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DESCRIPTION JPS62188599

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DESCRIPTION JPS62188599
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
ultrasonic probe which prevents the destruction of a piezoelectric element due to an overcurrent.
[Prior Art] In recent years, non-destructive testing of various structures 9 Ultrasonic waves have
come to be used as a general method for medical diagnostic etc., but ultrasound probes used for
these non-destructive testing It is desirable that the sensor has high sensitivity and high
resolution over a wide area of the object, so a voltage of several hundred volts is usually applied.
The piezoelectric element used for such an ultrasonic probe is made of zircon-lead titanate (PZT)
having a large electromechanical coupling coefficient, but since this material is also a
ferroelectric at the same time, By applying a high voltage, a hysteresis loop is drawn, and heat
generation due to normal dielectric loss (tan δ loss) and heat generation due to hysteresis loss
are superimposed. (Problems to be Solved by the Invention) On the other hand, recently, research
and development of ultrasound has been advanced, and one of them is pulse compression
technology. However, when this pulse compression technique is used, the heat generation of the
probe is further increased, and there is a possibility that the piezoelectric element may be broken
if the heat radiation countermeasure is insufficient. In particular, when the probe is inserted into
the body as in an ultrasound endoscope, the destruction of the piezoelectric element may cause
the structural destruction of the entire probe, and the function of the probe is merely stopped.
Not, it will be in an undesirable state for safety. In addition, even in the nondestructive inspection
of a structure, a current leak to the structure may occur due to the breakage of the piezoelectric
element, which may cause various adverse effects. The present invention has been made under
such circumstances, and an object thereof is to provide a highly reliable and safe ultrasonic
probe that prevents the breakage of a piezoelectric element due to an overcurrent. [Means for
Solving the Problems] In order to achieve the above object, the present invention provides
pressure i! A current limiting element for limiting a current value according to temperature is
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provided in a housing in which the element is accommodated, and the current limiting element is
connected in series to the piezoelectric element. [Operation] In the present invention, when the
heating layer of the probe becomes large, the current limiting element operates to limit the
current supplied to the piezoelectric element to I11, so that the piezoelectric element can be
prevented from being broken. The present invention will be described below based on the
embodiments shown in the drawings. FIG. 1 shows an embodiment of an ultrasonic probe
according to the present invention, in which a part of the probe is shown in cross section. In the
figure, reference numeral 1 denotes a piezoelectric element made of PZT or the like, and the
piezoelectric element 1 is accommodated in a housing 2 in which a metal such as stainless steel
is formed in a cylindrical (or rectangular) shape.
The acoustic matching layer 3 is disposed on the ultrasonic wave transmitting / receiving side of
the piezoelectric element 1, and the damping layer 4 and the sealing layer 5 are disposed on the
opposite side of the acoustic matching layer 3 in an overlapping state. An insulating layer 6 is
provided between the damping layer 4 and the housing 2. Both surfaces of the piezoelectric
element 1 are electrodes, and a lead wire 7 is connected to the electrode on the ultrasonic
transmission / reception side. The lead 417 is connected to the housing 2 so that the
piezoelectric element 1 is grounded to the housing 2. Also, a lead 1118 is connected to the
electrode opposite to the ultrasonic transmitting and receiving side. The lead la8 passes through
the inside of the damping layer 4 and the seal layer 5 and is connected to the coaxial cable 9
provided at the lower part of the housing 2, and a positive characteristic thermistor 1o is
provided between the coaxial cable 9 and the lead $ 18. It is inserted. The positive temperature
coefficient thermistor 10 is provided in the seal 1I15, and the seal layer 5 is formed of an
insulating seal material such as silicon. Next, the operation of the positive temperature coefficient
thermistor 1o will be described. FIG. 2 shows the relationship between the resistance value of the
positive characteristic thermistor and the temperature, and it can be seen from FIG. 2 that the
resistance value of the positive characteristic thermistor 10 rises sharply when it reaches a
certain constant temperature (Tc) or more. Therefore, by connecting the positive characteristic
thermistor 10 in series to the piezoelectric element 1 as in the third factor, the electric current
supplied to the piezoelectric element 1 is generated by the probe! It can be limited according to
(temperature). Although the positive temperature coefficient thermistor 1o is provided in the seal
layer 5 in the above embodiment, it may be provided in the damping layer 4 as shown in FIG.
However, in that case, it is necessary to encapsulate the whole of the positive temperature
coefficient thermistor 10 with the insulating material 11. Also, as shown in FIG. 5, the positive
temperature coefficient thermistor 10 is formed in a disk shape having substantially the same
diameter as the piezoelectric element 1, and electrodes are provided on the disk surface of the
back and front to use epoxy adhesive, conductive adhesive, and soldering. It may be directly
bonded to the piezoelectric element 1. In this way, the acoustic impedance of the positive
temperature coefficient thermistor 10 shows a value close to that of the piezoelectric element 1,
so the frequency of the ultrasonic wave is reduced by an amount corresponding to the thickness
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of the positive temperature coefficient thermistor. While being able to prevent the bending selfoscillation which generate | occur | produces when non-piezoelectric material is used, it is
effective in the ability to suppress the ringing image development by an electrical damping effect.
The seal 115 of the first embodiment shown in FIG. 1 and the insulating material 11 of the
second embodiment shown in FIG. 4 are preferably made of an elastic material such as silicon.
This is because the seal layer 5 or the insulating material 11 absorbs the stress due to the
thermal expansion when the temperature of the positive temperature coefficient thermistor
sharply rises. As described above, according to the present invention, a current limiting element
for limiting the current value according to the temperature is provided in the housing in which
the piezoelectric element is accommodated, and this current limiting element is connected in
series to the piezoelectric element. As a result, the current supplied to the piezoelectric element
can be limited according to the amount of heat generation of the probe, so destruction of the
piezoelectric element due to an overcurrent can be prevented, and an ultrasonic probe with high
reliability and safety can be provided.
[0002]
Brief description of the drawings
[0003]
FIG. 1 is a cross-sectional view of an ultrasonic probe showing a first embodiment of the present
invention, FIG. 2 is a graph showing temperature-resistance characteristics of a positive
characteristic thermistor, and FIG. 3 is a circuit of the first embodiment FIG. 4 is a cross-sectional
view of an ultrasound probe showing a second embodiment of the present invention, and FIG. 5
is a cross-sectional view of the ultrasound probe showing a third embodiment of the present
invention.
DESCRIPTION OF SYMBOLS 1 ... Piezoelectric element, 2 ... housing, 3 ... Acoustic matching layer,
4 ... Damping layer, 5 ... Seal layer, 6 ... Insulating layer, 7.8 ... Lead wire , 9 ... coaxial cable, 10 ...
positive characteristic thermistor. Applicant agent Patent attorney Atsushi Tsuboi Fig.1 Fig.2
Fig.3 '24 Fig.5
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