Ji Yao, Hu Shuangyan, Yan Wei, Zhang Wenyi

(Nanjing Institute of Agricultural Mechanization, Ministry of Agriculture and Rural Affairs, Nanjing 210014, China)

Abstract: In order to reduce the power consumption of sowing, improve the sowing quality and meet the agronomic requirements of precision seedling raising and sowing of hybrid rice, in this paper, the air suction double-layer cylinder seedling raising and sowing line was optimized and improved, and an independent air suction double-layer cylinder seedling raising and sowing device was designed.The working principle of the seeding device, the structure of key components and parameter design were described.The relationship between seeding quality and rotating speed of seed metering device and suction hole negative pressure was determined by force analysis of seeds in the process of seeding.The central composite test was carried out with qualified rate and repetition rate as experimental indexes, using vibration frequency, rotaing speed of seed metering device and suction hole negative pressure parameters as test factors.The results showed that the average qualified rate was 93.21% and the average repeat seeding rate was 3.97% when the vibration frequency was 51.8 Hz, the rotating speed of seed metering device was 8 r/min and suction hole negative pressure parameter 3.4 kPa.The seed sowing device could satisfy the requirements of hybrid rice seedling and sowing.

Keywords: double-layer cylinder; raising and sowing seedlings; seed sowing device; central composite face-centered design

0 Introduction

The annual planting area of rice in China is about 29 694 khm2.Rice production is not only responsible for ensuring the safety of food rations in China, but also shouldering the important mission of increasing rice farmers’ income and rural revitalization[1-3].In addition, it is one of the central tasks of China’s agricultural and rural economic development in the new stage.

At present, the planting area of hybrid rice accounts for about half of the total rice planting area in China, and the yield accounts for more than 60% of the total rice yield.The planting area of hybrid rice is mostly one-year multiple cropping system, and the seasonal rotation is relatively tight[4-6].In order to achieve high and stable yield, the main planting method of hybrid rice is seedling raising and transplanting.Hybrid rice has strong tillering ability, which requires 1-3 plants per hole.It is not possible to achieve precise planting of hybrid rice by reducing the seedling area per plant hole.Therefore, it is necessary to improve the sowing precision of sowing device to realize the accurate planting of hybrid rice.However, the existing seedling and sowing equipment is mainly for conventional rice seedling sowing.They are mainly mechanical sowing, with large sowing rate, low precision and easy to damage seeds, which cannot meet the requirements of hybrid rice seedling sowing and limit the popularization of hybrid rice mechanized planting technology[7-8].

The air suction seeding method has certain advantages in precision sowing[9-11].Although the air suction needle type can ensure the precise seed taking and seed dropping of single grain, the sowing efficiency is relatively low, and it is rarely used in rice precision sowing.The working efficiency of the whole disc suction type is much higher than that of the air suction needle type[12].Due to the frequent reciprocating movement of the seed tray for the whole disk suction, the positioning accuracy of the mechanism is required to be high, and the intermittent work limits the improvement of the working efficiency.The air suction roller type seeding device can work continuously and has high efficiency.Generally, the structure used is that the inner part of the roller is divided into negative pressure area and positive pressure area through a long strip seed dropping chamber.However, those methods increase the complexity and power consumption of the mechanism.The seed dropping chamber or sealing diaphragm and the roller are sliding friction[13-14].Those parts in the mechanism are easy to wear.By the way, foreign researches are mainly designed for vegetables, flowers and other designs, not applicable to rice[15-16].Therefore, in this paper, we have designed a double-layer pneumatic roller precision metering device[17], its structure is a double layer roller structure.When the device works, the seeds in the seed box are in a boiling state under the action of the vibration device, and then they are absorbed to the mold hole by the negative pressure and friction force and move up with the roller at a uniform speed.When the seeds absorbed by suction holes move to the bottom of the cylinder, the negative pressure is cut off and the seeds fall on the corresponding seedling tray by gravity.In this paper, we have explored the influence of suction hole negative pressure and operated speed and vibration frequency on seed metering performance.The optimal combination of working parameters has been sought, which provides reference for the design of air suction roller type seed metering device.

1 Materials and methods

1.1 Structure and working principle of seed metering device

The double cylinder precision seed sowing device with independent airway is composed of outer cylinder, inner cylinder, air distribution plate, seeds box, vibrator, etc.the structure is shown in Figure 1.

Figure 1 Structure of double cylinder precision seed sowing device with independent airway

1.1.1 Design of seed metering device

This air-suction cylinder seed sowing device is a double-layer independent airway type cylinder, and the specific structure is shown in Figure 2.The inner cylinder and the outer cylinder are integrated by glue and seal.Among them, each row of axial suction holes on the outer cylinder corresponds to each independent airway in the axial direction of the inner cylinder.One end of the cylinder is fixedly connected with a driving motor, which rotates at a constant speed under the drive of the motor.Both ends of the cylinder are respectively fixed with end covers, and the outside of each end cover is connected with an air distribution plate, which is shown in Figure 3.The seeds in the seed box are boiling under the action of pneumatic vibrator, which can reduce the friction between seeds and facilitate the seeds to be adsorbed on the cylinder suction hole.When the adsorbed seeds rotate to the lowest position along with the cylinder, the negative pressure is cut off.Then the seeds fall onto the seedling tray under the combined action of their gravity and seed clearing components.

Figure 2 Structure of double layer independent airway seedling device

The precision seedling and sowing of hybrid rice are for precisely transplanting.Therefore, the arrangement and distribution of suction holes of suction cylinder should be adapted to the area of rice transplanter.The size of the conventional seedling tray supporting the existing transplanter is 280 mm × 580 mm × 30 mm.According to the agronomic requirements of hybrid rice planting, the device designed in this paper is suitable for the transplanter with 16 horizontal and 34 vertical transplanter, so that the cutting area of the transplanter is 17 mm×17.5 mm=297.5 mm2.According to 1-3 hybrid rice seeds per seedling area, two suction holes are arranged in diagonal line.The local expansion of suction holes on the surface of seed suction roller is shown in Figure 4.

Figure 4 Sketch map of suction holes site

In order to facilitate the program control, the system starts sowing when it detects the seedling tray passing by and resets the cylinder after one tray sowing.At the same time, due to the same working efficiency, the smaller the diameter of the cylinder is, the higher the speed.When the rotation speed of the cylinder is too high, the sowing quality will be reduced.The specific design parameters are shown in Table 1.

Table 1 Design parameters

1.1.2 Design of component for removing excess seeds

In the process of seed suction by the seed metering device, the removal of redundant seeds play an important role in the qualified rate of seed suction for precisely sowing of 2±1 seeds per seedling area of super rice.In this paper, the mechanical method of brushing off seeds through comb teeth is adopted to remove excess seeds.According to the radial position of the suction holes on the cylinder, we set each comb tooth in the middle of the two rows of suction holes on the circumference of the cylinder.During installation, the comb teeth are fixed on the seed box through the extension connecting parts, and a circle of suction holes are corresponding between each two comb teeth, and the comb tooth tip maintains a 2 mm gap with the surface of the cylinder.The structure of seed cleaning comb is shown in Figure 5.

Figure 5 Schematic diagram of comb tooth structure for removing excess seeds

The significance of comb design includes two parts, on the one hand, it can straighten out the posture of seeds and increase the stability of seeds adsorbed on the air suction cylinder.On the other hand, it can remove excess seeds, and ensure the uniformity of sowing and improving the qualified rate of seed suction.

1.1.3 Design of the mechanism for automatic spreading seedling raising tray

The automatic seedling tray mechanism is mainly composed of swing tray table, pneumatic system, seedling tray monitoring sensor and seedling tray guiding mechanism.Among them, swing tray table is the core component to complete the action of automatic seedling tray.Before laying the seedling tray, the swing tray table is in the horizontal position.At this time, the seedling tray is stacked on the swing tray table with the support of the guide device.When the seedling tray is laid, the pneumatic system starts the air cylinders on both sides under the action of the excitation signal, and then drives the swing tray table to rotate inward.Under the action of four inclined support plates set on the tray table, the lowest seedling tray loses its support and falls into the conveyor belt, while the upper seedling tray is still clamped.After completing the action of laying seedling tray, the air cylinder moves in the opposite direction, and the swing tray table returns to the initial horizontal position to complete a cycle.The laid seedling tray moves with the conveyor belt.When the sensor detects that the seedling tray moves out of the seedling tray area, the pneumatic system will start the next cycle to automatically lay the seedling tray.The automatic seedling tray system is shown in the Figure 6.

Figure 6 Structural diagram of automatic seedling laying tray mechanism

1.1.4 Design of intelligent control system for automatic seeding

Automatic seeding mechanism is the core part of the whole seedling planter, which directly determines the working performance of the seedling planter.The seedling planter adopts an air suction cylinder seed metering device.As shown in Figure 7 and Figure 8, the servo motor is controlled by PLC to drive the transmission belt of the seedling tray and the rotation of the air suction seeding cylinder.When the seedling planter starts seedling raising and sowing, the air suction cylinder automatically completes the initialization setting and the air suction cylinder automatically completes the adsorption of seeds under the action of negative pressure and rotates to the set position.When the seedling tray reaches the monitoring position, the PLC enters the sowing delay state and the conveyor belt servo motor steps to the seed dropping position of the coupling air suction cylinder to start sowing.When sowing, PLC enters the sowing time state, and the air suction cylinder servo motor completes the sowing process according to the set step size.After sowing, checking the rotation position of the air suction cylinder and making the seed dropping position of the air suction cylinder to reach the specified position.

Figure 7 Servo motor and its driver

Figure 8 Air suction cylinder position sensor and travel switch

1.2 Test conditions

The seeds used in the test are all Y Liangyou 646 super hybrid rice seeds with a 1 000 grain weight of 24.9 g, and the seed state is dry seed without germination.The test is carried out on a self-designed test bench.Among them, MS-130ST-M10015 servo motor with power of 1.5 kW is selected as the motor, GHBH002341R5 industrial high-pressure fan is selected as the negative pressure fan, and FBV-0.17 piston air compressor is used for positive pressure distribution.The air pressure at the suction hole of cylinder is measured by digital manometer.

1.3 Test methods

The test is carried out according to the test method for precision seeder(GB/T 6973-2005), 1-3 seeds per hole are defined as qualified sowing, and more than 3 seeds per hole are regarded as repeated sowing.The qualified rate and repeat seeding rate are selected as the performance indexes to measure the seeding quality of the metering device.They are all taken as the test target.Combined with the research results of relevant scholars, the main parameters affecting the performance of seed metering, such as vibration frequency, rotating speed of seed metering device and negative pressure, have been selected as the three main test factors of the test, namely vibration frequency, rotating speed of seed metering device and negative pressure, which are represented byx1,x2andx3respectively.The seed number of each seedling area needs to be collected manually at the end of the test and recorded the seed number of 150 seedling areas.Tests are averaged in three replicates for each group to obtain the average value.

1.4 Central composite face-centered design

The influence law of experimental factors on test indexes has been studied.In order to ensure a good sequence of the test, to avoid the complexity caused by the increasing of level and to reduce the test error, the central composite face central design(CCF)method is used to optimize the test scheme.Based on the relationship between the coding value and the actual value of CCF scheme, as shown in Table 2, each group of tests is repeated three times to obtain the average value, and F-test is conducted at the level ofP= 0.05.

Table 2 Experimental values and coded level

2 Results and discussion

The field test is arranged according to the test scheme, and the test results are shown in Table 3.

Table 3 Experiment layout and results

2.1 Regression equation

(1)

Y2=4.43+1.2x1+x2-1.1x3+

(2)

2.2 Regression equation analysis of variance

Table 4 Variance analysis of regression equation for qualification rate Y1

Table 5 Variance analysis of regression equation for repeat seeding rate Y2

2.3 Influence regular pattern of each test factors on test indexes

The relationship between the interaction between factors and test indicators is shown in Figure 9.

(a)Interaction of x2 and x3 on Y1

According to Figure 9(a), whenx1is constant, the relationship betweenx3andY1tends to be linear.At the same time, whenx2is large, the relationship betweenx3andY1is negatively correlated, whenx2is small, the relationship betweenx3andY1is positively correlated, whenx1is constant, the relationship betweenx2andY1tends to be parabolic.At the same time,Y1first increases and then decreases with the increase ofx2, and the change rate in the latter half is greater.According to Figure 9(b), whenx2is constant, the relationship betweenx3andY2tends to be linear, showing a decreasing trend as a whole, and the relationship betweenx1andY2shows an increasing trend.According to Figure 9(c), whenx3is constant, the relationship betweenx1andY2tends to be linear, showing an increasing trend as a whole, and the relationship betweenx2andY2shows an increasing trend.

3 Comprehensive optimization

In order to make the working performance of the seed metering device reach the optimal state, the seed metering qualified rate must be the best and the repeat seeding rate must be the lowest.In this study, the design expert software is used for multi-objective optimization of the test data, in which the objective function and constraint interval are as follows.

The final optimization shows that the optimal level combination is that the vibration frequency is 51.79 Hz, the rotating speed of the seed metering device speed is 8.04 r/min and the suction hole negative pressure is 3.41 kPa.Under this condition, suction hole the qualified rate of the optimal index is 93.85% and the repeat seeding rate is 3.90%.

In order to verify the accuracy of the optimization results, the vibration frequency of the seed metering device is 51.8 Hz, the rotating speed of seed metering device is 8 r/min, and the suction hole negative pressure is 3.4 kPa.Through the test verification, the qualification rate and repeat seeding rate are 93.21% and 3.97% respectively, and the relative errors with the optimization results are 0.68% and 1.79% respectively.The test conditions are shown in Figure 10.

Figure 10 Seeding status

4 Conclusions

In this paper, an air suction double-layer cylinder precision seedling raising and sowing device has been designed, which adopts the combination of double-layer cylinder and independent airway technology to reduce the pneumatic parameter requirements of air suction cylinder precision rice seeding device.

Taking the vibration frequency, rotating speed of seed metering device and the suction hole negative pressure parameters as test factors, the central composite test of qualification rate and repeat seeding rate has been carried out, the regression equation has been fitted, and the optimization has been carried out based on the regression equation.The optimal level combination is obtained as vibration frequency of 51.79 Hz, rotating speed of seed metering device 8.04 r/min and the suction hole negative pressure parameter of 3.41 kPa.The corresponding qualification rate and repeat seeding rate of this combination are 93.85% and 3.90%.

The verification test results of the optimal level combination is as follows: the average qualified rate is 93.21%, the average repeat seeding rate is 3.97%, and the relative errors with the optimization results are 0.68% and 1.79% respectively, while indicating that the optimization results are credible.