Venting in the comparative study of flexural ultrasonic transducers to improve resilience at elevated environmental pressure levels

The classical form of a flexural ultrasonic transducer is a piezoelectric ceramic disc bonded to a circular metallic membrane. This ceramic induces vibration modes of the membrane for the generation and detection of ultrasound. The transducer has been popular for proximity sensing and metrology, particularly for industrial applications at ambient pressures around 1 bar. The classical flexural ultrasonic transducer is not designed for operation at elevated pressures, such as those associated with natural gas transportation or petrochemical processes. It is reliant on a rear seal which forms an internal air cavity, making the transducer susceptible to deformation through pressure imbalance. The application potential of the classical transducer is therefore severely limited. In this study, a venting strategy which balances the pressure between the internal transducer structure and the external environment is studied through experimental methods including electrical impedance analysis and pitch-catch ultrasound measurement. The vented transducer is compared with a commercial equivalent in air towards 90 bar. Venting is shown to be viable for a new generation of low cost and robust industrial ultrasonic transducers, suitable for operation at high environmental pressure levels

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The influence of air pressure on the dynamics of flexural ultrasonic transducers

The flexural ultrasonic transducer comprises a piezoelectric ceramic disc bonded to a membrane. The vibrations of the piezoelectric ceramic disc induce flexural modes in the membrane, producing ultrasound waves. The transducer is principally utilized for proximity or flow measurement, designed for operation at atmospheric pressure conditions. However, there is rapidly growing industrial demand for the flexural ultrasonic transducer in applications including water metering or in petrochemical plants where the pressure levels of the gas or liquid environment can approach 100 bar. In this study, characterization methods including electrical impedance analysis and pitch-catch ultrasound measurement are employed to demonstrate the dynamic performance of flexural ultrasonic transducers in air at elevated pressures approaching 100 bar. Measurement principles are discussed, in addition to modifications to the transducer design for ensuring resilience at increasing air pressure levels. The results highlight the importance of controlling the parameters of the measurement environment and show that although the conventional design of flexural ultrasonic transducer can exhibit functionality towards 100 bar, its dynamic performance is unsuitable for accurate ultrasound measurement. It is anticipated that this research will initiate new developments in ultrasound measurement systems for fluid environments at elevated pressures

Wideband electromagnetic dynamic acoustic transducer as a standard acoustic source for air-coupled ultrasonic sensors

To experimentally study the characteristics of ultrasonic sensors, a wideband air-coupled ultrasonic transducer, wideband electromagnetic dynamic acoustic transducer (WEMDAT), is designed and fabricated. Characterisation methods, including electrical impedance analysis, laser Doppler vibrometry and pressure-field microphone measurement, are used to examine the performance of the WEMDAT, which have shown that the transducer has a wide bandwidth ranging approximately from 47 kHz to 145 kHz and a good directivity with a beam angle of around 20˚ with no evident side lobes. A 40 kHz commercial flexural ultrasonic transducer (FUT) is then taken as an example to receive ultrasonic waves in a pitch-catch configuration to evaluate the performance of the WEMDAT as an acoustic source. Experiment results have demonstrated that the WEMDAT can maintain the most of the frequency content of a 5 cycle 40 kHz tone burst electric signal and convert it into an ultrasonic wave for studying the dynamic characteristic and the directivity pattern of the ultrasonic receiver. A comparison of the dynamic characteristics between the transmitting and the receiving processes of the same FUT reveals that the FUT has a wider bandwidth when operating as an ultrasonic receiver than operating as a transmitter, which indicates that it is necessary to quantitatively investigate the receiving process of an ultrasonic transducer, demonstrating a huge potential of the WEMDAT serving as a standard acoustic source for ultrasonic sensors for various air-coupled ultrasonic applications

Measurement using flexural ultrasonic transducers in high pressure environments

The flexural ultrasonic transducer comprises a metallic membrane to which an active element such as a piezoelectric ceramic is attached. The normal modes of the membrane are exploited to generate and receive the desired ultrasonic wave. Flexural ultrasonic transducers are popular due to their ability to couple to different media without requiring matching layers. There is growing demand for ultrasound measurement using flexural ultrasonic transducers in high pressure environments, such as in gas metering. However, their sealing increases the risk of transducer deformation as the pressure level is raised, due to pressure imbalance between the internal cavity of the transducer and the external environment. In this study, a novel form of flexural ultrasonic transducer for operation in high pressure environments, those above 100 bar, is shown alongside key measurement strategies. Different methods can be used to enable pressure equalization between the internal cavity and the external environment, one of which is venting and used in this study. Dynamic performance is monitored via pitch-catch ultrasound measurement in air up to 130 bar. The results suggest the suitability of the vented transducer for operation in high pressure environments compared to the classical flexural ultrasonic transducer, constituting a significant development in ultrasonic measurement

A novel mathematical model for transit-time ultrasonic flow measurement

The calculation of the averaged flow velocity along an ultrasonic path is the core step in ultrasonic transit-time flow measurement. The conventional model for calculating the path-averaged velocity does not consider the influence of the flow velocity on the propagation direction of the ultrasonic wave and can introduce error when the sound speed is not much greater than the flow velocity. To solve this problem, a new mathematical model covering the influence of the flow velocity is proposed. It has been found that the same mathematical expressions of the path-averaged flow velocity, as a function of the absolute time-of-flight (ToFs) of ultrasonic waves travelling upstream and downstream, can be derived based on either of the models. However, the expressions as a function of the time difference (the relative ToF) between the ultrasonic waves travelling upstream and downstream derived by the two models are completely different. Flow tests are conducted in a calibrated flow rig utilising air as flowing medium. Experimental results demonstrate that the path-averaged flow velocities, calculated using either the relative or the absolute ToFs based on the new model, are much more consistent and stable, whereas those calculated based on the conventional model have shown evident and increasing discrepancy when the flow velocity exceeds 15 m/s. When the flow velocity is around 39.45 m/s, the discrepancy is as high as 0.38 m/s. As the relative ToF can be more accurately, reliably and conveniently measured in real applications, the proposed mathematical model has a great potential for the increase of the accuracy of the ultrasonic transit-time flowmeters, especially for the applications such as the measurement of fluids with high flow velocities

Active damping of ultrasonic receiving sensors through engineered pressure waves

Transducers for ultrasonic sensing and measurement are often operated with a short burst signal, for example a few cycles at a specific excitation voltage and frequency on the generating transducer. The vibration response of a narrowband transducer in detection is usually dominated by resonant ringing, severely affecting its ability to detect two or more signals arriving at the receiver at similar times. Prior researchers have focused on strategies to damp the ringing of a transducer in transmission, to create a temporally short output pressure wave. However, if the receiving transducer is narrowband, the incident pressure waves can create significant ringing of this receiving transducer, irrespective of how temporally short the incident pressure waves are on the receiving transducer. This can reduce the accuracy of common measurement processes, as signals are temporally long and multiple wave arrivals can be difficult to distinguish from each other. In this research, a method of damping transducers in reception is demonstrated using a flexural ultrasonic transducer. This narrowband transducer can operate effectively as a transmitter or receiver of ultrasound, and due to its use in automotive applications, is the most common ultrasonic transducer in existence. An existing mathematical analog for the transducers is used to guide the design of an engineered pressure wave to actively damp the receiving flexural ultrasonic transducer. Experimental measurements on transducers show that ultrasonic receiver resonant ringing can be reduced by 80%, without significantly compromising sensitivity and only by using a suitable driving voltage waveform on the generating transducer

Oil filled flexural ultrasonic transducers for resilience in environments of elevated pressure

In recent years, flexural ultrasonic transducers (FUTs) have gained popularity in a wider scope of applications, due to their robust design and efficient coupling to different fluids. They comprise a metallic membrane with a piezoelectric ceramic bonded to its underside, typically protected with a silicone backing to seal the FUT from its environment. However, the sealed interior of the commercially available and widely used FUT has restricted its application in environments above 1 bar, where pressure imbalances are known to lead to unstable dynamic performance, and deformation of the piezoelectric-membrane structure and the housing of the transducer. The recently reported approach of venting, such as the removal of the hermetic seal, has been shown to boost the resilience of FUTs to environments of elevated pressure, but an alternative approach is needed to prevent exposure of sensitive internal structures within the transducer to an external fluid. In this study, a novel FUT design for ultrasound measurement in elevated pressure environments is proposed, where the vibrating membrane is backed with an incompressible fluid comprising a non-volatile oil. Prototype oil-filled flexural ultrasonic transducers (OFFUTs) are fabricated, and their dynamic performance monitored through acoustic microphone, electrical impedance, and pitch-catch ultrasound measurements. Enhanced resilience of the OFFUT to environmental pressures approaching 200 bar is displayed, expanding the potential applications of this device towards challenging flow and gas monitoring systems