A New Technique for Realizing Annular Arrays or Complex Shaped Transducers

Abstract

We propose here a new technique for making arrays of transducers with arbitrary shapes and specially annular arrays. It is well known that the simplest solution which consists in photoetching different electrodes on a unique piezoelectric plate does not give satisfying results. The main reason is that exciting localized stresses in a solid plate creates a lateral propagation in terms of Lamb modes which perturbs the emitted ultrasonic field, mainly in creating secondary lobes of high level. The mechanical separation of the transducers obtained by cutting the plate according to the contours of the different transducers is not often a satisfying solution. First, it may be a difficult and expensive technology if the contours are not straight. But, in general, unpleasant characteristics remain in the behaviour of each isolated transducer. If the width of the transducer is larger than the thickness, the electromechanical behaviour exhibits different modes which may be strongly excited simultaneously with a transient signal in such a way that the emitted field has much larger duration and directivity strongly different from the piston-like one. In the simplest kind of array, i.e. a linear array of identical transducers, a satisfying solution consists in cutting the piezoelectric plate with a pitch smaller than the thickness, each transducer resulting from the electrical association of adjacent blocks of ceramic.