Design and Verification of the Variable Capacity Roller-Wheel Precision Rice Direct Seed-Metering Device

Abstract

In view of the problems of the inability to continuously adjust the seeding rate, small adjustment range, and low precision of the rice direct seed-metering device, a variable capacity roller-wheel seed-metering device was designed, fabricated, and verified. With the variable capacity method, the seeding rate could be adjusted steplessly and with high precision through the seeding rate adjusting mechanism. Meanwhile, theoretical analysis and parametric design of the critical components, such as the seeding wheel and the seeding rate adjusting mechanism, were carried out. The rotation speed of the seed-metering device, the sphericity of rice seeds, and the adjustment depth were selected as the main influencing factors of the experiment. Then, the full-factor experiment was conducted, and the results of the bench validation experiments showed that the average seeds per hill error of the seed-metering device could be controlled within 2%. Meanwhile, the seed-metering device performed the best when the rotation speed of the seed meter device was 30 r/min, the sphericity of rice seeds was 52.7%, and the hole depth was 6 mm. Compared to the conventional mechanical precision seed-metering device, the designed seed-metering device offered better seeding performance. The research results provide theoretical and practical references for the design of a rice seed-metering device and the development of precision agriculture.

1. Introduction

Rice is one of the three major grain crops in China and an important staple food crop globally [1,2,3]. China’s rice output ranks first in the world, with a total rice output of over 200 million tons [4,5]. Rice is cultivated in a quarter of the arable land in China and supports more than half of its population [6,7,8]. Traditional rice planting involves many steps such as raising and transplanting seedlings [9]. However, rice direct seeding is a planting technology that directly seeds in the field for cultivation, thus eliminating the traditional seedling and transplanting process and reducing the prophase cost [10,11,12]. The cost of direct seeding of rice was lower than that of traditional rice transplanting by 2480 CNY/hm² [13]. Therefore, the research on direct seeding technology of rice is of great significance.

Seed-metering devices play an important role in rice direct seeding technology [14]. According to their working principle, these devices can be mechanical or pneumatic [15,16,17,18]. The pneumatic seed-metering device must maintain high-level airtightness, and its structure is complicated, making it difficult to adjust seeding rates widely [19,20]. Compared with the pneumatic seed-metering device, the mechanical seed-metering device has a relatively simple structure and is easy to mechanize and popularize in agricultural production [21,22]. Additionally, its seeding rate has a wide adjustment range [23]. Because of these advantages, the mechanical seed-metering device has become a focus and hotspot of domestic and foreign scholars.

At present, many experts and scholars are conducting studies on mechanical seed-metering devices. Maleki et al. [24,25] designed a screw-type seed-metering device. Through experiments, it has been seen that the seeding uniformity is good but the seeding hole formation is poor. Gaikwad et al. [26] designed a vibrating vegetable seedling-metering device using a vibrating device to improve the crowding phenomenon of seeds in the seed filling area, which effectively improved the seedling raising efficiency. The study results provided a theoretical reference for the research of mechanical seed-metering devices. Luo et al. [6,27,28] expanded the seeding mechanism of the mechanical seed-metering device through mechanical principle and established a theoretical model of seed spacing. Zhang et al. [29] designed a seed-metering device with a combined hole type, and the seeding rate was adjusted by changing the size of the combined hole type. However, the adjustment process of the seed-metering device is complicated, and the seeding rate cannot be adjusted continuously. Wang et al. [4] designed a precision seed-metering device with an adjustable seeding rate for different rice varieties. However, the structure of the whole machine is relatively complex, and the adjustment precision was low. Zhu et al. [30] developed a slide hole-wheel rice direct seeding-metering device. They simulated and analyzed the seeding process and obtained the best operating parameters. Tian et al. [31] designed a helical groove rice direct seeding-metering device, analyzed the movement track of rice bud seeds, determined the critical component parameters of the seed-metering device, and verified the performance of the device through experiments.

To sum up, the mechanical precision rice seed-metering device still has several problems, such as low adjustment precision, inability to adjust continuously, small adjustment range, complicated adjustment process, and high manufacturing cost. This study focused on the design, analysis, and experimental verification of a new precision rice direct seeding device to improve the seeding performance and stability under practical applications. The seeding rate adjustment mechanism designed in this study is different from the existing mechanism. It can realize continuous and extensive adjustment of the seeding rate and solve the problem that the adjustment process is complicated and the seeding rate cannot be adjusted infinitely. Furthermore, the seed clearing process is completed by the wheeled forced seeding mechanism, which solves the jamming problem in the existing seed-metering device and improves seeding efficiency.

This study aimed to design, fabricate, and verify a variable capacity roller-wheel precision rice direct seed-metering device. First, the configuration and working principle of the seed-metering device is described in detail. Then, theoretical analysis and parametric design of the critical components were carried out. Next, the factors affecting the performance of seed-metering devices were analyzed. Finally, the direct rice simulation experiment was conducted on the laboratory seeding platform by using three different types of rice seeds with different sphericity characteristics as experimental objects. This study provides technical references and practical references for the innovative design of rice precision seed-metering devices and provides an important machine innovation for the development of precision agriculture.

2. Materials and Methods

2.1. Configuration and Working Principle

2.1.1. Configuration of Seed-Metering Device

The seed-metering device (Figure 1) consists of a shell, a seeding wheel, a seeding shaft, a brush, a forced seed feeding mechanism, a fixed track, a seed tube, and a seeding rate adjusting mechanism. The seeding wheel and shell are connected by a supporting bearing. The fixed track is placed on the shell, and the seeding adjusting mechanism is mounted on the adjusting track by an adjusting handle. The push shaft and spring on the forced seeding mechanism are installed in the hole. The fixed track on the shell and the inner track on the seeding adjustment mechanism form a double track, and the rolling bearing on both sides of the forced seeding mechanism moves along the double track. By cooperating with the double tracks, the forced seeding mechanism moves along the slide groove in the hole, thus allowing the depth of the hole to be adjusted to meet different seeding rates.

2.1.2. Working Principle of the Seed-Metering Device

Before the seed-metering device works, the seeding rate adjusting mechanism is transferred to the theoretical seeding rate position. The seeding shaft drives the wheel to make a circular motion under the external power. The rice seeds fall from the seed box to the filling area and then fill into the hole under the action of gravity, the pressure between the seeds, and the rotation of the seeding wheel. The seeds with irregular postures falling into the hole are removed by the seed cleaning brush. The circumference of the seeding wheel is provided with a uniform spacing hole, and a forced seed feeding mechanism is installed in the hole. Under the joint action of the forced seeding mechanism and the seeding rate adjusting device, the adjustment depth of the hole is changed when the hole passes through the seed filling area, making different numbers of seeds be accommodated. When the seeding wheel passes through the seed protection area and turns to the seeding area, the forced seeding mechanism acts immediately under the action of the double track and ejects the seed in the hole into the seed furrow instantly to complete the seeding.

2.2. Design of Crucial Components

2.2.1. Structural Design of the Seeding Wheel

The seeding wheel is a critical component of the seed-metering device, and the design of the structural parameters directly determines the performance of the seed-metering device. The critical structural parameters of the seeding wheel include its thickness and diameter, the number of holes, the adjustable depth, and the form of the hole. The type of holes mainly includes round holes, basin holes, spoon holes, scoop-shaped holes, etc. [28,32]. According to the characteristics of rice seed materials, round holes were chosen in this paper for the seed-metering device.

• Design of the Diameter and Thickness of the Seeding Wheel

When designing the diameter of the seeding wheel, it is necessary to consider the curvature of the seeding wheel (Figure 2). The smaller the diameter of the seeding wheel, the greater the curvature of the seeding wheel, and the higher the leakage rate of rice seeds, which is adverse to seed filling. The larger the diameter of the seeding wheel, the larger the structure size and weight of the whole seed-metering device, which is also not conducive to field operations. Generally, the diameter of the seeding wheel of the hole seed-metering device ranges from 80 to 200 mm, and the thickness is less than 50 mm. Considering the agronomic requirements and seed dimensions of direct seeding rice, the diameter R of the seeding wheel was set to 140 mm, and the thickness h was set to 25 mm [33,34]. The sliding groove on the seeding wheel mainly enables the forced seeding mechanism to move freely in the hole, and the design size L₁ can meet the adjustment depth required by the maximum seeding rate. The specific dimensions of the hole diameter R₁ and the design size L₁ of the sliding groove are described in detail in design of diameter of hole and adjustment depth. The positioning size L₂ of the sliding groove cannot exceed the radius of the seed-metering device, and the installation size L₂ of the sliding groove was set to 48 mm in this study.

• Design of the Hole Number

The number of holes directly affects the seed-filling performance of the seed-metering device. Increasing the number of holes can reduce the linear velocity of the seeding wheel at work and help the seed to fall into the hole. Meanwhile, the number of holes is limited by the diameter of the seeding wheel and the spacing of the hole. The number of holes could be calculated as follows:

$$\mathrm{n}=\frac{\pi \mathrm{R}{v}_m}{\mathrm{L}{v}_0}$$

(1)

where, n is the number of the holes, R is the diameter of the seeding wheel (mm), v₀ is the line speed of the seeding wheel (m s⁻¹), v_m is the operation speed of the seeder (m s⁻¹), and L is the hill spacing of direct rice seeding (mm).

The seed-metering device designed in this paper was mainly used for direct water seeding. Currently, the power machinery suitable for direct water seeding is the rice transplanter. The field operation speed of the rice transplanter in the market ranges from 0.5 to 1 m s⁻¹ [35,36,37], and 0.8 m s⁻¹ was considered for the design. For different rice species and planting areas, the hill spacing for direct seeding is adjustable from 0.1 to 0.25 m, and 0.1 m [20,21] was considered for the design. For the vertical disc seed-metering device, to meet the requirements of seeding performance, the optimum linear velocity of the seed-metering device ranges from 0.2 to 0.35 m s⁻¹ [21,38], and 0.3 m s⁻¹ was considered for the design. The relevant values were substituted into Equation (1), and the number of holes was 12.

• Design of Diameter of Hole and Adjustment Depth

For the seed-metering device designed in this paper, the shape of the hole is circular. Circular holes with uniform spacing are arrayed on the circumference of the seed-metering device. The hole diameter and adjustment depth are related to the seed size and seeding rate. To obtain the best performance of the seed filler and seed delivery, the diameter of the hole should meet the following requirements [29,34]:

$$R_1=(5-10\%)L_{\max }$$

(2)

where, R₁ is the diameter of the hole (mm); L_max is the maximum length of the seed (mm).

According to the basic parameters of the experimental seeds in this design shown in

Table 1. The basic parameters of the experimental seeds.

Species	Long Axis (mm)	Short Axis (mm)	Thickness (mm)	Sphericity (%)	Moisture Content (%)
Huang Hua Zhan	9.79 ± 0.32	2.16 ± 0.12	1.91 ± 0.10	35.0	23.4
Xiang Zao Xian 45	7.92 ± 0.20	2.83 ± 0.14	2.17 ± 0.08	46.1	25.3
Long Dao 6	7.18 ± 0.30	3.39 ± 0.11	2.22 ± 0.06	52.7	24.3

, the values were substituted into Equation (2), and 10% of the maximum value in the design was taken. The hole diameter of the seed-metering device should be 12 mm.

Twelve holes were evenly distributed on the circumference of the seeding wheel. For the rice direct seeding-metering device, the probability of seeds entering the hole in a lying posture is the highest during seed filling, and each layer can hold 2 or 3 rice seeds, but the probability of holding 2 rice seeds is much higher than holding 3 rice seeds [17,29,38]. Thus, the number of rice seeds filled in each layer was set to 2 in this design.

Rice planting areas and varieties differ significantly in planting density and seeds per hill. Generally, the row spacing is a fixed value for precision rice direct hill seeding, and the hill spacing and the number of holes can be adjusted. In direct seeding rice, there are about 3 to 8 seeds per hill, and the adjustment depth depends on the number of holes and seed thickness [23,29,38,39]. The adjustment depth could be calculated as follows [4,29]:

$$\mathrm{H}=\frac{\mathrm{N}}{\mathrm{N}\prime }{t}_{\max }$$

(3)

where, H is the adjustment depth (mm); N is the seeds per hill; N′ is the number of seeds filled in each layer; t_max is the maximum seed thickness.

To obtain wider adaptability of the seed-metering device, the number of seeds per hill was set to 8 in the design. According to Equation (3), the depth required for seeding adjustment was 9 mm. Thus, the design size L₁ of the sliding groove was 9 mm. Assuming that the seeds enter the hole in a flat-lying posture in the calculation, the number of seeds filled in each layer was set to 2, which led to a significant result. Therefore, the adjustment depth of 3, 6, and 9 mm will be explored for the performance of the seed-metering device in a later experiment.

2.2.2. Design of Forced Seeding Mechanism

To solve the problems of seed sticking and unsmooth seed feeding in the operation of the hole seed-metering device, this paper proposes a forced seeding mechanism (Figure 3). As shown in Figure 3, the forced seeding mechanism is mainly composed of a pushing shaft and a spring. The spring is installed at the bottom of the pushing shaft. Once the seeding wheel is rotated to the seeding area, the spring acts immediately, and the pushing shaft ejects the seed in the hole to the seed furrow instantly. To improve the seed filling performance of the seed-metering device, a guiding socket was designed at the top of the pushing shaft. The depth of the guiding socket should not be greater than the minimum thickness of the seeds; otherwise, the accuracy of the seeding rate adjustment mechanism would be affected. The design of the guiding socket could be calculated as follows:

$$\tan \mathsf{\beta }=\frac{L_4}{L_3}$$

(4)

where, β is the cone angle of the guiding socket (°); L₃ is the radius of hole (mm); L₄ is the depth of the guiding socket (mm).

According to the data listed in Table 1, the minimum seed thickness in the experiment was 1.91 mm, which was chosen as the value of L₄. Meanwhile, calculated by Equation (4), the cone angle of the guiding socket was designed to be 17.6°. Based on the working principle of the seed-metering device and the structure of the seeding wheel, the outer diameter of the rolling bearing should be larger than the diameter of the hole. In this design, the FAG625 series rolling bearing was employed, and the outer diameter of the bearing was 16 mm. To reduce the possibility of seed injury, the stiffness coefficient of the spring should not be too large. It was shown that the germination rate of rice seeds is affected when the external pressure is greater than 10 N [40]. The maximum adjustment depth of the hole was 9 mm, i.e., the maximum compression amount of the spring was 9 mm. According to Hooke’s law, the stiffness coefficient of the selected spring cannot be greater than 1111 N m⁻¹. Therefore, the stiffness coefficient of the spring selected in this paper was set to 500 N m⁻¹.

2.2.3. Design of the Seeding Rate Adjusting Mechanism

The structure of the seeding rate adjusting mechanism is shown in Figure 4, which mainly consists of an outer arc and an inner arc. The inner arc is composed of four curves, namely BF, BC, CE, and EF, and the radii of the points on the outer arc are equal. As for installation and coordination, the radius of the outer circular arc of the adjusting mechanism should be smaller than that of the seeding wheel. Thus, the radius of the outer circle was designed to be 68 mm. The inner arc mainly plays a regulating role in the BC section. The radius of the point in the BC section changes with the angle and the radius decreases from point B to point C so that the forced seeding mechanism moves along the sliding groove to the center of the seeding wheel, and the adjustment depth of the hole changes to adjust the seeding rate.

Taking two points, M and Q, at any point in the BC section, the coordinate system is established, as shown in Figure 4b. In this figure, v₁, v₂, and v₃ represent the tangential velocity, combined velocity, and normal velocity of point M, respectively. 1 and ∠2 and ∠1 and γ are complementary angles, so ∠2 and γ are equal. The tangent of γ could be calculated as follows:

$$\tan \gamma =\frac{\overline{\mathrm{K}Q}}{\overline{OM}-\overline{O\mathrm{K}}}=\underset{\begin{array}{l}\mathrm{dr}\to 0\\ \mathrm{d}\mathsf{\theta }\to 0\end{array}}{\lim }\frac{(r_m-\mathrm{dr})\sin \mathrm{d}\mathsf{\theta }}{r_m-(r_m-\mathrm{dr})\cos \mathrm{d}\mathsf{\theta }}$$

(5)

where, $\overline{\mathrm{K}Q}$,$\overline{\mathrm{OM}}$, and $\overline{\mathrm{OK}}$ represent the length of the line segment (mm); dθ is the rotation angle from M to Q (°); dr is the polar radius variation from M to Q (mm); r_m is the initial polar radius point M (mm); r_m-dr is the polar radius after turning dθ (mm).

When the rotation angle of the adjusting mechanism is tiny, i.e., dr and dθ approach 0, sindθ/dθ tends to be 1. By using the limit thought in mathematics, we have:

$$\tan \gamma =\frac{r_{\mathrm{m}}}{\mathrm{dr}/\mathrm{d}\mathsf{\theta }}$$

(6)

This study assumed that the maximum seeding rate is adjusted within a fixed angle range and the seeding rate and the radius change equally when the adjusting mechanism rotates at the same angle every time. Based on the definition of the up-cut angles and related research results, the slip-tangent angle of any point on the curve is equal under the assumption of this study [41,42]. That is, the tangent value of the up-cut angles is constant. According to the properties of integrals and derivatives, Equation (6) can be rewritten as follows:

$$r_{\mathrm{m}}=\exp ({\mathsf{\theta }}_m+Z\mathrm{k})/\mathrm{Z}$$

(7)

where, θ_m is the polar angle of point M (°); Z and k are integral constants.

When the forced seeding mechanism slides on the BF section, the adjustment depth of the hole is zero. According to the working principle of the adjustment mechanism and the cooperation between the mechanisms, the radius of the curve BF could be calculated as follows:

$$r_2=L_2+r_4$$

(8)

where, r₂ is the radius of the curve BF (mm); L₂ is the positioning dimension of the sliding groove (mm); r₄ is the radius of the rolling bearing (mm).

According to design of the diameter and thickness of the seeding wheel, L₂ is 48 mm, and since the outer diameter of the rolling bearing is 16 mm, r₄ is 8 mm. Thus, the value of r₂ is 56 mm. To reduce the movement of seeds in the hole and meet the maximum seeding adjustment depth of 9 mm, r₃ was designed to be 47 mm. The EF curve is the connecting curve, and its primary function is to ensure the smooth seeding area of the seed-metering device.

The adjustment of the maximum seeding rate is completed within 90°. Since the adjustment mechanism adjusts the seeding rate from small to large when it turns counterclockwise, the larger the radius of the point on the curve, the smaller the adjustment depth of the hole, and the lower the seeding rate. In this case, the radius is 47 mm when the polar angle of the point on the curve is 0°, and the radius is 56 mm when the polar angle is 90°. According to Equation (7), the track curve BC in the adjusting mechanism can be represented as follows:

$$\mathrm{r}\prime =\exp (\mathsf{\theta }\prime +34.5)/8.96$$

(9)

where, r′ is the polar radius of the point on the curve BC (mm); θ′ is the polar angle of the point on curve BC (°).

The inner track curve of the adjusting mechanism is regarded as the fixed track curve on the shell. The fixed track on the shell and the inner track of the adjusting mechanism form a double track (Figure 5). The primary function of the fixed track is to ensure the immovability of the seeding position. Each forced seeding mechanism acts correspondingly when the seeding wheel rotates to the seed feeding area. The inner track of the adjusting mechanism is mainly used to adjust the seeding rate.

When the seed-metering device works, the adjustment mechanism rotates counterclockwise to adjust the seeding rate from small to large, and the seeding wheel rotates clockwise. When the seeding rate of the seed-metering device is zero, the adjustment mechanism of the inner track is consistent with that of the fixed track. According to the working principle of the seed-metering device and the angle design of the seeding rate adjusting mechanism (as shown in Figure 4a), when the adjusting mechanism rotates by an angle of α (α is the radian), the double tracks are misplaced at (12α − 5π)/12. At this time, the rolling bearings on the forced seeding mechanism slide along the fixed track between (12α − 5π)/12 and −π/2, and the other angles slide along the inner track of the adjustment mechanism. The cooperation between the double tracks can realize the dynamic adjustment of the seeding rate. It also ensures the fixed seeding position so that the seeds in the hole can be ejected to the seed ditch at the fixed seeding position.

2.3. Experimental Materials

To verify the seeding performance of the developed seed-metering device, the seeding experiments were carried out. The rice seeds with different sphericity were selected as the experiment material to verify the adaptability of the seed-metering device to different rice species. Before the experiment, rice seeds were cleaned and pretreated to make them meet the direct seeding standard. The basic parameters of the seeds used in the experiments are shown in Table 1.

2.4. Experimental Equipment

The seeding experiment was conducted in the Agricultural Machinery Equipment Laboratory of the National Agricultural Intelligent Equipment Engineering Technology Research Center (Beijing, China). The seeding experiment bench is shown in Figure 6, which is self-built and mainly consists of a bench, a conveyor belt, a seed-metering device, an image acquisition device, a control cabinet, and other components.

The seed-metering device should be set on the bench. The conveyor belt speed was used to simulate the ground speed of the machine, and the speed regulating motor speed was controlled by a knob on the control cabinet. Meanwhile, a stepping motor was used to drive the seed meter, and the pulse signal input to the motor was adjusted by configuring the drive so the speed of the seed metering wheel could be precisely controlled. The driving motor of the seed-metering device adopted a stepping motor, and the pulse signal input to the motor was controlled by adjusting the driver to accurately control the rotation speed of the seeding wheel. Moreover, an image acquisition device was used to record the experimental process and obtain rice seeds per hill. The image acquisition device was calibrated to acquire data at 240 f/s to facilitate later data processing.

2.5. Experimental Design

The adaptability of the seed-metering device was verified with rice seeds of different sphericity. Therefore, three types of rice seeds with different sphericity were selected as experimental factors, as shown in Table 1. This design was focused on the seeding rate adjustment device, and the change in adjustment depth was realized by the adjustment mechanism. A range of adjustment depths was chosen as an experiment factor to verify that the structural design meets the requirements and that the seeding rate can be adjusted in an extensive range. In addition to the sphericity of rice seeds and the adjustment depth of the hole, different speeds of the seed-metering device were also selected at the same time. The speed of the seed-metering device could be calculated as follows:

$$\mathrm{n}\prime =\frac{60v_m}{\mathrm{nL}}$$

(10)

where, n′ is the speed of the seed-metering device (r min⁻¹).

For different rice varieties and direct seeding conditions, the hill spacing is required to be adjustable from 0.1 to 0.25 m. According to Equation (10), the speed of the seed-metering device ranges from 16 to 40 r min⁻¹ during field operations. Therefore, four rotation speeds of 20, 25, 30, and 35 were considered in this paper, and the experimental factor levels are presented in

Table 2. The levels of the experiment factors.

Level	Factor
	Speed of the Seed-Metering Device X1 (r min−1)	Rice Seed Sphericity X2 (%)	Adjustment Depth X3 (mm)
1	20	35.0	3
2	25	46.1	6
3	30	52.7	9
4	35

.

2.6. Evaluation Indicators

According to the above experiment method, the all-factor seeding experiment was conducted on the seed-metering device. During post-data processing, the seeds per hill discharged by the seed-metering device were recorded continuously. Every 250 hills constituted a group, and a total of 36 groups were tested. Each group of experiments was repeated three times, and the average value was taken with 108 experiments. The qualified rate (y₁), missing rate (y₂), and reseeding rate (y₃) of seeds per hill were used as evaluation indexes to evaluate the seeding performance of the seed-metering device, and the average seeds per hill (y₄) and the coefficient of variation of seed number (y₅) were used as evaluation indexes to evaluate the seeding accuracy of the seed-metering device. Specifically, taking c as a theoretical number of seeds per hill, the case of seeds per hill of c ± 1 was regarded as qualified, the case of seeds per hill of less than c − 1 was regarded as missing, and the case of seeds per hill of greater than c + 1 was regarded as reseeding. According to Equation (3), when the adjustment depth was 3, 6, and 9 mm, the theoretical values of the seeds per hill of the seed-metering device were 3, 6, and 9, respectively. The valuation index can be calculated by the following Equation (11).

$$\left\{\begin{array}{l}{y}_1=\frac{n_1}{Q\prime }\times 100\%\\ y_2=\frac{n_2}{Q\prime }\times 100\%\\ y_3=\frac{n_3}{\mathrm{Q}\prime }\times 100\%\\ y_4=\frac{{\displaystyle \sum _{i=1}^{Q\prime }{N}_{\mathrm{i}}}}{\mathrm{Q}\prime }\times 100\%\\ y_5=\frac{{\mathrm{S}}_{\mathrm{d}}}{y_4}\times 100\%\end{array}\right.$$

(11)

where, Q′ is the number of theoretical seeding hills; n₁ is the number of qualified seeding hills; n₂ is the number of the missing hills; n₃ is the number of reseeding hills; N_i is the number of seeds in hill i; S_d is the standard deviation of the hill number.

3. Results and Analysis

3.1. Seeding Accuracy Experiment

The overall performance of the seed-metering device was verified and analyzed under different rotation speeds, rice seed sphericity, and adjustment depths. The experiment results are shown in

Table 3. Experimental results.

Indicators	Factors	X31			X32			X33
		X21	X22	X23	X21	X22	X23	X21	X22	X23
y1 (%)	X11	83.61 ± 1.44	86.53 ± 1.32	87.04 ± 1.28	83.23 ± 1.54	86.91 ± 1.43	87.29 ± 1.32	84.49 ± 1.87	86.79 ± 1.59	87.17 ± 1.44
	X12	85.64 ± 1.54	87.17 ± 1.41	88.82 ± 1.36	84.37 ± 1.65	87.8 ± 1.63	88.94 ± 1.57	84.88 ± 1.96	87.55 ± 1.73	88.31 ± 1.63
	X13	84.24 ± 1.67	88.44 ± 1.56	90.22 ± 1.44	85.39 ± 1.74	89.2 ± 1.75	90.34 ± 1.61	85.01 ± 2.15	88.18 ± 1.84	89.33 ± 1.76
	X14	81.07 ± 1.71	83.61 ± 1.64	84.5 ± 1.60	81.58 ± 1.91	83.87 ± 1.81	85.26 ± 1.69	81.70 ± 2.22	83.73 ± 1.96	84.75 ± 1.82
y2 (%)	X11	7.50 ± 0.71	4.19 ± 0.76	4.07 ± 0.82	8.13 ± 0.72	4.45 ± 0.81	3.94 ± 0.87	7.50 ± 0.88	4.32 ± 0.92	4.57 ± 0.95
	X12	8.01 ± 0.62	4.57 ± 0.72	4.45 ± 0.79	8.77 ± 0.69	4.07 ± 0.79	4.96 ± 0.83	8.64 ± 0.87	4.83 ± 0.81	4.70 ± 0.91
	X13	9.15 ± 0.59	5.08 ± 0.68	5.21 ± 0.77	8.26 ± 0.67	5.46 ± 0.76	6.23 ± 0.79	8.13 ± 0.78	6.48 ± 0.80	6.35 ± 0.88
	X14	10.04 ± 0.44	9.15 ± 0.65	8.77 ± 0.72	10.29 ± 0.56	8.89 ± 0.72	7.62 ± 0.76	10.55 ± 0.69	9.28 ± 0.78	8.39 ± 0.85
y3 (%)	X11	8.89 ± 0.73	9.28 ± 0.44	8.89 ± 0.46	8.64 ± 0.82	8.64 ± 0.62	8.77 ± 0.45	8.01 ± 0.99	8.89 ± 0.67	8.26 ± 0.49
	X12	6.35 ± 0.92	8.26 ± 0.69	6.73 ± 0.57	6.86 ± 0.96	8.13 ± 0.84	6.10 ± 0.74	6.48 ± 1.09	7.62 ± 0.92	6.99 ± 0.72
	X13	6.61 ± 1.08	6.48 ± 0.88	4.57 ± 0.67	6.35 ± 1.07	5.34 ± 0.99	3.43 ± 0.82	6.86 ± 1.37	5.34 ± 1.04	4.32 ± 0.88
	X14	8.89 ± 1.27	7.24 ± 0.99	6.38 ± 0.88	8.13 ± 1.35	7.24 ± 1.09	7.12 ± 0.93	7.75 ± 1.53	6.99 ± 1.18	6.86 ± 0.97
y4 (seed/hill)	X11	2.98 ± 0.12	3.16 ± 0.13	3.20 ± 0.15	5.92 ± 0.10	6.12 ± 0.14	6.16 ± 0.15	8.99 ± 0.09	9.11 ± 0.13	9.26 ± 0.14
	X12	2.95 ± 0.14	3.11 ± 0.16	3.14 ± 0.13	5.85 ± 0.12	6.08 ± 0.14	6.13 ± 0.13	8.82 ± 0.13	9.09 ± 0.15	9.18 ± 0.10
	X13	2.91 ± 0.15	3.08 ± 0.13	3.09 ± 0.13	5.83 ± 0.14	6.04 ± 0.12	6.09 ± 0.14	8.73 ± 0.13	9.06 ± 0.12	9.12 ± 0.14
	X14	2.85 ± 0.13	3.03 ± 0.12	3.06 ± 0.17	5.78 ± 0.11	6.02 ± 0.13	6.04 ± 0.16	8.72 ± 0.10	9.03 ± 0.13	9.07 ± 0.16
y5 (%)	X11	36.38 ± 1.20	31.20 ± 1.24	29.81 ± 1.29	22.36 ± 1.21	20.59 ± 1.28	19.85 ± 1.33	16.40 ± 1.34	15.41 ± 1.38	15.15 ± 1.40
	X12	37.92 ± 1.12	32.62 ± 1.21	32.42 ± 1.27	23.03 ± 1.18	21.07 ± 1.27	20.59 ± 1.30	18.95 ± 1.33	15.83 ± 1.28	16.45 ± 1.37
	X13	38.9 ± 1.10	36.36 ± 1.17	34.35 ± 1.25	23.73 ± 1.16	21.78 ± 1.24	20.75 ± 1.27	19.05 ± 1.26	17.21 ± 1.28	17.12 ± 1.34
	X14	39.61 ± 0.97	39.29 ± 1.15	35.62 ± 1.21	24.16 ± 1.07	22.75 ± 1.21	22.73 ± 1.24	21.07 ± 1.18	18.34 ± 1.26	17.81 ± 1.32

. The precision of the seeding rate adjusting mechanism directly affects the seeding performance. When the adjusting mechanism was transferred to different positions, the adjusting depth was different, and the theoretical seeding rate was also different. Taking the average seeds per hill and the variation coefficient of the seed number as the experimental indexes, the seeding accuracy of the seed-metering device was obtained, as shown in Table 3.

When the theoretical number of seeds per hill was 3, 6, and 9, respectively, the effects of the rotational speed and rice seed sphericity on the average seeds per hill and the variation coefficient of seed number are illustrated in Figure 7. The experimental results showed that when the theoretical seeds per hill were 3, 6, and 9, the average seeds per hill were 3.05, 6.01, and 9.02, and the average variation coefficients of the seed number were 35.37%, 21.95%, and 17.40%, respectively. When the rice seed sphericity and adjustment depth were constant, with the increase in the seeding wheel speed, the average seeds per hill decreased, and the variation coefficient of the seed number increased. This is because the time for the seeds to enter the hole was shortened due to the increased rotation speed. When the rotational speed and the adjustment depth were constant, with the increase in rice seed sphericity, the average seeds per hill increased and the coefficient of variation decreased. This is mainly because the more extensive the rice seed sphericity, the smaller its long axis size, the closer the shape of the rice seed to the spherical shape, the easier it is for seeds to enter the hole, and the more stable the posture of rice seeds entering the hole.

The variation coefficient of the seed number varied significantly with the change in the adjustment depth. The greater the adjustment depth, the longer the seeding rate adjustment section in the seed filling area, the bigger the pre-filling curve section, the more stable the number of seeds entering the hole, and the smaller the variation coefficient of the seed number. Similarly, the smaller the adjustment depth, the shorter the seeding rate adjustment curve section in the seed filling area, the smaller the pre-filling curve section, the more unstable the number of seeds entering the hole, and the greater the variation coefficient of the seed number. When the theoretical seeding rate was 3, 6, and 9 seeds, compared to the theoretical value, the average seeds per hill in holes changed slightly, and the variation coefficient of the seed number changed a little. The adjustment mechanism could adjust the seeding rate in the range of 3–9 seeds, and the seeding accuracy of the seed-metering device met the requirements.

3.2. Seeding Performance Experiment

The seeding performance of the seed-metering device was tested by taking the qualified rate, missing rate, and reseeding rate as experiment indexes. The experimental results in Table 3 show that when the rotation speed of the metering wheel was 30 r/min, the sphericity of the rice seeds was 52.7%, the hole depth was 6 mm, and the qualified rate, missing rate, and reseeding rate of the seed-metering device were 90.34%, 6.23%, and 3.43%, respectively. Under all hole depths, the qualified rate of the metering device reached 90%, the missing rate was less than 4.7%, and the reseeding rate was less than 4.57%. The adjustment of the hole depth had little influence on the experimental index of the seed-metering device.

The influence of different experiment factors on the seeding performance index is shown in Figure 8. With the increase in the rice seed sphericity, the qualified rate of the seed-metering device showed an upward trend, and the missing rate showed a downward trend. The main reason was that the higher the sphericity of the rice seeds, the closer the shape to the sphericity, and the easier the seeds enter the hole. When the rotation speed of the seeding wheel was 20–30 r/min, the qualified rate was on the rise, and the reseeding rate showed a downward trend. When the rotation speed exceeded 30 r/min, the qualified rate fell rapidly and the reseeding rate rose rapidly. The main reason was that when the seeding wheel turns to the seeding area with the change in the track arc, the forced seeding mechanism moves and has a specific vibration effect on the seed-metering device. The more vigorous the vibration, the better fluidity of the seeds, and the easier the rice seeds enter the hole. The influence of the vibration of the seeding wheel on the seeding performance will be further explored in later research. The missing rate was proportional to the rotation speed. Since the rotation speed was too high, the seed filling time was insufficient. In practical applications, a specific seeding efficiency is needed. Thus, it is necessary to control the rotation speed of the seeding wheel, adjust the hole depth, and improve the sphericity of rice seeds so that the performance of the seed-metering device can be optimized.

Univariate analysis of variance was conducted on the experiment results shown in

Table 4. Univariate analysis of variance.

Indicators	Factors	SS	Df	MS	F	p	Significance	R2
y1 (%)	X1	103.39	3.00	34.46	124.17	0.00	**	0.963
	X2	98.06	2.00	49.03	176.66	0.00	**
	X3	0.47	2.00	0.24	0.85	0.44
y2 (%)	X1	77.70	3.00	25.90	65.08	0.00	**	0.929
	X2	67.98	2.00	33.99	85.40	0.00	**
	X3	0.57	2.00	0.29	0.72	0.50
y2 (%)	X1	47.51	3.00	15.84	32.68	0.00	**	0.803
	X2	6.56	2.00	3.28	6.77	0.00	**
	X3	1.06	2.00	0.53	1.10	0.35

** representative significance (p < 0.01).

. The analysis results indicated that the adjustment depth had no significant influence on the performance index of the seed-metering device (p > 0.05). In contrast, the rotation speed of the seeding wheel and the seed sphericity had an extremely significant influence on the performance index of the seed-metering device (p < 0.01). The model R₂ between the qualified rate of the seed-metering device and the experimental factors was 0.963, indicating that the experimental factors could explain the change in the qualified rate by 96.3%. The model R₂ between the missing rate and the experimental factors was 0.929, indicating that the experimental factors could explain the change in the missing rate by 92.9%. The model R₂ between the reseeding rate and the experiment factor was 0.803, indicating that the experimental factors could explain the change in the reseeding rate by 80.3%.

According to the results of variance analysis, the rotation speed of the seeding wheel and the sphericity of rice seeds had significant differences in the results. It was demonstrated that the difference between the level averages was significant, but it was not determined whether there was a significant difference between any two averages. Therefore, the factors with significant differences were analyzed by multiple comparative analyses. The results are presented in Figure 9, where the letters indicate a significant difference of 5%.

4. Discussion

In conditions of high speed and low sphericity, the qualified rate of the designed seed meter was lower but still greater than 81%. When the regulated depth was small, the coefficient of variation was relatively higher, while when the regulated depth was deep, the average seeds per hill became larger than the theoretical seeding number, so it requires further optimization for the hole diameter and regulated depth.

It is important to note that the seed metering system is a complete system, and seeding performance should be determined by the number of holes, the seed filling degree, the seed throwing degree, the seed clearing mechanism, and the angle required to complete maximum seeding. In this paper, the main parameters of the seed-metering device were designed and tested, but further research is required to determine the optimal combination of parameters for other components. This forced seeding mechanism is an innovation in this paper and the spring performance can directly affect the seeding performance, so it needs further research on the spring effect on seeding performance.

This study selected rice considering different sphericities, while it lacks verification experiments for more varieties. To achieve longer-lasting results, it is necessary to strengthen the adaptability of the seed-metering device. The performance evaluation of the seed-metering device can be considered from multiple perspectives. In the later stage, the comprehensive performance of the seed-metering device should be studied from various perspectives, including the evaluation indicators of hole distance, hole diameter, and empty hole rate.

5. Conclusions

In this study, a direct seed-metering device was developed and the critical component parameters were investigated. The seed-metering device could meet the requirements of direct seeding rice in the rice seed sphericity range of 35.0 to 52.7% and could adjust the seeding amount continuously in the range of 3 to 9 rice seeds.

The results of the seeding accuracy experiment showed that when the theoretical seeding quantity were 3, 6, and 9, the average seeds per hill were 3.05, 6.01, and 9.02, and the average coefficient of variation of the seed number was 35.37%, 21.95%, and 17.40%, respectively. The results of the seeding performance experiment demonstrated that when the speed of the seeding wheel was 30 r/min, the sphericity of rice seed was 52.7%, and the hole depth was 6 mm, the qualified rate, missing rate, and reseeding rate of the seed-metering device were 90.34%, 6.23%, and 3.43%, respectively, and the performance of the seed meter reached the optimal level. When the seeding wheel’s speed increased, the average number of seeds per hill decreased, and the variation coefficient of seeds increased. As rice seed sphericity increased, the average number of seeds per hill increased, and the coefficient of variation decreased. The missing rate was proportional to the rotation speed. The qualified rate of the seed-metering device went up as rice seed sphericity increased, and the missing rate went down. As seeding wheel rotation speed decreased, qualified rates increased, and reseeding rates decreased. The qualified rate dropped rapidly when rotation speed exceeded 30 r/min, and the reseeding rate rose rapidly.

The variance analysis results indicated that the adjustment depth had no influence on the qualified rate, missing rate, and reseeding rate of the seed-metering device (p > 0.05). However, the speed of the seeding wheel and the sphericity of the rice seed had a significant influence on the qualified rate, missing rate, and reseeding rate of the seed-metering device (p < 0.01). Under different conditions, the seeding qualified rate of the seed-metering device was more than 81.07%, which met the requirements of precision direct seeding of rice and provide a reference for precision direct seeding of rice.