| As the largest food crop in China,maize plays a vital role in ensuring national food security.As a key link in the whole mechanization of maize,mechanized harvesting is of great significance to ensure the production and development of maize.The functions of direct maize grain harvesters include ear-picking,peeling,threshing,and cleaning.Higher efficiency and lower labor requirements can be achieved with direct harvesting than with ear-picking,which is becoming the main development direction of maize harvesters in China.The air-screen cleaning device can realize the blowing,migrating,and screening of maize mixture to separate grains and impurities.As the "digestive system" of maize grain combine harvester,the operating performance of the cleaning device directly affects the loss percentage of maize grain and impurity percentage of maize grain.As the working width of the combine harvester and the yield of crops per unit continues to increase year by year,the feeding mass continues to be increased,so the accumulation of maize mixture in the front of the screen is more and more serious during screening.The traditional planar reciprocating vibrating screen is a rigid integrated screen,and the movement mode is single,which causes the amplitude and frequency of each part of the screen to be the same when the screen moves.Under the large feeding mass of the maize mixture,the maize mixture could not be migrated backward rapidly in the front of the planar reciprocating vibrating screen,making the accumulation of the maize mixture more serious,resulting in a reduction in screening efficiency and the operating efficiency of the maize grain harvester.This research was based on the principle of bionics,and earthworm was used as bionic prototype,the motion characteristics of the earthworm and the operation principle of cleaning screen were integrated.The motion posture of the bionic screen was determined by the difference of concave position of the wavy geometry of the lower surface during the earthworm moving,and the conversion between different postures of the bionic screen was realized by designing the cam drive mechanism.The design of the cleaning screen based on the bionic earthworm motion characteristics was completed.The test results shown that the accumulation of maize mixture in front of the screen was reduced when maize mixture was fed continuously with a large feeding mass,and the cleaning efficiency and performance was improved.The physical and mechanical parameters of maize mixture provide a reference and basis for the establishment of simulation particle model and the setting of simulation parameters.The main contents and conclusions were as follows:Measurement of the physical and mechanical characteristic parameters of maize mixture.The main components of the maize mixture were maize grain,stalk,cob and lightweight impurity through field investigation and sampling.The moisture content and density of maize grain,stalk,and cob were measured using drying and liquid displacement method.The static friction coefficient between the maize mixture and screen was measured by the rotating disk.Based on the measuring platform of coefficient of restitution,the coefficient of restitution of maize mixture and screen was measured.The results provide a basis for the establishment of the simulation particle model of maize mixture.Bionic design of maize grain cleaning screen based on earthworm motion characteristics.Earthworm was used as the bionic prototype,and its motion characteristic was analyzed.The contour curve of the earthworm's head in an axial contracted state was extracted by image processing technology.The maximum variation of the height of the earthworm's head somite was3.48 mm,and the maximum concave depth of the bionic screen was 50 mm.The shapes of the wave geometry of the earthworm's lower surface were simplified.Based on the difference of the concave position of the wavy geometry of the lower surface during the earthworm moving,the motion of the bionic screen was divided into four postures,and the model of motion posture of the screen surface was obtained;The cam drive mechanism was designed to realize the continuous conversion between concave bending and flattening of the bionic screen.The "V-shaped" concave surface formed in the motion of the screen surface would produce the "backward-wave" of backward movement with the change of the screen surface in different postures by the cooperation of different cam drive mechanisms,and the design of cleaning screen based on earthworm motion characteristics was completed.The kinematics analysis of bionic screen was carried out based on ADAMS.The velocity characteristics of the follower have been analyzed to verify its motion law.The displacement of screen plates' geometric center over time and the displacements of screen plates' geometric centers at 0 s,0.093 s,0.260 s,and 0.389 s were analyzed.The results showed that the requirements of the motion posture of the screen during the bionic screen moving and the mutual conversion between different postures could be realized by the cooperation of cam drive mechanisms.Numerical simulation of gas-solid two-phase motion in cleaning device of the bionic screen.The motion equations of each screen of the bionic screen were obtained through ADAMS and MATLAB,and the non-harmonic motion of the bionic screen in EDEM(two movements and one rotation)was realized by using API.The change law of the airflow velocity on the screen surface was analyzed when the bionic screen was in the posture 0?3,and the concave screen played a certain role in blocking the airflow.The change laws of the horizontal velocity and vertical displacement of the maize mixture on the screen along the front to the rear of the screen under different concave depths and inclination angles of the screen surface were analyzed.The average horizontal velocity of maize grain,stalk,and cob increases with the increasing concave depth and installation angle.The horizontal velocity of stalk and cob was higher than that of maize grain,which was beneficial to the separation and dispersion of maize grain and impurity(stalk and cob)along the horizontal direction of the screen surface.The average horizontal velocity of the maize mixture in the front area of the bionic screen was the highest,which was conducive to the maize mixture migrated backward rapidly in the front of the bionic screen to reduce the accumulation of feed end.The average vertical displacement of maize grain,stalk,and cob gradually increases with the increasing of the concave depth of the screen surface,and progressively decreases with the increasing of the installation inclination of the screen surface.When the maize mixture moved from the middle to the tail of the screen,the maize mixture on the screen was maize grain,cob,and stalk,respectively.The grain and impurity appear obvious stratification in the vertical direction.The maize grain and impurity was separated horizontally and layered vertically on the bionic screen,which was conducive to the maize grain traverse the opening in the screen.Simulation test of the cleaning device of the bionic screen.The single factor test was carried out by taking the concave depth of the bionic screen surface,the installation angle of the screen surface,the rotation speed of the cam,the airflow velocity at the inlet of the cleaning device,and the airflow direction angle as the test factors.The results showed that the effects of the rotation speed of cam and airflow direction angle on the lost percentage of maize grain and the impurity percentage of maize grain were not significant.With the increase of concave depth and installation angle of the screen surface,the lost percentage of maize grain increased,and the impurity percentage of maize grain decreased.With the increase of inlet airflow velocity,the lost percentage of maize grain first decreased and then increased,while the impurity percentage of maize grain gradually decreased.Based on the results of the single-factor simulation test,the quadratic orthogonal rotation combination test with three factors and five levels was designed after the insignificant factors were eliminated.The optimal combination of parameters was obtained as follows: the concave depth of the screen surface was25 mm,the installation inclination angle of the screen surface was –5.5°,and the inlet airflow velocity was 12.3 m/s.At this time,the lost percentage of maize grain was 1.56%,and the impurity percentage of maize grain was 1.85%,which met the requirements of national standards.Bench test of the cleaning device of the bionic screen.The motion state of maize mixture on the bionic screen in the posture of 0?3 and their horizontal velocity changed was analyzed by high-speed camera,which was basically consistent with the results of numerical simulation.The mechanism of maize mixture migrated on bionic screen was revealed.When the optimal structure and operation parameters,the performance of maize mixture migrating and cleaning of the cleaning device of the bionic screen and the cleaning device of planar reciprocating vibrating screen under different feeding mass of maize mixture were compared through bench test.The test results showed that the time of maize mixture migrated in the front area of the screen,the total screening time,the loss percentage of maize grain,and the impurity percentage of maize grain of bionic screen were less than those of planar reciprocating vibrating screen when the feeding mass was same.Compared with the planar reciprocating vibrating screen,when the feeding mass was 6 kg/s,7 kg/s,and 8 kg/s,the time of maize mixture migrated in the front area of the screen was reduced by 1.81 s,2.56 s,and 3.27 s,the total screening time was reduced by 2.23 s,2.33 s,and 2.38 s,the loss percentage of maize grain was reduced by 1.20%,1.84%,and 2.78%,and the impurity percentage of maize grain of bionic screen was reduced by 1.74%,3.30%,and 5.40%,respectively,which indicated that the bionic screen had better adaptability to large feeding mass.In this study,a bionic screen for maize cleaning was designed based on earthworm motion characteristics.The motion of the screen surface,the migration and stratification of the maize mixture on the screen and the cleaning performance of the bionic screen were analyzed by kinematics analysis,numerical simulation of gas-solid two-phase and bench test.The problem of maize mixture accumulation in the front of the screen caused by the single movement mode of the traditional planar reciprocating vibrating screen was solved.It is beneficial to improve the operation efficiency and cleaning performance of maize combine harvester under the large feeding mass of maize mixture.The related contents and methods of this study provide a reference and basis for the innovative design and the exploration of screening mechanism of maize cleaning screen. |
PDF Full Text Request