Development of an Accurate and Robust Air-Coupled Ultrasonic Time-of-Flight Measurement Technique

Abstract

Ultrasonic time-of-flight (ToF) measurements enable the non-destructive characterization of material parameters as well as the reconstruction of scatterers inside a specimen. The time-consuming and potentially damaging procedure of applying a liquid couplant between specimen and transducer can be avoided by using air-coupled ultrasound. However, to obtain accurate ToF results, the waveform and travel time of the acoustic signal through the air, which are influenced by the ambient conditions, need to be considered. The placement of microphones as signal receivers is restricted to locations where they do not affect the sound field. This study presents a novel method for in-air ranging and ToF determination that is non-invasive and robust to changing ambient conditions or waveform variations. The in-air travel time was determined by utilizing the azimuthal directivity of a laser Doppler vibrometer operated in refracto-vibrometry (RV) mode. The time of entry of the acoustic signal was determined using the autocorrelation of the RV signal. The same signal was further used as a reference for determining the ToF through the specimen in transmission mode via cross-correlation. The derived signal processing procedure was verified in experiments on a polyamide specimen. Here, a ranging accuracy of <0.1 mm and a transmission ToF accuracy of 0.3μs were achieved. Thus, the proposed method enables fast and accurate non-invasive ToF measurements that do not require knowledge about transducer characteristics or ambient conditions.

Fluidic Ultrasound Generation for Non‐Destructive Testing

“…The resulting signals are then shifted and cross-correlated to obtain the ToF, as detailed in the experimental section. This approach refines the previously published through-transmission setup, [62] which employed only one LDV and one accelerometer, in order to allow true contactfree ToF measurements without requiring prior knowledge about the ambient speed of sound. The longitudinal velocity was then calculated from the ToF with knowledge of the specimen thickness.…”

“…Since the longitudinal acoustic velocity can be related to various material parameters of the specimen, this property was chosen as the quantity to be measured. For this purpose, the ToF of the signal was measured in a fully optical through‐transmission setup (see Figure a) based on a recently introduced method [ 62 ] that has been shown to achieve a ToF accuracy of less than 1 µs for transducer working distances larger than 40 mm. Two laser Doppler vibrometers, LDV 1 and LDV 2, are used to record the ultrasonic signal before entering the specimen and at its back wall, respectively.…”

“…A piezoelectric ACU transducer NCG100-S63 (Ultran Group, USA) with a center frequency of 80 kHz [40,62] was used for comparison with commercial systems. The same measurement setup was used.…”

Ultrasound and Nanomedicine for Cancer-Targeted Drug Delivery: Screening, Cellular Mechanisms and Therapeutic Opportunities

“…The ultrasonic wave is conducted at the interface of different medium densities, and the probe reflects the energy [26]. The strength of this reflection is proportional to the electric signal generated [27]. The resulting electric signal transformation is presented as a grayscale image for subsequent visual interpretation [28].…”

OsciCheck: A Novel Fluidic Transducer for Air-Coupled Ultrasonic Measurements

“…3a). To enable fully non-contact TOF measurements, a novel all-optical measurement technique [26] was used, that is sketched in Figure 3b. In this method, two laser Doppler vibrometers (LDV) are used.…”