Communications – electrical: acoustic wave systems and devices – Signal transducers – Underwater type
Patent
1997-01-06
1998-01-06
Eldred, J. Woodrow
Communications, electrical: acoustic wave systems and devices
Signal transducers
Underwater type
367157, 367162, 367151, 310334, 310335, 310337, H04R 1700
Patent
active
057062528
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to acoustic transducers capable of operating on several emission frequencies and/or of receiving with wide passbands around these frequencies. It makes it possible in underwater imaging to obtain long range for low frequency, but with low resolution, and high resolution for high frequency, but with short range. Low-frequency operation is then used first to pinpoint the objects which it is desired to identify. The boat carrying the sonar equipped with this type of transducer subsequently approaches the object thus detected, and when sufficiently near, the high frequency is used making it possible to obtain an accurate image of this object.
It is known from French Patent Application Number 8707814, filed by the applicant on 4 Jun., 1987 and granted on 9 Dec. 1988 under the number 2616240, to fabricate a multifrequency acoustic transducer essentially intended to be used in medical uses, by inserting between the active piezoelectric plate and the reflector of an ordinary probe, a half-wave plate with the natural resonant frequency of this plate. The probe can thus be used at two distinct frequencies, one being substantially equal to half the other. However, this system, although it is well suited to medical imaging, in particular so as to use one frequency in imaging mode and the other frequency to view blood flows, exhibits a number of drawbacks in underwater imaging. In particular, the bandwidth around one of the two resonant frequencies is relatively small. This is not very important in respect of the frequency used to view blood flows. In underwater imaging, by contrast, the processing operations used make it necessary to have a large bandwidth for both frequency ranges.
To alleviate these drawbacks, the invention proposes a wideband multifrequency acoustic transducer, of the type comprising a piezoelectric emitter plate of impedance Z and resonating in .lambda./2 mode at a fundamental frequency F0, a rear plate of impedance Z3 and a support forming a reflector of the type with substantially zero impedance, characterized in that the rear plate resonates in .lambda./4 mode at the frequency F0 so as to make it possible to obtain two resonant frequencies FA and FB of the assembled transducer, and in that this transducer furthermore comprises two front matcher plates whose impedances Z1 and Z2 are given by the formulae equal to .lambda./4 for respectively each of the frequencies FA and FB and to be substantially transparent for respectively each of the other frequencies; these thicknesses being optimized with the aid of a Mason type model.
According to another characteristic, the rear plate is formed from the same material as the active plate.
According to another characteristic, the material constituting the active layer and the rear plate is a ceramic of the PZT type for which Z is substantially equal to 21.times.10.sup.6 acoustic ohms, the matcher plates have respective impedances Z1=3.9.times.10.sup.6 acoustic ohms and Z2=6.times.10.sup.6 acoustic ohms, and the thicknesses of these plates are respectively equal as a function of the frequency which they are required to match to e1=.lambda./2.16 and e2=.lambda./5.04 at the 1st frequency, and to e1=.lambda./3.77 and e2=.lambda./8.81 at the 2nd frequency.
According to another characteristic, the active plate has a thickness such that it resonates in .lambda./2 mode at a frequency of 250 kHz and in that the two frequencies of emission for which the transducer is matched are substantially equal to 350 kHz and 150 kHz.
Other features and advantages of the invention will emerge clearly in the following description presented by way of non-limiting example with regard to the appended figures which represent:
FIG. 1, a sectional view of the structure of an antenna according to the invention;
FIG. 2, an exploded perspective view of the various layers, constituting this antenna; and
FIG. 3, a perspective view of such a transducer after slicing to obtain columns necessary in the case of an application to a sonar.
Represented in FIG. 1 is a
REFERENCES:
patent: 5410205 (1995-04-01), Gururaja
patent: 5418759 (1995-05-01), Fiebiger et al.
patent: 5629906 (1997-05-01), Sudol et al.
Le Verrier Bertrand
Ramos Alphonse
Roux Gerard
Tardy Bruno
"Thomson-CSF"
Eldred J. Woodrow
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