The high frequency flexural ultrasonic transducer for transmitting and receiving ultrasound in air

Flexural ultrasonic transducers are robust and low cost sensors that are typically used in industry for distance ranging, proximity sensing and flow measurement. The operating frequencies of currently available commercial flexural ultrasonic transducers are usually below 50 kHz. Higher operating frequencies would be particularly beneficial for measurement accuracy and detection sensitivity. In this paper, design principles of High Frequency Flexural Ultrasonic Transducers (HiFFUTs), guided by the classical plate theory and finite element analysis, are reported. The results show that the diameter of the piezoelectric disc element attached to the flexing plate of the HiFFUT has a significant influence on the transducer's resonant frequency, and that an optimal diameter for a HiFFUT transmitter alone is different from that for a pitch-catch ultrasonic system consisting of both a HiFFUT transmitter and a receiver. By adopting an optimal piezoelectric diameter, the HiFFUT pitch-catch system can produce an ultrasonic signal amplitude greater than that of a non-optimised system by an order of magnitude. The performance of a prototype HiFFUT is characterised through electrical impedance analysis, laser Doppler vibrometry, and pressure-field microphone measurement, before the performance of two new HiFFUTs in a pitch-catch configuration is compared with that of commercial transducers. The prototype HiFFUT can operate efficiently at a frequency of 102.1 kHz as either a transmitter or a receiver, with comparable output amplitude, wider bandwidth, and higher directivity than commercially available transducers of similar construction

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

Shear horizontal (SH) ultrasound wave propagation around smooth corners

Shear horizontal (SH) ultrasound guided waves are being used in an increasing number of non-destructive testing (NDT) applications. One advantage SH waves have over some wave types, is their ability to propagate around curved surfaces with little energy loss; to understand the geometries around which they could propagate, the wave reflection must be quantified. A 0.83 mm thick aluminium sheet was placed in a bending machine, and a shallow bend was introduced. Periodically-poled magnet (PPM) electromagnetic acoustic transducers (EMATs), for emission and reception of SH waves, were placed on the same side of the bend, so that reflected waves were received. Additional bending of the sheet demonstrated a clear relationship between bend angles and the reflected signal. Models suggest that the reflection is a linear superposition of the reflections from each bend segment, such that sharp turns lead to a larger peak-to-peak amplitude, in part due to increased phase coherence

Articulating Inequalities: a linguistic ethnographic account of race and class in an undergraduate architecture studio in England

In a political climate that has seen increasingly urgent and often separately articulated claims for social justice on the grounds of racial and class-based inequalities, Higher Education as a whole, and Architectural Education in particular, have consistently reported stark and persistent inequalities based on measures relating to class and ethnicity. Policy responses aiming to address these inequalities have been criticised for employing fixed and separate identity categories and for positioning students of colour as the deficient embodiment of social problems. More recently, these measures have been employed in government-commissioned reports and subsequent policy as justification for the denial of structural and institutional racism. In Higher Education, policy discourse around racial inequality has been displaced by a discourse of individual choice and agency. Addressing these issues, this thesis presents the findings of a linguistic ethnographic study into race and class in an undergraduate architecture studio. It employs a critical sociolinguistic approach that sees social structures and the categories of inequality they produce as reproduced and resisted in everyday social interaction (Silverstein, 2003; Bucholtz & Hall, 2005). Drawing on anti-essentialist study of race and class in Britain, the study engages Stuart Hall’s notion of articulation, to treat the discursive construction of race and class as co-constituted articulations in material conditions of inequality produced by specific histories (Hall, 2021 [1980]). Accordingly, the study is situated in specific histories of race and class in England (Shilliam, 2018; Virdee, 2014; Kundnani, 2021). The thesis finds the discursive and ideological conditions navigated in the architecture studio to be characterised by three interrelated centres of authority: racially hegemonic whiteness, self-responsible deservedness, and conviviality. In showing how Higher Education is hegemonically white, the findings suggest the importance of avoiding neoliberal meritocratic framings of choice and agency and of fostering the potential of convivial relations amidst racism

A closed-loop operation to improve GMR sensor accuracy

Giant magnetoresistance (GMR) magnetic field sensors are compact, low power, high sensitivity devices that are low cost and have very simple supporting electronics. One of the disadvantages of GMR sensors can be their nonlinearity, hysteresis, and temperature-dependent output, which can reduce measurement accuracy. This paper presents an approach to improve the measurement accuracy of GMR sensors using a closed-loop circuit, which includes the sensor, a biasing coil, and a feedback circuit. The current in the biasing coil is actively changed to ensure that the component of magnetic field along the sensitive axis of the device is held constant, so that as the external magnetic field or orientation of the GMR sensor changes, the output of GMR sensor remains stable. In this way, the external magnetic field component along the sensitive axis of the device can be calculated by measuring the current in the biasing coil surrounding the GMR sensor, regardless of the hysteresis and nonlinearly of GMR sensor. The linearity and the accuracy of magnetic field measurements using a GMR sensor are significantly improved and a hardware prototype has been constructed and tested under a reference magnetic field

Mode mixing in shear horizontal ultrasonic guided waves

SH guided waves are used increasingly for non-destructive testing (NDT) applications, particularly for pipes and pipe supports using circumferentially guided wave modes. In practical implementations, it is not always straightforward to ensure single-mode operation and this requires consideration when interpreting results. During shear horizontal (SH) wave generation or SH guided wave interaction with geometrical changes or defects, multiple SH guided wave modes may be produced, depending on the shear wave speed, the frequency of operation, the thickness of the sample and the transducer characteristics. This paper discusses the interference patterns created as the multiple SH modes mix (for both continuous tone generation and short bursts), and the problems caused by the interference patterns on applications such as NDT. In particular, the patterns can lead to defects being missed during an NDT inspection using SH waves, and a way to circumvent this problem is suggested

Analysis of electrical resonance distortion for inductive sensing applications

Resonating inductive sensors are increasingly popular for numerous measurement techniques, not least in non-destructive testing (NDT), due to the increased sensitivity obtained at frequencies approaching electrical resonance. The highly unstable nature of resonance limits the practical application of such methods while no comprehensive understanding exists of the resonance distorting behaviour in relation to typical measurements and environmental factors. In this paper, a study into the frequency spectrum behaviour of electrical resonance is carried out exploring the effect of key factors. These factors, known to distort the electrical resonance of inductive sensors, include proximity to (or lift-off from) a material surface, and the presence of discontinuities in the material surface. Critical features of resonance are used as metrics to evaluate the behaviour of resonance with lift-off and defects. Experimental results are compared to results from a 2D finite element analysis (FEA) model that geometrically mimics the inductive sensor used in the experiments, and to results predicted by an equivalent circuit transformer model. The findings conclusively define the physical phenomenon behind measurement techniques such as near electrical resonance signal enhancement (NERSE), and show that lift-off and defect resonance distortions are unique, measurable and can be equated to exclusive variations in the induced variables in the equivalence circuit model. The resulting understanding found from this investigation is critical to the future development and understanding of a complete model of electrical resonance behaviour, integral for the design of novel sensors, techniques and inversion models

Dynamic nonlinearity in piezoelectric flexural ultrasonic transducers

Recent studies of the electro-mechanical behavior of flexural ultrasonic transducers have shown that their response can be considered as three distinct characteristic regions, the first building towards a steady state, followed by oscillation at the driving frequency in the steady state, before an exponential decay from the steady state at the transducer's dominant resonance frequency, once the driving force is removed. Despite the widespread industrial use of these transducers as ultrasonic proximity sensors, there is little published information on their vibration characteristics under different operating conditions. Flexual transducers are composed of a piezoelectric ceramic disc bonded to the inner surface of a metallic cap, the membrane of which bends in response to the high-frequency ceramic vibrations of the ceramic. Piezoelectric devices can be subject to nonlinear behavior, but there is no reported detail of the nonlinearity in flexural transducers. Experimental investigation through laser Doppler vibrometry shows strong nonlinearity in the vibration response, where resonance frequency reduces with increasing vibration amplitude

Shear Wave EMAT Thickness Measurements of Low Carbon Steel at 450˚C without Cooling

Ensuring reliability of components operating at high temperature, such as pipelines and boilers, within a variety of industries is of importance in the asset management process, and is implemented via regular inspections and condition monitoring. Performing online inspections without the need for plant shutdown is highly desirable. Development of portable or permanently installed high temperature ultrasonic sensors without sample surface preparation remains a key challenge. There are examples of high temperature piezoelectric sensors operating without cooling, but these usually require welding or brazing. Actively cooled electromagnetic acoustic transducers (EMATs) have previously been used for thickness measurements and defect detection to over 1000 ˚C. High temperature EMAT operation requires active cooling for permanent magnet EMATs [1] or a large electromagnet [2], limiting their use in some industrial settings. Low carbon steel pipelines operating at elevated temperatures often develop a magnetostrictive oxide surface coating (magnetite), which greatly improves EMAT efficiency below the Curie point of the magnetite (~560 ˚C), and we are able to take advantage of this if we can indefinitely operate an EMAT at elevated temperature. In this work, a high temperature shear wave EMAT utilizing a proprietary high field, high Curie point, permanent magnet has been developed to generate ultrasonic thickness measurements on magnetite coated steel at temperatures up to 450 ˚C, without active cooling. Exploiting the high efficiency possible on magnetite coated surfaces, relatively high signal-to-noise ratios, in the region of 25 dB for single shot data, have been measured at 450 ˚C using this technique, despite increased ultrasound attenuation and reduced magnet field strength at elevated temperatures