| In the working process of precision planter,the final position of seed is determined by the comprehensive action of profiling mechanism,opener,seed metering device,soil-covering and soil-compacting device.Among them,the bouncing and rolling of seeds after contact with soil during dropping process and the change of seed position during soil covering and compacting are important reasons for the reduction of sowing uniformity.In the past,test methods have been used to study the bouncing displacement of seeds during sowing to optimize the structural and dimensional parameters of related mechanical components.The method,however,is not only time-consuming,labourious and plagued by seasonal limitations but also difficult to obtain information via,i.e.the interaction force acting between particles and boundary,particle displacement,and velocity,which is important for the design and optimization of agricultural machinery components.At present,the above problems can be solved by using the discrete element method(DEM)to analyse the contact between particles and mechanical components.To realize the simulation analysis of maize seed dropping process and soil covering and compacting process of the precision seeding unit,this paper analyses the shape and size of main varieties in three main maize planting areas,and then puts forward a general modeling method of maize seed based on multi-sphere method.Moreover,the parameters which are difficult to be directly measured are calibrated.The feasibility of the modelling method of the maize seed proposed in this paper is validated by comparing the experimental and simulation results of bulk density,angle of repose and "self-flow screening".Subsequently,the influence of multiple contacts on the dynamic behaviour and static behaviour of an assembly of seeds is investigated under different Young's modulus of seed particles.The soil particle model is established,and the Edinburgh Elasto-Plastic Adhesion model is used to calculate the contact forces between soil particles,between soil and seed particles,and between soil particles and boundary.Plackett-Burman test and central composite design are used to calibrate the contact parameters between soil particles,between soil and seed particles,and between soil particles and boundary.Through single factor simulation test,the factors affecting the bounce displacement of seed(namely,normal relative velocity between seed particle and soil,tangential relative velocity,soil slope,rotational speed of seed particle,moisture content and seed posture)are studied.The significant factors are analysed by orthogonal test,and the optimal parameter combination is obtained.The coupling calculation method of discrete element method(DEM)and multi rigid body dynamics(MBD)is used to simulate the soil covering and pressing process of seeding monomer.The main work and conclusions of this paper are as follows.(1)Nine major maize varieties are selected from the three major maize producing areas in Chinese mainland,and the shape analysis of the maize seeds of the nine varieties shows that the shape of maize seeds of the nine varieties can be represented by five shapes,i.e.,horse-tooth shape,truncated triangular pyramid shape,ellipsoid cone shape,spheroid shape and irregular shape.According to the larger planting area and the uniformity of seed shape distribution,three main varieties(Liangyu 99,Xianyu 335 and Zhengda 999)are selected for further study.By analysis,it is found that there is a certain functional relationship between the characteristic sizes of each shape of maize seeds.In addition,the physical parameters and contact parameters of maize seeds are measured.(2)A general modeling method of maize seed based on multi-sphere method is proposed.The conclusions are as follows:?According to the shape characteristics of each shape seed particle,each shape seed is simplified to the corresponding regular shape for modelling.This paper uses the method of inscribed on the corresponding regular shape to model that takes the centroid of the particle model of the maize seed calculated according to the formula of the centroid of the corresponding regular shape as the origin of the Cartesian coordinate system,and the position coordinates and the radius of each filling sphere are calculated according to the corresponding characteristic sizes.According to the filling accuracy,the particle models with different filling ball numbers are established.In addition,the artificial roughness of particle models is defined and calculated to represent the filling accuracy of the particle model.A method for calculating the mass and moment of inertia of seed particles according to the characteristic size of seed particles of various shapes is given.?By studying the sensitivity of the coefficients of rolling friction to the angle of repose,the calibration method of the coefficients of rolling friction of maize seeds is determined.The coefficient of rolling friction between seed particles and the boundary does not affect the angle of repose,but the angle of repose is highly sensitive to the coefficient of rolling friction between seed particles.It provides a basis for the calibration of the coefficients of rolling friction between seed particles and between seed particles and boundaries by a single-factor test.?The feasibility of the modelling method of the maize seed proposed in this paper is verified by comparing the experimental and simulation results of bulk density,angle of repose and "self-flow screening".The pros and cons of multi-sphere model of seed particle built by different researchers are analyzed by comparing the simulated results and experimental data in terms of the bulk density,angle of repose,and percentage passing.The results show that: the 12-sphere model of horse-tooth shaped seeds,the10-sphere model of ellipsoidal cone seeds,the 9-sphere model of spheroid-shaped seeds and the seed population composed of the 12-sphere model of horse-tooth shaped seeds,the 28-sphere model of truncated triangular pyramid-shaped seeds,the 10-sphere model of ellipsoidal cone seeds and the 9-sphere model of spheroid-shaped seeds can meet the test requirements better.?By comparing of the simulated results of the HM model and the HM-new model in terms of the percentage passing and porosity for maize seed particle models with different filling spheres,the effect of multiple contacts on the dynamic behaviour(the screening process)and static behaviour(the packing process)of an assembly of seed particles is analysed.The results show that although multiple contacts has a significant effect on the movement of a single particle,when the number of contacts is less than a certain value,multiple contacts will not affect the group behavior of seed particles.However,when the number of contacts exceeds a certain value,multiple contacts not only has a significant impact on the dynamic behavior of an assembly of seeds,but also has a great impact on the static behavior of an assembly of seeds.(3)The parameters between soil particles,between soil particles and seed particles,and between soil particles and boundary are calibrated.The conclusions are as follows.?The maximum adhesion force of normal contact between soil and seed and between soil and boundary under three moisture contents is analyzed.It is proved that the adhesion between seed and soil and between boundary and soil has a significant effect on contact collision.Therefore,the Edinburgh Elasto-Plastic Adhesion model(EEPA)is used to calculate the contact forces between soil particles,between soil particles and seed particles,and between soil particles and boundary.?The significance of input parameters in EEPA model is studied by PlackettBurman test.The results show that: the coefficient of restitution,coefficient of static friction,coefficient of rolling friction and surface energy between soil particles have significant effects on the angle of repose test;the coefficient of static friction,surface energy and the contact plasticity ratio between maize seeds and soil particles have a significant effect on the angle of repose test;the coefficient of static friction and the surface energy between soil particles and boundary has a significant effect on the sliding test.Through the central composite design,the parameters between soil particles,between soil particles and seed particles,and between soil particles and boundary are optimized,and the optimal parameter combination is obtained.(4)The bouncing displacement of seeds after contact the soil during dropping process is studied.The conclusions are as follows:?The single factor test is used to study the factors affecting the bouncing displacement of maize seeds after contact with soil.The results showed that the significant factors affecting the bouncing of seed particles are the normal relative velocity of seed particles,soil moisture content,tangential relative velocity of seed particles and soil slope?The significant factors affecting the offset of seed particles after contact with soil are studied by orthogonal design method,and the horizontal combination of various factors that minimize the bounce of seed particles after contact with soil is obtained:the normal relative velocity of seed particles is 1m/s,the soil moisture content is 20%or 25%,the tangential relative velocity of seed particles is 0.5 m/s,and the soil slope is10 deg.The optimized results are validated by the actual seed dropping process of mazie seeds.(5)The coupling calculation method of discrete element method(DEM)and multi rigid body dynamics(MBD)is used to simulate the soil covering and compacting process of seeding monomer.The relevant conclusions are as follows.?This paper studies the influence of different forward speeds of sowing monomer on the ditching of ditcher.The results show that the slope of seed ditch decreases with the increase of forward speed of opener.?With the increase of the covering speed of the seeding monomer,the displacement of the seed particles in the longitudinal direction increases significantly,while the displacement of the seed particles in the transverse and sowing depth directions decreases with the increase of the forward speed of the monomer;With the increase of the pressing speed of sowing monomer,the longitudinal and transverse displacement of each shape of seed particles gradually increases,and the displacement of each shape of seed particles in the direction of sowing depth decreases.?With the increase of operation speed,the sowing uniformity decreased. |
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