【作者】 赵湛；

【导师】 李耀明；

【作者基本信息】 江苏大学 ， 机械设计及理论， 2009， 博士

【摘要】 排种器是实现精密播种技术的核心部件,其工作性能的好坏直接影响着播种精度、均匀性、种子的出苗率等。由于气吸振动式排种器具有对种子尺寸要求不高、不伤种子、通用性好、适应性强的优点,且易于提高播种速度,实现自动控制,是一种较为先进的排种装置,已成为当前国内外精密排种器发展的主要方向之一。种子在种盘内的振动以及在吸排种气流场中的受力运动是影响排种器播种精度的主要因素,本文以油菜种子为试验对象,开展了气吸振动式排种器的工作机理研究和性能试验分析,主要工作包括:1、实验测量了油菜种子的几何尺寸、形状特征以及密度、接触刚度、摩擦系数等力学参数,建立了三维模型。根据电磁激振器的工作原理,分析了振动种盘的幅频特性。2、推导出种子与种盘二自由度碰撞振动系统周期运动的Poincar（?）映射,以振动频率为控制参数,计算得到映射Jacobi矩阵特征值的变化曲线,根据其穿越复平面单位圆的位置,分析了映射不动点的稳定性,获得种子由稳定周期运动通向混沌的过程,通过轨迹相图分析得到混沌运动状态有利于实现种群的离散。3、根据油菜种子的机械特性,采用线性弹簧-阻尼-滑动接触力学模型,编写离散元程序,模拟了振动种盘内种群的三维运动规律。为了描述种群空间运动状态,给出了重心波动和体积膨胀系数的计算方法。计算结果表明:碰撞接触时间和最大变形量随接触刚度的增大而减小;最大变形量随碰撞相对速度和恢复系数的增大而线性增加;接触时间随恢复系数的增加而减小,相对速度对其影响不显著。种群重心波动和体积膨胀系数的特征频率出现在种盘振动频率附近。种盘作小幅高频振动时,种子几乎在原地作垂直运动,水平位移较小,种群可以有效分离以减小摩擦力;体积平均膨胀系数H₀随着振动强度K_v的增加而增大,随种层厚度的增加而减小,在相同振动强度下,种盘振幅越大,种群离散程度越高;对于薄层种子,随着种子与种盘、种子与种子之间的碰撞恢复系数的增大,体积平均膨胀系数H₀有明显增大的趋势,随着层厚的增加,这一趋势逐渐减弱;在弱振动条件下,种子之间以摩擦接触为主,体积平均膨胀系数H₀随摩擦系数的增加而增大,在强振动时,摩擦系数的影响很小。4、运用Gambit软件建立滚筒正负气压腔的三维几何模型,将其划分为结构化四面体网络,在恒压力边界条件下,利用Fluent软件中的标准k—ε湍流模型和壁面函数法对滚筒内部全流道气流场分布进行了数值计算。结果表明:吸种过程中,滚筒负压腔为恒压区域,可以起到稳压稳流作用,在轴向不同位置的吸孔气流速度相对误差小于4%。滚筒与负压轴之间的气流通孔存在压力损失,并且随着气流通孔和孔吸种截面积比γ_s的增加,压力损失率η_p逐渐减小,当γ_s大于500%时,η_p小于5%。排种过程中,轴向不同位置的排种孔气流速度存在明显差异,且正对底板连接座的排种孔气流速度最大,随着γ_s的增大,流速差异逐渐减小,且当γ_s大于300%时,流速差异小于5%,正压差对流速差异影响不明显。5、基于气固两相流力学理论,分析了种子在吸种气流场中的受力情况。将种子受力在吸孔轴向和径向进行分解,由Fluent计算了离散空间节点上吸力的大小。运用插值运算法从垂直吸种距离z、径向吸种距离r和有效吸种空间体积V三个方面综合分析排种器的吸种性能,确定不同结构形式吸孔的有效吸种区域和瞬间吸种运动过程,并对吸孔结构参数进行了设计。在此基础上,通过假设种群在振动种盘内呈正四面体空间分布,分析了种群层数n、负压差Δp、间隙系数λ、吸孔直径d_k对种子受力的影响,揭示振动和负压吸种机理。结果表明:吸附运动主要是由压力梯度力所控制;锥孔的吸种能力优于直孔和沉孔;增加吸孔直径d_k比提高负压差Δp更有利于提高排种器吸种能力;随着吸孔直径d_k的增大,z、r随之线性增加,V成3次方增加;增加负压差Δp,z、r和V也随之增加,且在Δp较小时,增加速度较快,Δp较大时,增加速度较为缓慢;对于多层种子,随着间隙系数λ的增加,种子受力呈现增加趋势,且λ小于1.25时,受力增长较为迅速,可以有效提高排种器垂直吸种距离,λ大于1.25时,受力增长比较缓慢。6、根据湍动射流力学理论计算了排种过程中种子受力大小,分析正压差、排种角和滚筒转速对排种运动过程的影响,在JPS-12气吸振动式排种器性能试验台上,对排种过程种子运动状态进行了高速图像采集,采用颜色向量建立种子模型,提出用Mean shift算法编程进行目标跟踪,获取种子实际运动轨迹,验证了数值计算的正确性。分析得到:湍动射流产生的瞬间冲击加速度是决定排种过程种子运动状态的主要因素,正压差和排种角的波动是造成落种位置误差的重要原因,计算了不同工作参数下的落种速度和时间。当正压差在1～1.5kPa、排种角在0～-10°范围时,落种精度最高。7、在JPS-12气吸振动式排种器性能检测试台进行单因素和正交试验,建立合格指数与负压差、滚筒转速、种盘振动频率和吸种角的数学关系,通过自适应遗传算法对排种器工作参数进行优化。结果表明:锥孔的吸种能力优于直孔和沉孔,孔径对吸种性能影响显著,吸孔与种子的直径比应选定在0.4～0.6。增大孔径和负压差,在提高吸种能力的同时,又会造成重吸率的增加。初始油菜种层厚度约为8～10mm时,吸种精度较高。滚筒转速在15～25r/min范围内,排种器合理负压差和吸种角范围分别为2.8～3.0kPa、25.5～36.5°,合格指数能达到94～97%。落种位置误差随滚筒转速提高而线性增大,正压差在1.5kPa、排种角在-5～-10°范围的播种均匀性最高。试验结果和理论分析结论相互吻合,为气吸振动式精密排种器的理论设计提出了新方法。更多还原

【Abstract】 As a core component of precision sowing technique,seeder’s working performance directly influenced the sowing accuracy,seed spacing uniformity and emergence rate. Vacuum-vibration seeder has become the major development trend of precision seeder because of its advantages including lower requirement of seeds size,lower harmful to seeds,higher universality,widely adaptability,easy to improve the working efficiency and realize automatic control.The working performance of the seeder was majorly determined by the seeds motion states on vibration plate and in the vacuum gas field,so in this paper,taking rape seeds as experimental materials,the theoretical analysis on the working mechanism and the performance tests on the vacuum-vibration precision seeder were carried out,the main results were generalized as follows:1.A 3-D rape seed model was established according to the geometry size,shape features and mechanical parameters including density,rigidity and friction coefficient. Based on the working principle of electromagnetic vibration plate,the amplitude-frequency response characteristics were analyzed.2.A two degree-of-freedom vibro-impact motion model of seeds on the vibration plate was established and then a 4-dimensional Poincar（?） map of periodic motion was derived.With the increasing of vibration frequency,the variation process of Jacobi matrix eigenvalues was calculated.According to the position of eigenvalues crossing the unit circle in the complex plane,the stability of fixed point of the map was discussed and the route from periodic motion to chaos via bifurcation was received.From the trajectory phase of seeds,we found the chaotic motion state was helpful to seeds separation.3.A Matlab program based on the discrete element method was developed to simulate the seeds motion in a 3-dimensional vibration plate.The mechanical interaction forces were modeled by linear springs,dash-pots and friction sliders.In order to describe the states of seeds motion,we defined the fluctuation coefficients of gravity center and the volume expansion.The numerical calculation results showed that the impact time and the maximum deformation both decreased with the increasing of contact rigidity,the maximum deformation linear increased with the increasing of impact relative speed and the recovery coefficient,impact time decreased with the increasing of recovery coefficient and independent of relative speed.Characteristic frequencies of the fluctuation coefficients of gravity center and the volume expansion both appeared near the vibration frequency of plate.While the plate vibrating with high frequency and small amplitude,the seeds were almost vertical vibrating in the same place with small horizontal displacement,so the seeds could be separated effectively to reduce the friction forces.The average volume expansion coefficient,H₀,increased with the increasing vibration strength,K_v,and decreased with the seeds initial thickness,h_s.At the same K_v,the larger amplitude would lead to the higher distribution level.To thin-layer seeds,with the increasing of recovery coefficient between seeds and seeds with plate,H₀ increased significantly,but to thick-layer seeds,this increasing tendency was slightly.Under the condition of weak vibration,H₀ increased with the increasing of friction coefficient because the impact forces were mainly delivered through friction,while under the strong vibration,the friction coefficient was slight.4.3-D geometry models of air flow channel inside the cylinder cavities were built and then be formed with the tetrahedral grids using Gambit.Take the compressibility and the viscosity of the real gas into account,the N-S equations,standard k-εturbulence model and wall function were applied to calculate the airflow distribution.The results showed that,during the suction process,the negative differential pressure in the cavity keeps constant which is helpful to stabilize the air flow and the relative velocity errors between different axial nozzles were less than 4%.The pressure loss ratio,η_p,decreased continuously with the increasing of sectional area ratio,γ_s,between suction nozzle and through-hole,andη_p was less than 5%withγ_s larger than 500%.During the sowing process,the velocities in the nozzles under different axial position were different obviously,and this difference decreased to 5%continuously with the increasing ofγ_s up to 300%.The positive differential pressure affected slightly to the difference.5.The forces acting on seed in gas flow field were analyzed based on the theory of gas-solid two-phase.After decomposed the suction force in axial and radial direction of nozzle,the force values on discrete space nodes were calculated by Fluent.The effective pickup region of different structural nozzles and the seeds pickup trajectories were received utilizing the binary interpolation method.The axial pickup distance,z,the radial pickup distance,r,and the volume of effective pickup region,V,were proposed to comprehensively analyze the nozzles pickup ability.By assuming the seeds in the vibration plate were distributed with regular tetrahedron,the influences of seeds lay thickness,n,negative differential pressure,Δp,seeds gap coefficient,λ,nozzle diameter, d_k,on suction force values were analyzed.The results showed that the seeds motion during the suction process were major determined by the pressure gradient force.Pickup ability of conical nozzle was batter than the straight nozzle and the bore hole.With the increasing of d_k,z and r increased linearly and V increased cubicly.With the increasing ofΔp,z,r and V all increased quicker at first and then become slower and the force value increased quickly at large d_k and slowly at small d_k,so the pickup ability would be improved more effectively through increasing the diameter of nozzle than the negative differential pressure.With the increasing ofλ,the force values first increased rapidly withλ＜1.25 and then increased slowly.6.Effects of positive differential pressure,sowing angle and rotational speed of cylinder were analyzed according to the turbulent jet mechanical theory.On JPS-12 vacuum-vibration seeder performance test bed,the seeds sowing processes were acquired using high speed camera system and a Mean shift algorithm utilizing the color eigenvector was proposed to track the seed motion.The image trajectory tracking results verified the correctness of theoretical analysis and the results showed that seeds sowing states were mainly determined by the transient acceleration around the nozzle. The fluctuation of positive differential pressure and sowing angel were the key factors which lead to seeds landing position error,while the positive differential pressure and sowing angel in the range of 1～1.5kPa and 0～-10°,the seeds landing position error can reach the smallest value.7.The mathematical relationship between the single pickup ratio and negative differential pressure,rotational speed of cylinder,vibration frequency of plate and the suction angle was established though single factor and orthogonal experiments on the JPS-12 vacuum-vibration seeder performance test-bed,and the seeder’s working parameters were optimized by adaptive genetic algorithm.The results indicated that the pickup ability of conical nozzle was batter than the straight nozzle and the bore hole. The optimum diameter ratio of nozzle and seed should be selected between 0.4～0.6. Increasing the nozzle diameter and negative differential pressure can improve the pickup ability but also would lead to the increasing of seeds multiple pickup ratio. While the rotational speed of cylinder in the range of 15～25r/min,the optimum negative differential pressure and suction angle are in the range of 2.8～3.0kPa and 25.5～36.5° and the single pickup ratio can reach 94～97%.The landing position error linear increased with the increasing of rotational speed of cylinder and the sowing uniformity reached the maximum value with negative differential pressure of 1.5kPa and suction angle in the range of -5～-10o.The accordance of the experimental and theoretical analysis results demonstrates that the method provided in the paper is effective for vacuum-vibration precision seeder design.更多还原