Design and Experiment of a Wheel Precision Seed-Metering Device with Cells for Corn

: In this study, a wheel seed-metering device with cell s for corn was built to enhance seed-metering performance and shorten the design cycle. The actual seed-metering process was analyzed and the three main factors influencing seed metering were obtained: rotational speed of the seed-metering device, hole diameter, and hole pitch. With the three factors as the test factors, and the QFI, MISS and MULT index as the test indexes, the multiple quadratic rotation orthogonal combination test was carried out . Using Design-Expert 8.0.6 to analyze the test data, the mathematical model between the test factors and the test index was obtained. The parameter optimization results indicate that optimal performance of the seed-metering device can be achieved under a rotational speed of 30.32 rpm, hole diameter of 13 mm, and hole pitch of 45 mm, qualification rate, miss-seeding rate, and reseeding rate were 94.99%, 1.93%, and 3.08%, respectively. Under the optimal parameter combination, the field test proved that the qualification rate, miss-seeding rate, and reseeding rate were 91.81%, 3.08%, and 5.11%, respectively. The test results were close to the optimization results, which could show the agronomic requirements of the precision sowing of maize.


INTRODUCTION
Corn is one of three major grain crops in China suitable for precision sowing.Precision seeding techniques can increase both corn yields and revenues [1,2].In precision sowing, the seed metering device is a key component and its performance will have a direct impact on crop quality and labor costs [3][4][5].In this context, development and improvement of seed metering devices depend on convenient and rapid detection of several performance indices, including qualified, miss-seeding, and miss-filling rates [6][7][8].
At present, precision seed-metering devices commonly used in China include the horizontal disc, inclined disc, cell wheel, composite seed-filling, and pneumatic types.According to working principles, seed-metering devices can be classified as either mechanical or pneumatic [9][10][11][12].Pneumatic seed-metering devices are highly adaptive and can be universally applied since there are no strict requirements for seed shape or dimensions; however, shortcomings include complex structures, high processing costs, and frequent failures [13].In contrast, mechanical seed-metering devices are characterized by a simple structure and stable performance, and are easy to repair.Therefore, small mechanical precision seeders are the most prevalent precision seed-metering devices in China [14].Due to the high cost and huge size of existing seedmetering devices, small-scale experimental platforms provide the best way to optimize the quality and speed of precision seed-metering [15,16].
The study completed the construction of the device through the design of the key components of the cell-wheel corn precision seed-metering device.Through the analysis of the actual seed-metering process, the factors influencing seed metering were obtained.Furthermore, seed-metering performance tests were performed, working parameters of the seed-metering device were optimized and then the field tests were conducted to validate the results.

MATERIALS AND METHODS 2.1 Design of the Complete Machine
The cell wheel precision seed-metering device is comprised of a platform, seed-metering device, seedmetering device motor, transmission device, transmission device motor, belt, roller, and detection and control devices, as shown in Fig. 1.The seed-metering device was installed above the transmission device and on the right side of the platform.The motor driving the seed-metering device was mounted on the right side of the seed-metering device.The transmission device, located below the seed-metering device, was fixed to the bottom of the device via two rollers.Another motor, used to drive the transmission device via a belt, was placed on the bottom left side of the device.To ensure the reliability of the platform in service, the motor driving the transmission device was hinged on one side with the platform, and connected on the other side using long bolts, thus allowing the belt to be conveniently fastened when necessary.After the power is switched on, the motor powers the seed-metering device through direct coupling, while the other motor drives the transmission device via the belt.Seeds in the tank first fall into the seed-metering device, and are then transferred to the transmission device.Movement of the transmission device relative to the seedmetering device simulates movement of the seed-metering device in the field.Tachometers were used to monitor real-time speeds of the seed-metering device and the transmission device and the driving motors were used to match the working speeds, thereby simulating the entire seed-metering process.By recording the entire process and performing statistics on the qualified rate, reseeding rate, and miss-filling rate, seed metering-related indices of the cell wheel-type seed-metering device were obtained at different speeds, which provide a basis for design and optimization of the seed-metering device, as shown in Fig. 2.

Design of Key Parts 2.2.1 Seed-Metering Device
The cell wheel diameter is directly related to the overall size of the seed-metering device, linear speed of the cell wheel, centripetal force required to move the seeds, and other parameters [17,18].In relation to the cell wheel diameter, the following simultaneous equations can be established:  Specifically, the corn set of plants per hectare must be higher than 75000 plants/ha.Considering a row spacing of 0.5 m and an approximately square plot (M = N), when M > 273, the required planting density is satisfied, i.e., l < 0.027.In our test, we set l = 0.2 m, such that, In general, the cell wheel rotational speed n is 30-40 rpm, and the seeder working speed v m is 1-2 m/s, therefore, the cell wheel hole count z was set to 12.The cell wheel diameter D was 0.14 m.
The hole parameters are key design parameters that directly affect the seed filling, seed carrying, and seed charging processes of the seed-metering device and overall performance of the device [19].In our design, the types holes are hemispherical.For precision sowing, the outline dimensions of corn seeds were measured and the mode of successful entry of the seeds into the holes was analyzed to ensure only one seed enters each hole.
Zhengdan 958 coated corn seeds from Henan Province were measured and assumed to approximately trapezoidal with upper base length d l = 8.32 mm, lower base length d w = 3.04 mm, height h = 10.15 mm, and thickness t = 5.47 mm.Seeds can enter holes in one of two positions: flatlying posture and side-lying posture.Limit analysis was performed on the two postures using the measured seed dimensions, as shown in Fig. 3. Fig. 3a shows the minimum hole diameter when seeds enter holes in the flat-lying posture.In this case, the relationship between hole diameter and seed dimension can be expressed as: According to Eq. ( 5), when seeds are in the flat-lying posture, the minimum hole radius is 6 mm; that is, the minimum hole diameter is 12 mm.Fig. 3b shows the maximum hole diameter when seeds enter holes in the side-lying posture.In this case, the relationship between hole diameter and seed dimension can be expressed as According to Eq. ( 6), when seeds are in the flat-lying posture, the maximum hole radius is 6.79 mm; that is, the maximum hole diameter is 14 mm.
Based on the above analysis, to ensure only one seed enters each hole, the hole diameter should be 12 mm < d < 14 mm.
Working requirements for the seed-metering device are as follows: At a working speed of 7-10 km/h, the seedmetering device should be able to perform precision seedmetering with a plant spacing of 200 mm.A 12-hole cell wheel seed-metering device was adopted to develop the device.
The rotational speed V s of the seed-metering device is: where V s denotes the rotational speed of the seed-metering device, rpm; z is the hole count of the seed plate; k is the working speed of the seed-metering device (7-10 km/h), rpm; l denotes the spacing of corn plants, mm.At the highest speed of 10 km/h, the rotational speed of the seed plate is 37.9 rpm.The driving motor of the seedmetering device is directly connected to the seed-metering device, therefore, the rotational speed of the seed-metering device is equal to the rotational speed of the motor.To guarantee a certain allowance, a maximum possible rotational speed of 50 rpm was set.To achieve this, a 5IK120RGN-CF motor with a rated power of 120 W and maximum rotational speed of 54 rpm was selected.

Conveying Mechanism
When ejected from the cell wheel seed-metering device, corn seeds have high kinetic energy.To make sure seeds land on the conveying belt with only a small rebound and without rolling, and to ensure a reliable and stable conveying process, a conveying belt composed of PVC conveyor and plush fabric was selected as the transmission device of the device.As a specialized transmission device, the PVC conveyor ensures precision and reliability.The plush fabric provides excellent shock absorption and reliably ensures seeds land in the desired position with only a small amount of rebound and without rolling.In addition, to ensure there are more than five seeds on the conveying belt surface, dimensions of the PVC conveyor and the plush fabric were designed as follows: total conveyor length of 2600 mm; conveyor width of 400 mm; conveyor thickness of 2 mm; plush fabric perimeter of 2600 mm; plush fabric width of 300 mm.
Due to the frequent start/stop nature and high rotational speed of the conveying belt, a servo motor was selected to drive the transmission device [20].The rotational speed n of the conveying roller is: where n denotes the rotational speed of the roller, rpm; V t denotes the speed of the transmission device, set as 7-10 km/h; and D is the diameter of the roller, set as 114 mm.Based on the highest speed of the transmission device of 10 km/h, the rolling speed is 465.6 rpm.The conveying wheel has a transmission ratio of 1:1, therefore, the rotational speed of the roller is equal to the rotational speed of the conveying belt motor.To guarantee a certain allowance, the motor was set to the highest rotational speed of 500 rpm.Thus, the 5IK120RGN-CF motor with a rated power of 120 W was selected.

Detection Device
The sensor detection device is used to detect the speed of the transmission device and the rotational speed of the seed-metering device.The speed of the transmission device is equivalent to the speed of the advancing seedmetering device.Each time the seed-metering device makes one full rotation, 12 piles of seeds are ejected.Once the seeding speed of the seed-metering device is determined, a pile spacing of 200 mm is adopted to calculate the rotational speed.For simplicity and convenience, RC41B digital tachometer revolution meters was adopted and used in combination with the U-groove Ltype optoelectronic switches for detection [21,22].

Experimental Details
The proposed cell wheel seed-metering device for corn was tested in accordance with the Testing methods of single seed drills (precision drills) (GB/T 6973-2005) with rotational speed of the seed-metering device, hole diameter of the seed plate, and hole pitch of the seed plate as the independent variables.Response values were the qualification index, miss-seeding index, and reseeding index [23][24][25].The test was carried out in the Agricultural Machinery Laboratory of Northwest Agriculture & Forestry University using a self-made cell wheel precision seed-metering device, as shown in Fig. 4. Zhengdan 958 seeds with uniform dimensions and capable of adapting to the hole size of the seed plate were used as the test material to minimize the influence of seed dimensions on the results.
Considering the structural limitations of the seedmetering device and the factors influencing the actual seed metering process, the main influencing factors were identified as rotational speed of the seed-metering device, hole diameter of the seed plate, and hole pitch of the seed plate.During the test, the first step involved clearing residual seeds from the seed-metering device, installing the required seed plate, and debugging the seed-metering device.Next, the high-speed camera was turned on and adjusted to a suitable angle so that seeds ejected from the holes of the seed plate could be clearly captured.Afterwards, the rotational speed of the seed-metering device was allowed to stabilize under the specified conditions.A sufficient number of seeds (100 was adopted as the standard for statistical analyses) were quickly added to the seed-metering device.Then, the high-speed camera was used to record the actual seed-metering process.Finally, the steps were repeated three times for each of the test conditions listed in Tab. 1.
Statistics were performed on the qualification, missseeding, and reseeding rate.In the initial stage, since seed filling is non-uniform, the actual seed-filling effect cannot be accurately reflected, therefore, the first 20 seeds ejected from the seed-metering device were excluded from the statistical analysis.Starting from 21, a total of 100 seeds were considered as one group of data.The test results are presented in Tab. 2.

RESULTS
Test data were analyzed and fitted using Design-Expert (version 8.0.6).Regression equations were obtained for qualification index Y1, miss-seeding index Y2, and reseeding index Y3 [26].

Regression Analysis of Qualification Index
A multiple regression analysis was performed on the test data to obtain regression models for various factors relative to the qualification index Y1: Tab. 3 shows results of the analysis of variance of qualification index Y1.From Tab. 3, PL1 = 0.3137 > 0.05 indicating that no loss factor existed in the regression analysis, and the regression model exhibited a high fitting degree.The P-value of the model regression items, PM1 < 0.0001 < 0.01, indicated that the regression results were reliable to a certain extent.Among the interaction terms, X1, X2 2 and X3 2 were highly significant terms, X3 and X2X3 were significant terms.The model for qualified index was regressed by considering only the significant terms and shown as: 92.53 2.8 0.6 0.66

Regression Analysis of Miss-Seeding Index
A multiple regression analysis was performed to obtain regression models of the various factors relative to miss-seeding index Y 2: Tab. 3 shows results of the analysis of variance of miss-seeding index Y 2 .From Tab. 3, P L2 = 0.2232 > 0.05, indicating that no loss factor existed in the regression analysis, and the regression model exhibited a high fitting degree.The P value of the model regression items, P M2 = 0.0002 < 0.01, indicated that the regression results were reliable to a certain extent.Among the interaction terms, X 1 , X 2 , and X 2 2 were highly significant terms, X 3 , X 1 X 2 and X 1 2 were significant terms.The model for missseeding index was regressed by considering only the significant terms and shown as:

Regression Analysis of Reseeding Index
A multiple regression analysis was performed to obtain regression models of the various factors relative to reseeding index Y 3 : Tab. 5 shows results of the analysis of variance of reseeding index Y 3 .From Tab. 5, P L3 = 0.2965 > 0.05 indicating that no loss factor existed in the regression analysis, and the regression model exhibited a high fitting degree.The P value of the model regression items, P M3 = 0.001 < 0.01 indicating that the regression results were reliable to a certain extent.Among the interaction terms, X 1 , X 2 , and X 3 2 were highly significant terms, X 1 X 2 and X 2 2 were significant terms.The model for reseeding index was regressed by considering only the significant terms and shown as:

Influence of Various Factors on the Test Indexes
The influence of experimental factors on the model is directly proportional to the value of contribution rate K, and the calculation method is as follows: where, F is the F-value of each regression term in the regression equation;  is the assessment value of the regression item to the F value; K j is the contribution rate of each influencing factor.According to Eq. ( 14), the contribution indices of rotational speed, hole diameter, hole pitch to the qualification index are 1.62, 1.41 and 2.27, the contribution of the factors to the qualification index followed the order as follows: hole pitch X 3 > rotational speed X 1 > hole diameter X 2 ; the contribution indices of rotational speed, hole diameter, hole pitch to the miss-seeding index are 2.34, 2.39 and 1.24, the contribution of the factors to the missseeding index followed the order as follows: hole diameter X 2 > rotational speed X 1 > hole pitch X 3 ; the contribution indices of rotational speed, hole diameter, hole pitch to the reseeding index are 2.17, 2.64 and 1.31, the contribution of the factors to the reseeding index followed the order as follows: hole diameter X 2 > rotational speed X 1 > hole pitch X 3 .Technical Gazette 29, 5(2022), 1741-1748

Effects of Test Factors on Performance Indices
To express the interactive influence of each factor on the qualification index Y 1 , miss-seeding index Y 2 and reseeding index Y 3 , the above three quadratic regression equations of the evaluation indices were subjected to treatment.One of the factors was set to level 0, while the other two underwent interaction effect analysis (by removing the insignificant items: X 1 X 2 and X 1 X 3 in qualification index Y 1 , X 1 X 3 and X 2 X 3 in miss-seeding index Y 2 and X 1 X 3 and X 2 X 3 in reseeding index Y 3 ) to study the influence law on the evaluation indices Y 1 , Y 2 and Y 3 , and the corresponding response surfaces were generated, as illustrated in Fig. 5.

Figure 5 Effects of test factors on indices
Fig. 5a shows the effect of interaction between the hole diameter of the seed-metering device and hole pitch on the qualification index when the rotational speed of the seed-metering device is 34.11 rpm.For the given hole pitch, as the hole diameter increases, the qualification index first increases, then For the given hole diameter, as the hole pitch increases, the qualification index first increases, then decreases.Fig. 5b shows the effect of interaction between the rotational speed of the seed-metering device and the hole diameter on the missseeding index when the hole pitch is 40.4 mm.For the given hole diameter, as the rotational speed of the seedmetering device increases, the miss-seeding index gradually increases.At a constant rotational speed, as the hole diameter increases, the miss-seeding index first decreases, then increases.Fig. 5c shows the effect of interaction between the rotational speed of the seedmetering device and the hole diameter on reseeding index when the hole pitch is 40.4 mm.For the given hole diameter, as the rotational speed of the seed-metering device increases, the reseeding index gradually increases.For a constant rotational speed of the seed-metering device, as the hole diameter increases, the reseeding index gradually increases.
When the speed of the seed-metering device is low, the qualification index is better, because the seeds have enough time to enter the hole and complete the seed filling under the low-speed operation condition.However, as the speed of the seed-metering device increases, the seeds do not have enough time to enter the hole, resulting in increased miss-seeding index.On the other hand, the seeds rotate directly along the seed-metering wheel, leading to an increase in reseeding index.When the hole diameter is small, it is difficult for seeds to enter hole, which leads to lower qualification index and higher omission index.However, when the hole diameter is larger, the chance of seeds entering hole increases, and multiple seeds enter hole at the same time, which leads to the decline of qualified index and the increase of reseeding index.When the hole pitch is small, the time for seeds to enter holes is short, leading to a lower qualified index and a higher miss-seeding index.However, with the increase of the hole pitch, the time for seeds to enter holes increases, leading to an increase in reseeding index.

Parameter Optimization
To identify the optimal combination of test factors levels, boundary conditions of the test factors and regression equations of the qualification index Y 1 , the miss-seeding index Y 2 and the reseeding index Y 3 were combined to determine the optimal solution.The test indices were optimized for the maximum qualification index Y 1 and minimum miss-seeding index Y 2 and reseeding index Y 3 .
In the Design-Expert software, the optimal performance was achieved with a rotational speed of the seed-metering device of 30.32 rpm, hole diameter of 13 mm, and hole pitch of 45 mm; qualification rate, miss-seeding rate, and reseeding rate were 94.99%, 1.93%, and 3.08%, respectively.

Test Verification
To verify the optimized seeding performance parameters of the cell wheel seed-metering device for corn, seeding tests were carried out using Zhengdan 958 corn seeds on an experimental plot in Yangling agricultural high-tech industries demonstration zone in mid-March 2021.The plot is characterized by flat terrain and fertile soil.The single-seed qualification index, reseeding index, and miss-seeding index were measured in accordance with the Testing methods of single seed drills (precision drills) (GB/T 6973-2005).The field tests were carried out 5 times, and the detailed experimental data are shown in Tab. 4. The field test results are the average values of 5 repeated tests, and show that the single-seed qualification index, miss-seeding index, and reseeding index are 91.81%,3.08%, and 5.11%, respectively.Compared with the optimization results, the error rates of qualification index, miss-seeding index and reseeding index are 3.18%, 1.15% and 2.03%.

CONCLUSIONS
In this study, a cell wheel precision seed-metering device was designed and built.The following conclusions are drawn: 1) Through the test, the three test factors have an effect on the qualification index in the following order: hole pitch, rotational speed and hole diameter; at the same time, the hole diameter has the greatest influence on the miss-seeding index and reseeding index.Therefore, when designing a mechanical metering device, this can be used as a reference.
2) The results were fitted and optimized in the Design-expert data analysis software.The seed-metering device exhibited optimal performance under a rotational speed of the seed-metering device of 30.32 rpm, hole diameter of 13 mm, and hole pitch of 45 mm.The field test results showed that the qualification rate was 91.81%, the miss-seeding index was 3.08%, and the reseeding index was 5.11%.The field test results were consistent with the optimization results.
The device can realize the rapid detection of the performance of the seed-metering device, simplify the performance detection process of the seed-metering device, shorten the research and development cycle of the seed-metering device, and help promote the development of China's mechanical seed-metering device.However, only Zhengdan 958 corn seeds were used to verify the experiment.Caution should be taken when using the other seeds.

Figure 2
Figure 2 Working principles of cell wheel precision seed-metering device of plants per hectare, plants/ha; M denotes the number of rows; N denotes the number of columns; b denotes the row spacing, (m); l denotes the plant spacing, (m); n is the rotational speed of the cell wheel, rpm; w v is the linear speed of the cell wheel, (m/s); D is the cell wheel diameter, m; z represents the hole count of the cell wheel; and v m is the working speed of the seeder ( m/s).

Figure 3
Figure 3 Relationship between hole diameter and flat-lying posture 1. Hole; 2. Seed flat-lying posture; 3. Seed side-lying posture.dl is the upper base length, mm; dw is the lower base length, mm; t is the seed thickness, mm;h is the seed height, mm; R is the hole radius, mm; X is the distance from upper base to hole center, mm.

Figure 4
Figure 4 Structure of cell-wheel precision seed-metering deviceAmong them, the rotational speed of the seed-metering device fell within the range of 30.32-37.9 rpm.The driving motor and tachometers were used to realize the desired rotational speed of the device.The hole diameter range was 12-14 mm and the hole pitch range was 20.2-60.6 mm.

Table 1
Test factors and codes

Table 2
Test design and results

Table 3
Analysis of variance

Table 4
Field test results