PROGRESS IN STACKED PIEZOCOMPOSITE ULTRASONIC TRANSDUCERS FOR LOW-FREQUENCY APPLICATIONS

A stacked ultrasonic transducer comprises multiple individual layers c onnected mechanically in series and electrically in parallel to reduce the fundamental thickness mode resonance to a frequency corresponding to the transit time of the complete stack and the electrical impedanc e to a value which corresponds to that of the layers of the stack in p arallel. In turn: this allows lower frequency resonant operation than would be possible with a single layer, and facilitates electrical impe dance matching to typical transmission circuitry. On transmission, an ideal stack of uniform layers will have an output amplitude larger tha n that of the equivalent single layer by a factor equal to the. Howeve r, using conventional signal amplification circuitry on reception, the output voltage amplitude will be smaller than that of the equivalent single layer by a similar factor. In the past, stacks have commonly be en assembled from layers of conventional piezoceramic material but mor e recently there have been reports of stacks of 1-3 piezocomposites an d it is this type that is considered here. The work described in this paper is motivated by the need to operate at frequencies lower than ar e possible using conventional piezocomposite fabrication technology. P rogress in comparison of experimental and simulated results is outline d and the highlights of a theoretical design study are presented. Thes e show that although the general behaviour of a stacked structure is e asily predicted, a rigorous theoretical analysis is essential to under stand the detail of even a limited range of possible designs. (C) 1998 Elsevier Science B.V. All rights reserved.