Crushable crop stalk discrete element simulation model construction method

Abstract

The invention discloses a breakable crop stalk discrete element simulation model construction method, which comprises the following steps: firstly, measuring the actual feature size of a crop stalk, and layering the internal structure of the crop stalk based on a microstructure; then performing an experiment to obtain intrinsic parameters and contact parameters of the stalks, constructing a multi-layer geometry three-dimensional model according to measurement characteristics, filling each layer of the geometry three-dimensional model with different types of particles, establishing an initial discrete element simulation model, introducing a contact model, and constructing an initial breakable discrete element simulation model; and finally, adjusting the mesoscopic parameters of the simulation model according to the actually measured data until the mesoscopic parameters are kept consistent. According to the method, the contact model is introduced, different structural layers are filled with particles with different material attributes, the biological characteristics and mechanical characteristics of multiple layers of real stalks from outside to inside are restored, and errors are smaller when mechanical behaviors are simulated.

Description

Technical Field

[0001] The invention belongs to the technical field of plant modeling, and in particular relates to a method for constructing a discrete element model of a crop stem. Background Art

[0002] In the prior art, when constructing discrete element models of crop stems, crops are often regarded as isotropic and simplified into a tissue composition. The established EDEM simulation model is made of particles with similar properties glued together. When the stems have a multi-layer structure with anisotropy, the use of particles with similar properties for simulation will lead to larger errors.

[0003] When analyzing the interaction between materials and mechanical parts, numerical simulation methods are often used to reduce test costs and visualize results. However, the stalk models constructed by existing research institutes cannot simulate a series of mechanical behaviors such as compression, stretching, and shearing of crop stalks during the harvesting process. In order to simulate these mechanical processes, it is necessary to establish a discrete element model of breakable crop stalks, but there is currently no good solution in the existing technology.

[0004] Therefore, a modeling method with higher accuracy is needed that can simulate the mechanical behavior of stems. Summary of the invention

[0005] In view of this, in order to solve the deficiencies of the above-mentioned prior art, the purpose of the present invention is to provide a method for constructing a discrete element simulation model of breakable crop stalks, and to establish a discrete element model of breakable crop stalks with multi-layer bonding of the stalks, which can accurately simulate the interaction between mechanical parts and the stalks with higher accuracy.

[0006] To achieve the above object, the technical solution adopted by the present invention is: A method for constructing a discrete element simulation model of a breakable crop stalk comprises the following steps: S1. Based on the measurement of the characteristic dimensions of the actual crop stalks, the initial dimensions of the geometric body approximating the stalks are obtained; S2. Observe the microstructure of crop stems and determine the internal structural components of crop stems; S3, measuring the intrinsic parameters and contact parameters of crop stems through experiments; S4, generating a multi-layer geometric three-dimensional model of the crop stem based on the initial size of the crop stem obtained in S1; S5. Fill each layer of the three-dimensional model of the geometric body with different types of particles according to the different structural components of the crop stems, and establish an initial discrete element simulation model; S6. Based on the initial discrete element simulation model, a contact model is introduced into the initial discrete element simulation model of the breakable crop stems to construct an initial discrete element simulation model; S7. Through experiments, the microscopic parameters of the initial discrete element simulation model of the breakable crop stems in S5 are adjusted so that the force and deformation curves calculated by the discrete element simulation model are consistent with the force and deformation curves actually measured in S3, thereby obtaining a discrete element simulation model of the breakable crop stems.

[0007] Furthermore, after the discrete element model is established, the contact model is introduced to form bonding bonds between the particles. The force between the particles is determined by the contact model to construct the initial discrete element simulation model of breakable crop stems.

[0008] Further, the S7 includes the following steps: S71, setting bonding parameters and particle structure; S72, simulating the initial discrete element simulation model of the breakable crop stems to obtain a force and deformation curve; S73, measuring the physical and mechanical properties of the characteristic size data of the crop stems measured in S1; S74, comparing the simulated force in S73 with the actual measured value in S73, if they are consistent, obtaining a discrete element simulation model for crushing crop stems; otherwise, returning to step S71 to adjust the bonding parameters and particle structure until the comparison is consistent.

[0009] The beneficial effects of the present invention are: 1. The modeling method proposed in the present invention is obtained based on the parameter analysis of the multi-layered plant stems. Different layers of structures are filled with particles with different material properties, and a contact model is selected. It is suitable for modeling plants with multi-layered structures and anisotropy inside the stems, and restores the biological and mechanical properties of the real stems from the outside to the inside. The modeling method of the present invention simulates a series of mechanical behaviors such as compression, stretching, and shearing of crop stems during the harvesting process, and the error is smaller during simulation; 2. The present invention further takes tea stems as the research object, selects the Hertz-Mindlin with Bonding contact model according to the difference in mechanical properties inside the stems, uses three kinds of particles with different material properties for particle filling, and establishes a three-layer bonding discrete element simulation model of the stems; through physical and mechanical tests and simulation tests, the simulation parameters of each internal layer are calibrated, and the accuracy of the model is verified through tensile, puncture and shear simulation tests. The results show that the absolute error between the bonding enhancement model and the actual shear test results is smaller, and it has higher accuracy; 3. The tensile, puncture and shear simulation tests were carried out using the bonding reinforced stem model to further verify the effectiveness and accuracy of the stem model. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0011] Figure 1 A flow chart of the model building method of the present invention; Figure 2 The Hertz–Mindlin with bonding contact model and the bonding bond schematic diagram of the present invention; Figure 3 A schematic diagram of sampling locations of modeling test materials in Example 2 of the present invention; Figure 4 This is a schematic diagram of the microstructure of tea stems in Example 2 of the present invention; Figure 5 This is a cross-sectional view of a traditional filling method of a tea stem model in Example 2 of the present invention; Figure 6 This is a filled cross-sectional view of the optimized tea stem model in Example 2 of the present invention; Figure 7 The optimized three-layer stem filling model in Example 2 of the present invention; Figure 8 It is a schematic diagram of the structure of the pith particles in the three-layer stem filling model; Fig. 9 The schematic diagram of the structure of phloem particles in the three-layer stem filling model; Fig.10 It is the overall diagram of the discrete element model data of the present invention; Fig.11 This is a shear simulation test effect diagram of the present invention; Fig.12 This is a drawing showing the effect of a tensile simulation test of the present invention; Fig.13 This is a diagram showing the effect of a puncture simulation test of the present invention. DETAILED DESCRIPTION

[0012] The following specific examples are given to further clearly, completely and in detail illustrate the technical solution of the present invention. This example is the best example based on the technical solution of the present invention, but the protection scope of the present invention is not limited to the following examples.

[0013] Example 1 A method for constructing a discrete element simulation model of a breakable crop stalk comprises the following steps: S1. Based on the measurement of the characteristic dimensions of the actual crop stalks, the initial dimensions of the geometric body approximating the stalks are obtained; S2. Observe the microstructure of crop stems and determine the internal structural components of crop stems; S3, measuring the intrinsic parameters and contact parameters of crop stems through experiments; S4, generating a geometric three-dimensional model of the crop stalk based on the initial size of the crop stalk obtained in S1; S5. Filling the geometric three-dimensional model with different kinds of particles according to different internal structural components of crop stems, and establishing an initial discrete element simulation model of the crop stems; S6. Based on the initial discrete element simulation model, a contact model is introduced into the initial discrete element simulation model of the breakable crop stems to construct an initial discrete element simulation model; S7. Through experiments, the microscopic parameters of the initial discrete element simulation model of the breakable crop stems in S5 are adjusted so that the force and deformation curves calculated by the discrete element simulation model are consistent with the force and deformation curves actually measured in S3, thereby obtaining a discrete element simulation model of the breakable crop stems.

[0014] Compared with the traditional discrete element modeling method of stems, the present invention constructs a multi-layer bonding discrete element simulation model of stems by introducing a contact model and using particles with different material properties to fill different layers of stems. Particle models with different characteristics are stacked to restore the biological and mechanical properties of real stems from the outside to the inside.

[0015] Furthermore, after the discrete element model is established, the contact model is introduced to form bonding bonds between the particles. The force between the particles is determined by the contact model to construct the initial discrete element simulation model of breakable crop stems.

[0016] Furthermore, the intrinsic parameters and contact parameters include density ρ, Poisson's ratio μ, shear modulus G, restitution coefficient and friction coefficient, etc.

[0017] Further, the S7 includes the following steps: S71, setting bonding parameters and particle structure; S72, simulating the initial discrete element simulation model of the breakable crop stems to obtain a force and deformation curve; S73, measuring the physical and mechanical properties of the characteristic size data of the crop stems measured in S1; S74, comparing the simulated force in S73 with the actual measured value in S73, if they are consistent, obtaining a discrete element simulation model for crushing crop stems; otherwise, returning to step S71 to adjust the bonding parameters and particle structure until the comparison is consistent.

[0018] Furthermore, the contact mechanics model is introduced to accurately calculate the forces and bonding characteristics between discrete element models. The forces and moments on the model particles are calculated using the following formulas: Where: A—contact area, mm 2 ; and —Normal stiffness and shear stiffness per unit area, N·mm -3 ; δ t —Time step, s; v n and v t —Normal velocity and tangential velocity of the particle, m·s -1 ;ω n and ω t —Normal and tangential angular velocities of the particle, rad·s -1 ; δM n and δM t —Normal and tangential moments of the bond, N·m; δF n and δF t —Normal cohesion and tangential cohesion of the bond, N; J—moment of inertia, mm 4 ; The tangential and normal forces between the particles act on the bond. Due to the movement of the particles (translation or rotation), when the force exceeds a predetermined value, the bond breaks, as shown in the following formula:

[0019] Example 2 The present invention proposes a method for constructing a discrete element simulation model of a breakable crop stem. In this embodiment, tea stems are taken as an example, and a simulation test is performed on them according to the method in Example 1 to verify the effectiveness of the method. The specific steps are as follows: Step 1: Select the third section of the tea stem for research. The sampling position is as follows: Figure 3 As shown, based on the measurement of the characteristic dimensions of the actual tea stems, the initial dimensions of the geometric body approximating the stems are obtained.

[0020] Step 2: Observe the microstructure of the tea stems and determine the internal structure of the tea stems, namely the four main parts of epidermis, phloem, xylem and pith. However, the epidermis accounts for a small proportion of the internal structure and has low structural strength. The cortex and phloem are difficult to separate in reality and have similar physical properties, so they are combined into a layer for simulation, thus consisting of three parts: phloem, xylem and pith.

[0021] Step 3, measuring the intrinsic parameters and contact parameters of the tea stems through experiments, including density ρ, Poisson's ratio μ, shear modulus G, restitution coefficient and friction coefficient, etc.; In this embodiment, the density ρ of the sample is measured by the drainage method, and the Poisson's ratio has no significant effect on the simulation test, so the Poisson's ratio μ is set to 0.34 with reference to similar agricultural material stems. The elastic modulus E of the plant stem is measured by a tensile test, and its shear modulus G is calculated.

[0022] Furthermore, in order to accurately establish the discrete element model of the famous tea stems and provide reference values ​​for the calibration of bonding parameters, mechanical tests (tensile, puncture and shear) were carried out on the stems. The test results are shown in Table 1. Table 1 Biomechanical properties of tea stems

[0023] Step 4: Based on the average length and average diameter of the tea stems obtained in step 1, a geometric three-dimensional model of the tea stems is generated in solidworks three-dimensional design software, and the constructed three-dimensional model is imported into EDEM.

[0024] Step 5: Fill the stem model with three different types of particles according to the different internal structures of tea stems to establish an initial discrete element simulation model of tea stems. Figure 3 The schematic diagram of the microstructure of the tea stem shown in the figure shows that the tea stem mainly consists of three parts: phloem, xylem and pith. Therefore, the cylindrical space is divided into three layers according to the proportion of each part, and three different types of particles are filled in each layer respectively.

[0025] Since the texture of the pith is uneven and the internal spongy material is relatively soft, but it accounts for the largest proportion, large-diameter particles are used to occupy space and small particles increase the filling density, making the mechanical properties of the particle group closer to the actual situation.

[0026] Furthermore, the filling particles of the pith are loose particles. When filling the pith, a particle unit composed of 5 particles is used for filling. Large particles occupy the space position, and small particles increase the filling density, which can reduce the porosity of the discrete element model, enhance the bonding effect between particles, and make the mechanical properties of the particle group closer to the actual situation. The four groups of pith particle units are arranged in a rotating array to obtain the discrete element model of the pith, which is more standardized and has better simulation effect.

[0027] Alternatively, in this embodiment, the particle group is as follows Figure 8 As shown, it is composed of a small particle and four large particles stacked on top of each other, and the particle filling method is that the particles are arranged in a circular shape.

[0028] Furthermore, since the mechanical properties of the phloem and xylem of tea stems are significantly different from those of the pith, and the phloem and xylem account for a large proportion and have high structural strength, the discrete element particles that constitute the phloem and xylem are optimized. The phloem and xylem are simulated using composite particles of multi-sphere filling particles. However, the physical properties of the two, including critical law / tangential force, critical law / tangential stiffness, and contact radius, are different. The cross-section of the discrete element model of the tea stem before and after optimization is shown in the figure below. Figure 6 and Figure 7 shown.

[0029] Optionally, in this embodiment, the multi-spherical filling particles are composed of a large particle and a plurality of small particles stacked around it. Fig. 9 The new type of particles shown are formed by stacking multiple particles of different sizes to replace the traditional spherical particles, so that more bonding bonds are formed at the contact points of the particles, thereby improving the accuracy of tea stem modeling.

[0030] Step 6: Based on the initial discrete element simulation model of the tea stems obtained in step 5, a Hertz-Mindlin with bonding contact model is introduced into the model, which can form bonding bonds between particles, to construct an initial discrete element simulation model of breakable tea stems.

[0031] The bonding between particles is Figure 4 As shown. b is the resultant force of particle A on particle B; M n and M s are the normal torque and tangential torque respectively; n and t are the normal component and tangential component respectively; L b is the overlap between particle A and particle B; R b is the bonding radius; R is the particle radius; R contact is the contact radius. In order to ensure sufficient connection between particles, R contact It should be set to be 20%-30% larger than R. The force and moment on the particle are calculated by the following formula: The tangential and normal forces between the particles act on the bond. Due to the movement of the particles (translation or rotation), when the force exceeds a predetermined value, the bond breaks, as shown in the following formula:

[0032] Step 7: Adjust the microscopic parameters of the initial breakable tea stem discrete element simulation model obtained based on step 6 through tests such as the steepest climbing test and the Box-Behnken response surface test, so that the force and deformation curves calculated using the discrete element simulation model are consistent with the force and deformation curves actually measured, thereby obtaining a breakable tea stem discrete element simulation model.

[0033] According to the discrete element simulation model of tea stems established above, the following Fig.11 , Fig.12 and Fig.13 Shear, tensile and puncture simulation tests shown: In the shear simulation test of the tea stem discrete element model, the simulation test curve is compared with the physical test curve, such as Fig.10 As shown in the figure, it is found that the change trends of the two are basically the same. The established tea stem simulation model can reflect the change law of force over time during the shear process. The maximum shear force F of the bonding enhancement model max The maximum tensile force of the model is 3.83N, which is 2.41% relative to the actual test. The maximum puncture force of the model is 1.81N, which is 2.76% different from the actual test. This shows that the model can simulate the shearing, stretching and puncture process of tea stems.

[0034] In summary, compared with the traditional discrete element modeling method of stems, the present invention introduces the Hertz-Mindlinwith bonding contact model and uses particles with different material properties to fill different structural layers to construct a multi-layer bonding discrete element simulation model of stems. The constructed simulation model has been verified by tensile and puncture tests, proving the accuracy of the model and providing a reliable simulation model and parameter calibration method in the analysis of the interaction between mechanical parts and stems.

[0035] The above shows and describes the main features, basic principles and advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited by the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements according to actual conditions, and these changes and improvements fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for constructing a discrete element simulation model of breakable crop stems, characterized in that: The following steps are involved: S1. Based on the measurement of the characteristic dimensions of the actual crop stalks, the initial dimensions of the geometric body approximating the stalks are obtained; S2. Observe the microstructure of crop stems and determine the internal structural components of crop stems; S3, measuring the intrinsic parameters and contact parameters of crop stems through experiments; S4, generating a multi-layer geometric three-dimensional model of the crop stem based on the initial size of the crop stem obtained in S1; S5. Fill each layer of the three-dimensional model of the geometric body with different types of particles according to the different structural components of the crop stems, and establish an initial discrete element simulation model; S6. Based on the initial discrete element simulation model, a contact model is introduced into the initial discrete element simulation model of the breakable crop stems to construct an initial discrete element simulation model; S7. Through experiments, the microscopic parameters of the initial discrete element simulation model of breakable crop stems are adjusted so that the force and deformation curves calculated by the discrete element simulation model are consistent with the force and deformation curves actually measured in S3, thereby obtaining a discrete element simulation model of breakable crop stems.

2. The method for constructing a discrete element simulation model of a breakable crop stalk according to claim 1, characterized in that: In S5, when the crop is tea, the internal structure of the tea stem includes three parts: phloem, xylem and pith, and different parts of the stem model are filled with three different types of particles.

3. The method for constructing a discrete element simulation model of a breakable crop stalk according to claim 2, characterized in that: The filling particles of the pith are loose particles, and the particle filling method is that the particle clusters are arranged in a circle.

4. The method for constructing a discrete element simulation model of a breakable crop stalk according to claim 3, characterized in that: The particle cluster is formed by stacking one small particle and four large particles.

5. The method for constructing a discrete element simulation model of a breakable crop stalk according to claim 2, characterized in that: The filling particles in the xylem and phloem are both multi-spherical filling particles, which are composed of multiple particles of different sizes stacked on top of each other.

6. The method for constructing a discrete element simulation model of a breakable crop stalk according to claim 5, characterized in that: The multi-spherical filling particles are composed of a large particle and a plurality of small particles stacked around it.

7. The method for constructing a discrete element simulation model of a breakable crop stalk according to claim 5, characterized in that: The contact parameters of the xylem filling particles and the phloem filling particles are set differently, wherein the contact parameters include critical normal / tangential force, critical normal / tangential stiffness, and contact radius.

8. The method for constructing a discrete element simulation model of a breakable crop stalk according to claim 1, characterized in that: After the discrete element model in S6 is established, the contact model is introduced to form bonding bonds between the particles. The force between the particles is determined by the contact model to construct an initial discrete element simulation model of breakable crop stems.

9. The method for constructing a discrete element simulation model of a breakable crop stalk according to claim 1, characterized in that: The intrinsic parameters and contact parameters include density ρ, Poisson's ratio μ, shear modulus G, restitution coefficient and friction coefficient.

10. The method for constructing a discrete element simulation model of a breakable crop stalk according to claim 1, characterized in that: The S7 includes the following steps: S71, setting bonding parameters and particle structure; S72, simulating the initial discrete element simulation model of the breakable crop stems to obtain a force and deformation curve; S73, measuring the physical and mechanical properties of the characteristic size data of the crop stems measured in S1; S74, comparing the simulated force in S73 with the actual measured value in S73, if they are consistent, obtaining a discrete element simulation model for crushing crop stems; otherwise, returning to step S71 to adjust the bonding parameters and particle structure until the comparison is consistent.

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