Parameter Optimization and Experimental Study on Alfalfa Stem Flattening Process Based on DEM–MBD

Abstract

To address issues such as uneven flattening and high stem breakage rate in post-harvest alfalfa field conditioning operations, an adjustable-clearance flattening and modulating device was designed. The device incorporates a dual-spring floating pressure mechanism and preload adjustment mechanism to ensure the adaptive performance of conditioning rollers during alfalfa stem flattening. Based on the biological characteristics of alfalfa stems, a rigid–flexible coupling model between stems and the flattening and modulating device was established. Using the Discrete Element Method (DEM) and Multibody Dynamics (MBD) co-simulation technology, experiments were conducted with feeding amount, roller speed, and buffer spring preload force as test factors, while stem crushing rate and bonding key fracture rate served as evaluation indices. Box–Behnken experimental design was employed to simulate the dynamic conditioning process, followed by regression analysis of the simulation results. The findings revealed optimal parameter combinations as follows: feeding amount of 5.10 kg/s, modulation roller speed of 686.87 r/min, and buffer spring preload force of 670.02 N. According to the optimal combination of parameters to carry out field tests, the average flattening rate of stem and stem crushing rate were 95.71% and 1.73%, respectively, which showed small relative error with the predicted value and met the requirements of alfalfa steam flattening and modulation operation. These research findings provide theoretical basis and technical support for the design and optimization of alfalfa flattening and modulating devices.

1. Introduction

Alfalfa (Medicago sativa L.) is a perennial legume that serves as an ideal forage source for the livestock industry [1] due to its high nutritional value [2], high yield, good palatability, and adaptability to grow in loose sandy soils [3]. When harvested during the pre-bloom to early flowering stage, alfalfa stems contain high moisture content, which can lead to fermentation heat damage and mold growth during high-moisture storage [4,5]. To achieve uniform drying of stems, leaves, and flowers during harvest [6], flattening and modulating devices are commonly used to process the stems through node breaking, bark splitting, and flattening before field drying [7]. This process helps reduce nutrient loss and improve forage harvest quality.

Extensive research has been conducted on alfalfa flattening and modulating devices both domestically and internationally. The DISCO MOVE front-mounted mower conditioner developed by CLAAS (Germany) [8] features a motion-copying device that enables independent adjustment of the mowing unit from the ground, allowing three-dimensional adaptation to ground contours and enhancing operational capability on undulating terrain. Kverneland (Norway) developed the 4300 series mower conditioner [9] for conditioning alfalfa and similar crops, incorporating floating protection and dual suspension systems that enable independent operation of the disc system and flattening and modulating device from the frame, resulting in superior ground-following capability across various terrain conditions. Song et al. [10] developed a measurement and control system for an alfalfa conditioning test bench that enables precise control and real-time data acquisition through monitoring parameters, such as torque, rotational speed, and pressure. Chen et al. [11] designed a gap adjustment mechanism for forage conditioners that utilizes a screw rod to rotate a V-shaped floating frame around a hinged axis, accommodating variations in feed rate. Wu et al. [12] employed ADAMS software to create virtual prototypes of the header and plants for alfalfa mowing and feeding trials, validating the correlation between the breakage coefficient and actual breakage rate through parameter optimization and field experiments. Wang et al. [13] established a flexible model of alfalfa stems using the Discrete Element Method (DEM) to optimize parameters including feed rate, roller speed, and roller clearance, investigating their effects on stem flattening rate and loss rate. Currently, domestic and international research primarily focuses on structural design and static conditioning processes, with limited investigation into dynamic conditioning processes during field operations.

To address issues such as uneven flattening and high stem breakage rates in mechanized alfalfa harvesting, an adjustable-clearance flattening and modulating device was designed. Based on theoretical analysis and preliminary experiments of the alfalfa stem conditioning process, a simulation model of the flattening and modulating device-stem interaction was developed. Using MBD–DEM (Multi-body dynamics-discrete element) coupled simulation technology [14,15], the main factors affecting the operational performance of the flattening modulation device were identified and simulated to obtain the optimal parameter combinations affecting the modulation quality, which can be used to enhance the flattening rate of alfalfa steams and reduce the crushing rate during harvesting, so as to improve the operational efficiency of the harvesting equipment. The accuracy of the coupling model and the simulation results were verified through field tests, in order to provide theoretical basis and technical support for the design and optimization of alfalfa flattening and modulation equipment.

2. Materials and Methods

2.1. Overall Machine Structure

The designed suspended folding alfalfa mowing and flattening machine primarily consists of a suspension frame, mowing device, flattening and modulating device, hydraulic lifting system, contouring mechanism, and other components. The mowing device is installed at the bottom of the machine and consists of an elliptical cutter disc, specially shaped bending knives, alfalfa guiding rollers, and sliding arms. The flattening and modulating device is mounted above the mowing device and is made up of gap adjustment side plates, compression springs, floating frame, driving rollers, driven rollers, preload adjustment mechanism, cushioning springs, rotating linkages, and alfalfa strip width adjustment mechanism. The hydraulic lifting system is installed on the same side as the suspension frame and mainly includes a hydraulic cylinder, main links, auxiliary links, and stop plates. The overall structure of the machine is shown in Figure 1.

2.2. Working Principle and Main Technical Parameters

The suspended folding alfalfa mowing and flattening machine is connected to the tractor using a three-point side suspension system. Before operation, the contouring mechanism is adjusted to balance the pulling force on the spring with the gravitational forces acting on the mowing and flattening devices, thereby reducing the ground pressure during operation and enabling the machine to contour the ground. During operation, power is supplied by the tractor’s PTO and transmitted via a coupling to the mowing device and flattening adjustment device. The elliptical cutter disc of the mowing device performs unsupported mowing of the alfalfa plants within the working width [16]. The cut plants are pushed by the alfalfa guiding rollers into the flattening adjustment device, where they undergo flattening, cracking, and bending through the intermeshing action of the main and driven V-shaped rubber rollers. Finally, the alfalfa strips are laid in the field at the desired width with the help of the alfalfa strip width adjustment mechanism. After the operation, the hydraulic lifting system rotates and folds the mowing and flattening device at the gearbox pivot point, facilitating transportation for the machine’s relocation. The main technical parameters of the suspended folding alfalfa mowing and flattening machine are shown in

Table 1. Main technical parameters of the suspended folding alfalfa mowing and flattening machine.

Parameter	Value
Overall dimensions (L × W × H)/(mm × mm × mm)	3360 × 1200 × 830
Machine mass/kg	710
Number of discs/pc	4
Complete machine supporting power/kW	≥45
Effective modulating width/mm	1000
Modulating roller material	PU (polyurethane) rubber

.

2.3. Flattening and Modulating Device Structure

According to the requirements of alfalfa field mowing and flattening operation, the designed flattening and modulating device has an adjustable roller gap, which is mainly composed of an active roller, driven roller, adjustable side plate, compression spring, buffer spring, rotating linkage, floating block, rubber buffer block, etc.; the structure of the flattening and modulating device is shown in Figure 2.

The traditional flattening modulation device is mainly of a fixed transmission design, i.e., the gap between the modulating rollers does not change. In order to effectively reduce the modulation force between the modulating rollers if too large and realize the adaptive adjustment of the modulating roller gap, the flattening and modulating device adopts a dual-spring floating pressure mechanism, which is radially mounted on both sides of the modulation roller by compression springs and buffer springs, as shown in Figure 3.

The flattening and modulating device adopts a zigzag rubber-tooth engagement rotation method. When the device runs idle, the driven roller is driven by the active roller for meshing transmission. As the alfalfa stems continuously enter the device, the stacking of materials forces deformation h′ of the compression springs and buffer springs on both sides. The tangential gap of the modulation roller increases to a, to achieve adaptive adjustment between the modulation rollers, ensuring that the stems are constantly in a compressed state during modulation. According to the modulation flattening operation requirements of GB/T 21899-2008 Mowing and flattening machine [17], the adjustment range of the two modulation roller gaps a is 3.2 to 10 mm. The designed top circle radius R_a of the modulation roller is 107.5 mm, the root circle radius R_f is 82.5 mm, the pitch circle radius R is 92 mm, the tooth height H_a is 25 mm, and the tooth width W_a is 68 mm. Through the gap adjustment bolts, the precise adjustment of the floating distance H of the driven roller is achieved.

2.4. Analysis of the Modulating Roller Drive Process

According to the relationship between the modulation roller rubber-tooth engagement rotation direction and the rubber-tooth engagement, a mechanical analysis of the meshing transmission process is conducted. In the modulation roller rubber-tooth engagement transmission, the driven roller rubber teeth are helical in shape and, when engaged at an inclined contact line, the total normal force F_n is perpendicular to the rubber tooth surface and can be resolved into three components, as shown in Figure 4.

The total normal force F_n lies within the plane ab′d′c, forming the normal pressure angle α with the cylinder’s tangent plane aa′b′b. F_n is resolved into radial force Fr and tangential force F_k, further decomposed into circumferential force F_t and axial force F_a. These three components of force are

$$\left\{\begin{array}{l}{F}_t={\displaystyle \frac{2T_1}{D}}\\ F_a=F_t\tan \beta \\ F_r={\displaystyle \frac{F_t\tan \alpha }{\cos \beta }}\end{array}\right.$$

(1)

where F_t is the circumferential force of the driving roller, N; F_a is the axial force of the driving roller, N; F_r is the radial force of the driving roller, N; T₁ is the torque, N·m; D is the pitch circle diameter of the modulation roller, m; β is the spiral angle of the rubber roller segment, (°); α is the normal pressure angle of the rubber roller, (°).

The torque generated by the driving roller is

$$T_1={\displaystyle \frac{9550\lambda P_t}{n_g}}$$

(2)

where P_t is the modulating power of active rollers, kW; n_g is the active roller speed, r/min; λ is the power factor.

The total normal force F_n of the active roller is

$$F_n={\displaystyle \frac{F_{\mathrm{t}}}{\cos \alpha \cos \beta }}$$

(3)

Substituting Equations (1) and (2) into Equation (3) yields

$$F_n={\displaystyle \frac{\mathrm{19,100}\lambda P_s}{n_{g}D\cos \beta }}$$

(4)

When the roller is driven by the total normal force F_n, it tends towards uniform circular motion. Due to rotation, an inertial force F_g is generated. The normal component F_g1 of this force hinders the rotation of the driving roller. F_n1 and F_g1 act together on the alfalfa plants for crushing and flattening.

$$\left\{\begin{array}{l}{F}_g={\displaystyle \frac{MR{\omega }^2}{2}}\\ F_{g1}={\displaystyle \frac{F_g}{\cos \beta }}\\ \cos \beta ={\displaystyle \frac{1}{\sqrt{1+{\mu }^2}}}\end{array}\right.$$

(5)

where M is the Mass of follower rollers, kg; r is the radius of rotation of the driven roll, m; ω is the angular velocity of the slave roll, rad/s; μ is the friction coefficient between alfalfa stems and tempering rolls.

Deduction from the formula yields

$$F_{g1}=2\sqrt{1+{\mu }^2}MR{(\pi n_g)}^2$$

(6)

From Equations (4)–(6), it is evident that, during the flattening process, the force exerted by the driving roller is primarily related to the modulation power of the driving roller; the force exerted by the driven roller is associated with the modulation roller speed n_g and the mass M of the driven roller. As the modulation roller speed increases, the normal component force F_g1 of the modulation roller on the alfalfa stems increases, leading to potential stem crushing and fracture.

2.5. Analysis of Alfalfa Stem Flattening and Modulation Process

According to the analysis of the force of the rubber teeth meshing drive of the modulating roller, in the process of alfalfa steam flattening and modulation, the alfalfa steam is mainly subjected to the compression force and shear force generated by the modulating roller. In order to investigate the physical changes of alfalfa steams under the two forces, biomechanical characterization of alfalfa steams was carried out. Alfalfa stems with a diameter ≥ 3 mm and a length of 30 mm were selected as test samples. Radial compression and shear tests were performed using a TA.XT Plus texture analyzer (Designed and produced by Stable Micro Systems in the UK), as illustrated in Figure 5.

During the stem compression test, the testing probe descended at a speed of 6 mm/s. Upon contact with the alfalfa stem, compression was applied at a speed of 1 mm/s until the device displacement reached 90% of the measured stem diameter, at which point compression was halted. Subsequently, the testing probe ascended at a speed of 6 mm/s. The experimental results are depicted in Figure 6.

Alfalfa stems are essentially long, thin-walled circular tubes. During radial compression, the squeezing contact points on both sides experience radial compression forces, while the sides undergo lateral tensile forces. Since both compression and tensile forces have components pointing towards the center of the circular tube, this leads to the flattening of the circular cross-section, forming an elliptical shape. With increasing compression force on the alfalfa stem, stress concentration occurs at the lateral endpoints of the stem, ultimately resulting in structural failure of the stem.

During the stem shearing process, the motion parameters of the testing probe are the same as in the compression test. Shearing is stopped when the shearing displacement exceeds 1 mm of the stem diameter. The experimental results are shown in Figure 7.

The alfalfa stem diameters vary, resulting in significant differences in compression and shear forces. Twenty sets of alfalfa stems were tested for maximum compression and shear forces, and curve fitting was applied to the data, as illustrated in Figure 8.

The compression and shear tests on alfalfa stems indicate that the best modulation effect is achieved when the modulation force ranges from 136 to 157 N/cm, resulting in a flattening rate of ≥95% and a fracture rate of ≤5%. When the modulation rollers are in a balanced state, the relationship with the mutual pressure between the modulation rollers and the initial tension of the spring is expressed as

$$P={\displaystyle \frac{M\mathrm{g}+2k_h\Delta l+2k_y\Delta l}{L}}$$

(7)

where P is the modulating roller pressure, N/m; L is the flattening modulation width, m; M is the follower roll quality, kg; ∆l is the maximum compression of buffer and compression springs.

According to the positional relationship, the stiffness coefficient between the two springs is

$${\displaystyle \frac{k_h}{k_y}}={\displaystyle \frac{l_2}{l_1}}=0.42$$

(8)

where k_h is the buffer spring stiffness factor; k_y is the compression spring stiffness factor; l₁ is the horizontal distance between the center of the modulating roller and the mounting position of the buffer spring, mm; l₂ is the horizontal distance between the center of the modulating roller and the mounting position of the compression spring, mm.

By substituting the known quantities, the stiffness coefficients of the two springs are determined to be k_h = 7.36 N/mm and k_y = 17.03 N/mm, respectively. A cylindrical helical compression spring is chosen for the compression spring, and according to the GB/T 2089-2009 Cylindrical coiled compression spring dimensions and parameters standard [18], the main parameters of the compression spring are determined as follows: original length 110 mm, mean diameter 40 mm, wire diameter 8 mm, stiffness coefficient 17 N/mm. The buffer spring has an original length of 170 mm, mean diameter 30 mm, wire diameter 5 mm, and stiffness coefficient 8 N/mm.

2.6. Modeling of Multibody Dynamics

To investigate the dynamic changes to alfalfa stem during the flattening modulation process, a combined model of the stem’s flexibility and the rigid body model of the flattening and modulating device was integrated to conduct rigid–flexible (simulation method combining rigid and flexible body dynamics models) coupled dynamic analysis [19]. The MBD–DEM coupled simulation test method was employed to analyze the impact of various factors on the flattening modulation operation. A three-dimensional model of the flattening and modulating device was created in SolidWorks 2021 and saved as a .x_t format file, which was then imported into RecurDyn. Constraint relationships were added to each component based on the operational principles, as shown in

Table 2. Type of component motion constraints.

Component	Assembly Method
Adjustment of side panels to the floor	Fixation
Adjustment of the side plate and rotation of the linkage	Rotation
Rotating linkage and floating block	Rotation
Slave rollers and floating blocks	Rotation
Rubber buffer with adjustable side plate	Fixation

.

To ensure the quantitative feeding of alfalfa stems and the accuracy of the simulation model, the belt conveyor was invoked in the toolkit to transport the alfalfa stems to the flattening and modulating device. Springs were added between the rotating link and the adjusting side plate, as well as between the floating block and the adjusting side plate, with the spring parameters set and preload applied. Using the contact module, solid contacts (solid–solid) were established between the rotating link and the rubber buffer block, and between the rotating link and the sliding groove of the adjusting side plate. Non-contact components within the flattening and modulating device were omitted to simplify the model, enhancing the computational speed and efficiency of the DEM–MBD coupling. The dynamic simulation model of the flattening modulation process is illustrated in Figure 9.

2.7. Discrete Elemental Modeling and Parameterization of Alfalfa Stems

The alfalfa variety used in the experiment is ‘Zhongtian No.1’ purple alfalfa, harvested on 12 June 2023, in Xicha Town, Gaolan County, Lanzhou City, Gansu Province. Considering that the flattening modulation process mainly involves the flattening and rolling of alfalfa stems [20,21], without the need to process its flowers and leaves, simplification of the alfalfa plant’s flowers and leaves was carried out. The diameters of 30 alfalfa stems were statistically analyzed: stems with d > 3 mm were categorized as root stems, stems with 3 mm ≥ d ≥ 2 mm were considered middle stems, and stems with d < 2 mm were classified as top stems. The sorting results are shown in Figure 10.

The sorted alfalfa stem samples were weighed using an electronic analytical balance, and the basic dimensions of the alfalfa stems were measured using a digital caliper. The weight proportions of root stems, middle stems, and top stems were calculated to be 60%, 29%, and 11% respectively.

Table 3. Alfalfa stem geometry.

Project Title		Maximum Value/mm	Minimum Value/mm	Mean Value/mm	Standard Deviation/mm	Content/%
stem Height		655.3	443.2	560.72	0.15
Top stems	Diameter	2	1	1.82	1.23	60
	Wall Thickness	0.7	0.4	0.55	0.25
Middle stems	Diameter	3	2	2.85	0.11	29
	Wall Thickness	1.1	0.9	1.01	1.06
Root stems	Diameter	4	3	3.77	0.16	11
	Wall Thickness	1.5	1.1	1.35	1.33
Number of branches		4	1	3	0.06

presents the geometric dimensions of the alfalfa stems.

Based on the geometric dimensions of the alfalfa stems, the feeding amount is converted into the number of fed stem units, and the stem quantity is calculated. With a planting density of 400 plants per square meter for purple alfalfa, the total number of stems N is.

$$N={\mu }_{1}n$$

(9)

where P is the modulating roller pressure, N/m; L is the flattening modulation width, m; M is the follower roll quality, kg; ∆l is the maximum compression of buffer and compression springs.

After measuring an average branching count of 3 for alfalfa stems, the total number of alfalfa stems is 1200 per square meter. When the operating machine advances at speeds of 1.4 m/s, 2.8 m/s, and 4.2 m/s, corresponding feeding amounts are 3.47 kg/s, 6.94 kg/s, and 10.41 kg/s, respectively. The corresponding total numbers of top, middle, and root stems are 215, 430, and 645. Based on the proportions of the three segments, the number of fed stem units corresponding to different feeding amounts is shown in

Table 4. Alfalfa stems’ quantity conversion table.

Feed Amount (kg/s)	Top (60%)	Middle (29%)	Root (11%)
3.47	129	62.35	23.65
6.94	258	124.7	47.3
10.42	387	187.05	70.95

.

2.8. Simulation Design of Experiments for Coupled Simulation Modeling

To ensure the accuracy of the alfalfa stem simulation model, considering alfalfa stems are hollow in the middle [22], a hollow stem model was constructed in SolidWorks 2021 software with high-precision mesh division. The alfalfa stem model was established in EDEM using the Hertz–Mindlin with JKR and bonding contact models, and the API was utilized to replace the mesh nodes with small particles. As the alfalfa stem model was replaced, adjacent particles within the stem formed bonding contacts at the contact points [23], enabling a more precise simulation of the flexible behavior of alfalfa stems. The flexible model of the alfalfa stem is depicted in Figure 11.

By utilizing the bidirectional coupling interface of EDEM 2022 and RecurDyn 2023, a coupled simulation of the flattening and modulating device was conducted. In the coupling simulation, in order to reduce the simulation time [24], the main contact parts of alfalfa steams (modulating rollers, conveyor belts and ground) were exported as a *.wall coupling file by utilizing RecurDyn’s External SPI function module and the file was imported into EDEM by utilizing the Import Geometries from RecurDyn command. The coupling simulation is shown in Figure 12. In order to ensure the stability and accuracy of the data according to the relevant literature [25,26], the alfalfa steam particle parameters are set in EDEM, and then through the Particle Replacement function, to complete the replacement of the three “top, middle and bottom” kinds of steam model; the discrete element model simulation parameters are shown in

Table 5. Discrete element simulation parameters.

Material	Parameter	Value
Alfalfa stems	Poisson’s ratio	0.45
	Density/(kg·m−3)	996
	Shear modulus/MPa	17.5
	Modulus of elasticity/GPa	3.5
PU Rubber	Poisson’s ratio	0.4
	Density/(kg·m−3)	1200
	Shear modulus/MPa	20
	Modulus of elasticity/MPa	2.8
Alfalfa stems-PU rubber	Coefficient of recovery	0.4
	Static friction factor	0.24
	Rolling friction factor	0.3
Alfalfa stems-stems	Coefficient of recovery	0.44
	Static friction factor	0.39
	Rolling friction factor	0.13
	Normal contact stiffness/(N/m)	3.57×109
	Tangential contact stiffness/(N/m)	4.01×108
	Shear strength/MPa	2.32
	Tensile strength/MPa	7.16
Hertz-Mindlin with JKR	Surface Energy/(J/m2)	2

. The size of the buffer spring preload was adjusted by “Pre Load” in the Spring module of RecurDyn 2023 software, and the modulation force was finally determined to be in the range of 136–157 N/cm when the buffer spring preload was 240–1200 N. The “Fixed Time Step” of the model was set to 2.36 × 10⁻⁶, and the total time was 2 s for simulation calculation.

2.9. Test Factors and Evaluation Indicators

Based on the analysis of the flattening roller transmission process, the experimental factors selected are the feeding amount X₁, modulation roller speed X₂, and buffer spring preload force X₃. The factor coding is shown in

Table 6. Factor level coding.

Levels	Factor
	Feeding Amount X1/(kg/s)	Modulating Roller Speed X2/(r/min)	Buffer Spring Preload Force X3/(N)
−1	3.47	540	240
0	6.94	675	720
1	10.41	810	1200

.

Using the alfalfa stem discrete element flexible model, the crushing rate Y₁ and bonding key fracture rate Y₂ are chosen as evaluation criteria.

Crushing rate: During the flattening modulation process, the top, middle, and root sections of the alfalfa stem are separately introduced into the flattening modulation coupled simulation. By adjusting the modulation roller clearance and modulation roller speed, individual stem units are subjected to modulation treatment. The flattened state of the alfalfa stem during the modulation process is illustrated in Figure 13.

The initial deformation of the compression spring and the buffer spring is set to 15 mm, and the initial deformation of the buffer spring can be adjusted by the preload force adjustment device, with an adjustment range of 0~60 mm, so the buffer spring preload force (240, 720, 1200 N) is selected for the coupling test, which in turn controls the modulating inter-roller force. When alfalfa stems pass through the modulation rollers, the flexible structure of the stems undergo deformation due to compression forces, reaching a critical flattened modulation state. Upon removal of external forces, the alfalfa stems exhibit a structural state of node breakage, skin cracking, and bending (without fracture). If the compression force on the flexible stem structure exceeds the compression force at the critical flattened modulation state, the alfalfa stems are excessively flattened, leading to a fractured state where stem damage increases, resulting in greater harvesting losses. The critical fracture forces of alfalfa steams at the top, middle and roots of alfalfa steams were obtained by simulation as 26.5 N, 33 N and 75.1 N, respectively. According to the national standard GB/T 21899-2008 Mowing and flattening machine, alfalfa stems with a length < 7 cm after modulation testing are considered as crushed, and the crushing rate Y₁ is calculated using the formula:

$$Y_1={\displaystyle \frac{S_1}{S_0}}$$

(10)

where S₁ is the mass of chopped alfalfa, g; S₀ is the total mass of alfalfa stems, g.

Bonding key fracture rate: During the flattening modulation process, the bonding key fracture rate Y₂ is calculated by evaluating the ratio of the number of bonding key fractures to the total number of bonding keys. This metric assesses the modulation condition of alfalfa stems after flattening modulation treatment. The number of top, middle, and root bonding keys were: 1993, 3904, and 6392, respectively. The formula for calculating the bonding key fracture rate Y₂ is as follows:

$$Y_2={\displaystyle \frac{N_1}{N_0}}$$

(11)

where N₁ is the number of alfalfa stem bonding bonds broken; N₀ is the total number of alfalfa stem bonding bonds.

2.10. Alfalfa Stem Flattening Modulation Simulation Test

To visually demonstrate the motion state of the flattening and modulating device and the modulation effect on alfalfa stems when multiple stems are fed in, taking the operating conditions (feeding amount: 10.42 kg/s, modulation roller speed: 675 r/min, buffer spring preload force: 720 N) as an example, the flattening modulation process of the alfalfa stems is illustrated in Figure 14.

At 0.05 s, alfalfa stems are generated above the conveyor belt and fall due to gravity; at 0.15 s, the alfalfa stems complete particle replacement and the conveyor belt starts moving, transporting the stems to the modulation roller gap; at 0.25 s, some alfalfa stems have entered the modulation roller gap and are flattened, bent, and torn under the interaction forces of the modulation rollers; at 0.45 s, most alfalfa stems have undergone flattening modulation processing and, due to the high-speed rotation of the modulation rollers, the stems are propelled backward at a significant speed; at 0.55 s, the stems have completed the flattening modulation operation, and the modulated stems collide with the alfalfa collection plate, rapidly reducing their throwing speed; at 0.65 s, the modulation rollers stop rotating, and the stems start falling onto the sieve plate; at 1.0 s, the stems are completely stationary on the sieve plate, which begins vibrating at a frequency of 24 Hz with a displacement amplitude of 10 mm; at 1.3 s, due to the vibration, the broken alfalfa stems smaller than the sieve hole diameter fall and their mass is statistically analyzed in the falling area; at 1.5 s, no broken alfalfa stems fall, completing the broken alfalfa separation.

To analyze the dynamic changes of alfalfa stems entering the modulation roller gap, data from the period when the stems enter the modulation roller gap until the end of flattening modulation is selected for statistical analysis. The variation curves of the modulation roller gap, spring deformation, and the number of stem bonding key fractures are shown in Figure 15.

According to the variation curve of the spring deformation in Figure 15a, at 0.2 s when the stems enter the modulation roller gap, as the feeding amount increases, the stems overcome the gravity of the driven roller and the pressure of the modulation roller in the modulation roller gap, leading to an expansion of the modulation roller gap and deformation of the spring. The maximum spring deformation during this process is 0.33 mm. At 0.5 s, when the stems exit the modulation roller gap, the modulation roller gap continuously decreases, and the spring deformation returns to its initial state.

As per the variation curve of the number of bonding key fractures of alfalfa stems in Figure 15b, at 0.2 s, when the stems are fed into the modulation roller gap, the modulation rollers start flattening and bending the stems by meshing through involute rubber teeth, initiating the modulation process. Due to the inertia force at the beginning of the operation, damage is already inflicted on some parts of the stems, leading to the fracture of bonding keys. By 0.5 s, when the stems are fully propelled, the number of bonding key fractures stabilizes, indicating the completion of the flattening modulation process.

3. Results

3.1. Test Results and Analysis

Based on the Box–Behnken central composite design method, data analysis was conducted with a total of 17 sets of simulation experiments. The results of the simulation experiments are shown in

Table 7. Experimental results.

No.	Factors			Evaluation Indicators
	Feeding Amount X1	Modulating Roller Speed X2	Buffer Spring Preload Force X3	Crushing Rate Y1 (%)	Bonding Key Fracture Rate Y2 (%)
1	0	1	1	4.45	8.36
2	0	−1	−1	2.09	3.97
3	−1	1	0	1.54	5.46
4	−1	0	1	3.46	5.11
5	0	0	0	1.45	6.52
6	1	0	−1	2.42	6.36
7	0	0	0	1.48	6.415
8	−1	0	−1	1.42	3.35
9	1	−1	0	2.56	5.59
10	−1	−1	0	1.36	3.43
11	1	0	1	5.74	8.69
12	1	1	0	3.51	8.78
13	0	1	−1	2.42	6.06
14	0	0	0	1.75	6.517
15	0	0	0	1.83	6.619
16	0	−1	1	3.95	5.17
17	0	0	0	1.99	6.415

, where X₁, X₂, and X₃ are coded values.

The experimental results were subjected to regression analysis to obtain the analysis of variance results for the factors affecting the crushing rate Y₁, as shown in

Table 8. Analysis of crushing rate Y_1.

Source	Freedom	Sum of Squares	Mean Square	F Value	p-Value	Significance
Mode	9	24.78	2.75	38.76	<0.0001	**
X1	1	5.20	5.20	73.20	<0.0001	**
X2	1	0.4802	0.4802	6.76	0.0354	*
X3	1	10.70	10.70	150.55	<0.0001	**
X1×2	1	0.1482	0.1482	2.09	0.1918
X1X3	1	0.4096	0.4096	5.77	0.0474	*
X2X3	1	0.0072	0.0072	0.1017	0.7591
X12	1	0.3480	0.3480	4.90	0.0625
X22	1	0.2738	0.2738	3.85	0.0904
X32	1	6.82	6.82	95.97	<0.0001	**
Residual error	7	0.4973	0.0710
Lack-of-fit	3	0.2829	0.0943	1.76	0.2935
Pure Error	4	0.2144	0.0536
Cor Total	16	25.28

Note: ** means very significant (p < 0.01); * means significant (0.01 < p < 0.05).

.

According to Table 8, the significance of the effects of various factors and their interactions on the crushing rate Y₁ decreases in the following order: X₃, X₃², X₁, X₂, X₁X₃. After eliminating insignificant terms, the quadratic polynomial regression equation of the crushing rate Y₁ with respect to the feeding amount X₁, modulation roller speed X₂, and buffer spring preload force X₃ is established as

$$Y_1=1.16X_3+1.27{X_3}^2+0.863X_1+0.2450X_2+0.32X_1{X}_3+1.7$$

(12)

To provide a more intuitive analysis of the impact of the interaction effects of various factors on the crushing rate, response surface plots as shown in Figure 16 were generated. From Figure 16A, it can be observed that, when the feeding amount of alfalfa stems is constant, as the modulation roller speed and meshing force increase, resulting in a greater total modulation force on the stems, the crushing rate shows an increasing trend. When the modulation roller speed is fixed with a larger feeding amount of stems, due to the mismatch between the modulation roller gap and the feeding amount, the modulation force on the stems becomes excessive, leading to an increase in the crushing rate. Figure 16B indicates that, with a constant preload force on the buffer spring, as the feeding amount increases, the crushing rate initially decreases and then increases. This is due to the increased feeding amount resulting in more alfalfa stems being entrapped within the alfalfa pile, leading to a reduction in the pressure exerted by the modulation rollers due to the buffering within the flexible material, causing a decrease in the crushing rate. As the feeding amount of alfalfa stems continues to increase, blockages form at the adjustment roller gap, forcing an increase in the gap between the adjusting rollers. Consequently, the compression of the spring leads to an increase in the pressure between the adjusting rollers, pushing more stems into a critical flattening and modulation state, thereby increasing the crushing rate of the stems. From Figure 16C, it is evident that, with a constant modulation roller speed, the crushing rate increases with an increase in the preload force of the buffer spring. When the preload force of the buffer spring is constant, as the modulation roller speed accelerates, the force between the modulation rollers continues to increase, resulting in an increasing trend in the crushing rate. However, due to the rapid increase in speed, the duration of interaction between the stems and the modulation rollers decreases, leading to a decreasing trend in the crushing rate.

Variance analysis results for the bonding key fracture rate Y₂ are shown in

Table 9. Analysis of variance of bonding key fracture rate Y_2.

Source	Freedom	Sum of Squares	Mean Square	F Value	p-Value	Significance
Mode	9	41.36	4.60	141.55	<0.0001	**
X1	1	18.21	18.21	560.98	<0.0001	**
X2	1	13.78	13.78	424.53	<0.0001	**
X3	1	7.20	7.20	221.83	<0.0001	**
X1X2	1	0.3364	0.3364	10.36	0.0147	*
X1X3	1	0.0812	0.0812	2.50	0.1577
X2X3	1	0.3025	0.3025	9.32	0.0185	*
X12	1	0.5080	0.5080	15.65	0.0055	**
X22	1	0.4721	0.4721	14.54	0.0066	**
X32	1	0.3123	0.3123	9.62	0.0173	**
Residual error	7	0.2272	0.0325
Lack-of-fit	3	0.1980	0.0660	9.02	0.0297
Pure Error	4	0.0293	0.0073
Cor Total	16	41.58

Note: ** means very significant (p < 0.01); * means significant (0.01 < p < 0.05).

. According to Table 9, the significance of the effects of various factors on the bonding key fracture rate Y₂ decreases in the following order: X₁, X₂, X₃, X₁², X₂², X₁X₂, X₃², X₂X₃. After eliminating insignificant terms, the quadratic polynomial regression equation of the bonding key fracture rate Y₂ with respect to the feeding amount X₁, modulation roller speed X₂, and buffer spring preload force X₃ is established as

$$Y_2=1.51X_1+1.31X_2+0.95X_3-0.35{X_1}^2-0.33{X_2}^2+0.29X_1{X}_2-0.27{X_3}^2+0.28X_2{X}_3+6.5$$

(13)

As shown in Figure 17, the response surface plots of the interactions on the bonding key fracture rate Y₂ are presented. From Figure 17A, it can be observed that, when the feeding amount of alfalfa stems is constant, an increase in the modulation roller speed leads to a greater tangential force exerted by the modulation roller on the stems, resulting in an increase in the bonding key fracture rate. With a constant modulation roller speed, as the feeding amount increases, due to the mismatch between the modulation roller gap and the feeding amount, an excess of stems surpasses the critical flattening and modulation state, causing an increase in the bonding key fracture rate. Figure 17B indicates that, when the feeding amount is constant, the bonding key fracture rate increases with the increase in the preload force of the buffer spring. A higher preload force on the buffer spring results in greater forces between the modulation rollers, making the stems more prone to node breakage, splitting, and bending, thus leading to a higher bonding key fracture rate. When the preload force of the buffer spring is constant, the bonding key fracture rate increases with the feeding amount. From Figure 17C, it can be seen that, with a constant modulation roller speed, the bonding key fracture rate increases with the increase in the preload force of the buffer spring. Similarly, when the preload force of the buffer spring is constant, the bonding key fracture rate increases with the higher modulation roller speed.

3.2. Parameter Optimization

Using the minimum crushing rate and maximum bonding key fracture rate as optimization objectives, with the feeding amount, modulation roller speed, and buffer spring preload force as optimization targets, the established regression model was optimized using Design-Expert software. The objective and constraint equation are as follows:

$$\left\{\begin{array}{l}\min Y_1\left(X_1,X_2,X_3\right)\\ \max Y_2\left(X_1,X_2,X_3\right)\\ s.t.\left\{\begin{array}{l}3.47 \mathrm{kg}/\mathrm{s}\le X_1\le 10.41 \mathrm{kg}/\mathrm{s}\\ 540 \mathrm{r}/\min \le X_2\le 810 \mathrm{r}/\min \\ 240 \mathrm{N}\le X_3\le 1200 \mathrm{N}\end{array}\right.\end{array}\right.$$

(14)

Using the Optimization function of Design-Expert 11 software, the optimal combination of each influencing factor was determined to be feeding amount of 5.10 kg/s, modulation roller speed of 686.87 r/min, and buffer spring preload force of 670.02 N. Under this parameter combination, the crushing rate and bonding key fracture rate were found to be 1.28% and 95.60%, respectively. Simulation verification experiments based on the optimal parameters yielded a stem breakage rate of 1.35% and a stem flattening rate of 95.81%. The relative errors compared to the predicted values were small, confirming the reliability of the optimization results.

3.3. Test Conditions and Equipment

Based on the optimal parameter combination, a prototype was constructed, and field experiments were conducted in Zhangjiagou, Xicha Town, Gaolan County, Lanzhou City, Gansu Province in August 2023. This time period includes the ‘Zhongtian No.1’ alfalfa bud stage to the first flowering stage, the test object for the third crop of alfalfa. The experimental target was the third crop of alfalfa, with a John Deere 954 tractor (rated power of 69.9 kW) utilized for the operation. The mounted folding mower–flattener machine was connected to the tractor via three-point linkage, and the terrain of the test field was relatively flat; the test site is shown in Figure 18.

According to GB/T 21899-2008 Mowing and flattening machine operational standards, during the field trial process, after the prototype passed through the initial 20 m stabilization zone, five random test samples in the stabilization zone along the forward direction are selected; the length of each sample area is 1 m, each group of samples is repeated three times, and the test result is taken as the average value. The actual harvested alfalfa stem mass, flattened alfalfa stem mass, and shredded stem mass were measured. Evaluation criteria were the stem flattening rate G₁ and stem shredding rate G₂, calculated as the average of 10 measurements, with the formulas as follows:

$$\left\{\begin{array}{l}{G}_1={\displaystyle \frac{M_1}{M_0}}\times 100\%\\ G_2={\displaystyle \frac{M_2}{M_0}}\times 100\%\end{array}\right.$$

(15)

where G₁ is the stem flattening rate, %; G₂ is the stem crushing rate, %; M₁ is the mass of flattened alfalfa stems, g/m²; M₂ is the mass of crushed alfalfa stems, g/m²; M₀ is the actual mass of harvested alfalfa stems, g/m².

4. Discussion

The test results for the flattening rate and crushing rate are shown in

Table 10. Flattening rate and crushing rate for alfalfa steams.

No.	Forward Speed (m/s)	Stem Flattening Rate (%)	Stem Crushing Rate (%)
1	1.4	95.23%	1.35%
2		95.31%	1.28%
3		95.15%	1.77%
4	2.8	95.86%	1.76%
5		95.95%	1.83%
6		95.77%	1.68%
7	3.2	96.11%	2.04%
8		96.08%	1.96%
9		95.93%	1.89%

. The stem average flattening rate and stem crushing rate are 95.71% and 1.73% respectively, which are relatively small errors with predicted values, and the optimization results are reliable. This shows that the developed flattening and modulating device can adapt to different feeding amounts and can flatten and modulate alfalfa steams well.

5. Conclusions

A gap adjustable flattening modulation device was designed, and the theoretical analysis of alfalfa steam flattening modulation process, as well as the related mechanical test, were carried out; the analysis and parameter optimization of the flattening modulation device were completed by using MBD–DEM coupled simulation, and the main conclusions are as follows:

(1) Based on the requirements for alfalfa flattening and conditioning, the structural design of the flattening and modulating device was completed. Mechanical analysis of the operation process was conducted to determine the structural parameters of the compression spring and buffer spring. The main factors influencing conditioning quality were identified based on theoretical analysis.

(2) An DEM–MBD coupled simulation model between the flattening and modulating device and the flexible body of alfalfa stems was established. Experimental factors included feeding amount, modulation roller speed, and buffer spring preload force. Evaluation criteria were the shredding rate and bonding key fracture rate. Simulation experiments and regression fitting analysis were conducted. The optimal combination of influencing factors was determined as follows: feeding amount of 5.10 kg/s, modulation roller speed of 686.87 r/min, and buffer spring preload force of 670.02 N.

(3) Field trials were carried out based on the optimal parameter combination. Results indicated that the average flattening rate and breakage rate of the alfalfa stems were 95.71% and 1.73%, respectively, meeting the requirements for alfalfa stem flattening and conditioning operations.