Spiral Air-Coupled Ultrasonic Phased Array for High Resolution 3D Imaging

Spiral-shaped phased arrays are widely used for sound source localization and are trending in other fields, e.g. 3D medical ultrasound imaging. The non-uniform element geometry allows grating lobe free beamforming without an inter-element spacing of half wavelength. We demonstrate the 3D imaging capabilities of an air-coupled ultrasonic 40-kHz phased array based on 64 Murata MA40S4S transducers, spirally arranged on a 200 mm aperture. The 3D image is generated by sequentially transmitting beamformed pulses in discrete directions and capturing the corresponding echo signals, using all transducers. The GPU accelerated array signal processing consists of matched filtering, conventional beamforming and envelope extraction. The origins of the echos are visualized in B-Scans and 3D-Scans using OpenGL. We characterize the far-field transmit and receive directivity patterns, as well as the angular resolution in an anechoic chamber. The main lobe widths and maximum side lobe levels of the transmit (2.6°, -11.5 dB), the receive (2.6°, -11.6 dB) and the ideal model directivity patterns (2.4°, -13.4 dB) are in good agreement. Two laterally positioned rods (Ø10 mm) are separately detectable with an angular resolution of 2.3 °. The localization of multiple scatterers at the same range (2 m) is demonstrated by a test pattern of corner reflectors. Overall, the spiral array allows highly directional sound beam steering and localization, making it ideal for imaging and other in-air applications.