Close-Distance Object Tracking With Air-Coupled PMUT Arrays: Reducing Blind Zone and Improving Axial Resolution

In this paper, we report a set of methods to reduce the blind zone and improve the axial resolution and accuracy of an object tracking system based on air-coupled Piezoelectric Micromachined Ultrasonic Transducers (PMUTs). These methods require few computational resources and can therefore be deployed in the field to enable a self-calibrating measuring system. The proposed methodology comprises a calibration procedure and custom waveform design. The automatic calibration procedure identifies optimal inverted pulses to suppress the transmitter's free ringing (ringdown). The inverted pulses are found one at a time, by firstly sweeping the pulse timing, then sweeping the pulse duration. The ringdown energy is used as the objective function to be minimized and energy-based checks guarantee convergence. We achieved suppression in the -30 to -36 dB range. We then experimentally demonstrate that the obtained waveform, which includes a wait time between the driving pulses and the inverted pulses, and is designed to produce a beating effect with the receiver, reduces the blind-zone to less than 1 cm and improves the axial resolution and accuracy of an object-tracker with negligible impact on sensitivity.