M.Sc Thesis


M.Sc StudentEyal Bakalu
SubjectA Comparison Between Direct Monitoring of Vibrations in
Mechanical Systems and the Measurement of
Acoustical Noise
DepartmentDepartment of Mechanical Engineering
Supervisors Dr. Itzhak Porat
Full Professors Bucher Izhak
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

   Acoustic noise signals, measured at the vicinity of a machine, and mechanical vibration signals, measured on the outer surface of the machine, during operation, contain vital information about its inner processes and can supply valuable information about its state. The discipline of diagnostics, which combine vibration and acoustic diagnostics, is called vibroacoustical diagnostics.

  The aim of the current research is to compare between acoustical and vibration monitoring techniques. In addition, it includes the study of vibroacoustical diagnostics indexes (mathematical parameters which are calculated from the sampled signals), used to monitor the condition of a ball bearing in electrical motor, which was damaged intentionally, with unique surface defect.

   In the current research, a new and innovative simulation was developed. The simulation is capable of finding the coupling between vibration and acoustics. Using the simulation, one can calculate the acoustic pressure signal at a point in the space, which is caused by any vibration signal at the same acoustic medium. The simulation is based on modeling the acoustic propagation, using the approach of lumped model.  

   Through the research, it has been proved, that this simulation enables the exact solution of complicated acoustic problems, using simple, linear, canonical algebraic equations.

   In order to examine the simulation, at first, a one dimensional simulation was developed and compared to the development of the analytical solution, of 1D acoustic propagation. A very good agreement was achieved. After proving the ability of modeling vibroacoustical phenomena, using this unique approach, the 1D model was expanded to full 3D model.

   In order to examine the 3D simulation results, an experimental setup was built. A very good agreement between the results of the 3D numerical simulation and those of the experiments was achieved. Moreover, through all the development steps, a comparison between analytical, simulation and experimental results, were followed by acoustic calculation, using the acoustical package of the commercial software, named "COMSOL Multiphysics".

   During this research, the vibroacoustic index, suitable for distinguishing between normal state and surface damaged bearing, was found.

   In addition, a simulation of the expected acoustic pressure generated by the vibration of the motor was conducted. The measured vibration of the motor was inserted to the simulation as an input, and the acoustic pressure, created by it, was calculated using the new simulation that was developed. This simulation yielded a very good agreement between the calculated and measured pressure.