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- Development of a high-power multilayer PVDF acoustic projector for 40 to 80 kHz bandPublication . Silva, António; Hughes, Ashley; Pozzatti, Daniel; Zabel, Friedrich; Viegas, Rúben; Martins, MarcosA piston type projector using the PVDF piezoelectric polymer was developed for operating in underwater environment, below 100 kHz. For those frequencies PZT piezoelectric ceramic is usually a preferable choice and PVDF is only considered for frequencies above a few hundreds of kHz. This paper will show that efficient underwater acoustic projectors for frequencies below 100 kHz can be implemented regarding an appropriate impedance adapter is being used. The developed project presents a calibrated transmitting voltage response (TVR) of approximately 166, 160 and 175 dB at 40, 50 and 75 kHz, respectively. The PVDF TVR values are compatible with the PZT projectors available on market with the advantage of having a larger bandwidth than most PZT projectors. To the authors knowledge this is the first time that a PVDF projector attain such characteristics. Although theoretically the PVDF projector bandwidth is larger than 40 to 80 kHz, in practice it was observed that only between those frequencies the project presents a stable operation for the transmission of long-term signals.
- In-Lab demonstration of an underwater acoustic spiral sourcePublication . Viegas, Rúben; Zabel, Friedrich; Silva, AntónioUnderwater acoustic spiral sources can generate spiral acoustic fields where the phase depends on the bearing angle. This allows estimating the bearing angle of a single hydrophone relative to a single source and implementing localization equipment, e.g., for target detection or unmanned underwater vehicle navigation, without requiring an array of hydrophones and/or projectors. A spiral acoustic source prototype made out of a single standard piezoceramic cylinder, which is able to generate both spiral and circular fields, is presented. This paper reports the prototyping process and the multi-frequency acoustic tests performed in a water tank where the spiral source was characterized in terms of the transmitting voltage response, phase, and horizontal and vertical directivity patterns. A receiving calibration method for the spiral source is proposed and showed a maximum angle error of 3° when the calibration and the operation were carried out in the same conditions and a mean angle error of up to 6° for frequencies above 25 kHz when the same conditions were not fulfilled.
- Spiral beacon calibration and experiments for underwater localizationPublication . Viegas, Rúben; Zabel, Friedrich; Gomes, João; Silva, AntónioUnderwater localization and navigation are still challenging tasks due to the underwater acoustic channel characteristics. Spiral sources are underwater transducers that create structured acoustic fields from which the angle to the source can be readily obtained. The angle estimation is obtained from the phase difference between transmitted circular and spiral fields, but for reliable operation the transducers must be properly calibrated. This paper presents a spiral source calibration procedure with the integration of a stepper motor to measure phase and amplitude features of the transmitted circular and spiral fields, at multiple bearing angles. The calibration was performed for two developed prototypes, which in turn determined the most appropriate operating frequency range. For one of the prototypes, its linearity was confirmed at all the tested frequency ranges through homogeneity and additivity tests. In addition to calibration, acoustic localization experiments were carried out with the transmission of circular and spiral fields, with a comparative analysis against footage captured from the top of the test pool. The phase difference of the mobile hydrophone was subtracted to the phase difference of the reference hydrophone to compute the angle between the spiral beacon and the mobile hydrophone. The localization results revealed noteworthy angular errors, hypothesized to be associated with the Doppler effect induced by the movement of the mobile hydrophone. These calibration and localization experiments suggest that spiral sources could be an important enabling technology for safe and reliable localization of underwater vehicles.