Modeling and validation of the baling process in the compression chamber of a large square baler
The pressure-density relationship and the pressure distribution inside the compression chamber of a newly designed New Holland BB960 large square baler were studied for the baling of alfalfa, whole green barley, barley straw, and wheat straw. An analytical model was developed for the pressure distribution inside the compression chamber of the large square baler in the x-, y-, and z-directions by assuming isotropic linear elastic properties for forage materials. In order to validate this model, a tri-axial sensor was designed and used to measure the forces inside the compression chamber when whole green barley, barley straw, and wheat straw were baled. The experimental results proved that the developed analytical model for each of the tested forage materials had a good correlation with the experimental data with a reasonable coefficient of determination (0.95) and standard error (20.0 kPa). Test data were also used to develop an empirical model for the pressure distribution inside the compression chamber of the baler for each of the tested forage materials using least square method in regression analysis. These empirical models were simple equations which were only functions of the distance from the full extension point of the plunger along the compression chamber length.Analytical and empirical models were also developed for the pressure-density relationship of the baler for baling alfalfa and barley straw. Results showed that bale density initially decreased with distance from the plunger, and then remained almost constant up to the end of the compression chamber. The developed empirical model for both alfalfa and barley straw was a combination of a quadratic and an exponential equation. In order to validate the developed models, field tests were performed by baling alfalfa and barley straw of different moisture contents, flake sizes, and load settings. The forces on the plunger arms were recorded by a data acquisition system. The actual bale bulk density was calculated by measuring the bale dimensions and weight. Results showed that both load setting and flake size had a significant effect on the plunger force. The plunger force increased with increased load setting and flake size. Comparing analytical and empirical models for bale density as a function of the pressure on the plunger showed that the trend of variation of density with pressure in both models was similar, but the rate of change was different. The variation rate of density with pressure in the analytical model was higher than that of the empirical model. The analytical model underestimated the bale density at low plunger pressures but showed more accurate prediction at higher pressures, while the empirical model accurately predicted the bale density at both low and high pressures. Some crop properties such as coefficient of friction and modulus of elasticity were determined for the development of the pressure distribution model. Results showed that static coefficient of friction of alfalfa on a polished steel surface was a quadratic function of material moisture content, while the relationship between the coefficient of friction of barley straw on a polished steel surface and material moisture content was best expressed by a linear equation. Results of this study also proved that modulus of elasticity of alfalfa and barley straw was constant for the density range encountered in the large square baler.
Baling process, Pressure distribution, Modeling, Pressure-density relationship
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