Sparse spiral phased arrays are advantageous for many emerging air-coupled ultrasonic applications, since grating lobes are prevented without being constrained to the halfwavelength element spacing requirement of well-known dense arrays. As a result, the limitation on the maximum transducer diameter is omitted and the aperture can be enlarged for improving the beamforming precision without requiring the number of transducers to be increased. We demonstrate that in-air imaging, in particular, benefits from these features, enabling large-volume, unambiguous and high-resolution image formation. Therefore, we created an air-coupled ultrasonic phased array based on the Fermat spiral, capable of transmit, receive and pulse-echo operation, as well as 3D imaging. The array consists of 64 piezoelectric 40-kHz transducers (Murata MA40S4S), spanning an aperture of 200 mm. First, we provide an application-independent numerical and experimental characterization of the conventional beamforming performance of all operation modes for varying focal directions and distances. Second, we examine the resulting imaging capabilities using the single line transmission technique. Apart from the high maximum sound pressure level of 152 dB, we validate that unambiguous high-accuracy 3D imaging is possible in a wide field of view (±80 • ), long range (20 cm to 5 m+) and with a high angular resolution of up to 2.3 • . Additionally, we demonstrate that object shapes and patterns of multiple reflectors are recognizable in the images generated using a simple threshold for separation. In total, the imaging capabilities achieved are promising to open up further possibilities, e.g. robust object classification in harsh environments based on ultrasonic images.
In this work we combine a multipath ultrasonic gas flow meter (UFM) with an ultrasonic air-coupled phasedarray. This allows complementing the advantages of a multipath UFM, i.e. higher accuracy and more robustness to irregular flow, with the extended velocity measuring range due to sound drift compensation via a phased-array. We created a 3D-printed flow meter consisting of an 8 × 8 λ/2 phased-array for transmission and 14 individual receivers for seven upstream and seven downstream sound paths. Measurements were conducted in a test rig with a maximum gas flow rates of 8.3 m 3 s −1 (107 m s −1 ). A differential pressure nozzle was used as reference sensor. Three configurations were compared: Parallel sound paths with a single transmitter; parallel sound paths with the phasedarray as transmitter; and fan-shaped sound paths with the phased-array as transmitter. The signal-to-noise ratio (SNR) and deviation of measured flow were used as comparison criteria. In addition, we measured the optimum steering angles of the phased-array required to compensate the sound drift effect. Using the phased-array with the sound drift effect compensation enabled and disabled, the SNR increases by 10.6 dB and 4.95 dB, respectively, compared to the single transmitter setup at 83 m s −1 . Furthermore, the phased-array with compensation active, extends the velocity measuring range by 29%, from 83 m s −1 to 107 m s −1 , while maintaining a similar standard deviation of the flow measured. Besides demonstrating that a phased-array in a gas flow meter significantly extends the measurement range, our setup qualifies as versatile research platform for designing future high-velocity gas flow meters.
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