Duncan R. Billson

The use of broadband acoustic transducers and pulse-compression techniques for air-coupled ultrasonic imaging.

A pulse-compression technique has been applied to air-coupled testing of solid materials. Capacitance transducers were used to generate wide bandwidth swept-frequency (chirp) signals in air, which were then used to measure and image solid samples in through transmission. The results demonstrate that such signal processing techniques lead to an improvement in the signal to noise ratio and timing accuracy for air-coupled testing. Measurements of thickness and spectroscopic experiments are presented. Images of defects in a wide range of materials, including metals and carbon-fibre composites have also been obtained. This combination of capacitive transducers with pulse-compression techniques is shown to be a powerful tool for non-contact air-coupled ultrasonic measurements.

Preliminary studies of a novel air-coupled ultrasonic inspection system for food containers

Air-coupled ultrasound has been used to perform measurements on liquids and starch-based materials, within containers similar to those used in the food industry. The technique uses capacitance transducers with polymer membranes to generate ultrasonic signals in air over a reasonable bandwidth. Ultrasonic pulse-compression (UPC) is then applied to increase the sensitivity of signals transmitted through the containers. It will be demonstrated that various non-contact measurements are possible, including the detection of variations in consistency within starch-based liquids within a microwaveable food container, the detection of liquid level in polymer-based soft drink bottles, and the tomographic imaging of such containers to detect foreign objects.

Novel, wide bandwidth, micromachined ultrasonic transducers

Surface micromachined, capacitive ultrasonic transducers have been fabricated using a low thermal budget, CMOS-compatible process. This process allows inherent control of parameters such as membrane size and thickness, cavity size and the intrinsic stress in the membrane to be achieved. Devices fabricated using this process exhibit interesting properties for transduction in air at frequencies in excess of 1 MHz when driven from a standard ultrasonic voltage source. Experiments have been performed with devices containing silicon nitride membranes of variable thicknesses over a 2 /spl mu/m thick air cavity and with device dimensions of up to 5 mm square. This is much larger than has been reported for a device with a single membrane. Calibration measurements using 1/8 inch microphones in air, and miniature PVDF hyrdophones in water, have been performed. The dependence on d.c. bias voltage is examined, involving static membrane deflection measurements and received peak voltages. Pulse-echo and pitch-catch mode operation have been achieved. Interferometric measurements of membrane displacement have been performed in air to illustrate the membrane deflection characteristics. Operation in liquids is also discussed.

High-resolution, air-coupled ultrasonic imaging of thin materials

This paper describes the use of a focused air-coupled capacitance transducer combined with pulse compression techniques to form high-resolution images of thin materials in air. The focusing of the device is achieved by using an off-axis parabolic mirror. The lateral resolution of the focused transducer, operating over a bandwidth of 1.2 MHz, was found to be less than 0.5 mm. A combination of the focused transducer as a source and a planar receiver in through-transmission mode has been developed for the measurement of different features in paper products, with a lateral resolution in through-transmission imaging of /spl sim/0.4 mm. Images in air of thin samples such as bank notes, high-quality writing paper, stamps, and sealed joints were obtained without contact to the sample.

The use of air-coupled ultrasound to test paper

Capacitance transducers containing a thin polymer membrane have been used to transmit ultrasonic signals with frequencies in excess of 1 MHz through various paper products such as paper and cardboard. At normal incidence, a resonance was visible in thicker samples, the frequency of which could be correlated to parameters such as the thickness of the paper sample and the moisture content. It has also been demonstrated that images can be obtained of changes in structure across paper and card samples.

Surface metrology using reflected ultrasonic signals in air

Abstract An ultrasonic signal has been generated in air, using a capacitive transducer to produce a broad bandwidth transient signal. This was then focused using an off-axis parabolic mirror onto the surface of solid materials. The reflected signal was detected in terms of received amplitude and time of arrival, and images formed of the surface profile by scanning the focal region over metallic surfaces. Measurements of small steps formed from slip gauges have shown that 5 μm vertical features can be detected. The field of the transducer assembly has been measured with a miniature detector, and the width of the focal region studied. Scans of the surfaces of coins are used to illustrate the technique for non-contact surface profiling.

The characterization of capacitive micromachined ultrasonic transducers in air

Surface micromachined, capacitive ultrasonic transducers have been fabricated using a low thermal budget, CMOS-compatible process. They exhibit interesting properties for transduction in air at frequencies in excess of 1 MHz, when driven from a standard ultrasonic voltage source. Experiments are described using 1 mm square devices in air, operating in both pitch-catch and pulse-echo modes. The dependence on d.c. bias voltage is examined, together with calibration measurements using 1/8 in. microphones. The radiated beam profile, and the farfield directivity pattern, have been measured for both broad bandwidth and one-burst excitation, using a scanned miniature receiver. A 16 element square array is also presented, which has been used to measure the beam cross-sections from a focussed source.

Ultrasonic Lamb wave tomography using scanned EMATs and wavelet processing

A continuous wavelet transform has been used to analyse multi-mode Lamb wave propagation. This was applied to experimental data from non-contact electromagnetic acoustic transducers (EMATs) in thin metal plates. The technique was able to identify the presence of s 0, a 0, a 1, s 1 and s 2 modes in an aluminium plate from a single waveform, and hence to separate the modes. The technique has been applied to tomographic reconstruction of artificial defects in plates from individual modes to illustrate the usefulness of the approach.

Using a magnetite/thermoplastic composite in 3D printing of direct replacements for commercially available flow sensors

Flow sensing is an essential technique required for a wide range of application environments ranging from liquid dispensing to utility monitoring. A number of different methodologies and deployment strategies have been devised to cover the diverse range of potential application areas. The ability to easily create new bespoke sensors for new applications is therefore of natural interest. Fused deposition modelling is a 3D printing technology based upon the fabrication of 3D structures in a layer-by-layer fashion using extruded strands of molten thermoplastic. The technology was developed in the late 1980s but has only recently come to more wide-scale attention outside of specialist applications and rapid prototyping due to the advent of low-cost 3D printing platforms such as the RepRap. Due to the relatively low-cost of the printers and feedstock materials, these printers are ideal candidates for wide-scale installation as localized manufacturing platforms to quickly produce replacement parts when components fail. One of the current limitations with the technology is the availability of functional printing materials to facilitate production of complex functional 3D objects and devices beyond mere concept prototypes. This paper presents the formulation of a simple magnetite nanoparticle-loaded thermoplastic composite and its incorporation into a 3D printed flow-sensor in order to mimic the function of a commercially available flow-sensing device. Using the multi-material printing capability of the 3D printer allows a much smaller amount of functional material to be used in comparison to the commercial flow sensor by only placing the material where it is specifically required. Analysis of the printed sensor also revealed a much more linear response to increasing flow rate of water showing that 3D printed devices have the potential to at least perform as well as a conventionally produced sensor.

Novel silicon nitride micromachined wide bandwidth ultrasonic transducers

Silicon micromachined ultrasonic devices have been fabricated using a CMOS-compatible process. Devices of up to 1 mm in size have been tested, with silicon nitride membranes of 1 /spl mu/m and 2 /spl mu/m thickness. The work has investigated the response of these ultrasonic receivers, as a function of membrane thickness and lateral dimensions. It is shown that the resultant surface micromachined transducers can operate over a wide bandwidth in air, without the resonant behaviour associated with previous devices.