Abstract:
An ultrasonic sensor is provided at a bumper  31  of a vehicle  3  and used in a vehicle obstacle detector  1  that detects the presence of an obstacle within a detection area by transmitting and receiving ultrasonic waves. The ultrasonic sensor includes a piezoelectric element  21  configured to transmit and receive the ultrasonic waves by using vibrations of a piezoelectric substrate and a plate-shaped acoustic matching layer  22  on one surface  22   a  of which the piezoelectric element is attached. The ultrasonic sensor is inserted in a mounting hole formed through the mounting surface. Another surface  22   b  of the acoustic matching layer  22  opposite to the one surface  22   a  is inclined with respect to the one surface  22   a  on which the piezoelectric element is attached, and is formed to be parallel with the outer surface of the bumper  31.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an ultrasonic sensor for use in a vehicle obstacle detector that detects objects using ultrasonic waves. 
     BACKGROUND OF THE INVENTION 
     An ultrasonic sensor is used in a vehicle obstacle detector that is installed in a bumper of a vehicle and detects an obstacle by intermittently transmitting ultrasonic waves from an ultrasonic microphone and receiving reflected waves from the obstacle present within a detection area. Furthermore, an ultrasonic sensor has the characteristics of being able to limit the detection area thereof and to measure the distance. However, since ultrasonic waves have wide directivity, a system that is used in a detection area of a maximum distance of 2 m is commonly used. However, a vehicle obstacle detector has required an ultrasonic sensor having a long-distance detection area. 
       FIGS. 8A and 8B  illustrate the schematic configuration of a vehicle obstacle detector  1  having an ultrasonic sensor  11 ′ and detection areas.  FIG. 8A  is a plan view of a vehicle  3 , and illustrates the block configuration of a vehicle obstacle detector  1  and the horizontal detection areas Ah of the vehicle  3 .  FIG. 8B  is a side view of the vehicle  3 , and illustrates the vertical detection area Av of the vehicle  3 . 
     The vehicle obstacle detector  1  includes the ultrasonic sensor  11 ′, a controller unit  12 , and a display unit  13 . The ultrasonic sensor  11 ′ includes ultrasonic microphones  2 ′, and the ultrasonic microphones  2 ′ are provided at both side portions of a bumper  31  on the rear of a vehicle  3 . The display unit  13  notifies a user of the presence of an obstacle within the detection area using a buzzer or LEDs. The controller unit  12  monitors and controls ultrasonic waves transmitted and received by the ultrasonic sensor  11 ′, and outputs the results of the detection to the display unit  13 . 
     The operation of the vehicle obstacle detector  1  will now be described. First, the controller unit  12  intermittently generates an ultrasonic wave pulse signal and outputs it to the ultrasonic sensor  11 ′. The ultrasonic sensor  11 ′ amplifies the ultrasonic wave pulse signal, and the ultrasonic microphone  2 ′ transmits ultrasonic waves. Furthermore, when an obstacle is present in the detection area, the ultrasonic microphone  2 ′ receives reflected waves from the obstacle, and the ultrasonic sensor  11 ′ amplifies the received ultrasonic wave pulse signal and outputs it to the controller unit  12 . Furthermore, the controller unit  12  calculates a distance to the obstacle based on the difference between the time at which the ultrasonic waves were transmitted and the time at which the reflected waves were received. If the obstacle is present within predetermined range, the controller unit  12  outputs detection signal to the display unit  13 , and the display unit  13  notifies a user of the presence of the obstacle. 
     According to the principle by which ultrasonic waves are utilized, the detection area varies depending on the obstacle, that is, an object to be detected. The detection areas shown in  FIGS. 8A and 8B  are detection areas when a plastic column having an outer diameter of 60 mm (not shown) is assumed to be the object to be detected. 
     Next, the ultrasonic microphone  2 ′ that transmits and receives ultrasonic waves will be described. An ultrasonic microphone  2 ′ using a piezoelectric element  21  based on diffused vibration mode has been proposed (for example, see Patent document 1). 
       FIGS. 9A to 9C  illustrate the schematic configuration of the ultrasonic microphone  2 ′.  FIG. 9A  is a plan view of the ultrasonic microphone  2 ′,  FIG. 9B  is a sectional view showing section A-A′ of the ultrasonic microphone  2 ′, and  FIG. 9C  is a sectional view showing section B-B′ of the ultrasonic microphone  2 ′. 
     The ultrasonic microphone  2 ′ includes a piezoelectric element  21 , an acoustic matching layer  22 , a vibration casing  23 , a connector  25 , and a filler  26 . The piezoelectric element  21  is configured in the form of a plate in which a piezoelectric substrate (not shown) makes diffused vibrations. The acoustic matching layer  22  has a plate shape, the opposite surfaces  22   a  and  22   b  thereof are formed to be parallel, and the piezoelectric element  21  is attached to one surface  22   a  of the acoustic matching layer  22 . The acoustic matching layer  22  suppresses a reflection occurring on a boundary surface due to the difference between the sound impedance of the piezoelectric element  21  and the sound impedance of air, and increases the efficiency of transmission and reception of ultrasonic waves. Furthermore, the ring-shaped vibration casing  23  is attached to the periphery of one surface  22   a  of the acoustic matching layer  22  to surround the piezoelectric element  21 , and supports the acoustic matching layer  22 . Furthermore, lead wires  24   a  and  24   b  are extended from one surface  21   a  and the opposite surface  21   b  of the piezoelectric element  21 , respectively, and are connected to the connector  25 . Furthermore, the inside of the vibration casing  23  is filled with the filler  26 , and the piezoelectric element  21  is sealed. 
     In the above configuration, when an ultrasonic wave pulse signal is input through the connector  25 , the piezoelectric element  21  vibrates, and ultrasonic waves are output from the opposite surface  22   b  of the acoustic matching layer  22  via the acoustic matching layer  22 . Furthermore, the ultrasonic waves are transmitted in a direction that is perpendicular to one surface  21   a  of the piezoelectric element  21  that is attached to the acoustic matching layer  22 . Furthermore, the piezoelectric element  21  may be a piezoelectric element that uses a thickness vibration mode. 
       FIGS. 10A to 10C  are diagrams illustrating the schematic configuration of the ultrasonic sensor  11 ′ using the ultrasonic microphone  2 ′.  FIG. 10A  is a plan view of the ultrasonic sensor  11 ′,  FIG. 10B  is a sectional view of section A-A′ of the ultrasonic sensor  11 ′, and  FIG. 10C  is a sectional view of section B-B′ of the ultrasonic sensor  11 ′. The ultrasonic sensor  11 ′ includes a holding rubber  111  for securing the ultrasonic microphone  2 ′; an electronic circuit unit  112  for amplifying ultrasonic wave pulse signals output from the controller unit  12  and the ultrasonic microphone  2 ′; a housing  113  configured to accommodate the ultrasonic microphone  2 ′, the holding rubber  111  and the electronic circuit unit  112  and to be fitted into the mounting hole  31   a  of the bumper  31  of the vehicle  3 ; a signal line  114  for connecting the electronic circuit unit  112  and the controller unit  12 ; and a connector  115 . 
     The holding rubber  111  is formed in a cylindrical shape having a bottom, and the ultrasonic microphone  2 ′ is inserted into the holding rubber  111 , so that the outer surface of the ultrasonic microphone  2 ′ comes into contact with the inner surface of the holding rubber  111 . Accordingly, the holding rubber  111  holds the ultrasonic microphone  2 ′. 
     The electronic circuit unit  112  amplifies the ultrasonic wave pulse signal output from the controller unit via the signal line  114  and the connector  115 . The amplified ultrasonic wave pulse signal is then output to the piezoelectric element  21  via the connector  25  and the lead wires  24   a  and  24 B, so that ultrasonic waves can be transmitted from the piezoelectric element  21 . Furthermore, when an obstacle is present in the detection area, the piezoelectric element  21  receives ultrasonic waves reflected from the obstacle, and the electronic circuit unit  112  amplifies the received ultrasonic wave pulse signal and outputs it to the controller unit  12 . Furthermore, the controller unit  12  calculates the distance to the obstacle based on the difference between the time at which the ultrasonic waves were transmitted and the time at which the ultrasonic waves were received. 
     The housing  113  includes a cylindrical rear housing part  113   a , a substantially cylindrical front housing part  113   b  extending from one surface of the rear housing part  113   a , and a flange part  113   c  protruding around the opening of the front housing part  113   b . The cylindrical rear housing part  113   a  accommodates the electronic circuit unit  112  therein. The holding rubber  111  holding the ultrasonic microphone  2 ′ is fitted into the front housing part  113   b , and the front housing part  113   b  accommodates the holding rubber  111  in such a manner that the outer surface of the holding rubber  111  comes into contact with the inner surface of the front housing part  113   b . The flange part  113   c  is formed to protrude from the periphery of the opening of the front housing part  113   b  in the outward radial direction. Furthermore, the front surface  11   a  of the ultrasonic sensor  11 , which is formed by the flange part  113   c  of the housing  113 , the periphery of the opening of the holding rubber  111 , and the opposite surface  22   b  of the acoustic matching layer  22  of the ultrasonic microphone  2 ′, forms a single smooth surface. 
     The mounting hole  31   a  is formed through the bumper  31  so that the outside and the inside of the bumper  31  communicate with each other. The diameter of the mounting hole  31   a  is substantially the same as the outside diameter of the rear housing part  113   a  and the front housing part  113   b . Furthermore, the ultrasonic sensor  11 ′ is inserted into the mounting hole  31   a  from the outside of the bumper  31 , and the ultrasonic sensor  11 ′ is held on the bumper  31  by the flange part  113   c  of the housing  113 . As a result, only the flange part  113   c  of the ultrasonic sensor  11 ′ protrudes from the outside surface of the bumper  31 . The flange part  113   c  is formed to be thin, and the front surface  11   a  of the ultrasonic sensor  11 ′ and the outside surface of the bumper  31  are formed to be parallel. 
     In the above configuration, when an ultrasonic wave pulse signal is intermittently output from the controller unit  12  to the electronic circuit unit  112 , the electronic circuit unit  112  amplifies the ultrasonic wave pulse signal and outputs it to the ultrasonic microphone  2 ′. Furthermore, the piezoelectric element  21  of the ultrasonic microphone  2 ′ vibrates, and ultrasonic waves are transmitted from the opposite surface  22   b  of the acoustic matching layer  22 . The direction in which ultrasonic waves are transmitted is the direction perpendicular to one surface  21   a  of the piezoelectric element  21 . That is, ultrasonic waves are transmitted in the direction perpendicular to the outside surface of the bumper  31 . Furthermore, when an obstacle is present in a detection area, the ultrasonic microphone  2 ′ receives waves reflected from the obstacle, and the electronic circuit unit  112  amplifies the ultrasonic wave pulse signal and outputs it to the controller unit  12 . The controller unit  12  calculates the distance to the obstacle based on the difference between the time at which the ultrasonic waves were transmitted and the time at which the reflected waves were received. If the obstacle is within a predetermined distance, the controller unit  12  outputs a detection signal to the display unit  13 , and the display unit  13  notifies a user of the presence of the obstacle. 
     Next, the detection area within which the ultrasonic sensor  11 ′ can detect the presence of an obstacle will be described. The detection area is determined by the transmission direction and orientation angle of ultrasonic waves and the detection distance. In  FIG. 8A , if the detection area Ah in the horizontal direction of the vehicle  3  includes an area outside of the width  3   a  of the vehicle  3 , an obstacle that would not collide with the vehicle  3  may be detected, and therefore the defect of providing an unnecessary notification to the user may occur. Furthermore, in  FIG. 8B , if the detection area Av in the vertical direction of the vehicle  3  includes a road surface, the defect of detecting the road surface as an obstacle may occur. 
     Accordingly, an ultrasonic sensor arrangement has been proposed in which three ultrasonic sensors  11 ′ are provided at substantially equal intervals and the detection distance of the ultrasonic sensor  11 ′ at the center of the arrangement is set to a value that is longer than that of the ultrasonic sensors  11 ′ at both sides thereof, so that an area outside of the width  3   a  of the vehicle  3  is not detected (for example, see Patent document 2). In light of the characteristic in which the detection area of such an ultrasonic sensor  11 ′ is narrow for a short distance and wide for a long distance, the ultrasonic sensors  11 ′ with a short detection distance provided on both sides are made to cover only short-distance areas, and ultrasonic sensor  11 ′ with a long detection distance provided at the center is made to cover a long-distance area. Accordingly, the short-distance areas within the width  3   a  of the vehicle  3  are covered by the ultrasonic sensors  11 ′ provided on both sides, and the long-distance area within the width  3   a  of the vehicle  3  is covered by the ultrasonic sensor  11 ′ provided at the center. 
     Furthermore, an ultrasonic sensor  11 ′ has been proposed in which the orientation angle of ultrasonic waves is adjusted by forming an inclined surface by beveling the periphery of the opposite surface  22   b  of the acoustic matching layer  22  of the ultrasonic microphone  2 ′ (for example, see Patent document 3). As the magnitude of the beveling performed on the periphery of the surface  22   b  of the acoustic matching layer  22  increases, the orientation angle of ultrasonic waves increases. In contrast, as the magnitude of the beveling decreases, the orientation angle of ultrasonic waves decreases. As described above, the orientation angle of the ultrasonic waves can be adjusted by changing the magnitude of the beveling performed on the periphery of the surface  22   b  of the acoustic matching layer  22 . Accordingly, when the ultrasonic sensor  11 ′ is mounted on the vehicle  3 , the orientation angle of ultrasonic waves can be adjusted to prevent the area outside the width  3   a  of the vehicle  2  or a road surface from being detected. 
     Furthermore, an ultrasonic sensor  11 ′ has been proposed in which the thickness of opposite end portions of the acoustic matching layer  22  of the ultrasonic microphone  2 ′ in one direction is thinner than that of the center thereof and the thickness in the other direction is uniform, so that the orientation angles of ultrasonic waves in horizontal and vertical directions are adjusted (for example, see Patent document 4). The thicknesses of the acoustic matching layer  22  are set as described above, so that the orientation angle of ultrasonic waves in one direction is made narrow and the orientation angle of the ultrasonic waves in the other direction is made wide. Furthermore, the ultrasonic microphone  2 ′ is provided on the bumper  31  so that the one direction becomes the vertical direction of the vehicle  3  and the other direction becomes the horizontal direction of the vehicle  3 , and the area within the width  3   a  of the vehicle  3  in the horizontal direction is detected in the other direction having a wide orientation angle. Furthermore, since the area in the vertical direction is detected in the one direction having a narrow orientation angle, the road surface can be prevented from being detected. 
     As described above, the ultrasonic sensors disclosed in Patent documents 2 to 4 are configured to set detection areas in such a way as to adjust the detection distance or orientation angle of ultrasonic waves, thereby preventing the area outside of the width  3   a  of the vehicle  3  or the road surface from being detected. 
     (Patent document 1) Japanese Patent No. 2651140 
     (Patent document 2) Japanese Patent Publication No. H3-42635 
     (Patent document 3) Japanese Patent Application Publication No. S64-85000 
     (Patent document 4) Japanese Patent Application Publication No. 2003-163995 
     However, in accordance with the above-described conventional ultrasonic sensor, when the outside surface of the bumper  31  is arcuate or inclined, the direction in which ultrasonic waves are transmitted deviates from the longitudinal direction  3   b  of the vehicle  3 . 
       FIGS. 11A and 11B  illustrate detection areas when the bumper  31  is arcuate in the horizontal direction of the vehicle  3 .  FIG. 11A  is detection areas Ah in the horizontal direction, and  FIG. 11B  is a detection area Av in the vertical direction. 
     As shown in  FIG. 11A , in the horizontal direction of the vehicle  3 , the outside surface of the portion of the bumper  31  where the ultrasonic sensor  11 ′ is installed is inclined at an angle α with respect to the transverse direction  3   c  of the vehicle  3  that is perpendicular to the longitudinal direction  3   b  thereof. As described above, when the ultrasonic sensor  11 ′ is installed such that the front surface  11   a  of the ultrasonic sensor  11  and the outside surface of the bumper  31  are parallel to each other, the direction H in which ultrasonic waves are transmitted is laterally inclined at an angle α with respect to the longitudinal direction  3   b  of the vehicle  3  in the horizontal direction. As a result, areas outside the width  3   a  of the vehicle are included in the detection areas Ah in the horizontal direction, and therefore the defect of detecting an obstacle that would not collide with the vehicle and providing an unnecessary notification to the user occurs. Furthermore, the direction H in which ultrasonic waves are transmitted is laterally inclined with respect to the longitudinal direction  3   b  of the vehicle  3  and, therefore, behind the vehicle  3 , there is a gap between the two detection areas Ah of the ultrasonic microphone  2 ′ in the horizontal direction, so that there is an area where an obstacle cannot be detected. 
     Furthermore, when the ultrasonic sensor  11 ′ is installed into the bumper  31  that is arcuate or inclined with respect to the vertical direction of the vehicle  3 , the direction in which ultrasonic waves are transmitted is downwardly or upwardly inclined with respect to the longitudinal direction  3   b  of the vehicle  3 . 
       FIGS. 12A and 12B  illustrate detection areas when the bumper  31  is arcuate in the vertical direction of the vehicle  3 .  FIG. 12A  is detection areas Ah in the horizontal direction, and  FIG. 12B  is a detection area Av in the vertical direction. 
     As shown in  FIG. 12B , in the vertical direction of the vehicle  3 , the outside surface of the portion of the bumper where the ultrasonic sensor  11 ′ is installed is downwardly inclined at an angle β with respect to the vertical direction  3   c  of the vehicle  3 . As described above, when the ultrasonic sensor  11 ′ is installed such that the front surface  11   a  of the ultrasonic sensor  11  and the outside surface of the bumper  31  are parallel to each other, the direction V in which ultrasonic waves are transmitted is downwardly inclined at an angle β with respect to the longitudinal direction  3   b  of the vehicle  3  in the vertical direction. As a result, a road surface is included in the detection area Av in the vertical direction, and therefore the defect of providing an unnecessary notification to the user occurs. 
     Accordingly, when the conventional ultrasonic sensor  11 ′ is installed on the bumper  31  which is inclined at an angle α, the direction in which the ultrasonic sensor  11 ′ is installed is adjusted so that the front surface  11   a  of the ultrasonic sensor  11  is perpendicular to the longitudinal direction  3   b , thereby overcoming the above problem. FIGS.  13 A to  13 C illustrate schematic configurations when the conventional ultrasonic sensor  11 ′ is installed onto the inclined bumper  31 .  FIG. 13A  is a plan view of the ultrasonic sensor  11 ′,  FIG. 13B  is a sectional view showing section A-A′ of the ultrasonic sensor  11 ′, and  FIG. 13C  is a sectional view showing section B-B′ of the ultrasonic sensor  11 ′. 
     However, when the ultrasonic sensor  11 ′ is installed as described above, the front surface  11   a  of the ultrasonic sensor  11  and the outside surface of the bumper  31  are not parallel to each other, as shown in  FIG. 13B . Furthermore, the thickness of the flange part  113   c  of the housing  113  increases in the direction from A to A′, and therefore the extent of the protrusion of the ultrasonic sensor  11 ′ with respect to the bumper  31  increases. Accordingly, the ultrasonic sensor  11 ′ is noticeable from the bumper  31 , and deteriorates the design of the bumper  31 . 
     SUMMARY OF THE INVENTION 
     In view of the above, the present invention provides an ultrasonic sensor that does not deteriorate the design of a vehicle and can set detection areas where erroneous detection hardly occurs. 
     In accordance with an aspect of the present invention, there is provided an ultrasonic sensor for use in a vehicle obstacle detector that is provided at a mounting surface of a vehicle to detect an obstacle in a detection area by transmitting and receiving ultrasonic waves, the ultrasonic sensor including: a piezoelectric element configured to transmit and receive the ultrasonic waves by using vibrations of a piezoelectric substrate; and an acoustic matching layer on one surface of which the piezoelectric element is attached, wherein the ultrasonic sensor is inserted in a mounting hole formed through the mounting surface; and wherein another surface of the acoustic matching layer opposite to the one surface is inclined with respect to the one surface on which the piezoelectric element is attached, and is formed to be parallel with the mounting surface. 
     With such configuration, it is possible to set detection areas where erroneous detection hardly occurs without deteriorating the design of the vehicle. 
     Further, the opposite surface of the acoustic matching layer may be inclined in a horizontal direction of the vehicle with respect to the one surface of the acoustic matching layer on which the piezoelectric element is attached. 
     With such configuration, it is possible to set detection areas where erroneous detection hardly occurs without deteriorating the design of the vehicle even when the mounting surface for the ultrasonic sensor is curved or inclined in the horizontal direction. 
     In addition, the opposite surface of the acoustic matching layer may be inclined in a vertical direction of the vehicle with respect to the one surface of the acoustic matching layer on which the piezoelectric element is attached. 
     With such configuration, it is possible to set detection areas where erroneous detection hardly occurs without deteriorating the design of the vehicle even when the mounting surface for the ultrasonic sensor is curved or inclined in the vertical direction. 
     Moreover, the opposite surface of the acoustic matching layer may be inclined in horizontal and vertical directions of the vehicle with respect to the one surface of the acoustic matching layer on which the piezoelectric element is attached. 
     With such configuration, it is possible to set detection areas where erroneous detection hardly occurs without deteriorating the design of the vehicle even when the mounting surface for the ultrasonic sensor is curved or inclined in the horizontal and vertical directions. 
     As described above, in accordance with an aspect of the present invention, it is possible to set detection areas where erroneous detection hardly occurs without deteriorating the design of the vehicle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects and features of the present invention will become apparent from the following description of embodiments, given in conjunction with the accompanying drawings, in which: 
         FIGS. 1A to 1C  are diagrams showing the schematic configuration of an ultrasonic microphone according to a first embodiment of the present invention, wherein  FIG. 1A  is a plan view of the ultrasonic microphone,  FIG. 1B  is a sectional view showing section A-A′ of the ultrasonic microphone, and  FIG. 1C  is a sectional view showing section B-B′ of the ultrasonic microphone; 
         FIGS. 2A to 2C  are diagrams showing the schematic configuration of an ultrasonic sensor having the ultrasonic microphone according to the first embodiment of the present invention, wherein  FIG. 2A  is a plan view of the ultrasonic sensor,  FIG. 2B  is a sectional view showing section A-A′ of the ultrasonic sensor, and  FIG. 2C  is a sectional view showing section B-B′ of the ultrasonic sensor; 
         FIGS. 3A and 3B  are diagrams showing the block configuration of a vehicle obstacle detector and the detection areas of the ultrasonic sensor according to the first embodiment of the present invention, wherein  FIG. 3A  is a plan view of a vehicle showing the block configuration of the vehicle obstacle detector and detection areas Ah in the horizontal direction, and  FIG. 3B  is a side view of the vehicle showing a detection area Av in the vertical direction; 
         FIGS. 4A to 4C  are diagrams showing the schematic configuration of an ultrasonic microphone according to a second embodiment of the present invention, wherein  FIG. 4A  is a plan view of the ultrasonic microphone,  FIG. 4B  is a sectional view showing section A-A′ of the ultrasonic microphone, and  FIG. 4C  is a sectional view showing section B-B′ of the ultrasonic microphone; 
         FIGS. 5A to 5C  are diagrams showing the schematic configuration of an ultrasonic sensor having the ultrasonic microphone according to the first embodiment of the present invention, wherein  FIG. 5A  is a plan view of the ultrasonic sensor,  FIG. 5B  is a sectional view showing section A-A′ of the ultrasonic sensor, and  FIG. 5C  is a sectional view showing section B-B′ of the ultrasonic sensor; 
         FIGS. 6A to 6C  are diagrams showing the schematic configuration of an ultrasonic microphone according to a third embodiment of the present invention, wherein  FIG. 6A  is a plan view of the ultrasonic microphone,  FIG. 6B  is a sectional view showing section A-A′ of the ultrasonic microphone, and  FIG. 6C  is a sectional view showing section B-B′ of the ultrasonic microphone; 
         FIGS. 7A to 7C  are diagrams showing the schematic configuration of an ultrasonic sensor having the ultrasonic microphone according to the third embodiment of the present invention, wherein  FIG. 7A  is a plan view of the ultrasonic sensor,  FIG. 7B  is a sectional view showing section A-A′ of the ultrasonic sensor, and  FIG. 7C  is a sectional view showing section B-B′ of the ultrasonic sensor; 
         FIGS. 8A and 8B  are diagrams showing the block configuration of a vehicle obstacle detector and the detection areas of a conventional ultrasonic sensor; 
         FIGS. 9A to 9C  are diagrams the top and sections of a conventional ultrasonic microphone; 
         FIGS. 10A to 10C  are diagrams the top and sections of the conventional ultrasonic sensor; 
         FIGS. 11A and 11B  are diagrams showing the block configuration of the vehicle obstacle detector and the detection areas of the conventional ultrasonic sensor; 
         FIGS. 12A and 12B  are diagrams showing the block configuration of a vehicle obstacle detector and the detection areas of a conventional ultrasonic sensor; and 
         FIGS. 13A to 13C  are diagrams the top and sections of the conventional ultrasonic sensor. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will now be described with reference to the accompanying drawings. In the drawings, the same or similar components are assigned by like reference numerals and redundant descriptions thereof will be omitted. 
     First Embodiment 
     The first embodiment of the present invention will be described with reference to  FIGS. 1A to 1C ,  FIGS. 2A to 2C , and  FIGS. 3A and 3B . 
     The present embodiment is directed to a case where an ultrasonic sensor  11  is installed on a bumper  31  which is arcuate in the horizontal direction, as shown in  FIG. 3A , and the present embodiment will now be described in detail. In the horizontal direction of a vehicle  3 , the outside surface of the portion of a bumper  31  where the ultrasonic sensor  11  is installed is inclined at an angle α with respect to the transverse direction  3   c  of the vehicle  3 . That is, the angle α is the angle that is formed by the transverse direction  3   c  and the tangent line that is one of the tangent lines of the bumper  31  to which the sensor  11  is attached and that forms a plane along with the transverse direction  3   c . Furthermore, the basic configuration of a vehicle obstacle detector  1  including the ultrasonic sensor  11 , a controller unit  12  and a display unit  13  is the same as that of the conventional vehicle obstacle detector  1 . The same reference numerals will be assigned to the same components, and redundant descriptions thereof will be omitted. 
     The ultrasonic microphone  2  of the present embodiment, as shown in  FIGS. 1A to 1C , includes a piezoelectric element  21 , an acoustic matching layer  22 , a vibration casing  23 , lead wires  24   a  and  24   b , a connector  25 , and a filler  26 . The ultrasonic microphone  2  of the present embodiment is different from the conventional ultrasonic microphone  2 ′ in that the shapes of the acoustic matching layers  22  are different from each other. The other components of the ultrasonic microphone  2  of the present embodiment are the same as those of the conventional ultrasonic microphone  2 ′, so that the same reference numerals will be assigned to these components and redundant descriptions thereof will be omitted. 
     In the section A-A′ shown in  FIG. 1B , with respect to one surface  22   a  of the acoustic matching layer  22  on which the piezoelectric element  21  is attached, the opposite surface  22   b  is inclined at an angle α so that the opposite surface  22   b  approaches the one surface  22   a  along the direction from A to A′. This angle α is designed to be equal to the angle at which the bumper  31  is inclined. 
     In accordance with the above configuration, in the ultrasonic microphone  2 , when an ultrasonic wave pulse signal is input via the connector  25 , the piezoelectric element  21  vibrates, and ultrasonic waves are output from the opposite surface  22   b  of the acoustic matching layer  22  via the acoustic matching layer  22 . Furthermore, ultrasonic waves are transmitted in the direction perpendicular to the one surface  21   a  of the piezoelectric element  21  that is attached to the acoustic matching layer  22 . Accordingly, the direction perpendicular to the opposite surface  22   b  of the acoustic matching layer  22  is inclined at the angle α with respect to the direction in which ultrasonic waves are transmitted. 
     The ultrasonic sensor  11 , as shown in  FIGS. 2A to 2C , includes the ultrasonic microphone  2 , a holding rubber  111 , an electronic circuit unit  112 , a housing  113 , a signal line  114 , and a connector  115 . The ultrasonic sensor  11  is different from the conventional ultrasonic sensor  11 ′ in that the shapes of the ultrasonic microphone  2 , the holding rubber  111  and the housing  113  are different from each other. The other components of the ultrasonic sensor  11  are the same as those of the conventional ultrasonic sensor  11 ′, so that the same reference numerals will be assigned to these components and redundant descriptions thereof will be omitted. 
     The peripheries of the openings of the holding rubber  111  and the front housing part  113   b  are inclined in conformity with the inclined angle α of the opposite surface  22   b  of the acoustic matching layer  22  so that the front surface  11   a  of the ultrasonic sensor  11 , which is formed by the opposite surface  22   b  of the acoustic matching layer  22  of the ultrasonic microphone  2 , the periphery of the opening of the holding rubber  111  and the flange part  113   c  of the housing  113 , can form a single smooth surface. 
     The ultrasonic sensor  11  is inserted into the mounting hole  31   a  of the bumper  31  so that direction A-A′ of the ultrasonic microphone  2  becomes the transverse direction  3   c  of the vehicle  3  (that is, the one surface  22   a  of the acoustic matching layer  22  is parallel with the transverse direction  3   c  of the vehicle  3 ), and the ultrasonic sensor  11  is held on the bumper  31  by the flange part  113   c  of the housing  113 . Consequently, the direction perpendicular to the one surface  21   a  of the piezoelectric element  21  is consistent with the longitudinal direction  3   b  of the vehicle  3 , so that the direction in which ultrasonic waves are transmitted becomes the longitudinal direction of the vehicle  3 , as shown in  FIG. 3A . 
     Furthermore, since the front surface  11   a  of the ultrasonic sensor  11  and the outside surface of the bumper  31  are all inclined at the same angle α with respect to the transverse direction  3   c , the front surface  11   a  of the ultrasonic sensor  11  and the outside surface of the bumper  31  are parallel to each other. Furthermore, only the flange part  113   c  of the ultrasonic sensor  11  protrudes from the outside surface of the bumper  31  with the thickness of the flange part  113   c  kept small and uniform. For this reason, the ultrasonic sensor  11  is not noticeable from the bumper  31 , and does not deteriorate the design of the bumper  31 . 
     In accordance with the above-described configuration, even when the bumper  31  is inclined at an angle α with respect to the transverse direction  3   c , the direction in which ultrasonic waves are transmitted can be set to the longitudinal direction  3   b  of the vehicle without deteriorating the design of the bumper  31 . For this reason, an area outside the width  3   a  of the vehicle is not included in a detection area Ah in the horizontal direction, so that the defect of detecting an obstacle that would not collide with the vehicle and then providing an unnecessary notification to the user can be prevented. 
     Second Embodiment 
     A second embodiment of the present invention will be described with reference to  FIGS. 4A to 4C  and  FIGS. 5A to 5C . 
     The present embodiment is directed to a case where an ultrasonic sensor  11  is installed on a bumper  31  which is arcuate in the vertical direction  3   d , which will be described in detail below. The outside surface of the portion of the bumper  31  where the ultrasonic sensor  11  is installed is inclined at an angle β with respect to the vertical direction  3   d  of the vehicle  3 . That is, the angle β is the angle that is formed by the vertical direction  3   d  and a tangent line which is one of the tangent lines of the bumper  31  to which a sensor  11  is attached and which forms a plane along with the vertical direction  3   d.    
     As shown in  FIGS. 4A to 4C  and  FIG. 5A to 5C , the basic configurations of the ultrasonic microphone  2  and ultrasonic sensor  11  of the present embodiment are the same as those of the first embodiment, so that the same reference numerals will be assigned to these components and redundant descriptions thereof will be omitted. 
     The present embodiment is different from the first embodiment in that since the ultrasonic microphone  2  of the present embodiment is installed into the bumper  31  that is inclined at an angle β with respect to the vertical direction  3   d  of the vehicle  3 , the opposite surface  22   b  of the acoustic matching layer  22  of the ultrasonic microphone  2  is inclined in the direction B-B′ (that is, inclined so that the vertical direction of the opposite surface  22   b  of the acoustic matching layer  22  forms an angle β with respect to the vertical direction  3   d  of the vehicle  3 ). As in the first embodiment, in the section B-B′ shown in  FIG. 4C , with respect to one surface  22   a  of the acoustic matching layer  22  on which the piezoelectric element  21  is attached, the opposite surface  22   b  is inclined at an inclined angle β so that the opposite surface  22   b  approaches the one surface  22   a  along the direction from B′ to B. Since ultrasonic waves are transmitted toward the direction perpendicular to one surface  21   a  of the piezoelectric element  21  that is attached to the acoustic matching layer  22 , the direction perpendicular to the opposite surface  22   b  of the acoustic matching layer  22  is inclined at the angle α with respect to the direction in which ultrasonic waves are transmitted. 
     Furthermore, the peripheries of the openings of the holding rubber  111  and the front housing part  113   b  are inclined in conformity with the inclined angle β of the opposite surface  22   b  of the acoustic matching layer  22  so that in the ultrasonic sensor  11  having the ultrasonic microphone  2 , the front surface  11   a  of the ultrasonic sensor  11 , which is formed by the opposite surface  22   b  of the acoustic matching layer  22 , the periphery of the opening of the holding rubber  111  and the flange part  113   c  of the housing  113 , can form a single smooth surface. 
     The ultrasonic sensor  11  is inserted into the mounting hole  31   a  of the bumper  31  so that the direction B-B′ of the ultrasonic microphone  2  becomes the vertical direction  3   c  of the vehicle  3 , and the ultrasonic sensor  11  is held on the bumper  31  by the flange part  113   c  of the housing  113 . Consequently, the direction perpendicular to the one surface  21   a  of the piezoelectric element  21  is consistent with the longitudinal direction  3   b  of the vehicle  3 , so that the direction in which ultrasonic waves are transmitted becomes the longitudinal direction  3   b  of the vehicle  3 . 
     Furthermore, since the front surface  11   a  of the ultrasonic sensor  11  and the outside surface of the bumper are all inclined at the angle β with respect to the vertical direction  3   d , the front surface  11   a  of the ultrasonic sensor  11  and the outside surface of the bumper  31  are parallel to each other. Furthermore, only the flange part  113   c  of the ultrasonic sensor  11  protrudes from the outside surface of the bumper  31  with the thickness of the flange part  113   c  kept small and uniform. For this reason, the ultrasonic sensor  11  is not noticeable from the bumper  31 , and does not deteriorate the design of the bumper  31 . 
     In accordance with the above-described configuration, even when the bumper  31  is inclined at the angle β with respect to the vertical direction  3   d , the direction in which ultrasonic waves are transmitted can be set to the longitudinal direction  3   b  of the vehicle without deteriorating the design of the bumper  31 . For this reason, a road surface is not included in a detection area Av in the vertical direction, so that the defect of providing an unnecessary notification to the user can be prevented. 
     Third Embodiment 
     A third embodiment of the present invention will now be described with reference to  FIGS. 6A to 6C  and  FIGS. 7A to 7C . 
     The present embodiment is directed to a case where an ultrasonic sensor  11  is installed on a bumper  31  which are arcuate in both the horizontal and vertical directions, and the present embodiment will now be described in detail. The outside surface of the portion of the bumper  31  where the ultrasonic sensor  11  is installed is inclined at an angle α with respect to the transverse direction  3   c  of the vehicle  3  and at an angle β with respect to the vertical direction  3   d.    
     As shown in  FIGS. 6A to 6C  and  FIGS. 7A to 7C , the basic configurations of the ultrasonic microphone  2  and ultrasonic sensor  11  of the present embodiment are the same as those of the first and second embodiments, so that the same reference numerals will be assigned to these components and redundant descriptions thereof will be omitted. 
     The present embodiment is different from the first and second embodiments in that since the ultrasonic microphone  2  of the present embodiment is installed into the bumper  31  that is inclined at the angle α with respect to the transverse direction  3   c  and at the angle β with respect to the vertical direction  3   d , the opposite surface  22   b  of the acoustic matching layer  22  of the ultrasonic microphone  2  is inclined in both direction A-A′ and direction B-B′. As in the first and second embodiments, in the section A-A′ shown in  FIG. 6B , with respect to one surface  22   a  of the acoustic matching layer  22  on which the piezoelectric element  21  is attached, the opposite surface  22   b  is inclined at the inclined angle α so that the opposite surface  22   b  approaches the one surface  22   a  along the direction from A to A′. Furthermore, in the section B-B′ shown in  FIG. 6C , the opposite surface  22   b  of the acoustic matching layer  22  is inclined at an inclined angle β so that the opposite surface  22   b  approaches the one surface  22   a  along the direction from B′ to B. Since ultrasonic waves are output and transmitted toward the direction perpendicular to one surface  21   a  of the piezoelectric element  21  that is attached to the acoustic matching layer  22 , the direction perpendicular to the opposite surface  22   b  of the acoustic matching layer  22  is inclined at the angle α in the horizontal direction and at the angle β in the vertical direction with respect to the direction in which ultrasonic waves are transmitted. 
     Furthermore, in the ultrasonic sensor  11  having the ultrasonic microphone  2 , the peripheries of the openings of the holding rubber  111  and the front housing part  113   b  are inclined in conformity with the inclined angles α and β of the opposite surface  22   b  of the acoustic matching layer  22  so that the front surface  11   a  of the ultrasonic sensor  11 , which is formed by the opposite surface  22   b  of the acoustic matching layer  22 , the periphery of the opening of the holding rubber  111  and the flange part  113   c  of the housing  113 , can form a single smooth surface. 
     The ultrasonic sensor  11  is inserted into the mounting hole  31   a  of the bumper  31  so that direction A-A′ of the ultrasonic microphone  2  becomes the horizontal direction of the vehicle  3  and direction B-B′ of the ultrasonic microphone  2  becomes the vertical direction of the vehicle  3 , and the ultrasonic sensor  11  is held on the bumper  31  by the flange part  113   c  of the housing  113 . Consequently, the direction perpendicular to the one surface  21   a  of the piezoelectric element  21  is consistent with the longitudinal direction  3   b  of the vehicle  3 , so that the direction in which ultrasonic waves are transmitted becomes the longitudinal direction  3   b  of the vehicle  3 . 
     Furthermore, since the front surface  11   a  of the ultrasonic sensor  11  and the outside surface of the bumper are all inclined at the angle α with respect to the transverse direction  3   c  and at the angle β with respect to the vertical direction  3   d , the front surface  11   a  of the ultrasonic sensor  11  and the outside surface of the bumper  31  are parallel to each other. Furthermore, only the flange part  113   c  of the ultrasonic sensor  11  protrudes from the outside surface of the bumper  31  with the thickness of the flange part  113   c  kept small and uniform. For this reason, the ultrasonic sensor  11  is not noticeable from the bumper  31 , and does not deteriorate the design of the bumper  31 . 
     In accordance with the above-described configuration, even when the bumper  31  is inclined at the angle α with respect to the transverse direction  3   c  and at the angle β with respect to the vertical direction  3   d , the direction in which ultrasonic waves are transmitted can be set to the longitudinal direction  3   b  of the vehicle without deteriorating the design of the bumper  31 . For this reason, an area outside the width  3   a  of the vehicle is not included in a detection area Ah in the horizontal direction and a road surface is not included in a detection area Av in the vertical direction, so that the defect of detecting an obstacle that would not collide with the vehicle and then providing an unnecessary notification to the user can be prevented. 
     While the invention has been shown and described with respect to the embodiments, it will be understood by those skilled in the art that various changes and modification may be made without departing from the scope of the invention as defined in the following claims.