The evaluation and calibration of ultrasonic transducers

The evaluation and calibration of ultrasonic transducers

Conl renee reporl: The evaluation and calibration of ultrasonic transdu London, 11th and 12th May, 1977 This two day meeting organized by the Material...

204KB Sizes 0 Downloads 11 Views

Conl renee reporl: The evaluation and calibration of ultrasonic transdu London, 11th and 12th May, 1977 This two day meeting organized by the Materials and Testing Group of the Institute of Physics was concerned with various aspects of the standardization of ultrasonic transducers. It attracted over 110 delegates. An aim of the meeting was to bring together workers from the universities, medical establishments and from nondestructive testing laboratories and this spread of interest was reflected both in the papers presented and in the delegate list. It was also pleasing to note a high level of support for the meeting from the rest of Europe with five of the 18 papers and some 20 per cent of the delegates coming from abroad. The first paper by Dr Coffey (CEGB) and Dr Wickham (Manchester University) described some current work aimed at improving the theoretical understanding of the operation of shear wave probes especially for the near field region. Ray theory was used and the contribution to the ultrasonic field due to the toroidal diffracted wave from the edge of the crystal was included. The failure of ray theory near shadow boundaries was recognized and an allowance for this made. A telling indication of the improvement to be gained from this approach was provided by a comparison between the interfacial stress pattern at the steel surface as predicted by this theory and the 'top hat' stress function commonly assumed. A comparison of experiment with theory is underway and this has highlighted the presence of a surface wave which passes across the probe face. Since this wave reflects at the probe extremity and its velocity does not coincide with normal modes in the probe or the specimen materials, we must presumably look for an origin in some form of Stoneley mode. The following paper by Dr Wustenberg (BAM Berlin) was also concerned with the sound field from angled shear wave probes. In this case a simplified approach had been made to the problem in which the piezoelectric membrane is described as part of two long strip emitters set at right angles. Single probe and transmit-receive operation was considered and it was shown, by comparison with experiment, that good qualitative agreement was possible in the far field. Interference between the head wave and the normal shear wave in low angle shear wave probes (45 ° and less) was also reported. A change of background and approach was provided by the next paper by Dr Gore and Dr Leeman of the Royal Postgraduate Medical School. In this case the probe was considered as part of the wider ultrasonic system and the problem discussed was that of quantifying results obtained on a test object and relating these to the practical situation. An approach was proposed and described whereby the step from the real situation to the observed ultrasonic image could be represented in terms of mathematical operators, proceeding via an idealized image. From the same mathematical

236

basis a quantitative index of resolution was proposed. No experimental evidence was presented but it is clear that an approach on these hnes could be relevant to other fields, although the contention that absorption improves resolution will no doubt continue to be debated. The paper by Dr Weight and Dr Hayman (City University) presented the results from a programme of experimental work aimed at verifying theoretical predictions of the transient field from short-pulsed probes. Visualization of the ultrasonic wave fronts using a Schlieren system was described and experimental results presented - some on videotape. In addition the use of a very small probe (150/am diameter, 40/am thick) to scan the sound field was described. The results obtained were in agreement with earlier theoretical data. The final paper of this session was by Dr Chivers (University of Surrey) and Dr Duck (Institute of Cancer Research) and described the det'mition of reference spectra for pulse excited transducers. The mathematics of the process could be kept in a reasonably simple form provided a system of circular symmetry was assumed - not an onerous assumption in most ultrasonic work. The problem of absorption could be treated by inserting it into the transfer function. A number of computed examples were shown. The second session began with a paper by Dr. K.G. Hall (British Rail, Derby) who described the use of a photoelastic visualization system to determine the stress in ultrasonic waves. Tardy compensation was used to remove the image of the various ultrasonic wavefronts. Calibration against static torsional and compressive stresses provided a quantitative estimate of stress. As a test the system had been used to confirm the reflection and transmission laws. Both shear waves and longitudinal waves had been studied. The following paper by Dr Wustenberg was concerned with the definition of the frequency of probes used in day-to-day NDT. This was an important quantity even for short-pulsed probes and the use of expensive calibration equipment was difficult to justify for the probes used in manual testing. The use of a number of simpler techniques based on the deterruination of cycle length, absorption and interference was discussed and compared. The paper by Dr Whittingham and Dr Farmery (Newcastle and General Hospital) described the construction and use of a simple probe beam plotter and a portable pressure balance for the absolute calibration of transducer power outputs. The latter instrument was sensitive to ~ 0.2 mW cm-2 if used with care. It was clear at times during this amusing presentation that there was a serious shortage of finances for some of the medical programmes in this area. There was no shortage of enthusiasm, however, and the production of

U L T R A S O N I C S . S E P T E M B E R 1977

a portable version of a sophisticated instrument for a few hundred pounds is a very creditable achievement. The f'mal paper in this session by Dr E. Borloo and Mr A. Pozzo described some of the activities of the Community Bureau of Reference with particular reference to the working sub-group on ultrasonics. The third session began with a paper by Mr F.J. Clinik (MOD(PE)) who described the work being carried on by the Admiralty to produce a better ultrasonic probe. It is perhaps particularly relevant to note that, when commercial sources of of piezoelectric material were used, 50 per cent of the ceramic discs were rejected before probe construction began. This theme was continued in the next paper by Dr M.J. Whittle and Dr H Smallman (CEGB), where it was mentioned that 22 out of 80 commercial single crystal probes and 23 out of 50 twin crystal probes were found to be defective in some way. The need for standards for probe specification was emphasized and many examples of faulty operation were given. The CEGB is now setting acceptance standards and this step should perhaps be seriously considered by other users.

A paper by Dr Jehenson and Dr Bredael (ISPRA) again considered the practical means of characterizing probes, particularly for medical applications. Some reasons for probe variability were considered and the equipment available at ISPRA for calibrating ultrasonic probes was described. The construction of multi-element arrays for real-time twodimensional ultrasonic imaging was then described in a paper by Dr. Willson, Dr Leeman (Royal Postgraduate Medical School) and Dr Lloyd (City University). The need for rapid mechanical or electronic scanning was described and with reference to the latter, the poor performance of some conventional arrays described. The development of an etched array was expected to give the advantages of minimal mechanical damage, repeatability and versatility while other possible advantages could be foreseen. The final paper of this session by Dr. Walton and Dr. Chivers (University of Surrey) described the use of a small piezoelectric microphone in assessing the beam profile of ultrasonic transducers. This element was some 0.05 mm thick and about ½ mm diameter. Among possible applications the lack of well documented information on transducer ageing was mentioned.

The first paper in the final session by Dr J. Speake (NDT Centre, HarweU) described the use of laser interferometry for absolute calibration of transducers. This work had originally sprung from an interest in acoustic emission responses. Although the interferometer is less sensitive than piezoelectric transducers in detecting surface movement, the technique had the advantage of a fiat ultrasonic response. The difference in response between the centre and edge of a piezoelectric disc was clearly shown. The advantages of the system were listed as portability, ease of operation, absolute measurement, no contact or special reflector and long range, although the cost was a possible disadvantage at £8000. A paper on speckle interferometry by Dr Jones and Dr Bergqvist (Loughborough University) followed. This utilized a 3-path system to compensate for bench movements. The pressure sensitivity was 8 - 9 kilopascals. At the present time the frequencies studied had been in the kiloHertz range, but it was hoped to extend this to higher frequencies. The paper by Dr. Babonx, Dr l.akestani and Dr Perdnx (INSA France) was concerned with absolute calibration using impulse techniques. The step pressure is generated by a thick piezoelectric transducer and the receiving sensitivity of the transducer is thus measured and the transmitting sensitivity computed using the reciprocity theorem. Experimental measurements were also carried out. The final paper by Dr G. Torr (University of Aberdeen) described attempts to measure the absolute power output from transducers using constant flow calorimeters. Two calorimeters were described, neither of which is regarded as being optimized. In the most accurate version the apparent power fell as a function of increasing frequency and this was ascribed to the fact that the majority of ultrasonic heating at high frequencies takes place above the level of the calibration heater. A better design is planned which should lead towards the goal of a portable instrument. In conclusion, the meeting brought together workers from different areas who have this fundamental problem in common. If the stimulating question sessions and high attendance levels for all papers are any guide, the meeting succeeded in its aims. It is hoped that the proceedings of the meeting will be available between covers shortly. M.G. Silk

2nd international symposium on ultrasonic tissue characterization Gaithersburg, Maryland, USA, 13-15 June 1977 Ultrasound is coming to play an increasingly important role in clinical diagnosis. In consequence, diagnostic ultrasound instruments together with their maintenance and care, are becoming a major component in health care costs. In the United States alone, ~64 M will be spent in 1977 on diagnostic ultrasound equipment, and this amount is now increasing annually by about 31 per cent. Efforts are currently underway in many hospitals and research laboratories to extend this diagnostic modality to tissues and organs hitherto disregarded. Many investigators are attempting to enhance the information

ULTRASONICS. SEPTEMBER 1977

extraction from the established methods as well as to develop new techniques. In particular, a great deal of work is underway to improve the identification of different tissues and their many possible pathologies by non-invasive ultrasound methods. Thus the field of diagnostic ultrasound is marked by a challenging ferment. The future looks assured but the exact trajectory of future development is far from decided. Some of the component aspects of that trajectory are generally accepted:

237