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DESCRIPTION JP2004072416

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DESCRIPTION JP2004072416
To improve the corrosion resistance of a vibration case of an ultrasonic sensor and to improve
sound pressure characteristics. An ultrasonic sensor (10) comprises a bottomed cylindrical
vibration case (1), a piezoelectric element (2) joined to an inner surface of a bottom portion
forming a diaphragm (1a) of the vibration case (1), and a terminal (3). There is. Electrodes are
provided on both sides of the piezoelectric element 2 and are electrically connected to the pair of
terminals 3 respectively. The vibration case 1 is formed by forging an aluminum material having
an Al content of 99.7 wt%. [Selected figure] Figure 1
Ultrasonic sensor
[0001] The present invention relates to an ultrasonic sensor using a piezoelectric element. 2.
Description of the Related Art In recent years, various sensors have been used in automobiles as
various electronic control systems are realized. In particular, ultrasonic sensors are used for
ultrasonic transducers such as back sonars. An ultrasonic sensor of this type is usually composed
of a cylindrical vibration case with a bottom and a piezoelectric element joined to the inner
surface of the bottom of the vibration case of the vibration case. In this ultrasonic sensor, a
voltage is applied to the piezoelectric element to vibrate the piezoelectric element to resonate the
diaphragm, whereby a sound wave is transmitted. In addition, the vibration of the diaphragm
upon receiving the sound wave generates a voltage in the piezoelectric element and reception is
performed. This ultrasonic sensor is required to have a narrow directivity in the vertical direction
and a wide directivity in the horizontal direction. In order to make the vibration case into a shape
suitable for this directivity characteristic, it has been necessary to cut with a machining center or
the like. Therefore, an aluminum material is used for the vibration case, and among them, an
aluminum material having a relatively low viscosity and good machinability and an Al content of
about 94.5 wt% is used. SUMMARY OF THE INVENTION However, an aluminum material having
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good machinability generally has a high Cu content and is inferior in corrosion resistance. In
particular, in an ultrasonic sensor for a back sonar of a car, the surface of the vibration case is
exposed to the outside. For this reason, the vibration case using an aluminum material having
poor corrosion resistance is easily corroded. In this ultrasonic sensor, the resonant frequency is
determined by the thickness of the vibration case. Therefore, for example, in a vibration case
having a thickness of 0.3 mm at the bottom diaphragm of the vibration case and a resonance
frequency of 40 KHz, the resonance frequency drops to 39 KHz when the thickness of the bottom
diaphragm is reduced by 20 μm. On the other hand, since the circuit is designed at 40 KHz, it
operates at a sound pressure value lower than the sound pressure peak value at the resonance
frequency of 39 KHz. As described above, there is a problem that the sound pressure of the
ultrasonic sensor is reduced due to the corrosion of the vibration case. For this reason, the
corrosion prevention measures, such as apply | coating a paint on the surface of a vibration case,
were needed, and it had caused the cost increase. In addition, since burrs are generated by
cutting, unnecessary vibration is generated, which causes deterioration of the sound pressure
characteristic. The present invention has been made in view of the above problems, and aims to
improve the corrosion resistance of the vibration case of the ultrasonic sensor and to improve
the sound pressure characteristics.
Another object of the present invention is to provide an ultrasonic sensor at low cost. In order to
achieve the above object, an ultrasonic sensor according to the present invention comprises a
bottomed cylindrical vibration case and a piezoelectric element joined to the inner surface of the
bottom of the vibration case. The ultrasonic sensor is characterized in that the vibration case is
formed of an aluminum material having an Al content of 99.0 wt% or more. Further, the vibration
case is formed by forging. Thereby, the corrosion resistance of the vibration case of the
ultrasonic sensor can be improved, and the sound pressure characteristic can be improved. In
addition, ultrasonic sensors can be provided at low cost. BEST MODE FOR CARRYING OUT THE
INVENTION Hereinafter, an embodiment of the present invention will be described based on FIG.
As shown in FIG. 1, the ultrasonic sensor 10 is configured of a bottomed cylindrical vibration
case 1, a piezoelectric element 2, and a pair of terminals 3. The bottom portion of the vibration
case 1 forming the diaphragm 1a is formed so as to have a thick central portion, and the
piezoelectric element 2 is joined to the inner surface of the thick bottom portion. Electrodes are
provided on both sides of the piezoelectric element 2, and one is electrically connected to the
terminal 3 through the vibration case 1 by a lead. Furthermore, the terminal 3 is fixed to a lid
(not shown) of the vibration case 1. The vibration case 1 is formed by forging an aluminum
material having an Al content of 99.7 wt% as indicated by the material A in Table 1. The
dimensions of the cylindrical portion of the vibration case 1 are, for example, 14.0 mm in outer
diameter, and the thickness of the bottom diaphragm 1a is approximately 0.8 mm at the center
and 0.3 mm at the periphery. The ultrasonic sensor 10 operates at, for example, 40 KHz, and
transmission is performed by applying a voltage to the terminal 3, and a voltage is output from
the terminal 3 upon reception. The vibration case 1 may be formed by forging an aluminum
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material having an Al content of 99.0 wt% as indicated by the material B in Table 1. Material C of
Table 1 is a material used in the following comparative example. Table 1 <Img class =
“EMIRef” id = “1978153583-00003” /> Here, the chemical components are indicated by
wt% for Al, Cu, Mg and Si. Material A has an Al content of 99.7 wt%, and the other chemical
components are 0.3 wt%. The material B has an Al content of 99.0 wt%, an Mg content of 0.5
wt%, an Si content of 0.4 wt%, and the other chemical components of 0.1 wt%.
The material C has an Al content of 94.5 wt%, a Cu content of 5.0 wt%, and the other chemical
components of 0.5 wt%. The aluminum material of this material A is sticky, suitable for forging,
and has good corrosion resistance. On the other hand, the aluminum material of material C is less
sticky, is suitable for cutting, and has poor corrosion resistance. The aluminum material of the
material B is a characteristic close to the material A among these. FIG. 2 is a characteristic
diagram showing a change in sound pressure of an ultrasonic sensor by a salt spray test. This
characteristic diagram shows the change of standing time (hr) versus the change in sound
pressure (dB) when the ultrasonic sensor having the vibration case is left in a 5 wt% salt water
spray at 35 ° C. atmosphere. This vibration case is made of each material shown in Table 1 and
does not apply a corrosion preventing paint. Also, the operating frequency of this ultrasonic
sensor is 40 KHz. In the ultrasonic sensor Sa having a vibration case made of the material A, the
change in sound pressure from the initial value is −0.5 dB in the standing time of 1000 hr and
−1.0 dB in the 2000 hr. The ultrasonic sensor Sb having a vibration case made of the material B
has a sound pressure change of -1.0 dB at a standing time of 1000 hr and -1.5 dB at a 2000 hr.
The ultrasonic sensor Sc having a vibration case made of the material C of the comparative
example has a change in sound pressure of −2.0 dB at a standing time of 1000 hr and −4.0 dB
at 2000 hr. As described above, the ultrasonic sensors Sa and Sb having the vibration case made
of the material A or the material B having a high Al content have a small change in sound
pressure. This is because the vibration case made of the material A or the material B is less likely
to be corroded, and the resonance frequency does not decrease significantly. Therefore, by
selecting a material having an Al content of 99.0 wt% or more, an ultrasonic sensor with a small
change in sound pressure can be obtained. Next, forging of the vibration case will be described.
In general, when forging an aluminum material, a phenomenon called work hardening occurs
and becomes hard. This hardness changes with processing conditions, such as a size of a pellet of
a material, for example. Further, this hardness is expressed by Al proof stress (tensile strength) N
/ mm <2>, and the harder the hardness is, the larger the Al proof stress becomes. On the other
hand, the Al proof stress of the vibration case formed by cutting is the same as the Al proof stress
of the aluminum material of the raw material, and does not change. FIG. 3 is a characteristic
diagram showing the relationship between the Al resistance of the vibration case formed by
forging and the sound pressure difference of the ultrasonic sensor.
In the forging process of the vibration case, four kinds of processing conditions are set to
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prepare a sample. The four types of samples obtained by this are each represented by the
magnitude | size of Al proof stress. The horizontal axis shows this Al resistance (N / mm <2>).
The vertical axis represents the sound pressure difference (dB) obtained by subtracting the sound
pressure of the ultrasonic sensor having the vibration case formed by cutting from the sound
pressure of the ultrasonic sensor having the vibration case formed by forging. The vibration case
is formed of the material A in both ultrasonic sensors. Also, the operating frequency of this
ultrasonic sensor is 40 KHz. The Al proof stress of the vibration case formed by cutting is 100 N
/ mm <2>. On the other hand, as shown in the example of FIG. 3, the Al proof stress of the
vibration case formed by forging is 120 to 180 N / mm <2>. Thus, it can be seen that the Al
resistance is increased. The sound pressure difference is 1.0 dB for Al tolerance 120N / mm <2>,
the sound pressure difference is 2.0 dB for Al tolerance 140N / mm <2>, and the sound pressure
difference is 2.5 dB for Al tolerance 160N / mm <2>. The sound pressure difference is 3.0 dB at
180 N / mm <2>. As described above, by forming the vibration case by forging, work hardening
occurs to increase the vibration efficiency, so that the sound pressure of the ultrasonic sensor
can be improved. Also, the sound pressure can be changed by changing the processing
conditions of the forging process. According to the configuration of the embodiment in the
present invention, the corrosion resistance of the vibration case can be improved because an
aluminum material having an Al content of 99.0 wt% or more is used for the vibration case of the
ultrasonic sensor. It is possible to prevent the temporal decrease in sound pressure of the
ultrasonic sensor. As a result, no corrosion prevention measures are required, and an ultrasonic
sensor can be provided at low cost. Further, by forming the vibration case by forging, work
hardening occurs and the vibration efficiency becomes high, so that the sound pressure of the
ultrasonic sensor can be improved. Furthermore, since a vibration case having a complicated
shape can be formed with high precision by forging, unnecessary vibration due to burrs and the
like can be suppressed. As a result, the sound pressure characteristics of the ultrasonic sensor
can be improved. As described above, according to the present invention, since an aluminum
material having an Al content of 99.0 wt% or more is used for the vibration case of the ultrasonic
sensor, the temporal noise of the ultrasonic sensor is generated. It is possible to prevent pressure
drop.
Furthermore, the sound pressure characteristics of the ultrasonic sensor can be improved by
adopting forging processing. As a result, an ultrasonic sensor with good characteristics can be
provided at low cost. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view
showing a schematic configuration of an ultrasonic sensor according to an embodiment of the
present invention. FIG. 2 is a characteristic diagram showing a change in sound pressure when a
salt water spray test is performed on the ultrasonic sensor in the above-described example and
the comparative example. FIG. 3 is a characteristic diagram showing the relationship between the
Al resistance of the vibration case formed by forging and the sound pressure difference of the
ultrasonic sensor. Explanation of symbols 1-----Vibration case 2----Piezoelectric element 3---Terminal 10-----Ultrasonic sensor
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