper is reciprocated by the combined effects of normal force F_a, friction f and gravity G. Decomposing the above forces into the xOz coordinate system, as shown in Figure 6b, the resultant force F_x of the soil particle along the X-axis is

$$F_x=F_a\cos a+G\sin 15°-f\cos \beta \sin a$$

(3)

Where, the angle β is the included angle of A_z projection on the surface (°).

Taking the formula of friction f into formula(3)

$$F_x=F_a\cos \alpha +G\sin 15°-\mu F_a\cos \beta \sin a$$

(4)

Where:

μ is the friction coefficient between soil particle and operating surface of shaper

G is gravity (N)

In formula (4), the friction coefficient μ between the soil particle and the operating surface of the shaper can be measured. The included angle β is related to the motion direction A_z at the moment of slip, The installation angle α can be determined by artificial adjustment, Soil particles will generally move in the positive direction of the x axis, and the mechanical condition is F_x>0.

Figure 6. Spatial force analysis at the moment of soil particle slipping under the action of shaper.

Figure 6. Spatial force analysis at the moment of soil particle slipping under the action of shaper.

According to the requirements of agronomy, the shaper should not cause soil to be piled-up greatly during its operation. When the curved form of the scraper is determined, the installation angle α is the key factor for the formation of the seedbed and the soil-covering belt of seedlings. Field experiments show that the width of seedbed can be guaranteed when the maximum installation angle α of ridge and furrow soil collection is 60° and the width of shaper is 330mm.

3.3. Rotary tillage soil-covering device

Rotary tillage knife can throw soil from lateral side[20,21], while in operation, the tractor will transfer power to the gearbox to change the speed of the power and drive rotary tillage soil-covering device work, the rotary tillage knife would break soil on the seedling soil-covering belt, ditch and throw soil on both sides of the seedbed to complete the seedling soil-covering operation. In order to ensure that soil covered on both sides of the seedbed is uniform in the process of rotary tillage, the rotary tillage knife on the cutter head adopts the external installation method, and the rotary tillage knife at both ends of the cutter head bends outwards. The rotary tillage knife arrangement is shown in Figure 9.

3.3.1. Analysis of soil-covering amount of rotary tillage knife

The rotating direction of the rotary tillage knife is the direction of soil throwing[22]. The process of the rotary tillage knife throwing soil is shown in Figure 10. The advance speed of the machine is V_m, the rotary tillage knife rotates at an angular speed ${\omega }_1$, and the soil is thrown up by the rotary tillage knife to position and cover the seedlings. In order to ensure that the thickness of covering-soil on seedlings is 40~80mm, it is necessary to analyze the soil-covering amount of rotary tillage knife on seedbed. Firstly, the soil cutting area of a single rotary tillage knife in a single rotation period is analyzed[23].

In order to establish the parameter equation of the soil disturbed by rotary tillage knife, the area of soil(S) cut by a single rotary tillage knife in a certain cycle is determined at first, then the thickness of soil(T) cut by a single rotary tillage knife in a certain cycle is obtained, and finally the area of soil(V) disturbed by rotary tillage knife is obtained. A rectangular coordinate system was established as shown in Figure 11. The motion curve of the blade end of the adjacent rotary tillage knife in the same direction in a certain cycle is established. The blade pivot(O₁) are taken as the center, the forward direction of the machine is the x-axis, and the vertical downward axis is the y-axis. The absolute motion of rotary tillage knife’ s end is the combination of rotation motion of blade pivot and forward motion of the machine, and the motion process of the end of rotary tillage knife meets the requirements of trochoid [24].

As shown in Figure 11, since the area enclosed by points P₂, P₃, and P₄ is torn apart by the cutter teeth during high-speed rotation, the soil cutting area S is enclosed by points P₁, P₃, P₄ and P₅, the area of soil(S) cut by blade is enclosed by points P₁, P₃, P₄ and P₅. The soil cutting area of a single rotary tillage knife in a single rotation period is expressed as

$$S={\displaystyle {\int }_0^{h_a}\mathit{dy}{\displaystyle {\int }_{x_{1\left(y\right)}}^{x_{2\left(y\right)}}d}x}$$

(13)

Where:

$x_1(y)$ is the Equation of motion curve of the first rotary tillage cutter end

$x_2(y)$ is the Equation of motion curve of the second rotary tillage cutter end in the same period

The equation of motion curve of the first rotary tillage cutter end is

$$\{\begin{array}{l}x=R\cos ({\omega }_{1}t){+V}_{m}t\\ y=R\sin ({\omega }_{1}t)\end{array}$$

(14)

If the number of blades in the same direction is 3, then the lag soil-covering angle of the second rotary tillage knife in the same direction is$\frac{2\pi }{3}$, and the equation of motion curve of the second rotary tillage cutter end in the same direction is

$$\{\begin{array}{l}x=R\cos ({\omega }_{1}t-\frac{2\pi }{3}){+V}_m{t+x}_0\\ y=R\sin ({\omega }_{1}t-\frac{2\pi }{3})\end{array}$$

(15)

Where $x_0$is the pitch of soil cutting, and its coordinates meet

$$x_0=\frac{2\pi V_m}{{Z\omega }_1}$$

(16)

Where:

Z is the number of rotary tillage knife in the same direction,3

By synthesizing Equations (13)~(16), it can be calculated that the soil cutting area S is

$$S={\displaystyle {\int }_0^{h_a}{x}_{2\left(y\right)}\mathit{dy}-{\displaystyle {\int }_0^{h_a}{x}_{1\left(y\right)}d}y}$$

(17)

From the equation, the time t₁, t₃, t₄ and t₅ of rotary tillage knife pass through P₁, P₃, P₄ and P₅ can be calculated as

$$\{\begin{array}{l}{\omega }_1{t}_1={\sin }^{-1}(1-\frac{h_a}{R})\\ t_3=\frac{\pi }{2{\omega }_1}\\ t_4=\frac{7\pi }{6{\omega }_1}\\ t_5=t_1+\frac{2\pi }{3{\omega }_1}\end{array}$$

(18)

Where:t is time (s)

h_a is the tillage depth (mm)

R is the radius of rotary tillage knife (mm)

${\omega }_1$ is the angular velocity of rotary tillage blade (rad/s)

Combined with Equations (17) and (18), the soil cutting area S can be calculated as

$$S=\frac{A\cos 2{\omega }_1{t}_1+B\sin 2{\omega }_1{t}_1+C\cos {\omega }_1{t}_1+D\sin {\omega }_1{t}_1-\frac{1}{2}{R}^2{t}_1+E}{{\omega }_1}$$

(19)

Where:

$$\{\begin{array}{l}A=\frac{({\omega }_1+\sqrt{3})R^2}{8{\omega }_1}\\ B=\frac{(1+\sqrt{3}{\omega }_1{)R}^2}{8{\omega }_1}\\ C=\frac{3\sqrt{3}+3x_{0}R+3{\mathrm{RV}}_m-3\sqrt{3}{\omega }_1{\mathrm{RV}}_m{t}_1-2\sqrt{3}{\mathrm{RV}}_m}{6{\omega }_1}\\ D=\frac{3-3\sqrt{3}{\mathrm{RV}}_m{t}_1-3\sqrt{3}{x}_{0}R+2\pi {\mathrm{RV}}_m+3{\omega \mathrm{RV}}_m{t}_1+3\sqrt{3}{\mathrm{RV}}_m}{6{\omega }_1}\\ E=\frac{12\sqrt{3}+14\pi {\mathrm{RV}}_m-(6\pi +3\sqrt{3}+3{\omega }_1)R^2-12\sqrt{3}{x}_{0}R}{24{\omega }_1}-\frac{2{\mathrm{RV}}_m+\sqrt{3}\pi {\mathrm{RV}}_m}{4{\omega }_1{}^2}\end{array}$$

(20)

The area of soil cut by a single rotary tillage knife in one rotation cycle can be approximately calculated as the product of the area of soil cutting and the thickness of soil cutting T, and the formula for calculating the thickness of soil cutting T is[25]

$${T=L}_1\sin (\pi -\theta )$$

(21)

Where L₁ is the cutting length of the rotary tillage knife’s tangent edge, mm

$\theta$ is the Angle of tangent edge

According to Equations (19) and (21), the covering volume V of a seedbed soil-covering belt for one rotation of blade pivot is

$$\begin{array}{l}V=\mathit{TSZ}\\ =\frac{3L_1\sin \theta }{{\omega }_1}\left[A\cos 2{\omega }_1{t}_1+B\sin 2{\omega }_1{t}_1+C\cos {\omega }_1{t}_1+D\sin {\omega }_1{t}_1-\frac{1}{2}{R}^2{t}_1+E\right]\end{array}$$

(22)

According to Equations (18), (20) and (22), the volume of disturbed soil V is related to such parameters as ω₁, R, L₁, θ, h_a, Z and V_m. When the parameters of rotary tillage knife are determined, R, L₁ and θ are fixed values, so the soil-covering amount of seedbed is mainly affected by the depth of tillage h_a, rotary angular velocity ω₁, numbers of rotary tillage knife Z, forward speed of machine V_m. When the forward speed of machine V_m is certain, the rotation angular speed and tillage depth are the main factors affecting the soil-covering thickness. Therefore, the adjustment of the depth of rotary tillage is taken into consideration in the design of the component.

3.3.2. Analysis of axial soil-transporting performance of rotary tillage knife

In order to ensure the axial covering range of soil while rotary tillage knife tilling soil, it is necessary to determine the mechanical conditions of soil axial movement. The force of rotary tillage is mainly determined by the structural parameters and soil parameters of rotary tillage. Suppose that there is no relative displacement of soil particles on the rotary tillage knife and there is no interaction between the soil particles. Figure 12 shows the spatial mechanical analysis of a single soil particle at the moment of separation. Taking the center line of the blade axis as the origin$O_1$, the direction of the center line of the blade axis as$x_1$, the direction of the machine as $y_1$and the direction of the vertical horizontal plane as Z₁, the origin of the coordinate system O₁ is translated along the plane $y_1{O}_1{z}_1$ to the center of the soil particle $O_2$ to obtain the spatial coordinate system. Suppose the instantaneous motion direction of the soil particle when it breaks away from the tangent plane of the rotary tillage blade be$A_z{}_1$, it can be concluded that the moment it breaks away from the tangent plane of the rotary tillage knife, it is affected by the comprehensive action of normal force$F_n$, friction force f₁, gravity G₁ and centrifugal force$F_c$.

The stress of soil particles in $x_2{o}_2{z}_2$ coordinate system is decomposed into the coordinate system, as shown in Figure 13, the resultant force of soil particles along the direction of $x_2$ and $z_2$ are respectively

$$\{\begin{array}{l}{F}_{x2}=\frac{F_c\cos {\alpha }_1{+G}_1}{\cos {\beta }_2}\sin {\beta }_2{-f}_1\cos {\beta }_2\sin {\gamma }_1\\ F_{z2}=F_n\cos {\beta }_2{-F}_c\cos {\alpha }_1{-G}_1\end{array}$$

(23)

Where ${\alpha }_1$ is rotation angle of cutter shaft when soil particle is detached,(°)

${\beta }_2$ is angle between surface of $x_2{o}_2{z}_2$ and tangent edge of rotary tillage blade,(°)

$\gamma$₁ is angle between projection of$A_z{}_1$on the surface of $x_2{o}_2{z}_2$ and $y_2$axis,(°)

F_x2 is the resultant force of soil particles along the $x_2$axis, (N)

F_z2 is the resultant force of soil particles along the $z_2$axis, (N)

Figure 13. Stress of soil particles in $x_2{o}_2{z}_2$ coordinate system.

Figure 13. Stress of soil particles in $x_2{o}_2{z}_2$ coordinate system.

The expressions of centrifugal force F_c, gravity G₁ and friction f₁ are put into Equation (23) to obtain

$$F_{x2}=-\left({m\omega }_1{}^{2}R\cos {\alpha }_1+\mathit{mg}\right)\cot {\theta -\mu }_1\left({m\omega }_1{}^{2}R\cos {\alpha }_1+\mathit{mg}\right)\sin {\gamma }_1$$

(24)

Where m is mass of soil particles (kg)

R is rotation radius of soil particle movement (mm)

${\mu }_1$ is friction factor between soil particle and surface of rotary tillage knife

g is acceleration of gravity(m/s²)

$\theta$ is angle of tangent edge bend

In Equation (24), the relation between ${\beta }_2$ ( the surface of $x_2{o}_2{y}_2$and the tangent edge of the rotary tillage knife) and $\theta$ (the bending angle of the tangent edge) is${\beta }_2=\theta -\pi /2$, and the angle of the tool shaft α₁ ranges from 0° to 90°[25].

In order to determine the axial motion displacement conditions of soil particles, it is necessary to determine the motion process after the action of rotary tillage knife on soil particles. Setting the action time of rotary tillage knife on soil particles as t₁, and the component velocity along the direction of $x_2$ at the moment when the soil particles leave the positive section of rotary tillage blade as $v_x$and the component velocity along the direction of $z_2$ as $v_z$, it can be obtained from the momentum theorem

$$\{\begin{array}{l}{F}_x{}_2{t}_1{=\mathit{mv}}_x\\ F_z{}_2{t}_1{=\mathit{mv}}_z\end{array}$$

(25)

The motion process of soil particles on the surface of $x_2{o}_2{z}_2$ is shown in Figure 14. Suppose the vertical height between the moment when soil particles are thrown out and the head of the seedling is h, the component velocity on the surface of $x_2{o}_2{z}_2$ is V₁, the rising height of soil particles after being thrown out is h_z, and the displacement of soil particles along the direction of $x_2$is X₁, and without considering the air resistance, then the time displacement relation is

$$\{\begin{array}{l}{X}_1{=v}_x{t}_z\\ h_z=v_z{t}_{z1}-\frac{1}{2}g{t}_{z1}{}^2\\ t_{z1}=\frac{v_z}{g}\\ t_{z2}=\sqrt{\frac{2\left(h+h_z\right)}{g}}\\ t_z=t_{z1}+t_{z2}\end{array}$$

(26)

Where t_z is the time it takes for soil particles to fall to the head of the seedling after being thrown out (s)

t_z1 is the time it takes for soil particles to rise to the highest point after being thrown out (s)

t_z2 is the time it takes for soil particles to fall from the highest point to the head of the seedling (s)

g is acceleration of gravity(m/s²)

According to agronomic requirements, the maximum displacement of soil particles$X_1$along the direction of$x_2$after being thrown out should meet

$$X_1\ge l_1/2$$

(27)

l₁ is spacing of wide line, which is set to be 700mm according to agronomic requirements

In combination with Equation (25) and (26), the displacement of soil particles$X_1$along the direction of $x_2$ (covering width) can be calculated as

$$X_1=\frac{F_{x2}{t}_1}{m}(\frac{F_{z2}{t}_1}{\mathit{mg}}+\sqrt{2(\frac{h}{g}+\frac{F_{z2}{}^2{t}_1{}^2}{2m^2{g}^2})}$$

(28)

As can be seen from formula (28), factors affecting the landing position of soil particles are the vertical height between the moment when soil particles are thrown out and the head of the seedling(h), the resultant force F_z2 of soil particles along the$z_2$axis, the resultant force F_x2 of soil particles along the$x_2$axis, and the action time t₁ of rotary tillage knife on soil particles.

Figure 14. Movement process of soil particles on the surface of $x_2{o}_2{z}_2$.

Figure 14. Movement process of soil particles on the surface of $x_2{o}_2{z}_2$.

4. Simulation of rotary tillage covering operation process

5. Field experiment

6. Conclusion

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