Investigation of a planar active focusing air-coupled ultrasonic transducer using multiple concentric ring electrode patterns

Abstract

Air-coupled ultrasonic transducers (ACUTs) have been applied in industrial non-destructive testing, structural health monitoring, and medical ultrasound. However, conventional passive focusing methods often result in undesired effects on sensitivity due to variations in acoustic impedance matching conditions, which are critical in ACUT design, where sensitivity is the top priority. Accordingly, a novel active focusing method for ACUTs is proposed in this study. The key idea is to create multiple concentric ring electrode patterns on a bulk planar piezoelectric plate so that a quarter-wavelength acoustic impedance matching layer of uniform thickness can be attached onto the plate. The initial structural parameters of the electrode patterns are determined based on the design methodology of a Fresnel zone plate (FZP). Those parameters are optimized through finite element simulation, with the transducer’s sensitivity as the objective function, while ensuring only slight variations to the focal length and lateral resolution. Single-sided multiple concentric ring electrode patterns are then fabricated on 1–3 piezoelectric fiber/epoxy resin composite plate by screen printing and combined with a high-performance acoustic impedance matching layer made from epoxy resin composite filled with hollow glass microspheres. The planar active focusing ACUT is developed, while two types of conventional passive focusing ACUTs using FZP and concave lens are fabricated with the same piezoelectric and acoustic matching materials. Comparative experimental testing is carried out. The developed planar active-focusing ACUT achieves significant sensitivity improvements of 7.1 and 17.4 dB, respectively, while maintaining comparable radial and axial full-width at half maximum. The results of this study offer a novel approach for the design of high-performance ACUTs.

Graphical abstract

Keywords

air-coupled ultrasonic transducer / planar active focusing method / multiple concentric ring electrode pattern

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