Walter Galbraith

Characterization of air-coupled transducers

This paper describes a theoretical and experimental study for determination of the through-air system impulse response and insertion loss with different air-coupled ultrasonic transducers. Wide-band piezopolymer transducers (PVDF) are employed in both transmission and reception modes and their behavior assessed by means of mathematical modeling and experiment. Specifically, a linear systems approach, modified to include the influence of attenuation in the propagation medium, was used to design suitable PVDF transducers for wide-band operation in air. Suitable devices were then manufactured for determination of the transmission and reception response characteristics of piezocomposite and electrostatic transducers when operating in the air environment. A range of transducers was evaluated, including 1-3 connectivity composites of different ceramic volume fraction and mechanical matching conditions, in addition to electrostatic devices of varying design. To complement the investigation, relative performances for narrow-band operation are also presented under transmission and transmit-receive conditions. Despite the obvious measurement difficulties, good agreement between theory and experiment was observed and the methodology is shown to provide a convenient and robust procedure for comparison of through-air transducers operating in the frequency range 50 KHz to 2 MHz. Although highly resonant, the most effective composite transducers under consideration demonstrate an improvement in two-way insertion loss of 22.4 dB and 11.5 dB over a corresponding electrostatic pair, under narrow-band and wide-band operation, respectively.

Micromachining of a piezocomposite transducer using a copper vapor laser

A 1-3 piezocomposite transducer with front face dimensions of 2/spl times/2 mm has been micromachined using a copper vapor laser. The device consists of PZT5A piezoceramic pillars with a 65-/spl mu/m pitch suspended in a low viscosity thermosetting polymer. The kerf width is 13 /spl mu/m, and the transducer thickness is 170 /spl mu/m, making the device suitable for ultrasonic reception at frequencies close to 10 MHz.

A Noncontact Ultrasonic Platform for Structural Inspection

Miniature robotic vehicles are receiving increasing attention for use in nondestructive testing (NDE) due to their attractiveness in terms of cost, safety, and their accessibility to areas where manual inspection is not practical. Conventional ultrasonic inspection requires the provision of a suitable coupling liquid between the probe and the structure under test. This necessitates either an on board reservoir or umbilical providing a constant flow of coupling fluid, neither of which are practical for a fleet of miniature robotic inspection vehicles. Air-coupled ultrasound offers the possibility of couplant-free ultrasonic inspection. This paper describes the sensing methodology, hardware platform and algorithms used to integrate an air-coupled ultrasonic inspection payload into a miniature robotic vehicle platform. The work takes account of the robots inherent positional uncertainty when constructing an image of the test specimen from aggregated sensor measurements. This paper concludes with the results of an automatic inspection of a aluminium sample.

Simulation of ultrasonic lamb wave generation, propagation and detection for a reconfigurable air coupled scanner.

A computer simulator, to facilitate the design and assessment of a reconfigurable, air-coupled ultrasonic scanner is described and evaluated. The specific scanning system comprises a team of remote sensing agents, in the form of miniature robotic platforms that can reposition non-contact Lamb wave transducers over a plate type of structure, for the purpose of non-destructive evaluation (NDE). The overall objective is to implement reconfigurable array scanning, where transmission and reception are facilitated by different sensing agents which can be organised in a variety of pulse-echo and pitch-catch configurations, with guided waves used to generate data in the form of 2-D and 3-D images. The ability to reconfigure the scanner adaptively requires an understanding of the ultrasonic wave generation, its propagation and interaction with potential defects and boundaries. Transducer behaviour has been simulated using a linear systems approximation, with wave propagation in the structure modelled using the local interaction simulation approach (LISA). Integration of the linear systems and LISA approaches are validated for use in Lamb wave scanning by comparison with both analytic techniques and more computationally intensive commercial finite element/difference codes. Starting with fundamental dispersion data, the paper goes on to describe the simulation of wave propagation and the subsequent interaction with artificial defects and plate boundaries, before presenting a theoretical image obtained from a team of sensing agents based on the current generation of sensors and instrumentation.

Miniature Mobile Sensor Platforms for Condition Monitoring of Structures

In this paper, a wireless, multisensor inspection system for nondestructive evaluation (NDE) of materials is described. The sensor configuration enables two inspection modes-magnetic (flux leakage and eddy current) and noncontact ultrasound. Each is designed to function in a complementary manner, maximizing the potential for detection of both surface and internal defects. Particular emphasis is placed on the generic architecture of a novel, intelligent sensor platform, and its positioning on the structure under test. The sensor units are capable of wireless communication with a remote host computer, which controls manipulation and data interpretation. Results are presented in the form of automatic scans with different NDE sensors in a series of experiments on thin plate structures. To highlight the advantage of utilizing multiple inspection modalities, data fusion approaches are employed to combine data collected by complementary sensor systems. Fusion of data is shown to demonstrate the potential for improved inspection reliability.

The radiation field characteristics of piezoelectric polymer membrane transducers when operating into air

This paper describes the evaluation of the radiation field characteristics of PVDF membrane transducers, whereby the active area is defined by an appropriate electrode pattern when operating into air. Measurement, including conventional scanning of the fields and laser vibrometry, is supplemented by finite element modeling over the frequency range 500 kHz to 1.1 MHz. Contrary to expectation, the surface displacements are nonuniform, giving rise to premature axial focusing of the field. This is shown to be caused by generation of the zero order symmetrical Lamb wave, arising from the electric field discontinuity at the electrode boundary of the transducer.

4I-1 Autonomous Mobile Robots for Ultrasonic NDE

Robotic vehicles are becoming increasingly attractive for non-destructive evaluation (NDE) of engineering structures, particularly for those areas where access is difficult or too hazardous for human intervention. This paper describes the design philosophy and prototype evaluation of a team of heterogeneous miniature climbing robots, which perform a range of NDE tasks, including magnetic flux leakage, eddy-current, visual and ultrasonic inspection. A host computer is used for overall supervision and inter-vehicle communication is achieved using wireless links. Each autonomous robot is located in 3D space by an ultrasonic positioning system in a listener-beacon configuration and is supplemented by data derived from optical wheel encoders and an on-board gyroscope. Results for non-contact ultrasonic inspection are presented, where the vehicle NDE payload comprises a pair of air coupled transducers, inclined at the appropriate angle to a test plate, for generation and reception of the A0 Lamb wave. C-scan images are shown, using the vehicle and the local positioning system to provide relative positional information

Small inspection vehicles for non-destructive testing applications

Miniature autonomous vehicles offer high potential for inspection tasks in areas where access is difficult or that are hazardous for human intervention. This paper focuses on the design of small mobile robots for nondestructive testing (NDT) of ferromagnetic materials. The major challenge in this connection is miniaturisation, as climbing skills are required in order to cope with various types of inclined or curved surfaces and ceilings. The proposed compact design involves permanent magnets that provide both the holding force and the source of the magnetic field for surface inspection using Hall Elements. Experiments show that artificial defects up to 1 mm width can be detected reliably with this passive inspection method.

P1I-6 A New Electrostatic Transducer Incorporating Fluidic Amplification

A methodology for improving the performance of electrostatic transducers that comprise a thin membrane positioned above a conducting backplate is presented. The concept has its foundations in the normally tubular sections used for amplification of sound in musical instruments, where the resonant frequency of a fluid tilled conduit is determined by the conduit length. Finite element modelling was used to evaluate a range of potential configurations, including conduit aspect ratio and the membrane - cavity structure immediately above the conduit (ie membrane dimensions, cavity width and depth). Results are presented that reveal the potential variation in amplification and these demonstrate that under certain conditions, a very significant improvement can be obtained. For example, at a centre frequency of 215 kHz, a conduit with an aspect ratio of 10:1 will produce a tenfold increase in displacement at the membrane end of the system. Although this is achievable over a wide frequency range, the best overall performance in terms of absolute sound pressure level was obtained when the length of the conduit was matched to the centre frequency of the vibrating membrane. Proof of concept devices were manufactured and tested experimentally, for both quarter wavelength and half wavelength conditions. Laser vibrometry was used to measure the displacement of the membrane, while a calibrated PVDF hydrophone and electret microphone were employed to measure the acoustic output at the end of the conduit. The presence of the vibrating air conduits was observed to increase the measured acoustic output by a factor greater than 20 dB. The results confirm that the prototype devices can operate efficiently at both ends of the system and the membrane can be excited effectively by the air column. Such transducers are ideally matched to air and also offer the potential to be used with other load media

Micromachined unimorphs and bimorphs

A frequency doubled copper vapour laser (CVL) operating at an ultraviolet wavelength of 271 nm has been used to cut miniature cantilevers in laminated composites incorporating at least one layer of the ferroelectric polymer polyvinylidene fluoride (PVDF). Devices have been fabricated from PVDF with thickness values in the range 9 /spl mu/m to 38 /spl mu/m. Laser vibrometer measurements show that the fundamental resonance frequencies of these devices correspond almost exactly with values obtained from mechanical beam theory. Ultrasonic transducers consisting of bimorph and unimorph arrays are currently being designed with the aid of a finite element model, which is not discussed.