Design and analysis of a portable greenhouse weeding machine

Abstract

With the development of modern agriculture, more and more greenhouse was used in agricultural production. According to the requirement of small-scale agricultural devices in greenhouse production, this paper putted forward a design idea of portable greenhouse weeding machine. Its structure and function were designed and analyzed. The weeding machine consists of five parts: the power part, the transmission part, the control part, the execution part and the shell part. The resistance analysis of different soil quality and humidity was carried out by statics analysis method. The mathematical model was established to calculate that the optimum weeding effect could be achieved when the cutting tool rotated at a speed of 200 r/min. Execution part of weeding machine was composed of roller and cutting tool, transmission part was made up of steel wire flexible shaft, couplings and bevel gear. The three-dimensional model of the weeding machine was established and a prototype was made. The static and modal analysis of the roller was carried out by using ANSYS. The results of simulation and prototype experiments showed that the structure of the device was simple and stable, with strong adaptability, high efficiency and no pollution, and the working effect meets the requirements, which provided a new way of thinking and method for the development of a lightweight weeding machine.

Keywords

Portable lightweight greenhouse production weeding machine finite element analysis

1. Introduction

In recent years, the application of greenhouse planting in agriculture is more and more extensive. Weeds are easy to grow in the field because of favorable climatic conditions during greenhouse production. Mechanical weeding is a key production technology for sustainable development of dry farming, and it is also an ideal weeding method. It overcomes the disadvantages of low efficiency and high labor intensity of manual weeding. Moreover it doesn’t have the problem of chemical weeding, which easily pollutes the environment and seeds, weeds are easy to produce resistance to insecticides. At the same time it also has good ventilation effect and soil loosening. It not only meets the needs of green agriculture production, but also adapts to the development trend of conservation tillage [1, 2]. At present, the products of Europe and America are mainly used in horticultural work, such as rotary tillage, grass cutting, shattering and so on. As for Japan, due to the early start of basic industry, the level of research and development of micro-tillage machine is relatively high. However, the research on the mechanical devices used in the special environment of greenhouse is relatively few. and there is no mature weeding machine in the market.

For the special working environment of greenhouse, the large agricultural machinery weeding operation is restricted in space first. The weeds between crops are mainly small gramineous plants, and the distribution is scattered, so it is difficult to ensure the effect of weeding in the work of ordinary agricultural machinery. A rotary tiller developed by Guo et al. was a pure mechanical system [3], but couldn’t be used for interline and interplant weeding. Han et al. developed a medium tillage weeding machine [4] which had great improvement in the field weeding effect but the device needs tractor driving and it had not been used in the greenhouse environment. An electric micro-tillage machine developed by Zhao et al. was small in volume and less in pollution [5], but it was unable to realize weeding in greenhouse. Akhalaya and Shogenov developed a automated multifunctional tillage machine and simplified the fuselage structure [6], which can be applied to weeding and digging soil between crops, and had great innovation. Zeng et al. developed a micro tillage machine which works smoothly and operates simply [7]. However, the device is mainly used in orchards and tea gardens.In order to achieve clean and pollution-free weeding operation and adapt to the limited working environment of the greenhouse, this paper presented a design scheme of portable greenhouse weeding machine, and analyzed its structure and function. The lithium battery was used as the power, and the bevel gear and flexible shaft were used to work with knife roller [8]. The device has the characteristics of simple and compact structure, stable and reliable operation, and the strength check and feasibility analysis of the roller were carried out, and the prototype was tested by experiment.

2. Analysis of weeding in greenhouse

At present, artificial and chemical weeding methods are mainly used in most areas, but with the development of agricultural modernization and the improvement of the quality of agricultural products, the disadvantages of these two kinds of weeding methods are becoming more and more prominent. The average efficiency of a small weeding machine is 0.3–0.5 ha/day, which is eight times that of manual weeding. The yield increase of mechanical weeding to crops is about 36.2%, which is slightly lower than that of chemical herbicides. However, improper use of chemical herbicides can easily damage crops and pollute the environment. The problems of pesticide residues and weed resistance caused by the long-term use of herbicides are becoming more and more serious [9, 10]. In order to meet the needs of the development of green agriculture production, it is more and more important to develop the greenhouse weeding machine [11].

The existing mechanical weeding equipment mainly includes two kinds of manual weeding machine and power weeding machine. The manual mower is driven by the user and has multiple gears installed in its transmission mechanism. As a result, the size of the manual mower is larger, and its transmission mechanism is more complex. The components are easily damaged and difficult to maintain. The power weeding machine is equipped with an engine, which drives the cutting tool to rotate, and it can work automatically when pushing the weeding machine on the lawn.Most of the existing power lawn mowers are powered by gasoline. The gasoline lawn mower has a large amount of exhaust gas and heat in use, and it is large in size, high in weight and inconvenient to carry. Therefore, it is great significance to develop a battery-driven lightweight small-sized weeding machine. The device can not only ensure the working efficiency, reduce the manpower intensity, but also reduce the pollution to the environment of the greenhouse during the weeding operation in the greenhouse [12].

3. Design and analysis of portable greenhouse weeding machine

3.1 Design requirements

Considering the actual working environment of portable greenhouse weeding machine and ensuring the work effect. After analysis, the device should be designed to met the following requirements:

1.
In order to ensure the best working effect, we analyzed and calculated that the cutting tool speed should be about 180 r/min, so the design of the cutting tool speed was 200 r/min;
2.
Considering the portability and lightness of the device, in order to adapt to its working environment, the maximum size of the device was not more than 150 cm, and the total weight of the equipment should not be too high;
3.
The device could adapt to many kinds of terrain and ensured the effect of weeding;
4.
The weeding efficiency of the devices was not less than 90%;
5.
The device works stably and had high safety factor;

3.2 The structure design of the weeding machine

3.2.1 Design of the overall scheme

In order to adapt to the limitations and particularity of greenhouse space and ensure the work effect, the whole device was designed optimally. The weeding machine consists of five parts: the power part, the transmission part, the control part, the execution part and the shell part.

The power part contains batteries and motors. The use of lithium battery instead of gasoline engine as a power source, on the one hand to reduced the weight of the body and easy to transport devices, on the other hand, to avoid damaged to the growing environment of crops. The transmission part was composed of bevel gear, coupling and flexible shaft, which simplified transmission mechanism to the maximum extent and improved transmission efficiency. The control part was mainly composed of governor and switch. The governor could adjust the motor speed according to the actual demand in a certain range. The execution part included roller and cutting tools. The roller was made of aluminium rods, as the fixing device for the cutter, at the same time, cutting tools were embedded into the soil with the help of the roller’s own gravity. The shell part included a device housing, the extendable rod and the handle which were the support of the whole device.

Figure 1.

Schematic diagram of device. 1. Handle; 2. Extendable rod; 3. Linkage unit; 4. Roller bracket; 5. Roller.

3.2.2 Design of transmission scheme for weeding machine

The transmission part of the weeding device adopted the steel wire flexible shaft, which should ensure that it could work at the minimum bending angle. Under the premise of meeting the requirement of weeding operation, the bending times of the flexible shaft should be reduced as much as possible [13]. The key technology of this design was to use steel wire flexible shaft with coupling and bevel gear to change the direction transmission. Soft shaft transmission not only simplified the structure of transmission system, but also minimized the weight of the device.

Figure 2.

The structure diagram of the transmission part of the prototype.

Figure 3.

Prototype integral diagram.

3.3 Working principle of portable greenhouse weeding machine

The device was powered by a lithium battery and drove the motor to rotate. The power was transferred from the power output shaft to the coupling, and through the coupling to the steel wire flexible shaft, which transmitted the power to the weeding roller. The weeding roller was fixed on the tool holder by bearing. The cutter was mounted on the roller, and when the weeding roller rotated, the cutting tool on the roller rotated and cut into the soil for weeding and digging the soil [14, 15].

The weeding parts depend on the motor power output shaft to provide the power to work. When the speed of the weeding roller needs to be adjusted, the speed of the motor output shaft can be changed by adjusting the speed regulator to obtain the suitable operating speed.

4. Establishment and force analysis of mechanism modal

4.1 Establishment and analysis of soft shaft model

Aiming at the insensitive action of soft shaft when it is used in concrete application, after in-depth analysis, and thought that the problem was caused by the following three reasons: First, the manufacturing process of flexible shaft is not perfect, the precision is not high enough; second, it is related to the planning and design of the control system; third, there are some deviations in the installation of flexible shaft. Therefore, Li et al. introduced the empirical formula of the friction force of soft shaft in detail [16]. By using LS and PSO theory and combined with a large number of experiments, they put forward a modified formula which is more consistent with the soft axis force.

$f=C_{f}\frac{l^{X_{f}}d^{Y_{f}}}{\rho^{Z_{f}}}$ (1)

In the formula: $f$ is the soft shaft friction; $C_{f}$ is the modified friction coefficient related to material friction coefficient; $l$ is the length of mandrel; $d$ is the diameter of mandrel; $\rho$ is the radius of curvature; $X_{f}$ , $Y_{f}$ , $Z_{f}$ , are the length coefficient, diameter coefficient, radius of curvature coefficient.

From the Euler equation of tribology, the force driving the soft shaft could be obtained:

$P_{1}=P_{0}e^{f(\alpha_{1}+\alpha_{2})}$ (2)

In the formula: $P_{1}$ is the active tension; $P_{0}$ is the initial binding force; $e$ is the natural logarithm; $f$ is the friction factor; $\alpha_{1}$ is the first bending angle; $\alpha_{2}$ is the second bending angle.

It can be seen from the above formula that when the $P_{1}$ of pulling the soft shaft is greater than $e^{f\left({\alpha_{1}+\alpha_{2}}\right)}$ of the initial binding $P_{0}$ at the other end of the flexible shaft, the soft shaft will be pulled. Therefore, it is concluded that the increase of the friction coefficient $f$ and the bending angle $\alpha$ of each segment of the flexible shaft makes the force of pulling the soft shaft increase accordingly. The friction force can be reduced by reducing the bending degree of the flexible shaft and selecting the soft shaft with small friction coefficient.

4.2 Analysis of soil moisture

4.2.1 Effect of soil moisture on tillage efficiency and traction resistance

Soil relative humidity is an important factor to analyze the influence of soil on the working effect of the device. It was defined as the ratio of natural soil water content to total field water holding capacity.

$w_{0}=\frac{w}{w_{n}}\times 100\%$ (3)

In the formula: $w$ is the natural soil water content; $w_{n}$ is the total field water holding capacity.

Relative humidity was used to compare soil moisture content in different environments and conditions. For dryland, the relative humidity of the soil is in the range of 40% to 60%, which is more suitable for cultivation. so the soil moisture within this range is called suitable soil moisture.

With the decrease of $w$ , the soil is too dry and hard, the soil block is difficult to be broken, and the tillage resistance is high.

With the increase of $w$ , the soil is too wet and sticky, so it is easy to appear congestion and incomplete overturning of soil ridges and so on.

Most of the agricultural greenhouses belong to dryland planting, the soil is sandy loam, which is suitable for soil loosening, so the torque and grip of the weeding wheel need not be too large.

4.3 Analysis of soil resistance

Soil compaction: Under vertical loading, the compressive capacity of the soil at different depths is called Compaction.

The value of compaction can be calculated by the following formula:

$\displaystyle q=q^{0}\times h$ (4)

In the formula: $q^{0}$ – Unit pressure strength; $h$ – Depth of plunger depression.

The experimental results showed that the unit pressure strength of different soils was showed in the following Table 1.

Table 1

Unit pressure strength of various soils

Soil types and	Loose soil	Medium sedimentary	Viscous sedimentary	Viscous and dry hard
	states	soil	soil	sedimentary soil
Unit pressure strength	0.5–1.5	3–8	6–10	12–20

Table 2

Friction coefficient between different soil and cutting tools

Soil species	Friction factor	Angle of friction
Sandy loam (loose)	0.25–0.35	14 ${}^{\circ}$ –19 ${}^{\circ}$ 30 ${}^{\prime}$
Sandy loam (Viscous)	0.50–0.70	26 ${}^{\circ}$ 30 $\prime$ –35 ${}^{\circ}$
Medium sandy clay	0.35–0.5	19 ${}^{\circ}$ 30 $\prime$ –26 ${}^{\circ}$ 30 ${}^{\prime}$
Heavy viscous soil	0.6–0.90	31 ${}^{\circ}$ –42 ${}^{\circ}$

According to the data in the table, From the relevant research between grinding edge angle and static cutting angle [17, 18], it could be concluded that the maximum force of the cutting tool was when it rotates to the depth angle of 10 ${}^{\circ}$ –15 ${}^{\circ}$ . In different bending angle. According to the study of the relationship between the quality and stress distribution of rotary cutting tools, it was known the optimum range of bending angle of rotary cutting tool is 125 ${}^{\circ}$ –130 ${}^{\circ}$ .

4.4 Calculation of motor power and torque

According to the torque formula, the driving torque of the motor can be obtained as follows:

Figure 4.

Finite element mesh model of roller.

Motor rated power: $P=$ 200 W

Motor speed: $n=$ 200 $r$ /min

$T=9550\times\frac{P}{n}=9.55∼{}N\cdot m$ (5)

From the analysis of the bending angle and the rotary angle of the cutting tool, it could be seen that when the cutting tool works in the optimum angle range, the power, speed and torque of the motor can ensure the best weeding effect.

5. Force and modal analysis based on ANSYS

5.1 Build finite element model

The structure of the portable greenhouse weeding machine was relatively simple, the key part of which lies in the soft shaft and roller. In the working process, the roller was the most important force component, so it was necessary to analyze by finite element method. The 3D model of roller was built by CREO and imported into ANSYS software.

Setting related parameters, the roller was made of solid aluminum ingots. Elastic modulus $E=$ 71.7 GPa, Poisson’s ratio $\mu=$ 0.33, Density $\rho=$ 2.7 $\times$ 10 ${}^{3}$ kg/m ${}^{3}$ . The weeding tool holder was made of HT200, Elastic modulus $E=$ 130 GPa, Poisson’s ratio $\mu=$ 0.25, Density $\rho=$ 7.93 $\times$ 10 ${}^{3}$ kg/m ${}^{3}$ , Using the system to mesh the model, the mesh size was 1 mm, the roller was divided into 486759 nodes and 299632 elements tetrahedron, and the finite element mesh model was obtained as shown in Fig. 4.

5.2 Static analysis of roller

The fixed constraint was established in the position of the roller axis, and the torsional torque was 9.5 N $\cdot$ m. From the analysis results, The analysis results showed that the stress change of the weeding tool holder was small, and the strain of the tool holder was also small. The results of statics analysis of roller were showed in Fig. 5. The maximum stress and strain were in the roller bracket, and the maximum stress was 0.261 MPa, which was less than the allowable stress of the material. The maximum strain was 3.8 $\times$ 10 ${}^{-5}$ mm and the deformation was very small. So the whole structure design completely met the requirements.

Figure 5.

Stress nephogram and strain contours of roller.

5.3 Modal analysis of roller

Modal analysis is to transform the physical coordinates of the vibration differential equations of linear time-invariant systems into modal coordinates and decouple the equations into a set of independent equations described by modal coordinates and modal parameters. In order to find out the modal parameters of the system [19, 20].

Because the roller was in the state of rapid rolling, it would inevitably produce vibration, and the vibration of the roller, tool holder and other components would have a great impact on the weeding effect of the cutting tool and the stability of the whole device. Therefore, it was necessary to study its dynamic characteristics, and measured the feasibility of portable greenhouse weeding machine by modal parameters such as natural frequency, damping, damping ratio and vibration shape.

In the process of modal analysis, ANSYS provided six modal extraction methods: Subspace method, block method, power dynamics method, reduced method, asymmetric matrix method and damped method. The block method was suitable for solving eigenvalue problems of large symmetric matrices. This method has high accuracy and fast convergence speed. Therefore, this paper used the block method to analyze the modal of roller structures.

The first six order modals of roller were calculated by finite element analysis with ANSYS, and the first six orders of mode were showed in Fig. 6. The results of the roller modal analysis were derived, as shown in Table 3.

Figure 6.

Modal analysis of roller.

Table 3

Modal analysis results of roller

Order	Natural frequency/Hz	Description of dynamic performance
1	1163	In a rigid state
2	3894.6	Roller slight shimmy
3	3896.6	Roller slight shimmy
4	5695.9	Fixed support and roller shimmy
5	5698	Fixed support and roller shimmy
6	5952.1	Integral support and roller shimmy

According to the vibration pattern of roller and modal analysis, it could be seen that the first order natural frequency of roller working mode was relatively low, and the latter five frequency was higher. However, all the frequencies were much higher than the working frequency of the small weeding machine studied in this paper, so there would be no resonance. It could be seen from the mode diagram that the main feature of the weeding machine was the slight swinging of the tool holder, while the roller and its connecting part were not large deformed, and had good stability, which would not affect the work of the roller and the cutting tool.

6. Field experiment and result analysis

6.1 Experimental method

Test of weeding and turning soil using prototype in Maize Experimental Field. The maize in the experimental field was planted in an equidistant line with a width of 200 mm. the absolute moisture content of the soil in the experimental field was 50% and the soil type was Sandy loam. The average height of maize seedling was 150 mm, and the artificial weeding was performed once during the seedling period. There were many kinds of weeds on the ridge surface and they were randomly distributed, and the height of the grass was less than 70 mm. Each test area is 10 meters long and two meters wide, and the prototype is operated by a single person in the experiment. Set the rotation speed of the cutting tool to 180 r/min, 200 r/min, 220 r/min respectively, and carried out the field experiment with different travel speed and weeding depth [21].

6.2 Evaluation of experimental results

6.2.1 Determination of weeding rate

The weeding rate is the ratio of the number of weeds removed by prototype operation and the total number of weeds before weeding. The formula is as follows

$\eta_{1}=\frac{C_{1}-C_{2}}{C_{1}}$ (6)

In the formula: $\eta_{1}$ is the weeding rate; $C_{1}$ is the total number of weeds before weeding; $C_{2}$ is the total number of weeds after weeding.

6.2.2 Determination of seedling injury rate

Seedling injury rate is the ratio of the number of damaged seedlings by prototype operation and the total number of seedlings before weeding. The formula is as follows:

$\eta_{2}=\frac{Z_{2}}{Z_{1}}$ (7)

In the formula: $\eta_{1}$ is the seedling injury rate; $Z_{1}$ is the number of maize seedlings in experiment area before the experiment; $Z_{2}$ is the number of injured seedlings after operation.

6.2.3 Determination of broken soil rate

The experimental plots were sampled, collected, measured and determined the rate of broken soil, and the depth measured was the same as the depth of weeding. The rate of broken soil was generally the weight ratio of soil under 20 mm in diameter after operation, and the rate of broken soil was more than 70% as the standard [22].

Each group of experiments was measured three times in three plots with the same parameters, and the results were averaged. The experimental results were shown in following tables.

Table 4
Experimental results of prototype machine when the cutting tool speed is 180 r/min

Experiment	Working speed/	Weeds depth/	Broken soil	Weeding	Seedling
No.	$m\cdot s^{-1}$	$m m$	rate/%	rate/%	injury rate/%
1	0.4	30	87.1	92.7	1.1
2	0.4	40	86.4	92.0	1.3
3	0.6	30	84.3	92.2	1.7
4	0.6	40	83.5	91.8	2.2
5	0.8	30	81.4	87.5	3.7
6	0.8	40	80.3	88.9	4.6

Table 5

Experimental results of prototype machine when the cutting tool speed is 200 r/min

Experiment	Working speed/	Weeds depth/	Broken soil	Weeding	Seedling injury
No.	$m\cdot s^{-1}$	$m m$	rate/%	rate/%	rate/%
1	0.4	30	89.6	94.9	1.3
2	0.4	40	87.2	93.8	1.6
3	0.6	30	86.5	94.2	2.1
4	0.6	40	85.8	94.0	2.9
5	0.8	30	83.4	89.3	4.6
6	0.8	40	82.1	91.6	5.1

Table 6

Experimental results of prototype machine when the cutting tool speed is 220 r/min

Experiment	Working speed/	Weeds depth/	Broken soil	Weeding	Seedling injury
No.	$m\cdot s^{-1}$	$m m$	rate/%	rate/%	rate/%
1	0.4	30	91.4	94.8	2.1
2	0.4	40	90.1	94.2	2.5
3	0.6	30	87.9	93.7	4.3
4	0.6	40	86.8	93.8	4.9
5	0.8	30	85.3	90.2	6.7
6	0.8	40	84.1	91.3	7.4

It can be seen from the experimental results that increasing the rotation speed of the cutting tool of the weeding machine is beneficial to the removal of weeds between rows and increasing the rate of soil fragmentation, and at the same time, it will also lead to the increase of the rate of seedling injury. This is because with the increase of the rotation speed of the weeding tool, the impact force of the cutter on the soil increases, the weeds with shallow roots are easily removed, and the crop roots are also easily damaged when the weeding machine is moved.

When the weeding machine is working, the root of the weed is chopped out, which speeds up the death of the weed and prolongs the cycle of weed regeneration. The experimental results showed that the herbicidal efficiency was higher and the rate of seedling injury was lower when the speed of weeding machine was 0.6 m/s. and the depth of weeding was 30 mm, which mainly depends on the depth and toughness of maize seedling root and weed root. The roots of weeds were generally softer and shallower, and when the weeding depth was shallow and the speed was faster, the weeds could be chopped and uprooted. However, if the weeding speed had set too slow and the depth had set too deep, the deep roots of maize seedlings would have damaged. Because the speed and power of the machine were fixed, if the weeding depth increased, the total amount of broken soil would increased, and the rate of broken soil would decreased.

7. Conclusions

According to the requirement of working environment and the development of small weeding machine, this paper putted forward a design scheme of portable greenhouse weeding machine. Under the special working environment of greenhouse, the design meets the requirements of no pollution, strong adaptability and high working efficiency of the weeding device. The three-dimensional solid model of weeding machine was established and finite element analysis was carried out on it by ANSYS. The results showed that the stress of cutting tool, roller and soft shaft, the strain produced by cutting tool and the stress of fixed support were smaller, and the frequency of the first three steps of the cutting tool roller was lower. The vibration was not obvious. The prototype was tested in the test field and obtained the statistical data. The results showed that the effective weeding rate was over 93%, and the rate of seedling injury was less than 4% its working effect met the requirements.

In the process of experiment, the prototype was operated by a single persons it worked stably and had no sharp jitter. Since the device was powered by a lithium battery, no harmful gases were emitted during the operation. The device not only realized the omnidirectional weeding, but also cut off the root of weeds by knife roller, which greatly prolongs the cycle of weed regeneration. Through the theoretical analysis of the device and the analysis of the experimental results of the prototype, it is shown that the design scheme of the weeding machine is feasible and has a great application prospect in the weeding operation in greenhouse. At the same time, the device was portable and efficient, and the mechanization level of weeding in greenhouse was improved remarkably. The analytical method used in this paper was scientific and effective, which shortens the period of product design, and provided new methods and ideas for the research of related mechanical devices in other fields. It was of great significance for reference.

Footnotes

Acknowledgments

The research is funded partially by the students scientific research innovation training project of Nanjing Agricultural University (1730A08), and the six major talent summit in Jiangsu province (2015-zbzz-011).

Conflict of interest

The authors declare that there are no competing interests regarding the publication of this paper.

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Design and analysis of a portable greenhouse weeding machine

Abstract

Keywords

1. Introduction

2. Analysis of weeding in greenhouse

3. Design and analysis of portable greenhouse weeding machine

3.1 Design requirements

3.2.1 Design of the overall scheme

4. Establishment and force analysis of mechanism modal

4.1 Establishment and analysis of soft shaft model

4.2.1 Effect of soil moisture on tillage efficiency and traction resistance

5.1 Build finite element model

5.2 Static analysis of roller

6.1 Experimental method

6.2 Evaluation of experimental results

6.2.1 Determination of weeding rate

Table 4 Experimental results of prototype machine when the cutting tool speed is 180 r/min

Footnotes

Acknowledgments

Conflict of interest

References

Table 4
Experimental results of prototype machine when the cutting tool speed is 180 r/min