High Accuracy Ultrasound Micro-Distance Measurements with PMUTs under Liquid Operation

Abstract

Ultrasonic systems driven by multi-frequency continuous waves (MFCW) have been used for range distance measurement, offering high accuracy in long and medium range distance estimation. However, the use of continuous waves in very short-distance measurements causes large errors due to multipath reflections. This paper presents a new strategy to estimate very short relative distances with high accuracy based on the use of multi-frequency pulsed waves (MFPW). The proposed strategy allows to avoid the multipath reflections that appear when continuous waves are used, and it improves the achieved accuracy compared to the original MFCW method. To validate it, an 80 µm square AlScN piezoelectric micromachined ultrasonic transducer (PMUT) was chosen as a transmitter while a hydrophone was utilized as a target and receiver, immersed in fluorinert (FC-70) as a propagation medium. Three independent and consecutive tone-burst signals were transmitted successively. The selected frequencies are f1 = 2.3962 MHz, f2 = 2.327 MHz and f3 = 2.1195 MHz, giving first and second-order resolutions of 6.88 and 0.79 µm/°, respectively. Experimental results show a ±6.2 μm measured range error in a range of 3.5 mm, and therefore it represents a good candidate for ultrasound micro-profilometer applications under liquid operation.

Review of Piezoelectric Micromachined Ultrasonic Transducers for Rangefinders

“…Noises and interferences in a pMUT rangefinder system blur the signal and thus reduce the range-finding accuracy. Primary noises and interferences include the thermal noise caused by the random motion of the medium molecules received by the pMUT, the thermal noise of the piezoelectric element, the noises from the receiver circuit, the crosstalk between adjacent pMUT elements, the environmental reflections, and the possible multipath fading if the signal is continuous [ 44 , 45 , 46 ]. The signal-to-noise ratio (SNR) describes and evaluates the effects of as well as the noise and interference level on the accuracy.…”

“…For instance, a pMUT rangefinder first achieving more than a five-meter detection range with over 11.5 dB SNR was proposed by Yang et al [ 31 ], where the working frequency was 66 kHz, which is lower than most previous designs. For in-liquid range-finding, the acoustic absorption is low, and the working frequency is usually in the MHz range [ 46 ] for higher bandwidth and larger Rayleigh distance.…”

“…MFCW-based pMUT rangefinder systems cannot achieve ideally high performance due to multipath reflections that are caused by the standing waves formed between the pMUT device and the target [ 45 ]. To eliminate this side effect, Zamora et al proposed a novel system configuration for pMUT rangefinders named a multi-frequency pulsed wave (MFPW) system in 2021 [ 46 ], where three continuous waves are replaced by three pulses, each of which contains a single sinusoidal wave with several cycles. Additionally, the number of cycles, as well as the interval between distinct pulses, are well designed to make different pulses distinguishable and have a time-of-flight that is more than the total length of the transmitted signal, as shown in Figure 19 .…”

“…An MFPW system has a similar calculation process as an MFCW system, but it avoids interferences caused by multipath fading because discontinuous signals cannot form standing waves. The pMUT rangefinder system in [ 46 ] has an extremely low error: lower than 6.2 μm within a 3.5 mm transmitter-receiver distance.…”

Monolithic Sensor Integration in CMOS Technologies

“…For example a chemical sensor can be fabricated by depositing an absorbing material sensitive to a specific chemical over a resonator, and then detecting its mass change [29], [30]. If we add piezoelectric materials to the manufacturing process, like the SilTerra MEMS-on-Top PMUT MEMS process [31], we can build Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) for applications like fluid properties monitoring [32], micron-range distance measurements [33] or for microimaging [34].…”