Investigation of vibration of polyvinlidine fluoride cylindrical film by use of acoustic probe

Abstract

Ultrasonic pen and white board digitizing systems are a rapidly growing market for PVDF transducers, utilizing an ultrasonic transmitter in the pen tip and two stationary ultrasonic receivers. The transmitter is constructed of piezoelectric PVDF welded into a cylindrical shape. Radius vibration (breathing mode) creates an 80-kHz airborne acoustic wave. The initial cycle is used to determine the time of flight, used to calculate the pen position by triangulation. An ideal cylindrical transmitter would have a uniform 360 horizontal radiation pattern. However, practical devices have a 65% to 85% minimum/maximum ratio for the initial pressure peak. A novel acoustic probe was devised to investigate this nonuniformity. The probe tip is a needle with 0.5-mm through hole, and the acoustic wave propagates through the needle to a conventional reference microphone. The probe was positioned using a precision XYZ stage. Investigations with this probe revealed the details of the nonuniformity in horizontal radiated pressure. It was found that (1) vibration at the welded seam of the PVDF cylinder is a maximum, opposite the expected behavior, and (2) the pressure distribution profiled along the height of the cylinder has a central plateau and decreases linearly at the cylinder ends.