Patent Publication Number: US-8111146-B2

Title: Ultrasonic sensor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2008-084918, filed on Mar. 27, 2008, the entire content of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     This invention generally relates to an ultrasonic sensor. 
     BACKGROUND 
     A known ultrasonic sensor is provided at a flip-up rear hatch (i.e., back door) that opens and closes via a pivot shaft provided at the rear top of a vehicle so that a surface of the back door is included in a detection range of the sensor. The ultrasonic sensor detects an object or an obstacle in the vicinity of the vehicle, i.e., performs an obstacle detection by intermittently transmitting an ultrasonic wave, which hits the obstacle to thereby generate a reflected wave as a reflection of the ultrasonic wave. An example of an opening and closing control system for a vehicle door equipped with such ultrasonic sensor is disclosed in JP2007-138566A (which will be hereinafter referred to as Reference 1). According to the opening and closing control system disclosed in Reference 1, an ultrasonic sensor is provided at a back door for a vehicle to transmit an ultrasonic wave so that a detection range of the sensor extends substantially along a surface of a vehicle body (specifically, the back door). The ultrasonic sensor detects a distance therefrom to an object or an obstacle and a direction where the obstacle is present based on a time interval between the ultrasonic sensor transmitting the ultrasonic wave and receiving a reflected wave from the obstacle. When the distance to the obstacle detected by the ultrasonic sensor is within a predetermined range, the opening and closing control system controls, for example, the back door to stop opening or closing. Because the ultrasonic sensor moves along with the back door when it opens or closes, an edge portion of the back door that is most away from the vehicle body upon opening and closing of the back door is constantly included within the detection range of the ultrasonic sensor. Accordingly, the opening and closing control system successfully performs the obstacle detection when the back door opens or closes, thereby opening or closing an opening portion of the vehicle based on the detection result of the ultrasonic sensor. 
     JP2007-139570A (which will be hereinafter referred to as Reference 2) discloses a sensor unit used for such opening and closing control system for a vehicle, an obstacle detection system, and the like. The sensor unit disclosed in Reference 2 includes a sensor head arranged next to a license plate lamp of a back door for a vehicle. In addition, JP2007-057527A (which will be hereinafter referred to as Reference 3) discloses an ultrasonic sensor of which sensor head is arranged in the vicinity of a hinge provided at an upper portion of a back door, at a vehicle emblem of the back door in view of design, and the like. 
     A position where the sensor head is arranged according to Reference 2 is determined, focusing on a routing of wiring, ease of connection with CAN (Controller Area Network), and the like. In addition, a position where the sensor head is arranged according to Reference 3 is determined so that the detection range of the ultrasonic sensor extends along a surface of the back door or determined in view of design. However, none of the above References discloses a method for arranging the sensor head at an appropriate position by excluding a small aboveground structure such as a curbstone and a wheel stopper provided on a road, a parking surface, and the like from the detection range of the ultrasonic sensor while securing the detection range of the ultrasonic sensor. Further, based on the aforementioned standpoint, none of the above References includes considerations or proposals on the arrangement of the sensor head at the appropriate position. 
     A need thus exists for an ultrasonic sensor which is not susceptible to the drawback mentioned above. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, an ultrasonic sensor provided at a flip-up back door that opens and closes via a pivot shaft provided at a rear top of a vehicle, a surface of the back door being included in a detection range of the ultrasonic sensor, the ultrasonic sensor detecting an object in a vicinity of the vehicle by intermittently transmitting an ultrasonic wave, which hits the object to thereby generate a reflected wave as a reflection of the ultrasonic wave, the ultrasonic sensor including a sensor head provided at the back door and including a transmitter transmitting the ultrasonic wave and a receiver receiving the reflected wave, wherein the sensor head is provided at a position in a vicinity of a centerline of the vehicle in a width direction thereof and where a distance from a corner of an edge portion of the back door to the sensor head is shorter than a distance from the sensor head to a small aboveground structure provided on the ground. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and additional features and characteristics of the present invention will become more apparent from the following detailed description considered with reference to the accompanying drawings, wherein: 
         FIG. 1  is a rear view of a vehicle for illustrating a detection range of an ultrasonic sensor provided at an opening and closing control system for a vehicle according to an embodiment of the present invention; 
         FIG. 2  is a side view of the vehicle for illustrating the detection range shown in  FIG. 1 ; 
         FIG. 3  is a side view of the vehicle for illustrating the detection range when a back door shown in  FIG. 1  is opened; 
         FIG. 4  is a block diagram schematically illustrating an example of a structure of the opening and closing control system according to the embodiment of the present invention; 
         FIG. 5  is a timing chart illustrating a basic operation of an obstacle detection performed by the ultrasonic sensor according to the embodiment of the present invention; 
         FIGS. 6A to 6C  are waveform charts each explaining a wave detection performed by the ultrasonic sensor according to the embodiment of the present invention; 
         FIG. 7  is a block diagram schematically illustrating another example of the structure of the opening and closing control system according to the embodiment of the present invention; 
         FIG. 8  is an explanatory view illustrating an example of a sensor head of the ultrasonic sensor; 
         FIG. 9  is a side view of the vehicle for illustrating an appropriate mounting position of the sensor head; and 
         FIG. 10  is a rear view of the vehicle for illustrating the appropriate mounting position of the sensor head. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of an ultrasonic sensor will be explained with reference to the attached drawings. In the following, an opening and closing control system that opens and closes a rear hatch (i.e., back door) for a vehicle and where an ultrasonic sensor according to the present embodiment is mounted will be explained as an example.  FIG. 1  is a rear view of a vehicle  1  for illustrating a detection range  8  of an ultrasonic sensor  2  provided at the opening and closing control system.  FIG. 2  is a side view of the vehicle  1  for illustrating the detection range  8  shown in  FIG. 1 . A back door  11  for the vehicle  1  is a pivotal door opening and closing in a vertical direction of the vehicle  1 . The vehicle  1  is placed on a ground G in  FIGS. 1 and 2 . 
     The ultrasonic sensor  2  transmits an ultrasonic wave, which hits an object or an obstacle such as an obstacle X to thereby generate a reflected wave as a reflection of the ultrasonic wave. The reflected wave is then received back by the ultrasonic sensor  2 . The ultrasonic sensor  2  detects a distance therefrom to the obstacle X based on a time interval from the transmission of the ultrasonic wave to the reception of the reflected wave. A control portion, i.e., a door ECU  3  (which will be explained in detail later) determines that the obstacle X is present when the distance detected by the ultrasonic sensor  2  is within a predetermined range. The control portion alerts, on the basis of the determination result of the ultrasonic sensor  2 , a driver or a user of a presence of the obstacle X by a buzzer, a warning display, and the like, or controls a drive portion, i.e., a door actuator  4 . For example, in a case where it is determined that the obstacle X is present, the control portion controls the back door  11  to stop opening or closing, and the like. As a result, the back door  11  is prevented from making contact with the obstacle X. 
     As illustrated in  FIG. 1 , a sensor head  20  of the ultrasonic sensor  2  is arranged at a substantially center portion of the back door  11  in the vicinity of a portion where a vehicle emblem  15  is generally provided, i.e., in the vicinity of the vehicle emblem  15  provided at a slightly upper side of a license plate  50 . The back door  11  rotates about a hinge  13  serving as a pivot shaft. A detailed position where the sensor head  20  is arranged will be explained in detail later. A center axis C of the sensor head  20  in a transmission direction thereof inclines downward so as to extend substantially along a surface  12 A of the back door  11  as illustrated in  FIG. 2 . The term “the center axis C extends substantially along the surface  12 A of the back door  11 ” specifically means that an angle formed between the center axis C and the surface  12 A is within 45 degrees. The detection range  8  is formed into a fan shape in a planar view as illustrated in  FIG. 1  and is formed into a frusto-conical shape in a spatial view as illustrated in  FIG. 2 . Because of a principle of the ultrasonic sensor  2 , the ultrasonic sensor  2  includes a non-detection range  9  in the vicinity of the sensor head  20  where resonance of a transmitted wave from the ultrasonic sensor  2  changes to a received wave and thus an obstacle is not detectable. 
     In a case where an elevation angle α in  FIG. 2  is defined in a range from 0 degree (inclusive) to 60 degrees (inclusive), even the low-output ultrasonic sensor is applicable. In a case where the elevation angle α is defined in a range from 0 degree (inclusive) to 45 degrees (inclusive), the detection sensitivity of the ultrasonic sensor  2  is further improved. The elevation angle α may be around 90 degrees. In this case, the surface  12 A of the back door  11  is fully and entirely detectable. 
     Because the sensor head  20  of the ultrasonic sensor  2  is arranged at the back door  11 , the ultrasonic sensor  2  moves along with the back door  11  when the back door  11  opens or closes. Thus, as illustrated in  FIG. 3 , the center axis C of the ultrasonic sensor  2  is constantly positioned along the surface  12 A of the back door  11 . The detection range  8  moves along with the opening of the back door  11 . Thus, in a case where the obstacle X is present along a path of the back door  11  that is opening, the obstacle X is easily detectable. 
     Further, as illustrated in  FIGS. 1 to 3 , the detection range  8  of the ultrasonic sensor  2  includes an edge portion  12   a  (i.e., edge of the surface  12 A) of the back door  11  at an opening and closing side opposite from a side where the hinge  13  is provided. The edge portion  12   a  of the back door  11  first starts moving when the back door  11  starts opening, i.e., the edge portion  12   a  is most likely to make contact with the obstacle X. Accordingly, the edge portion  12   a  is included in the detection range  8  of the ultrasonic sensor  2  to thereby further secure an obstacle detection thereof while the back door  11  is opening. 
       FIG. 4  is a block diagram schematically illustrating a structure example of the opening and closing control system according to the present embodiment. As illustrated in  FIG. 4 , the opening and closing control system includes the ultrasonic sensor  2 , the door actuator  4  for driving the back door  11  to open and close, and the door ECU  3  serving as the control portion for driving the door actuator  4  based on the detection result of the ultrasonic sensor  2  and controlling the back door  11  to open and close with an assistance of a gas spring  16 . The ultrasonic sensor  2  includes the sensor head  20  constituted by a transmitter  21  and a receiver  22 , a transmitting portion  23 , a receiving portion  24 , a detecting portion  25 , and an operating portion  26 . The transmitting portion  23  is a functional portion for causing the transmitter  21  to transmit the ultrasonic wave (i.e., transmitted wave) based on a transmission command output from the operating portion  26 . The receiving portion  24  is a functional portion for receiving the ultrasonic wave received by the receiver  22  (i.e., received wave) as an electrical signal. The detecting portion  25  is a functional portion for detecting the received wave. The operating portion  26  is a functional portion for causing the transmitting portion  23  to send the transmitted wave. In addition, the operating portion  26  detects whether or not an obstacle such as the obstacle X is present on the basis of the detection result by the detecting portion  25  and calculates a distance to the obstacle X when it is detected that the obstacle X is present. 
     Next, a principle for detecting the obstacle X by the ultrasonic sensor  2 , and an overview of the opening and closing control system by which the back door  11  is controlled to open and close on the basis of the detection result of the ultrasonic sensor  2  will be explained below.  FIG. 5  is a timing chart illustrating a basic operation of the obstacle detection performed by the ultrasonic sensor  2 .  FIGS. 6A to 6C  are waveform charts each explaining a wave detection by the detecting portion  25 . Each of the functional portions of the ultrasonic sensor  2  will be explained in detail below with reference to  FIGS. 5 and 6 . 
     As illustrated in (a) of  FIG. 5 , the operating portion  26  outputs a timing signal to the transmitting portion  23  at time t 0 . The timing signal is repeatedly output at a predetermined time interval (for example, 10 ms to 100 ms). Upon reception of the timing signal, the transmitting portion  23  outputs an oscillation signal corresponding to the predetermined number of pulse signals to the transmitter  21  of the sensor head  20  as illustrated in (b) of  FIG. 5 . The transmitter  21  oscillates when receiving the oscillation signal and outputs a burst ultrasonic wave (i.e., a burst wave) as a transmitted wave W 1  as illustrated in (c) of  FIG. 5 . In (c) of  FIG. 5 , the greater burst wave is output relative to the oscillation signal because of the resonance of the transmitter  21 . 
     The receiver  22  and the transmitter  21  are arranged adjacent to each other, constituting the sensor head  20 . Thus, the receiver  22  arranged in the vicinity of the transmitter  21  directly receives the ultrasonic wave transmitted by the transmitter  21  as illustrated in (d) of  FIG. 5 . In addition, a bumper  14  serving as a projection portion is provided at a portion of the vehicle  1  relatively close to the sensor head  20 . More specifically, the bumper  14  faces the edge portion  12   a  and extends from the vehicle  1  (specifically, a vehicle body  12 ) relative to the edge portion  12   a . Thus, the transmitted wave from the transmitter  21  is reflected by the bumper  14  and the resulting reflected wave is input to the receiver  22 . 
     An example shown in (d) of  FIG. 5  simulates a case where the reflected wave from the bumper  14  reaches the receiver  22  during a time interval where the resonance of the transmitter  21  still remains. A duration of a received wave W 2  shown in (d) of  FIG. 5  is longer than a duration of the transmitted wave W 1  shown in (c) of  FIG. 5  accordingly. A received wave W 3  shown in (d) of  FIG. 5  simulates the reflected wave from the obstacle X. In order to distinguish the received wave W 2  from the received wave W 3  serving as the reflected wave from the obstacle X, the received wave W 2  including the transmitted wave W 1  and the reflected wave from the bumper  14  will be hereinafter referred to as an initial received wave. 
     A mask signal shown in (e) of  FIG. 5  is a recognition signal for preventing the detecting portion  25  and the operating portion  26  from detecting the initial received wave W 2  including the transmitted wave W 1  and the reflected wave from the bumper  14  as the reflected wave from the obstacle X and from performing the obstacle detection. According to the present embodiment, the detection by the detecting portion  25  is not performed on the initial received wave W 2 . Alternatively, the detection by the detecting portion  25  may be performed on all received waves and the operating portion  26  may determine whether or not the obstacle X is present on the basis of the mask signal. The mask signal is active within a time interval from time t 0  time t 1 , which is called a mask period. 
     In (d) of  FIG. 5 , the received wave W 3  is the reflected wave from the obstacle X. The detecting portion  25  is the functional portion for detecting the received wave W 3 . The received wave W 3  shown in  FIG. 6A  is full wave rectified by the detecting portion  25  to thereby obtain a waveform W 4  as illustrated by a wavy line (a dashed line) in  FIG. 6B . Then, an envelope processing is performed on the waveform W 4  to thereby obtain an envelope curve W 5 . Next, the detecting portion  25  performs a comparison calculation on the envelope curve W 5  with a threshold value P 1 , for example, thereby obtaining a detection signal W 6  as illustrated in  FIG. 6C  and (f) of  FIG. 5 . A rising of the detection signal W 6  corresponds to a detection start time, which is time t 2  in (f) of  FIG. 5 . 
     According to the present embodiment, for the purpose of easy understanding, the detecting portion  25  determines the detection start time by the comparison between the envelope curve W 5  and the threshold value P 1 . However, the determination of the detection start time is not limited to the aforementioned method. For example, a straight-line approximation may be performed on an increase of the envelope curve W 5 . Then, timing when the thus-obtained straight line and a predetermined threshold value intersect each other is used for determining the detection start time. The aforementioned threshold value P 1 , an amplitude center of the received wave W 3 , and the like may be appropriately specified as the predetermined threshold value. 
     The operating portion  26  calculates a distance from the sensor head  20  to the obstacle X based on a time difference between time t 0  at which the timing signal is output and time t 2  at which the detection is started (i.e., detection start time t 2 ). That is, the ultrasonic wave makes a round-trip between the sensor head  20  and the obstacle X during the time difference between t 0  and t 2 . Accordingly, by an estimation of the product of the sonic speed and a half of the time for the round-trip of the ultrasonic wave between the sensor head  20  and the obstacle X, i.e., time for one-way trip, the distance from the sensor head  20  to the obstacle X is obtained. The operating portion  26  determines the presence of the obstacle X by means of the detection signal W 6  and calculates the distance to the obstacle X when it is detected that the obstacle X is present. 
     The calculation result of the operating portion  26  is output to the door ECU  3 . When it is detected that the obstacle X is present in the vicinity of the back door  11 , the door ECU  3  stops the operation of the door actuator  4 , for example. Accordingly, the back door  11  is controlled to stop before making contact with the obstacle X. 
     With reference to  FIGS. 5 and 6 , the principle for detecting the obstacle X by the ultrasonic sensor  2  and the overview of the opening and closing control system by which the back door  11  is controlled to open and close on the basis of the detection result of the ultrasonic sensor  2  are explained in the above. For the purpose of easy explanation, one receiver  22  is provided in the aforementioned explanation. However, even though the distance to the obstacle X is obtained by the calculation, a position or a direction where the obstacle X is present cannot be obtained by only the one receiver  22 . At least two of the receivers  22  are necessary for detecting the direction of the obstacle X in addition to the distance thereto. 
       FIGS. 7 and 8  each illustrate an ultrasonic sensor  2 A that also detects a direction where the obstacle X is present.  FIG. 7  is a block diagram schematically illustrating another structure example of the opening and closing control system including the ultrasonic sensor  2 A.  FIG. 8  is an explanatory view illustrating a sensor head  20 A of the ultrasonic sensor  2 A. 
     As illustrated in  FIGS. 7 and 8 , the ultrasonic sensor  2 A includes four oscillating portions, i.e., one transmitter  21  and three receivers  22   a ,  22   b , and  22   c . The four oscillating portions are arranged at respective corners, i.e., in the vicinity of respective vertexes, of a square shape of the sensor head  20 A. In  FIG. 8 , an x-axis corresponds to a vehicle width direction, a z-axis extends along the center axis C of the ultrasonic sensor  2 A in the transmission direction thereof, and a y-axis is perpendicular to the x-axis and the y-axis. A half wavelength of the transmitted wave from the transmitter  21  is sufficiently smaller than a distance to each of the adjacent oscillating portions in a direction of each side of the square shape of the sensor head  20 A. The ultrasonic sensor  2 A includes the three receivers  22   a ,  22   b , and  22   c  to thereby three-dimensionally detect a position of the obstacle X. 
     The ultrasonic sensor  2 A includes three receiving portions  24   a ,  24   b , and  24   c  corresponding to the three receivers  22   a ,  22   b , and  22   c , respectively, as illustrated in  FIG. 7 . In addition, the ultrasonic sensor  2 A includes three detecting portions  25   a ,  25   b , and  25   c  corresponding to the three receiving portions  24   a ,  24   b , and  24   c , respectively. Because the three receivers  22   a ,  22   b , and  22   c  are located in different positions from one another, distances to the obstacle X from the receivers  22   a ,  22   b , and  22   c  are different from one another. That is, the three receiving portions  24   a ,  24   b , and  24   c  receive the reflected wave from the object X at different timings. The detection start times that the three detecting portions  25   a ,  25   b , and  25   c  determine, respectively, are different from one another accordingly. A geometrical relationship among the three receivers  22   a ,  22   b , and  22   c  is a known information for the operating portion  26 . Thus, the operating portion  26  obtains the direction where the obstacle X is present by a known triangulation calculation based on the distance to the obstacle X from each of the receivers  22   a ,  22   b , and  22   c , and the geometrical relationship among the receivers  22   a ,  22   b , and  22   c.    
     As mentioned above, the ultrasonic sensor  2  according to the present embodiment mounted onto the opening and closing control system for a vehicle excellently detects the obstacle X present in the vicinity of the vehicle  1 . In order to exercise a performance of the ultrasonic sensor  2  at a maximum, the sensor head  20  is desirably arranged at an appropriate position. An appropriate mounting position of the sensor head  20  will be explained in detail below. 
       FIG. 9  is a side view of the vehicle  1  for illustrating the appropriate mounting position of the sensor head  20 .  FIG. 10  is a rear view of the vehicle  1  for illustrating the appropriate mounting position of the sensor head  20 . In  FIGS. 9 and 10 , the sensor head  20  is mounted at a mounting position  20   c . The mounting position  20   c  of the sensor head  20 A shown in  FIG. 8 , for example, is desirably provided at an intersection between the z-axis and the sensor head  20 A. In  FIGS. 9 and 10 , a small aboveground structure such as a curbstone and a wheel stopper (hereinafter comprehensively referred to as a curbstone S) is provided on the ground such as a road, a parking surface, and the like. The ultrasonic sensor  2  is desirably arranged at a position so as to detect the obstacle X that may influence the opening or closing operation of the back door  11 , without detecting the curbstone S as an obstacle. 
     In  FIG. 10 , a value D 1  is a distance from the ground G to the edge portion  12   a  of the back door  11 . A value D 2  is a half-length of the edge portion  12   a  of the back door  11  in the vehicle width direction. In this case, the value D 2  may be replaced by a value D 2 ′, which is a half-length of the maximum width of the back door  11 . A value D 3  is a length from the mounting position  20   c  of the sensor head  20  to a corner  12   b  of the edge portion  12   a  of the back door  11 . A value D 4  is a length from the mounting position  20   c  of the sensor head  20  to the edge portion  12   a  of the back door  11  in the vertical direction. A value D 5  is a height of the curbstone S from the ground G. 
     The mounting position  20   c  of the sensor head  20  desirably satisfies a following mathematical expression I while being specified in the vicinity of a centerline M in the vehicle width direction of the vehicle  1 .
 
√{square root over (( D 2 2   +D 4 2 ))}&lt; D 4 +D 1 −D 5  [Mathematical expression I]
 
     In this case, according to the Pythagorean theorem, a left-hand side of the aforementioned mathematical expression I (√{square root over ((D 2   2 +D 4   2 ))}) is equal to the value D 3 . Further, when a distance from the edge portion  12   a  of the back door  11  to the curbstone S, i.e., “D 1 -D 5 ”, is specified to be a value D 6 , the aforementioned mathematical expression I is replaced by a mathematical expression II as below.
 
 D 3 &lt;D 4+ D 6  [Mathematical expression II]
 
     The above mathematical expression II indicates a condition for arranging the mounting position  20   c  of the sensor head  20  in which the detection range  8  includes the coiner  12   b  of the edge portion  12   a  of the back door  11  and excludes, specifically, does not reach, the curbstone S in a direction of the ground G in a case where the sensor head  20  is provided at the mounting position  20   c  of the sensor head  20  as illustrated in  FIG. 10 . As seen from  FIG. 10 , a right-hand side of the mathematical expression II, i.e., “D 4 +D 6 ” is equal to a value D 7  that is a distance from the mounting position  20   c  of the sensor head  20  to the curbstone S. Thus, the mounting position  20   c  of the sensor head  20  is desirably determined so as to satisfy a condition where the distance D 3  from the corner  12   b  of the edge portion  12   a  of the back door  11  to the sensor head  20  is shorter than the distance D 7  from the sensor head  20  to the curbstone S. 
     In the mathematical expression II, the value D 3  is variable depending on the mounting position  20   c  of the sensor head  20 . Thus, the value D 3  should be appropriately defined. The value D 3  is determined on the basis of the values D 2  and D 4  in the mathematical expression I. The value D 2  is determined by the vehicle  1 , i.e., the value D 2  is specific to the vehicle  1 , and therefore the value D 4  is a variable value. Thus, the appropriate determination of the value D 4  enables the mounting position  20   c  of the sensor head  20  to be provided at the appropriate position. In order to obtain the value D 4  based on the mathematical expressions I and II, a mathematical expression III is acquired as below. 
     
       
         
           
             
               
                 
                   
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                   [ 
                   
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                     III 
                   
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     The value D 4  is a minimum distance that should be maintained above the edge portion  12   a  of the back door  11  upon mounting of the sensor head  20 . Thus, the mounting position  20   c  of the sensor head  20  is appropriately provided higher than the edge portion  12   a  of the back door  11  by the distance D 4  or greater. For example, in a case where the height of the curbstone S (D 5 ) is 20 cm, the width of the back door  11  (D 2 ×2) is 138 cm, and the height from the ground G to the edge portion  12   a  of the back door  11  (D 1 ) is 68 cm, then the distance D 4  is equal to or greater than 26 cm. 
     Accordingly, the sensor head  20  is desirably provided at a position in the vicinity of the centerline M in the vehicle width direction of the vehicle  1  and where the distance from the corner  12   b  of the edge portion  12   a  of the back door  11  to the sensor head  20 A is shorter than the distance from the sensor head  20 A to the small aboveground structure such as the curbstone S that may be provided on the ground. 
     As illustrated in  FIG. 9 , the sensor head  20  is further desirably provided at a position between a top portion  12   c  of a curved shape of the back door  11  in a vertical direction of the vehicle  1  and the edge portion  12   a  of the back door  11 . Generally, the back door  11  of the vehicle  1  is curved in a vertical direction and a right-left direction of the vehicle  1  so as to extend in a rearward direction thereof. Thus, if the sensor head  20  is arranged at an upper portion of the back door  11 , the edge portion  12   a  of the back door  11  may become a blind spot of the sensor head  20  because of the curved shape of the back door  11 . When the sensor head  20  is provided between the peak portion  12   c  of the curved shape of the back door  11  and the edge portion  12   a  of the back door  11 , the sensor head  20  faces the edge portion  12   a  of the back door  11 . Thus, the edge portion  12   a  to which the obstacle detection is most desired is included in the detection range  8 . 
     Accordingly, the sensor head  20  of the ultrasonic sensor  2  that detects the obstacle X present in the vicinity of the vehicle is provided at the appropriate position. 
     In a case where the sensor head  20  is arranged in the vicinity of the centerline M in the vehicle width direction, distances from the sensor head  20  to edges of the back door  11  on the light side and the left side are equal to each other, thereby effectively utilizing the detection range  8  of the ultrasonic sensor  2 . In addition, as long as the sensor head  20  is arranged in such a way that at least the corner  12   b  of the edge portion  12   a  of the back door  11  is included in the detection range  8 , the edge portion  12   a  of the back door  11  is substantially fully included in the detection range  8 . Accordingly, a portion that moves the longest distance when the back door  11  opens or closes, i.e., the edge portion  12   a  of the back door  1 , is included in the detection range  8  of the ultrasonic sensor  2 . In addition, the sensor head  20  is provided at the position so that the distance from the corner  12   b  of the edge portion  12   a  of the back door  11  to the sensor head  20  is shorter than the distance from the sensor head  20  to the small aboveground structure that may be provided on the ground. Therefore, even when the detection range  8  is specified to be small so as not to detect the small aboveground structure as an obstacle, the edge portion  12   a  of the back door  11  is securely included in the detection range  8 . According to the structure of the present embodiment, the sensor head  20  of the ultrasonic sensor  2  that detects the obstacle in the vicinity of the vehicle  1  is provided at the appropriate position. 
     The back door  11  is formed into a curved shape to extend in a rearward direction of the vehicle  1 , and the sensor head  20  or  20 A is provided at the position between the top portion  20   c  of the curved shape of the back door  11  in a vertical direction of the vehicle  1  and the edge portion  12   a  of the back door  11 . 
     Because the sensor head  20  is positioned in the aforementioned manner, the sensor head  20  faces the edge portion  12   a  of the back door  11 , which is included in the detection range  8 . 
     The small aboveground structure includes the curbstone S provided on the ground. 
     The back door  11  is driven to open or close after the vehicle  1  is parked, mostly at a roadside or a parking lot. In such place, the curbstone S serving as the small aboveground structure is most likely to exist. Thus, considering the curbstone S as the basis for the small aboveground structure for determining the position where the sensor head  20  is provided, the versatile ultrasonic sensor  2  is achievable. 
     The position where the sensor head  20  or  20 A is provided is in the vicinity of the license plate  50  provided at the surface  12 A of the back door  11 . 
     The vicinity of the license plate  50  is positioned close to the hinge  13  relative to the license plate  50 . 
     The position where the sensor head  20  or  20 A is provided is in the vicinity of the vehicle emblem  15  arranged at an upper side of the license plate  50  provided on the surface  12 A of the back door  11 . 
     The principles, preferred embodiment and mode of operation of the present invention have been described in the foregoing specification. However, the invention which is intended to be protected is not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. Variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present invention. Accordingly, it is expressly intended that all such variations, changes and equivalents which fall within the spirit and scope of the present invention as defined in the claims, be embraced thereby.