Sivaram Nishal Ramadas

Tapered transmission line technique based graded matching layers for thickness mode piezoelectric transducers

Conventionally, in order to acoustically match thickness mode piezoelectric transducers to a low acoustic impedance load medium, multiple quarter wavelength (QW) matching layers are employed at the front face of the device. Typically a number of layers, 2–4 in number, are employed resulting in discrete impedance steps within the acoustic matching scheme. This can result in impedance matching with limited bandwidth characteristics. This paper investigates the application of tapered transmission line filter theory to implement a graded impedance profile, through the thickness of the matching layer scheme, to solve the impedance mismatch problem whilst accounting for enhanced transducer sensitivity and bandwidth.

Versatile air-coupled phased array transducer for sensor applications

We introduce a versatile one-dimensional (1D) air-coupled phased array transducer device operating at 40 kHz. It allows both - transmit beam steered ultrasound at high sound pressure level and it can sense beam steered ultrasound. This device opens the door to many new sensory applications. Conventionally, low frequency sensor array design for air-coupled application was not possible due to the inter element spacing (also referred as pitch) criteria that needs to be satisfied for given applications. We overcome this limitation by introducing a smart packaging layer that separates the acoustic aperture from the actual radiating aperture of the ultrasonic transducer. By doing this, we ensure that the half wavelength criteria is fulfilled. The proposed new design leads to a wide operation range without any undesirable grating lobes. A proof of concept prototype device was built and its performance evaluated in the laboratory. An impressive 130 ± 1 dB sound pressure level (SPL) was observed at a distance of 1m. The prototype also was capable of steering approximately 110° in total, in both transmit and receive mode of operation.

Phased array transducer for emitting 40-kHz air-coupled ultrasound without grating lobes

We introduce an ultrasonic one-dimensional (1D) air-coupled phased array transducer, operating at 40 kHz without any grating lobes. It is well known, that this achievement is only possible when each transducer element is small enough for an element pitch that is equal to or less than half of the wavelength, i.e. ≤ 4.3 mm for 40 kHz operation frequency. As far as we know, conventionally low frequency array designs for air-coupled transducer applications were not possible at 40 kHz due to this inter element spacing requirement. The main idea is to separate the acoustic aperture from the actual radiating aperture of the ultrasonic transducer. We use acoustic waveguides, forming a smart packaging layer, to fulfill the half-wavelength criteria for the pitch and to benefit from concentrating the acoustic energy from several single ultrasonic transducers into a smaller effective aperture. A proof-of-concept prototype array was built and characterized. An impressive 130 ± 1 dB sound pressure level (SPL) is observed at a distance of 1 m. The opening angle of the array is 110° in total, without any grating lobes. For future work, we plan extending the approach to build an air-coupled fully populated 2D phased array transducer as well.

The use of fractal geometry in the design of piezoelectric ultrasonic transducers

The geometry of composite piezoelectric ultrasonic transducers is typically regular and periodic with one dominant length scale. In many applications there is motivation to design transducers that operate over a wide bandwidth so that, for example, signals containing a broad frequency content can be received. The devices length scale will dictate the central operating frequency of the device and so, in order to construct a wide bandwidth device, it would seem natural to design a device that contains a range of length scales. The objective of this article therefore is to consider one such transducer design and build a theoretical model to assess its performance. For the composite geometry a fractal medium is chosen as this contains a wide range of length scales. Numerical results of a theoretical model are presented. They suggest that this device would have a three-fold improvement in the reception sensitivity bandwidth as compared to a conventional composite design. Finite-element analysis provides information on the effect of poling on the devices performance. A preliminary experimental investigation was undertaken, with a Sierpinski gasket fractal transducer design, and good correlation between the simulated and experimentally measured operation was observed.

Flexural mode metal cap transducer design for specific frequency air coupled ultrasound generation

Flexural transducers are effective ultrasonic generators in fluid media, where standard piezoelectric transducers suffer a significant performance loss due to a large impedance mismatch. The flexural modes of piezoelectrically actuated metal caps are routinely used to make low frequency (typically 40 kHz) air coupled transducers for simple distance measurements. Such transducer types offer many intrinsic advantages including an integrated metal buffer for environmental shielding, good fluid coupling for generation and detection of ultrasound, and large amplitude signals for a low driving voltage. In this work, we investigate the design of arbitrary and specifically higher frequency (> 100 kHz) flexural metal cap probes. The analytical theory of vibrating plates was used to determine how the geometry of the cap affects the frequencies of its normal modes. Finite element modelling (FEM) was used to simulate a more realistic system. A first set of prototype transducers was built and investigated. The prototype behaviour is in general agreement with the theoretical and FEM models, but with shifted modal frequencies. The prototype transducers have a strong mode at 140 kHz, which can be used to generate ultrasound in air.

A plate waveguide design for ultrasonic flow measurements in hostile environments

A waveguide bundle consisting of parallel, thin, metallic plates has been designed to be used as a thermal buffer in an ultrasonic flow meter, where each of the plates acts as an independent, low dispersion waveguide. An initial prototype has been built and characterized using both finite element modeling and experimental measurements. Initial experiments in water have shown that even with little optimization, it is possible to use these waveguides in transmit-receive type experiments.

A wideband annular piezoelectric composite transducer configuration with a graded active layer profile

Ultrasound technology is routinely used in many application areas including underwater sonar, biomedicine, non destructive evaluation (NDE), materials characterisation and process control - all with direct routes into the vital economic sectors of energy, transportation, healthcare and food and drink. As technology demands have increased, device manufacturers are faced with a constant need to extend bandwidth and/or frequency response, while at the same time improve sensitivity, all combined with minimisation of size, complexity and cost. It is becoming increasingly apparent that the current transducer technology, with its emphasis on multi-element array systems, is nearing saturation and new approaches to ultrasonic system design and operation are needed to satisfy many future demands. This paper presents wideband annular piezoelectric configuration with graded composite active layer.

Knowledge based approach for design optimization of ultrasonic transducers and arrays

Ultrasonic transducers and arrays are used routinely in a diverse range of applications, including biomedicine, non-destructive testing, sonar and process monitoring. This paper presents a knowledge based approach for design and optimization of complex ultrasonic transducer systems. Examples of the design optimization process are illustrated. The first of these relates to the implementation of a front face matching layer, designed to promote bandwidth and sensitivity in relatively straightforward thickness mode applications. This is followed by a multi-layered structure comprising different zones of piezoelectric polarisation. A mathematical model has been developed and used for simulation purposes, with the number and dimensions of the active layers, along with passive matching layer information being fed as variable parameters for the optimizer. The results are shown to compare very favourably with conventional methods and serve to demonstrate that the knowledge-based approach is a very efficient and attractive alternative. It may be implemented for design and optimization of any kind of thickness mode transducer system, subject to the availability of an appropriate simulator.

Extending the receive performance of phased ultrasonic transducer arrays in air down to 40 kHz and below

We present a versatile ultrasonic one-dimensional (1D) air-coupled phased array transducer with a focus on its receive performance. In addition to its beam steering capability in transmit mode, it can be used to listen to ultrasonic waves at 40 kHz and below. We fulfill the half-wavelength criteria for the element spacing by introducing an additional layer, consisting of many tapered sound tubes (waveguides). These sound tubes separate the effective acoustic aperture from the actual receiving aperture of several ultrasonic transducers. This ensures that the array is not affected by grating lobes over a wide range of steering angles, and, thus, the array is capable of receiving directed ultrasound with only one primary receive direction for an adjustable angle (receive-beam forming). Based on this approach, we built and characterized a proof-of-concept prototype device. Our results prove that the array is able to receive ultrasonic waves over a wide range of 110° in total, with a receive sensitivity of -55.9 dB (0 dB equal 1 V/Pa). Thus, the device has huge potential for new sensory applications and the approach is compatible with micromachined ultrasonic transducers as well.

Matching layers design for a plate waveguide ultrasonic transducer for flow measurement in hostile environments

A wetted ultrasonic transducer with an integrated thermal buffer could allow ultrasonic techniques to be used for flow measurement in harsh environments. We have previously presented one such buffer design, consisting of parallel stainless steel strip waveguides. One disadvantage of such a design is the large impedance mismatch which is created between the buffer material and the test fluid, reducing the transmitted energy. The addition of a matching layer, with an intermediate acoustic impedance, to the waveguide can assist in reducing this effect. Such matching layers are commonly used in conventional ultrasonic transducer design, however for this application additional consideration must be given to the material selection to ensure they are suitable for use in hostile environments. In this work we have investigated several materials, selected for their thermal stability, which were then modified by loading, in order to tailor the acoustic properties to best suit our application. These materials have then been investigated using both finite element modelling and experimental techniques quantify their effectiveness as matching layers for a strip waveguide transducer system. This work has shown that and increase in the maximum emitted pressure by a factor of 3.4 may be obtained using a tungsten loaded matching layer.