A frequency selective acoustic transducer for directional Lamb wave Sensing

Abstract

A frequency selective acoustic transducer (FSAT) is proposed for directional sensing of guided waves. The considered FSAT design is characterized by a spiral configuration in wavenumber domain, which leads to a spatial arrangement of the sensing material producing output signals whose dominant frequency component is uniquely associated with the direction of incoming waves. The resulting spiral FSAT can be employed both for directional sensing and generation of guided waves, without relying on phasing and control of a large number of channels. The analytical expression of the shape of the spiral FSAT is obtained through the theoretical formulation for continuously distributed active material as part of a shaped piezoelectric device. Testing is performed by forming a discrete array through the points of the measurement grid of a scanning laser Doppler vibrometer. The discrete array approximates the continuous spiral FSAT geometry, and provides the flexibility to test several configurations. The experimental results demonstrate the strong frequency dependent directionality of the spiral FSAT and suggest its application for frequency selective acoustic sensors, to be employed for the localization of broadband acoustic events, or for the directional generation of Lamb waves for active interrogation of structural health.

Experimental demonstration of spiral frequency-steerable acoustic sensors

“…A piezoelectric patch used for GW actuation or sensing can be represented in the (k x , k y ) wavenumber domain by 2D Fourier Transform (FT) of its spatial geometry. It is shown in [7,8] that, for a given frequency, maximum generation or sensing performance of the patch occur at wavenumbers k * where dispersion circumferences intersect the device (k x , k y ) distribution. Each wavenumber k * selects a specific propagation direction.…”

“…In sensing, FSAT output has a dominant spectral component uniquely associated with the direction of the incoming wave. The desired transducer geometry in space domain x = x i x + y i y is achieved by inverse 2D FT of the specified wavenumber distribution and can be analytically expressed as [8]:…”

“…Alternative devices with inherent directional properties include the CLoVER transducer [4] featuring individually activated wedge sections of a piezo-composite ring pattern, as well as rosette configurations of microfiber composite (MFC) devices [5]. Whereas these solutions still involve multiple channels, recently proposed FSATs incorporate directional capabilities into one-or two-channel devices [6,7,8] with peculiar geometries.…”

Experimental demonstration of directional GW generation through wavenumber-spiral Frequency Steerable Acoustic Actuators

“…In [4], a procedure to design acoustic transducers with frequency steerable capabilities was presented, based on the definition of the desired directivity properties in the wavenumber domain. Such procedure stems from the observation that maximum transducer response is identified in such domain by intersections between the dispersion characteristics of the medium in which guided waves propagate and the wavenumber representation of the device electrodes' geometry.…”

Inkjet fabrication of spiral frequency-steerable acoustic transducers (FSATs)

“…As an alternative, active and passive structure interrogation through devices with inherent directional capabilities was proposed, respectively by firing individual wedge sectors of a ring-shaped composite long-range variable-direction emitting radar (CLoVER) transducer, 5 and by directional sensing of incoming waves using macro-fiber composite piezoelectric rosettes. 6 The wavenumber spiral frequency steerable acoustic transducer (WS-FSAT) recently proposed by the authors 7,8 provides an interesting option for directional structure inspection with further reduction of hardware requirements down to a single, differential channel. The device concept is based on a direction-dependent spatial filtering effect whereby each direction of GW propagation within the [0 • , 180 • ] range is uniquely associated with a dominant frequency component in the spectrum of the actuated/recorded signal.…”