Design of and Experiment on a Cleaning Mechanism of the Pneumatic Single Seed Metering Device for Coated Hybrid Rice

Abstract

In order to improve the single-grain seeding rate of the pneumatic single seed metering device, an airflow seed cleaning device was designed in combination with positive pressure airflow. The influence of the position of the seed cleaning mechanism on the seed cleaning effect is theoretically analyzed and a flow field simulation test analysis of different nozzle structures was carried out by using Fluent software (ANSYS, Inc., Canonsburg, PA, USA). The results of this test show that a nozzle with a Witoszynski curve has good airflow concentration and uniform air pressure distribution. In order to verify the performance of the seed cleaning mechanism, a 0.7 times coated seed (hybrid rice Wuyou 1179) was used as the test material and a quadratic regression test with three levels was carried out with the rotation speed of the seed plate, the negative pressure of the suction chamber, and the positive pressure of the seed cleaning as the test factors. The results showed that when the speed of sucking plate was 30 r/min, the negative pressure of the suction chamber was 1.8 kPa and the positive pressure of the seed cleaning was 0.2 kPa; the seeding effect was at its best and the qualified rate of the seed metering device was the highest at 86.43%, the minimum leakage rate was 3.81%, and the multiple rate was 9.76%. The proposed seed cleaning mechanism effectively improves the accuracy of seeding and provides a certain theoretical basis for the single-grain sowing of hybrid rice.

1. Introduction

According to the agronomic requirements of rice planting, the technology of the direct seeding of rice into a precise amount of holes is intended to evenly distribute the rice seeds into the field at precise and fixed distances [1,2,3,4,5]. The types of direct seeding rice are mainly conventional rice and hybrid rice. The seeding rate per hole of conventional rice is generally 5–10 grains, while for hybrid rice with strong tillering ability, the seeding rate per hole is generally 2–4 grains during direct seeding [6,7,8]. With the continuous advancement of seed production technology, the quality of hybrid rice seeds has also improved and the number of seeds that is used is getting lower and lower. Single-grain seeding technology has the characteristics of saving time, labor, and seeds, which are conducive to promoting the process of the mechanization of rice production and improving the precision of seeding [9]. The current research on single-seed sowing technology focuses on crops with high seed sphericity, such as soybean, corn, and rapeseed, and there is little research on rice single-grain sowing technology. Therefore, it is particularly important to carry out research on rice single-grain sowing technology.

The air suction-type precision seed meter has the advantages of high sowing accuracy, low requirements for seed size, strong adaptability, and high operating speed [10,11,12,13]; it is the main way that is used to achieve precise and small seeding. Existing studies have found that the seeding accuracy of the pneumatic seed metering device is not only related to the seeds’ suction performance, but its seed cleaning performance is also one of the key factors affecting the seed metering performance [14,15]. Seed cleaning describes the process of removing the excess seeds at the suction hole, so as to ensure a single hole and a single seed. Over-cleaning or leakage will lead to a decrease in the accuracy of the seed metering device. The existing seed cleaning devices are mainly mechanical, such as lever-type plates, scraper-type plates and serrated cleaning devices [16]. Aiming at the dense planting pattern of Panax notoginseng in Wenshan, Yunnan, Lai et al. [17] designed a six-row air-blowing centralized precision seed metering device and they studied the effects of the air nozzle’s outlet pressure, seed metering wheel speed, and hole taper on the performance of the seed metering device and they then explored the best combination of these parameters. Wang et al. [18] designed a central drum vacuum seed metering device and installed a double-sided curved seed scraping mechanism that was designed to remove the excess seeds from the suction holes on both sides of the axial direction; this combination design can be used as a reference for improving the sowing precision of hybrid rice seeds. Based on the problem of the poor seed cleaning effect of the precision seed metering device under high-speed working conditions, Wang et al. [19] designed a wheel seed cleaning mechanism, which improved the seed cleaning performance of the corn seed metering device during high-speed operation. In order to solve the problems of high damage rate, low sowing precision and poor sowing effect in the process of mechanical sowing, Xiong et al. [20] designed a sowing device with a combination of a sheave wheel and air blowing. Single-factor experiments were performed on seeds (e.g., soybeans, corn, and rape-seeds) in order to verify the general properties of the seed metering device. However, the cleaned seeds could not fall back into the seed filling chamber, which affected the machine’s operational efficiency. In order to solve the problems of the high replay index of the air suction corn seed metering device and the difficulty in ensuring the rationality of the design of the seed cleaning mechanism, Ding et al. [21] created a sawtooth seed cleaning mechanism. However, the sawtooth seed cleaning mechanism easily caused mechanical damage to the seeds during high-speed operation.

The research on existing seed cleaning mechanism is mainly based on mechanical seed cleaning, which belongs to the category of forced seed cleaning and often takes seeds with high sphericity, such as rapeseed, corn, and so on, as the research object [22]. For rice seeds with low sphericity and an irregular shape, due to their changeable adsorption posture over-cleaning or leakage often occurs and the seed cleaning effect is poor. In order to improve the single-grain sowing accuracy of coated hybrid rice, combined with the previous research on the structure of the seed metering device, a flexible seed cleaning method for removing reabsorbed seeds by the application of a positive pressure airflow is proposed and a positive pressure airflow seed cleaning mechanism structure was designed. The purpose is to reduce the seed reabsorption rate of the seed meter and improve the seed metering accuracy.

2. Overall Structure and Design of the Seed Cleaning Mechanism

2.1. The Overall Structure of the Seed Meter

The overall structure of the single-grain seed metering device for the pneumatic distribution of coated hybrid rice is shown in Figure 1. The structure includes a seed box (labeled 1), seed box connector (2), seed cleaning mechanism (3), sealing ring (4), seed chamber shell (5), suction chamber shell (6), baffle (7), sucking plate (8), shaft (9), flange (10), seed unloading device (11), and seed tube (12).

When the seed metering device is in operation, the rice seeds flow from the seed box (1) through the seed box connector (2) into the seed suction chamber in the seed chamber shell (5) and they are subjected to the action of the flow field of the negative pressure flow channel of the suction chamber shell (6) in order for them to be absorbed into the seed suction chamber. Once the seeds have been adsorbed onto the suction hole of the sucking plate (8)—the sucking plate is fixed onto the flange 10 by bolts—the flange and the shaft (9) are rotated synchronously. Through key connection, the adsorbed rice seeds are rotated with the sucking plate to the seed cleaning area and, then, through the positive pressure air flow of the seed cleaning mechanism (3) they are fixed in the installation hole of the seed metering housing while the reabsorbed rice seeds are blown down from the suction hole and they fall back into the seed suction chamber under the isolation effect of the baffle (7). The remaining single seed rotates with the sucking plate (8) to the seeding area, leave the sucking plate (8) under the positive pressure blowing action of the suction chamber shell (6) after which the seed falls into the seeding ditch after passing through the seed tube (12), thus completing the seeding process.

2.2. The Design of the Seed Cleaning Mechanism

2.2.1. Theoretical Analysis

The change of the adsorption posture is the main factor affecting the effect of seed cleaning. In order to clarify the adsorption attitude of the rice seeds, a FASTCAM Super 10K high-speed camera, produced by the American Photron company, San Diego, CA, USA, was used to photograph the adsorption attitude of the rice seeds on the sucking plate. It was found that the number of the reabsorbed rice seeds was mainly two and there were a few cases of three or more. The adsorption posture of two rice seeds is roughly divided into three types (Figure 2): (1) the two rice seeds are parallel to the auxiliary seed suction device; (2) one rice seed is perpendicular to the auxiliary seed suction device and the other one is parallel to the auxiliary seed suction device; and (3) both of the rice seeds are perpendicular to the auxiliary seed suction device. Since the adsorption posture of the reabsorbed rice seeds is more variable, the mechanical seed cleaning mechanism is likely to cause over-cleaning or leakage and it is difficult to obtain a better seed cleaning effect. Therefore, a scheme design of air-flow seed cleaning is proposed and the location of the seed cleaning is analyzed in order to determine the installation location.

The type of seed metering device that was used was an air suction vertical disc-type seed metering device. Since the force of the rice seeds keeps changing during the rotation of the sucking plate, the unstable adsorption state will cause over-cleaning or leakage. In order to obtain the stable position of the adsorption state of the rice seeds and determine the installation position of the seed cleaning mechanism, the stress of the rice seeds in the seed-carrying area was analyzed. In order to display the force situation more intuitively, the three-dimensional coordinates, as shown in Figure 3, were constructed. The x axis is consistent with the tangential direction of the movement of the rice seed, the y axis is consistent with the radial direction of the rice seed’s movement, the z axis points to the direction of the suction force P₁ of the suction hole, and the center of the circle O′ is at the centroids of the rice seeds. When the rice seeds move to the seed-carrying area with the seed suction tray, assuming that the coated rice seeds are adsorbed stably in the seed-carrying area, they will not be affected by the force of the auxiliary seed suction device on the coated rice seeds. The force is the suction force P₁ at the suction hole, the seed gravity G, the inertial force J caused by the seed acceleration, the N force is the normal reaction force of the suction hole to the seed, and R is the radius of the circle where the suction hole is located.

Rice seeds can be stably adsorbed by the suction holes and will not be detached with the synchronous rotation of the sucking plate; therefore, the following force balance conditions should be satisfied:

$$P_1\frac{d}{2}\ge QC$$

(1)

where Q is the guaranteed minimum resultant force of the coated rice seeds that are adsorbed by the suction holes N; P₁ is the suction force at the air outlet of the suction hole N; d is the suction hole’s diameter in mm; and C is the distance between the centroid of the rice seed and the plane of the suction hole in mm.

Through the force analysis of the coated rice seeds on the sucking plate, G and J are synthesized into force Q:

$$Q=\sqrt{G^2+J^2+2GJ\cos \alpha }$$

(2)

where G is the rice seed’s gravity N; J is the centrifugal inertial force that is generated by the rotation of the rice seeds with the plate N; and α is the angle between the G and J forces.

Simultaneously to Formulas (1) and (2), the following can be known:

$$P_1\frac{d}{2}\ge C\sqrt{G^2+J^2+2GJ\cos \alpha }$$

(3)

According to formula (3), the larger the difference between the force of the left airflow and the force of the right rice seed on the suction tray, the more stable the adsorption state of the rice seed on the suction hole. When the working conditions remain unchanged, the rice seeds are adsorbed from the seed suction chamber by the suction holes and when the sucking plate rotates, P₁ d/2, C, G, and J remain unchanged. Therefore, α is the only factor that affects the above formula. When the rice seed moves to the uppermost end with the suction hole (α is 180°), as is shown in formula (4), the value of the right-hand side of the inequality is at its smallest value and the rice seed has the most stability under the negative pressure of the seed suction, which is beneficial in its ability to reduce the occurrence of over-cleaning. Therefore, the action point of the airflow seed cleaning device is selected at the upper intersection of the circle where the suction hole is located and the vertical line passes through the axis (Figure 4).

$$P_1\frac{d}{2}\ge C\left|G-J\right|$$

(4)

2.2.2. Structural Design of Seed Cleaning Mechanism

In order to effectively remove the reabsorbed rice seeds, the impact effect of the airflow needs to be improved. Therefore, the airflow seed cleaning mechanism needs to gather the airflow into an airflow beam for precise seed cleaning operations. The existing literature points out that a conical nozzle can effectively concentrate airflow in order to form an airflow beam and that the resulting jet has good concentration [23]. When the reabsorbed rice seeds pass through the action area of the airflow beam, the equilibrium state between the adsorbed rice seeds is broken and the inferior rice seeds are blown off, so as to achieve single seeding (Figure 5). If air flow is too small, the reabsorbed rice seeds cannot be blown off.

The cleaning mechanism is composed of a nozzle and a connecting pipe, which is installed at the position of the cleaning mechanism that is determined by the above theoretical analysis. The nozzle is oriented perpendicular to the auxiliary seed suction device and it points to the suction hole’s position. Since the internal space of the seed metering device is small and the air pressure requirement of the cleaning mechanism is small, referring to the relevant literature [24], the inlet radius is 5 mm, the wall thickness is 1 mm, the length of the connecting pipe is 20 mm, and the length of the nozzle is 10 mm. The existing research has found that, when the constriction angle of the conical nozzle is between 10° and 30°, the clustering is better [25]; therefore this paper has selected a constriction angle of 15° and an exit radius of 2.5 mm. In order to better show the installation position of the cleaning mechanism, the upper intersection of the circle where the suction hole is located and the vertical line passing through the axis O position was set as the center O′, the three-dimensional coordinates that are shown in Figure 6 were constructed, the x-axis direction was parallel to the surface of the sucking plate, the y-axis direction coincided with the vertical line passing through the axis position, and the z-axis direction was perpendicular to the surface of the sucking plate. The cleaning mechanism was offset by 35° to the y-axis on the xy plane and 30° to the y-axis on the yz plane. The nozzle was oriented perpendicularly to the auxiliary seed suction device.

3. Simulation Analysis of Nozzle Structure Flow Field

3.1. Nozzle Structure Parameters

Using the nozzle structure to turn the positive pressure airflow into an airflow beam can improve the concentration and impact force of the positive pressure airflow, thereby effectively cleaning multiple seeds. The different shrinkage curves of the nozzles have a great influence on the structure of the external flow field of each nozzle. In order to obtain a stable and well-converged airflow beam, research literature studies using three different types of shrinkage, namely the Witoszynski curve, the bicubic curve and a straight line, have carried out simulation analysis [26]. The parameters of each shrinkage curve equation were determined by the inlet radius S₁, the nozzle length L, and the outlet radius S₂ from Figure 7. According to the above introduction, the inlet radius S₁ was 5 mm; the nozzle length L was 10 mm; and the outlet radius S₂ was 2.5 mm.

The curves of the three curve equations were depicted by MATLAB software (The Mathworks, Natick, MA, USA), as is shown in Figure 8. By comparison, it was found that, in the inlet section, the Witoszynski curve shrunk faster and the straight line and bicubic curve were gentler. In the outlet section, the Witoszynski curve and the bicubic curve were flatter than the straight line and the Witoszynski curve was the flattest.

3.2. Simulation Model Construction

In order to analyze the influence of the three shrinkage curves on the distributions of the pressure and velocity in the flow field, Fluent software (ANSYS, Inc., Canonsburg, PA, USA) was used to simulate the flow field. Figure 9a shows the simulation model, wherein the inlet is a cylinder with a diameter of 10 mm and a length of 10 mm and the length of the nozzle is 10 mm in order to ensure stable flow. In order to fully observe the airflow from the nozzle, a cylinder with a diameter of 30 mm and a length of 100 mm was selected for the external flow field. Figure 9b shows the simulation mesh. Due to the regular shape of the nozzle, a hexahedral mesh was used to improve the simulation’s efficiency. The number of mesh nodes for the three nozzle structures was about 230,000 and the number of meshes was about 260,000. The pressure inlet was selected for the inlet and the air pressure was set to 0.15 kPa, according to the previous test. The right side was the external flow field area, which was set as the pressure outlet, and the air pressure was the standard atmospheric pressure of 0 kPa. The working process of the seed cleaning nozzle belongs to the category of jet flow. In order to better simulate the air movement conditions, the k–ω turbulence model was selected. Considering the narrow nozzle flow channel and the existence of near-wall effects, the SST (Shear Stress Transfer) k–ω turbulence model was used.

3.3. Simulation Results and Analysis

After the airflow passes through the nozzle structure, it is in a free outflow state and its pressure value will continue to decrease with its distance from the outflow. Figure 10 shows the total pressure nephogram of the three types of nozzles with their shrinking curves. It can be seen from the figure that the pressure value of the linear nozzle decreased the fastest, the central high pressure area was small, and the pressure field distribution was relatively disordered. It was also found that the pressure value of the Witoszynski curve nozzle decreased the slowest, the distance from the central high pressure area was the longest, and the transition of the pressure field was the smoothest. Figure 11 shows the velocity nephograms of the nozzles with their three contraction curves. It can be seen from the figure that, due to the reduction of the gas flow area in the nozzle’s contraction section, the flow velocity increases rapidly and the gas velocity reaches its maximum value at the nozzle. The maximum flow velocity of the Witoszynski curve nozzle was much higher than that of the straight nozzle and its maximum velocity was 15.7 m/s. The core jet area of the Witoszynski curve nozzle was the longest and its airflow state was the most stable.

According to the flow velocity of the nozzle axis position of the different shrinkage curves (Figure 12), it can be seen that the 0–2 cm section of the nozzle is its shrinkage section. The Witoszynski curve nozzle structure shrinks the earliest and its airflow velocity rises the fastest, reaching a maximum flow rate of 15.15 m/s at 1 cm. The velocity of the straight line nozzle rose the slowest and it reached a maximum velocity of 11.15 m/s at 2 cm, which is lower than that of the remaining two nozzles. After a distance of 2 cm, the airflow leaves the nozzle area and it is in a free outflow state. Due to air resistance, the airflow’s velocity gradually decreases. Among the nozzles, the velocity of the Witoszynski curve nozzle decreased the most gently and the average velocity of the air after leaving the nozzle was 8.48 m/s (the highest of the tested nozzles). The average flow velocity of the bicubic curve nozzle was 7.97 m/s and the average velocity of the straight line nozzle was 7.09 m/s (the lowest of the tested nozzles); the flow velocities of both the bicubic curve and the straight line nozzles fluctuate and the straight nozzle was found to be the most unstable. In summary, the Witoszynski curve was selected for the structural design of the seed cleaning nozzle.

4. Test and Analysis

4.1. Test Materials and Methods

4.1.1. Coating Treatment of Rice Seeds

Previous studies have found that naked rice seeds are relatively large in length and width and that they have awns on the surface. The force between multiple naked rice seeds on suction holes is large and it is difficult to clean these seeds. By coating the rice seeds (Figure 13), the coating material increases the surface area of the rice seeds and improves the surface characteristics of the rice seeds (by increasing the sphericity of the seeds and making the surface smooth), which is beneficial in reducing the interaction force between the reabsorbed seeds and improving the seed cleaning effect of the seed metering device [27,28,29]. Through experimental analysis of the seed sucking and cleaning performance with different coating times in the early stage, it was found that the 0.7 times coating rice seeds of the hybrid rice Wuyou 1179 had a low missing index and good seed cleaning effect; therefore, 0.7 times coated rice seed was selected as the test material, the 1000-seeds weight of which is 35.92 g. The coated seeds were provided by Plant Protection Research Institute Guangdong Academy of Agricultural Sciences.

4.1.2. Test Device

The test device is shown in Figure 14, it mainly includes a seed metering device, drive motor, fan, and electronic barometer, among other apparatuses. In the test, a differential-pressure electronic barometer was used to monitor the air pressure and the effect of the seed cleaning mechanism was observed by cutting the seed metering device while the FASTCAM Super 10K high-speed camera, produced by the American Photron Company, San Diego, CA, USA, was used to continuously record and statistically analyze the cleaning situation of the seed metering device.

4.1.3. Evaluation Indicators

In order to achieve single-grain sowing, the experimental design was carried out according to the single-grain (precision) planter test method and the national standard of the P.R.C. [30], wherein 1 rice seed adsorbed by each suction hole in each group is the qualified index, ≥2 grains/hole is the multiple index, 0 seed is the missing index, and 250 seeds were counted in each group of experiments (which were repeated 3 times).

4.1.4. Test Design

In order to verify the feasibility of the airflow seed cleaning mechanism, we had to study the relationship between the working parameters of the seed metering device and the seed metering performance and then determine the optimal working parameters. This was done by carrying out a quadratic regression rotation orthogonal combination test analysis with three factors and three levels. According to the requirements of the field operation speed, the rotation speed of the sucking plate of a pneumatic seed metering device is generally between 20 and 40 r/min (

Table 1. Test factor and level table.

Coding	Factor
	Rotation Speed of Sucking Plate A/r·min−1	Negative Pressure B/kPa	Cleaning Pressure C/kPa
−1	20	1.0	0.1
0	30	1.4	0.2
1	40	1.8	0.3

) [31]. Through the preliminary experimental research, we selected the vertical installation angle of the seed cleaning mechanism and the auxiliary seed suction device and the vertical distance between the nozzle orifice and the suction hole was set to 15 mm (Figure 15).

4.2. Results and Analysis

The test results are shown in

Table 2. Test results.

Test Number	Test Factors			Test Results
	A	B	C	Qualified Index	Missing Index	Multiple Index
1	0	0	0	86.41%	6.5%	7.09%
2	1	−1	0	76.60%	21.31%	2.08%
3	0	1	−1	75.82%	2.52%	21.66%
4	0	0	0	84.76%	7.16%	8.08%
5	0	0	0	84.15%	9.69%	6.15%
6	−1	0	−1	84.37%	7.88%	7.88%
7	1	1	0	86.99%	5.57%	7.74%
8	−1	−1	0	76.07%	21.65%	2.29%
9	0	0	0	83.95%	9.47%	6.58%
10	0	−1	−1	80.09%	12.56%	7.35%
11	0	0	0	84.29%	8.77%	6.94%
12	−1	0	1	66.67%	31.02%	2.31%
13	−1	1	0	85.22%	4.03%	10.75%
14	1	0	−1	77.65%	7.12%	15.23%
15	0	−1	1	54.47%	44.36%	1.17%
16	1	0	1	74.72%	23.54%	1.74%
17	0	1	1	81.99%	13.62%	4.39%

. There is a large difference in the qualification rate of the seed metering device under different working conditions, which is distributed between 50% and 90%. In order to improve the qualified index and reduce the missing index, a regression model was established by variance analysis and the insignificant factors were removed. The fitting equations for obtaining the qualified index Y₁, the missing index Y₂, and multiple index Y₃ are as follows and the fitting degrees of these were found to be extremely significant (p < 0.01):

$$\{\begin{array}{l}{Y}_1=84.71+5.35B-5.01C+3.69AC+7.95BC-3.13B^2-8.49C^2\\ Y_2=8.32-9.27B+10.31C-5.17BC+2.85B^2+7.10C^2\\ Y_3=6.97+3.96B-5.31C-2.77BC\end{array}$$

(5)

The optimal solution combination that was predicted by the experimental design and analysis software Design-Expert is as follows: the speed of the sucking plate should be 30 r/min, the negative pressure of the suction chamber should be 1.8 kPa, and the positive pressure of the seed cleaning should be 0.2 kPa. Under these proposed conditions, the qualified index was 87.01%, the missing index was 2.21%, and the multiple index was 10.61%. Under these conditions, using 0.7 times coated hybrid rice Wuyou 1179 as the test material, we were able to obtain statistics on the suction of rice seeds in 750 holes in each experiment. The test results showed that the qualified index was 86.43%, the missing index was 3.81%, and the multiple index was 9.76%; these are results which are similar to the results of the prediction model.

Through variance analyses, it can be seen that when A is 20, 30, and 40 r/min the effect of A on the qualified index is not significant; for B and C, the interaction terms AC and BC and the square terms B² and C² have a significant impact; and the order of influence of the three experimental factors on the qualified index is B > C > A. Among these factors, the effect of B is the most significant. From the variance analysis of the missing index, B, C, BC, and C² have significant effects. The order of the influence of the three experimental factors on the missing index is C > B > A. The effect of C is the most significant and the reason for this is that the sucking plate has the structure of the guide groove and the auxiliary seed suction device and it reduces the influence of the speed of the sucking plate [32]. So that the influence on the test index is not significant within the selected parameter range, the B factor affects the seed sucking performance of the seed metering device because it affects the adsorption force of the rice seeds in the seed suction chamber. The air pressure of the seed cleaning mechanism has a great influence on the seed carrying performance. The multiple index of the seed metering device decreased and the missing index of the seed sucking gradually increased. Therefore, B and C had significant effects on each test index.

In order to analyze the influence of interaction factors on the test indicators, Design-Expert software (Stat-Ease, Inc., Minneapolis, MN, USA) was used to process the test data and draw a response surface graph, as is shown in Figure 16. From the influence of each interaction factor on the qualified index, it can be seen that when C is at the center level (+0.2 kPa) the qualified index gradually increases with the increase in B and A has little influence on it; when B is at the center level (−1.4 kPa), with the increase in C, the qualified index first increases and then decreases and A has little influence on it; when A is at the center level (30 r/min), with the increase in C, the qualified index first increases and then decreases; and when C is +0.3 kPa, with the increase in B, the qualified index increases significantly.

From the influence of each interaction factor on the missing index, when C is at the central level (+0.2 kPa), the leakage rate decreases with the increase in B and A has little influence on the missing index. When B is at the central level (−1.4 kPa), the missing index increases with the increase in C and A has little effect on it. When A is at the central level (30 r/min), the missing index increases with the increase in C, when C is +0.3 kPa, and the missing index decreases with the increase in B.

From the influence of each interaction factor on the multiple index, when C is at the center level (+0.2 kPa), the resorption rate increases with the increase in B and the multiple index increases first and then decreases with the increase in A. When B is located in the center at the level −1.4 kPa, the multiple index decreases with the increase in C and A has little effect on the multiple index. When A is at the central level (30 r/min) and C is +0.1 kPa, the multiple index increased significantly with increasing B and when B was −1.8 kPa the multiple index decreased significantly with the increase in C (

Table 3. Analysis of variance.

Source	Qualified Index				Missing Index				Multiple Index
	SS	df	F	p-Value *	SS	df	F	p-Value	SS	df	F	p-Value
Model	1100.88	9	143.37	<0.0001	1945.56	9	55.07	<0.0001	418.64	9	17.66	0.0005
A	1.65	1	1.93	0.2073	6.20	1	1.58	0.2493	1.58	1	0.60	0.4634
B	228.87	1	268.26	<0.0001	687.09	1	175.05	<0.0001	125.22	1	47.55	0.0002
C	200.80	1	235.36	<0.0001	849.96	1	216.54	<0.0001	225.89	1	85.78	<0.0001
AB	0.38	1	0.45	0.5236	0.88	1	0.23	0.6496	1.96	1	0.74	0.4169
AC	54.54	1	63.92	<0.0001	11.29	1	288	0.1337	15.68	1	5.95	0.0447
BC	252.65	1	296.13	<0.0001	107.12	1	27.29	0.0012	30.75	1	11.68	0.0112
A2	0.56	1	0.66	0.4429	16.40	1	4.18	0.0802	10.15	1	0.0904	0.0904
B2	41.14	1	48.23	0.0002	34.16	1	8.70	0.0214	0.38	1	0.7159	0.7159
C2	303.75	1	356.02	<0.0001	212.16	1	54.05	0.0002	7.96	1	0.1257	0.1257
Residual	5.97	7			27.48	7			18.43	7
Lack of fit	2.01	3	0.68	0.6099	19.42	3	3.21	0.1446	16.36	3	0.0228	0.0228
Error	3.96	4			8.06	4			2.07	4
Total	1106.85	16			1973.04	16			437.08	16

* p < 0.01 means extremely significant, 0.01 < p < 0.05 means significant, p > 0.05 means not significant, SS means Sum of Squares, df means Degree of Freedom, F means F-value.

).

5. Discussion

In this paper, the 0.7 times coated hybrid rice seed of Wuyou 1179 was used as the test material, the theoretical and experimental analysis of the seed cleaning mechanism was carried out, and the feasibility of flexible cleaning seed sucking by positive pressure air flow was verified. The proposed method was able to reduce the occurrence of over-cleaning or leakage and greatly improve the rate of single seed sucking.

The reason for these finding is that the rice seeds were adsorbed onto the suction holes under negative pressure. The adsorption state of a single rice seed is more stable than that of multiple seeds. The airflow seed cleaning mechanism forms an airflow beam pointing to the suction hole position through the utilization of a nozzle structure. When the additional rice seeds that were adsorbed onto the suction hole rotate through the airflow beam action area with the suction disc, under the impact of the airflow the rice seeds that are less affected by the suction hole’s negative pressure are blown off by the airflow and the remaining single grain rice seed firmly occupies the suction hole position, achieving effective seed cleaning. In addition, this proposed seed cleaning method is less affected by space and position, has good applicability to reabsorbed rice seeds with different shapes, and can effectively improve the single seed rate of seeding.

In this paper, a flexible seed cleaning method of airflow impact was selected and the shape, the angle, and the pressure values were studied such that the appropriate parameters for the seed clearing device were obtained in order to improve the sowing performance of the pneumatic single seed metering device for coated hybrid rice. In the follow-up research, the combination of various devices should be considered in order to further improve the seeding accuracy. For example, Liu et al. [15] used a combination of cleaning pressure and vibration frequency to improve the seeding accuracy of vegetable seeding; Han et al. [33] designed an inside-filling air-blowing seed metering device, which uses a combined nozzle structure to complete the process of seed-filling and seed cleaning–pressing. By refining the functions of each component and combining and cooperating with each other, we will be able to further improve seeding accuracy.

This paper only uses one type of rice seed as the test material and fails to establish the relationship model between different rice seed types and seed cleaning pressures; hence, further research is needed in terms of applicability. Moreover, the above studies were all carried out as a bench test, but the vibration of the overall machine and the fluctuation of air pressure in the actual workplace will have a certain impact on the cleaning situation of the seed metering device; these are influences which need to be considered in future experimental research.

6. Conclusions

In order to improve the sowing accuracy of the pneumatic single seed metering device and to reduce the multiple index, high-speed photography technology was used to photograph the adsorption postures of rice seeds in the seed-carrying area. Based on the analysis of the seed cleaning mechanism, an airflow seed cleaning mechanism that uses positive pressure airflow to blow off and re-suck the seeds has been designed and a simulation test was carried out for the nozzle structure of the airflow seed cleaning mechanism. From this analysis, the core of the Witoszynski curve nozzle was obtained. For this nozzle, the jet area was found to be the longest and the air velocity rose the fastest and fell the slowest.

In order to study the relationship between the speed of the sucking plate, the negative pressure and the cleaning pressure, a three-factor, three-level quadratic regression orthogonal rotation combination test was carried out. When the negative pressure was 1.8 kPa and the cleaning pressure was 0.2 kPa, the qualified index was at its highest at 86.43%, the minimum missing index was 3.81%, and the multiple index was 9.76%. In order to improve the seeding performance of the seed meter, this research provides the basis for the optimization of the mechanism.