Calibration of rototilled soil discrete element parameters after rotary tillage in the preparation process of rapeseed mechanized direct seeding micro-ridge seed bed

This study aimed to establish, calibrate and verify a discrete element method (DEM) particle model of rototilled soil particles for investigating the particle migration and machine soil interaction mechanism when forming micro-ridge seed beds by the rapeseed direct seeding machine. The soil type was rototilled yellow-brown soil, which is widely distributed in the mid-lower Yangtze River. The study consisted of two parts: parameter calibration and application verification. The parameter calibration followed these steps: (1) testing the soil plastic limit and selecting the contact model accordingly; (2) measuring the basic parameters, such as the soil particle size distribution and repose angle after rototilling; (3) using the EDEM particle filling and particle factory function to reproduce the particle bed based on the particle size distribution; (4) solving the optimal contact parameter combination by using factorial design, steepest ascent method, and quadratic orthogonal rotation regression method. The application verification test used two sets of structural parameters of a rotary-cutting micro-ridge seedbed preparation device (RMSD, which is the key device of rapeseed direct seeding machine to forming micro-ridge seed beds) to conduct DEM simulation and field tests, and compared the particles migration patterns and the landform after operation. A soil plastic limit tester was used to determine the plastic limit of the soil. The plastic limit of the sampled soil was 24.79%, which is close to the upper limit of the moisture content for rototiller operation. Hence, the Hertze-Mindlin (no slip) contact model was selected. The particle size range and mass ratio of the rototilled soil particles from the rapeseed direct seeding machine were sieved and counted. The rototilled soil particles of each particle size range were scanned and modelled using the EinScan-Pro 3D scanner. Based on the 3D models of the soil particles, the EDEM particle filling function was used to reconstruct the particles with different particle sizes and mass ratios. The soil particles bed was re-generated in EDEM according to the particles` mass ratio. The contact parameter calibration test includes a two-level factorization test, steepest ascent test, and quadratic orthogonal regression test, respectively, to perform a preliminary analysis of test factors and eliminate non-significant tone system, narrow down the range of factors, and solve the combination of contact parameters. The contact parameter calibration test uses the repose angle as an indicator. The two-level factorization test showed that the impact restitution coefficient had no significant effect on the repose angle; the rolling friction coefficient, static friction coefficient, and the interaction term of the two significantly affected the repose angle. The steepest ascent test was designed based on the model regression equation from a two-level factorization test. The steepest ascent test reduced the range of factors tested 0.20-0.60 (static friction coefficient) and 0.05-0.40 (rolling friction coefficient) to 0.32-0.51 and 0.11-0.27, a reduction of greater than 50%, providing effective support for obtaining accurate regression models. Since the restitution coefficient had an insignificant effect on the repose angle, the impact recovery coefficient was set to an intermediate level of 0.350 for the quadratic orthogonal regression test. The calibrations resulted in restitution, static friction and rolling friction coefficients of 0.350, 0.351 and 0.257, respectively. The verification consists of simulations and field tests conducted on the RMSD with two structural parameters. The field verification test used the PIVlab tool and the Trimble TX8 3D laser scanner to obtain the soil particles` velocity and the landform. The simulation of 2 structural parameters was consistent with field experiments. The error of micro-ridge distance is 8.25%, which increases with the number of vanes of RMSD. The calibration parameters are accurate. This paper provides fundamentals for the DEM simulation and structural improvement for rapeseed micro-ridge seedbed preparation device.