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JP2005027186

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DESCRIPTION JP2005027186
The present invention provides an ultrasonic transducer capable of making unnecessary or lower
a DC bias voltage used for adsorbing a diaphragm to a lower electrode in an electrostatic
ultrasonic transducer. SOLUTION: A vibrating film 11 vibrating in an ultrasonic frequency band,
an upper electrode 12 which is a ferromagnetic material such as a nickel thin film integrally
formed on the upper surface of the vibrating film 11 by deposition or the like, and a lower
surface of the vibrating film 11 face each other. And a lower electrode 13 having characteristics
as a permanent magnet such as a plastic magnet having conductivity, and the like. [Selected
figure] Figure 1
Ultrasonic transducer
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
ultrasonic transducer for converting an electrical signal and an ultrasonic signal. [0002]
Conventional main ultrasonic transducers (ultrasonic transducers) are of a resonance type and an
electrostatic type (electrostatic type). The resonant ultrasonic transducer converts an electrical
signal and an ultrasonic signal using the resonance phenomenon of piezoelectric ceramic.
Therefore, the transmission and reception characteristics of the ultrasonic signal become good in
a relatively narrow frequency band around the resonance frequency. On the other hand, the
electrostatic ultrasonic transducer converts an electrical signal and an ultrasonic signal by
vibration due to the electrostatic effect. At that time, a wide band type frequency characteristic is
realized by forming a plurality of electrostatic capacitances for producing the electrostatic effect
and making each capacitance different (see, for example, Patent Documents 1 and 2 and Nonpatent Document 1). ). Here, an example of the configuration of a conventional electrostatic
ultrasonic transducer will be described with reference to FIG. 6 and FIG. The electrostatic
ultrasonic transducer shown in FIG. 6 uses, as a vibrator, a dielectric 81 (insulator) such as PET
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(polyethylene terephthalate resin) having a thickness of about several μm (about 3 to 10 μm).
For the dielectric 81, an upper electrode 82 formed as a metal foil is integrally formed on the
upper surface thereof by a process such as vapor deposition, and a brass lower electrode 83 is
provided in contact with the lower surface. . The lower electrode 83 is connected to the lead 84
and fixed to a base plate 85 made of Bakelite or the like. The dielectric 81 and the upper
electrode 82 and the base plate 85 are crimped by the case 80 together with the metal rings 86,
87 and 88 and the mesh 89. A DC power supply 90 and an AC power supply 91 are connected
between the upper electrode 82 and the lower electrode 83. The DC power supply 90 is for
generating a bias voltage for adsorption of the upper electrode 82, and generates a voltage of
about 50 to 150 V DC. The AC power supply 91 is for generating an AC voltage of a frequency of
20 kHz or more for generating an ultrasonic signal, and generates a voltage of about 50 to 150 V
at peak-peak. Further, on the surface of the lower electrode 83 on the side of the dielectric 81, a
plurality of microgrooves 83a of about several tens to several hundreds of μm each having an
uneven shape are formed. The random minute grooves 83a are formed by, for example, manually
roughening the surface of the lower electrode 83 with a file.
As shown in FIG. 7, in the dielectric 81, when a DC voltage is applied between the upper
electrode 82 and the lower electrode 83, the upper electrode 82 and the lower electrode 83
attract each other by electrostatic force, that is, the lower electrode 83. It adsorbs to the convex
part 83b of. Each minute groove 83a forms an air gap between the lower electrode 83 and the
dielectric 81, and each has a minute capacity. Therefore, the distribution of capacitance between
the upper electrode 82 and the lower electrode 83 slightly changes. When an AC voltage is
applied in a state in which a DC bias is applied, each portion corresponding to the groove 83 a of
the dielectric 81 vibrates as shown by a broken line with different resonance frequencies. In the
electrostatic ultrasonic transducer, the frequency characteristic of the ultrasonic transducer is
made to be a wide band by thus forming an infinite number of capacitors having different sizes
and depths of air gaps. [Patent Document 1] JP-A-2000-50387 (pages 9-10, FIGS. 11 and 12)
[Patent Document 2] JP-A-2000-50392 (pages 5 and 3) Non-Patent Document 1 “Electrode
Surface Shape and Output Sound Pressure of Electrostatic Ultrasonic Transducer” January
1995, Toshigen Aoki, The University of Electro-Communications [0009] Problems to be Solved
by the Invention The ultrasonic transducer of the above-mentioned type required a DC bias
voltage (DC bias voltage) of several tens to several hundreds of V to adsorb the thin film of the
upper electrode and the bulk material of the lower electrode. The DC bias voltage is a high
voltage, which causes the size increase / power increase / cost increase of the device as well as
the danger of electric shock and the like. The present invention has been made in view of such
circumstances, and in an electrostatic ultrasonic transducer, an ultrasonic transducer capable of
making a DC bias voltage unnecessary or reducing the voltage (ultrasonic transducer The
purpose is to provide SUMMARY OF THE INVENTION In order to solve the above problems,
according to the present invention, there is provided a vibrating membrane vibrating in an
ultrasonic frequency band and a conductor which is a ferromagnetic material formed on one
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surface of the vibrating membrane. And a magnet disposed opposite to the other surface of the
vibrating membrane. For example, a vibrating film is made of a polymeric material forming an
insulator such as PET (polyethylene terephthalate), and a thin film of a conductor such as a metal
of a ferromagnetic material such as nickel is formed on one surface thereof by a process such as
plating. The vibrating membrane is attracted to the magnet by the attraction between the
conductor of the ferromagnetic body and the magnet disposed on the other surface of the
vibrating membrane.
Therefore, even without using a direct current bias voltage, it is possible to obtain the necessary
adsorption power for the vibrating film. Alternatively, even in the case where a DC bias is
required, a desired adsorption power can be obtained with a DC bias with a smaller voltage value
than in the prior art by using the adsorption power by the magnet and the adsorption power by
the DC bias in combination. Still another invention is characterized in that the magnet is made of
a mixture of a polymer compound and a ferromagnetic powder and is unipolar-magnetized, and
is a conductor. I assume. In this configuration, the magnet is made of a mixture of a polymer
compound such as a plastic magnet and a rubber magnet and a ferromagnetic powder, and
conductivity is ensured by the ferromagnetic powder, and the magnet is used as a conductor. Do.
An ultrasonic signal can be generated by applying an alternating voltage to the conductor formed
by the magnet and the conductor formed on the vibrating membrane. Moreover, magnetization
can be simplified by using single-pole magnetization. Still another invention is characterized in
that the magnet is made of a mixture of a polymer compound and a ferromagnetic powder and is
multipolar-magnetized, and is a conductor. I assume. In this configuration, the magnet is made of
a mixture of a polymer compound such as a plastic magnet and a rubber magnet and a
ferromagnetic powder, and conductivity is ensured by the ferromagnetic powder, and the magnet
is used as a conductor. Do. An ultrasonic signal can be generated by applying an alternating
voltage to the conductor formed by the magnet and the conductor formed on the vibrating
membrane. Moreover, the magnetic force of a magnet can be raised by setting it as multipolar
magnetization. Still another invention is characterized in that the magnet is a single-pole
magnetized permanent magnet, and further comprising a nonmagnetic conductor between the
magnet and the vibrating membrane. In this configuration, an ultrasonic signal can be generated
by applying an alternating voltage to a nonmagnetic conductor such as aluminum, copper, brass
or the like and a conductor formed on a vibrating film. Moreover, magnetization can be simplified
by using single-pole magnetization. Here, as the permanent magnet, it is desirable to use one
having a relatively strong magnetic force, such as a rare earth magnet. [0015] Still another
invention is characterized in that the magnet is a multipole magnetized permanent magnet, and
further comprising a nonmagnetic conductor between the magnet and the diaphragm. In this
configuration, an ultrasonic signal can be generated by applying an alternating voltage to a
nonmagnetic conductor such as aluminum, copper, brass or the like and a conductor formed on a
vibrating film.
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Moreover, the magnetic force of a magnet can be raised by setting it as multipolar magnetization.
Here, as the permanent magnet, it is desirable to use one having a relatively strong magnetic
force, such as a rare earth magnet. Further, in the above configuration, a plurality of grooves
having different depths may be provided on the surface of the magnet on the vibrating film side,
or a plurality of grooves having different depths on the vibrating film side of the nonmagnetic
conductor A groove may be provided, or a spacer may be additionally provided to form a
plurality of air gaps of different depths between the conductor which is a magnet or a
nonmagnetic material and the vibrating film. According to this, it is possible to easily broaden the
frequency characteristics of the ultrasonic signal. BEST MODE FOR CARRYING OUT THE
INVENTION An embodiment of an ultrasonic transducer according to the present invention will
be described below with reference to the drawings. Each embodiment of the ultrasonic
transducer of the present invention described below is an electrostatic ultrasonic transducer. The
characteristic part is a configuration for generating an adsorption force to a vibrating film made
of a dielectric as a vibrator of ultrasonic waves and a configuration of a pair of electrodes for
applying a voltage. FIG. 1 is a cross-sectional view of a portion including an ultrasonic vibration
film and a pair of electrodes sandwiching the vibration film in the first embodiment of the
ultrasonic transducer according to the present invention. The vibrating film 11 is a dielectric
(insulator) such as PET (polyethylene terephthalate resin) having a thickness of about several
μm (about 3 to 10 μm). The thickness of the vibrating film 11 is preferably 10 μm or less in
view of its vibration characteristics. The upper electrode 12 is formed on the upper surface of the
vibrating film 11 as a metal foil. The upper electrode 12 is a conductor made of a ferromagnetic
material such as nickel (Ni), iron (Fe), cobalt (Co), an alloy thereof, an amorphous alloy and the
like and having conductivity. . The upper electrode 12 can be formed on the upper surface of the
vibrating film 11 by, for example, a plating process, ie, a process of immersing in a molten metal
or vacuum evaporation. The lower electrode 13 is disposed on the lower surface of the vibrating
membrane 11 so as to face and contact the lower surface. The lower electrode 13 is a plastic
magnet, a rubber magnet or the like in which a polymer compound such as plastic or rubber and
a ferromagnetic powder such as ferrite powder or rare earth magnet powder are mixed, and it is
formed to further form a conductor. It is an electrode. For example, a conductive plastic is usually
a resin bound with a conductive material powder. Therefore, by mixing magnetic materials such
as nickel, iron, cobalt and alloy powder of these composite materials in the binding material, it is
possible to secure conductivity and magnetism, and a conductive plastic magnet is obtained.
Alternatively, an electrode layer made of a material having conductivity in the vicinity of the
surface of a plastic magnet or rubber magnet is integrally molded with a polymer compound in
such a shape as to be partially dispersed over the entire surface. It may be used as a conductor.
The lower electrode 13 is unipolarly magnetized and has characteristics as a permanent magnet.
Arrows in the figure indicate the magnetizing direction of the magnet. Magnetic lines of force as
shown by broken lines are generated by the lower electrode 13. Therefore, the upper electrode
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12 made of a ferromagnetic material and the lower electrode 13 constituting the permanent
magnet attract each other, and the vibrating film 11 is adsorbed to the lower electrode 13. The
planar shape of the diaphragm 11, the upper electrode 12, and the lower electrode 13 has a disk
shape, an annular shape, a polygonal shape, or the like. The vibrating film 11 shown in FIG. 1 is
attracted by the magnetic force between the upper electrode 12 and the lower electrode 13, and
by applying an alternating voltage of the frequency of the ultrasonic signal between the upper
electrode 12 and the lower electrode 13, It vibrates at a frequency. The applied voltage is, for
example, an alternating voltage of 50 to 150 V peak-to-peak. However, the attraction force for
attracting the vibrating film 11 to the lower electrode 13 is not generated by the magnetic force
at all, but a DC bias voltage is applied between the upper electrode 12 and the lower electrode 13
to combine the attraction force by static electricity. You may do it. Further, the lower electrode
13 is further roughened on the surface on the vibrating film 11 side to form a plurality of
grooves having different depths, or the depths between the lower electrode 13 and the vibrating
film 11 are different. A spacer may be additionally provided to form a plurality of air gaps.
According to this, it is possible to further broaden the frequency characteristics of the ultrasonic
signal. However, the surface finish may be mirror-like. FIG. 2 is a cross-sectional view of a portion
including an ultrasonic vibration film and a pair of electrodes sandwiching the vibration film in
the second embodiment of the ultrasonic transducer according to the present invention. The
vibrating membrane 11 and the upper electrode 12 are configured similarly to those shown in
FIG. The lower electrode 14 is disposed on the lower surface of the vibrating membrane 11 so as
to face and contact the lower surface. The lower electrode 14 is, like the lower electrode 13 in
FIG. 1, a plastic magnet, a rubber magnet, etc. in which a polymer compound such as plastic or
rubber and a ferromagnetic powder such as ferrite powder or rare earth magnet powder are
mixed. And an electrode further formed to be a conductor. Unlike the lower electrode 13 of FIG.
1, the lower electrode 14 is multipolarly magnetized and has characteristics as a permanent
magnet.
That is, the difference between the configuration of FIG. 1 and the configuration of FIG. 2 is the
magnetization pattern. Arrows in the figure indicate the magnetizing direction of the magnet. In
the configuration shown in FIG. 2, magnetization is partially reversed in the opposite direction. In
this case, the lower electrode 14 generates a plurality of magnetic lines of force as indicated by
broken lines in correspondence with the arrows. As a result, the magnetic circuit can be
shortened and the attractive force can be strengthened. FIG. 3 is a cross-sectional view of a
portion including an ultrasonic vibration film and a pair of electrodes sandwiching the vibration
film in the third embodiment of the ultrasonic transducer according to the present invention. The
vibrating membrane 11 and the upper electrode 12 are configured similarly to those shown in
FIGS. 1 and 2. The lower electrode 15 is disposed on the lower surface of the vibrating
membrane 11 so as to face and contact the lower surface, and the magnet 16 is disposed on the
lower surface of the lower electrode 15. The lower electrode 15 is made of a nonmagnetic
material and a conductive material. The nonmagnetic material is a magnetic material other than a
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ferromagnetic material. Examples of nonmagnetic metals include materials such as gold (Au),
silver (Ag), copper (Cu), aluminum (Al), platinum (Pt), and brass. Alternatively, conductive plastic
can be used. Here, the substance which is a conductive material has a property as a conductor (a
substance having a large electric conductivity). A magnet 16 is disposed below the lower
electrode 15. The magnet 16 and the lower electrode 15 may be fixed to each other by adhesion,
pressure bonding or the like, or may be held so as to be movable in the vertical direction. The
magnet 16 is a permanent magnet, and there is no limitation on the type of magnet material.
However, it is preferable to use a plastic magnet having a strong magnetic force, a rare earth
magnet having a strong magnetic force, or the like rather than a ferrite magnet having a weak
magnetic force. Further, in the configuration shown in FIG. 3, it is unipolarly magnetized and has
a magnetization pattern in one direction. Therefore, the upper electrode 12 made of a
ferromagnetic material and the magnet 16 constituting the permanent magnet attract each other,
and the vibrating film 11 is adsorbed to the lower electrode 15. The planar shape of the vibrating
film 11, the upper electrode 12, the lower electrode 15, and the magnet 16 is a disk shape, an
annular shape, a polygonal shape, or the like. The vibrating membrane 11 shown in FIG. 3 is
attracted by the magnetic force between the upper electrode 12 and the magnet 16, and an
alternating current voltage of the frequency of the ultrasonic signal is applied between the upper
electrode 12 and the lower electrode 15. Vibrate at The applied voltage is, for example, an
alternating voltage of 50 to 150 V peak-to-peak.
However, the attraction force for attracting the vibrating film 11 to the lower electrode 15 is not
generated by the magnetic force at all, but a DC bias voltage is applied between the upper
electrode 12 and the lower electrode 15 to combine the attraction force by static electricity. You
may do it. The lower electrode 15 is further roughened on the surface on the vibrating film 11
side to form a plurality of grooves having different depths, or the depth between the lower
electrode 15 and the vibrating film 11 is different. A spacer may be additionally provided to form
a plurality of air gaps. According to this, it is possible to further broaden the frequency
characteristics of the ultrasonic signal. However, the surface finish may be mirror-like. FIG. 4 is a
cross-sectional view of a portion including an ultrasonic vibration film and a pair of electrodes
sandwiching the vibration film in the fourth embodiment of the ultrasonic transducer according
to the present invention. The vibrating membrane 11 and the upper electrode 12 are configured
in the same manner as those shown in FIG. 1, FIG. 2 and FIG. The magnet 17 is a magnet
disposed below the lower electrode 15 in the same manner as the magnet 16 of FIG. The magnet
17 is a permanent magnet, and there is no limitation on the type of magnet material. However, it
is preferable to use a plastic magnet having a strong magnetic force, a rare earth magnet having
a strong magnetic force, or the like rather than a ferrite magnet having a weak magnetic force.
Unlike the magnet 16 of FIG. 3, the magnet 17 is multipolarly magnetized. That is, the difference
between the configuration of FIG. 3 and the configuration of FIG. 4 is the magnetization pattern.
Arrows in the figure indicate the magnetizing direction of the magnet. In the configuration shown
in FIG. 4, magnetization is partially reversed in the opposite direction. As a result, the magnetic
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circuit can be shortened and the attractive force can be strengthened. In this case, a plurality of
magnetic force lines as shown by broken lines are generated by the magnet 17 corresponding to
the arrows. FIG. 5 is a diagram showing an example of the frequency characteristic of the
ultrasonic transducer having the structure shown in FIG. 1 to FIG. The horizontal axis represents
frequency, and the vertical axis represents sound pressure. Also, for reference, an example of the
frequency characteristic of a conventional resonant ultrasonic transducer is shown. According to
each embodiment of the present invention, a constant high sound pressure can be generated (in a
frequency band of about 40 kHz or more) over a wide frequency band. As described above,
according to each embodiment of the present invention, the upper electrode 12 which is a
ferromagnetic material attracts the lower electrode 13 or 14 which is a permanent magnet or the
magnet 16 or 17 by a magnetic force. The vibrating membrane 11 is adsorbed to the lower
electrode 13, 14 or 15.
Therefore, the DC bias voltage can be eliminated or the voltage value of the DC bias voltage can
be lowered. Therefore, downsizing / cost reduction of the device can be easily realized. The
ultrasonic transducer according to the present invention can be applied, for example, as various
sensors, sound sources for directional speakers, and impulse signal sources. In this case, if used
as an ultrasonic sensor, a wide frequency band can be realized by a simple drive circuit. When
used as an impulse signal generation source, an impulse signal close to an ideal waveform can be
generated. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view showing a
first embodiment of an ultrasonic transducer according to the present invention. FIG. 2 is a crosssectional view showing a second embodiment of the ultrasonic transducer according to the
present invention. FIG. 3 is a cross-sectional view showing a third embodiment of the ultrasonic
transducer according to the present invention. FIG. 4 is a cross-sectional view showing a fourth
embodiment of the ultrasonic transducer according to the present invention. FIG. 5 is a frequency
characteristic diagram showing the characteristics of the first to fourth embodiments. FIG. 6 is a
cross-sectional view showing a configuration example of a conventional electrostatic ultrasonic
transducer. FIG. 7 is a partial enlarged view of FIG. [Explanation of the code] 11 ... vibrating
membrane, 12 ... upper electrode, 13, 14, 15 ... lower electrode, 16, 17 ... magnet
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