|TABLE OF CONTENTS|
Illustration 1 presents ultrasound simulations achieved by the CEA for the ideal definition of this transducer :
|Illustration 1 : Applying the ultrasound simulation to the design of a array transducer|
These predictions permit us to determine the dimensions of the ultrasound beam and to check them against the desired dimensioning precision of the desired defaults. Starting with the specification elements obtained in this way (geometric description and cut of the transducer, nominal frequency and pass-band, damping, impedance adaptation, absence of acoustic and electrical cross talk between the elements of the array, pressure and temperature holding, associated interconnection technique) solutions which respond better to the desired functions have been considered.
At this stage, the choices deal with the piezoelectric material, the acoustic adaptation and damping materials, the electrical adaptation and cabling elements, the determination of the dimensional parameters as well as the machining procedures, set-up and testing to be implemented. Here again, simulation tools are used in order to optimise the choices made : piezoelectric composite or passive design support tool, equivalent electrical circuits of transducers, radiation models of geometrical elemental surfaces. An example of a simulation result achieved by Imasonic is furnished in Illustration 2, which represents the impulse response and the frequency spectrum of the considered sensor modelled by an equivalent electrical circuit of a transducer.
|Illustration 2 : Impulse response and frequency spectrum|
|Pulse-Echo (Two-way) Impulse Response - V(Rx)/V(Tx)|
- 6dB center frequency: 1.98 MHz|
- 6dB relative bandwidth: 68%
- 6dB pulse length: 0.61 µs
After manufacturing the sensor, a characterisation step is necessary in order to validate the design choice. Illustration 3 shows by example the measured impulse response of the sensor in transmission/reception mode on a reflecting plane target.
Illustration 4 : Validation of the transducer in operating conditions : measurement of the field transmitted by the transducer.
|Illustration 3 : measured impulse response|
- 6dB center frequency: 2.12 MHz
- 6dB relative bandwidth: 81%
- 6dB pulse length: 0.62 µs
The transducer was then tested under operating conditions which were close to real conditions. Illustration 4 presents a comparison between the predictions of the ultrasound field given by the Field-Sound model and the measurements performed in transmission using an EMAT probe in contact with the outer wall. These amplitude responses correspond to the measurement of the maximal amplitude of the ultrasound field, in each displacement point of the EMAT receiving flux valve. The comparisons between theory and experience, carried out on blocks of different thickness' have shown an excellent accord and have permitted the validation of the whole testing procedure : test assembly and behaviour of the transducer in terms of acoustic and electrical response.
| Rémi Berriet |
15 Rue Alain Savary, 25000 Besançon
Web Page on NDTnet
| Steve Mahaut|
CE Saclay, 91191 Gif sur Yvette