Standard Ultrasonic Transducers Research Articles

An in vitro demonstration of a passive, acoustic metamaterial as a temperature sensor with mK resolution for implantable applications

Wireless medical sensors typically utilize electromagnetic coupling or ultrasound for energy transfer and sensor interrogation. Energy transfer and management is a complex aspect that often limits the applicability of implantable sensor systems. In this work, we report a new passive temperature sensing scheme based on an acoustic metamaterial made of silicon embedded in a polydimethylsiloxane matrix. Compared to other approaches, this concept is implemented without additional electrical components in situ or the need for a customized receiving unit. A standard ultrasonic transducer is used for this demonstration to directly excite and collect the reflected signal. The metamaterial resonates at a frequency close to a typical medical value (5 MHz) and exhibits a high-quality factor. Combining the design features of the metamaterial with the high-temperature sensitivity of the polydimethylsiloxane matrix, we achieve a temperature resolution of 30 mK. This value is below the current standard resolution required in infrared thermometry for monitoring postoperative complications (0.1 K). We fabricated, simulated, in vitro tested, and compared three acoustic sensor designs in the 29–43 °C (~302–316 K) temperature range. With this concept, we demonstrate how our passive metamaterial sensor can open the way toward new zero-power smart medical implant concepts based on acoustic interrogation.

Structural model of standard ultrasonic transducer array developed for FEM analysis of mechanical crosstalk

One of the reasons of distortions in ultrasonic imaging are crosstalk effects. They can be divided into groups according to the way of their formation. One of them is constituted by mechanical crosstalk, which is propagated by a construction of a multi-element array of piezoelectric transducers. When an individual transducer is excited, mechanical vibrations are transferred to adjacent construction components, thereby stimulating neighboring transducers to an undesired operation. In order to explore ways of the propagation of such vibrations, the authors developed the FEM model of the array of piezoelectric transducers designed for calculations in COMSOL Multiphysics software. Simulations of activating individual transducers and calculated electrical voltages appearing on transducers unstimulated intentionally, were performed in the time domain in order to assess the propagation velocity of different vibration modes through the construction elements. On this basis, conclusions were drawn in terms of the participation of various construction parts of the array of piezoelectric transducers in the process of creating the mechanical crosstalk. The elaborated FEM model allowed also to examine the ways aimed at reducing the transmission of mechanical crosstalk vibrations through the components of the array. Studies showed that correct cuts in the fasteners and the front layer improve the reduction of the mechanical crosstalk effect. The model can become a helpful tool in the process of design and modifications of manufactured ultrasonic arrays particularly in terms of mechanical crosstalk reduction.

Investigation of fabricated 1 MHz lithium niobate transfer standard ultrasonic transducer

The fabrication of a single element transducer made from Lithium Niobate (LiNbO3) operating at 1 MHz is focused on this paper. The air-backed LiNbO3 transducer is developed to be used as the standard transfer ultrasonic transducer to calibrate the ultrasound power-meter, which is measured the total emitted acoustic power radiated from the medical equipment. To clarify the precision of the acoustic power, the primary standard calibration measurement (radiation force balance, RFB) based on IEC 61161 is used to investigate the fabricated transducer. The geometry of the piezoelectric active element was first designed by the prediction of Krimholtz, Leedom, and Matthaei (KLM) simulation technique. The electrical impedance measurements of the LiNbO3 element, before and after assembling into the transducer, were checked and compared. The results of electrical impedance show that the operating frequency is in the range from 1 MHz to 10 MHz by forming harmonics. The evaluations of total emitted power and radiation conductance of fabricated transducer were also revealed. Results of acoustic power have been responding up to 2.1 W, which can be assessed within 6% of expanded uncertainty (k = 2).

Ultrasound Doppler measurements inside a diaphragm valve using novel transducer technologies

In this project, velocity profiles were measured in a diaphragm valve using an ultrasonic velocity profiling (UVP) technique. A non-Newtonian CMC model fluid was tested in this highly complex geometry and velocity profiles were measured at four different positions at the centre (contraction) of a specially manufactured 50% open diaphragm valve. The coordinates of the complex geometry and velocity magnitudes were analysed and compared to the bulk flow rate measured using an electromagnetic flow meter. Two different ultrasonic transducers (standard and delay line) were used and results were compared in order to assess velocity data close to wall interfaces as well as the accuracy and magnitude of measured velocities. The difference between calculated and measured flow rates was 32% when using the standard ultrasonic transducers. The error difference decreased to 18% when delay line transducers were introduced to the measurements. The velocity data obtained in the diaphragm valve showed a significant improvement close to the wall interfaces when using the delay line transducers. The main limitation when using delay line transducers is that beam refraction can significantly complicate measurements in a highly complex geometry such as a diaphragm valve. A new delay line transducer with no beam refraction could provide a solution. The introduction of delay line transducers showed that UVP can be used as a powerful tool for detailed flow behaviour measurements in complex geometries.

High frequency guided ultrasonic waves for defect monitoring

High frequency guided ultrasonic waves offer a good compromise between area coverage and defect detection sensitivity for the health monitoring of critical sections of structures. Using standard ultrasonic transducers with single sided access to the structure, guided wave modes were generated that penetrate through the complete thickness of the structure. The wave propagation and interference of the guided wave modes depends on the frequency thickness product. Laboratory experiments were conducted using a laser interferometer and the wave propagation dependence on the wall thickness reduction verified against theoretical predictions. Measurements were conducted using accelerated corrosion in a salt water bath. From the measured signal changes due to the wave mode interference the wall thickness reduction due to corrosion was monitored. The energy transfer between the plate surfaces results in a good sensitivity for the detection of small defects on both surfaces. Experimentally the reflected wave was measured using standard pulse-echo equipment. Using a combination of evaluation in the time and frequency domain, the defect location and damaged plate side can be accurately determined. The high frequency guided waves have the potential for damage monitoring at critical and difficult to access locations from a stand-off distance.

Thick aluminium nitride films deposited by room-temperature sputtering for ultrasonic applications

Materials in film form for electromechanical transduction have a number of potential applications in ultrasound. They are presently under investigation in flexural transducers for air-coupled ultrasound and underwater sonar operating at frequencies up to a few megahertz. At higher frequencies, they have the potential to be integrated with electronics for applications of ultrasound requiring high spatial resolution. However, a number of fabrication difficulties have arisen in studies of such films. These include the high temperatures required in many thick and thin film deposition processes, making them incompatible with other stages in transducer fabrication, and difficulties maintaining film quality when thin film––typically sub-1 μm––processes are extended to higher thicknesses. In this paper, we first outline a process which has allowed us to deposit aluminium nitride (AlN) films capable of electromechanical transduction at thicknesses up to more than 5 μm without substrate heating. As an ultrasonic transduction material, AlN has functional disadvantages, particularly a high acoustic velocity and weak electromechanical transduction. However, it also has a number of advantages relating to practicality of fabrication and functionality. These include the ability to be deposited on a variety of amorphous substrates, a very high Curie temperature, low permittivity, and low electrical and mechanical losses. Here, we present experimental results highlighting the transduction capabilities of AlN deposited on aluminium electrodes on glass and lithium niobate. We compare the results with those from standard simulation processes, highlighting the reasons for discrepancies and discussing the implications for incorporation of AlN into standard ultrasonic transducer design processes.

Ultrasonic study for detection of inner diameter cracking in pipeline girth welds using creeping waves

The assessment of pipeline girth welds in power plant equipment using a fitness for purpose approach relies heavily on accurate detection and estimation of defect size [1,2]. This is difficult to achieve with commonly used techniques employing standard ultrasonic transducers. Defects, particularly tight cracks located at the inner diameter (ID) of a girth weld and very close to the root of the weld, are difficult to detect and resolve from indications due to severe root undercut and other geometric defects. This could be overcome by the application of creeping wave probes. The secondary creeping wave, which is generated at the ID of the pipe and propagates on the ID surface, will respond to only ID cracking close to the root of the weld without generating any interfering echoes from the geometric defects. The high strength creep resistant martensitic steel ‘X-20’ used in power cycle piping is highly susceptible to cracking either due to the welding cycle or due to stress corrosion. Recently, the failure of girth welds in X-20 pipes revealed tight cracks at the ID very close to the root. The special ultrasonic techniques using the creeping wave probes revealed the problem of detection and sizing of the cracks. This paper describes our experience with the use of creeping wave probes.