Simulation Analysis and Test on the Effect of Picking Up the Residual Film of Typical Film Lifting Parts

Abstract

The agronomic technology of whole plastic-film mulching on double ridges has been widely promoted in the arid region of northwest China, which has caused serious residual film pollution in the field. Mechanized residual plastic-film collecting is the main way, but this way of residual plastic-film collecting is more complicated. To prove the effect of typical film-lifting parts on residual plastic-film collecting performance by a numerical simulation method, further evaluate and select the working parts suitable for residual plastic-film collecting and improve the collecting efficiency, virtual whole plastic-film mulching on double ridges needs to be established. In this study, the film-lifting parts and the double ridge with flexible, deformable, mulching, film interaction model was developed using the discrete element method (DEM). The interaction model was used to simulate the operation process of a typical third-order shovel, hug plate and elastic teeth film-lifting parts. Furthermore, the operation resistance, seed bed disturbance and collecting residual film form under different working conditions were analyzed. The simulation results show that: the disturbance of the ridge from strong to weak is the third-order shovel, elastic teeth and hug plate film-lifting parts; the crushing of the ridge soil from strong to weak is the elastic teeth, the hug plate and the third-order shovel film-lifting parts; the collecting film from strong to weak is the hug plate, elastic teeth and the third-order shovel film-lifting parts; the median value of the horizontal force fluctuation range of the ridge soil on the film-lifting parts is increased, and the increase trend is from large to small in the third-order shovel, elastic teeth and hug plate film-lifting parts (the third-order shovel is 5250 N, 6250 N and 7750 N; the hug plate is 3250 N, 3500 N and 4000 N; and the elastic teeth are 4150 N, 4550 N and 5450 N). The comparative analysis results show that the hug plate film-lifting parts are more suitable for the residual-film collecting of whole plastic-film mulching on double ridges. Field trials were carried out to verify the operational effectiveness of the hug plate film-lifting parts. The results show that the morphological changes of the recovered residual film and the simulated mulching film during the operation of the hug plate-type residual film machine, and the distribution of the surface properties of the ridge after the residual-film collecting are basically consistent with the simulation results, and the film pick-up rate of hug plate film-lifting parts reaches 95.20%, and the established discrete element interaction model is reliable. The research results can provide reference for the research and development of full film duopoly furrow-mechanized residual film-collecting agricultural equipment in the arid region of northwest China.

1. Introduction

Plastic-film mulching technology has the functions of increasing temperature and preserving soil moisture, preventing diseases and insects, and inhibiting weeds [1,2]. This technology is widely used in northwest and northeast China and parts of North China, and it has made great contributions to increasing farmers’ production and income and ensuring food security [3]. However, the popularization and application of plastic-film mulching technology in a large area cause natural environment pollution, which is not conducive to the sustainable development of agricultural production [4,5]. Development of residual-film-collecting technology is one of the main ways to prevent and control residual film pollution, and it is also an important guarantee to maintain the sustainable popularization and application of plastic-film mulching technology [1]. In July 2020, the Ministry of Agriculture and Rural Affairs and the State Administration for Market Regulation again issued the Measures for the Management of Agricultural Films, which require agricultural film users to recycle agricultural mulching film waste during their service period [6].

The whole plastic-film mulching on double ridges is a typical agronomic technology of plastic-film mulching, which is widely used in the arid region of northwest China and has a remarkable effect on drought resistance and income increase [1]. In the early stage, our group has carried out some research on mechanized ridging and film mulching, seeding on film, crop harvest and residual-film collecting [7,8]. In recent years, the discrete element method (DEM) has been widely used in the field of agricultural engineering [9], and it optimizes and screens agricultural machinery and equipment [10,11]. It has obvious advantages in proving the coupling operation mechanism of soil and machinery [12,13,14]. Previous studies in relation to residual-film collecting mainly focused on the collecting process and machine design [15,16,17,18], while there are relatively few studies on the performance of the film-lifting parts combined with the discrete element method. Guo Wensong et al. applied the discrete element method to establish a flexible and deformable virtual plastic-film model in the YADE software, and they used this model to perform the tensile and tear test [19]. Jin Wei et al. proposed an EDEM-Fluent coupling-based method for the suspension separation of residual films and impurities based on the differences in specific gravity characteristics, suspension velocity and flow characteristics between residual films and impurities, such as soil [20].

As one of the core parts of the residual-film-collecting machine, the plastic-film lifting operation effect has a trade-off relationship with the energy consumption and efficiency of residual-film collecting. To find out the influence of film-lifting components on the collecting performance of residual film, this study, in combination with the relevant scholars’ research on soil and polyethylene film (mulching film) in the northwest arid area [12,21,22], established a film-lifting parts -mulching film-soil simulation interaction model by compiling a particle coordinate file with discrete element software EDEM (the mulching film discrete element model was established by the particle aggregation method). The model is used to simulate the operation process of typical film-lifting components (third-order shovel, hug plate and elastic teeth film-lifting parts). The disturbance of third-order shovel, hug plate and elastic teeth film-lifting parts in the soil of a film-covered ridge with whole plastic-film mulching on double ridges; the shape of mulching film; and the stress on film-lifting parts during the operation of different film-lifting parts are discussed. Finally, field trials were conducted to verify the effectiveness of the hug plate film-lifting parts. It is expected to provide references for the construction of a discrete element simulation model of whole plastic-film mulching on double ridges, the optimal screening of film starting components and the research and development of mechanized residual-film-collecting agricultural equipment.

2. Simulation Model

2.1. Discrete Element Model of Whole Plastic-Film Mulching on Double Ridges

2.1.1. Contact Model

The discrete element model of whole plastic-film mulching on a double ridges seed bed is composed of ridge-body soil and PE plastic film. Collecting of a residual film with whole plastic-film mulching on double ridges is mainly divided into autumn uncovering film and top uncovering film [1]. According to the requirements of the mechanized film-collecting depth, the film-lifting parts operate in the tillage layer, where the soil moisture content generally ranges from 11% to 15%, and the soil particles are agglomerated into blocks and have the characteristics of vibration-induced depolymerization [23].

The Hertz–Mindlin with Bonding model is used, which can constrain the relative motion of particles in normal and tangential directions and replace the liquid bridge with cylindrical bonding bonds between soil particles, which can withstand certain forces and moments, and can simulate soil agglomerates and inter-particle interactions well, and the model is less affected by the shape of the unit particles [12], so mono-spherical particles can be used to represent particle units. The current methods for constructing virtual mulch are the particle aggregation method and the Minkowski sums method [19]. The discrete element model of flexible PE mulch built by the particle aggregation method can be formed by generating spherical particles arranged in a single layer in the discrete element software EDEM and completing the connection with the Hertz–Mindlin with bonding contact model.

Therefore, in this study, the Hertz–Mindlin with bonding contact model was added between soil particles, between mulching film particles and between soil particles-mulching film particles, and the Hertz–Mindlin (no slip) contact model was used for the rest of the contact models. The microscopic parameters of the Hertz–Mindlin with bonding contact model are the normal stiffness coefficient K_n, the tangential stiffness coefficient K_s, the critical normal stress σ, the critical tangential stress γ₁ and the bonding radius R. Among them, the normal stiffness coefficient K_n can be obtained from Equation (1), and the tangential stiffness coefficient K_s, is 2/3~1 of the normal stiffness coefficient K_n [24], and the tangential stiffness coefficient K_s is chosen to be equal to the normal stiffness coefficient K_n for the study. The critical normal stress σ can be obtained from Equation (2), and with reference to the literature [25,26], the critical tangential stress γ₁ is chosen to be equal to the critical normal stress σ in this study. The bond radius is generally 1.2 to 1.3 times the particle radius.

This is the normal stiffness coefficient K_n:

$$K_{\mathrm{n}}=\frac{4}{3}{(\frac{1-{\mu }_{\mathrm{a}}^2}{E_{\mathrm{a}}}+\frac{1-{\mu }_{\mathrm{b}}^2}{E_{\mathrm{b}}})}^{-1}{(\frac{r_{\mathrm{a}}+r_{\mathrm{b}}}{r_{\mathrm{a}}{r}_b})}^{-\frac{1}{2}}$$

(1)

where μ_a and μ_b are the Poisson’s ratio of the particles; E_a and E_b are the modulus of elasticity of the particles, MPa; and r_a and r_b are the radii of the particles, mm.

This is the critical normal stress σ:

$$\sigma =\frac{F}{A}$$

(2)

where σ is the critical normal stress, MPa; F is the tensile critical load, N; and A is the original cross-sectional area of the specimen, m².

2.1.2. Discrete Element Model of Whole Plastic-Film Mulching on Double Ridges

Previous studies have shown that too large a particle size for the particle model will increase the error of the simulation results, while too small a particle size will cause the simulation time to be too long [27]. Combining the requirements of computer performance and simulation accuracy, the particle radius for the mulching film model was established at 2.5 mm and the soil particle radius at 8 mm. The particle aggregation method was used to build a discrete element model for PE mulch, as shown in Figure 1.

According to the agronomic and mechanized operation requirements of whole plastic-film mulching on double ridges technology, combined with the computing power, simulation efficiency and test requirements, the model size of whole plastic-film mulching on double ridges was established as 1100 × 1340 × 310 mm³, where the height of the small ridges was 160 mm and the height of the large ridges was 120 mm, and the mulching film model was overlaid on the small ridges, both sides and both ridges along the center of small ridges, with measurements of 1200 mm wide and 5 mm thick (single layer particles).

To improve the efficiency of the simulation and to avoid problems, such as multiple adjustments of the gravitational acceleration direction, which cause excessive experimental errors due to particle instability, the whole plastic-film mulching on double ridges model was established using the particle rapid-filling method. The process is as follows: a 3D model of a whole plastic-film mulching on double ridges is created in SolidWorks and saved in .stp format, which is used to partition and constrain the relative positions of the particles. The dynamic particle factory was used in EDEM to fill the ridge model, and the position information and ID number were derived after the ridge soil particles were stabilized. Considering the orderliness of the particles in the virtual film constructed by the particle aggregation method, the ANSYS Workbench platform was used to complete the meshing of the mulching film geometry model and the acquisition of the mesh coordinate point.txt file. The mulching film geometry model was meshed in Mesh (with a mesh size of 5 mm), and the mesh coordinate point.txt file was obtained in Fluent by loading the CalcRadius.c file and performing the CalcRadius Volume operation. The obtained soil particle location file and the mulching film grid coordinate file are compiled into the Block_Factory_Data.txt file at the same time. The ridge soil and mulching film intrinsic parameters and contact parameters were set in EDEM. Combined with previous research and experiments by the group and related scholars, the main parameters of the discrete element model were selected as shown in

Table 1. Main parameters of discrete element model.

Type	Parameter	Unit	Value	Source
Soil intrinsic parameters	Moisture content	%	11~15	Measurement
	Compactness	Pa	1.3	Measurement
	Density	kg·m−3	2600	[22]
	Poisson’s ratio	\	0.3	[22]
	Shear modulus	Pa	5.0 × 107	[22]
Soil contact parameters	Recovery coefficient	\	0.21	[22]
	Static friction coefficient	\	0.68	[22]
	Dynamic friction coefficient	\	0.27	[22]
Soil bonding parameters	Bond stiffness	N·m−3	5.0 × 107	[12,15]
	Bond critical stress	Pa	3.0 × 104	[12,15]
	Bonding radius	mm	8.54	[28]
Mulching film intrinsic parameters	Density	kg·m−3	930~950	Measurement [29,30]
	Poisson’s ratio	\	0.41	Measurement [29,30,31,32]
	Average tensile critical load	N	1.45	Measurement [29,30,31,32]
	Shear modulus	Pa	5.50~8.65 × 108	Measurement [29,30,31,32]
Mulching film bonding parameters	Bond stiffness	N·m−3	3.12 × 107	Measurement
	Bond critical stress	Pa	7.25 × 106	Measurement
	Bonding radius	mm	3.2	Measurement

.

In the EDEM, the direction of the gravity acceleration was set to the Z-axis negative direction, the BlockFactory_x64.dll file was loaded to achieve rapid particle generation, and the Bonding key was added after the particles reached stability. In the post-processing, the Simulation Time was set to 0 and the Simulation Desk file was exported. To this end, the discrete element model of whole plastic-film mulching on double ridges was created (Figure 2).

2.2. Typical Film Lifting Parts

The film-lifting parts of the whole plastic-film mulching on double ridges collecting machine in previous studies were divided into integral film-lifting parts and partial film-lifting parts according to the operation mode. The integral film-lifting parts are less damaging to the film, mainly flat shovels, step shovels and profiling shovels, etc.; the cross-sectional size of the partial film-lifting parts is smaller in the film-raising direction, mainly popping teeth type and straddling plate type, etc.

To conduct the simulation reasonably and effectively and ensure the accuracy of the results, the modeling software SolidWorks was used to establish the 3D models of three typical film-lifting parts (third-order shovel, hug plate and elastic teeth) in a scale of 1:1. The parts unrelated to the working process are simplified, and the simplified models were saved as .stp files for later simulation (Figure 3). Among them, the width of the third-order shovel, hug plate and elastic teeth film-lifting parts were 1100 mm. Third-order shovel film-lifting parts were composed of 9 groups of film-lifting monomer, and the width of the film-lifting monomer was 110 mm. Hug plate film-lifting parts were composed of 10 groups of film-lifting monomer, the width of the film-lifting monomer was 10 mm and the elastic teeth film-lifting parts were composed of 3 rows of film-lifting teeth: the first row of 11 groups, the diameter of the monomer was 10 mm; the second row of 14 groups, the diameter of the monomer was 8 mm; and the third row of 20 groups, the diameter of the monomer was 6mm; and the spacing of each row was 500 mm.

2.3. Interaction Model Establishment and Simulation Process

To explore the influence of the film-lifting parts on the operational effect of the mechanized collecting operation, the film-lifting parts and whole plastic-film mulching on double ridges interaction model was established and simulation tests were carried out. The third-order shovel, hug plate and elastic teeth film-lifting parts were introduced into the discrete element model of the whole plastic-film mulching on double ridges and were initially positioned on the negative side of the whole plastic-film mulching on double ridges in the Y-axis, aligned with the center of the small ridges (Figure 4).

In practice, the hug plate film-lifting parts can adjust the depth of penetration by adjusting the compression distance of the spring preloading device, which was simplified to a rigid model. In order to simplify the simulation model, the effect of the tunability of the residual-film-collecting machine, the effect of the elastic teeth elasticity on the residual film collection and the effect on the crops or residual stubble on the effect of the residual film-picking operation were all ignored. The process of inserting the soil of the film-lifting parts is not set to simplify the simulation process. The depth of entry for the film-lifting operation was set to 65 mm, the forward speed was set to 0.46 m·s⁻¹ [1,12], the start time was set to 0, the simulation time step was set to 7.405 × 10⁻⁶ s, the Rayleigh time step was set to 9%, the data saving interval was set to 0.01 s and the simulation duration was set to 2.5 s. The positive X-axis of the simulation coordinate system is specified as the amplitude direction, the positive Y-axis is specified as the horizontal operation direction, and the negative Z-axis is specified as the gravitational acceleration direction.

3. Results and Discussion

3.1. Effect Analysis of Residual Film Picking Operation

3.1.1. Ridge Disturbance Analysis

To analyze the disturbance of three typical film-lifting parts to the soil particles of the whole plastic-film mulching on double ridges ridge body during the operation, the ridge body is symmetrical, so the longitudinal profile position in the middle of the ridge body of the small ridge in the horizontal operation direction (X-axis direction) of the third-order shovel, hug plate and elastic teeth film-lifting parts was selected to analyze the disturbance and migration of soil particles, as shown in Figure 5, Figure 6 and Figure 7.

As shown in Figure 5, in 0.2~2.4 s, during the operation of the third-order shovel film-lifting parts, the soil disaggregation behavior is weak, the soil disturbance behavior is strong, and the operation state of the third-order shovel film-lifting parts is unstable. Within 0.2~0.6 s, the volume of the discrete element model for digging the third-order shovel film-lifting parts gradually increases until the film-lifting parts completely enter the model. As the particle velocity increases from 0 to 0.4~0.6 m/s, the soil particles above the film-lifting parts are lifted along the direction perpendicular to the shovel surface, and the distance to be lifted gradually increases, and the particles generally migrate backward. The velocity of soil particles above the film-lifting parts is kept at 0.4~0.6 m/s. Most soil particles exist in the form of aggregates, and the disaggregation behavior of aggregates is not obvious. After 0.8 s, under the compression of soil particles in the front, most of the undifferentiated soil aggregates obtain greater kinetic energy and migrate backward and are backfilled to the cavity area below the tail of the film-lifting parts at a speed of 0.8~1.0 m/s.

The third-order shovel film-lifting parts lifts the soil and the film on the ridge together to achieve film lifting, which reduces the amount of film breakage caused by the direct contact between the film-lifting parts and the film, and improves the film-lifting rate to a certain extent. However, the crushing effect of the third-order shovel film-lifting parts of the soil is not significant (disaggregation of soil aggregates). This film-lifting part needs to be used in conjunction with the screening type residual-film-collecting device to complete the integrated operation of harvesting crops under the film and residual-film collecting. The simulation analysis shows that in the process of residual-film collecting, the third-order shovel film-lifting parts easily causes the piled-up earth, and the film is not easily separated from the soil and impurities directly. The residual film can be recovered by the film-rolling device.

As shown in Figure 6, the disturbance and the crushing behavior of the hug plate film-lifting parts of soil are mainly concentrated around the film-lifting parts, and the soil lifting is less during the film-lifting parts operation. From 0.2 to 2.4 s, the hug plate film-lifting parts gradually enter a stable operation state and act with the ridge from the front to the back. The soil particle velocity near the front section of the film-lifting parts is stable at 0.4~0.6 m/s, where the soil particle velocity is greater than the upper soil particle velocity, and the soil particle aggregates have a more significant disaggregation behavior under the effect of the film-lifting parts. After 0.6 s, smaller soil particle aggregates and depolymerized soil particles are backfilled to the cavity area behind the film-lifting parts through the gap between the film-lifting parts, and the particle velocity of the backfilled particles is within 0.8~1.0 m/s. The larger soil particle aggregates continue to move forward and are gradually disaggregated under the action of the film-lifting parts, and they finally pass through the gap between the film-lifting parts. The soil particles in front of the film-forming parts with a velocity of more than 0.4 m/s are significantly more than those behind the film-forming parts. The hug plate film-lifting parts gather the mulch film at the ridge forward to realize film lifting. The film-lifting parts can be used with the residual-film-collecting machine with film-rolling and film-conveying devices to complete the operation of “first gathering film, then gathering film” [33].

As shown in Figure 7, the elastic teeth film-lifting parts are designed as three rows, which have strong disturbance behavior to the soil and strong disaggregation behavior of soil aggregates. The speed of soil particles in the operation area of the film-lifting parts is obviously higher than that in other areas, and the soil particle aggregates can pass through the gap between the film-lifting parts more smoothly. Within 0.2 s~1.0 s, the first row of film-lifting elastic teeth gradually entered the simulation model and interacted with the ridge body, which gradually enhanced the soil disturbance, and the velocity of most soil particles within the disturbance range was stable within 0.4~0.6 m/s. The sectional area of a single film-lifting elastic tooth in the operation direction is small, and the cavity area behind the elastic teeth after operation is small, so there are fewer soil particles with a speed of 0.8~1.0 m/s. At 1.2~2.2 s, the first row and the second row of film-lifting elastic teeth operate at the same time, and the particle number is obviously more than that of one row of film-lifting elastic teeth within the speed of 0.4~0.6 m/s. After 2.4 s, the three rows of film-lifting elastic tooth operate at the same time, and the soil disturbance behavior is the strongest. The film-collecting effect of a single row of film-lifting elastic teeth is poor, and multiple rows of film-lifting elastic teeth with different spacing need to be set, that is, the elastic teeth film-lifting parts gather the film forward, and the film lifting is completed under the joint action of three rows of film-lifting elastic teeth.

3.1.2. Analysis of Morphological Changes of Plastic Film

To analyze the film-lifting effect of third-order shovel, hug plate and elastic teeth film-lifting parts in the operation process, the film shapes of each film-lifting part at different times in the operation process are selected as the research object, as shown in Figure 8, Figure 9 and Figure 10.

As shown in Figure 8, in 0.3~0.5 s, the mulch film is gradually lifted, the plastic-film deformation is gradually increased, and the velocity of most particles at the deformation location is within 1.5~2.5 m/s. In 0.5~2.5 s, the mulch film is lifted along the Z-axis, which shows that the mulch film is relatively complete, but the effect of gathering is not obvious in the horizontal operation direction (positive Y-axis direction). The maximum speed of mulch film particles mainly appears at the upper position in front of the film parts, and the change of film particle velocity in the small ridge is more obvious. The main reason is that the gap between the third-order shovel film-lifting parts is small, and the film-lifting parts will lift the soil above it as a whole during the operation. With the continuous progress of the film-lifting parts, the soil will accumulate on the upper surface of the film-lifting parts in large quantities, and the accumulated soil will lift the film (Figure 6). There is little direct contact between the film-lifting parts and the film, so the film is less damaged.

The simulation process shows that the third-order shovel film-lifting parts can lift the mulch film as a whole, alleviate the damage of the mulch film and facilitate the orderly collecting in the later period. It is mainly applied to the integrated harvesting of rhizome crops under the film and the residual film, but the harvesting process needs to complete the soil crushing, crop screening and separation from the residual film, which requires a more complex separation device.

As shown in Figure 9, in 0.3~0.5 s, the mulch film is gradually lifted, and the deformation of the mulch film is gradually increased, that is, the hug plate film-lifting parts enter the simulation model. The velocity of the film particles is higher the closer they are to the film-lifting parts, and the maximum velocity of the film particles mainly occurs at the front of the film-lifting parts. In 0.7~2.5 s, the mulch film is gathered forward, curled and gathered in front of the film-lifting parts, and the amount of aggregation gradually increases, and the damage of the mulch film is relatively small. The main reason is that, during the operation, the mulch film was pushed forward under the joint action of the hug plate film-lifting parts and the soil particle aggregates that did not penetrate the gaps between the film-lifting parts. The gap between the individuals of the hug plate film-lifting parts is larger than that between the third-order shovel film-lifting parts. Compared with the third-order shovel film-lifting parts operation, the hug plate film-lifting parts operation process lifts less soil, and the soil particle aggregates flow to the rear through the gap between monomers after disaggregation, which has less impact on the promotion of the mulch film, making the larger area of the mulching film subject to the gathered effect of the film-lifting parts, and the gathered effect was obvious in the horizontal operation direction (positive Y-axis direction). The simulation process shows that the hug plate film-lifting parts can lift the mulch film as a whole, and the film-lifting process contains less soil, which is convenient to use with the eccentric film-lifting roller, which can better complete the collection of the mulch film roll and is more conducive to simplifying the subsequent plastic-film impurity separation device.

As shown in Figure 10, in 0.3~0.5 s, One side of the mulch film is cut off and lifted. The velocity of most lifted mulch film particles is within 1.5~2.5 m/s. The lifting effect of the mulch film in the vertical direction is not obvious. In 0.7~1.3 s, the mulch film is gradually lifted. In 1.3~2.5 s, the mulch film has Z-axis direction lifting and the horizontal operation direction (Y-axis positive direction) gathered effect. The simulation process shows that the mulch film was moved forward by the elastic teeth film-lifting parts, the film is wrapped forward as a whole, and part of the film was damaged and passes through the gap between the elastic teeth film-lifting parts. The overall velocity of the mulch film was relatively high, but the film does not lift significantly along the Z-axis, which was not conducive to the separation of the mulch film impurities. The elastic teeth film-lifting parts frequently contact with the mulch film, causing more damage to the mulch film, which makes it difficult to recover the residual film later.

3.2. Resistance Analysis of Residual Film Picking Operation

The data on the horizontal and vertical forces acting on the third-order shovel, hug plate and elastic teeth film-lifting parts in the simulation process vary with the distance derived. The diagrams of the forces on the film-lifting parts at different distances were made on the origin, as shown in Figure 11.

The curves of the horizontal and vertical forces for each film-lifting parts were depicted in Figure 11 and exhibit a tendency of quick growth to a specific value, fluctuated within a specific range, and sudden increase. The third-order shovel film-lifting parts have a large section area and a narrow gap between its film-lifting parts. Due to the shear effect of the cutting edge during operation, the bonding between particles is weak, making it difficult for the particles to quickly reach the rear of the film-lifting parts. The horizontal force gradually rises as a result of the frontal accumulation of many particles, and the fluctuation range’s median value also rises noticeably (Figure 11a).

The horizontal force acting on each film-lifting parts tends to increase, constrained by the border size of the model. Comparatively speaking, it is discovered that during operation, the hug plate film-lifting parts were subjected to the steadiest horizontal force (Figure 11b). The third-order shovel, elastic teeth and hug plate film-lifting parts were in the order of horizontal force from large to small.

It is evident that the horizontal force exerted on the film-lifting parts is larger than the vertical force. The primary reason is that the vertical force is mostly generated by the pressure of the particles on the upper part of the film-lifting parts, while the vertical sectional area of each film-lifting parts is small. The horizontal force acting on particles like dirt prevents the film-lifting parts functioning in the horizontal direction from moving.

With an increase in the volume of the film-lifting parts entering the whole film double ridge model throughout the simulation process, the squeezing impact of the film-lifting parts of the model grows. The force operating on the particles approaches the range of the force acting on the steady operation after the film-lifting parts have fully entered the model.

The upper bound value and lower bound value of the first time that the horizontal force of the ridge soil on the film-lifting parts reaches the fluctuation range are as follows. The upper bound value of the third-order shovel film-lifting part is 6750 N, and the lower bound value is 4000 N (Figure 11a). The upper bound value of the hug plate film-lifting parts is 4450 N, and the lower bound value is 2550 N (Figure 11b). The upper bound value of the elastic teeth film-lifting parts is 4900 N, and the lower bound value is 4050 N (Figure 11c). The median value of the horizontal force fluctuation range of the ridge soil on the film-lifting parts is increased, and the increased trend is from large to small in the third-order shovel, elastic teeth and hug plate film-lifting parts (the third-order shovel is 5250 N, 6250 N and 7750 N; the hug plate is 3250 N, 3500 N and 4000 N; the elastic teeth are 4150 N, 4550 N and 5450 N).

The horizontal and vertical forces acting on the film-lifting parts fluctuate within a specific range during the stable operation stage. One explanation is that the bonding bond causes the soil particles to aggregate into aggregates. The force acting on the film-lifting parts causes the movement of the film-lifting parts, which gradually increases the bonding bond between the particles. The force that the particle aggregates are exerting on the film-lifting parts is also gradually increasing at the same time. The bonding bond breaks when the force exerted on the film-lifting parts reaches its critical stress [25]. The aggregate particles that have lost their bonding force are no longer bound together, and the depolymerization of the particle aggregates lowers the force acting on the film-lifting parts. Another reason is that as the accumulation of particles in front of the film-lifting parts increases, the force of particle aggregates on those parts increases. When the accumulation reaches a certain level, the film-lifting parts exert a weaker force to move the accumulated particles to the working area behind them. To put it another way, the model particles pass through the opening or upper portion of the film-lifting monomer and fill the area after the operation. The particle group in front of the film-lifting component exerts less force [34].

The accumulation and backward movement of the particle groups prior to the film-lifting parts, as well as the growth and breakdown of the bonding bond, are all changing concurrently, causing the horizontal and vertical forces on the film-lifting parts to fluctuate within a specific range. The film-lifting parts gradually approach the simulation model’s edge and move through it at a constant speed once the displacement reaches 1 m. Particles build up between the model’s edge and the film-lifting parts because the particle group cannot pass through the geometry. The forces acting on the film-lifting parts in the horizontal and vertical directions to contact particles or surround them increase sharply as the distance between the model edge and the film-lifting parts gradually decreases. This process cannot more accurately simulate the actual operation state, and the simulation data are not referential. As a result, the simulation model developed generally reflects the actual operation state, and the stable operation process of the film-lifting parts is the continuous fluctuation state of horizontal force and vertical force.

3.3. Ridge Body Changes

The Y-axis sections were selected in the Clipping option of the EDEM post-processing interface to discuss the ridge body state after simulation of the different film-lifting parts (Figure 12).

The large and small ridges were cut, the ridge soil was moved forward, and the seed bed was relatively flat after the operation of the third-order shovel film-lifting parts, as shown in Figure 12a. The ridge body retains the approximate shape of double ridges after the hug plate film-lifting parts were operated, as shown in Figure 12b, and the amount of soil depolymerization of the ridge body was small. Following the operation of the elastic teeth film-lifting parts, as shown in Figure 12c, the ridge and ditch were filled with a large amount of soil particles after depolymerization, and the seed bed was relatively flat.

The simulation results were summarized from the aspects of the disturbance of ridge soil, the change of film form, the resistance during the simulation operation and the change of the ridge after the operation. The disturbance degree of seed bed was in the order of large to small for the third-order shovel, hug plate and elastic teeth film-lifting parts. The collection effect in the operation direction from strong to weak was as follows: hug plate, elastic teeth and third-order shovel film-lifting parts. The simulation operation resistance from large to small was: third-order shovel, elastic teeth and hug plate film-lifting parts.

The harvesting process of root and stem crops under the film of “digging device & screening device” can be used to complete the picking up of residual film and the digging of crops and soil under the film at the same time, so the third-order shovel film-lifting parts was suitable for the integrated operation of root and stem crops under the film harvesting and residual film. The hug plate film-lifting parts have a weak soil disturbance, small resistance, significant raking film effect and good ridge retention after the operation. Therefore, the hug plate film-lifting parts were more suitable for the residual film picking-up operation of whole plastic-film mulching on double ridges planting crops. Further field tests were carried out on the hug plate film-lifting parts to verify the reliability of the simulation model and the accuracy of the simulation results.

4. Field Verification Test

To test the operation effect of the film-lifting parts and verify the reliability of the simulation model, field tests were carried out on the plastic-film residue collector installed with the hug plate film-lifting parts after evaluation and selection. The test field is the stubble field after corn harvest. The previous corn planting adopts the whole plastic-film mulching on double ridges planting agronomic mode, with the planting row spacing of 400 mm and the plant spacing of 280 mm. Before the residual-film-collecting operation, the corn straw has been cleaned, and the stubble height is 70~110 mm. The covered film is the standard white mulch (thickness is 0.01 mm and width is 1200 mm) purchased by the Gansu Provincial government. Collecting of residual film with whole plastic-film mulching on double ridges is mainly divided into autumn uncovering film and top uncovering film [1]. The test was autumn uncovering, the soil was cultivated loessial soils, and the measured soil moisture content was 11%~15% The coupling power of the working machine is Dongfanghong-300 tractor, the rated power is 22.1 kW, and the forward working speed of the prototype is 0.40~0.80 m/s. The test process control tractor driving speed was 0.46 m/s, and adjusting the operating depth of the stripping part of the residual film recovery machine to stabilize the average operating depth at 65 mm made it consistent with the simulated operating depth. The test effect is shown in Figure 13.

The overall operation situation of the machine and tools and the film-collecting effect of the film parts were investigated, and the film-lifting rate was used as the evaluation index of the film-collecting effect of the film-lifting parts.

Film lifting rate T is:

$$T=(1-\frac{m_1}{m_0})\times 100\%$$

(3)

In the formula, T is the film-lifting rate, %, m₀ is the total mass of the mulching film in the selected test cell, and g; m1 is the residual film mass of the film-lifting parts, g.

The results of the field test show that the whole machine was stable in the operation process, and the effect of picking up plastic film by the hug plate film-lifting parts was remarkable, which was beneficial to the subsequent picking up and conveying of plastic film and rolling up of residual film. After the test, the film-lifting rate of residual film by the hug plate film-lifting parts reached 95.20%. The hug plate film-lifting parts and eccentric drum type residual-film-collecting parts are well matched. After the residual-film collecting, the soil disturbance of the ridge body was small, and the residual film contained less soil, which meets the basic requirements of the whole plastic-film mulching on double ridges residual-film-collecting technology.

5. Conclusions

The simulation interaction model of film-lifting parts and whole plastic-film mulching on double ridges was established in the discrete element software EDEM by compiling the particle coordinate file (the polyethylene plastic film (PE plastic film) discrete element model was established by the particle aggregation method) to explore the impact of film-lifting parts on the collecting performance of residual film. The disturbance of the third-order shovel, hug plate and elastic teeth film-lifting parts to the ridge body of whole plastic-film mulching on double ridges; the change of the plastic-film shape during the operation of the film-lifting parts; and the resistance change of the film-lifting parts were simulated and analyzed.

The disturbance of the ridge from strong to weak is the third-order shovel, elastic teeth and hug plate film-lifting parts. The crushing of the ridge soil from strong to weak is the elastic teeth, the hug plate and the third-order shovel film-lifting parts. The collecting film from strong to weak is the hug plate, elastic teeth and the third-order shovel film-lifting parts. The upper bound value and lower bound value of the first time that the horizontal force of the ridge soil on the film-lifting parts reaches the fluctuation range are: the upper bound value of the third-order shovel film-lifting part is 6750 N, and the lower bound value is 4000 N; the upper bound value of the hug plate film-lifting parts is 4450 N, and the lower bound value is 2550 N; and the upper bound value of the elastic teeth film-lifting parts is 4900 N, and the lower bound value is 4050 N. The median value of the horizontal force fluctuation range of the ridge soil on the film-lifting parts is increased, and the increased trend is for large to small in the third-order shovel, elastic teeth and hug plate film-lifting parts (the third-order shovel is 5250 N, 6250 N and 7750 N; the hug plate is 3250 N, 3500 N and 4000 N; and the elastic teeth are 4150 N, 4550 N and 5450 N). The increase of the bonding force and bonds breaking, and the accumulation and backward movement of the particle groups in front of the film-lifting parts are all periodic changes, which are the main reasons for the fluctuation of the horizontal and vertical forces on the film-lifting parts in a certain range. The hug plate film-lifting parts has good operation stability, small resistance and a remarkable film collection effect, so it is more suitable for picking up the residual film of whole plastic-film mulching on double ridges planting crops.

The change of film shape and ridge shape in the field experiment process and the discrete element simulation process are basically consistent. During the operation, the hug plate film-lifting parts has a good effect in picking up the mulching film. After the residual film is recovered, the ridge soil disturbance is small, and the residual film picked up is less mixed with soil and the film-lifting rate of residual film by the hug plate film-lifting parts reaches 95.20%. The operation process of the machine is stable, and the collecting of the residual film meets the basic requirements of the whole plastic-film mulching on double ridges residual-film-collecting technology.