Surface vibration studies of piezoelectric transducers
In this thesis, surface vibration amplitudes and modes of piezoelectric transducers are studied. A system is developed to measure these amplitudes and observe the modes in different media. Piezoelectric transducers are used for underwater sound transmission. They are energized individually or in array form by continuous or pulsed electrical power and the acoustic energy is transmitted from the radiating front surface. This acoustic transmission and its beam pattern correspond to the vibration amplitudes and modes of transducer radiating surface when the electrical energy is applied. Optical techniques are the most common methods used to determine the small vibration amplitudes of objects and the Electronic Speckle Pattern Interferometry Technique (ESPI) has been used to detect the small movements of objects down to 10-20A in real time. Hogmoen and Lokberg have described a method for real time detection and have carried out measurements of small vibrations at frequencies up to 30 KHz . They claimed detection limits of 20A by visual observation and O.lA by photoelectric measurements using a lock in technique. In the thesis, the reference beam modulation is performed by an e1ectro-optic modulator in the path of the reference beam. This allowed ESPI to be used to measure high frequency vibration amplitudes into the MHz region semi-automatically. A video integrator gated by a digital scanner is used to sample the video information directly from a television screen. Various vibration modes of a PZT-4 transduter radiating surface were studied at its resonant frequencies in 3 different media. By using an automatic calibration procedure and a phase-sensitive detection technique, absolute vibration amplitude measurements were successfully carried out. Various experimental surface vibration modes are related to the theoretical values by using simple plate vibration theory which suggests that a PZT-4 transducer behaves like a simply-supported thin plate at low power levels. The relationship between transducer radiation beam patterns and the surface vibration amplitudes under water is presented in Chapter 8. An optical study of the Michelson Interferometer has been given alongside the speckle interferometer and the results of early experiments with the Miche1sori is presented. The reasons for abandoning the Miche1son Interferometer and using the speckle interferometer are given extensively in Chapter 1.