Theoretical And Experimental Study On Vacuum-vibration Precision Seeder

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.