CN113297749B - Corn threshing process simulation method based on elastic-plastic connection mechanical model - Google Patents
Corn threshing process simulation method based on elastic-plastic connection mechanical model Download PDFInfo
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Abstract
The invention belongs to the technical field of connection mechanical models, and particularly relates to a corn threshing process simulation method based on an elastic-plastic connection mechanical model, which adopts a multi-time continuous loading and unloading mode, external forces acting on grains are loaded along the X, Y, Z axial direction respectively until the grains fall off from fruit stalks, and different connection rigidity coefficients are set in the loading and unloading (reloading) processes to represent the energy dissipation of the grain and the fruit stalks in the threshing process. The invention adopts a discrete element method to research a corn threshing process, establishes an analysis method and simulation software of the corn threshing process, provides a corn ear modeling method based on a particle aggregation method, develops a connection mechanical model based on elastoplasticity, and preliminarily realizes simulation analysis of the corn threshing process.
Description
Technical Field
The invention relates to the technical field of connection mechanical models, in particular to a corn threshing process simulation method based on an elastic-plastic connection mechanical model.
Background
Corn is the first major food crop in China, the annual output exceeds 2.6 hundred million tons, and as a key link of corn production, the influence of the corn threshing process is important. Therefore, the deep research on the corn threshing process and mechanism has great significance in the aspects of improving the corn production quality, reducing the labor cost, accelerating the corn harvesting mechanization process, realizing the corn full-value harvesting and the like.
Due to the complexity of the geometric structure and the mechanical behavior of the corn ears, the performance analysis or the optimized design of the threshing device is mostly carried out at home and abroad by adopting an empirical method, a test method, a statistical analysis method or a continuous medium mechanical analysis method so far. The test method and the statistical analysis method rely on a large number of tests, the production period is long, and the obtained result is generally not of universal significance; the continuous medium mechanics analysis method can only analyze the stress and motion of a single corn ear, corn cob or corn kernel, and has a great difference from the contact action and motion process of the corn ear, corn cob and corn kernel group and the threshing part in the actual corn threshing process.
Disclosure of Invention
Technical problem to be solved
Aiming at the defects of the prior art, the invention provides a corn threshing process simulation method based on an elastic-plastic connection mechanical model, and solves the problems that the connection mechanical model in the existing corn ear modeling method based on a particle aggregation method is simpler, and defects exist in a test mechanism and a model algorithm, so that the precision of an analysis method and the practical application value of simulation software are influenced.
The second technical proposal.
The invention specifically adopts the following technical scheme for realizing the purpose:
a corn threshing process simulation method based on an elastic-plastic connection mechanical model comprises a linear damping mechanical model and an elastic-plastic mechanical model part;
the linear damping mechanics model specifically comprises:
the mechanical properties of the corn kernels and the corn cobs are tested and analyzed, a local coordinate system between the kernels and the corn cobs is shown in figure 1, external force acting on the kernels is loaded to fall off along the X, Y, Z axial direction respectively, stress and displacement curves of the kernels and the corn cobs are approximately linear, a linear damping mechanical model is adopted to calculate connecting force between the corn kernels and the corn cobs along the X, Y, Z axis, and the concrete expression is as follows:
in the formula
The connecting force between the seed grains and the core at the time t along the X, Y, Z axial direction,
the elastic force is connected along the X, Y, Z axial direction;
in the formula
The elastic force is connected along the axis direction of X, Y, Z in the last time step; k is a radical of X(Y,Z) Is the stiffness coefficient of the connection along the X, Y, Z axial direction (OP section approximate straight line slope); Δ u X(Y,Z) The relative displacement increment between the seeds and the core at the time t along the X, Y, Z axial direction;
the damping force is connected along the X, Y, Z axial direction;
in the formula c X(Y,Z) The damping coefficient is connected along the X, Y, Z axial direction; Δ t is the calculation time step.
The model is adopted to preliminarily realize the calculation of the connecting force between grains and corncobs and the simulation analysis of the corn threshing process, but the model is simpler, and some problems in the aspects of test mechanism and model algorithm need to be deeply researched: on one hand, the test mechanism that the seeds directly fall off by loading has larger difference from the falling off of the seeds after repeated beating or squeezing and rubbing by a threshing part in the actual threshing process; on the other hand, the feasibility of representing the energy dissipation of the seed and the stalk in the threshing process through a damping model and a method for determining the damping coefficient are difficult.
The elastoplasticity mechanics model specifically comprises:
aiming at the problems, the invention improves the mechanical property test method of the seed and the fruit stem, simulates the actual threshing process, adopts a multiple continuous loading and unloading mode, and finally loads until the fruit stem is broken, thereby obtaining the relation curve of the connecting force and the displacement (see figure 2).
Analysis shows that the curve is approximately linear when loading, the slope of the curve is increased when unloading, a semi-closed area is formed by the curve and the loading curve, and energy dissipation exists, so that the elastic-plastic change of the seed and the fruit stem occurs in the loading and unloading processes, and the connection action is gradually attenuated.
Therefore, the invention improves the algorithms of the elastic force in the mechanical model formulas (1) and (2), sets different connection rigidity coefficients in the loading and unloading processes to represent the energy dissipation of the seed and the fruit stem in the threshing process, and the specific expression is as follows:
in the formula k X(Y,Z)1 、k X(Y,Z)2 Respectively the loading and unloading stiffness coefficients connected along the X, Y, Z axial direction,
is the relative displacement between the kernel and the core along the X, Y, Z axial direction at the time t,
for the relative residual displacement at time t, updating needs to be performed at each time step, which is specifically represented as:
in order to counteract the low-amplitude oscillation of the loading or unloading process, a damping force related to the speed is added to the model, and the damping force is expressed as follows:
where c is the damping coefficient of the connection,
is the relative speed between the kernel and the core along the X, Y, Z axis direction at the time t.
(III) advantageous effects
Compared with the prior art, the invention provides a corn threshing process simulation method based on an elastic-plastic connection mechanical model, which has the following beneficial effects:
the invention adopts a discrete element method to research a corn threshing process, establishes a corn threshing process analysis method and simulation software, provides a corn ear modeling method based on a particle aggregation method, develops a corn threshing process simulation method based on an elastic-plastic connection mechanical model, and preliminarily realizes the simulation analysis of the corn threshing process.
Drawings
FIG. 1 is a schematic view of a local coordinate system between a corn kernel and a corn cob according to the present invention;
FIG. 2 is a schematic view of a connection force versus displacement curve according to the present invention;
FIG. 3 is a schematic diagram of a combined experimental study and simulation analysis module of the present invention;
FIG. 4 is a photograph of the grain and corn cob in different force application directions during the test of the present invention;
FIG. 5 is a schematic view showing the mechanical properties of the connection between the seeds and the stalks in different directions of application of force when the water content is 28.23%;
FIG. 6 is a schematic view showing the mechanical properties of the connection between the seeds and the stalks in different directions of application of force when the water content is 18.44%;
FIG. 7 is a schematic illustration of a dynamic display of an axial shear test simulation analysis of the present invention;
FIG. 8 is a schematic diagram showing the comparison between the simulated values and the test values of the mechanical properties of the fruit stalks under different force application directions and loading speeds when the water content is 28.23%;
FIG. 9 is a schematic diagram showing a comparison structure between simulated values and test values of mechanical properties of fruit stalks under different force application directions and loading speeds when the water content is 18.44%;
FIG. 10 is a schematic diagram showing the effect of the loading speed on the damping coefficient under different water contents according to the present invention;
FIG. 11 is a schematic illustration of the effect of damping coefficient on low amplitude oscillation during loading and unloading in accordance with the present invention;
FIG. 12 is a schematic view of a three-dimensional discrete element analysis model of the corn thresher according to the present invention;
FIG. 13 is a schematic view of an analysis model of an ear of corn according to the present invention;
FIG. 14 is a schematic view of a dynamic simulation display of the corn threshing process of the present invention;
FIG. 15 is a schematic diagram of simulated threshing performance and crushing rate at different drum speeds and moisture contents in accordance with the present invention;
FIG. 16 is a schematic view showing the variation curve of the kernel shedding amount and the simulation time at different drum rotation speeds and water contents according to the present invention;
FIG. 17 is a schematic view of a variation curve of total pressure and simulation time of seeds under different drum rotation speeds and water contents.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Examples
As shown in fig. 1 to 17, the method for simulating a corn threshing process based on an elastic-plastic connection mechanical model according to an embodiment of the present invention employs a multiple continuous loading and unloading manner, and external forces acting on kernels are loaded along the X, Y, Z axial direction respectively until the kernels fall off from fruit stalks, and is characterized in that: different connection rigidity coefficients are set in the loading and unloading processes to represent the energy dissipation of the seed and the stalk in the threshing process, and the specific expression is as follows:
in the formula k X(Y,Z)1 、k X(Y,Z)2 Respectively the loading and unloading stiffness coefficients connected along the X, Y, Z axial direction,
is the relative displacement between the kernel and the core along the X, Y, Z axial direction at the time t,
for the relative residual displacement at time t, updating needs to be performed at each time step, which is specifically represented as:
in some embodiments, to counteract low amplitude oscillations of the loading or unloading process, a velocity-dependent damping force is added to the model, which is embodied as:
where c is the damping coefficient of the connection,
is the relative speed between the kernel and the core along the X, Y, Z axis direction at the time t.
The model verification method comprises the following steps:
the accuracy of the proposed connection mechanics model is verified by adopting a method of combining experimental research with simulation analysis, and the influence factors, the change rule, the determination method and the like of parameters in the research model are shown in figure 3.
The method comprises the steps of researching mechanical behaviors and influence factors of the seed and fruit stalks by a test method, obtaining macroscopic test data of a loading rigidity coefficient k1 and an unloading rigidity coefficient k2, using the macroscopic test data as simulation parameter input values, carrying out simulation analysis on a test process by adopting agriDEM software, obtaining microscopic simulation results of the k1, the k2 and a damping coefficient c, researching the influence factors and change rules of the simulation results, establishing association between microscopic simulation parameters and macroscopic test data by comparing the microscopic simulation parameters and the test data, and researching a determination method of the simulation parameter input values.
Connection model test study:
1. test set-up
Analysis shows that the mechanical properties of the corn kernel stalks are greatly influenced by a force application mode and the moisture content of the kernels in the test, so that the mechanical behavior and the mechanical properties of the connection between the kernels and the corn cobs are tested and researched by taking a local pioneer No. 8 corn ear as an example in a quasi-static test mode.
Selecting the middle section of the corn ear as a research object, and adjusting the water content of the corn ear in a natural air drying mode. When the threshing process is analyzed by the discrete element method, the contact action such as extrusion, friction and the like between the corn kernels on the corn ears is calculated by adopting a contact model, and the connection action between the corn kernels and the corn cobs is calculated by adopting a connection model, so that the corn ears are made into single-kernel samples in the graph 1 without considering the influence of other kernels.
The testing equipment is a WDW-20 type electronic universal testing machine, the loading and unloading speed is 0.1mm/s, the force application direction during the test comprises radial (Z-axis) stretching and compression, axial (Y-axis) shearing and tangential (X-axis) shearing (see figure 4), the loading and unloading are carried out twice continuously, then the loading is carried out until grains fall off, the relation curve of the connecting force and the displacement is shown in figure 3, the curve is fitted to obtain the mechanical characteristics of the connection between the grains and the corncobs, which are shown in figure 5 (t.s.: tangential shearing; l.s.: axial shearing; r.c.: radial compression; r.t.: radial stretching), the relation curve comprises k1, k2 and e, wherein k1 is the slope after the loading curve is fitted, k2 is the average value of the slope after the 2-time unloading and reloading curve is fitted, e is the recovery coefficient of the loading and unloading process, and is determined by k1 and k2, and is specifically represented as:
2. analysis of test results
The invention researches the influence of the force application direction and the moisture content of grains on the mechanical properties of the fruit handle connection such as k1, k2, e and the like, and the test results are shown in fig. 5 and 6.
From the figures 5 and 6, the mechanical properties of the seed and the fruit stalk have the characteristic of anisotropy, and under 2 water contents, k1 is sequentially increased in the tangential shearing direction, the axial shearing direction, the radial compression direction and the radial stretching direction, and is relatively close to each other in the tangential shearing direction and the axial shearing direction and is obviously smaller than the radial stretching direction and the radial compression direction; and in different force application directions, the change rule of k2 is consistent with that of k 1.
As can be seen from fig. 5, when the seed is loaded and then unloaded, the strength of the stalk of the seed gradually decays, so that k2 is increased to about 2 times of k1 compared with k 1. At the moment, the water content of the grains is higher, the toughness of the stems of the grains is stronger, the connection recovery e is larger and ranges from 0.68 to 0.71, and small plastic change and residual displacement are generated during unloading. As can be seen from FIG. 6, as the water content decreases, k2 and k1 in different force application directions and the difference between the two increase, and the increment increases in sequence according to the tangential shear direction, the axial shear direction, the radial compression direction and the radial stretching direction, and k2 is 3-4 times of k 1. The reduction of the water content leads to the reduction of the toughness and the strong brittleness of the seed and the fruit stem, the connection recovery e is reduced and is between 0.51 and 0.58, and the plastic change and the residual displacement are increased during the unloading. Therefore, along with the reduction of the moisture content of the grains, the strength attenuation degree of the stems of the grains is gradually increased in the loading and unloading processes, and the grains are easy to fall off.
Connection model simulation analysis
1. Simulation setup
And (3) performing simulation analysis on the experimental study of the mechanical behavior of the seed and fruit stalks by adopting agriDEM software, and setting a simulation analysis model, parameters and a method according to the experimental process. And (3) generating a corn ear analysis model according to the sample in the figure 1, and only analyzing the stress and displacement between grains and corncobs without considering the whole stress of the corn ears during simulation. And loading and unloading the boundary once, wherein the loading distance is 1mm, and the total stress and displacement of the boundary at each time step are stored. Wherein, the connection model simulation parameters are determined according to the test data, and the contact model simulation parameters can be obtained by calibration. In order to consider the influence of different loading speeds on the simulation result, different loading speeds of 1mm/s, 10mm/s and 100mm/s are added.
Taking an axial shear test as an example, the dynamic display of the simulation analysis is shown in fig. 7, and the color of the kernel is displayed according to the axial connecting force between the kernel and the corncob. It can be seen from the figure that the boundary contacts with the kernel to start the loading process, the axial connecting force between the kernel and the corncob is gradually increased, the unloading process is started when the loading distance is 1mm, the connecting force is gradually reduced, the residual displacement exists between the kernel and the corncob after the unloading process is finished, and the phenomena are relatively close to the actual test process.
2. Method for determining simulation result
And fitting the stress and displacement scatter diagrams in the boundary loading and unloading processes respectively, wherein the slopes of the fitted straight lines are simulation results of k1 and k 2.
When the loading speed is higher in the simulation process, the grains have low-amplitude oscillation during loading or unloading, the oscillation can be gradually attenuated or disappear along with the increase of the damping coefficient, the dispersion degree of data is reduced, and the influence on the simulation result is caused by overlarge values of the damping coefficient. To this end, a method for determining a damping coefficient is proposed: fitting the scattered data of each time step stress and displacement, and taking the 2 norm of the difference value of the original data and the fitting data of each point as the evaluation index of the data dispersion degree, wherein the specific expression is as follows:
where y and z are the original and fitted values of the scatter data, respectively. The damping coefficient is increased by 0.1 when simulating, and when N is c 1 or less or
When the damping coefficient is larger than the set damping coefficient, c is the damping coefficient under the simulation condition.
3. Analysis of simulation results
The influence of the force application direction, the grain water content and the loading speed on simulation results such as k1, k2, e and c is researched, and the pair of the simulation results and test data is shown in figures 8 and 9 (e.r.: test value; s.r.: simulated value).
(1) Influence of direction of application of force
As can be seen from FIG. 8, when the loading speed is 0.1mm/s, the simulation results of k1 and k2 in different force application directions are the same as the variation trend of the test data, and are both smaller than the test data. The error between the simulation result of k1 and k2 and the test data is greatly influenced by the stiffness coefficient, is relatively small in tangential shear and axial shear and is within 8%, the error is gradually increased along with the increase of the stiffness coefficient, and the error is maximum in radial stretching and is respectively 15.27% and 27.30%.
The result shows that when the rigidity coefficient is larger, the error between the simulation result and the test data is larger. Analysis shows that the grain and the boundary generate overlapping amount due to the connecting force between the grain and the corn cob during simulation, the overlapping amount is increased along with the increase of the connecting rigidity coefficient, so that the simulation results of k1 and k2 are smaller than the input values during simulation, and errors exist between the simulation results and test data. Therefore, in order to reduce errors, the input values of the connection stiffness coefficients k1 and k2 can be properly increased during simulation, but the overlapping amount between the kernel and the boundary can be further increased, so that the simulation process is influenced.
(2) Influence of Water cut
The simulation result when the water content is 18.44% is shown in fig. 9, when the loading speed is 0.1mm/s, the simulation results of k1 and k2 have the same change trend with the test data, and are basically consistent with the change rule when the water content is 28.23%, the error between the simulation results and the test data increases in sequence according to the tangential shear, axial shear, radial compression and radial stretching directions, and the error is the largest during radial stretching and is respectively 15.60% and 41.19%. In order to consider the influence of the water content, the contact rigidity coefficient between the grains and the boundary is increased during simulation. By comparison, the error between the k1 simulation result and the test data during radial stretching is substantially consistent with that when the water content is 28.23%, and the error does not continue to expand along with the increase of the stiffness coefficient.
The result shows that the contact rigidity coefficient between the grain and the boundary can influence the simulation result. Analysis shows that the contact rigidity coefficient between the grains and the boundary is increased, the contact acting force between the grains and the boundary can be increased, the superposition between the grains and the boundary is reduced, and further the error between a simulation result and test data is reduced. Therefore, in order to reduce errors, the input value of the contact stiffness coefficient between the grain and the boundary can be properly increased during simulation, but the calculation time is increased when the simulation time step needs to be reduced.
(3) Influence of the Loading speed
As can be seen from FIG. 10, the influence of the loading speed on the simulation results of k1 and k2 is small, and when the loading speed is increased to 100mm/s, the simulation results are slightly reduced compared with 0.1 mm/s; the loading speed has a great influence on the damping coefficient c, when the loading speed is lower than 10mm/s, the damping coefficient approaches to 0 and increases along with the increase of the loading speed, and when the loading speed is 100mm/s, the damping coefficient is 1.1Ns/m. Taking tangential shear as an example, when the water content is 28.23% and the loading speed is 100m/s, the influence of the damping coefficient on low-amplitude oscillation in the loading and unloading process is as shown in fig. 11, the oscillation is gradually attenuated along with the increase of the damping coefficient, but the oscillation still exists in the initial stage of loading and unloading, and the oscillation gradually disappears when the loading and unloading distance exceeds 0.2 mm.
Simulation analysis of the corn threshing process:
1. simulation setup
In order to further verify the accuracy of the model and the parameters thereof and the practical application value thereof, taking a local YT-3 type roller type corn thresher and pioneer No. 8 corn ears as examples, the corn threshing process is analyzed by agriDEM software, and the influence of the rotating speed of a roller of the thresher and the water content of the corn ears on the threshing performance such as threshing performance, crushing rate and the like is researched from the macroscopic and microscopic angles. The corn thresher and the corn ear analysis model are respectively shown in fig. 12 and fig. 13.
For example, the moisture content is 28.23%, the drum rotation speed is 187rpm, the dynamic simulation of the corn threshing process is shown in fig. 14, and the color of the corn threshing process is displayed according to the seed pressure. As can be seen from the figure: the corn ears are generated and enter a threshing area from the beginning, the corn ears and the threshing part are contacted, grains gradually fall off, and the whole process is close to an actual threshing bench test after the last threshing process is finished. When the threshing element is contacted with corn ears, the pressure on the corn ears is large, the corn ears are easy to crush, part of the corn ears which are not completely threshed are discharged from the corn ear discharge hole during threshing, and part of the corn ears which are not completely threshed are remained in a threshing area after the threshing is finished, so that the threshing difficulty is large, the threshing rate is low, and the threshing performance is basically consistent with the actual threshing performance of the corn with high water content.
2. Analysis of simulation results
(1) Analysis of macroscopic Properties
Firstly, the corn ear threshing rate and the kernel breakage rate are analyzed from a macroscopic view, the breakage rate is represented by the pressure borne by the kernel and a set threshold value, and the simulation result is shown in fig. 15. The comparison shows that the threshing performance difference of the corn ears with 2 water contents is obvious, when the rotating speed of a roller is increased from 187.5rpm to 252.1rpm, the threshing rate of the corn ears with the water content of 28.23 percent is increased from 84.15 percent to 91.07 percent, and the crushing rate is increased from 8.13 percent to 12.99 percent; the corn ear threshing rate of 18.44 percent of water content is increased from 96.65 percent to 98.08 percent, and the breakage rate is increased from 2.86 percent to 3.48 percent.
Analysis shows that when the water content is 28.23%, the connection strength of the seed and the stalk is high, the crushing strength of the seed is low, the seed is not easy to fall off, at the moment, the threshing performance is low, the crushing rate is high, the impact on the corn ear is enhanced along with the increase of the rotating speed of the roller, and the threshing performance and the crushing rate are both improved; when the water content is 18.44%, the connection strength of the seed and the fruit stalk is reduced, the crushing strength is improved, the influence of the rotating speed of the roller on the threshing performance is gradually reduced, the simulation result is basically consistent with the analysis in the mechanical characteristic test, and the feasibility and the effectiveness of the proposed mechanical model and the parameters thereof are verified.
(2) Microscopic Performance analysis
From the viewpoint of microscomia, the kernel shedding amount and the total pressure borne by the kernels in the simulation time are analyzed, data are recorded once at intervals of 0.003s, the data are 1000 data points in total, and the simulation results are respectively shown in fig. 16 and 17.
As can be seen from fig. 16, the grains start to fall off at 0.2s, the corn threshing number difference is small under different conditions from 0.2s to 0.6s, the threshing number difference increases with the lapse of simulation time, and after 2.5s, the threshing number basically reaches the limit, the corn ear threshing rate difference is significant at different water contents, and the influence of the rotating speed of the drum is large; as can be seen from fig. 17, the seed pressure gradually increases from 0.1s to about 0.6s and reaches a peak value, the threshing number gradually increases along with the lapse of the simulation time, the threshing area is separated through the sieve holes, the seed pressure gradually attenuates, the seed pressure difference of the ears and the corn with different water contents is obvious, and the feasibility and the effectiveness of the model and the parameters are further verified from the microscopic view point by the simulation result.
Finally, it should be noted that: although the present invention has been described in detail with reference to the foregoing embodiments, it will be apparent to those skilled in the art that changes may be made in the embodiments and/or equivalents thereof without departing from the spirit and scope of the invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (2)
1. A corn threshing process simulation method based on an elastic-plastic connection mechanical model adopts a multi-time continuous loading and unloading mode, and external force acting on grains is loaded to the grains to fall off from fruit stalks along the X, Y, Z axial direction respectively, and is characterized in that: different connection rigidity coefficients are set in the loading and unloading processes to represent the energy dissipation of the seed and the stalk in the threshing process, and the specific expression is as follows:
in the formula k X(Y,Z)1 、k X(Y,Z)2 Respectively the loading and unloading stiffness coefficients connected along the X, Y, Z axial direction,
is the relative displacement between the kernel and the core along the X, Y, Z axial direction at the time t,
for the relative residual displacement at time t, updating needs to be performed at each time step, which is specifically represented as:
2. the corn threshing process simulation method based on the elastic-plastic connection mechanical model, according to claim 1, is characterized in that: in order to counteract the low-amplitude oscillation of the loading or unloading process, a damping force related to the speed is added to the model, and the damping force is expressed as follows:
where c is the damping coefficient of the connection,
is the relative speed between the kernel and the core along the X, Y, Z axis direction at the time t.
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