Optimal Design and Experiment of Critical Components of Hand-Pushing Corn Plot Precision Planter

Abstract

The mechanized sowing operation of corn breeding can improve test accuracy and efficiency and speed up the test process. In view of the problems of low flexibility, high price, and poor sowing quality of the current seeder in corn breeding experiments, the critical components of a hand-pushing corn plot precision planter were optimized. Through the analysis of the relative motion between corn seed and the hole seed-metering wheel, the hole diameter range and hole depth range of the hole seed-metering wheel were determined as 13~19 mm and 6~12 mm, respectively. The chamber structure was optimized to improve the seed filling performance under the condition of a small number of seeds. The angle range between the connecting plate and the horizontal line (ACP) and the angle range between the back plate and the horizontal line (ABP) were 35~55° and 40~70°, respectively. EDEM was used to simulate the seeding circumstances of hole seed-metering wheel with different holes sizes. The optimal combination of hole diameter and depth under different seeds’ sizes and the appropriate angle combination of ACP and ABP were obtained through the orthogonal test. The results of the simulation experiment showed that the optimal hole diameter–depth combinations of the small, medium, and large corn seeds were 13–8, 15–10, and 17–8 mm, respectively. The angle ACP and the angle ABP were, respectively, 45°and 70°. The soil bin test was carried out to further verify the actual seeding effect and the influence of the rotating speed of hole seed-metering wheel on seeding. The results showed that when the rotating speed of hole seed-metering wheel was less than 16.0 r/min, the qualified seeding rates and the double seeds rates of the small, medium, and large corn seeds were higher than 90% and 85%, respectively. The missing seeding rates were 0%. The variation coefficients of plant spacing were less than 5%, indicating good stability of plant spacing, which met the design requirements. The study results can provide a theoretical reference for the optimization design of maize breeding equipment in a plot.

1. Introduction

Breeding improved corn varieties is one of the most effective means of increasing corn yield [1,2]. Experiments aimed at breeding improved varieties can make breeding materials show authentic and accurate genetic differences in the selection and identification process. Suitable field breeding equipment for plots is of great significance for realizing the mechanization of production for plot breeding, shortening the breeding cycle, and improving breeding efficiency and precision [3].

The seeding characteristic of the plot experiment is a “large number of plots, small amount of seed sowing demand of each plot.” Compared with a field seeding operation, the planter used in the plot breeding experiment has more strict requirements, including quantitative seed delivery, thorough seed clearing, uniform spacing of seeds, and consistent sowing depth [4]. Some scholars have conducted many studies on seeding machinery for special use in breeding experiments and have established a relatively complete seeding technology system that integrates mechanization and breeding intelligence [5,6]. The 500 and 600 Series Plot Seed Drills of SRES (US), the Dynamic Disc single-seed seeder of Wintersteiger (Austria), and the SB-25, SR-30, and SNT-25 series seeders of Haldrup (Germany) can all be applied to corn plot seeding operations [7,8,9]. The above seeders are equipped with advanced technologies such as automatic seed supply, seeding positioning controller, GPS positioning technology, etc., which can achieve accurate plant spacing (one seed per hole), and no mixed precision seeding [10]. However, the above planters are mainly large, expensive, and complex. It is difficult to replace and repair spare parts, which is unsuitable for the basic national conditions of small plots in China and difficult to apply in a widespread manner.

The breeding experiment in China adopted the traditional manual seeding method, which seriously affected the breeding experiment’s accuracy and efficiency. Some scholars have conducted much research on plot seeders in recent years to replace traditional manual sowing with mature and stable seeders in China. Yang Wei et al. [11] designed an air-suction plot precision seed meter of corn with a self-clearing seed chamber. The process of seed discharge and seed clearing did not interfere with each other. The missing seeding rate and the reseeding rate were less than 1% and 4%, respectively. The working width of the seeder is 1.8 m with four rows. Cui Gongpei et al. [12] designed a canvas cone belt plot seeder with six rows and a width of 1.8 m, which could achieve continuous field experiments with different sowing quantities and varieties in the same experimental plot. Zhou Jiapeng et al. [13] developed a precision corn single-seed drill for plot breeding, with four rows, 1.6 m in width, with a 4.63% missing rate. Most of the seed meters of the above plot planters are air suction or cone canvas belt. In addition, the plot planters’ rows exceed 4, and the width exceeds 1.5 m, and there was a missing seeding phenomenon. The principle of the plot planter would rather involve repeating-seeding than missing-seeding. If there is leakage during the sowing, the whole plot of the test would be scrapped. However, single-seed precision sowing cannot ensure that there is no missed sowing and seed-emergence rate, which will increase the scrappage rate of the plot. When applied to the sowing operation of plot breeding, there are still some problems, such as poor flexibility, high price, and difficulty in meeting the needs of low-cost and high-quality for small- and medium-sized breeding research institutes in China. Therefore, it is necessary to develop a simple and flexible seeder for maize breeding [14,15]. The hand-pushing plot seeder has the advantages of high sowing efficiency, high emergence rate, and good sowing effect, which can meet the requirements of corn plot breeding experiments. However, few mature products are in the market [16,17].

Given the above problems, according to the seeding requirements of the corn plot breeding experiment in China, this paper optimized the critical components of the hand-pushing precision corn plot seeder. It can achieve precision sowing of a small number of seeds and promote sowing mechanization in plot breeding experiments.

2. Materials and Methods

2.1. Structure and Principle of Hand-Pushing Plot Seeder

The overall structure of the hand-pushing corn plot precision planter is shown in Figure 1, which mainly consists of the rotary bin type seed changer, the hole seed-metering wheel, the closing wheels, the seed-clearing brush, the seeder shell, and the duckbill devices. The hole seed-metering wheel is the critical component of the plot seeder to realize precision seeding. The seven duckbills are evenly arranged around the planter to play the role of fixed point and fixed distance sowing. The rotary bin type seed changer is connected to the shell through the top two ear handles. The chamber structure formed by the rotary bin type seed changer separated the filling area from the column-type seed clearing area, which can reduce the filling area space, increase the degree of seed aggregation, and improve the filling quality. Meanwhile, the rotary bin type seed changer can concentrate the remaining seeds to clear seeds after the sowing operation, avoiding the “mixed seeds” phenomenon in a breeding experiment. After sowing, the closing wheels would close the seeding ditch, and the duckbill devices can ensure the same sowing depth and plant spacing.

The operation process of the hole seed-metering wheel is divided into four stages: seed filling, seed clearing, seed carrying, and seed discharging, as shown in Figure 2. Before the sowing, rotate the rotary bin type seed changer at a certain angle to separate the seed clearing area and filling area, preventing seeds from falling into the seed changer and reducing the sowing quality. During operation, push the hand push rod to drive the duckbill devices to rotate. The duckbill is provided with a lever, which is turned at a certain angle to drive the transmission gear of the hole seed-metering wheel and the transmission gear of the seed cleaning brush to rotate in the same direction. The seed cleaning brush removes the surplus seeds in the hole. The duck’s beak is inserted into the ground to open a hole, and the seeds fall into the hole to complete the sowing. Repeat the above process to complete the seeding operation of the entire plot. After completing the sowing for a plot, rotate the rotary bin type seed changer to concentrate all the remaining seeds in the storage chamber of the rotary bin type seed changer. Then, pull out the rotary bin type seed changer, and remove the seeds from the rotary bin type seed changer.

2.2. Determination of Physical Characteristics Parameters of Corn Seeds

Fourteen representative maize varieties were selected for triaxial size measurement. During the measurement, 500 seeds were randomly selected for each variety. We used a vernier caliper with an accuracy of 0.01 mm to measure the size parameters of corn seeds in three directions: length (L), width (S), and thickness (T). The mean values of the size parameters of 14 different varieties of corn seeds were summarized, as shown in

Table 1. Size parameters of corn seeds of different varieties.

Index	Corn Varieties
	1	2	3	4	5	6	7
Length (mm)	10.90	10.58	12.96	11.16	10.34	12.55	11.24
Width (mm)	8.33	7.61	7.61	6.50	7.45	7.19	6.17
Thickness (mm)	6.17	6.00	4.13	4.55	5.55	4.51	3.64
Index	Corn Varieties
	8	9	10	11	12	13	14
Length (mm)	13.54	9.40	12.27	10.22	9.69	7.80	10.38
Width (mm)	7.31	6.36	8.63	7.38	7.24	7.58	8.09
Thickness (mm)	4.21	4.81	4.84	5.14	5.30	6.33	6.51

.

2.3. Structure Design of the Hole Seed-Metering Wheel

Based on the size parameters of corn seeds of 14 varieties, combined with the seeding requirements of the plot breeding experiment [18] and the precision seeding requirements of the optimal number of seeds in the hole (two seeds), the hole seed-metering wheel was designed through the kinematic analysis of the hole filling process.

During sowing, when the hole seed-metering wheel rotates, there is a relative velocity v of the seed relative to the hole seed-metering wheel for forced seed filling into the hole. The kinematic analysis of seed filling in the hole is shown in

Table 2. Relative motion analysis of corn seed and the hole seed-metering wheel.

Filling Gesture	Schematic Diagram	Relative Motion Analysis
Horizontal filling into the hole		The tangential direction of the hole seed-metering wheel:
		$W-\mathrm{A}=2\pi nrt$	(2)
		Axis direction of hole seed-metering wheel:
		$\mathrm{B}=\frac{1}{2}g{t}^2$	(3)
		Required diameter of seed filling into hole horizontally:
		$W=\mathrm{A}+2\pi nr\sqrt{\frac{2\mathrm{B}}{g}}$	(4)
Turn 90° filling into the hole		The tangential direction of the hole seed-metering wheel:
		$W-\mathrm{B}=2\pi nrt$	(5)
		Axis direction of hole seed-metering wheel:
		$\mathrm{B}=\frac{1}{2}g{t}^2$	(6)
		Required diameter of the seed filling into hole by turn 90°:
		$W=\mathrm{B}+2\pi nr\sqrt{\frac{2\mathrm{B}}{g}}$	(7)
1: Hole seed-metering wheel, 2: corn seed Where: W is the hole diameter, mm; A is half width of the corn seed section, mm; B is half height of the corn seed section, mm; r is the radius of the hole seed-metering wheel, mm; v is the relative velocity between corn seed and hole seed-metering wheel, m/s; g is the acceleration of gravity, m/s2; t is the time when seeds are successfully filled into the hole, s; n is the rotating speed of hole seed-metering wheel, r/min.

, where the relative velocity v is

$$v=2\pi nr.$$

(1)

where v is the relative velocity between corn seed and hole seed-metering wheel, m/s; n is the rotating speed of hole seed-metering wheel, r/min; r is the radius of the hole seed-metering wheel, mm.

According to the Agricultural Machinery Design Manual [19], the shape of the hole on the hole seed-metering wheel is cylindrical, including two design parameters: the diameter (DMH) and the depth (DH) of the hole.

2.3.1. Hole Diameter

During sowing, corn seeds are filled into the hole by the relative movement between the hole seed-metering wheel and corn seeds to achieve efficient seed filling [20,21]. According to the statistics of the size parameters of 14 different varieties of corn seeds, most corn seeds used in breeding experiments were dent and spheroid seeds. There is no significant difference in the size parameters of length, width, and thickness of spheroid corn seeds. The spherical corn seeds were considered a particular case of dent corn seeds when the kinematic analysis of the seed filling process was carried out. In the process of seed filling, corn seeds would fill the hole in various states. The relationship between the required hole diameter and the size of the corn seed was analyzed by selecting two seed filling methods: horizontal filling hole and 90° turning filling hole.

When corn seed is horizontally filled into the hole, there may be six filling poses of seeds in the cross-section direction of the hole seed-metering wheel, as shown in Figure 3. When the corn seed is turned 90° to fill the hole, there may be three filling poses in the cross-section direction, as shown in Figure 4.

According to the kinematics analysis of the seed filling process of 14 different varieties of corn seeds, the diameter of the hole required by nine filling poses of different varieties was calculated. The hole’s maximum and minimum required diameters were selected as the required range of the hole diameter to ensure that 14 different varieties of corn seeds can be successfully filled into the hole, as shown in

Table 3. Summary of the hole diameter range of different maize varieties.

Index	Corn Varieties
	1	2	3	4	5	6	7
Upper limit (mm)	16.00	15.68	17.98	16.25	14.46	17.60	16.33
Lower limit (mm)	10.58	10.83	8.54	9.12	10.31	9.04	7.92
Index	Corn Varieties
	8	9	10	11	12	13	14
Upper limit (mm)	18.52	14.50	17.33	15.32	14.79	12.82	15.48
Lower limit (mm)	8.66	9.47	12.87	9.83	10.00	11.21	11.41

.

To ensure that each seed is successfully filled into the hole at any pose, the selection method of target hole diameter was selected as shown in Equation (8) [21]:

$$W_{lmax}\le W\le W_{hmax,}$$

(8)

where $W_{lmax}$ is the maximum of the lower limit of the hole’s required diameter of 14 different varieties of corn seeds, mm, $W_{hmax}$ is the maximum of the upper limit of the hole’s required diameter of 14 different varieties of corn seeds, mm.

It can be seen from Table 3 that the hole required diameter ranges from 12.87 mm ($W_{lmax}$) to 18.52 mm ($W_{hmax}$). Based on the theoretical and practical requirements, it is concluded that the range of hole diameter was 13~19 mm.

2.3.2. Hole Depth

The hole depth should satisfy that the seeds can enter the hole smoothly and be stored stably in the hole. There is at least one corn seed in the hole after passing through the seed filling area. When more than three seeds appear in the hole, the cleaning brush clears the excess seeds out of the hole. In the seed discharging area, it is necessary to ensure that the corn seeds in the hole are separated from the holes in time to prevent seeds from getting stuck and causing missed sowing.

1. Hole minimum depth.

The selection of hole depth should meet the following requirements: at least one corn seed should be stored in the hole, and the seed should not be cleared in the seed cleaning area. Therefore, the minimum hole depth must meet the following conditions:

$$H_{min}\ge T_{ave,}$$

(9)

where H_min is the minimum required hole depth (mm) and T_ave is the average thickness of corn seed (mm).

According to the measurement results of size parameters of corn seed in Table 1, T_ave = 5.12 mm. Based on the thickness of various varieties of corn seeds and the practical processing requirements, H_min = 6 mm was chosen as the minimum hole depth.

2. Hole maximum depth.

The maximum hole depth should meet the requirements that the hole can store at most three corn seeds. Excess corn seeds of the hole need to be cleaned out by the seed cleaning brush in the seed cleaning area. When there are three corn seeds in the hole, the seeds are most arranged with two laid flat and one laid upright [19], as shown in Figure 5. Therefore, the maximum hole depth should meet the following conditions,

$$L_{ave}\le H_{max}\le 2T_{max}$$

(10)

where L_ave is the average corn seed length (mm), H_max is the maximum hole depth (mm), and T_max is the maximum thickness of corn seed (mm).

According to the measurement results of size parameters of corn seed in Table 1, L_ave = 10.9 mm, and T_max = 6.51 mm. Based on the thickness of various varieties of corn seeds and the practical processing requirements, H_max = 12 mm was chosen as the maximum hole depth.

Therefore, it is concluded that the range of hole depth is as follows: 6~12 mm.

2.3.3. Number of Holes

The more holes on the hole seed-metering wheel, the linear speed of the hole seed-metering wheel is lower, which is conducive to improving the hole-filling performance. However, the number of holes is limited by the diameter of the hole. According to the Agricultural Machinery Design Manual [19], the diameter of the hole seed-metering wheel was designed to be 120 mm. Combined with the maximum diameter of 19 mm and the maximum depth of 12 mm, the maximum number of holes is 15.9, according to Equation (11). Considering that there should be a space between the holes, the number of holes is determined to be 15. A three-dimensional model of the hole seed-metering wheel was established in SolidWorks, as shown in Figure 6. We have

$$m_{max}=\frac{\pi \left(d-2H_{max}\right)}{W_{max}},$$

(11)

where m_max is the maximum number of holes, d is the diameter of the hole seed-metering wheel (mm), and W_max is the largest hole diameter (mm).

2.4. Determination of Rotating Speed of Hole Seed-Metering Wheel

The normal walking speed of the human body is the moving forward speed of the hand-pushing maize plot precision planter. According to the transmission mechanism of the plot precision planter [22], the rotating speed calculation formula of the hole seed-metering wheel is as follows [19],

$$n=\frac{6000V}{m\times l},$$

(12)

where m is the number of holes, l is the theoretical plant spacing (cm), and V is the forward speed of the planter (m/s).

The theoretical plant spacing of the hand-pushing precision seeder was set as 25 cm. The normal walking speed range was assumed to be 0.6~1.2 m/s. The rotating speed range of the hole seed-metering wheel calculated from Equation (12) was 9.6~19.2 r/min. It can realize the seeding operation of different plant spacing by increasing or decreasing the number of duckbills and replacing the hole seed-metering wheel with a different number of holes.

2.5. Optimization Design of Seeder Chamber Structure

The chamber of the seeder needs to concentrate the seeds as much as possible to facilitate seed filling, so it should not be too large. Therefore, it is necessary to determine the distribution regularities of corn seeds in the chamber of the seeder to ensure that the hole can still effectively fill seeds in the case of a small number of seeds.

The sowing experiment of corn breeding in the plot is carried out in quantitative seeds and fixed area blocks. In the breeding experiment, the minimum sowing area of the block is 6 m², that is, two rows, 5 m per row. Therefore, the demand for sowing seeds in the minimum block can be calculated according to Equation (13),

$$M=\frac{S}{L}\times H\times e,$$

(13)

where M is the theoretical minimum demand for corn seed in the sown block, S is the sowing line length (cm), and L is the theoretical plant spacing (cm). According to the density setting of corn plot breeding experiments in the past decade, a planting density of 67,500 plants per hectare was selected, which meant that the plant spacing was 25 cm and the row spacing was 60 cm. H is the number of seeding rows, and e is the optimal number of seeds in the sowing hole. Two seeds in each hole were selected in this paper to reduce the plot’s scrap rate and ensure the experimental plot’s emergence rate.

According to Equation (13), when the sowing area of the block is 6 m², the theoretical minimum demand for sowing seeds is 80 per plot. The number of corn seeds is relatively small. Totals of 80, 90, 100, and 110 corn seeds were put into the planter to analyze the distribution law after seeds fall into the seed chamber, respectively, as shown in Figure 7.

It can be seen from Figure 7 that corn seeds were mainly distributed on both sides of the hole seed-metering wheel. The corn seeds fall into the holes by their gravity during the filling process. If the corn seeds were concentrated on both sides of the hole seed-metering wheel, the seeds might rely on the component force of their gravity to complete seed filling, which would reduce the quality of seed filling, resulting in missed sowing.

Increasing the length of the hole seed-metering wheel in the seed filling area can increase the rotating time of the hole seed-metering wheel in the seed filling area, improving the disturbance to the seed and the seed filling effect. Based on the existing chamber structure of the seeder, the chamber structure was optimized to realize effective seed filling with a small number of seeds. The schematic diagram of the optimized chamber structure is shown in Figure 8.

The rotary bin type seed changer separates the seed-filling area from the column-type seeds-changing area. The angle between connecting plate and the horizontal direction (ACP) and the angle between the back plate and the horizontal direction (ABP) significantly influence the seed filling quality of corn seeds. If the ACP and ABP are too small, the corn seed cannot slide down, resulting in poor filling quality. If the ACP and ABP are too large, it would affect the seeds clearing and changing. The ABP and ACP must be greater than the static friction angle of corn seeds, which is generally 20~30° [23]. Based on the shell size and the static friction coefficient of the seeds, it is concluded that the angle range of ACP is 35~55°, and the angle range of ABP is 40~70°.

2.6. Performance Simulation Test of Critical Components Based on EDEM

2.6.1. Establishment of Discrete Element Model of Corn Seeds

Through the appearance observation and size parameter measurement of 14 different varieties of corn seeds, the corn seeds were divided into large, medium, and small levels according to the shape and size during the modeling of corn seeds. Large and small seeds are mostly the shape of dent, and medium corn seeds are primarily spherical. The mean values of size parameters of corn seeds are shown in

Table 4. Average value of size parameters of corn seeds after classification.

Index	Size Parameters of Corn Seeds
	Length/mm	Width/mm	Thickness/mm
Large seed	12.51	7.39	4.27
Medium seed	10.67	7.60	5.76
Small seed	9.51	7.14	5.40

. Discrete models were established for the three sizes of seeds based on EDEM, as shown in Figure 9.

2.6.2. Planter Model Establishment

We design and establish the 3D model of the seeding apparatus with Solidworks, eliminate unnecessary and non-contacting structures, and save it as a .step file. We import the .step file to EDEM, and grid the model, as shown in Figure 10. Seed particle properties and the interaction parameters between corn seeds and planter components were obtained by referring to relevant references [24], as shown in

Table 5. Simulation parameter setting.

Item	Variable	Value
Corn seeds	Poisson’s ratio	0.4
	Shear modules/MPa	1.37 × 108
	Solid density/(kg·m−3)	1.197
Seed box and hole seed-metering wheel	Poisson’s ratio	0.35
	Shear modules/MPa	1.3 × 109
	Solid density/(kg·m−3)	1.20
Seeds to seeds	Coefficient of restitution	0.182
	Coefficient of static friction	0.0338
	Coefficient of rolling friction	0.0021
Seeds to seed box and hole seed-metering wheel	Coefficient of restitution	0.709
	Coefficient of static friction	0.459
	Coefficient of rolling friction	0.0931

.

2.6.3. Simulation Test on the Performance of the Hole Seed-Metering Wheel

An orthogonal experiment was designed to study the effects of hole diameter and depth on the filling performance of corn seeds of different sizes. The factors and levels are shown in

Table 6. Factor level table of orthogonal test.

Levels	Diameter of Hole A/mm	Depth of Hole B/mm	Seeds
1	13	6	Large seed
2	15	8	Medium seed
3	17	10	Small seed
4	19	12

. Due to the small demand for seeds in the breeding experiment (seed demand of the minor plot (6 m²) in the breeding experiment: 80 seeds), a 20% margin was increased based on the minimum demand in the simulation experiment to prevent the missing sowing. Therefore, the total number of corn seeds was 96 in the simulation experiment.

2.6.4. Simulation Test of Chamber Structure Optimization

Aiming at the problems of complex structure and poor seed filling quality of the hole in the case of a small number of seeds, the structure of the existing plot planter chamber was optimized by changing the angle combination of the ACP and the ABP, as shown in

Table 7. Factor level table of orthogonal test.

Levels	ACP (°)	ABP (°)
1	35°	40°
2	45°	55°
3	55°	70°

.

Medium corn seeds were selected for the simulation test. The test parameters were set as follows: 96 corn seeds, the rotating speed of the hole seed-metering wheel 16.0 r/min, hole diameter 15 mm, and hole depth 10 mm.

2.6.5. Performance Evaluation Indices

The sowing quality was evaluated according to the China Agricultural Industry Standard NY/T 1628-2013 [25] and China National Standard GB/T 6973-2005 [26].

The main performance evaluation indices of the hole seed-metering wheel were the qualified seeding rate, missing seeding rate, and multiple seeding rate [27]. During the simulation test, the number of seeds in the hole was counted when the seed was ready to fall off in the seed discharging area. If the number of seeds was 1–3, it was qualified seeding. If the number of seeds was less than 1, it was missing seeding. If the number was more than 3, it was repeating seeding. According to the seeding requirements of the breeding experiment, the double seed rate was added as an evaluation index to evaluate the seed performance of the hole seed-metering wheel. Statistical formulas are displayed as follows:

$$R=\frac{n_0}{N_1}\times 100\%$$

(14)

$$Q_1=\frac{n_1}{N_1}\times 100\%$$

(15)

$$M=\frac{n_2}{N_1}\times 100\%$$

(16)

$$D_1=\frac{n_3}{N_1}\times 100\%,$$

(17)

where Q₁ is the qualified seeding rate (%), M is the missing seeding rate (%), R is the multiple seeding rate (%); D₁ is the double seed rate (%), n₀ is the number of holes that are filled with than 4 seeds, n₁ is the number of holes that are filled with 1~3 seeds, n₂ is the number of missed sowing (the number of seeds in holes: 0), n₃ is the number of 2 seeds per hole, and N₁ is the total number of seeds sown.

The main performance evaluation indices of the optimized chamber structure were the seed filling rate Q₂, and double seed rate D₁. The calculation formula is offered in Equation (17) and Equation (18). We have

$$Q_2=\frac{n_4}{N_1}\times 100\%,$$

(18)

where Q₂ is the seed filling rate of the hole (%), and n₄ is the number of holes that are filled with at least one seed.

2.7. Soil Bin Test

2.7.1. Experimental Arrangement

The corn seeds used in the experiment were Zhengdan 958, Jingke 528, and Jinongda 568. The size parameters of Jinongda 568, Jingke 528, and Zhengdan 958 belong to small corn seeds, medium corn seeds, and large corn seeds, respectively. The hole seed-metering wheel with the hole diameter of 13 mm and the hole depth of 8 mm was selected when sowing Jinongda 568 corn seeds. The hole seed-metering wheel with the hole diameter of 15 mm and the hole depth of 10 mm was selected when sowing Jingke 528 corn seeds. The hole seed-metering wheel with the hole diameter of 17 mm and the hole depth of 8 mm was selected when sowing Zhengdan 958 corn seeds. According to the simulation test results of the chamber structure optimization, the optimal chamber structure (the ACP of 45° and ABP of 70°) was selected during the test.

The soil bin was at the Key Laboratory of “Soil-Machine-Plant”, China Agricultural University, in November 2019. The experimental machine was a hand-pushing precision planter for plot breeding designed independently. A rotary tiller was used to break the soil into small particles, requiring consistent soil conditions for every treatment. After rotary tillage, the soil would naturally settle in the air for three days, to be as consistent as possible with the field sowing conditions. Before sowing, the plot area was divided according to the minimum sowing plot area of 6 m² in the breeding experiment.

According to the simulation test results, the seeding performance of the designed hole seed-metering wheel and the optimized chamber structure was verified at the rotating speed of the hole seed-metering wheel (RSP) of 9.6 r/min, 12.8 r/min, 16.0 r/min, and 19.2 r/min. The test factors and levels are shown in

Table 8. Test factors and levels.

Levels	RSP (r/min)	DMH-DH (mm)	ACP (°)	ABP (°)
1	9.6	Small seed 13–8	45°	70°
2	12.8	Medium seed 15–10
3	16.0	Large seed 17–8
4	19.2

RSP, rotating speed of the hole seed-metering wheel; DMH-DH, hole diameter- hole depth; ACP, angle between the connecting plate and the horizontal line; ABP, angle between the back plate and the horizontal line.

.

As shown in Figure 11, the plot area of each experimental plot was set as 12, i.e., 10 m long × 1.2 m wide. In the length direction of each test plot, the middle 5 m was taken as the sowing area, and 2.5 m was left at both ends as the buffer area to complete the filling and clearing seeds. A completely randomized design experiment with four replications was conducted according to Table 8. After sowing, the sowing quality can be known by digging seeds from the soil (Figure 12) and counting the number of seeds per hole.

2.7.2. Evaluation Indicators

Seeding quality evaluation indicators of the plot seeder mainly include metering performance indicators, which include qualified seeding rate, missing seeding rate, multiple seeding rate, and double seeds rate, and plant spacing evaluation indicators, which include average plant spacing, and the variation coefficient of plant spacing.

The calculation formulas of four evaluation indices of metering performance indicators are shown in Equations (14)–(17).

The plant spacing of 25 cm was selected for the sowing experiment. Five repeated sowing experiments were conducted for different levels of seeds. A 5-m standard measuring tape was laid out along the strip, and accumulated spacing readings were recorded. The variation coefficient of plant spacing is used to explain the consistency of plant spacing. The smaller the variation coefficient is, the better the designed seeder has the stability of plant spacing. The calculation formulas of average plant spacing and variation coefficient of plant spacing are shown in Equations (19)–(21),

$$S_M=\frac{1}{N}{\displaystyle \sum }_{i=1}^N{x}_i$$

(19)

$$\mathsf{\sigma }=\sqrt{\frac{1}{N}{\displaystyle \sum }_{i=1}^N{\left(x_i-S_M\right)}^2}$$

(20)

$$S_{\mathit{CV}}=\frac{\mathsf{\sigma }}{S_M} \times 100\%$$

(21)

where S_M is the average plant spacing (mm), x_i is the ith plant spacing value (mm), N is the total number of plant spacing data, σ is the standard deviation of plant spacing (mm), and S_CV is the variation coefficient of plant spacing (%).

3. Results and Discussion

3.1. Simulation Results of the Hole Seed-Metering Wheel under the Corn Seeds of Different Levels

3.1.1. Small Corn Seeds

A and B are factor level values. The range analysis was used with the experimental data to obtain the best parameter combination. The results of the range analysis are shown in

Table 9. Range analysis of small corn seeds.

Evaluation Indices		Factors
		A	B
Qualified seeding rate/%	k1	91.14	89.28
	k2	85.00	90.00
	k3	72.14	72.85
	k4	62.44	59.58
	Rj	28.7	30.42
Missing seeding rate/%	k1	2.86	5.00
	k2	1.43	0.00
	k3	0.72	0.00
	k4	0.00	0.00
	Rj	2.86	5.00
Multiple seeding rate/%	k1	5.00	5.72
	k2	13.57	10.00
	k3	27.85	27.85
	k4	37.55	40.42
	Rj	32.55	34.7
Double seeds rate/%	k1	59.28	43.57
	k2	55.71	65.71
	k3	33.57	35.71
	k4	23.35	26.93
	Rj	35.93	38.78

. The influences of different levels of hole diameter and depth on evaluation indices are shown in Figure 13. The larger the range of each factor is, the greater is this factor’s influence on the performance evaluation indices.

The variance analysis was used with the experimental data to clarify the significance of the influence of hole diameter and depth on the performance evaluation indices. The analysis results are shown in

Table 10. Variance analysis results of small seeds.

Evaluation Indices	Source	Sum of Squares	df	Mean Square	F-Value	p-Value
Qualified seeding rate/%	A	2101.04	3	700.35	4.379	0.037 *
	B	2547.19	3	849.07	5.309	0.022 *
	Error	1439.45	9	159.94
Missing seeding rate/%	A	17.857	3	5.952	1.00	0.436
	B	75.00	3	25.00	4.20	0.041 *
	Error	53.571	9	5.952
Multiple seeding rate/%	A	2528.9	3	842.9	6.414	0.013 *
	B	3113.8	3	1037.9	7.898	0.007 **
	Error	1182.7	9	131.4
Double seeds rate/%	A	3606.8	3	1202.3	6.47	0.013 *
	B	3311.4	3	1103.8	5.94	0.016 *
	Error	1672.7	9	185.8

Significance: **, p ≤ 0.01; *, p ≤ 0.05.

. Hole diameter is significant to the qualified seeding rate, the multiple seeding rate, and the double seed rate but not to the missing seeding rate. The above results are obtained because only when the hole depth is 6 mm, there is some missing seeding, as shown in Figure 13. When the hole depth is 8, 10, and 12 mm under any hole diameter, the missing seeding rates were 0. The hole depth has a significant influence on the four evaluation indices.

According to the variance analysis and range analysis, when sowing small corn seeds, the influence of the two factors (hole diameter and hole depth) on four evaluation indices, from big to small, were hole depth B and hole diameter A. With the increase of the hole diameter and hole depth, the seed filling space increased, leading to a rise in the multiple seeding rate and a decrease in the missing seeding rate. Considering the influence of the two factors on the sowing effect, as shown in Figure 13, B₂A₁ was the combination with the best sowing effect. When the combination was B₂A₁ (8 mm of the hole depth and 13 mm of the hole diameter), the qualified seeding rate and double seed rates were 97.14% and 88.57%, respectively. There was no missing seeding, meeting the requirements of the national variety test [28].

3.1.2. Medium Corn Seeds

The range analysis was used with the experimental data to obtain the best parameter combination. The results of the range analysis are shown in

Table 11. Range analysis of each index of medium seeds.

Evaluation Indices		Factors
		A	B
Qualified seeding rate/%	k1	89.58	88.54
	k2	94.79	91.14
	k3	88.57	92.19
	k4	85.94	86.98
	Rj	8.85	5.21
Missing seeding rate/%	k1	8.33	8.32
	k2	3.125	4.69
	k3	3.125	1.56
	k4	0.52	0.52
	Rj	7.81	7.80
Multiple seeding rate/%	k1	2.09	3.13
	k2	2.08	4.17
	k3	8.33	6.25
	k4	13.54	12.50
	Rj	11.46	9.37
Double seeds rate/%	k1	33.33	34.38
	k2	65.63	54.17
	k3	58.85	65.62
	k4	44.27	47.92
	Rj	32.3	31.24

. The influences of different levels of hole diameter and depth on seed metering performance evaluation indices are shown in Figure 14.

The variance analysis was used with the experimental data of the medium corn seeds. The analysis results are shown in

Table 12. Variance analysis results of medium seeds.

Evaluation Indices	Source	Sum of Squares	df	Mean Square	F-Value	p-Value
Qualified seeding rate/%	A	165.77	3	55.26	6.02	0.016 *
	B	68.07	3	22.70	2.47	0.128
	Error	82.61	9	9.18
Missing seeding rate/%	A	128.84	3	42.95	10.39	0.003 **
	B	148.30	3	49.43	11.96	0.002 **
	Error	37.19	9	4.13
Multiple seeding rate/%	A	367.79	3	677.95	59.89	0.00 ***
	B	211.46	3	122.59	34.43	0.00 ***
	Error	18.42	9	70.49
Double seeds rate/%	A	2525.30	3	842.77	2.87	0.096
	B	2034.98	3	678.32	2.31	0.145
	Error	2645.80	9	293.98

Significance: “***”: p ≤ 0.001; “**”: p ≤ 0.01; “*”: p ≤ 0.05.

. It can be seen that the hole diameter and hole depth are highly significant for the missing seeding rate and the multiple seeding rate but not for the double seed rate. The hole diameter also substantially impacts the seeding qualified seeding rate.

According to the variance analysis and range analysis, when sowing medium corn seeds, the two factors (hole diameter and hole depth) influencing four evaluation indices, from big to small, were hole diameter A and hole depth B. According to the requirements of the national variety test and the influence of two factors on the sowing effect, as shown in Figure 14, combination A₂B₃ (15 mm of the hole diameter and 10 mm of the hole depth) had better performance in seed metering quality than other combinations. When the combination was A₂B₃, the qualified seeding rate, double seeds rate, and multiple seeding rate were 97.92%, 89.58%, and 2.08%, respectively. More importantly, there was no missing seeding.

3.1.3. Large Corn Seeds

The range analysis was used with the experimental data of large corn seeds to obtain the best parameter combination. The results of the range analysis are shown in

Table 13. Range analysis of each index of large seeds.

Evaluation Indices		Factors
		A	B
Qualified seeding rate/%	k1	84.13	85.83
	k2	89.73	90.26
	k3	92.53	89.74
	k4	87.58	88.14
	Rj	8.4	4.43
Missing seeding rate/%	k1	14.91	11.83
	k2	8.27	6.81
	k3	1.96	5.01
	k4	0.91	2.41
	Rj	14.0	9.42
Multiple seeding rate/%	k1	0.96	2.34
	k2	2.01	2.93
	k3	5.51	5.26
	k4	11.52	9.46
	Rj	10.56	7.12
Double seed rate/%	k1	35.40	40.28
	k2	54.58	58.62
	k3	71.22	56.81
	k4	51.71	57.20
	Rj	35.82	16.92

. The influences of different levels of hole diameter and depth on seed metering performance evaluation indices are shown in Figure 15.

Variance analysis was carried out on the orthogonal experiment of large corn seeds, and the analysis results are shown in

Table 14. Variance analysis results of large seeds.

Evaluation Indices	Source	Sum of Squares	df	Mean Square	F-Value	p-Value
Qualified seeding rate/%	A	150.69	3	50.23	3.097	0.082
	B	47.57	3	15.86	0.978	0.445
	Error	145.97	9	16.22
Missing seeding rate/%	A	502.68	3	167.56	25.573	0.000 ***
	B	189.92	3	63.31	9.67	0.004 **
	Error	58.97	9	6.55
Multiple seeding rate/%	A	271.79	3	90.60	27.67	0.000 ***
	B	125.32	3	41.78	12.76	0.001 ***
	Error	29.48	9	3.28
Double seeds rate/%	A	2583.03	3	861.01	3.026	0.086
	B	901.55	3	300.52	1.056	0.415
	Error	2560.81	9	284.53

Significance: “***”: p ≤ 0.001; “**”: p ≤ 0.01.

. According to the results of variance analysis, it can be seen that the hole diameter and hole depth had a very significant influence on the missing seeding rate and multiple seeding rate but not on the qualified seeding rate and double seed rate.

According to the variance analysis and range analysis, when sowing large corn seeds, two factors (hole diameter and hole depth) influencing four evaluation indices, from big to small, were hole diameter A and hole depth B. Considering the influence of the two factors on the sowing effect, as shown in Figure 15, the combination A₃B₂ (17 mm of the hole diameter and 8 mm of the hole depth) had the most excellent seed metering quality compared with other combinations. When the combination was A₃B₂, the qualified seeding rate, double seed rate, missing seeding rate, and multiple seeding rate were 98.08%, 86.54%, 0%, and 1.92%, respectively.

3.2. Simulation Results of Chamber Structure Optimization

The simulation results of chamber structure optimization are shown in Figure 16. The ACP and the ABP are both key factors affecting the seed filling quality of the hole. The seed filling rate Q₂ and double seed rate D₁ increased with the increase of the ABP and showed a trend of increasing first and then decreasing with the increase of the ACP. When the ACP and the ABP were 45° and 70°, respectively, it had the best performance in the two indices of seed filling rate and double seeds rate compared with other combinations. The seed filling rate and double seeds rate were 99.56% and 88.68%, respectively, which indicated that the designed hole seed-metering wheel and the optimized chamber had an excellent seed filling effect under a small number of seeds and reached the expected design objective.

3.3. Results of Soil Bin Test

3.3.1. Plant Spacing

The average plant spacing and variation coefficient of plant spacing of three different levels of corn seeds are shown in

Table 15. Results of plant spacing.

	Mean Plant Spacing/mm	Variation Coefficient/%
Small seed	24.80	3.37
Medium seed	24.89	3.44
Large seed	24.97	3.79

. The average plant spacing of the three levels of seeds fluctuated around the theoretical plant spacing of 25 mm. The biggest difference from the theoretical plant spacing is the average plant spacing of small seeds, with a difference of 0.2 mm. The variation coefficient of plant spacing of large, medium, and small seeds were all less than 5%, which were 3.79%, 3.44%, and 3.37%, respectively. The small variation coefficient of plant spacing indicated that the designed hand-pushing plot seeder had good stability of plant spacing, which met the requirement of uniformity of plant spacing in the breeding experiment.

3.3.2. Metering Performance Indicators

A single-factor experiment was conducted to verify the seeding performance of the seeder under different rotating speeds of the hole seed-metering wheel. The test results of the large, medium, and small seeds at different rotating speeds are shown in Figure 17.

It can be seen from Figure 17a,b, with the continuous increase of the hole seed-metering wheel rotating speed, the qualified seeding rates, and double seed rates of the small, medium, and large corn seeds increased first and then decreased. When the rotating speed of the hole seed-metering wheel was 16.0 r/min, the sowing effects of the hole seed-metering wheels were better than that of other rotating speeds and had the best qualified seeding rate and double seeds rate. It can be seen from Figure 17c that when the hole seed-metering wheel rotating speed was lower than 16.0 r/min, the missing seeding rates of the small, medium, and large corn seeds were 0. When the rotating speed of the hole seed-metering wheel was 19.2 r/min, the corn seeds of three sizes all had some missed sowing, but the missing seeding rate was lower than 1.5%. It can be concluded from Figure 17d with the continuous increase of the rotating speed of the hole seed-metering wheel, the multiple seeding rate of small, medium, and large corn seeds had a decreasing trend. When the rotating speed of the hole seed-metering wheel was higher than 16 r/min, the downward trend became slower. The trend is because the seed filling time becomes more prolonged, and the seed filling effect increases with the decrease of the rotating speed of the hole seed-metering wheel. Therefore, when the rotating speed of the hole seed-metering wheel was 16 r/min, the seeding quality was optimal and there is no missing seeding phenomenon, as shown in

Table 16. Results of plant spacing.

Evaluation Indices	Small Seed	Medium Seed	Large Seed
Qualified seeding rate/%	98.22	97.96	98.25
Missing seeding rate/%	0.00	0.00	0.00
Multiple seeding rate/%	1.78	2.04	1.75
Double seeds rate/%	88.00	88.33	88.21

.

4. Conclusions

This paper optimized the critical components of the hand-pushing precision corn plot seeder to solve the problems, such as the difficulty of guaranteeing the quality of artificial sowing and the high cost of existing mature seeding machinery not meeting the needs of Chinese breeding. The optimal hole parameters and the chamber structural parameters were selected through the EDEM simulation test. A soil bin test was conducted to evaluate the actual operation effect of the optimized planter under the different rotating speed of the hole seed-metering wheel. The main research conclusions are as follows.

(1) Based on the analysis of sowing demand, a hole seed-metering wheel was designed and the chamber structure was optimized to improve the seed filling performance and achieve effective seed filling under the condition of a small number of seeds. Considering the overall size parameters of corn seeds, we determine the parameter range of the hole diameter as 13~19 mm, the hole depth as 6~12 mm, the hole seed-metering wheel diameter as 120 mm, the number of holes as 15, and the rotating speed range of the hole seed-metering wheel as 9.6~19.2 r/min. The angle range between the connecting plate and the horizontal line and the angle range between the back plate and the horizontal line were 35~55° and 40~70°, respectively.

(2) Based on the discrete element simulation analysis of EDEM under a small number of 96 seeds, the optimal combination of diameter–depth of hole corresponding to three different levels of corn seeds were 13–8 mm, 15–10 mm, and 17–8 mm, respectively. The angle between the connecting plate and the horizontal line and the angle between the back plate and the horizontal line were 45° and 70°, respectively.

(3) The soil bin test was used to evaluate the seeding performance of the optimized plot planter when a small number of seeds were sown. The results showed that when the rotating speed of the hole seed-metering wheel was 19.2 r/min, there was a phenomenon of missing sowing, which did not meet the requirements of the breeding test. When the rotating speed of the hole seed-metering wheel was less than or equal to 16.0 r/min, the qualified seeding rates and double seed rates of three different levels of corn seeds were higher than 90.0% and 85%, respectively. the missing seeding rates were 0%, which met the requirements of the breeding experiment. The measurement results of plant spacing in different levels of seeds showed that the variation coefficient of plant spacing of different levels of seeds were 3.37%, 3.44%, and 3.79%, respectively, which were all less than 5%, indicating good stability of plant spacing. It can provide a good theoretical reference for the research and development of mature and stable plot breeding seeder.