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JPH05183996

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DESCRIPTION JPH05183996
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
broadband ultrasonic probe which uses water as a propagation medium.
[0002]
2. Description of the Related Art Conventionally, as an ultrasonic probe used for a water
immersion type ultrasonic fluid probe, for example, the one shown in FIG. 14 is used. In the
ultrasonic probe of FIG. 14, reference numeral 1 denotes a piezoelectric vibrator using, for
example, PZT, which is fixed to the back plate 2 and has two impedances close to the acoustic
impedance of water serving as a propagation medium on the opposite side. Conversion blocks 3
and 4 are provided.
[0003]
Here, the thickness l of the piezoelectric vibrator 1 is set to l = λ / 2, and the thicknesses of the
impedance conversion blocks 3 and 4 are set to λ / 4. However, in such a conventional
ultrasonic probe, there is no problem if the use center frequency fo is high, but if the use center
frequency fo is lowered to increase the propagation distance in water, for example, fo = In the
case of a low 30 kHz, the thickness l of the piezoelectric vibrator 1 is l = λ / 2 = V / (2fo) = 67
mm, assuming that the sound velocity V of the piezoelectric vibrator is 4000 m / s. It is
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impossible to manufacture such a thick piezoelectric vibrator, and it is too thick to make
polarization difficult and can not be used at low frequency. In order to solve this problem, the
Langevin oscillator shown in FIG. 15 has been put to practical use.
[0004]
In the Langevin vibrator of FIG. 15, 1a and 1b are a pair of piezoelectric vibrators having a
predetermined thickness, and duralmin (acoustic impedance 17 × 106 [kg / m 2 / sec] on both
sides of the piezoelectric vibrators 1a and 1b. Is a symmetrical structure in which the resonance
blocks 5a and 5b are integrally provided, and the total length of the vibrator is set to λ / 2.
Aluminum or another metal may be used for the resonance blocks 5a and 5b.
[0005]
According to such a Langevin vibrator, the use center frequency fo can be lowered, for example,
to fo = 30 kHz without thickening the piezoelectric vibrator, and the underwater propagation
distance of ultrasonic waves can be lengthened. However, in such a conventional Langevin
transducer, since the Q indicating the sharpness of the transfer function with respect to the used
center frequency is very large, the frequency band width of the ultrasonic vibration propagating
in the water becomes a narrow band, As a result, there is a problem that the received pulse width
of ultrasonic vibration becomes long and the distance resolution is poor.
[0006]
Fig. 16 shows the time change of received voltage when ultrasonic waves are emitted and
received in water using a conventional Langevin oscillator, and it is a tone-burst electric wave
including one cycle of a 27 kHz sine wave. When a pulse is applied, the reception voltage shown
as sensitivity falls below the −20 dB line of the reception voltage peak value after τ time, and
the time τ until it falls below the −20 dB line is defined as the pulse width τ.
[0007]
In the case of the conventional Langevin oscillator, the pulse width τ widens, for example, to τ
= 337 [μsec], so the distance resolution dres in water is dres = 0.25 [m]
[0008]
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FIG. 17 shows the relationship of the transfer function to the frequency of the conventional
Langevin oscillator. When the operating center frequency fo = 26.6 kHz, the relationship of the
transfer function to the illustrated frequency is obtained.
From this characteristic, the frequencies f1 and f2 determined by the −6 dB line of the peak
amplitude are f1 = 25.4 kHz F2 = 27.8 kHz. Therefore, the bandwidth Δf is Δf = 2.4 kHz.
Therefore, when the ratio band (Δf / fo) is determined, Δf / fo = 0.09, and the sharpness Q has a
narrow band characteristic exhibiting a very high value of Q = fo / Δf = 11.1. The width
increases and the distance resolution decreases.
[0009]
In order to solve the problem that the frequency band width of ultrasonic vibration propagating
in water in such a conventional Langevin transducer becomes narrow and the reception pulse
width of ultrasonic vibration becomes long and the distance resolution is poor. As shown in FIG.
18, in a broadband ultrasonic probe of a Langevin structure in which resonance blocks 10a and
10b are symmetrically provided on both sides of a pair of piezoelectric vibrators 1a and 1b, the
water of the Langeban transducer is used as a propagation medium The acoustic impedance ZB
of the resonance blocks 10a and 10b is determined so that the value of the ratio band Δf / fo at
a predetermined use center frequency fo determined from the transfer function T (f) of the
transmission / reception system is 0.2 or more. Japanese Patent Application No. 3-186745
proposes a broadband ultrasonic probe characterized by
[0010]
Here, as the resonance blocks 10a and 10b for providing wide band characteristics in which the
value of the relative band Δf / fo is 0.2 or more, for example, an epoxy compound having an
acoustic impedance ZB of 4.2 × 10 6 [kg / m 2 / sec]. It may be made of plastic using a material.
The pair of piezoelectric vibrators 1a and 1b may be only a single piezoelectric vibrator 1a.
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[0011]
According to the broadband ultrasonic probe of FIG. 18 having such a configuration, by
specifying the electroacoustic equivalent circuit of the Langevin transducer with a four-terminal
circuit, for example, a Langevin transducer using water as a propagation medium The transfer
function T (f) of the transmission / reception system is obtained, and the ratio band (Δf / fo) can
be calculated from this transfer function T (f). Therefore, the relationship between the acoustic
impedance ZB of the resonant block and the relative band is plotted, and the maximum value of
the acoustic impedance ZB of the resonant block corresponding to the minimum value 0.2 of the
relative band (Δf / fo) where the sharpness Q can be sufficiently reduced The resonant block
may be constructed using a material which is determined and whose acoustic impedance ZB is
less than the maximum value.
[0012]
The maximum value of the acoustic impedance Z M of the resonance block for which this ratio
band is 0.2 or more is, for example, 9 × 10 6 [kg / m 2 / sec] in the case of the use center
frequency fo = 31.7 kHz. do it. By using, for example, an epoxy compound material having an
acoustic impedance Z M of 4.2 × 10 6 [kg / m 2 / sec] as a material satisfying this condition, the
pulse width τ = 117 shown in FIG. [Μsec] can be shortened to about one third, and as shown in
the transfer function of FIG. 19, a wide band characteristic of sharpness Q = 2.09 is obtained at a
relative band Δf / f = 0.48, and as a result, underwater The distance resolution at can be
increased by nearly three times.
[0013]
However, in the case of the low-frequency broadband ultrasonic probe shown in FIG. 18, as
shown by the absolute value of the transfer function up to the higher order in FIG. There is a
problem that the sensitivity of the center frequency (fundamental frequency) fO of the vibration
mode in the above is too low compared to the sensitivity of the resonance frequency fT of the
piezoelectric material near 400 KHz.
[0014]
These problems do not exist in the Langevin oscillator in which the resonance blocks 5a and 5b
shown in FIG. 15 are duralmin, and the absolute value (calculated value) of the transfer function
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in water is as shown in FIG.
Looking at the frequency characteristics of the transfer function in FIG. 22, the sensitivity is
sufficiently high also near the fundamental frequency fO. However, as described above, since the
sharpness Q is high and narrow, there is a problem that the bandwidth must be broadened
without reducing the sensitivity of the fundamental frequency.
[0015]
The present invention has been made in view of such conventional problems, and provides a
broadband ultrasonic probe which can be broadened while obtaining sufficiently high sensitivity
in the vicinity of a fundamental frequency which is a use center frequency. The purpose is
[0016]
In order to achieve this object, the present invention is configured as shown in FIG.
In addition, the code | symbol in an Example drawing is shown collectively. First, the present
invention is directed to a broadband ultrasonic probe provided with a Langevin vibrator structure
in which metal resonance blocks 5a and 5b are provided on both sides of a pair of piezoelectric
vibrators 1a and 1b.
[0017]
In the present invention as such a broad band ultrasonic probe, one or more acoustic matching
layers 6 of 1⁄4 wavelength are provided on one acoustic emission surface of a Langevin
transducer. Furthermore, as shown in FIG. 7, on the other side of the acoustic emission surface
provided with the 1⁄4 wavelength acoustic matching layer 6, a block member 7 made of a
material having a very low acoustic impedance compared to water is provided. It features. As the
block member 7, a closed cell sponge or a polystyrene foam is used.
[0018]
According to such a broad band ultrasonic probe of the present invention, the acoustic emission
surface of the Langevin transducer has a sufficiently low acoustic impedance compared to water,
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for example, only by providing a plastic acoustic matching layer. It is possible to achieve both
wide band and high sensitivity at the fundamental frequency of the frequency.
[0019]
Further, by providing a block member such as a closed cell sponge having a very low acoustic
impedance as compared with water on the opposite side of the Langevin vibrator, the back
reflection can be increased to improve the reception sensitivity.
[0020]
DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a block diagram showing an
embodiment of the present invention.
In FIG. 1, reference numeral 10 denotes a low frequency Langevin vibrator, which is provided
with metal resonance blocks 5a and 5b such as duralmin on both sides of a pair of piezoelectric
vibrators 1a and 1b.
The piezoelectric vibrators 1a and 1b are in close contact with each other via the central hot side
electrode 8. The ground side electrodes 9a and 9b are provided on the opposite side, and the
resonance blocks 5a and 5b are provided outside the ground side electrodes 9a and 9b. ing. In
addition, the length of the Langevin oscillator 10 is half of the operating wavelength λ.
[0021]
In the present invention, in addition to such a low frequency Langevin vibrator 10, the acoustic
matching layer 6 of 1⁄4 wavelength of n layer is attached to one acoustic radiation surface 11a of
the Langevin vibrator 10 It features. The number of layers of the acoustic matching layer 6 may
be appropriately determined, such as one, two, three, etc., as necessary. The broadband
ultrasonic probe of the present invention having such a structure transmits and receives low
frequency ultrasonic waves of around 30 kHz through water. In order to evaluate the
characteristics of low-frequency ultrasonic wave propagation using water as a medium in such a
broad band ultrasonic probe of the present invention, a configuration as shown in FIG. 2 is
considered.
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[0022]
In FIG. 2, reference numeral 12 denotes a transmission circuit, which can be expressed using a
signal voltage source 14 generating a transmission voltage Vs at a use center frequency fo and
the internal impedance Zs, and as shown in FIG. The transmission vibrator 15 using 6 is driven.
Here, the acoustic emission surface on the opposite side to the matching layer 6 in the
transmission vibrator 15 is terminated by air by providing a closed-cell sponge 7, and the
termination by the closed-cell foam 7 increases reflection on the back surface. It is intended to
increase the reception sensitivity. It is to be noted that, for example, expanded polystyrene or the
like may be used as the closed cell sponge 7 as long as the acoustic impedance is sufficiently
small compared to water as a propagation medium.
[0023]
The transmitting vibrator 15 is contained in water 16 as a propagation medium, and the
receiving vibrator 17 is disposed in the water 16 opposed to the transmitting vibrator 15. The
receiving vibrator 17 also uses the Langevin vibrator 10 and the matching layer 6 as shown in
FIG. 1, and the receiving vibrator 17 is connected to the receiving circuit 18 provided with the
receiving impedance ZL and receives at both ends of the receiving impedance ZL. The voltage VL
is to be obtained.
[0024]
Furthermore, as for the receiving vibrator 17, as in the case of the transmitting vibrator 15, the
independent bubble sponge 7 is provided on the acoustic emission surface on the opposite side
to the matching layer 6 and terminated with air to increase back reflection to enhance receiving
sensitivity. ing. Piezoelectric transducers 1a and 1b, resonant blocks 5a and 5b using duralmin of
the broadband ultrasonic probe according to the present invention shown in FIG. 1 used for the
transmitting transducer 15 and the receiving transducer 17 of FIG. The cable signal source
impedance, the reception impedance, and the vibrator drive waveform in the configuration of are
as follows. [Piezoelectric vibrator 1a, 1b] density PT = 7.4 [g / cm2] sound velocity VT = 2952.0
[m / sec] acoustic impedance ZT = 21.8 [106 kg / m2 / sec] electromechanical coupling factor
K33 = 0.63 dielectric constant ε 33 T = 1400 thickness LT = 5.0 [mm] diameter D = 20.0 [mm]
[resonance block 5a, 5b (duralmin)] density PB = 2.79 [g / cm 2] Sound velocity VB = 6135.0 [m
/ sec] acoustic impedance ZB = 17.1 [106 kg / m2 / sec] length LB = 41.0 [mm] impedance ZC =
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50.0 [Ω] signal propagation velocity VC = 2.0 * 108 [m / sec] length LC = 2.0 [m] [signal source
impedance] ZS = 50.0 [Ω] [reception impedance] ZL = 10.0 * 103 [Ω] [oscillator Drive waveform
(tone bar Carrier frequency) Freq = 30.0 [kHz] 1 amplitude Amp = 100.0 [V] wave number
Nburst = 1 Furthermore, as a matching layer 6 of the transmitting vibrator 15 and the receiving
vibrator 17, a single matching layer, two Taking the example of the double matching layer and
the triple matching layer, the density and acoustic impedance in these matching layers are as
shown in FIG.
[0025]
Transfer functions and reception waveforms obtained in the configuration of FIG. 2 under such
conditions are shown in FIGS. First, FIG. 4 shows the case where the matching layer 6 is not
provided, and the transmission numbers of a single matching layer, a double matching layer, and
a triple matching layer as the matching layer 6 up to the higher order mode. As apparent from
FIG. 4, the broadband ultrasonic probe provided with the matching layer (single matching layer,
double matching layer and triple matching layer) of the present invention as compared with the
case without the matching layer is The sensitivity is only lowered by about -3 to -6 dB at the
resonance frequency which is three or five times the fundamental vibration as well as the
fundamental vibration, and the relative band can be broadened by about three to six times.
[0026]
The following is a detailed description of each of the single matching layer, the double matching
layer, and the triple matching layer. FIG. 5 shows the absolute value of the transfer function of
the inventive broadband ultrasound probe with a single matching layer for a fundamental
frequency fo = 30.3 kHz, and the bandwidth Δf1 of the −3 dB line is Δf1 = 7. It is 4 kHz.
[0027]
FIG. 6 shows the time change of the received voltage in the broadband ultrasonic probe of a
single matching layer, and the time τ until the received waveform returns again beyond the -20
dB line, that is, the pulse width τ is τ = 151. It is [μsec]. FIG. 7 shows the absolute value of the
transfer function in the case of the double matching layer for the fundamental frequency fo =
31.5 kHz, and in this case, the bandwidth Δf2 of the −3 dB line is Δf2 = 11.8 kHz and a single
matching layer. It is spreading compared with. FIG. 8 shows the time change of the reception
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voltage in the case of the double matching layer, and the pulse width τ determined by the −20
dB line is shorter than that of the single matching layer in FIG. .
[0028]
Further, FIG. 9 shows the absolute value of the transfer function in the case of the triple
matching layer for the fundamental frequency fo = 32.8 kHz, and the bandwidth Δf3 given by
the −3 dB line further extends to Δf3 = 15.2 kHz. FIG. 10 shows the time change of the
reception voltage in the case of the triple matching layer, and the pulse width τ determined by
the −20 dB line is further shortened to τ = 87 [μsec].
[0029]
The broadband ultrasonic probe according to the present invention determined from the absolute
value of the transfer function of each of the single matching layer, the double matching layer,
and the triple matching layer shown in FIGS. Is summarized in FIG. FIG. 11 also shows one having
no matching layer in FIG. 15 and one having no metal in the resonant block.
[0030]
As apparent from FIG. 11, as the characteristics of the transfer function, as the number of
matching layers increases, the relative band Δf / fo increases and the bandwidth becomes wider,
while the sensitivity decreases. In addition, as the characteristics of the reception waveform, as
the number of matching layers increases, the pulse width becomes shorter, and as a result, the
distance resolution dres is improved. Further, when the present invention is compared with the
one without the matching layer in FIG. 15, the relative bandwidth is broadened to 3 to 6 times,
and the sensitivity to this wide bandwidth is 0.6 to 0.4 The degree of reduction is sufficient, and
the effectiveness of the broadband ultrasonic probe provided with the matching layer according
to the present invention has been confirmed.
[0031]
In addition, the sensitivity is substantially improved to 25 to 16 times although the relative band
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is almost the same as in the case where no metal is used for the resonance block in FIG.
Furthermore, with regard to the characteristics of the reception waveform, the pulse width is
about one third or less as compared with the one without the matching layer in FIG. 15, and
sufficient distance resolution is realized, and the resonance block in FIG. Even in the case of the
double matching layer, substantially equivalent distance resolution can be realized compared to
the above.
[0032]
FIG. 12 shows the absolute value of the transfer function of the ultrasonic probe without the
matching layer shown in FIG. 15 for the fundamental frequency fo = 28.9 KHz, and the pulse
width Δf4 of the −3 dB line is Δf4 = 2 .2 and narrow. Also, FIG. 13 shows the time change of
the reception voltage in the case of not having the matching layer of FIG. 15 similarly for the
fundamental frequency fo = 33.4 KHz, and the pulse width τ given by the −20 dB line is τ =
507 It is spread as [μsec], and the distance resolution is extremely low as shown in FIG.
[0033]
In the above embodiment, duralmin is taken as an example of the resonance blocks 5a and 5b,
but any other metal may be used if it is close to the acoustic impedance of water as a
propagation medium, for example, aluminum. You may use it. In the above embodiment, the pair
of piezoelectric vibrators 1a and 1b is used as an example, but the same is true even if only a
single piezoelectric vibrator 1a is used. is there.
[0034]
Furthermore, in the above embodiment, the case where up to three matching layers 6 are
provided is taken as an example, but if it is desired to further broaden the bandwidth by lowering
the receiving sensitivity, the matching exceeding three layers is performed. The number of layers
may be set, and the number of matching layers 6 can be appropriately determined according to
the reception sensitivity and the wide band characteristic.
[0035]
As described above, according to the present invention, the acoustic radiation surface of the
Langevin vibrator is provided with an acoustic matching layer such as plastic, and the acoustic
radiation surface on the opposite side of the acoustic matching layer is provided with water.
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Compared with the above, it is possible to achieve both wide band and high sensitivity at the use
center frequency of low frequency by a simple configuration in which a block of, for example, a
closed cell sponge having sufficiently low acoustic impedance is provided.
[0036]
Brief description of the drawings
[0037]
1 is a block diagram of an embodiment of the present invention
[0038]
Fig. 2 Configuration explanatory diagram for obtaining the transfer function of the transmission /
reception system using the probe of the present invention
[0039]
Fig. 3 An illustration showing the conditions of the matching layer used in the present invention
[0040]
Fig. 4 An illustration showing the frequency characteristics of the absolute value of the transfer
function of each of a single matching layer, a single matching layer, a 20 matching layer and a
triple matching layer without a matching layer up to the high order
[0041]
Explanatory diagram showing the frequency characteristic of the absolute value of the transfer
function near the used center frequency fo in the case of FIG.
[0042]
Explanatory diagram showing the time change of the received voltage in the case of FIG.
[0043]
Fig. 7 is an explanatory diagram showing frequency characteristics of the absolute value of the
transfer function near the use center frequency fo in the case of the double matching layer
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[0044]
Fig. 8 An explanatory view showing the time change of the received voltage in the case of the
double matching layer
[0045]
Fig. 9 is an explanatory diagram showing the frequency characteristic of the absolute value of the
transfer function near the use center frequency fo in the case of the triple matching layer
[0046]
Fig. 10 An explanatory diagram showing the time change of the received voltage in the case of
the triple matching layer
[0047]
Explanatory diagram showing characteristics of the transfer function and the received waveform
of the present invention in comparison with the conventional example
[0048]
Explanatory diagram showing the frequency characteristic of the absolute value of the transfer
function near the used center frequency fo without the matching layer in FIG.
[0049]
13 is an explanatory view showing the time change of the received voltage in the case without
the matching layer in FIG.
[0050]
Fig. 14: an illustration of a conventional water immersion type ultrasonic probe
[0051]
Fig.15 An illustration of a conventional Langevin oscillator
[0052]
Explanatory diagram showing the reception waveform of the conventional Langevin oscillator
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[0053]
Fig. 17 An explanatory diagram showing the frequency characteristic of the transfer function of
the conventional Langevin oscillator
[0054]
Fig. 18 An illustration of a Langevin oscillator according to the prior invention.
[0055]
Explanatory diagram showing the reception waveform of the Langevin oscillator of Figure 19
prior application
[0056]
Explanatory diagram showing the frequency characteristic of the transfer function of the
Langevin oscillator of Fig. 20 prior application
[0057]
Explanatory diagram showing the frequency characteristics of the transfer function of the
Langevin oscillator of the prior application of FIG. 21 to a high order
[0058]
Fig. 22 is an explanatory diagram showing the frequency characteristics of the transfer function
of the conventional Langevin oscillator of Fig. 15 up to the high order
[0059]
Explanation of sign
[0060]
1a, 1b: Piezoelectric vibrators 5a, 5b: Resonant block (made of metal) 6: Matching layer (plastic
layer) 7: Closed bubble sponge 8: Hot side electrode 9a, 9b: Ground side electrode 10: Langevin
vibrator 11a, 11b : Sound radiation surface 12: Transmission circuit 14: Signal voltage source 15:
Transmission vibrator 16: Water 17: Reception vibrator 18: Reception circuit
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