عنوان مقاله [English]
Introduction An important process in grain harvesting with the combine harvester is threshing the materials using the thresher unit. An ideal thresher is one that carries out complete threshing with maximum crop input and best grain separation while saves the shape and quality of the grain and minimizes grain loss. The vibrations of this unit cause the threshing action to fail and combine harvester loss to increase; therefore, it is highly important to study the vibration produced in the threshing unit. Since measuring the vibration in all working conditions on the farm is expensive, one of the ways to achieve the mentioned objectives is to use a vibration model by the simulation methods in order to examine the effects of vibration on the machine’s and the operator’s performance. Using vibration modeling and dynamic analysis of the structure through a mathematical model, finite element, and modal analysis, the causes and effects of the vibration in different working conditions can be examined with minimum cost. The present study was aimed to carry out the dynamic analysis of combine harvester using operational modal analysis. Therefore, the nonparametric and frequency decomposition methods were used in order to extract values of natural frequencies and damping coefficients, and the information obtained from the modal analysis was utilized to design and update the finite element model of the thresher. Afterward, the vibration of the thresher was adjusted as much as possible by modifying the structure through the weight modification method.
Materials and Methods To measure the vibration of the thresher in practical conditions, a piezoelectric accelerometer sensor DYTRAN/MODEL3255A2, an analyzer device, and a signal processing software MEscopeVES were employed. In order to carry out the analysis, the combine harvester was started in its normal conditions and all parts were set in operation. Due to the geometry of the structure, four points were chosen on the bearings of the threshing drum.
Afterward, de-noising signals used in MATLAB were utilized to calculate the response power spectral matrix, and singular values decomposition method was applied to it. Finally, by drawing each singular value, resonance peaks of the system were determined with respect to different frequencies, and the system’s damping was estimated.
In order to carry out the geometric modeling and the simulation of the thresher by finite element method, ABAQUS finite element software was employed.
In order to compare the analytical and the experimental results of NFD value, the predicted and measured natural frequencies were calculated.
Modifying the structure as one of the applications of the modal analysis is a technique to consider the effect of physical parameters of a structure on its dynamic properties, i.e. natural frequencies and mode shape in order to improve the structure’s dynamic behavior. In the present study, therefore, modification of the threshing unit was aimed to decrease its vibrations by changing the natural frequency. Due to its complexity, the process of modifying the structure can be carried out by changing the mass and hardness. As presented in the study, modification of the structure was conducted by changing the mass on the finite element model.
Results and Discussion The purpose of this study is to determine the vibration characteristics and present a vibration model of the thresher. The vibration responses of the thresher were recorded in working conditions on the bearings of the thresher. Through investigating signal parameters, including root mean square, energy, and entropy in different speeds of the thresher, it was specified that these parameters had significantly higher values in the rotation speed of 1000 rpm compared to other speeds, which proved the disturbance in the rotation speed of 1000 rpm.
By examining the range of the natural and excitation frequencies of the threshing unit and also considering the diagram obtained from decomposing the singular values of the power spectral density matrix and Campbell diagram, a resonance frequency was found for the given structure, which is the major cause of vibration in the thresher. Moreover, the speed of 1000 rpm was determined as the critical speed of the thresher.
In order to reduce the level of vibrations, the thresher’s excitation frequency should be far enough from the natural frequency; therefore, the process of modifying the structure is carried out by changing the mass applied to the finite element model, and it was observed that the natural frequency of the first mode changed from 16.98 Hz to 12.4 Hz.