William M. D. Wright

Air-coupled Lamb wave tomography

An entirely air-coupled inspection system using a pair of micromachined silicon capacitance transducers has been used to image defects in thin plates of different materials (0.7 mm to 2.22 mm thick) using air-coupled Lamb wave tomography. A filtered back projection algorithm was used in a form of difference tomography to reconstruct images of defects up to 10 mm diameter machined in aluminium and perspex (Plexiglas) plates, as well as in samples of carbon fiber reinforced polymer (CFRP). The technique was able to resolve non-central defects as well as multiple flaws within the scan area. This flexible tomographic system was able to produce images of the change in a variety of different acoustic variables from only one set of experimental data, with success dependent on the size, shape, and location of the defect in the scan area.

A full vectorial contrast source inversion scheme for three-dimensional acoustic imaging of both compressibility and density profiles

Imaging the two acoustic medium parameters density and compressibility requires the use of both the acoustic pressure and velocity wave fields, described via integral equations. Imaging is based on solving for the unknown medium parameters using known measured scattered wave fields, and it is difficult to solve this ill-posed inverse problem directly using a conjugate gradient inversion scheme. Here, a contrast source inversion method is used in which the contrast sources, defined via the product of changes in compressibility and density with the pressure and velocity wave fields, respectively, are computed iteratively. After each update of the contrast sources, an update of the medium parameters is obtained. Total variation as multiplicative regularization is used to minimize blurring in the reconstructed contrasts. The method successfully reconstructed three-dimensional contrast profiles based on changes in both density and compressibility, using synthetic data both with and without 50% white noise. The results were compared with imaging based only on the pressure wave field, where speed of sound profiles were solely based on changes in compressibility. It was found that the results improved significantly by using the full vectorial method when changes in speed of sound depended on changes in both compressibility and density.

Ultrasonic measurements in polymeric materials using air‐coupled capacitance transducers

Wide bandwidth capacitance devices have been designed which have a high transduction efficiency in air at frequencies into the MHz range. They are sufficiently sensitive to allow transmission through solid samples, including polymers and fiber‐reinforced composites. Examples are presented of experiments showing how such transducers may be used to measure material properties and to perform ultrasonic imaging.

Air-coupled piezoelectric detection of laser-generated ultrasound

A pulsed laser has been used to generate ultrasonic transients in samples of metal and fiber-reinforced polymer composite material. These have been detected using an air-coupled piezoelectric transducer. It is demonstrated that such a transduction system can be used for longitudinal waves in bulk material, Rayleigh waves at solid surfaces and Lamb waves in thin plates. >

Studies of laser‐generated ultrasound using a micromachined silicon electrostatic transducer in air

An air‐coupled micromachined silicon electrostatic transducer has been used to detect ultrasound generated by a pulsed laser in metals and polymers. Bulk longitudinal and shear wave modes have been observed, as have Rayleigh waves in thick samples and Lamb waves in thinner plates. The results are in good agreement with surface displacement waveforms obtained using a contact capacitance device as a reference. The result is a flexible noncontact ultrasonic system with potential uses in materials characterization.

Advances in ultrasonic electrostatic transduction

This paper describes some recent advances in the use of electrostatic transducers for performing ultrasonic measurements in both air and water. It will be shown that through-transmission imaging can be performed in metals, polymers and fibre-reinforced composites. Also possible are various forms of imaging in air, including tomographic reconstruction of flow and temperature fields in gases.

Ultrasonic measurement of gas flow using electrostatic transducers

Ultrasonic gas flowmeters typically use narrowband piezoelectric transducer arrangements for interrogating the flow of gas in a pipe. In this work, the suitability of broadband electrostatic transducers operating at frequencies of up to 1 MHz for ultrasonic measurement of gas flow has been investigated. The transit time method of ultrasonic gas flow measurement was adopted and experiments were carried out using a laboratory test rig capable of producing a range of gas flowrates up to 17.5 m/s. The test rig also allowed easy interchange of different prototype flowmetering sections. Times of flight of ultrasonic waves interrogating the gas flow were measured using separate send/receive electrostatic transducer arrangements. Two flowmeter configurations were considered. The first interrogated the flow at 45 degrees in contra-propagating upstream and downstream directions. The second consisted of an up-stream interrogation at 45 degrees to the gas flow and an interrogation made normal to the flow direction. k factors correlating the fluid velocity along the ultrasonic path with the mean fluid velocity in the pipe were calculated using experimental ultrasonic data and anemometer measurements. All transducer configurations were numerically modelled using the computational fluid dynamics software package FLOTRAN (ANSYS Inc.). Theoretical gas flow velocities for both transducer arrangements were subsequently compared with experimental values and found to be in excellent agreement. A flow-dependent frequency shift of the received ultrasonic signals was also observed simultaneously with the transit time measurement.

Field characterization of an air-coupled micromachined ultrasonic capacitance transducer

Investigations into the field characteristics of an air-coupled ultrasonic capacitance transducer have been performed for a range of transducer configurations. The field of a 2-MHz bandwidth silicon backplate capacitance transducer has been scanned in air using a 1-mm-diam miniature detector at frequencies of up to 1.5 MHz. The radiated peak sound-pressure field is compared to theory based on a plane piston approach for various driving signals, namely pulsed and tone burst excitation. Aperture modifications, such as an annulus and a zone plate, have also been investigated and the devices have been shown to behave as predicted by theory.

Ultrasonic tomographic imaging of temperature and flow fields in gases using air-coupled capacitance transducers

A pair of air-coupled ultrasonic capacitance transducers with polished metal backplates have been used to image temperature and flow fields in gases using ultrasonic tomography. Using a filtered back-projection algorithm and a difference technique, cross-sectional images of spatially variant changes in ultrasonic attenuation and slowness caused by the presence of temperature and flow fields were reconstructed. Temperature fields were produced in air by a commercial soldering iron, and the subsequent images of slowness variations used to reconstruct the air temperature at various heights above the iron. When compared to measurements made with a thermocouple, the tomographically reconstructed temperatures were found to be accurate to within 5%. The technique was also able to resolve multiple heat sources within the scan area. Attenuation and velocity images were likewise produced for flow fields created by an air-jet from a 1-mm-diam nozzle, at both 90 and 45 degrees to the scanning plane. The fact that tem...

Multi-channel indoor wireless data communication using high-k capacitive ultrasonic transducers in air

This work has demonstrated multi-channel wireless communication using a pair of broadband high-k capacitive ultrasonic transducers. Modulated ultrasound was used to carry digitally coded data signal over short distances using Amplitude Shift Keying (ASK) and On-Off Keying (OOK) modulation schemes. The over-all system data rate achieved was up to 420 kbps with ultrasonic wireless synchronization. Signal-to-Noise Ratio (SNR) and bit error rate (BER) characteristics were analysed and compared between different modulation schemes. It was shown that OOK can be used to transmit signals with higher power efficiency and lower BER than ASK.