Most works that address 2-D array ultrasonic transducers for underwater applications are about the geometry aspects of the array and beamforming techniques to make 3-D images. They look for techniques to reduce the number of elements from wide apertures, maintaining the side lobes and the grating lobes at acceptable levels, but not many details about the materials and fabrication processes are described. To overcome these gaps, this paper presents in detail the development of a 2-D array ultrasonic transducer prototype that can individually emit and receive ultrasonic pulses to make 3-D images of immersed reflectors within a volume of interest (VOI). It consists of a 4 × 4 matrix ultrasonic transducer with a central frequency of 480 kHz. Each element is a 5 mm sided square cut into a 1–3 piezocomposite. The center-to-center distance of two contiguous elements (pitch) was chosen to be greater than half wavelength, to increase the amplitude of emission and reception of signals with larger elements. Artifacts generated by grating lobes were avoided by restricting the field of view in the azimuth and elevation directions within 40° × 40° and applying the sign coherence factor (SCF) filter. Two types of backing layer materials were tested, one with air and another made of epoxy resin, on the transducers called T1 and T2, respectively. The pulse echoes measured with T1 had 2.6 dB higher amplitude than those measured with T2, and the bandwidths were 54% and 50% @ −6 dB, respectively, exciting the element with a single rectangular negative pulse. The 3-D images obtained with full matrix capture (FMC) data sets acquired of objects from 0.2 to 1.15 m motivate the development of a 2-D array transducer with more elements, to increase the angular resolution and the range.
In a previous work, the ultrasonic measurement of longitudinal strain in a plate using the Time Reversal technique was proved. One drawback of this measurement is the low sensitivity of the signal against changes in strain. This problem can be solved using the inverse filter signal processing. This technique increases sensitivity but also reduces the energy of the signal and, consequently, the signal to noise ratio. Thus, a physical solution is presented in order to improve the sensitivity of the system. Additionally, the one-bit time reversal is introduced in order to simplify the hardware used in this technique. The strain sensing system is composed of a pair of piezocomposite transducers bonded to the surface of the tested plate and used to generate and sense the ultrasonic waves guided through the specimen. The use of time reversal provides phase compensation for dispersion and edge reflections in the propagation of the guided waves in the plate, allowing time recompression of the waves. The measurement principle is based on the detection of changes in the amplitude and time-of-flight of the focused signal when the plate is subjected to longitudinal strains. System sensitivity is improved by using 2-2 piezocomposite transducers designed to operate between 0.2 to 3.0 MHz. In the signal processing, the one-bit time reversal is compared with the conventional time reversal in twelve-bit resolution. A figure of merit is introduced in order to evaluate the influence of the transfer function on strain sensitivity. This figure of merit relates the energy concentrated at the time reversal focus with the total energy of the signal. This value represents the ability of the time reversal process to recompress the signal at the focus. Experiments were conducted by applying strains up to 150 μm/m. Results show a linear response in the change of the focus amplitude. The sensitivity depends on the transducers and it can be related to the proposed figure of merit. The focus quality is kept when one-bit time reversal is used, showing to be also feasible for the measuring technique. All the results agreed with the numerical time-reversal implementation.
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