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DESCRIPTION JPH10303684

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DESCRIPTION JPH10303684
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
air ultrasonic transducer used for detecting the presence or absence of a car in a parking lot and
for detecting a person in an entrance or a corridor or the like.
[0002]
2. Description of the Related Art FIG. 1 shows a cross-sectional view of a conventionally used air
ultrasonic transducer. In FIG. 6, electrodes are formed on both sides, and the piezoelectric
ceramic disc 1 polarized in the thickness direction is bonded to a metal plate 2 having a diameter
larger than this, thereby constituting a piezoelectric disc bending vibrator. . At the center of the
metal disk 2 is attached a horn 3 for enhancing the efficiency of transmission and reception of
ultrasonic waves in the shape of a funnel formed of an aluminum plate. Further, on the back side
of the metal plate 2, the cylindrical holder 4 is bonded at the position of the nodal circle of the
bending vibrator. Furthermore, these structures are inserted into a cylindrical case 5 having an
opening in the ultrasonic wave transmission direction. The opening is provided with a mesh 6 for
preventing the entry of dust.
[0003]
By the way, when this air ultrasonic transducer is used underwater, since the Q of resonance is
substantially reduced to about several tens, the effect of transmission using resonance and
reception using anti-resonance disappears. , Limited to the air.
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[0004]
In the conventional airborne ultrasonic transducer shown in FIG. 1, the electrical equivalent
circuit of the piezoelectric vibrator including the horn has capacitances Cd and R, L, C as shown
in FIG. A series resonant circuit is provided by a two-terminal circuit connected in parallel.
From the circuit of FIG. 2, the resonance frequency fr in which the impedance is minimized and
the antiresonance frequency fa in which the impedance is maximized are given by Equation 1.
[0005]
Generally, in an airborne ultrasonic transducer, at the time of transmission, it is driven at the
resonance frequency fr so that electrical power can be input to the vibrator even if the drive
voltage is low, but at the time of wave reception When receiving waves, receiving sensitivity can
be increased by receiving at the antiresonant frequency fa with high impedance. Therefore,
conventionally, two resonant ultrasonic transducers are used to match the resonant frequency fr
of the transmitter to the antiresonant frequency fa of the receiver, or one aerial ultrasonic wave
is received. Is used to transmit at an antiresonance frequency fa and to receive a reflected wave
from an object.
[0006]
However, the use of two airborne ultrasonic transducers has the disadvantage that the shape of
the device becomes large and the cost is increased, and a single airborne ultrasonic wave is used.
In the case of a transducer, although the device is small, since it is driven at an anti-resonance
frequency fa with a high impedance, a very high voltage (100 V or more) is necessary to input
the necessary electrical power to the vibrator. As a result, the drive circuit needs a transformer
for boosting.
[0007]
The technical object (object) of the present invention is to provide an air ultrasonic transducer
which can be driven by a single air ultrasonic transducer and at a low voltage of about 10 V and
which does not require a transformer for boosting. It is.
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[0008]
According to the present invention, ultrasonic waves are radiated and propagated in air using a
piezoelectric bending vibrator directly utilizing the vibrating surface of the bending vibrator, and
In an airborne ultrasonic transducer configured to attach a horn for acoustic impedance
matching to a large portion of vibration amplitude and receive an ultrasonic wave that radiates
and propagates an ultrasonic wave in air through the horn. The electrical equivalent circuit of the
air ultrasonic transducer has capacitances having capacitances Cd of approximately equal values
between both terminals on the input / output side of the series resonant circuit represented by L,
C, R and the common terminal. Is represented by a symmetrical three-terminal circuit in which
each is connected in parallel.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION An air ultrasonic transducer according to an
embodiment of the present invention will be described.
The feature of the present invention is to use a piezoelectric bending vibrator formed by bonding
a single piezoelectric material or a plate-like piezoelectric material and a metal plate, a ceramic
plate or the like.
[0010]
Directly use the vibration plane of this bending oscillator to radiate ultrasonic waves into the air,
receive ultrasonic waves propagating in the air, or match the acoustic impedance to the large
part of the vibration amplitude of this bending oscillator Wear a horn and radiate ultrasonic
waves into the air through the horn or receive ultrasonic waves propagating in the air.
The air ultrasonic transducer thus configured has capacitances Cd of substantially equal values
on both sides of the series resonant circuit in which the electrical equivalent circuit of the air
ultrasonic transducer is represented by L, L, C, R. Are represented by a symmetrical threeterminal circuit connected in parallel to the common terminal.
As a piezoelectric bending vibrator, on at least one surface of a piezoelectric ceramic rectangular
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plate, interdigital electrodes parallel to the longitudinal direction of the piezoelectric ceramic
rectangular plate are symmetrical with respect to the center line in the width direction of the
piezoelectric ceramic rectangular plate It is characterized in that the formed symmetrical threeterminal type piezoelectric bending vibrator is used.
[0011]
Also, as a piezoelectric bending vibrator, electrodes are formed on both sides of a rectangular
plate made of metal or ceramic, a piezoelectric ceramic rectangular plate polarized in the
thickness direction is bonded, and the electrode on the opposite side to the bonding surface is
approximately 2 It is also possible to use a symmetrical three-terminal type piezoelectric bending
vibrator which is divided into equal parts and configured.
[0012]
Note that one terminal and the common terminal may be used as a transmission / reception
terminal, the other terminal may be opened when transmitting, and the terminal may be shortcircuited with the common terminal when receiving.
[0013]
The position of the node of vibration in the case of a rectangular plate is linear in the width
direction at a position of 22.4% of the total length from both ends in the case of one-wavelength
resonance.
When a disk is used, it differs from the diameter of the metal plate to be bonded and the
piezoelectric ceramic plate and the thickness of each plate.
In the case of a disc of uniform material, in the case of the basic resonance mode, the diameter is
a nodal circle of about 70% of the disc.
[0014]
In the present invention, as shown in FIG. 3, the equivalent circuit of the piezoelectric vibrator
used in the air ultrasonic transducer includes the horn if the horn for impedance matching with
air is mounted. It is based on the configuration that the capacitance Cd of approximately equal
value is represented by a symmetrical three-terminal circuit connected in parallel to the common
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terminal on both sides of the series resonant circuit represented by L, C, R.
[0015]
In FIG. 3, when − ′ is an input terminal and − ′ is a control terminal, the resonance frequency
frs and the antiresonance frequency fas viewed from the − ′ terminal when the − ′ is shorted
are given by the equation 2.
[0016]
On the other hand, the resonant frequency fro and the antiresonant frequency fao viewed from
the-'terminal when the-' is opened are given by the following equation (3).
[0017]
As can be seen from the equations (2) and (3), in FIG. 3, the anti-resonance frequency fas seen
from the-'terminal when the-' terminal is shorted is the-'terminal when the-' terminal is opened. It
is equal to the resonance frequency fro seen from.
That is, by opening the '-' terminal when transmitting and shorting the '-terminal when receiving,
the resonance frequency is lowered as in the case of using two air ultrasonic transducers
conventionally. It is possible to drive with the drive voltage and receive the same frequency as
the high anti-resonance frequency of the impedance.
[0018]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The air ultrasonic transducer
according to one embodiment of the present invention shown in FIG. 4 has a substantially
cylindrical shape as a whole, and the symmetrical three-terminal type piezoelectric bending
vibrator 12 used therein is a piezoelectric ceramic rectangular plate 11. Is equipped.
As indicated by hatching in FIG. 1, the interdigital electrodes are connected to every other one of
the seven strip electrodes on one surface of this piezoelectric ceramic rectangular plate 11, and
the width direction of this piezoelectric ceramic rectangular plate The common electrodes 21 and
31 of the common electrodes 21, 22 and 31 and 33 of the respective interdigital electrodes are
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connected to be connected to the common terminal 41, and the rest are formed symmetrically
with respect to the center line of the The two common electrodes 22 and 32 are connected to the
input terminal 42 and the control terminal 43, respectively.
In the piezoelectric vibrator 12 shown in FIG. 4, since the interdigital electrode is formed only on
one side, and the piezoelectric ceramic rectangular plate 11 is polarized using this interdigital
electrode, the same electrode as at the time of polarization is used. When used and driven,
flexural vibration is excited. Further, the interdigital electrodes are formed symmetrically with
respect to the center line in the width direction of the piezoelectric ceramic rectangular plate 11,
and one of the common electrodes 21, 22 and 31, 32 of the interdigital electrodes is common.
And 31 are connected and connected to the common terminal 41, so that a symmetrical threeterminal type piezoelectric vibrator 12 having the remaining two common electrodes 22 and 32
as input / output terminals is configured. As an example, when the dimensions of the vibrator:
width: 6 mm, length: 12 mm, thickness: 1 mm, electrode dimensions: electrode width: 0.4 mm,
electrode length: 7.2 mm, gap width: 0.4 mm An impedance characteristic chart of measurement
according to the oscillator characteristic is shown in FIG. fro almost matches with fas.
[0019]
FIG. 5 is a cross-sectional view showing the structure of the air ultrasonic transducer configured
using the symmetric three-terminal type piezoelectric vibrator 12 shown in FIG. As shown in FIG.
5, the symmetrical three-terminal type piezoelectric vibrator 12 has its electrode face down, and
the position of the node of the flexural vibration is bonded to a holder 51 made of a soft elastic
material. The holder 51 is joined to a stem 55 on which lead terminals 52, 53, 54 for input and
control are integrally formed. The input and control lead terminals 52, 53 and 54 are connected
to the common terminal 41, the input terminal 42 and the control terminal 43 of the symmetric
three-terminal type piezoelectric vibrator 12 by lead wires. Furthermore, a funnel-shaped horn
56 formed of aluminum for enhancing the efficiency of transmission and reception of ultrasonic
waves is mounted at the center of the ultrasonic radiation surface of the symmetrical threeterminal type piezoelectric vibrator 12 and these structures are provided. Is inserted into a
cylindrical case 57 having an opening in the ultrasonic wave transmission direction.
[0020]
FIG. 6 is a perspective view showing the structure of a symmetrical three-terminal type
piezoelectric bending vibrator 60 used in an air ultrasonic transducer according to another
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embodiment of the present invention. Electrodes are formed on both sides of the metal
rectangular plate 61, and a piezoelectric ceramic rectangular plate 62 polarized in the thickness
direction is adhered. A full surface electrode 65 is formed on the side of the piezoelectric ceramic
plate 62 in contact with the metal plate 61. The electrode on the surface opposite to the surface
bonded to the metal rectangular plate 61 of the piezoelectric ceramic rectangular plate 62 is
divided into approximately two equal parts into two electrodes 63 and 64. As can be easily
understood from the structure shown in FIG. 6, this piezoelectric bending vibrator 60 is
symmetrical 3 in which the metal rectangular plate 61 is a common terminal, and two bisected
electrodes on the surface are input / output terminals. It becomes a terminal type vibrator.
[0021]
Although the above description has been given of the example using the horn for impedance
matching with air as an example, even when the horn is not used, the reflecting plate should be
installed approximately at the center of the opening face of the case. Substantially the same
effect as in the case of using a horn.
[0022]
As described above, according to the present invention, since the symmetrical three-terminal type
piezoelectric bending vibrator is used, the control terminal (output terminal) and the common
terminal are shorted or opened to open the circuit. It is possible to substantially equalize the
resonance frequency fro of and the antiresonance frequency fas at the time of short circuit,
thereby obtaining an air ultrasonic transducer which can be driven at a low voltage by using one
piezoelectric bending vibrator. be able to.
[0023]
Brief description of the drawings
[0024]
1 is a plan view showing an electrode pattern of a symmetrical three-terminal type piezoelectric
bending vibrator 12 used for an air ultrasonic transducer according to an embodiment of the
present invention.
[0025]
2 is a cross-sectional view showing the structure of an air ultrasonic transducer according to an
embodiment of the present invention configured using the symmetric three-terminal type
piezoelectric vibrator 12 shown in FIG.
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[0026]
3 is a perspective view showing a structure of a symmetrical three-terminal type piezoelectric
bending vibrator 60 used for an air ultrasonic transducer according to another embodiment of
the present invention.
[0027]
4 is an impedance characteristic diagram of the measurement according to the vibrator 12 of
FIG.
[0028]
FIG. 52 is an electrical equivalent circuit of the terminal type piezoelectric vibrator.
[0029]
6 is an equivalent circuit of a symmetrical three-terminal type piezoelectric vibrator.
[0030]
7 is a cross-sectional view of a conventional airborne ultrasonic transducer.
[0031]
Reference Signs List 1 piezoelectric ceramic disk 2 metal disk 3, 56 funnel-shaped horn 4
cylindrical holder 5, 57 cylindrical case 6 mesh 11 piezoelectric ceramic rectangular plate 12
symmetrical three-terminal type piezoelectric bending vibrators 21, 22, 31, 32 Interdigitated
finger electrode common electrode 41 Common terminal 42 Input terminal 43 Control terminal
(Output terminal) 51 Holder 52 Input lead terminal 53 Control lead terminal 54 Common
terminal lead terminal 55 Stem 60 Symmetrical 3-terminal piezoelectric bending vibrator 61
Metal Rectangular plate 62 Piezoelectric ceramic rectangular plate 63, 64 Divided electrode for
input / output 65 Full surface electrode
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