|Case study of the piezoelectric transducer vibrations analysis by laser vibrometry and acoustic holography.|
Thesis january 2006, University of Claude Bernard, Lyon, France
The surface of piezoelectric transducers vibrates in a nonuniform way either because of manufacturing defects inherent or because of surface waves generated by each interface. The knowledge of the space distribution of the speed is essential if one wants to calculate in a precise way the field radiated by the transducer.
The objective of this work is to answer measure the pressure on the surface of the transducers when they are plunged in a liquid. In the thesis, the applications of the ultrasounds will be presented successively, principal technologies of transducers used to generate these waves and finally the various imaging techniques, optics and acoustics developed to measure the surface vibrations. The holographic method is at the base of work suggested.
This method requires to measure the pressure in amplitude and phase of any point of a surface located on the transducer. By using the integral of Rayleigh and opposite wave propagation, it is possible to calculate the distribution of speed on the surface of the piezo. This method is tested in experiments on various sources including piézocomposites, piezoceramics and the transducers of an unspecified nature with or without defects. Highlighted is the importance of Lamb waves which modifies in an important way the distribution of speed on the surface of the transducers. Knowing the distribution of speed it is then possible to calculate the ultrasonic field using the Rayleigh method. By comparing the measured field and the one calculated knowing the speed, it is possible on to validate the holographic method and to show that the used piston model allows only a rough coarse calculation of the field of pressure space distribution.
The purpose of the second part of the work, is to determine the measurement of the distribution of speed on the surface of piézocomposite or piezoceramic in the air in water and glycerin by a method of vibrometry Laser. The experimental unit is described: and a resolution of about 10 micrometers has been reached. If the measures in the air do not seem contestable, it appears that the measures in the liquids are sullied with important errors because of the acousto-optic interactions. It is possible to take account of this interaction only if the field of pressure is known. Which is the case only for transducers generating plane waves.
The induced errors are analyzed theoretically and then measured in monochromatic and pulsated mode experiments. Possibilities to correct these errors induced by the optical acousto effect are proposed. The measurement technique using the laser vibrometer is used to record the temporal form in each point of various transducers. It is thus possible to highlight in a clear way the generation and the propagation of the waves of Lamb in monochromatic and impulse mode. In conclusion, the laser vibrometry seems an invaluable tool to determine the behavior of transducers piézocomposite or piezoceramic when measurements are taken in the air.
This technique makes it possible to highlight disturbing modes of vibrations such as the surface waves. The laser vibrometry cannot be employed when measurements are taken in a liquid medium because of the acousto-optics interaction. A theoretical study gives the importance of this interaction which can be precisely given for the particular case of a plane wave.
The holographic method suggested in this work makes it possible to measure the distribution of speed on the surface of any immersed transducer. The confirmation of the exact measurment value could be made by re-propagating the wave starting from the measurements calculated on the surface of the transducer.