Calibration of Air-Coupled Ultrasonic Phased Arrays. Is it worth it?

Abstract

Air-coupled ultrasonic phased arrays are utilized for many emerging applications and typically consist of bending-plate piezoelectric ultrasonic transducers (PUT). Due to their narrow bandwidth, the system response of PUTs is highly sensitive to manufacturing tolerances causing relative amplitude and phase errors in an array configuration. We analyze the impact of these errors for different deviation limits on the transmit beam pattern (BP) using Monte-Carlo (MC) simulations. In addition, we conduct an experimental calibration of a real-world 40-kHz PUT array consisting of 64 pre-selected Murata MA40S4S transducers. Here, the transmit amplitude and phase errors are measured in the free-field and compensated by individually pulse-width modulated and time-delayed excitation signals. The BPs before and after the calibration are analyzed and compared to the ideal BP and the improvements are highlighted. The MC simulations show that the amplitude and phase errors can severely affect the BP, e.g. an increase in maximum side lobe level (MSLL) up to 75%. After the experimental calibration, the BP measured is in good agreement to the ideal simulated pattern with a minor MSLL increase of 14%. We conclude with recommendations for the calibration and pre-selection methodology and discuss the pros and cons.