A.G. Bashford

Micromachined ultrasonic capacitance transducers for immersion applications

Investigations into the characteristics of water-coupled ultrasonic capacitance transducers have been undertaken for a range of transducer configurations. The radiated fields have been scanned in water using a miniature hydrophone detector, and the results compared to theory based on a plane piston approach. Micromachined backplates in conjunction with thin Mylar and mica membranes have been investigated, together with aperture modifications such as an annulus and Fresnel zone plate. The measured results agree well with theory, thus demonstrating that wideband predictable performance (>8 MHz) is obtainable with such transducers. Additionally, pulse-echo C-scans of a Plexiglas plate containing an artificial defect have been undertaken with the capacitance transducer, in order to demonstrate that the capacitance transducer has sufficient sensitivity to allow routine nondestructive testing within immersion applications.

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.

Focussing of ultrasonic waves in air using a micromachined Fresnel zone-plate

An ultrasonic Fresnel zone-plate has been micromachined from a thin stainless steel disk for the purpose of focussing highfrequency ultrasonic waves in air. This zone-plate, which was designed with a focal length of 7.5 mm, consisted of five concentric stainless steel rings that were separated from one another by narrow air-channels. The zone plate was fitted to a micromachined air-coupled capacitance transducer source, and the radiation profile measured by a second 1 mm diameter air-coupled capacitance receiver. It is shown that the resultant spot size (for an operating frequency of 580 kHz) was 0.8 mm, with a 3 mm depth-of-field. Comparison to an existing theoretical model shows that the zone-plate is operating as might be expected, but that improvements might be obtained by further research.

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.

Radiated fields of an air-coupled ultrasonic capacitance transducer

The use and development of ultrasonic air-coupled transducers that can operate above 100 kHz have been of recent interest. Applications of these devices include location and imaging of objects for production engineering and non destructive evaluation of materials. In previously published work, investigation into the field characteristics of ultrasonic transducers has produced a mathematical theory that models the pressure field variations for a plane piston transducer. With modification to the mathematical theory a model of the pressure field variations for a range of transducer geometries and driving signals has been developed. The pressure field of a capacitance air-coupled transducer was measured by scanning a miniature detector through the field. Thus, it was possible to compare the experimentally obtained pressure field with the theoretical pressure field. This has enabled us to design air-coupled ultrasonic transducers with optimum characteristics for air-coupled non-contact imaging of materials.

Chromatic aberration of an air-coupled ultrasonic Fresnel zone-plate

A micromachined Fresnel zone-plate has been used to focus ultrasonic waves in air over a range of frequencies (450 to 900 kHz). The zone-plate was mounted upon a planar micromachined air-coupled capacitance transducer, which was capable of generating toneburst ultrasonic waves in air over a wide frequency bandwidth (<100 kHz to 2 MHz). A second air-coupled capacitance detector (apertured to 200 /spl mu/m) was scanned in the field of the zone-plate source in order to image the generated ultrasonic field at various frequencies of operation. It was found that the /spl sim/680 /spl mu/m spot size of the experimental zone-plate did not vary appreciably with changing frequency, whereas the focal length increased markedly with increasing frequency (from /spl sim/5 mm at 450 kHz up to /spl sim/15 mm at 900 kHz). These findings are shown to be in excellent agreement with previously reported theoretical predictions by the authors.

Characteristics of ultrasonic micromachined capacitance transducers in water

Investigations into the characteristics of water-coupled ultrasonic capacitance transducers have been performed for a range of transducer configurations. The radiated fields have been scanned in water using a miniature hydrophone detector, and the results compared to theory based on a plane piston approach. Additionally, the transducer has been used in pulse-echo mode and a C-scan performed to image an artificial defect in a Plexiglas plate.

Non-destructive evaluation of green-state ceramics using micromachined air-coupled capacitance transducers

Abstract Air-coupled ultrasound has been used to study the manufacture of ceramic components by both slip casting and injection moulding. A capacitance transducer, designed to operate at high temperatures, was used as both a source and receiver with a 500 kHz bandwidth to pass ultrasound through 3 mm thick green-state injection-moulded and slip-cast samples of silicon nitride. The transducers were C-scanned over the entire area of the sample in order to locate density changes which could lead to problems in the firing stage. Additionally, through-transmitted waveforms at 150 C were obtained, indicating that the technique may be useful for performing measurements at high temperatures.

Advances in wide bandwidth air-coupled capacitance transducers

Investigations into the field characteristics of an air-coupled ultrasonic capacitance transducer have been performed for a range of transducer configurations. The pressure field of a wide bandwidth silicon backplate capacitance transducer has been scanned in air, and compared to theory based on a plane piston approach. New devices have been fabricated, using laser-machined backplates and an electret film, and are described. The devices have also been used in an air-coupled tomographic imaging experiment.

New mica‐based capacitance ultrasonic immersion transducers

A new type of ultrasonic immersion transducer has been designed and constructed that has a wide bandwidth when used in water. It is a capacitive (electrostatic) device, using a micromachined backplate and a thin mica metallized membrane. It will be demonstrated that the field pattern of the device can be plotted experimentally using a miniature hydrophone, and the results will be compared favorably to theoretical predictions. The device has been produced as a plane piston, and also as a focusing device using both an annular transducer and a zone plate. The device has none of the ringing associated with piezoelectric devices, and hence is suitable for a wide range of applications.