Ultrasonic sensor

An ultrasonic sensor includes a cylindrical case having a bottom and a side wall, a piezoelectric element attached to an inner bottom surface of the case, a terminal retainer configured to hold outer terminals and inner terminals, and wires connected to the inner terminals and configured to feed power to the piezoelectric element. The side wall of the case has a thin portion adjacent to an opening of the case and a thick portion adjacent to the bottom of the case. The elastic member is provided between the thick portion and the terminal retainer. An opening region surrounded by the thick portion is preferably covered with the elastic member. The case is preferably internally filled with a filler.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to ultrasonic sensors. In particular, the present invention relates to an ultrasonic sensor which includes a piezoelectric element and input-output terminals electrically connected to the piezoelectric element and is used, for example, as a corner sonar or a back sonar of a car.

2. Description of the Related Art

An ultrasonic sensor is a sensor that uses ultrasonic waves to perform sensing. The ultrasonic sensor intermittently transmits ultrasonic pulse signals and receives reflected waves from surrounding obstacles to detect an object. For example, the ultrasonic sensor is used in a car as a corner sonar, a back sonar, or a parking spot sensor that detects a space between the car and an obstacle, such as a side wall, in parallel parking.

An ultrasonic sensor of this type is disclosed in Japanese Unexamined Patent Application Publication No. 2007-318742.FIG. 1is a cross-sectional view of an ultrasonic sensor10disclosed in this document. The ultrasonic sensor10includes a case12having a bottom portion14and a tubular portion16. The bottom portion14is made of metal, such as aluminum, and has a closed surface. The tubular portion16is made of metal, such as zinc, and fitted into and bonded by an adhesive to the bottom portion14.

A piezoelectric element18is bonded, by a conductive adhesive, to an inner bottom of the bottom portion14of the case12.

First and second input-output terminals20and22made of metal are electrically connected to the piezoelectric element18. The first and second input-output terminals20and22are extracted from the inside to the outside of the case12. The first input-output terminal20is electrically connected to an electrode on an upper principal surface of the piezoelectric element18. The first input-output terminal20includes a spring terminal20ahaving spring properties, a middle portion20b, and a pin-like extraction portion20c.

The second input-output terminal22is electrically connected, through the case12, to an electrode on a lower principal surface of the piezoelectric element18. The second input-output terminal22includes a connection portion22a, a middle portion22b, and an extraction portion22c.

The first and second input-output terminals20and22are supported by a substantially columnar support member24made of insulating synthetic resin. The first and second input-output terminals20and22are embedded, at their middle portions20band22b, in the support member24and secured to be integral with the support member24.

In the interior of the case12, the support member24is placed adjacent to the upper principal surface of the piezoelectric element18and secured to the tubular portion16of the case12.

In the case12, a damping member26is disposed on the closed surface where the piezoelectric element18is placed. In the interior of the case12, an opening side of the support member24is sealed with an expandable filler (not shown).

In the ultrasonic sensor10of related art illustrated inFIG. 1, the support member24that supports the first and second input-output terminals20and22is directly attached to the tubular portion16that vibrates. As a result, vibrations of the tubular portion16are transmitted to the first and second input-output terminals20and22and vibrate a substrate on which the first and second input-output terminals20and22are mounted (hereinafter referred to as “vibration leakage”). This vibration leakage causes a long reverberation time (i.e., deteriorates reverberation characteristics). If reverberation is prolonged in detection of a nearby object, a reflected signal is received while reverberation of a transmission signal (burst wave) continues. This makes it difficult to detect a nearby object. Additionally, in the structure illustrated inFIG. 1, where a boundary between the bottom portion14and the tubular portion16is exposed to a side face of the case12, it is necessary to take measures to prevent entry of moisture and corrosion under high humidity conditions.

An object of the present invention is to provide an ultrasonic sensor which has reverberation characteristics improved by preventing vibration leakage.

SUMMARY OF THE INVENTION

According to preferred embodiments of the present invention, an ultrasonic sensor includes a cylindrical case having a bottom and a side wall, a piezoelectric element attached to an inner bottom surface of the case, terminals extracted to the outside of the case, a terminal retainer configured to hold the terminals, and conductive members connected to the terminals and configured to feed power to the piezoelectric element. The side wall of the case has a thin portion adjacent to an opening of the case and a thick portion adjacent to the bottom of the case. The ultrasonic sensor further includes an elastic member disposed between the thick portion and the terminal retainer.

With this structure, vibrations from the case are attenuated in the elastic member and mostly prevented from being transmitted through the terminal retainer to the terminals. It is thus possible to significantly reduce vibration leakage that occurs when the terminals are mounted on a substrate.

An opening region surrounded by the thick portion may be covered with the elastic member. With this structure, sonic waves emitted from the piezoelectric element toward the interior of the case can be blocked from directly reaching the terminal retainer. It is thus possible to further reduce vibration leakage.

A space between the thin portion of the side wall and a side face of the elastic member may be filled with a filler. With this structure, the filler is in contact with the side wall of the case over a wide area. Therefore, as compared to a structure in which the thin portion and the elastic member are in contact with each other, vibrations in the side wall of the case can be suppressed and reverberation can be reduced.

A reinforcing member (weight) having an acoustic impedance higher than that of the case may be formed on the thick portion. This structure enhances rigidity of a portion around the inner bottom surface of the case, suppresses transmission of vibrations from the bottom to the side wall of the case, and thus improves sensitivity of the ultrasonic sensor.

A space may be created between the piezoelectric element and the elastic member, and a sound absorbing member may be provided on a surface of the elastic member adjacent to the piezoelectric element. With this structure, unwanted sonic waves are absorbed by the sound absorbing member. Therefore, it is possible to efficiently attenuate unwanted sonic waves transmitted from the piezoelectric element toward the interior of the case.

It may be preferable that the bottom of the case have stepped portions that produce anisotropy in a major axis direction and a minor axis direction, the elastic member have first engagement portions that engage with the respective stepped portions, the elastic member have a second engagement portion that engages with the terminal retainer, and the terminal retainer have an engagement portion that engages with the second engagement portion.

With this structure, where the case, the elastic member, and the terminal retainer are positioned with respect to each other, the terminals and the terminal retainer can be secured stably. To produce anisotropy in directivity of ultrasonic waves to be transmitted and received (i.e., to make a directivity angle in a vertical direction different from that in a horizontal direction), a thin portion with major and minor axes is typically created at the bottom of the case. However, the directivity of the thin portion cannot be identified from outside the case. Therefore, in related art, the terminal retainer is secured onto an end face of the case adjacent to the opening of the case. However, if the terminal retainer is secured onto the end face of the case, for example, entry of moisture through a boundary between the end face and the terminal retainer may cause degradation of sensitivity. In the structure described above, when the case, the elastic member, and the terminal retainer are brought into engagement with each other, the terminals are secured inside the case in accordance with the directional property of a vibrating surface of the case. Therefore, the directional property of the vibrating surface can be identified on the basis of the positions of the terminals exposed to the outside of the case.

According to the preferred embodiments of the present invention, vibrations from the case are attenuated in the elastic member and mostly prevented from being transmitted through the terminal retainer to the terminals. It is thus possible to significantly reduce vibration leakage that occurs when the terminals are mounted on a substrate. Therefore, deterioration of reverberation characteristics caused by vibration leakage can be prevented, and detection of a nearby object becomes possible.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

FIG. 2Ais a cross-sectional view of an ultrasonic sensor101according to a first embodiment of the present invention.FIG. 2Bis a plan view of a state before a case31of the ultrasonic sensor101is internally filled with a filler. The ultrasonic sensor101includes the case31of substantially cylindrical shape having a bottom31band a side wall31a, a piezoelectric element32attached to an inner bottom surface of the case31, a terminal retainer41configured to hold outer terminals43and inner terminals42, and wires (conductive members)34and35connected to the inner terminals42and configured to feed power to the piezoelectric element32.

The side wall31aof the case31has a thin portion31tadjacent to an opening of the case31and a thick portion31hadjacent to the bottom31bof the case31. The bottom31bof the case31has stepped portions31ST. Broken lines inFIG. 2Bindicate positions of the stepped portions31ST at the bottom31bof the case31. An elastic member33is disposed between the thick portion31hand the terminal retainer41. In other words, an opening region surrounded by the thick portion31his covered with the elastic member33.

A region surrounded by the stepped portions31ST (indicated by the broken lines inFIG. 2B) at the bottom31bof the case31and the inner periphery of the case31where the stepped portions31ST are not present is a main vibration region, which is substantially equivalent to the inner bottom surface of the case31. The main vibration region of the case31has a major (longer) axis corresponding to a direction parallel to the broken lines inFIG. 2Band a minor (shorter) axis corresponding to a direction perpendicular to the broken lines inFIG. 2B. Thus, since the main vibration region is anisotropic, anisotropy is produced in directivity of ultrasonic waves. Specifically, the directivity angle of ultrasonic waves is narrow in the major axis direction (i.e., the vertical direction inFIG. 2B) and wide in the minor axis direction (i.e., the horizontal direction inFIG. 2B).

The case31is internally filled with a filler36which is an elastic body made of silicon resin, urethane resin, or the like. The filler36is bonded to the inner surface of the case31. Since the opening region surrounded by the thick portion31his covered with the elastic member33, a space is created between the piezoelectric element32and the elastic member33.

Since the outside diameter of the elastic member33is smaller than the inside diameter of the thin portion31tof the side wall31aof the case31, the filler36is present between the thin portion31tand the side face of the elastic member33.

The case31is, for example, an aluminum forged body. The elastic member33is an elastic molded body made of silicon rubber, urethane resin, or the like. First engagement portions33ethat engage with the respective stepped portions31ST of the case31are formed in the lower part of the elastic member33. A second engagement portion33dwith which the terminal retainer41engages is formed in the upper part of the elastic member33. The elastic member33is not open in the center.

The terminal retainer41is a molded body of resin, such as polybutylene terephthalate (PBT). The terminal retainer41holds about two pins, which serve as the outer terminals43at one end and the inner terminals42at the other end. The terminal retainer41has a flange-like engagement portion (hereinafter referred to as a “flange”)41fat a lower end thereof. The flange41fengages with the second engagement portion33din the upper surface of the elastic member33. An upper surface41sof the flange41fof the terminal retainer41is covered with the filler36.

As described above, since there is the elastic member33between the thick portion31hof the case31and the terminal retainer41, vibrations from the case31are attenuated in the elastic member33and mostly prevented from being transmitted through the terminal retainer41to the outer terminals43. It is thus possible to significantly reduce vibration leakage that occurs when the outer terminals43are mounted on a substrate. In particular, since the elastic member33is not open in the center, sonic waves emitted from the piezoelectric element32toward the interior of the case31hit the elastic member33, instead of directly hitting the terminal retainer41. Thus, sonic waves emitted from the piezoelectric element32toward the interior of the case31are attenuated in the elastic member33. Therefore, it is possible to effectively prevent vibration leakage.

Additionally, since the upper surface41sof the flange41fof the terminal retainer41is covered with the filler36, the terminal retainer41can be held firmly in place. Thus, the terminal retainer41is made more resistant to removal and peeling.

In terms of material nature, whereas the elastic member33is less prone to transmit vibrations, the filler36suppresses (damps) vibrations of the case31. That is, it is preferable that the elastic modulus of the elastic member33be lower than that of the filler36. More specifically, the elastic modulus can be divided into a storage modulus and a loss modulus. The elastic member33preferably has a lower storage modulus, and the filler36preferably has a higher loss modulus. For example, the elastic member33is preferably made of silicon resin (silicon rubber), and the filler36is preferably made of urethane resin.

As described above, a space between the thin portion31tof the side wall31aof the case31and the side face of the elastic member33is filled with the filler36. Since the filler36is bonded to the side wall31aof the case31over a wide area (large depth range), the vibration damping effect of the side wall31aof the case31is improved. It is thus possible to reduce reverberations.

FIG. 3is an exploded perspective view illustrating a structure of the case31, the elastic member33, and the terminal retainer41of the ultrasonic sensor101. As described above, to make the main vibration region of the case31anisotropic, the case31has a pair of the stepped portions31ST at the bottom31b.FIG. 3shows only one of the stepped portions31ST. In the lower part of the elastic member33, there is a pair of the first engagement portions33ethat engage with the respective stepped portions31ST at the bottom31bof the case31. In the upper part of the elastic member33, there is the second engagement portion33dwith which the flange41fof the terminal retainer41engages. A protrusion33bis formed in a part of the second engagement portion33d.

A recess41dwith which the protrusion33bof the elastic member33engages is formed in a part of the flange41fof the terminal retainer41.

The first engagement portions33eof the elastic member33engage with the stepped portions31ST of the case31, and the flange41fof the terminal retainer41engages with the second engagement portion33dof the elastic member33. Thus, by sequentially bringing these three parts into engagement, the orientation of the terminal retainer41with respect to the case31becomes stable. Therefore, even when the terminal retainer41is not positioned on the end face of the case31adjacent to the opening, the directional property of a vibrating surface of the case31can be identified by the positions of the outer terminals43exposed to the outside of the case31.

If the case31, the elastic member33, and the terminal retainer41are temporarily secured to each other by engagement of the stepped portions31ST with the first engagement portions33eand by engagement of the second engagement portion33dwith the flange41f, the filler36can be easily placed in the case31.

Since the about two pins (which serve as the outer terminals43at one end and the inner terminals42at the other end) held by the terminal retainer41are molded into a substantially L-shape, there is a large space around the inner terminals42before the case31is filled with the filler36. This allows easy connection of wires to the inner terminals42.

FIG. 4Ais a graph showing reverberation characteristics of the ultrasonic sensor101according to the first embodiment.FIG. 4Bis a graph showing reverberation characteristics of the ultrasonic sensor10illustrated as a comparative example inFIG. 1. In the graphs, each division of the horizontal axis represents about 500 μs and each division of the vertical axis represents about 1 V. In both the first embodiment and the comparative example, the outer terminals were secured onto a substrate (not shown) by soldering, about eight burst waves were transmitted during the transmission time, and a voltage waveform appearing in the piezoelectric element was amplified and observed. Although attenuation of amplitude actually started immediately after the end of transmission, the waveform was saturated for a while, during which the dynamic range of the amplifying circuit was exceeded.

As is apparent from the comparison betweenFIG. 4AandFIG. 4B, in the ultrasonic sensor101of the first embodiment where the amplitude converges faster, vibration leakage and reverberations are suppressed.

In the first embodiment, unlike the structure ofFIG. 1, no boundary is present in the outer periphery of the case31. Therefore, it is possible to prevent degradation of sensitivity caused by entry of water through such a boundary, and to prevent corrosion between different metals.

Second Embodiment

FIG. 5Ais a cross-sectional view of an ultrasonic sensor102according to a second embodiment of the present invention.FIG. 5Bis a plan view of a state before the case31of the ultrasonic sensor102is internally filled with the filler36.

The ultrasonic sensor102includes a reinforcing member (weight)37on the thick portion31hof the case31. The reinforcing member37is located at a position not in contact with the inner periphery of the thin portion31tof the side wall31a. The reinforcing member37may be any molded body with high acoustic impedance. For example, a molded body made of the same material as that of the case31(aluminum) may be used as the reinforcing member37by adjusting the size, such as the thickness. However, it is preferable that the reinforcing member37be a molded body made of material (such as stainless steel (SUS) or zinc) higher in density than that of the material of the case31. Broken lines inFIG. 5Bindicate positions of the stepped portions31ST at the bottom31bof the case31. A region surrounded by the stepped portions31ST (indicated by the broken lines inFIG. 5B) at the bottom31bof the case31and the inner periphery of the case31where the stepped portions31ST are not present is a main vibration region, which is substantially equivalent to the inner bottom surface of the case31. The main vibration region of the case31is longer in a direction parallel to the broken lines inFIG. 5Band shorter in a direction perpendicular to the broken lines inFIG. 5B. Thus, anisotropy is produced in directivity of ultrasonic waves.

FIG. 6is an exploded perspective view illustrating a structure of the case31, the reinforcing member37, the elastic member33, and the terminal retainer41of the ultrasonic sensor102illustrated inFIG. 5A. The reinforcing member37is a substantially annular molded body having a substantially rectangular opening37hin the center thereof. In the lower part of the elastic member33, there are the first engagement portions33ethat engage with the opening37hof the reinforcing member37. The other configuration is the same as that described in the first embodiment.

The effect of the reinforcing member37enhances rigidity of a portion around the inner bottom surface of the case31. This can not only suppress transmission of vibrations from the bottom31bto the side wall31aof the case31, but can also allow the bottom31b(vibrating surface) of the case31to efficiently vibrate. The sensitivity of the ultrasonic sensor102can thus be improved.

Since the opening37hof the reinforcing member37and the first engagement portions33eof the elastic member33are substantially noncircular, it is possible to maintain the directional property of the terminal retainer41with respect to the case31.

Third Embodiment

FIG. 7is a cross-sectional view of an ultrasonic sensor103according to a third embodiment of the present invention. The ultrasonic sensor103includes the case31, the piezoelectric element32, the terminal retainer41that holds the outer terminals43and the inner terminals42, the wires (conductive members)34and35connected to the inner terminals42and configured to feed power to the piezoelectric element32, the reinforcing member37, a sound absorbing member38, and the filler36. The ultrasonic sensor103is obtained by adding the sound absorbing member38to the lower surface of the elastic member33(i.e., the surface adjacent to the piezoelectric element32) of the ultrasonic sensor102illustrated inFIG. 5A. The sound absorbing member38is, for example, a polyester felt and is bonded to the elastic member33by an adhesive.

As described above, since the sound absorbing member38is provided on the lower surface of the elastic member33adjacent to the piezoelectric element32, unwanted sonic waves are absorbed and attenuated by the sound absorbing member38before reaching the elastic member33and being attenuated inside the elastic member33. Therefore, it is possible to efficiently attenuate unwanted sonic waves transmitted from the piezoelectric element32toward the interior of the case31. Also, positioning of the sound absorbing member38can be easily made.

Fourth Embodiment

FIG. 8is a perspective view illustrating shapes of the terminal retainer41and the inner and outer terminals42and43included in an ultrasonic sensor according to a fourth embodiment of the present invention. About two pins held by the terminal retainer41serve as the outer terminals43at one end and the inner terminals42at the other end. As illustrated inFIG. 8, it is not necessary that the inner terminals42be bent inside the terminal retainer41, as long as they are exposed to allow connection of wires thereto.