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JPH0537999

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DESCRIPTION JPH0537999
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
broadband ultrasonic probe using a Langevin transducer structure that uses water as a
propagation medium.
[0002]
2. Description of the Related Art Heretofore, as an ultrasonic probe used in a water immersion
type ultrasonic fluid probe, for example, the one shown in FIG. 9 is used. In the ultrasonic probe
shown in FIG. 9, 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 frequency fo is high, but if the use frequency fo is
lowered to increase the propagation distance in water, for example, fo = 30 kHz In the case of
reducing the thickness, the thickness l of the piezoelectric vibrator 1 is l = λ / 2 ≒ V / (2fo) = 67
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mm, assuming that the sound velocity V of the piezoelectric vibrator is 4000 [m / s]. It is
impossible to manufacture such a thick piezoelectric vibrator, and it is too thick to make it
difficult to polarize when a voltage is applied, and it can not be used at low frequencies.
[0004]
In order to solve this problem, the Langevin oscillator shown in FIG. 10 has been put to practical
use. In the Langevin vibrator of FIG. 10, 1a and 1b are a pair of piezoelectric vibrators having a
predetermined thickness, and duralmin (acoustic impedance 17 × 10 6 [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.
[0005]
Aluminum may be used for the resonance blocks 5a and 5b. According to such a Langevin
vibrator, the operating 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.
[0006]
However, in such a conventional Langevin oscillator, since the Q indicating the sharpness of the
transfer function with respect to the used center frequency is very large, ultrasonic vibration
propagating in water is caused. As a result, there is a problem that the received pulse width of
the ultrasonic vibration becomes long and the distance resolution is poor.
[0007]
Fig. 11 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
containing one cycle of a 27 kHz sine wave. When a pulse is applied, the received voltage shown
as sensitivity falls below the −20 dB line after τ time, and the time τ until it falls below the
−20 dB line is defined as the pulse width τ.
[0008]
In the case of the conventional Langevin oscillator, the pulse width τ widens, for example, to τ
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= 337 [μsec], so the distance resolution dres in water is dres = 0.25 [m]
[0009]
FIG. 12 shows the relationship of the transfer function to the frequency of the conventional
Langevin oscillator. When the use 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.
[0010]
The present invention has been made in view of such conventional problems, and is directed to
an ultrasonic probe having a structure as a Langevin transducer, and has a sufficiently high
distance in water even if the use center frequency fo is low. It is an object of the present
invention to provide a wide band ultrasonic probe capable of obtaining resolution.
[0011]
In order to achieve this object, the present invention is configured as follows.
In addition, the code ¦ symbol in an Example drawing is shown collectively.
First, as shown in FIG. 1, the present invention is directed to a wideband ultrasonic probe of a
Langevin structure in which resonant blocks 10a and 10b are symmetrically provided on both
sides of a pair of piezoelectric vibrators 1a and 1b.
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[0012]
In the present invention as such a broad band ultrasonic probe, the relative bandwidth Δf at a
predetermined use center frequency fo determined from the transfer function T (f) of the
transmission and reception system using the water of the Langevin transducer as the
propagation medium The acoustic impedance ZB of the resonance blocks 10a and 10b is
determined so that the value of / fo is 0.2 or more. A wide band characteristic having a value of
the relative band Δf / fo of 0.2 or more The resonance blocks 10a and 10b to be provided may
be made of, for example, a plastic using an epoxy compound material having an acoustic
impedance ZB of 4.2 × 10 6 [kg / m 2 / sec].
[0013]
Furthermore, as shown in FIG. 13, the pair of piezoelectric vibrators 1a and 1b may be only a
single piezoelectric vibrator 1a.
[0014]
According to the broadband ultrasonic probe of the present invention having such a
configuration, by specifying the electroacoustic equivalent circuit of the Langevin transducer
with a four-terminal circuit, for example, the Langeban using water as a propagation medium The
transfer function T (f) of the transmission / reception system of the vibrator is determined, 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 formed using a material which is
determined and whose acoustic impedance ZB is equal to or less than the maximum value.
[0015]
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.
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By using an epoxy compound material with an acoustic impedance ZM of 4.2 × 10 6 [kg / m 2 /
sec] as a material that satisfies this condition, for example, the distance resolution in water is
nearly tripled. It can be enhanced.
[0016]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a block diagram showing
an embodiment of a wide band ultrasonic probe according to the present invention. In FIG. 1, 1a
and 1b are piezoelectric vibrators, for example, PZT is used, and the acoustic impedance Zo of
PZT is approximately Zo = 40 × 10 6 [kg / m 2 / sec]
[0017]
The pair of piezoelectric vibrators 1a and 1b are integrally bonded and fixed, and the resonance
blocks 10a and 10b are symmetrically fixed on the outer sides of the respective piezoelectric
vibrators 1a and 1b. That is, the wide band ultrasonic probe of the present invention has the
same transducer structure as the conventional Langevin transducer shown in FIG. A difference
from the conventional Langevin transducer is that the frequency characteristics of the transfer
function T (f) when, for example, water is used as a propagation medium are compared as the
resonance blocks 10a and 10b in the broadband ultrasonic probe of the present invention The
acoustic impedance Z M is set so as to obtain a wide band characteristic of 0.2 or more in the
band (Δf / fo). As a material for realizing the acoustic impedance ZM having the specific band
(.DELTA.f / fo) of 0.2 or more, for example, plastic resonance blocks 10a and 10b made of an
epoxy compound material may be used.
[0018]
In the embodiment of FIG. 1, when the use center frequency fo is set to fo = 31.7 kHz, the total
length l of the broadband ultrasonic probe is as shown. The thicknesses l1 and diameters D1 of
the piezoelectric vibrators 1a and 1b at this time are l1 = 2 mm and D1 = 24 mm, and the lengths
l2 and diameters D2 of the resonance blocks 10a and 10b are l2 = 17 mm, D2 = 20 mm
[0019]
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Next, as shown in the embodiment of FIG. 1, a relative band (Δf / fo) is 0.2 in the resonance
blocks 10a and 10b of the broadband ultrasonic probe having the Langevin transducer structure
of the present invention. The principle of making a wide band characteristic by using a material
which becomes the above-mentioned acoustic impedance Z M will be described in detail. First,
the inventors of the present application considered the equivalent circuit shown in FIG. 2 as an
equivalent circuit of an ultrasonic probe using a Langevin transducer.
[0020]
First, as shown in FIG. 2, the inventors of the present application put together an ultrasonic probe
using a Langevin transducer into a simple 4-terminal equivalent circuit using F parameters (A, B,
C, D). Next, a configuration as shown in FIG. 3 will be considered in order to evaluate the relative
bandwidth of the transmission / reception system using the Langevin oscillator and the pulse
width τ of the reception waveform. In FIG. 3, reference numeral 13 denotes a transmission
circuit, which can be expressed using a constant voltage power supply 14 generating a
transmission voltage Vs of a use center frequency fo and an internal impedance Zs, and driving a
transmission vibrator 16 using a Langevin vibrator There is. The transmitting vibrator 16 is
contained in water 17 as a propagation medium, and the receiving vibrator 18 is disposed in the
water 17 opposed to the transmitting vibrator 16. The receiving vibrator 18 is also a Langevin
vibrator, and the receiving vibrator 18 is connected to the receiving circuit 19 having the
receiving impedance Zl so that the receiving voltage Vl is obtained across the receiving
impedance Zl.
[0021]
If the equivalent circuit represented by the F parameter shown in FIG. 2 is applied to the
transmission / reception system of the Langevin vibrator using water as a propagation medium
shown in FIG. 3, an equivalent circuit of the transmission / reception system shown in FIG. 4 can
be obtained. With respect to the equivalent circuit of the transmission / reception system of FIG.
4, F parameters (A, B, C, D) of the equivalent circuit of the Langevin oscillator using transfer
function T (f) for transmission oscillator 16 and reception oscillator 18 are used. If it represents,
it will become like following Formula. T (f) = Vl / Vs = (2ZMZl) / [AZM + B + Zs (CZM + D)] [AZM +
B + Zl (CZM + D)] Thus, the transfer function T (f) of the transmission / reception system using
the Langevin oscillator Is obtained, the ratio band (.DELTA.f / fo) can be calculated by obtaining
the values of the F parameters (A, B, C, D) of the equivalent circuit of FIG. 2 at the use frequency
fo. The ratio band (Δf / fo) is calculated by obtaining the transfer function T (f) at the use
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frequency fo in the equation (1), and using this value as a peak value, the value of the transfer
function T (f) lowered by -6 dB The frequencies f1 and f2 before and after the use center
frequency fo giving F.sub.0 are obtained and calculated as .DELTA.f / fo = (f2-f1) / fo.
[0022]
Next, when the value of the ratio band (Δf / fo) which can be calculated from the transfer
function T (f) of the equation (1) is plotted for the case of the use center frequency fo = 31.7 kHz,
the characteristic shown in FIG. 5 is obtained. In FIG. 5, as the acoustic impedance ZB of the
resonance blocks 10a and 10b becomes lower, the value of the relative band increases.
[0023]
Here, the relative band (Δf / fo) indicates the frequency characteristic of ultrasonic vibration
propagating in water, and the larger the value of the relative band (Δf / fo), the broader the band
characteristics. Since the pulse width τ is shortened and the distance resolution in water can
also be improved if the propagation characteristic of ultrasonic vibration is a wide band
characteristic, in the present invention, for example, the ratio band (Δf / fo) is Δf. The acoustic
impedance ZB of the resonance blocks 10a and 10b is determined such that /fo=0.2 is a
minimum value and a ratio band of 0.2 or more is obtained.
[0024]
Assuming that the acoustic impedance of the piezoelectric vibrator is 40 × 10 6 [kg / m 2 / s],
the acoustic impedance Z B of the resonance blocks 10 a and 10 b giving the ratio band (Δf / fo)
= 0.2 is Z B = 10 × 10 6 kg / m 2 / sec]. Therefore, the resonant blocks 10a and 10b may be
constructed using a material having an acoustic impedance ZB less than this value.
[0025]
Specifically, as shown in the embodiment of FIG. 1, an epoxy compound material can be used,
and the acoustic impedance ZB of the epoxy compound material is ZB = 4.2 × 10 6 [kg / m 2 /
sec]. When adjusted, it is included in the range 20 of the vibrator of the present invention shown
in FIG. 5, and in this case, the specific band (Δf / fo) is as high as 0.48, and sufficient broadband
characteristics can be obtained.
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[0026]
Incidentally, the acoustic impedance of the resonance blocks 5a and 5b using duralmin or
aluminum in the conventional Langevin vibrator shown in FIG. 10 is 16 × 10 −6 [kg / m 2 /
sec] or more, and the conventional impedance shown in FIG. The ratio band (Δf / fo) in the range
30 of the Langevin transducer is slightly larger than 0.1, the propagation characteristic of
ultrasonic vibration becomes a narrow band characteristic, the pulse width τ becomes long, and
the distance resolution is extremely low. It will be.
[0027]
Of course, even if the relative band (Δf / fo) is less than 0.2, if it is larger than the range 30 of
the conventional Langevin oscillator, a considerable improvement effect can be expected, but it is
not so remarkable. It can be said that it is effective to form the resonance blocks 10a and 10b
with a material that provides an acoustic impedance ZB that makes .DELTA.f / fo 0.2 or more.
[0028]
FIG. 6 shows the operating frequency fo = 31 when molded using the epoxy compound material
of acoustic impedance ZB = 4.2 × 10 −6 [kg / m 2 / sec] for the resonance blocks 10 a and 10
b in the embodiment of FIG. 1. 7 shows a received waveform of ultrasonic vibration propagated
in water at 7 kHz.
In FIG. 6, the reception waveform of the ultrasonic wave by the wideband ultrasonic probe of the
present invention provided with the resonance blocks 10a and 10b made of the epoxy compound
material is until the reception waveform returns again beyond the -20 dB line. The pulse width τ
becomes τ = 117 [μsec], and the use center frequency fo slightly differs, but the pulse width τ
is reduced to about one third of that of the conventional Langevin oscillator shown in FIG. ing.
Also, the distance resolution dres in water in this case is dres = 0.088 [m], and the distance
resolution could be improved by nearly three times compared to the conventional example of
FIG.
[0029]
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FIG. 7 shows a transfer function with respect to the frequency f when propagating in water when
the center frequency fo = 31.7 kHz is used in the embodiment of FIG. 1 in which the resonance
blocks 10a and 10b are made of an epoxy compound material.
In FIG. 7, the frequencies f1 and f2 of −6 dB lines on both sides with respect to the center
frequency fo = 31.7 kHz are f1 = 24.1 kHz f2 = 39.3 kHz, and thus Δf is Δf = f2−f1 = 15.2 kHz.
This bandwidth .DELTA.f is expanded five times or more as compared with the conventional
example of FIG.
[0030]
Specifically, when the ratio band (Δf / fo) is determined, Δf / fo = 0.48, and therefore the
sharpness Q is obtained, and a sufficiently wide band characteristic is obtained in the wide band
ultrasonic probe of the present invention, As a result, it has been confirmed that the distance
resolution in water has been significantly improved.
[0031]
FIG. 9 is a block diagram showing another embodiment of the present invention. In the
embodiment shown in FIG. 1, a single piezoelectric vibrator 1a provided with a pair of
piezoelectric vibrators 1a and 1b is shown. The resonance blocks 10a and 10b are symmetrically
fixed on both sides of the piezoelectric vibrator 1a.
Also in the embodiment of FIG. 9, the resonance blocks 10a and 10b are made of an acoustic
impedance ZB material such as an epoxy compound material which has a specific band (.DELTA.f
/ fo) at an operating frequency fo of 0.2 or more.
[0032]
In the above embodiment, an epoxy compound material is taken as an example of the material
constituting the resonance blocks 10a and 10b, but the specific band (Δf) determined from T (f)
given by the above equation (1) Resonant blocks 10a and 10b can be made of appropriate
materials as long as the material has an acoustic impedance ZB such that / fo) is 0.2 or more. Of
course, it is desirable to construct the resonant blocks 10a, 10b with a material of acoustic
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impedance ZB, where the fractional band 0.2 is a minimum and results in a larger fractional
band.
[0033]
As described above, according to the present invention, the resonant block of the Langevin
vibrator structure is constructed using a material having an acoustic impedance with a relative
band of 0.2 or more, such as underwater. As the band characteristics of the propagating
ultrasonic wave with respect to the use center frequency of the ultrasonic wave as a broad band
characteristic, the pulse width of the ultrasonic vibration can be narrowed, and the distance
resolution of the ultrasonic wave in water can be greatly improved.
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