Design of intelligent transplanting system for vegetable pot seedling based on PLC control

Abstract

Pot seedling transplanting is the main way of vegetable planting, there were some problems of labor intensity and low level of automation in the process of mechanized transplanting of vegetable pot seedling. Aiming at this problem, this paper designed an intelligent transplanting system for vegetable pot seedling, which included the function of the seeding storage mechanism, the seedling feeding mechanism, the ejecting seedling mechanism, the pinch mechanism and the planting mechanism, the system monitors the whole movement process of planting through pressure sensing, stroke detection and limit switch, obtains each planting stage of the seedlings during planting, and uses the CCD camera to identify the key steps of planting and seedlings to determine whether it is missing. Three broccoli seedlings with different moisture content and the age of 42d were used as the experimental object of seedlings planting experiment. Results indicated that: Under the conditions of meeting the agronomic requirements of vegetable pot seedlings, when the speed of picking-up seedlings was 75 seedlings per minute, which resulted in a 95% success rate of picking-up seedlings and 5% leaked seedlings rate, the planting effect of vegetable pot seedlings was the best. The research can provide useful reference for the development of intelligent transplanting technology of vegetable pot seedling.

Keywords

Vegetable pot seedling intelligent transplanting system PLC control design

1 Introduction

Vegetable pot seedling transplanting is labor-intensive, time-consuming and laborious, and is gradually being replaced by mechanized transplanting equipment [1 –3]. At present, the research on mechanized transplanting of potted seedlings in China has just started, and it is mainly semi-automatic transplanting machine. It needs to complete the process of seedling collection by hand, the process of seedling throwing, the labor intensity is large, and the degree of automation is low, which restricts the improvement of transplanting efficiency [4 –8]. Therefore, it is urgent to develop intelligent transplanting equipment to reduce labor intensity and improve automation level of seedling transplanting machine [9].

By analyzing the artificial transplanting process of vegetable pot seedling, the design requirements of transplanting device are put forward, and the intelligent transplanting device of vegetable bowl seedling is designed. The structure characteristics and working process are expounded, and the design process of key components is explained. The control principle of transplanting system was analyzed by function-action process-action method, and the intelligent transplanting system was designed.

This paper designs an intelligent transplanting system based on PLC control by analyzing the process of artificial transplanting and planting. The system can realize automatic transplanting process, reduce labor intensity and improve automation degree. The innovation lies in the application of PLC control in the field of vegetable transplanting and the realization of automatic transplanting.

The Section 2 introduces the summary of the research progress at home and abroad. In the Section 3, we review the materials and methods of the intelligent transplanting feedback system. Such as the design requirements, structural characteristics, working principles and key components of the intelligent transplanting device, as well as the design of the intelligent transplanting control system. Section 4 gives the experiment process and analysis of transplanting effect of intelligent transplanting system. some conclusions are given in Section 5.

2 Related work

The study of transplanting mechanism started earlier in the foreign countries. The representative of machines was large, high automation and mechanical-joint, such as Italy Ferrari, American FMC, British Pearson and Australia HDl44 automatic transplanting machine [10], although with a high efficiency of picking-up seedlings, the machines were difficult to promote with a larger size, complex structure and expensive cost. Yanmar automatic transplanting machine [11], produced in Japan, was mature in technology but also not suitable for the Chinese vegetable farmer because of the complex structure and the restriction of only working in small plots of ridge. The research and development of automatic and intelligent transplanting machinery was slow in our country. Most of them were aimed at the research of the method and mechanism of seedling collection, but there was no fully automatic intelligent transplanting equipment. For example, Ye, B.L. et al. [12] and Yu, G.H. et al. [13] developed two kinds of non-circular gear planetary system based on rotary pick-up mechanism, which has a high pick-up efficiency with a relatively simple structure. But the disadvantages are the high manufacturing cost and poor mechanical properties in high-speed operating. Wu, J.M. et al. [14] designed a feeding device with automatically identifying system of seedling, which can effectively solve the problem of the leakage of seedlings. but the system can only be used for single point detection that was very low efficiency; Yang, C.H. et al. [15] designed an automatic transportation device for vegetable pot seedling transplanter controlled by programmable logic controller (PLC), and selected and delivered seedlings by means of high-speed servo electric cylinder pushing bar. Zhao, X.E. et al. [16] developed a kind of automatic transplanting machine for pepper pot-disc seedling. The experiment showed that the planting success rate of this machine reached 96.97%, which met the requirements of transplanting holes on the film.

Based on the above analysis, In order to emancipate the labor force, reduce the intensity of manual labor, and improve the intelligent transplanting level of potted seedlings, a kind of intelligent transplanting system for vegetable pot seedling was designed in this paper, which combined with mechanical transmission, sensor technology and PLC control technology. It can provide reference for the development of transplanting technology.

3 Design of intelligent transplanting system

3.1 Design requirement analysis of intelligent transplanting device

In this study, we focused on the 128 holes plug-seedling which was widely used for vegetable pot seedlings transplanting. Based on analysis of the artificial planting process of vegetable pot seedlings, the pot seedlings were removed smoothly from the plantlet plate by machine instead of artificial ones, then put into the planter of the planting mechanism, and the pot seedlings were implanted into the field by the planting mechanism. we designed a device to achieve the automatic feeding, clamping, throwing and planting process of the whole seedlings by a mechanical way. The specific design demands of the device were as follows [17 –21]:

To keep seedling tray orderly transporting, precise feeding and accurate locating with the designed device, and continuous supplying without error accumulation in the progress of operating.

To prevent the damage of the pot body and seedling body caused by movable composition of the device in the process of taking seedlings, reducing the influence on the growth and development of seedling in the late stages.

To prevent the empty seedling tray to interrupt the machine, improving the quality of operation.

To increase the speed of picking-up seedlings to 60 seedings per minute, ensuring it higher than the speed of manually transplanting.

To keep the structure simple and reliability high.

3.2 Structure characteristics and working process of intelligent transplanting device

According to the demands, an intelligent transplanting device for vegetable pot seedling was designed in Fig. 1. The structure was mainly composed of the seedling storage mechanism, the seeding feeding mechanism, the ejecting seeding mechanism, the pinch mechanism, the throwing mechanism and the planting mechanism. The seedling storage mechanism and the feeding seeding mechanism were placed horizontally to the rack, and the seedling tray recycling mechanism was placed in the behind of the seedling feeding mechanism. The ejecting seeding mechanism and the seedling clamping mechanism were vertically assembled to the seedling tray rack, and the seedling throwing mechanism was installed horizontally at the rear of the rack, and the planting mechanism was fixed below the seedling throwing mechanism.

Fig.1

Schematic diagram of intelligent transplanting device for vegetable pot seedling.

Before the running of the device, the full-filled seedling tray was settled in advance. And then, the seedling tray moves downward to keep the bottom in a flat with the surface of the seedling feeding mechanism. The seedling tray was pushed to the seedling feeding mechanism with a pushout plate, placed in the position above the ejecting seeding mechanism, and the pot seedlings was ejecting out using a thimble to get through the permeable hole in the bottom of the seedling tray. At the same time, the clamping mechanism clamped the seedling pot, and then throwed it to the top of the seedling throwing cylinder. As the seedling reaching the top of the throwing hole, the seedling canister opened, and the pot seedling dropped into the planting mechanism, and the work progress of pot seedling planting was finished.

3.3 Design of key components

3.3.1 Seeding storage mechanism

As shown in Fig. 2, the seedling storage mechanism was mainly composed of several symmetrical pallets, chain drives, gear drives, belt drives, seedling tray push plates and stepper motors. Its function was to store a certain amount of seedling tray and automatically transfer these seedlings plates to the seedling feeding mechanism. The main carrier by a plurality of symmetrical component tray, the tray by tray clamp fixed on both sides of the chain is arranged symmetrically. The power source was on the right side of the figure of the stepper motor, through a pair of steering gear and a belt pulley on the power transmission to the shaft on both sides, so that the two axis synchronous reverse rotation, and the chains on both sides simultaneously Until the bottom of the seedling tray was aligned with the surface of the seedling feeding mechanism, the stepper motor stops running, the push plate motor starts, drives the seedling tray to push the plate movement, and pushes the seedling tray to the seedling feeding mechanism.

Fig.2

Seeding storage mechanism.

3.3.2 Seedling feeding mechanism

The Seedling feeding mechanism was mainly composed of a chain, step motor, seedling recovery mechanism, limiting iron bar and a push rod, as shown in Fig. 3, the key role was from the seedling storing mechanism of seedling tray carrying top seedling position, and the empty seedling unified recovery. When the seedling tray was pushed to the seedling feeding mechanism, the seedling tray spacing baffle guide slot seedling middle card into the limiting iron bars, so as to ensure the seedling only along the longitudinal direction, the stepping motor rotates, the chain began with seedling and putting forward movement, until the first row into the seedling tray the ejecting seedling position, the top was permeable holes and the lower part of the ejecting seedlings mechanism, the motor stopped running, automatic transmission function of the seeding tray.

Fig.3

Seedling feeding mechanism.

3.3.3 Ejecting seeding mechanism

When working, the stepping motor drives the crank to make the ejector rod move up and down through the trajectory of the sliding bearing, which was placed above, and eject out the pot seedlings. The research shows that during the ejecting seedling stage, the ejection displacement and the diameter of the ejector rod have an enormous influence on the integrity of the seedling after the ejection of the seedlings. To ensure that the seedling pot can be fully ejector, the appropriate size and displacement of the ejector rod must be selected when designing. The motion diagram of the ejecting seeding mechanism was shown in Fig. 4.

Fig.4

Motion diagram of the ejecting seeding mechanism.

The displacement equations of the A and B points in the graph are ${\begin{matrix} x_{A} = l_{OA} cos α_{1} \\ y_{A} = l_{OA} sin α_{1} \end{matrix}$ (1) ${\begin{matrix} x_{B} = x_{B} + l_{OA} cos α_{2} \\ y_{B} = y_{B} + l_{OA} sin α_{2} \end{matrix}$ (2)

l_OA— the length of the crank OA, mm

l_OB— the Length of connecting rod AB, mm

α₁— the angular displacement of crank OA, rad

α₂— the angular displacement of connecting rod AB, rad

From α₁ = ωt, l_OAsinα₁ = l_ABsinα₂ + e, formula (3) can be got: $cos α_{2} = \sqrt{1 - {(\frac{l_{OA} sin α_{1} - e}{l_{AB}})}^{2}}$ (3)

ω— the angular velocity of crank OA, per rad

e— eccentricity, mm

The motion equation of the centroid of B point is ${\begin{matrix} x_{B} = l_{OA} cos (ω t) + l_{AB} cos [\sqrt{1 - {(\frac{l_{OA} sin (ω t) - e}{l_{AB}})}^{2}}] \\ y_{B} = e \end{matrix}$ (4)

B point velocity equation is: ${\begin{matrix} {\dot{x}}_{B} = - l_{OA} ω cos (ω t) - \frac{l_{OA} ω [sin (ω t) - e] cos (ω t)}{l_{AB} \sqrt{1 - {(\frac{l_{OA} sin (ω t) - e}{l_{AB}})}^{2}}} \\ {\dot{y}}_{B} = 0 \end{matrix}$ (5)

It can be seen from the above formula that the movement of the ejecting seeding mechanism was mainly related to the length, speed and eccentricity of the crank OA and the connecting rod AB rod.

3.3.4 Pinch mechanism

The clamping mechanism needs to carry out the action of clamping and releasing the pot seedlings. As shown in Fig. 5, the principle was mainly through adjusting the relative position between the optical axis and the two parallel optical axes on the back of the claw box through the sliding bearing installed on the small slide block.

Fig.5

Pinch mechanism.

First, the heavy duty linear guide rail was mounted on the claw box, and a large claw body was mounted on it. Two limit screws are installed on the guide rail to limit the extreme position of the upper and lower movement of the large jaw body, which was mainly used to achieve the pulling action. Secondly, in the large claw body middle groove placement of a heavy linear guide, linear guide can drive the small body moved a short distance in the groove of the large claw, used for seedling gripping action. Then, install the small body and seedling claw on it, and the optical axis passes through the bearing body on small transverse arrangement. Finally, at the bottom of each box, the top of the positioning box at both ends and the middle position of the claw, the spring with different stiffness coefficient was installed. The middle position of the spring to the seedling claw maintained in clamping state, through lever principle, provide clamping force, through the realization of seedling claw grab action relative position adjustment in small groove large claw in the body, namely small grooves in the bottom position, seedling claws open, and the contraction of grab. The upper and lower two pull springs can maintain a stable relative position between the claw body and the claw box, and achieve the action of pulling out by adjusting the relative position of the large claw body and the claw box. To sum up, the adjustment of the relative position of each component was mainly realized by the power provided by the spring, adjusting the relative position between the two parallel optical axes of the middle lateral adjusting optical axis and the back of the claw box, to realize the action of the paw seedling’s claw taking and pulling out simultaneously.

3.3.5 Planting mechanism

This study adopts the planetary wheel planting mechanism [22], effect of planetary gear train make in the mouth with the planting mechanism of rotation always keep down attitude, in each side planting mouth are equipped with convex design, its role was to enable the duckbill with rotational position to achieve open and close in the process of rotation that is, the state received seedlings from the top was closed, when rotating to the bottom, the boss will open the top of the duckbill, seedlings dropped, and then travels a distance close to return the highest point for the next grafting.

3.4 Design of Intelligent transplanting system

3.4.1 Transplanting action analysis

Based on the function-action process-action method [23](F-P-A, the function to be realized by machinery is to conceive the technics action process, and then the technics action process is decomposed to get the corresponding execution action), the intelligent transplanting device for vegetable pot seedling can be decomposed into the actions of the automatic seedling feeding, the automatic seedling picking-up and the automatic planting. The function of the automatic seedling feeding action was mainly to continuously and automatically transport the seedling tray stored in the seedling storage mechanism to the designated location, and no interference can occur between the front and rear seedling trays during the entire process. The control process was shown in Fig. 6(a). The key point of automatic seedling picking-up action was to achieve precise alignment of the seeding hole and the ejector pin, and the seedling clamping mechanism can cooperate with the ejecting seedings mechanism to achieve the coordinated operation of pots seedling ejection, clamping, transporting and putting into operation. The control flowchart was shown in Fig. 6(b). The automatic planting system was the last control link in the process of seedling planting, and it was necessary to cooperate with the seedling mechanism to avoid the problems of leakage. The control flowchart was shown inFig. 6(c).

Fig.6

Flow chart of control system.

3.4.2 Intelligent transplanting control system

The intelligent transplanting control system consisted of driving motor, sensor module, controller module, microcomputer and system software. The co-processor and the master controller were used to complete the monitoring and automatic conveying control process of vegetable pot seedling. The co-processor converted the analog output signal of the sensor into the digital switch quantity input to the main controller. It was convenient to realize the monitoring of analog signal in the system, the signal monitoring was stable and reliable, and the maintenance cost of the system was reduced. The main controller communicates with the microcomputer through the RS485 interface, displayed the relevant information of monitoring and control in the interactive interface, and realized the man-machine. Interactive function, system structure block diagram as shown in Fig. 7.

Fig.7

Structure block diagram of intelligent transplanting control system for vegetable pot seedlingTransplanting experimental design and analysis.

The motion of all the components in the intelligent transplanting device for vegetable pot seedling was completed by the sensor and the stepper motor, the position of each sensor and step motor was shown in Fig. 7, the stepping motor as the power source of the whole movement of a device, the sensor can obtain accurate signals in each part of the action, in order to control the parts of the stepper motor running and stopping. The Intelligent transplanting Control system of the whole device mainly included seedling feeding monitoring module, seedling sampling monitoring module and planting monitoring module. The seedling sending monitoring module consisted of pressure sensor and lack of seedling indicator lamp. A PVDF sticker pressure sensor (response time <1 ms, response point <200 g, range 0–50 kg) was arranged on the slot wheel seedling storage plate, which was used to detect whether there was a vegetable pot seedling tray waiting for transportation on the seedling storage plate. According to the change of pressure signal, the controller guided the motor to drive the sprocket of seedling storage disk to carry out the transmission of seedling disk, and detected the conveying position of seedling disk by setting the stroke switch on the conveyor belt of the sprocket wheel, and triggered the push-rod mechanism to perform the push disk action. To move the seedling disc from the grooved wheeled seedling storage mechanism to the seedling disc conveyor belt; to install a finite position sensor at the junction between the seedling disc conveyor belt and the push rod mechanism; when the seedling disc was detected to enter the seedling disc conveyor belt from the slot wheel seedling storage mechanism, sending the signal to the controller, stopping the push disc movement, ensuring the stability of the conveying action; detecting the pressure change of the ejecting seedling mechanism working through the pressure sensor, obtaining the abnormal information of the sampling seedling; removing and transporting the vegetable pot seedling from the seedling tray to the planting mechanism, A CCD camera was arranged above the conveying track of the cup to monitor the leakage and full load of the vegetable pot seedling in the cup to obtain the conveying information.

The purpose of this experiment was to verify the performance of the designed seeding device, to provide a basis for further improvement of the machine.

3.5 Testing conditions

Testing equipment and instruments: Test prototype of pot seedling high efficiency planting device, variable frequency speed regulating motor, DZF-6250 vacuum drying box, stopwatch etc.

Experimental materials: Selected seedling specifications for the 128 points, age 42d, FNZ substrate (peat and perlite component volume ratio of 2 : 1, after sowing by vermiculite covering) broccoli seedling number plate, by measuring the average seedling height of pot body seedlings was 128 mm, the width of leaf spread of seedling was 79 mm, the average quality of single plant was 11.85 g, the pot body as square, the length of caliber border was 21×20 mm, height pot was 56 mm.

3.6 Evaluation index of testing

The seedling planting device prototype test, due to the use of the top of the way from the seedling, it may cause the seedlings not to be ejecting or problems such as the failure of the top out and the clip in coordination and the failure of the seedling picking-up, and between each part with the device was not in place due to leakage phenomenon of seedling, so in this experiment, the operation performance and the success rate of picking-up seedlings and the leakage rate to examine device.

The success rate of picking-up seedlings $η_{1} = \frac{N_{1}}{N_{0}} \times 100$ (6)N₁–The number of the seedlings in the seedling tube for the seedling of the pot

N₀–The total number of seedlings in the seedling tray

Rate of leakage of seedlings $η_{2} = \frac{N_{2}}{N_{0}} \times 100$ (7)N₂– The number of the seedlings that have not entered the planter

3.7 Testing experiment

In this experiment, two factors including moisture content and the speed of picking-up seedlings were selected for three factors. The average water content of plug seedlings was 23.4%, 32.7% and 40.1% (After weighing pot seedling matrix, placed in 80 drying oven drying, every 2 hours to take out the measuring its quality, until the two measurement results were basically the same, and then into the formula, pot seedling moisture content were obtained), respectively. The rate of picking-up seedlings was 60, 75 and 90 seedlings per minute, respectively. A total of three groups of seedlings were needed in the whole test (three sets of seedlings in each group, 128 seedlings per set, and the rate of emergence was 100%). Three groups of plug seedlings with three moisture contents were automatically transported by pot seedling transplanting with different the speed of picking-up seedlings. During the experiment, we observed the number of pot seedlings successfully ejecting and sending to the seedling basket and recorded the number of seedlings. Figure 8 was a test prototype.

Fig.8

Test prototype.

3.8 Results and analysis

The test results of the success rate of picking-up seedlings and the leakage rate of the seedlings were known as Table 1. It can be found that the effect of water content on the success rate of picking-up seedlings was very significant, and the effect of the speed of picking-up seedlings on the leaked seedlings rate was very significant. At the same time, due to the effect of the working principle of the device, the rate of seedling leakage was related to the success rate of picking-up seedlings of, and it was influenced by the speed of picking-up seedlings.

Table 1
Results of the success rate of picking-up seedlings and the leaked seedings rate

Water content/% Seedling (rate/seedlings·min-1) N0/number N1/number N2/number η1/% η2/%

23.4 60 128 128 0 100.00 0

75 128 126 3 98.44 2.34

90 128 125 6 97.66 4.69

32.7 60 128 126 3 98.44 2.34

75 128 124 6 96.88 4.69

90 128 120 11 93.75 8.59

40.1 60 128 124 5 96.88 3.91

75 128 122 8 95.31 6.25

90 128 117 13 91.41 10.16

Water content/%	Seedling (rate/seedlings·min-1)	N0/number	N1/number	N2/number	η1/%	η2/%
23.4	60	128	128	0	100.00	0
	75	128	126	3	98.44	2.34
	90	128	125	6	97.66	4.69
32.7	60	128	126	3	98.44	2.34
	75	128	124	6	96.88	4.69
	90	128	120	11	93.75	8.59
40.1	60	128	124	5	96.88	3.91
	75	128	122	8	95.31	6.25
	90	128	117	13	91.41	10.16

As can be seen from Fig. 9a, the high efficiency planting device of the pot seedlings had a higher the success rate of picking-up seedlings than the artificial planting. Through observation, it was found that the effect of water content and the speed of picking-up seedlings on the success rate of picking-up seedlings of seedlings decreased in the range of test. The greater the moisture content of pot seedlings, the greater the adhesion between seedling matrix and seedling tray, and even greater than the binding force between roots, resulting in a large number of matrix residual holes in the ejecting seedling process. At the same time, the speed of picking seedlings was too large, and the ejecting seedings mechanism will produce a certain amplitude of vibration, which will affect the accuracy of the ejecting seedlings. Therefore, when the moisture content was 40.1% and the speed of picking-up seedlings was 90 per minute, the success rate of picking-up seedlings of seedling picking was the lowest.

Fig.9

Effect of test factors on quality of work.

From Fig. 9b, we can see that the seedling leakage rate was relatively low under the condition of low the speed of picking-up seedlings and low moisture content. Through observation, it was found that the influence of moisture content and the speed of picking-up seedlings on seedling leakage rate was increasing. Because the faster the speed of picking-up seedlings as, the more probability of mismatch will happen between the device placement and planting institutions, resulting in higher rate of seedling leakage. Therefore, when the moisture content was 40.1% and the speed of picking-up seedlings was 90 seedlings per minute, the rate of seedling leakage was the highest.

Based on the analysis, the design of the automatic vegetable pot seedling transplanting machine in the speed of picking-up seedlings was 75 seedlings per minute, the success rate of picking-up seedlings of picking-up seedlings over was 95%, while the seedling leakage rate was relatively low, more than manual work efficiency; and to improve the feeding the success rate of picking-up seedlings and reduce the leakage rate of seedlings, in order to meet the requirements of vegetable planting technology under the water should be used as far as possible to reduce the rate of seedling matrix device.

4 Conclusions

(1) According to the vegetable pot seedling planting agronomic requirements, designed a kind of simple structure, convenient control of vegetable pot seedling planting device efficiently, the chain transmission mechanism to realize the seedling tray automatically feed, by the movements of the slider-crank mechanism to achieve ejecting seedlings, used lever principle and spring to realize opening and closing of the mechanism of the action, are done by planetary wheel planting vegetables pot seedling planting.

(2) An intelligent transplanting system of vegetable pot seedling based on PLC control was designed, the system monitors the whole movement process of planting through pressure sensing, stroke detection and limit switch, obtains each planting stage of the seedlings during planting, and uses the CCD camera to identify the key steps of planting and seedlings to determine whether it is missing. Through the feedback of the display device planting information to the operator, the whole system can realize the orderly delivery and accurate planting of the seedlings.

(3) The experiment designed for testing the performance of the device was carried out, and the results indicated that: in the test range, water content and the speed of picking-up seedlings have a remarkable effect on the success rate of picking-up seedlings of picking-up seedlings, especially the water content; when the speed of picking-up seedlings was 75 seedlings per minute, the success rate of picking-up seedlings was over 95%. Under the condition of the agronomic requirements of vegetable planting, the lower the water content, the higher the success rate of picking-up seedlings, the lower the leakage rate, the better the quality of the vegetable planting. It can provide reference for the intelligent transplanting technique of vegetable pot seedling.

Footnotes

Acknowledgments

The work was sponsored by the National Natural Science Foundation of China (No. 51875175), the National Key Research and Development Program of China Sub-projects (No. 2016YFD0700103), the Innovation Scientists and Technicians Troop Construction Projects of Henan Province (No. 184200510017), and the Innovation Scientists and Technicians Talent Projects of Henan Provincial Department of Education (19HASTIT021).

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