GB2149254A

GB2149254A - Ultrasonic transducer assembly

Info

Publication number: GB2149254A
Application number: GB08426086A
Authority: GB
Inventors: Geoffrey Hugh Broomfield
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1983-10-27
Filing date: 1984-10-16
Publication date: 1985-06-05
Also published as: DE3439328A1; GB2149254B; GB8426086D0; JPS60173997A; FR2558676A1; GB8328789D0

Abstract

An ultrasonic transducer assembly 10 comprises a piezo- electric transducer 12 connected by a cable 20 of length "I" to an inductor 26 of inductance "L". The values of the length "I" and the inductance "L" are adjusted so that the frequency at which the impedance of the assembly 10 is a minimum is equal to the frequency at which the impedance of the transducer 12 alone is a minimum. The transducer 12 may be a lithium niobate transducer, with a Q value of about 5000. <IMAGE>

Description

SPECIFICATION Ultrasonic transducer assembly This invention relates to ultrasonic transducer assemblies incorporating piezo-electric ultrasonic transducers, and in particular to assemblies incorporating ultrasonic transducers with a relatively high Q value i.e. a relatively narrow resonant frequency bandwidth.

Transducers made of refractory piezo-electric material such as lithium niobate can operate at temperatures above 600"C, and so can be used within a core of a fast reactor.

However the ultrasonic signal strength obtainable hitherto from lithium niobate transducers has not been considered adequate for such use. Lithium niobate transducers generally have a very narrow frequency resonance, with a Q value (defined as the ratio of the resonant frequency to the difference between the frequencies on either side of the resonant frequency at which he ultrasonic signal is 3dB less than at the resonant frequency) which might for example be 5000.

The transducer assembly of the present invention provides a greater improvement in ultrasonic signal strength than can be achieved by tuning with an inductor in parallel with the transducer, as described by R.D.

Legge in CEGB report NW/SSD/RR/154/79. It has been suggested, for example by J. van Randeraat and R.E. Setterington, in "Piezo-electric Ceramics", Mullard 1974, that a transducer be tuned with an inductor in series, the maximum impedance of the circuit being tuned to occur at the frequency of maximum impedance of the transducer itself. This however requires very precise tuning to achieve satisfactory signal strength.

According to the present invention there is provided an ultrasonic transducer assembly comprising a piezo-electric transducer, a cable, and an inductor connected electrically in series with each other, the inductance of the inductor and the length of the cable being such that the frequency at which the impedance of the transducer assembly is a minimum is substantially the same as the natural resonant frequency of the piezo-electric transducer, i.e. the frequency at which the impedance of the piezo-electric transducer is a minimum.

The piezo-electric transducer is preferably a lithium niobate transducer.

The invention will now be further described by way of example only and with reference to the accompanying drawings, in which: Figure 1 shows a circuit diagram of a transducer assembly; Figure 2 is a graphical representation of the variation of admittance of the assembly of Fig.

1 with frequency; and Figure 3 is a graphical representation of the variation of signal strength from the assembly of Fig. 1 with inductance and length of cable.

Referring to Fig. 1, a transducer assembly 10 comprises a lithium niobate Z-plate transducer 1 2 of nominal resonant frequency 4 MHz, enclosed within a steel housing 1 4 with a base 1 6. One side of the transducer 1 2 is bonded to the base 16, which acts as a halfwavelength steel diaphragm, and an electrode 1 8 is provided on the other side of the transducer 1 2. Electrical connections to the two sides of the transducer 1 2 are made via a co-axial cable 20 with an outer conductor 22 and a core 24 connected to the housing 14 and the electrode 18, respectively.At a distance "1" along the cable 20 the core 24 is connected to one terminal of an inductor 26 of inductance "L", and the outer conductor 22 is connected to a metal casing 28 of the inductor 26.

A second co-axial cable 30 with an outer conductor 32 and a core 34 connects the transducer assembly 10 to a signal generator 36, the outer conductor 32 being connected to the casing 28, and the core 34 to the other terminal of the inductor 26.

In operation of the transducer assembly 10, excitation of the transducer 1 2 by the signal generator 36 causes the emission of ultrasonic waves, the amplitude of the ultrasonic waves depending upon the frequency of excitation, on the distance "1" and on the inductance "L".

Referring also to Fig. 2, for any given values of the length "I" and the inductance "L", as the frequency of the signal generator 36 is varied the admittance of the assembly 10 has a maximum (i.e. the impedance has a minimum) at a frequency "fas"' and the admittance has a minimum at a frequency "fp", the frequencies "f," and "fp" depending upon the length "I" and inductance "L" At the frequency "f," the current and voltage are in phase.In the absence of the inductor 26, the admittance of the transducer 1 2 alone would show a similar variation with frequency, with an admittance maximum at the natural resonant frequency of the transducer 12, and an admittance minimum at a slightly different frequency, but the frequencies at which these would occur would be much closer together than are "f," and "fp", and the difference between the admittance maximum and minimum values would be much smaller than the difference in admittance between "f," and "fop".

P In the presence of the inductor 26, the overall response is thus a large admittance A at frequency "f," and an admittance mini mum A' at frequemcy "fp", and also, generally, a smaller admittance maximum B, and an admittance minimum B' corresponding to the transducer 12 alone. In Fig. 2 however is shown the case where the frequency "f," is adjusted to coincide with the natural resonant frequency of the transducer 12, so the maximum admittance A coincides with the natural resonant frequency of the transducer 1 2.

Hence maxima A and B are coincident.

Fig. 3, to which reference is now made, shows the experimentally determined maximum signal strength of the ultrasonic signals emitted by the transducer 1 2 for various values of length "I" and inductance "L", in comparison to the signal strength in the absence of the inductor 26. For any combination of the length "i" and the inductance "L", the maximum signal strength is achieved when the signal generator 36 (see Fig. 1) excites the transducer 1 2 at the frequency "f," (see Fig. 2) of maximum admittance of the transducer assembly 10. The optimum signal strength, which in this case is + 28dB, is achieved when the length "I" and the inductance "L" are adjusted so that the frequency ''fs" coincides with the natural resonant frequency of the transducer 1 2 alone.

Claims

1. An ultrasonic transducer assembly comprising a piezo-electric transducer, a cable, and an inductor connected electrically in series with each other, the inductance of the inductor and the length of the cable being such that the frequency at which the impedance of the transducer assembly is a minimum is substantially the same as the natural resonant frequency of the piezo-electric transducer.

2. An assembly as claimed in Claim 1 wherein the transducer has a Q-value of the order of 5000.

3. An assembly as claimed in Claim 1 or Claim 2 wherein the transducer is of lithium niobate.

4. An ultrasonic transducer assembly substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.