Tobias J. R. Eriksson

Experimental and simulation characterisation of flexural vibration modes in unimorph ultrasound transducers

A unimorph flexural transducer design is proposed and tested with regard to mode shapes and frequencies. The transducers consist of a passive metal cap structure, and a thin piezoelectric disc, rigidly bonded to the inside. Extensive finite element (FE) modelling, and experimental 2D, time-resolved displacement measurements were done to characterise the transducers flexural properties, and to compare them to the analytical solutions of thin vibrating plates. Emphasis was put on characterising the passive layer of the unimorph structure, before bonding the piezoelectric element, to understand how the active element affects the behaviour of the flexing plate. A high power Nd:YAG laser was used to actuate the metal plate (non-contact), and the frequency content of the resulting displacement signal was analysed to identify the flexural modes. The non-axisymmetric modes, which are conventionally disregarded because of their unfavourable acoustic properties, were also taken into account. There was excellent agreement between the experimental results and the FE simulation data. There was good agreement with the analytical edge clamped plate model, but with some notable deviations, which have not previously been identified or commented upon. Specifically, the second axisymmetric mode is split into three separate modes, which is not explained by the traditional theory of vibrating plates.

Experimental Evaluation of Three Designs of Electrodynamic Flexural Transducers

Three designs for electrodynamic flexural transducers (EDFT) for air-coupled ultrasonics are presented and compared. An all-metal housing was used for robustness, which makes the designs more suitable for industrial applications. The housing is designed such that there is a thin metal plate at the front, with a fundamental flexural vibration mode at ∼50 kHz. By using a flexural resonance mode, good coupling to the load medium was achieved without the use of matching layers. The front radiating plate is actuated electrodynamically by a spiral coil inside the transducer, which produces an induced magnetic field when an AC current is applied to it. The transducers operate without the use of piezoelectric materials, which can simplify manufacturing and prolong the lifetime of the transducers, as well as open up possibilities for high-temperature applications. The results show that different designs perform best for the generation and reception of ultrasound. All three designs produced large acoustic pressure outputs, with a recorded sound pressure level (SPL) above 120 dB at a 40 cm distance from the highest output transducer. The sensitivity of the transducers was low, however, with single shot signal-to-noise ratio (SNR)≃15 dB in transmit–receive mode, with transmitter and receiver 40 cm apart.

Flexural transducer arrays for industrial non-contact applications

A new method of constructing robust air-coupled flexural ultrasound arrays is suggested, and results from a prototype transducer are presented. The flexural elements are defined on a single sheet of metal by bonding a baffle structure to the back of the sheet. Piezoelectric elements were attached directly to the metal plate in the centre of the holes in the baffle. The aim was to find if the baffle could sufficiently separate the elements, such that each element behaved like a flexural transducer with radius equal to the radius of the holes in the baffle. By measuring the front face displacement of the array the vibration mode of individual elements could be characterised and compared to those of a single element transducer. In general the baffle sufficiently separated the flexural elements, such that the vibration modes observed corresponded to those of an edge clamped plate with a radius 1.5 mm larger than that of the holes in the baffle. This is a small shift, which is explained by the outer boundary being less well defined for the array elements, compared to a single element flexural transducer.

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.

Design of flexural ultrasonic phased array for fluid-coupled applications

A design of a 4×4 ultrasonic phased array working in flexural mode is presented. The array consists of an elastic metal sheet, a baffle with 16 holes, a back plate and 16 piezoelectric discs. The active area of each flexural array element is defined by the diameter of the holes of the baffle and the back plate provides an additional clamped-edge-like boundary condition through the baffle for each flexural element. A finite element analysis is utilized to investigate the influence of the dimensions and the materials on the performance of the array. Optimal ratios of the radius of piezo disc to the radius of elastic element working at (0, 0) mode and (1, 0) mode are obtained. A direct comparison of the radiation patterns of the flexural transducers working at (0, 0) and (1, 0) modes proves that the (0, 0) mode is preferable as working mode for the array. The dimensions and the materials of the baffle and the back plate are chosen to effectively reduce the mechanical cross talk between the neighboring array elements. A flexural ultrasonic phased array prototype is fabricated and characterized. Experiments indicate that the ultrasonic beam of the array can be continuously steered from 0° to 50°. This proof-of-concept design demonstrates our low-cost flexural ultrasonic phased arrays design to be sufficiently robust for various fluid-coupled applications.

Air-coupled flexural electrodynamic acoustic transducers

Flexural transducers use the bending modes in a plate or membrane to produce sound in low acoustic impedance media. Traditionally, piezoelectrically actuated flexural transducers have been used to generate ultrasound with large amplitude for a relatively low excitation voltage. In this work, the use of electrodynamic forces generated by a current carrying coil is investigated, as an alternative method for generating ultrasound by flexural vibrations. Using a coil instead of a piezoelectric element makes the transducer easier to manufacture, and able to operate at high temperatures. The analytical theory of vibrating plates as well as finite element modelling was used to predict transducer behaviour, i.e. mode frequencies and shapes of the vibrating front face. Prototype transducers were made from aluminium with a pancake copper coil at the back for generation. A Polytec laser vibrometer was used to measure the front face displacement of these prototype transducers. The displacement measurements revealed a frequency spectrum with narrowband (~3 kHz full width half maximum) modal frequency peaks, and a dominant fundamental mode at ~50 kHz. The spectrum is in good agreement with calculated frequency values, and the experimental mode shapes are similar to those predicted by theory.

Ultrasound visualization using polymer dispersed liquid crystal sensors

The acousto-optic effect in liquid crystals (LCs) has previously been exploited to build large area acoustic sensors for visualising ultrasound fields, opening up the field of acoustography. There is an opportunity to simplify this technique and open new application areas by employing polymer dispersed LC (PDLC) thin films instead of aligned LC layers. In PDLCs, the normally opaque film becomes transparent under the influence of an acoustic field (e.g. when surface acoustic waves are propagating in the material under the film). This is called acoustic clearing and is visible by eye. There is potential for producing ultrasonic sensors which can be ‘painted on’ to a component, giving direct visualisation of the ultrasonic field without requiring scanning. We demonstrate the effect by using PDLC films to characterise a resonant mode of a flexural air-coupled transducer. Visualisation was quick, with a switching time of a few seconds. The effect shows promise for ultrasound sensing applications for transducer...

High temperature flexural ultrasonic transducer for non-contact measurement applications

A prototype flexural ultrasound transducer capable of operating at high temperatures was designed for noncontact measurement applications. A doped bismuth titanate was used as the piezoelectric element; the construction of the transducer was designed using materials and bonding capable of operating at temperatures up to 500°C. The bismuth titanate was characterised by X-ray diffraction, differential thermal analysis and impedance analysis; the transducer response was measured using laser interferometry at room temperature. The resulting frequency spectrum showed clear resonance peaks, indicative of an operational flexural transducer.

Metal cap flexural transducers for air-coupled ultrasonics

Ultrasonic generation and detection in fluids is inefficient due to the large difference in acoustic impedance between the piezoelectric element and the propagation medium, leading to large internal reflections and energy loss. One way of addressing the problem is to use a flexural transducer, which uses the bending modes in a thin plate or membrane. As the plate bends, it displaces the medium in front of it, hence producing sound waves. A piezoelectric flexural transducer can generate large amplitude displacements in fluid media for relatively low excitation voltages. Commercially available flexural transducers for air applications operate at 40 kHz, but there exists ultrasound applications that require significantly higher frequencies, e.g. flow measurements. Relatively little work has been done to date to understand the underlying physics of the flexural transducer, and hence how to design it to have specific properties suitable for particular applications. This paper investigates the potential of the ...