Optimising Rice Straw Bale Quality Through Vibration-Assisted Compression

Abstract

This study focuses on enhancing the comprehensive utilisation of rice straw by proposing a vibration-assisted compression technology, with the aim of resolving inherent issues in traditional baling, such as uneven compression and low density. This study designed a multi-point vibration-assisted compression test rig and established a vibration-enhanced compression mechanical model based on the physical properties of rice straw. By integrating discrete element method (DEM) simulations with bench testing, the optimal length-to-width ratio of 1:1 was identified for achieving superior compaction quality. A systematic analysis was conducted to evaluate the effects of vibration point configuration, frequency, and amplitude control on straw bale integrity. The results of the DEM simulations demonstrated that vibration-assisted compression significantly enhanced the compaction uniformity and stability of rice straw. The dimensional stability coefficient and pressure transmission rates of the straw bales reached 88.25% and 58.04%, respectively, validating the efficacy of the vibration-assisted compression technique. This study provides innovative concepts and theoretical foundations for optimising the design of straw baling and in-field collection equipment. It holds critical significance for advancing the resource-efficient utilisation of agricultural residues and promoting sustainable agricultural practices.

1. Introduction

Against the current global backdrop of advancing low-carbon economies and sustainable development, biomass stands as a key renewable resource with versatile applications across multiple sectors including energy production, industrial processes, pharmaceuticals, and food [1,2]. As a form of biomass, rice straw contains substantial cellulose and can effectively substitute for wood in paper production and the manufacturing of disposable food containers, among other applications [3]. Northern China harbours abundant rice straw resources, yet prevalent issues persist—including indiscriminate disposal, open-field burning, and low utilisation rates of this biomass [4,5]. With rapid economic and technological development, as well as evolving production methods and structures, the comprehensive management and utilisation of straw have emerged as urgent social, environmental, and resource-related issues requiring resolution.

In recent years, China has introduced the “Five Modernisations” technology for comprehensive structural utilisation of crop straw. However, challenges such as the material’s inherent seasonality and low bulk density have resulted in suboptimal utilisation rates [6,7]. Therefore, establishing an effective and rational technical system and development model for “collection, storage, and transportation” is crucial to overcome the seasonal constraints of straw and ensure its off-field removal, storage, and transportation. This serves as a foundational guarantee for addressing the comprehensive utilisation of straw as both an environmental and resource-related challenge [8]. Mirko Maraldi et al. investigated the effects of straw type, bale density, orientation, baling process, and loading rate on the mechanical properties of straw bales [9]. By analysing force–displacement curves from uniaxial compression tests, he established relationships between mechanical properties and geometric–densimetric parameters. Turner et al. conducted uniaxial compression tests to examine the required pressure and stress relaxation of formed bales when compressing different materials to fixed densities under varying moisture contents [10]. Deformation characteristics of feedstock materials were quantitatively assessed pre-compression, with a material pulverisation strategy subsequently developed to optimise compressive force reduction.

Straw baling technology is the primary, most efficient, and direct method for straw collection and off-field removal. As a critical component of the “collection, storage, and transportation” technical system, it constitutes an indispensable step toward achieving comprehensive straw utilisation [11,12]. Kenny Nona et al. established a predictive pressure distribution model for triaxial directions in the compression chamber of rectangular balers, which enhances the optimised design and operational efficiency of baler compression components [13]. Du et al. analysed the influence of moisture content, straw length, compression density, and compression speed on the relaxation characteristics of sorghum straw, with results demonstrating that while compression speed had a minimal impact on the elastic modulus, the other three factors exerted a significant influence [14]. Wang et al. explored the interaction between the pickup mechanism and straw using a coupled ADAMS-DEM and analysed the motion trajectory and morphological changes in the straw [15]. Current straw baling machinery primarily includes rectangular balers, round balers, and vertical balers. Compared to round balers, rectangular and vertical balers offer advantages such as simpler operation, lower manufacturing costs, and greater user-friendliness. Additionally, rectangular straw bales exhibit higher compactness, optimised volumetric capacity, and broader applicability, significantly reducing transportation and handling costs [16,17].

Contemporary vertical balers predominantly employ ram-driven compression methodologies, operating through either singular or cyclical compaction modes. These systems exhibit inherent operational constraints, particularly manifesting as non-uniform straw distribution and inadequate structural consolidation within compression chambers. Such deficiencies directly contribute to significant densification heterogeneity in processed straw bales, thereby compromising both mechanical integrity and dimensional stability of the final compacted products.

The discrete element method (DEM) is a key tool for analysing the microscopic mechanisms of straw compression. Zhang et al. calibrated the contact parameters for straw cubic compression using DEM, establishing a parameter basis for high-density baling [18]. Mei et al. proposed a flexible model of wheat straw based on cylindrical elements and developed a method for constructing a destructible discrete element model (DEM) of straw bales [19]. Xie et al. simulated the interaction among straw, a travelling mechanism, and soil using the discrete element method (DEM) and analysed the motion trajectory of the straw as well as the transmission of contact forces [20].

Against this contextual backdrop, the present study addresses the common challenges of low compression density and suboptimal forming stability inherent in the compression technologies of vertical balers. A multi-frequency vibration-assisted compression technology is proposed to enhance the stability of compressed straw. To this end, a straw compression test bench featuring adjustable frequency and amplitude was established by integrating a multi-frequency excitation device with a hydraulic compression system. By applying controlled vibrations at specific frequencies and amplitudes during the compression process, this approach aims to enhance the quality of straw compaction and improve the uniformity of density within the formed straw bales. Using DEM simulation technology, this study conducts a comparative analysis of the compression mechanisms of straw under vibration, focusing on force distribution, velocity distribution, stress distribution, and compression quality effects. The proposed research methods and findings provide theoretical underpinnings and technical references for the innovative design of key components in rice straw baling equipment. These insights hold significant implications for ensuring the efficient and rational utilisation of straw resources and promoting the sustainable development of the national green agriculture.

2. Materials and Methods

2.1. Overall Structure and Working Principle

2.1.1. Structural Configuration of the Multi-Point Vibration-Assisted Straw Compression Test Rig

The overall structure of the multi-point vibration-assisted straw compression test rig is illustrated in Figure 1a. It primarily consists of a main frame, compression chamber, compression plate, double-acting hydraulic cylinder, and a multi-point variable-amplitude frequency-modulation vibration device. The multi-point variable-amplitude frequency-modulation vibration device (Figure 1b) comprises an electric motor, sprocket, chain, vertical bearing housing, lifting platform, linear bearings, linear bearing limit frame, vibration units, and supporting frame. The structural details of a single vibration unit are shown in Figure 1c. Each unit is composed of a circular disc, eccentric adjustment screw, screw support, universal joint, eccentric shaft, hexagonal limit linkage, driving linkage, and vibration plate.

The double-acting hydraulic cylinder was vertically mounted above the main frame via a fixed mounting frame and connected to the hydraulic power unit and oil tank through hydraulic lines. A supporting frame was installed at the upper end of the lifting platform. Above the frame, the electric motor, motor bracket, and bearing frame were sequentially arranged from left to right. Four bearing housings were coaxially mounted on the bearing frame and motor bracket, with input and support shafts rotatably assembled within these housings. Multiple vibration units were serially connected between the power input shaft and support shaft. The motor was coupled to the active vibration unit group via a coupling, while the active and passive vibration unit groups were linked by sprockets and chains with a transmission ratio of 1:1. The rotating discs of the vibration units were designed with a diameter of 160 mm based on structural layout requirements and operational demands. The eccentric adjustment screw was secured to rotating disc A through screw supports, with an eccentric square brass nut threaded onto the screw. The eccentric shaft was connected to both the eccentric square brass nut and the eccentric slider, which interfaced with rotating disc B via slide rails. The vibration plate was linked to the eccentric shaft through a hexagonal limit linkage, universal joint, and driving linkage. Vibration amplitude adjustment was achieved by rotating the eccentric adjustment screw to alter the vertical position of the eccentric square brass nut, thereby increasing or decreasing the eccentricity of the shaft (minimum amplitude: 0 mm, corresponding to concentric alignment of the eccentric shaft and rotating disc). Vibration frequency was modulated by adjusting the rotational speed of the discs. Key dimensional parameters included a hexagonal limit linkage length of 200 mm, driving linkage length of 80 mm, and inter-disc spacing of 120 mm, all adaptively designed based on the vibration plate stroke, cross-sectional dimensions of the compression chamber, and spacing between adjacent vibration units.

2.1.2. Working Principle of the Multi-Point Vibration-Assisted Straw Compression Test Rig

The working principle of the rice straw multi-point vibration-assisted compression test rig is illustrated in Figure 2. Prior to operation, the lifting platform was positioned directly beneath the compression chamber. The platform height was adjusted to ensure full contact between the upper surface of the multi-point variable-amplitude adjustment device and the lower surface of the compression chamber. Rice straw was fed into the compression chamber through the feeding port. During operation, the hydraulic power unit was activated to drive the hydraulic cylinder, which pushed the compression plate downward to compress the straw. Simultaneously, the multi-point variable-amplitude frequency-modulation vibration device was engaged. The electric motor-initiated rotation of the driving shaft, thereby activating the primary vibration unit group. The driving sprocket mounted on the driving shaft transmitted synchronous rotation to the driven sprocket via a chain, which subsequently activated the secondary vibration unit group on the driven shaft. Vibration amplitude and frequency were adjusted by modulating the vibration units to meet specific test requirements. Upon completion of compression, the lifting platform was lowered, and the multi-point variable-amplitude frequency-modulation vibration device was retracted. The compression plate was then reactivated to eject the straw bale, accomplishing the discharge process.

To obtain the pressure distribution within the vibration zone of the multi-point variable-amplitude frequency-modulation vibration device, this study employed thin-film piezoelectric sensors for multi-point pressure acquisition under actual operational conditions. Specifically, IMS-S40A thin-film pressure sensors (sensitivity range: 5–100 kg; sensing area: 40 × 40 mm; 5 V power input; 0–5 V analogue output) were deployed. These sensors integrate piezoresistive elements, where the internal resistance increases proportionally with the applied normal-direction (z-axis) pressure, enabling localised pressure measurement. The sensor arrangement is detailed in Figure 2.

To align with practical operational requirements, the cross-sectional geometry of the compression chamber was designed as a rectangular shape. This decision was based on the standardised cross-sectional configurations of small-to-medium-sized straw balers and vertical balers, coupled with the need for transport efficiency of rectangular bales and their logistical advantages in storage and utilisation under small-scale agricultural and agro-pastoral conditions. The chamber height was determined as 800 mm through adaptive coordination with the feed port dimensions and hydraulic cylinder stroke. To accommodate the multi-point vibration design, rectangular vibration plates were selected (using 4 vibration points as a representative case), matching the chamber’s cross-sectional profile. Retaining plates (height: 30 mm) were installed around the lower periphery of the vibration plates to prevent material leakage and clogging during operation, designed based on the maximum allowable extension distance of the plates at peak vibration amplitude. The preliminary three-dimensional dimensions of the multi-point variable-amplitude frequency-modulation vibration device were established as 1221 × 526 × 462 mm (excluding the lifting platform). Critical operational parameters for the motor, hydraulic cylinder, and hydraulic power unit integrated into the rice straw multi-point vibration compression test rig were configured as summarised in

Table 1. The basic parameters of the configured power components.

Basic Parameters	Value	Unit
Motor rated speed	1300	r/min
Motor rated power	0.25	kW
Maximum stroke of hydraulic cylinder	500	mm
Maximum pressure of hydraulic cylinder	20,000	N
Maximum power of hydraulic power unit	0.75	kW

.

2.2. Structural Design of Key Components in the Multi-Point Vibration-Assisted Straw Compression Test Rig

2.2.1. Kinematic Analysis of Vibration Units

A kinematic analysis of the vibration process within the vibration units was conducted, as illustrated in Figure 3.

(1) Displacement of the vibration plate

When the vibration plate reaches point A, as shown in Figure 3a, the eccentric rotation angle is α, and the rotating disc rotates at an angular velocity ω in the direction indicated in the diagram. At this position, the swing angle β of the hexagonal limit linkage relative to the central axis of the vibration unit is defined. The displacement x of the vibration plate at point A is expressed as:

$$x=L_2\left(1-\cos \beta \right)+\Delta R\left(1-\cos \alpha \right)$$

(1)

$$\lambda =\Delta R/L_2$$

(2)

$$x=\Delta R\left[1-\cos \alpha +\left(1-\cos \beta \right)/\lambda \right]$$

(3)

where λ is eccentric linkage ratio, x is displacement of the vibration plate at point A, mm, L₂ is active connecting rod length, mm, ∆R is eccentric distance of eccentric axis, mm, α is eccentric rotation angle, (°), and β is swing angle of the hexagonal limit linkage relative to the central axis of the vibration unit, (°).

The above equation was further simplified. As illustrated in Figure 3a:

$$\Delta R\sin \alpha =L_2\sin \beta$$

(4)

$$\sin \beta ={\displaystyle \frac{\Delta R}{L_1}}\sin \alpha =\lambda \sin \alpha$$

(5)

$$\cos \beta =\sqrt{1-{\lambda }^2{\sin }^2\alpha }$$

(6)

where L₁ is hexagonal limit connecting rod length, mm.

Expanding the above equation yields:

$$\cos \beta =1-R^2{\sin }^2\alpha /2L_1^2-R^2{\sin }^4\alpha /8L_1^2$$

(7)

Since terms from the third onward are higher-order infinitesimals and thus negligible, the displacement of the vibration plate can be approximated as:

$$x_1=\Delta R\left[1-\cos \alpha +\lambda \left(1-\cos 2\alpha \right)/4\right]$$

(8)

where x₁ is approximate displacement of the vibration plate, mm.

The displacement variation curve of the reciprocating motion of the vibration plate is shown in Figure 3b. Based on the analysis derived from Equation (8), when the eccentric rotation angle α = 0°, the vibration plate reaches the upper dead centre position A₁. Conversely, when the linkage swing angle β = 0°, the vibration plate moves to the lower dead centre position A₂. The distance between the upper and lower dead centres (A₁ and A₂) is 2∆R, indicating that the vibration amplitude of the plate equals the eccentricity of the eccentric shaft.

(2) Motion velocity of the vibration plate

An analysis of the kinematic behaviour of the vibration units reveals that their motion can be described as a conversion from rotational to reciprocating motion. Let ω denote the angular velocity of the rotating disc. The displacement of the vibration plate is related to the deflection angle of the rotating disc. The velocity of the vibration plate during its reciprocating motion varies with time, and its instantaneous velocity at any given moment corresponds to the derivative of the displacement with respect to time, expressed as:

$$v=\mathit{d}x/\mathit{d}{t}_1=\mathit{d}x/\mathit{d}\alpha \cdot \mathit{d}\alpha /\mathit{d}{t}_1$$

(9)

$$\mathit{d}\alpha /\mathit{d}{t}_1=\omega$$

(10)

where v is rotation time, mm/min.

Therefore, the velocity of the vibration plate is:

$$v=\Delta R\omega \sin (\alpha +\beta )/\cos \beta$$

(11)

The velocity of the vibration plate was approximated by taking the time derivative of Equation (8).

$$v_1=\Delta R\cdot \omega \cdot \left(\sin \alpha +\lambda \sin 2\alpha /2\right)$$

(12)

where v₁ is the approximate velocity of the vibration plate, mm/s.

The velocity variation curve generated by the reciprocating motion of the vibration plate is shown in Figure 3c. As demonstrated by the aforementioned analysis, the motion of the vibration plate results from the superposition of two harmonic vibrations. Furthermore, the combined motion can still be regarded as a harmonic vibration, with its period expressed as:

$$T=2\pi /\omega$$

(13)

where T is the period of the vibration plate, min.

The vibration frequency of the vibration plate was calculated as follows:

$$\left\{\begin{array}{l}f=\omega /2\pi \\ \omega =2\pi n\\ T=1/f\end{array}\right.$$

(14)

where f is vibration plate motion frequency, cycles/min, and n is motor speed, rpm.

The relationship between the vibration frequency of the vibration plate and the motor speed can be analysed from Equation (14), which demonstrates that the vibration frequency of the vibration plate equals the motor speed.

(3) Vibration plate acceleration

The acceleration of the vibration plate can be expressed as the time derivative of Equation (11):

$$j=\Delta R\cdot {\omega }^2\cdot \left[\cos (\alpha +\beta )/\cos \beta +\lambda \cdot {\cos }^2\alpha /{\cos }^2\beta \right]$$

(15)

where j is acceleration of the vibration plate, mm/s.

The acceleration of the vibration plate is approximated as the time derivative of Equation (12):

$$j_1=\Delta R\cdot {\omega }^2\cdot \left(\cos \alpha +\lambda \cdot \cos 2\alpha \right)$$

(16)

where j₁ is the approximate acceleration of the vibration plate, mm/s.

2.2.2. Vibration Unit Arrangement Design

To realise the coordinated pulsating vibration function among multiple vibration units, an arrangement design was implemented for the vibration units of the multi-point variable-amplitude frequency-modulation vibration device with varying numbers of vibration points. Since the displacement of the vibration plate depends on the eccentric distance ΔR and rotational angle α of the eccentric shaft, and the vibration amplitude and frequency of each plate are consistent during operation, this indicates that the eccentric distances of the eccentric shafts in all vibration units are equal. Therefore, the collaborative pulsating vibration function among multiple vibration units can be achieved by adjusting the deflection angle between the rotating discs of adjacent vibration units. Taking four vibration points as an example, the arrangement is illustrated in Figure 4. The deflection angle of the rotating discs of adjacent vibration units is 90°.

2.2.3. Force Analysis of the Vibration Unit

The oscillatory action of the vibration plate on straw directly influences the stacking posture of straw during the compression process. Therefore, a kinematic and force analysis of the vibration plate was conducted, as illustrated in Figure 5.

A force analysis was performed on the vibration plate along the straw compression direction, and the equilibrium equation was formulated as follows:

$$F_1-F_2-G=m{j}_1$$

(17)

where F₁ is thrust of the vibration plate, N, and F₂ is applied force on the vibration plate, N, and G is gravity of the vibration plate, N, and m is mass of the vibration plate, kg.

The pressure F₂ acting on the vibration plate is expressed by Equation (18).

$$F_1=4m\Delta R{\pi }^2{n}^2\left(\cos 2\pi nt+\Delta R\cos 4\pi nt/L_2\right)+\left(F_L+m_{c}g/n_1\right)\cdot k+G$$

(18)

where F_L is the force on the compression plate, N, m_c is the mass of the vibration plate, kg, and k is the compressive force transmission coefficient in the normal direction (z-axis direction), which ranges between 0.5 and 0.6.

By combining, Equations (16)–(18), the thrust equation of the vibration plate can be derived as:

$$F_1=4m\Delta R{\pi }^2{n}^2\left(\cos 2\pi n{t}_1+\Delta R\cos 4\pi n{t}_1/L_2\right)+\left(F_L+m_{c}g/n_1\right)\cdot k+G$$

(19)

During a single vibration cycle, the average thrust F_m of the vibration plate is expressed as:

$$F_{\mathrm{m}}=\left({\displaystyle {\int }_0^{1/\pi }4m\Delta R{\pi }^2{n}^2\left(\cos 2\pi n{t}_1+\Delta R\cos 4\pi n{t}_1/L_2\right)+\left(F_L+m_{c}g/n_1\right)\cdot k+G}\right)/60$$

(20)

As demonstrated by the above analysis, the vibration force of the vibration plate is dependent on the number of vibration points, vibration frequency, and vibration amplitude of the multi-point variable-amplitude frequency-modulation vibration device. These parameters directly influence the compression quality of the multi-point vibration compression test rig. Subsequent virtual simulations were conducted to compare operational parameters, optimise structural configurations, and determine the optimal working parameters.

2.3. Numerical Simulation Study of the Multi-Point Vibration-Assisted Straw Compression Test Rig

Vibration-assisted compression can directly enhance the compressibility of straw and improve its compaction quality. In previous studies conducted by the research team, a comprehensive analysis was performed on the mechanistic effects of vibration on straw particle velocity, mechanical forces, and stress deformation during compression [21]. By investigating the impact of straw dimensional variations on compression performance, the optimal straw length was identified. To further optimise the cross-sectional dimensions of the compressed material and enhance the stability of compacted bales, this study will systematically investigate the post-compaction stability of straw.

2.3.1. Determination of Compression Chamber Cross-Sectional Dimensions

The research team has previously conducted a series of fundamental studies on the vibrational compression of straw. Using the discrete element method (DEM), the team compared the effects of different straw lengths on compression characteristics under vibration and examined the changes in the internal porosity of straw bales. The results demonstrated that vibration-assisted compression improves the uniformity and stability of the straw compression process along the normal direction (i.e., the z-axis) and established the corresponding mechanical loading mode. Furthermore, the cross-sectional dimensions of the compression chamber significantly influenced the uniformity of straw compression. The compression uniformity was evaluated through two aspects: the mechanical forces exerted on the straw and its velocity distribution during compression. To investigate and identify the optimal cross-sectional configuration, five categories of cross-sectional dimensions were defined. To ensure the universality of the selected cross-sections, cross-sectional dimension ratios were chosen as test factors, specifically 1:1, 1:1.5, 1:2, 1:2.5, and 1:3. The standard deviations of the average velocity and average pressure were selected as evaluation criteria. Each cross-sectional type was evenly divided into five regions along the y-axis direction. The standard deviations of the pressure exerted on the straw and the straw movement velocity within these five regions were calculated, as shown in Figure 6.

The equation for calculating the standard deviation of the average speed is as follows:

$${\sigma }_{vi}=\sqrt{{\displaystyle \sum _{m=1}^t{\left(v_m-v_t\right)}^2/t-1}}$$

(21)

where σ_vi is standard deviation of average velocity, mm/s, t is total sampling step duration, s, and v_m is average velocity at moment m, mm/s.

The average velocity v_t of rice straw within a specific region of the compression chamber over a time period t is expressed as:

$$v_t={\displaystyle \sum _0^t{v}_{ti}}/t$$

(22)

Let v_i denote the velocity of the i-th rice straw in a specific region of the compression chamber at a given time instant. The average velocity v_ti of all rice straws within the compression chamber at that time instant can be expressed as:

$$v_i={\displaystyle \sum _0^n{v}_{ti}}/n$$

(23)

The standard deviation of the mean pressure was calculated using the following equation:

$${\sigma }_i=\sqrt{{\displaystyle \sum _{m=1}^t{\left(f_m-f_t\right)}^2/t-1}}$$

(24)

where σ_i is standard deviation of average pressure, N, t is total sampling step duration, s, f_m is average pressure at moment m, N.

The average pressure f_t of rice straw within a defined region of the compression chamber over a time period t can be expressed as:

$$f_t={\displaystyle \sum _0^t{f}_{ti}}/t$$

(25)

Let f_i denote the force acting on the i-th rice straw in a specific region of the compression chamber at a given time instant. The average pressure f_ti exerted by all rice straws within the compression chamber at that time instant can then be expressed as:

$$f_{ti}={\displaystyle \sum _0^n{f}_i}/n$$

(26)

2.3.2. Numerical Simulation of Rice Straw Compression Assisted by Multi-Point Variable-Amplitude Frequency-Modulation Vibration

(1) Boundary condition setting

Previous studies indicate that a straw length of 200 mm results in bales with relatively good stability [22]. Therefore, for the EDEM simulation, a discrete element model of a 200 mm rice straw was constructed, comprising 39 overlapping spheres, each with a radius of 2.5 mm. Since the Bonding V2 model can effectively describe the mechanical properties of straw under bending, torsion, and shear, its parallel bond feature was used to connect adjacent spheres, with a bond radius of 2.8 mm. Straw particles were generated at a fixed initial position (0, 0, 600). The simulation model is presented in Figure 7, and the corresponding parameters are listed in

Table 2. Bonding V2 model parameters.

Parameters	Values	Reference
Normal stiffness per unit area (N m−3)	4.1551 × 109	[23,24]
Shear stiffness per unit area (N m−3)	8.0749 × 108
Normal strength (Pa)	1.0 × 107
Shear strength (Pa)	1.3 × 106
Bonded disc scale	1.0

.

Vibration loads were applied by the vibration plates at the bottom of the compression chamber. Their motion was governed by a sinusoidal displacement function (Equation (27)), where the phase angle ϕ was set to 0° to ensure the synchronous vibration of all plates. The vibration direction was aligned with the compression direction (i.e., the z-axis). The displacement of the plates per second was calculated using Equation (27). This motion was then implemented in EDEM, with the start times configured so that adjacent plates along the x-axis began vibrating sequentially at 1 s intervals. The configurations for 0 to 16 vibration points are shown in Figure 7. The gravitational acceleration was set to 9.81 m/s, and the “rainfall method” was employed to allow the rice straw to accumulate within the compression chamber, with an initial velocity of 2 m/s. To replicate the operation of a vertical baler, straw models were continuously fed into the simulation in a randomised manner. The time step was determined using the Rayleigh method with a 10% criterion, and data was saved every 0.01 s. The grid size was set to 3R. The specific simulation parameters used in this study are listed in

Table 3. Simulation parameter setting.

Parameters	Values	Reference
Discrete Element Contact Model Between Compressed Straw Particle	Edinburgh Elasto-Plastic Adhesion (EEPA)	[25]
Contact Model Between Compressed Particles and the Mould	Hertz-Mindlin (no slip)
Rice straw density/(kg/m3)	241	[26]
Poisson’s ratio of rice straw	0.5
Elastic modulus of rice straw/(Mpa)	312
Static friction coefficient between rice straw and rice straw	0.445
Dynamic friction coefficient between rice straw and rice straw	0.07
Collision Coefficient of Restitution between rice straw and rice straw	0.357
Static friction coefficient between rice straw and steel plate	0.317	[18,27]
Dynamic friction coefficient between rice straw and steel plate	0.028
Collision Coefficient of Restitution between rice straw and steel Plate	0.407

.

$$y=A\sin \left(2\pi ft+\varphi \right)$$

(27)

where f is the vibration frequency, cycles/min; t is compression time, s; and ϕ is phase angle, °.

In the compression chamber, the initial height of the rice straw simulation test was set to 600 mm. The compression chamber compressed the material along the vertical (z-axis direction) at a speed of 20 mm/s, with a compression stroke of 300 mm.

(2) Single-factor simulation test

Multi-point vibration generates a superimposed stress field through spatial phase differences, effectively eliminating the stress concentration phenomenon caused by traditional single-point excitation. Meanwhile, appropriate vibration frequencies and amplitudes can reduce the friction coefficient between straw materials, promote material reorganisation, and enhance compression density. To investigate the influence of different test factors on the compression quality of rice straw, single-factor simulation tests were conducted, with the levels of each test factor listed in

Table 4. Single-factor test factor level.

Level	Test Factors
	Number of Vibration Points x1	Vibration Frequency x2/(Cycles/min)	Vibration Amplitude x3/(mm)
1	0	100	1
2	1	200	1.5
3	4	300	2
4	9	400	2.5
5	16	500	3

.

Dimensional stability coefficient and pressure transmission rate were selected as evaluation indicators. A higher-pressure transmission rate contributes to enhancing the quality and structural stability of straw compression. A higher-pressure transmission rate indicates more efficient straw accumulation within the compression chamber, which leads to a higher final bale density. The calculation equation for pressure transmission rate is shown in Equation (28). To more accurately quantify the pressure on the straw during compression—specifically from the vibration plates and the compression plate—the chamber was partitioned into five equal-height zones along the z-axis. The force acting on the straw in each zone was recorded separately and calculated using Equation (28). The stability of the formed bale is quantified by the dimensional stability coefficient (Equation (29)), where a larger value denotes greater stability and greater resistance to loosening [28]. To minimise test errors, five repeated trials were conducted for each factor, and the results were expressed as mean values with standard deviations.

$${\lambda }_1=\left(F_b-M_g\right)/F_a$$

(28)

where λ₁ is pressure transmission rate, %; F_a is pressure applied to the compression plate during compaction, N; F_b is pressure applied to the base plate during compaction, N; and M_g is gravitational force acting on the material, N.

$${\lambda }_2=\left(1-L_{rs}/L_c\right)\times 100\%$$

(29)

where λ₂ is the dimensional stability coefficient, %; L_rs is rebound displacement of straw, mm; and L_c is constant-rate compression displacement, mm.

(3) Multi-factor test

In the single-factor simulation tests on straw compression, the effects of the number of vibration plates, vibration amplitude, and vibration frequency on the dimensional stability coefficient and pressure transmission rate of compressed bales were investigated. To further explore the optimal combination of operational and structural parameters for rice straw compression under multi-point variable-amplitude frequency-modulation vibration assistance, a three-factor three-level orthogonal test was conducted. The coding table for the factor levels of the multi-factor orthogonal test is shown in

Table 5. Multi-factor test factor level coding table.

Level Coding	Test Factors
	Number of Vibration Points A	Vibration Frequency B/(Cycles/min)	Vibration Amplitude C/(mm)
1	1	200	1
2	4	250	1.5
3	9	300	2

.

Where A represents the number of vibration points, B denotes the vibration frequency of the vibration plate, and C indicates the vibration amplitude of the vibration plate. Nine groups of orthogonal tests were conducted, and the test results were expressed as mean values.

2.4. Bench Verification Test

Longjing 29, a rice variety widely cultivated in the major rice-producing regions of Northeast China, was selected as the test material. Rice straw samples (with a moisture content of 30%) were obtained by threshing with a combine harvester. The straw was manually trimmed to the required lengths prior to testing. The straw was initially fed into the compression chamber in a randomly arranged, uncompacted state. Based on the optimisation results from multi-factor discrete element simulation tests, bench validation tests were conducted using the self-designed multi-point vibration-assisted straw compression test rig developed in this study. The test setup is illustrated in Figure 8.

To validate the optimal operational and structural parameters derived from the virtual multi-factor simulation tests, the bench validation test was conducted with the following selected factors: 4 vibration points, 300 cycles/min vibration frequency, and 2 mm vibration amplitude. The pressure transmission rate and dimensional stability coefficient were adopted as test indicators. The pressure transmission rate was measured by thin-film piezoelectric sensors installed on the test rig. Key parameters of the bench validation test are summarised in

Table 6. Main parameters of bench verification test.

Basic Parameters	Values	Unit
Rotational speed of the motor	300	rpm
Stroke of the hydraulic cylinder	300	mm
Length of a single straw	200	mm
Moisture content of straw	30.61	%

.

During the test, the lifting platform was elevated to align the upper surface of the multi-point device with the lower surface of the compression chamber. A fixed mass of rice straw was then fed into the compression chamber through the feed port, followed by the simultaneous activation of the multi-point variable-amplitude frequency-modulation vibration device and the double-acting hydraulic cylinder. The pressure values of the four vibration plates and the compression plate at the end of the compression phase were recorded using a real-time pressure monitoring system. Following compression, the rice straw underwent a shape-retention treatment within the compression chamber. The lifting platform was then lowered to eject the compressed straw bale. The rebound displacement of the formed bale was recorded after its removal. To ensure the reliability of test results, three replicate tests were conducted, and average values were calculated.

3. Results and Discussion

3.1. Analysis of Optimisation Design Results for Optimal Interface Types in Simulation Test

A 200 mm-long rice straw model was developed using the discrete element method (DEM). The compression chamber was configured with five different cross-sectional aspect ratios, ranging from 1:1 to 1:3, for the simulations. The standard deviations of average velocity and pressure during compression for different cross-sectional dimension types under two operating conditions were analysed, and the results are shown in Figure 9.

Under both compression conditions, the standard deviation of the tangential zone mean velocity exhibited a consistent trend with increasing aspect ratio of the cross-sectional dimensions. Within the aspect ratio range of 1:1 to 1:2, the standard deviation of the average velocity across tangential cross-sectional regions increased with rising aspect ratios, indicating degraded uniformity. Conversely, in the aspect ratio range of 1:2 to 1:3, the standard deviation first decreased and then increased as the aspect ratio increased, corresponding to an initial improvement followed by a subsequent deterioration in uniformity. Vibration compression demonstrated a reduced standard deviation in the average velocity of straw and enhanced uniformity compared to non-vibration compression.

Under non-vibration conditions, the standard deviation of average pressure initially increased and then decreased within the cross-sectional type range of 1:1 to 1:2, while it exhibited a gradual increase in the range of 1:2 to 1:3. After applying vibration to the compression process, the standard deviation of the average pressure for cross-sectional types in the 1:1 to 1:2 range initially increased and then decreased, whereas for cross-sectional types within the 1:2 to 1:3 range, it first decreased and subsequently increased. When the cross-section aspect ratio was 1:1, the standard deviation of average pressure reached its minimum value, consistent with non-vibration conditions. Under vibration conditions, the maximum pressure standard deviation occurred at a cross-section aspect ratio of 1:2, whereas in non-vibration cases, the peak standard deviation was observed at an aspect ratio of 1:3. Therefore, the optimal cross-section aspect ratio was determined to be 1:1.

The results demonstrate that vibration compression reduces the standard deviation of average velocity and enhances uniformity, which is primarily attributed to the vibration-induced improvement in particle fluidity. This mechanism increases the frequency of particle collisions, thereby effectively diminishing both the magnitude and variability of particle velocity [29]. However, with variations in cross-sectional types, the standard deviation of the average velocity exhibited a consistent trend during both vibration-assisted and non-vibration compression simulation tests, while that of the average pressure showed significant differences. This discrepancy was primarily attributed to the vibration-assisted effect, which induced changes in the interparticle forces among straw particles. Under vibration-assisted compression conditions, the variations in the standard deviation of average velocity exhibited a certain degree of linear correlation with those of average pressure. This phenomenon was primarily attributed to the higher degree of disorder in straw stacking and greater variability in interparticle interactions under non-vibratory compression scenarios. The variations in standard deviations of pressure and velocity were primarily influenced by straw arrangement [30]. During the vibratory compression process, the oscillatory effects adjusted the straw distribution, resulting in a linear correlation between the standard deviations of average velocity and average pressure. When the cross-sectional type exhibited an aspect ratio of 1:1, the standard deviations of average velocity across all regions under both operating conditions reached their minimum values, indicating the most uniform compression effect.

3.2. Analysis of Single-Factor Simulation Results for Rice Straw Compression Assisted by Multi-Point Variable-Amplitude Frequency-Modulation Vibration

3.2.1. Effect of the Number of Vibration Points on Compression Quality

The effects of various test factors on the dimensional stability coefficient and pressure transmission rate during straw compression were analysed. Taking the mechanical behaviour of straw under compression with different numbers of vibration points as an example, the stress distribution phenomena are illustrated in Figure 10.

The straw stacking completion time was set to 0 s, during which the compression plate descended to initiate the compaction process. At this stage, a higher number of vibration points resulted in more pronounced stress concentration on the straw near the vibration plate. As the compression plate continued descending, the stress distribution became most uniform at the compaction phase (15 s) when four vibration points were activated, with comparable stress magnitudes observed between the compression plate side and vibration plate side. When the number of vibration points was 0 or 16, uneven stress distribution was observed on the side adjacent to the compression plate. After the compression plate was raised, a gradual rebound occurred in the compacted straw bale. The smallest rebound displacement was achieved with 4 vibration points. Therefore, the analysis demonstrates that applying appropriate vibration during the compression process enhances the uniformity of stress distribution in the straw compaction and improves the forming quality of the straw bales.

A single-factor test was conducted to investigate the effect of vibration point quantity on straw compression quality. The vibration frequency of the vibration plate was set to 200 cycles/min, with an amplitude of 2 mm, and the number of vibration points was configured to 0, 1, 4, 9, and 16. To investigate the effects of varying numbers of vibration points on the dimensional stability coefficient and pressure transmission rate, the mechanical behaviour of straw during compression and shape-retaining rebound processes under different vibration point configurations was analysed. Repeated test results were processed, and the dimensional stability coefficient and pressure transmission rate were expressed as mean values with standard deviations, as illustrated in Figure 11.

Analysis of the single-factor test results on the effect of vibration plate quantity on compression quality revealed that the dimensional stability coefficient exhibited an increasing trend within the range of 0–4 vibration plates, followed by a decreasing trend within the range of 4–16 vibration plates. The dimensional stability coefficient, which reflects the springback of a compressed straw bale, is defined as follows: a higher coefficient corresponds to lower springback and thus serves as an indicator of reduced internal stress in the bale. When the number of vibration plates was set to 4, the dimensional stability coefficient reached its maximum value, with an average of 86.48% across five replicate tests, indicating optimal compression quality of the straw. The springback of the straw bales was minimised when four vibration plates were used. Under this condition, the internal stress also reached its minimum, demonstrating that vibration effectively reduces the residual stress within the compressed straw. In contrast, the lowest average dimensional stability coefficient (83.07%) was observed when no vibration plates were activated. Furthermore, repeated trials revealed fluctuations in the dimensional stability coefficient during non-vibratory compression processes. Analysis of the effect of the number of vibration plates on pressure transmission rate revealed that the highest-pressure transmission rate was observed when four vibration plates were employed. Pressure is transmitted from the upper compression plate through the straw bale to the lower vibration plates. A higher-pressure transmission rate indicates a greater degree of compaction and, consequently, a higher density of the straw bale within the chamber. Both increasing and decreasing the number of vibration plates beyond this optimal configuration resulted in a decline in pressure transmission rate. Notably, the pressure transmission rate was considerably lower in configurations with 0 or 16 vibration plates. The analysis demonstrated that during the compression process, the absence of vibration or the use of a single vibration plate led to insufficient adjustment of straw orientation within the compression chamber, resulting in inadequate compaction. Conversely, an excessive number of vibration plates induced over-oscillation impacts on the straw, thereby disrupting the structural integrity of the fully compacted straw bales. Based on the test results, the vibration plate quantity within the range of 1–9 demonstrated optimal performance. Therefore, this range (1–9 vibration plates) was selected as the test factor range for subsequent multi-factor trials.

3.2.2. Effect of Vibration Frequency on Compression Quality

During the simulation tests, the number of vibration plates was set to 4 with a vibration amplitude of 2 mm. A single-factor test on straw compression was conducted by varying the vibration frequency of the plates from 100 to 500 cycles/min. To minimise test errors, five repeated trials were performed for each frequency level. To investigate the effect of vibration frequency on the dimensional stability coefficient and pressure transmission rate, the average values from five trials were used to plot the comparisons of rice straw compression dimensional stability coefficients and pressure transmission rates under different vibration-assisted compression conditions. The results are shown in Figure 12.

At a vibration plate frequency of 200 cycles/min, the dimensional stability coefficient of the straw bale reached its maximum value of 86.48%. However, as the vibration frequency increased, the stability coefficient exhibited an overall declining trend. This phenomenon is primarily attributed to the reduced time for straw posture adjustment under single-cycle vibrations at higher frequencies, leading to insufficient compaction and a subsequent decrease in the structural stability of the straw bale. The pressure transmission rate exhibited an initial increase followed by a decrease with increasing vibration frequency, reaching its maximum at 200 cycles/min, while lower values were observed at 100 cycles/min and 500 cycles/min. Analysis indicates that a higher vibration frequency can effectively compact straw, enhance baling density, and thereby improve compression quality. While increased vibration frequency accelerates the baling speed, excessively high frequencies may lead to insufficient compaction of straw, negatively impacting the final compression quality. Therefore, the vibration frequency (x₂) of the vibration plate demonstrated superior performance within the range of 200–300 cycles/min.

3.2.3. Effect of Vibration Amplitude on Compression Quality

In the simulation tests, four vibration plates were configured with a vibration frequency of 200 cycles/min and amplitudes ranging from 1 to 3 mm. A single-factor test on straw compression was conducted with vibration amplitude as the variable, using the dimensional stability coefficient and pressure transmission rate as evaluation metrics. To minimise test errors, five replicate trials were performed for each amplitude level. To investigate the effect of vibration amplitude on the dimensional stability coefficient and pressure transmission rate, the average values from five trials were used to plot the comparison of rice straw compression characteristics under different vibration-assisted compression amplitudes, including dimensional stability coefficient and pressure transmission rate, as shown in Figure 13.

Within the vibration amplitude range of 1–2 mm, the dimensional stability coefficient exhibited a positive correlation with increasing amplitude, reaching its maximum average value of 86.48% at 2 mm. Within the vibration amplitude range of 2 mm–3 mm, the dimensional stability coefficient exhibited a trend of initial decrease followed by an increase with increasing amplitude. The lowest mean value of the dimensional stability coefficient (82.7%) was observed at a vibration amplitude of 2.5 mm. The pressure transmission rate peaked at a vibration amplitude of 1 mm, with an average value of 68.35%. As the vibration amplitude increased, the pressure transmission rate presented an overall downward trend, reaching its lowest value of 29.93% when the vibration amplitude was 3 mm. The analysis indicates that the amplitude influences the force distribution during the compression process. An appropriate amplitude ensures uniform distribution of compressive forces, thereby preventing localised over-compaction or insufficient densification. Excessive amplitude may induce higher mechanical stress on the equipment, accelerating component wear and potentially reducing operational lifespan while increasing maintenance costs [31]. Therefore, the optimal vibration plate amplitude (x₃) should be maintained within the range of 1–2 mm.

3.3. Multifactor Simulation Analysis of Rice Straw Compression Under Multi-Point Variable-Amplitude Frequency-Modulation Vibration Assistance

3.3.1. Orthogonal Simulation Test Results and Analysis of the Effects of Different Test Factors on Dimensional Stability Coefficient

Based on the findings from the single-factor simulation experiments, the influence of the optimal parameter combination on the dimensional stability coefficient was investigated. Subsequently, SPSS 22.0 software (SPSS, Chicago, IL, USA) was employed to statistically analyse the multi-factor simulation results, including a range analysis of the dimensional stability coefficient. The orthogonal experimental array and the corresponding range analysis results are provided in Figure 14a.

The range analysis of the effects of vibration plate quantity (A), vibration frequency (B), and vibration amplitude (C) on the dimensional stability coefficient of straw compression revealed the following contribution order: A > B > C. Based on the influence of each factor on the average dimensional stability coefficient, the optimal parameter combination was determined as A₂B₃C₃, corresponding to 4 vibration plates, a vibration frequency of 300 cycles/min, and a vibration amplitude of 2 mm.

To ensure a more accurate analysis of the experimental results, analysis of variance (ANOVA) was employed for further statistical evaluation. The ANOVA results are presented in Figure 14b. When the dimensional stability coefficient was selected as the evaluation index, the p-value for factor A was 0.035 (range: 0.01–0.05), indicating a significant difference on the response variable. Similarly, factor B exhibited a significant difference with a p-value of 0.018 (range: 0.01–0.05). In contrast, factor C demonstrated a marginally significant difference, as evidenced by its p-value of 0.065 (range: 0.05–0.1).

3.3.2. Orthogonal Simulation Test Results and Analysis of Different Test Factors on Pressure Transmission Rate

The range analysis of the effects of vibration plate quantity (A), vibration frequency (B), and vibration amplitude (C) on the pressure transmission rate is presented in Figure 15a.

The contribution order of these factors was determined as C > A > B. Based on the influence of each factor on the average pressure transmission rate, the optimal parameter combination was identified as A₂B₃C₃, corresponding to a vibration plate quantity of 4, a vibration frequency of 300 cycles/min, and a vibration plate amplitude of 2 mm.

The analysis of variance (ANOVA) results for different factors influencing the pressure transmission rate are presented in Figure 15b.

The p-value of influencing factor A was 0.055 (range: 0.05–0.1), indicating a marginally significant effect on the response variable. Similarly, factor B had a p-value of 0.06 (range: 0.05–0.1), suggesting its marginally significant effect. In contrast, factor C demonstrated a p-value of 0.033 (range: 0.01–0.05), thus confirming a significant effect on the evaluation index.

As demonstrated by the above analysis, the optimal parameter combination was determined to be A₂B₃C₃, corresponding to 4 vibration points, a vibration frequency of 300 cycles/min, and a vibration amplitude of 2 mm. Simulation tests were conducted under these parameters using the previously described configuration. The results revealed a dimensional stability coefficient of 89.36% and a pressure transmission rate of 59.47% for the optimal parameter combination.

3.4. Analysis of Bench Verification Test Results

To verify the compression reliability of the multi-point vibration test bench for rice straw, experiments were conducted using the independently developed apparatus employing the optimal parameters derived from DEM simulations. The measurement procedures for the pressure transmission rate and the dimensional stability coefficient, as well as the results of the bench validation test, are presented in Figure 16.

At the conclusion of the compression process, the average pressure transmission rate reached 58.04%, and the dimensional stability coefficient of the compressed bales averaged 88.25%. In the bench tests, the measured values of the pressure transmission rate and the dimensional stability coefficient showed errors of 2.4% and 1.2%, respectively, when compared with the simulation test data. Significantly, both error values were below 5%. The pressure variation over time obtained from the simulation was compared with the results from the compression test. The accuracy of the simulation was verified by analysing the absolute error between the two datasets. As shown in Figure 16e, the compressive stress errors are all below 5%, confirming that the simulation results are accurate and reliable. The bench test results demonstrated that the multi-point vibration-assisted compression test rig for rice straw exhibited stable operational performance, with satisfactory compression and vibration characteristics. However, the compression quality of the straw bales obtained from actual bench tests was slightly lower than the simulated results. The discrepancy may be attributed to the highly controlled operating conditions in the simulation tests, whereas in the bench tests, factors such as straw deformation, bending, variations in moisture content, and heterogeneous distribution collectively reduced the pressure transfer rate and dimensional stability coefficient.

3.5. Discussion

This study uncovers the mechanism through which vibration-assisted compression technology improves the uniformity and stability of rice straw bales. The underlying core mechanism is that the vibrational energy reduces the static friction coefficient between particles via periodic impacts. This, in turn, promotes the interlayer slip-reorganisation of straw, balances the stress distribution, and averts the loose structure resulting from local stress concentration in traditional compression processes. Collectively, these effects optimise the compression behaviour of straw comprehensively. Chen et al. carried out comparative tests on two stabilisation processes, namely, shape-retaining and pressure-retaining, under different compression conditions [32]. The results indicated that the shape-retaining stabilisation process consistently outperformed the pressure-retaining one. Therefore, in this study, vibrational assistance was introduced during the uniaxial compression process. This measure was taken to enhance the shape-retaining quality of the bales, leading to an improved dimensional stability coefficient of the compressed straw blocks. The implementation of a multi-point vibration mode, consisting of four vibration plates with a vibration frequency of 300 cycles/min and a vibration amplitude of 2 mm, increased the dimensional stability coefficient of the compressed straw bales to 88.25%. Under these vibration parameters, the inertial and frictional forces acting on the straw particles reached a dynamic equilibrium. This effectively circumvented the issue of insufficient particle compaction due to low-frequency vibrations (>200 cycles/min) and, simultaneously, curbed energy dissipation and structural fragmentation brought about by high-frequency vibrations (<400 cycles/min).

Current research on straw compression primarily focuses on uniaxial compression forms, which fails to ensure uniformity in the compression density of straw bales after compaction. The multi-point vibration compression technology proposed in this study can enhance the dimensional stability coefficient of straw bales to 88%, ensuring that the formed bales are less prone to deformation and exhibit uniform density across all layers within the bale. The multi-point vibration compression technology developed in this study exhibited lower requirements for equipment precision and energy consumption compared to the internationally advanced ultrasonic vibration technology (with a compression density of 450 kg/m³) [33]. Although steam-softening-coupled compression technology enhances the density of straw bales, it escalates the pretreatment costs by 30% and is incapable of processing straw with a high moisture content (exceeding 25%) [34]. In sharp contrast, the technology devised in this study omits the additional pretreatment procedures and directly compresses the field-harvested straw, which has a moisture content of 30%, without the need for drying processes. This approach meets the economic viability and operational simplicity requisites of small-to-medium-sized farms, manifesting remarkable adaptability to direct field operations.

The limitations of this study predominantly originate from model simplifications and the control of environmental variables. The DEM model failed to consider the microstructural characteristics of stems and leaves, giving rise to inaccuracies in the simulated stress distributions. Moreover, the disparity between the constant temperature and humidity conditions within laboratory environments and the dynamic conditions in the field led to a 1.3% decrease in the dimensional stability coefficient of the compressed bales during bench tests when compared to the simulation results. Future research efforts will be concentrated on the following three aspects: First, performing full-factorial vibration compression simulation tests using various rice varieties, including japonica and indica, across a broad spectrum of moisture content levels ranging from 20% to 45%, and creating a database for the adaptive control of vibration parameters. Second, the multi-point vibration compression test rig developed in this study is equipped with pressure-variable sensors on both the compression plate and vibration plate, enabling dynamic monitoring of the forces exerted on straw during the compression process. In subsequent work, an adaptive algorithm will be developed to achieve real-time matching between vibration parameters and straw characteristics (such as moisture content and fibre content). Third, the test bench designed in this study facilitates the independent adjustment of vibration frequency and amplitude via the motor speed and eccentric shaft, allowing the parameters to be customised based on the specific material properties of different straws (such as moisture content and length). Subsequent studies will involve a detailed investigation to determine the optimal operational parameters for various crop straws. Furthermore, a comparative analysis of the physical and mechanical properties of different straws against rice straw will be conducted using virtual simulation technology, thereby expanding the engineering applicability of the multi-point vibration compression method. The technological iterations and interdisciplinary innovations within the agricultural industry carry substantial significance for propelling the development of straw compression from “single-function optimisation” to a “resource-efficient ecological chain.” This approach offers a scientifically sound and engineering-practical solution for the management of agricultural waste.

4. Conclusions

This study designed a multi-point vibration-assisted compression test rig. Based on DEM technology, the vibration parameters of the rig were systematically designed and optimised. The stress variation in rice straw during compression and the dimensional stability coefficient of straw bales were analysed. The accuracy of the simulation results was validated through bench tests. The key findings are summarised as follows:

(1) For a variety of compression cross-sectional configuration aspect ratios, the standard deviations of average velocity and pressure in distinct regions manifested a pattern of initially ascending, then descending, and subsequently ascending once more as the aspect ratio increased. The standard deviations of both average pressure and velocity reached their minimums at an aspect ratio of 1:1. At this crucial ratio, the straw demonstrated the most optimal uniformity throughout all regions, and the formed bales exhibited the utmost stability.

(2) With the number of vibration plates ranging from 0 to 16, both the dimensional stability of straw bales and the pressure transmission rate initially increased and then decreased as the number of plates increased. Similar patterns emerged when considering the vibration frequency in the range of 100–500 cycles/min, with both metrics peaking at intermediate values. Regarding the vibration amplitudes ranging from 1 to 3 mm, the stability coefficient initially rose, then sharply plunged, and finally stabilised slightly. Meanwhile, the pressure transmission rate exhibited a three-phase variation: decrease-increase-decrease as the vibration amplitude increased.

(3) In the test of the rice straw multi-point vibration-assisted compression test rig, under the conditions of four vibration plates, a vibration frequency of 300 cycles/min, and a vibration amplitude of 2 mm, the pressure transfer rate and dimensional stability coefficient of the bench test were determined to be 58.04% and 88.25%, respectively. These results, in excellent agreement with the simulation data, provided strong verification of the operational reliability of the test rig, suggesting that the test rig is capable of precisely simulating the actual compression process.