Abstract:
An ultrasonic sensor for detecting an object includes: a substrate; a transmission device for transmitting an ultrasonic wave; a plurality of reception devices for receiving the ultrasonic wave; and a circuit for processing received ultrasonic waves, which are received by the reception devices after the ultrasonic wave transmitted from the transmission device is reflected by the object. The transmission device and the reception devices are integrated into the substrate. The dimensions of the sensor are minimized, and detection accuracy of the sensor is improved.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on Japanese Patent Application No. 2005-56639 filed on Mar. 1, 2005, the disclosure of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to an ultrasonic sensor. 
       BACKGROUND OF THE INVENTION 
       [0003]    An ultrasonic sensor is mounted on an automotive vehicle, for example. The sensor detects a distance between the sensor, i.e., the vehicle and an obstruction when a driver parks the vehicle or when the driver turns the vehicle. The ultrasonic sensor is disclosed in, for example, JP-A-2001-16694. The sensor for detecting the obstruction includes a transmission device and a reception device, which transmits an ultrasonic wave and receives the ultrasonic wave. The sensor may include a transmitting/receiving device. When the transmission device transmits the ultrasonic wave, the ultrasonic wave hits the obstruction. The obstruction reflects the ultrasonic wave; and then, the reflected ultrasonic wave is received by the reception device. On the basis of the received ultrasonic wave by the reception device, an acoustic pressure of the ultrasonic wave, a time lag and/or a phase difference are detected so that a direction to the obstruction and a distance between the obstruction and the vehicle are calculated. Further, a concavity and a convexity of the obstruction can be detected. 
         [0004]    The reception device of the ultrasonic wave is for example, an ultrasonic element having a vibrator formed of a piezoelectric thin film disposed on a membrane as a thin portion of a substrate. The ultrasonic element with a membrane structure is disclosed in, for example, JP-A-2003-284182. This element is formed by a micro machining method so that the element is called a MEMS (i.e., micro electro mechanical system) type ultrasonic element. JP-A-2003-284182 also discloses an ultrasonic array sensor including the MEMS type ultrasonic elements. 
         [0005]    The MEMS type ultrasonic element  90 R is shown in  FIG. 13A . In the element  90 R, a PZT ceramics thin film layer  2  as a ferroelectric substance is sandwiched by a pair of electrodes  3   a ,  3   b . The element  90 R further includes a piezoelectric sensor having a predetermined resonance frequency for detecting the ultrasonic wave. When the element  90 R operates, a predetermined bias voltage is applied between two electrodes  3   a ,  3   b  so that the resonant frequency of the element  90 R is changed, i.e., controlled. 
         [0006]      FIG. 13B  explains a positioning measurement method by using the ultrasonic wave, which is disclosed in JP-A-2003-284182. An ultrasonic sensor  900  includes an ultrasonic wave source  40  as a transmission device of the ultrasonic wave and an ultrasonic array device A 90 R as a reception device of the ultrasonic wave. The ultrasonic array device A 90 R includes multiple MEMS type ultrasonic elements  90 R, which are arrayed. In the sensor  900 , the source  40  is adjacent to the sensing device A 90 R, and transmits the ultrasonic wave. The ultrasonic wave hits an object  51 ,  52  as an obstacle; and then, the ultrasonic wave is reflected by the object  51 ,  52 . Thus, the ultrasonic wave is returned to the sensor  900 . The returned ultrasonic wave is received by each sensing element  90 R in the sensing device A 90 R. On the basis of the received ultrasonic wave, the position of the object  51 ,  52  including an orientation angle to the object  51 ,  52  is determined. Specifically, on the basis of a transmission time of the ultrasonic wave in each incident direction of the sensing element  90 R, the distance between the sensing element  90 R and the object  51 ,  52  in the incident direction is calculated. Thus, distribution of the distance in different incident directions is determined. Accordingly, the distance between the object  51 ,  52  and the sensing element  90 R in a depth direction of the object  51 ,  52  is determined. Here, the transmission time of the ultrasonic wave is a time from a transmission time when the ultrasonic wave is transmitted from the source  40  to a returning time when the ultrasonic wave is returned to the sensing element  90 R. 
         [0007]    Here, the source  40  and the sensing device A 90 R are separated each other. Therefore, a manufacturing cost of each of the source  40  and the sensing device A 90 R is necessitated. Further, when the source  40  and the sensing device A 90 R are mounted on a bumper of the vehicle, mounting accuracy of each of the source  40  and the sensing device A 90 R affects detection accuracy of the direction and the distance of the object. Furthermore, the mounting distance between the source  40  and the sensing device A 90 R may be increased. 
         [0008]    Further, in general, when an ultrasonic sensing device is directly mounted on the bumper of the vehicle, the sensing device cannot detect the distance to the object accurately by a water drop or a dust attached on a surface of the sensing element. Furthermore, attenuation of the ultrasonic wave transmitting through air depends on temperature and humidity of the air. These temperature and humidity are changeable in accordance with the environment around the vehicle. Thus, the detection accuracy of the object may depend on temperature change and humidity change. Specifically, the environmental temperature around the vehicle can be detected by an external temperature sensor or the like. However, there is no appropriate external humidity sensor mounted on the outside of the vehicle. Thus, the environmental humidity around the vehicle cannot be detected. 
       SUMMARY OF THE INVENTION 
       [0009]    In view of the above-described problem, it is an object of the present invention to provide an ultrasonic sensor having a transmission device and a reception device of an ultrasonic wave. 
         [0010]    An ultrasonic sensor for detecting an object includes: a substrate; a transmission device for transmitting an ultrasonic wave; a plurality of reception devices for receiving the ultrasonic wave; and a circuit for processing received ultrasonic waves, which are received by the reception devices after the ultrasonic wave transmitted from the transmission device is reflected by the object. The transmission device and the reception devices are integrated into the substrate. 
         [0011]    The dimensions of the above sensor are minimized, compared with a conventional sensor. Further, a manufacturing cost of the sensor is reduced. Furthermore, a positioning relationship between the transmission device and the reception device is accurately determined; and therefore, detection accuracy of the sensor is not affected by mounting accuracy of the sensor. 
         [0012]    Alternatively, the number of the reception devices may be equal to or larger than three so that the circuit is capable of detecting an operation failure. Further, each of the transmission device and the three reception devices has a surface for transmitting or receiving the ultrasonic wave, the surface being perpendicular to a ground. The three reception devices are composed of a first to a third reception devices. The first reception device is disposed above the third reception device, and disposed on a left side of the second reception device. The circuit is capable of calculating a horizontal plane distance between the object and the sensor in a horizontal plane parallel to the ground and a horizontal plane direction angle from the sensor to the object in the horizontal plane on the basis of the received ultrasonic waves received by the first and the second reception devices. The circuit is further capable of calculating a vertical plane distance between the object and the sensor in a vertical plane perpendicular to the ground and a vertical plane direction angle from the sensor to the object in the vertical plane on the basis of the received ultrasonic waves received by the first and the third reception devices. The circuit is capable of checking the horizontal and the vertical plane distances and the horizontal and the vertical plane direction angles on the basis of the received ultrasonic waves received by the second and the third reception devices so that the circuit is capable of detecting the operation failure. 
         [0013]    Alternatively, the number of the reception devices may be equal to or larger than four. Further, each of the transmission device and the four reception devices has a surface for transmitting or receiving the ultrasonic wave, the surface being perpendicular to a ground. The four reception devices are composed of a first to a fourth reception devices. The first reception device is disposed above the third reception device, and disposed on a left side of the second reception device. The fourth reception device is disposed under the second reception device, and disposed on a right side of the third reception device. The circuit is capable of calculating a horizontal plane distance between the object and the sensor in a horizontal plane parallel to the ground and a horizontal plane direction angle from the sensor to the object in the horizontal plane on the basis of the received ultrasonic waves received by the first and the second reception devices, and further capable of calculating a vertical plane distance between the object and the sensor in a vertical plane perpendicular to the ground and a vertical plane direction angle from the sensor to the object in the vertical plane on the basis of the received ultrasonic waves received by the first and the third reception devices, so that a first data of the object is obtained. The circuit is capable of calculating the horizontal plane distance and the horizontal plane direction angle on the basis of the received ultrasonic waves received by the third and the fourth reception devices, and further capable of calculating the vertical plane distance and the vertical plane direction angle on the basis of the received ultrasonic waves receive by the second and the fourth reception devices, so that a second data of the object is obtained. The circuit is capable of checking the first data and the second data so that the circuit is capable of detecting the operation failure. 
         [0014]    Alternatively, the transmission device may be capable of transmitting multiple ultrasonic waves having different frequencies so that the circuit is capable of compensating humidity. Further, the transmission device is capable of a first ultrasonic wave having a first frequency and a second ultrasonic wave having a second frequency. The number of the reception devices is equal to or larger than three. Each of the transmission device and the three reception devices has a surface for transmitting or receiving the ultrasonic wave, the surface being perpendicular to a ground. The three reception devices are composed of a first to a third reception devices. The first reception device is disposed above the third reception device, and disposed on a left side of the second reception device. The circuit is capable of calculating a horizontal plane distance between the object and the sensor in a horizontal plane parallel to the ground and a horizontal plane direction angle from the sensor to the object in the horizontal plane on the basis of the received ultrasonic waves having the first frequency received by the first and the second reception devices, and further capable of calculating a vertical plane distance between the object and the sensor in a vertical plane perpendicular to the ground and a vertical plane direction angle from the sensor to the object in the vertical plane on the basis of the received ultrasonic waves having the first frequency received by the first and the third reception devices. The circuit is capable of calculating a first attenuation loss between the transmitted ultrasonic wave and the received ultrasonic waves having the first frequency. The circuit is capable of calculating a second attenuation loss between the transmitted ultrasonic wave and the received ultrasonic waves having the second frequency. The circuit is capable of calculating the humidity of environment on the basis of the first and the second attenuation losses and a temperature obtained from an external temperature sensor. 
         [0015]    Alternatively, each of the transmission device and the reception devices may be provided by an ultrasonic element. The ultrasonic element is disposed on a membrane of the substrate. The ultrasonic element includes a piezoelectric thin film and a pair of metallic electrodes so that a piezoelectric vibrator is provided. The piezoelectric thin film is sandwiched by the metallic electrodes. The piezoelectric vibrator is capable of resonating together with the membrane at a predetermined ultrasonic frequency. Further, the piezoelectric thin film of the transmission device includes a partial cutting pattern, which is disposed on a stress concentration region of a radial direction vibration of the membrane. Furthermore, the membrane is separated by the partial cutting pattern into four pieces. The membrane has a square planar shape, and each piece of the membrane has a square planar shape. The partial cutting pattern penetrates one of the metallic electrodes and the piezoelectric thin film. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
           [0017]      FIG. 1A  is a top view showing an ultrasonic sensor according to a preferred embodiment of the present invention, and  FIG. 1B  is a schematic perspective view showing the sensor mounted on a circuit board; 
           [0018]      FIG. 2A  is a plan view showing an ultrasonic element in the sensor, and  FIG. 2B  is a cross sectional view showing the element taken along line IIB-IIB in  FIG. 2A ; 
           [0019]      FIG. 3  is a plan view showing another ultrasonic sensor, according to a modification of the embodiment; 
           [0020]      FIG. 4A  is a plan view showing an ultrasonic element according to a second modification of the embodiment, and  FIG. 4B  is a cross sectional view showing the element taken along line IVB-IVB in  FIG. 4A ; 
           [0021]      FIG. 5A  is a plan view showing an ultrasonic element according to a third modification of the embodiment, and  FIG. 5B  is a cross sectional view showing the element taken along line VB-VB in  FIG. 5A ; 
           [0022]      FIG. 6A  is a plan view showing an ultrasonic element according to a fourth modification of the embodiment, and  FIG. 6B  is a cross sectional view showing the element taken along line VIB-VIB in  FIG. 6A , and  FIG. 6C  is a partially enlarged cross sectional view showing a part VIC of the element in  FIG. 6B ; 
           [0023]      FIG. 7A  is a plan view showing an ultrasonic element according to a fifth modification of the embodiment, and  FIG. 7B  is a cross sectional view showing the element taken along line VIIB-VIIB in  FIG. 7A ; 
           [0024]      FIG. 8A  is a plan view showing an ultrasonic element according to a sixth modification of the embodiment,  FIG. 8B  is a cross sectional view showing the element taken along line VIIIB-VIIIB in  FIG. 8A , and  FIG. 8C  is a cross sectional view showing the element taken along line VIIIC-VIIIC in  FIG. 8A ; 
           [0025]      FIG. 9A  is a schematic view explaining a reception ultrasonic wave in a X-Y plane received by reception devices,  FIG. 9B  is a schematic view explaining the reception ultrasonic wave in a Z plane received by the reception devices, and  FIG. 9C  is a timing chart showing signals from a transmission device and four reception devices; 
           [0026]      FIG. 10  is a timing chart showing signals having two different frequencies from a transmission device and four reception devices, according to a seventh modification of the embodiment; 
           [0027]      FIG. 11A  is a top view showing an ultrasonic sensor according to an eighth modification of the embodiment, and  FIG. 11B  is a timing chart showing signals having two different frequencies from two transmission devices and four reception devices, according to the eighth modification; 
           [0028]      FIG. 12  is a top view showing an ultrasonic sensor according to a ninth modification of the embodiment; and 
           [0029]      FIG. 13A  is a partially enlarged cross sectional view showing an ultrasonic element according to a prior art, and  FIG. 13B  is a schematic view explaining a method for detecting an object by using an ultrasonic wave, according to the prior art. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0030]    An ultrasonic sensor  100  according to a preferred embodiment of the present invention is shown in  FIGS. 1A and 1B .  FIG. 1B  shows the sensor  100  mounted on a circuit board K. The sensor  100  includes one transmission device S 1  and four reception devices R 1 -R 4 , which are integrated on the same semiconductor substrate  10 .  FIG. 2A  shows an ultrasonic element  90  for providing each of the transmission device S 1  and the reception device R 1 -R 4 . 
         [0031]    The ultrasonic element  90  is similar to the MEMS type ultrasonic element  90 R as the reception device shown in  FIG. 13A . The transmission device S 1  of the ultrasonic element  90  has the same construction of the reception device R 1 -R 4  of the ultrasonic element  90 . 
         [0032]    The ultrasonic element  90  is formed of a SOI (i.e., silicon-on-insulator) semiconductor substrate  10 . The substrate  10  includes a first semiconductor layer  1   a  as a supporting layer, an embedded oxide layer  1   b , a second semiconductor layer  1   c  and a protection oxide film  1   d . A membrane M as a thin portion of the substrate  10  is formed by using a semiconductor micromachining method. A piezoelectric vibrator  20  is formed on the membrane M to cover the membrane M. The piezoelectric vibrator  20  includes a piezoelectric thin film  2  and a pair of metallic electrodes  3   a ,  3   b . Specifically, the piezoelectric thin film  2  is sandwiched by a pair of the metallic electrodes  3   a ,  3   b , which are formed of a metallic film. 
         [0033]    When the ultrasonic element  90  is used as the transmission device S 1 , alternating voltage is applied to the metallic electrodes  3   a ,  3   b  of the piezoelectric vibrator  20  so that the membrane M together with the piezoelectric vibrator  20  is resonated with a predetermined ultrasonic frequency. Thus, the ultrasonic wave is transmitted. When the ultrasonic element  90  is used as the reception device R 1 -R 4 , the returned ultrasonic wave reflected by the object to be measured resonates the membrane M together with the piezoelectric vibrator  20  so that the returned ultrasonic wave is converted to an electric signal by the piezoelectric vibrator  20 . Thus, the ultrasonic wave is received. 
         [0034]    When the ultrasonic element  90  is used as the transmission device S 1 , it is preferred that a planar area of the membrane M in the transmission device S 1  is comparatively large. This is because it is required to generate large ultrasonic sound pressure outputted from the transmission device S 1 . Thus, it is preferred that the planar area of the membrane M in the transmission device S 1  is larger than that in the reception device R 1 -R 4 . Thus, the transmission device S 1  can transmit the ultrasonic wave having large sound pressure. However, the planar area of the membrane M in the reception device R 1 -R 4  may be comparatively small as long as the reception device R 1 -R 4  has sufficient sensitivity of the ultrasonic wave. 
         [0035]      FIG. 3  shows another ultrasonic sensor  100   a  according to the preferred embodiment of the present invention. In this case, the planar area of the membrane Ms in the transmission device S 1   a  is larger than the planar area of the membrane Mr in the reception device R 1 -R 4 . 
         [0036]      FIGS. 4A to 8C  show other ultrasonic elements  91 - 95  for using as the transmission device S. 
         [0037]    The ultrasonic element  91  shown in  FIGS. 4A and 4B  includes the semiconductor substrate having SOI structure. The piezoelectric vibrator  21  is formed on the membrane M formed to be a thin portion of the substrate  10 . The piezoelectric vibrator  21  covers the membrane M. The piezoelectric vibrator  21  also includes the piezoelectric thin film  2  and the metallic electrodes  3   a ,  3   b . The piezoelectric thin film  2  is sandwiched by the metallic electrodes  3   a ,  3   b.    
         [0038]    In the piezoelectric vibrator  21  of the ultrasonic element  91 , the piezoelectric thin film  2  includes a partial cutting pattern  2   a  as a groove, which separates the piezoelectric thin film  4  into four parts. This partial cutting pattern  2   a  is obtained by removing a part of the piezoelectric thin film  2 , at which a stress caused by radial direction vibration of the membrane M is concentrated. Therefore, rigidity of the part of the piezoelectric thin film  2  as a stress concentration region is reduced, so that the membrane M is easily bent, i.e., the flexibility of the membrane M is increased. Accordingly, the piezoelectric vibrator  21  can transmit, i.e., output the ultrasonic wave having sufficient sound pressure. 
         [0039]    In the piezoelectric vibrator  22  of the ultrasonic element  92  shown in  FIGS. 5A and 5B , the piezoelectric thin film  2  includes a partial concavity pattern  2   b  as a partial groove. The thickness of the part of the piezoelectric thin film  2 , which is the stress concentration region of the radial direction vibration of the membrane M, is reduced so that the partial concavity pattern  2   b  is formed. Thus, the flexibility of the membrane M is increased so that the piezoelectric vibrator  21  can output the ultrasonic wave having sufficient sound pressure. 
         [0040]    The piezoelectric vibrator  23  of the ultrasonic element  93  shown in  FIGS. 6A to 6C  is formed of multiple layers composed of multiple piezoelectric thin films  2  and multiple metallic electrodes  3   a - 3   c , which are alternately stacked. When the voltage is applied to the piezoelectric vibrator  23 , deformation of the piezoelectric vibrator  23  is increased. Thus, vibration amplitude of the membrane M is increased so that the vibrator  23  outputs the ultrasonic wave having sufficient sound pressure. 
         [0041]    In the piezoelectric vibrator  24  of the ultrasonic element  94  shown in  FIGS. 7A and 7B , the piezoelectric vibrator  24  and the membrane Md are cantilevered with the substrate  10 . Thus, the membrane Md can be deformed sufficiently, i.e., no portion of the membrane Md, which prevents the membrane Md from deforming, exists in the membrane Md. Thus, when the voltage is applied to the piezoelectric vibrator  24  so that the piezoelectric vibrator  24  is deformed, the membrane Md is also deformed largely. Accordingly, the vibrator  24  outputs the ultrasonic wave having sufficient sound pressure. 
         [0042]    In the piezoelectric vibrator  25  of the ultrasonic element  95  shown in  FIGS. 8A to 8C , the membrane Me is formed in such a manner that a part of the embedded oxide layer  1   b  of the substrate  10  is hollowed, i.e., cut from a top surface side of the substrate  10  by a sacrifice etching method. A hole H for the sacrifice etching method is formed around the membrane Me and a beam Ha. Accordingly, the periphery of the membrane Me is partially supported on the substrate  10  through the beam Ha. Thus, the interference part of the membrane Me, which prevents the membrane Me from deforming, becomes small. When the voltage is applied to the piezoelectric vibrator  25  so that the membrane Me is deformed, distortion of the beam Ha is generated and the beam Ha is deformed largely; and therefore, the membrane Me is largely deformed. Thus, the vibrator  25  outputs the ultrasonic wave having sufficient sound pressure. 
         [0043]    Since each ultrasonic element  91 - 95  can output the ultrasonic wave having sufficient sound pressure, the element  91 - 95  can provide the transmission device S 1  of the ultrasonic sensor  100  having high detection accuracy. Here, the element  91 - 95  may also provide the reception device R 1 -R 4  of the ultrasonic sensor  100 . 
         [0044]    Next, a method for detecting the object by using the ultrasonic sensor  100  is explained with reference to  FIGS. 9A to 9C . In  FIGS. 9A to 9C , the substrate surface of the ultrasonic sensor  100  is disposed to be perpendicular to the ground. Specifically, the surface of the transmission device S 1  is perpendicular to the ground. Here, a X-Y plane in  FIG. 9A  is parallel to the ground. A Z-axis in  FIG. 9B  is perpendicular to the ground.  FIG. 9A  shows the reception devices R 1 , R 2  of the ultrasonic sensor  100  and the reception ultrasonic wave in the X-Y plane. Specifically, the ultrasonic wave transmitted from the transmission device S 1  is reflected by the obstacle  50 , and then the reflected ultrasonic wave is received by the reception device R 1 , R 2  as the reception ultrasonic wave.  FIG. 9B  shows the reception devices R 1 , R 3  of the ultrasonic sensor  100  and the reception ultrasonic wave in the Z-X, Y plane. Here, the Z-X, Y plane in  FIG. 9B  is perpendicular to the ground. ΔL represents difference of a path of the reception ultrasonic wave.  FIG. 9C  is a timing chart showing an alternate pulse signal of the ultrasonic wave outputted from the transmission device S 1  and four alternate pulse signals of the ultrasonic wave received by four reception devices R 1 -R 4 . 
         [0045]    In  FIG. 9A , Dx represents a distance between the center of the ultrasonic sensor  100  and the obstacle  50  in the X-Y plane. The distance Dx is calculated on the basis of a S signal No.  1  outputted from the transmission device S 1 , a R signal No.  1  received by the reception device R 1  and a R signal No.  2  received by the reception device R 2 . The reception devices R 1 , R 2  are disposed on an upper side of the sensor  100  in  FIG. 1 . Specifically, the distance Dx is calculated from an average time difference ΔTx between reception times (i.e., an arrival time) of the R signals No.  1  and No.  2  and a transmission time (i.e., an output time) of the S signal No.  1 . 
         [0046]    In  FIG. 9A , θx represents a direction angle to the obstacle  50  in the X-Y plane. The direction angle θx is measured from the X-axis as a reference axis. The direction angle θx is obtained on the basis of the R signals No.  1  and No.  2  from the reception devices R 1  and R 2 . Specifically, the direction angle θx is calculated from a phase difference ΔPx between the R signal No.  1  and the R signal No.  2 . 
         [0047]    In  FIG. 9B , Dz represents a distance between the center of the ultrasonic sensor  100  and the obstacle  50  in the Z-X, Y plane, which is perpendicular to the ground. The distance Dz is calculated on the basis of the S signal No.  1  from the transmission device S 1 , the R signal No.  1  from the reception device R 1  and a R signal No.  3  received by the reception device R 3 . The reception devices R 1 , R 3  are disposed on a left side of the sensor  100  in  FIG. 1 . Specifically, the distance Dz is calculated from an average time difference ΔTz between reception times of the R signals No.  1  and No.  3  and the transmission time of the S signal No.  1 . 
         [0048]    In  FIG. 9B , θz represents a direction angle to the obstacle  50  in the Z-X, Y plane. The direction angle θz is measured from the X-Y plane as a reference plane. The direction angle θz is obtained on the basis of the R signals No.  1  and No.  3  from the reception devices R 1  and R 3 . Specifically, the direction angle θz is calculated from a phase difference ΔPz between the R signal No.  1  and the R signal No.  3 . 
         [0049]    On the basis of the distances Dx, Dz and the direction angles θx, θz, the distance between the obstacle  50  and the sensor  100  and the direction to the obstacle  50  are determined. Thus, the sensor  100  detects the obstacle  50 . 
         [0050]    In the sensor  100 , the transmission device S 1  and the reception devices R 1 -R 4  are integrated into the same substrate  10 . Accordingly, the dimensions of the sensor  100  and the manufacturing cost of the sensor  100  are reduced, compared with the sensor  900  shown in  FIG. 13B , in which the transmission device S 1  and the ultrasonic allay device A 90 R are independently formed. Further, since the positioning relationship between the transmission device S 1  and the reception device R 1 -R 4  is accurately designed, i.e., determined on the substrate  10 . Thus, even when the sensor  100  is mounted on a bumper of an automotive vehicle, mounting accuracy of the sensor  100  on the bumper does not affect the detection accuracy of the sensor  100 . 
         [0051]    Even when the number of the transmission devices S 1  and/or the number of the reception devices R 1 -R 4  are increased or reduced, and/or even when the dimensions of the transmission device S 1  and/or the dimensions of the reception device R 1 -R 4  are changed, the sensor  100  can be formed only by changing a mask. Thus, the manufacturing cost of the sensor  100  is almost the same. 
         [0052]    Although the sensor  100  includes four reception devices R 1 -R 4 , the obstacle  50  can be detected by using three reception devices R 1 -R 3 . Specifically, the distance Dx in the X-Y plane and the direction angle θx measured from the X-axis are obtained by using two reception devices R 1 , R 2 , which are disposed on the upper side of the sensor  100 . The distance Dz in the Z-X, Y plane and the direction angle θ z measured from the X-Y plane are obtained by using two reception devices R 1 , R 3 , which are disposed on the left side of the sensor  100 . 
         [0053]    However, the distance Dx in the X-Y plane and the direction angle θx measured from the X-axis can be obtained by using two reception devices R 3 , R 4 , which are disposed on a lower side of the sensor  100 . The distance Dz in the Z-X, Y plane and the direction angle θz measured from the X-Y plane can be obtained by using two reception devices R 2 , R 4 , which are disposed on the right side of the sensor  100 . Thus, the obstacle  50  can be detected by three reception devices R 2 -R 4 . 
         [0054]    Accordingly, in the sensor  100 , two different distances and two different direction angles to the obstacle  50  are obtained. By comparing these two data of the obstacle  50 , operation failure of the sensor  100  is judged. Specifically, when two data of the obstacle do not coincide, the operation failure of the sensor  100  occurs. Accordingly, the sensor  100  has operation failure detection function. 
         [0055]    If the sensor  100  determines that only one reception device R 1 -R 4  acts up the operation failure, the sensor  100  can detect the obstacle  50  by using other three reception devices R 1 -R 4 . Accordingly, the sensor  100  has fail safe function. 
         [0056]    Further, even when the sensor  100  includes only three reception devices R 1 -R 3 , the sensor  100  can have the operation failure detection function. Specifically, the distance Dx and the direction angle θx are obtained from two reception devices R 1 , R 2 , and the distance Dz and the direction angle θ z are obtained by using two reception devices R 1 , R 3 . Accordingly, the obstacle  50  is detected on the basis of two combination data, one of which is obtained from the reception devices R 1 , R 2 , and the other one of which is obtained from the reception devices R 1 , R 3 . The other combination data obtained from the reception devices R 2 , R 3  can be used for checking the calculation of detection of the obstacle  50 . Thus, even when the sensor  100  includes three reception devices R 1 -R 3 , the sensor  100  can have the operation failure function. 
         [0057]    Thus, when the sensor  100  includes three or more reception devices R 1 -R 3 , the sensor  100  has the operation failure function. When the sensor  100  includes four or more reception devices R 1 -R 4 , the sensor  100  has the fail safe function. Thus, if the operation failure of the sensor  100  is occurred by waterdrop or dust, which is attached to the sensor  100 , the sensor  100  can avoid the operation failure. 
         [0058]    The sensor  100  can output two or more different ultrasonic waves having different frequencies, which are transmitted from one transmission device S 1  by controlling the frequency of the alternate pulse signal in terms of time, the pulse signal being applied to the transmission device S 1 . By using two different ultrasonic waves, the sensor  100  can detect the obstacle  50  with humidity compensation function. Here, the input voltage is controlled to have a frequency range other than the resonant frequency of the membrane M so that the ultrasonic waves having two different frequencies are transmitted. 
         [0059]      FIG. 10  explains the method for compensating the humidity. In  FIG. 10 , the transmission device S 1  outputs two different ultrasonic waves having two different frequencies f 1 , f 2 . The transmission device S 1  transmits the first ultrasonic wave having the first frequency f 1 , and then, the device S 1  transmits the second ultrasonic wave having the second frequency f 2 . The first and the second ultrasonic waves are periodically, i.e., with a predetermined time interval, outputted. In four reception devices R 1 -R 4 , the first R signal No.  1  corresponding to the first ultrasonic wave and the second R signal No.  1  corresponding to the second ultrasonic wave to the first R signal No.  4  corresponding to the first ultrasonic wave and the second R signal No.  4  corresponding to the second ultrasonic wave are detected. The relationship among the first R signals No.  1 - 4  and the first S signal No.  1  corresponding to the first ultrasonic wave in  FIG. 10  is the same as that in  FIG. 9C . Further, the relationship among the second R signals No.  1 - 4  and the second S signal No.  1  corresponding to the second ultrasonic wave in  FIG. 10  is the same as that in  FIG. 9C . 
         [0060]    In  FIG. 10 , the height of the alternate pulse signal of the first S signal No.  1  of the first frequency f 1  is equal to that of the second S signal No.  1  of the second frequency f 2 . However, the height of the first R signal No.  1  of the first frequency f 1  is higher than that of the second frequency f 2 , i.e., the second R signal No.  1  of the second frequency f 2  is largely attenuated, compared with the first R signal No.  1  of the first frequency f 1 . Similarly, the second R signals No.  2 - 4  are largely attenuated, i.e., reduced. 
         [0061]    Here, attenuation loss P, i.e., absorption loss of the ultrasonic wave is obtained by the following formula. 
         [0000]    
       
         
           
             
               
                 
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                           π 
                            
                           
                               
                           
                            
                           f 
                         
                         + 
                         
                           2 
                            
                           π 
                            
                           
                               
                           
                            
                           
                             f 
                             / 
                             k 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     F 
                      
                     
                         
                     
                      
                     2 
                   
                   ) 
                 
               
             
             
               
                 
                   k 
                   = 
                   
                     1.92 
                     × 
                     
                       
                         ( 
                         
                           
                             
                               G 
                               0 
                             
                             G 
                           
                           × 
                           h 
                         
                         ) 
                       
                       1.3 
                     
                     × 
                     
                       10 
                       5 
                     
                   
                 
               
               
                 
                   ( 
                   
                     F 
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                     3 
                   
                   ) 
                 
               
             
           
         
       
     
         [0062]    Here, m represents absorption coefficient, r represents transmission distance, M represents a predetermined coefficient, f represents a frequency, T represents a temperature, GO represents a saturated vapor pressure, G represents a total air pressure, and h represents a humidity. 
         [0063]    From the above formula F 1 , the attenuation loss P depends on the frequency f. As the frequency f of the ultrasonic wave becomes larger, the attenuation loss becomes larger. Further, the attenuation loss P depends on not only the frequency but also the temperature T and the humidity h of the transmission environment. The frequency f of the ultrasonic wave is preliminarily determined. The temperature T of the environment can be detected by an external temperature sensor or the like. When the sensor  100  is mounted on the vehicle, the temperature T, i.e., the external temperature can be detected easily. However, the humidity h of the environment, i.e., the external humidity h is not detected easily by a humidity sensor. This is because there is no appropriate humidity sensor for detecting the external humidity around the vehicle. 
         [0064]    However, since the received ultrasonic waves having two different frequencies f 1 , f 2  are measured, the humidity h can be calculated on the basis of the difference of two attenuation losses P obtained from two different frequencies f 1 , f 2 . This calculated humidity h is used for compensating the standard humidity, which is preliminarily determined and memorized in the sensor  100 . Thus, the sensor  100  has the humidity compensation function. In this case, the detection accuracy of the sensor  100  is much improved regarding the humidity change. 
         [0065]    Although the sensor  100  includes only one transmission device S 1 , it is preferred that the sensor  100  includes two or more transmission devices S 1 . When the sensor  100  includes two transmission devices S 1 , each transmission device S 1  can output the ultrasonic wave having different frequency with high Q value, the device S 1  outputting the wave by using different resonant frequency of the membrane M. 
         [0066]      FIG. 11  shows an ultrasonic sensor  101  having two transmission devices S 1 , S 2 . The sensor  101  can output two ultrasonic waves having different frequencies f 1 , f 2  simultaneously by using two transmission devices S 1 , S 2  for outputting two different ultrasonic waves. Thus, no compensation for compensating motion of the vehicle is necessitated. Here, since the ultrasonic waves having different frequencies f 1 , f 2  have the same transmission velocity, the reflected ultrasonic waves are arrived at the sensor  100  at the same time. Accordingly, frequency analysis for decomposing the reception ultrasonic waves into the component having the first frequency f 1  and the component having the second frequency f 2  is required. 
         [0067]      FIG. 12  shows an ultrasonic sensor  102  having the transmission device S 1  and eight reception devices R 1 -R 8 . The transmission device S 1  is surrounded with eight reception devices R 1 -R 8 . In this case, it is preferred that two reception devices R 1 -R 8  are arranged to be symmetrically with respect to the transmission device S 1 . Specifically, a pair of the reception devices R 1 , R 8 , a pair of the reception devices R 2 , R 7 , a pair of the reception devices R 3 , R 6 , and a pair of the reception devices R 4 , R 5  are arranged to be symmetrically with respect to the transmission device S 1  so that each pair of the reception devices R 1 -R 8  surrounds the transmission device S 1 . 
         [0068]    In this case, since each pair of the reception devices R 1 -R 8  is symmetrically disposed, the reflected ultrasonic wave outputted from the transmission device S 1  is returned to the pair of the reception devices R 1 -R 8  in such a manner that the sound pressure of the received ultrasonic wave received by one of the pair of the reception devices R 1 -R 8  is almost the same as the other one of the pair of the reception devices R 1 -R 8 . Accordingly, the detection accuracy of the obstacle  50  is improved. 
         [0069]    Thus, each sensor  100 ,  100   a ,  101 ,  102  has small dimensions and low manufacturing cost, and the detection accuracy of the sensor  100 ,  100   a ,  101 ,  102  is not affected by mounting accuracy of the sensor on the vehicle. Further, the sensor  100 ,  100   a ,  101 ,  102  has high detection accuracy, even if the waterdrop or the dust is adhered to the sensor  100 ,  100   a ,  101 ,  102  and even if the humidity around the sensor  100 ,  100   a ,  101 ,  102  changes. 
         [0070]    Although the sensor  100 ,  100   a ,  101 ,  102  includes one transmission device S 1  and four or eight reception devices R 1 -R 8 , the sensor may includes one or more transmission devices S 1  and two or more reception devices. When the sensor includes multiple transmission devices and multiple reception devices, the information from the sensor is increased. Further, when the sensor includes two or more transmission devices, the sound pressure of the ultrasonic wave becomes larger, and the directivity of the ultrasonic wave is controlled. 
         [0071]    Alternatively, the reception devices in the sensor may be arrayed so that a transmission signal is received by multiple reception devices in order to cancel the transmission signal, since the transmission signal may cause noise of the sensor. Specifically, when the transmission device and the reception device are integrated into one substrate, the transmission signal may input into the reception device so that the transmission signal may cause the noise of the sensor. Thus, by canceling the inputted transmission signal, the noise of the sensor is reduced. Accordingly, when the obstacle is disposed near the sensor, the S/N ratio of the signal is improved for detecting the obstacle. 
         [0072]    Although the reception device includes the piezoelectric thin film so that the reception device provides a piezoelectric type device, the reception device may be a capacitance type device for detecting a capacitance change between electrodes. Further, the reception device may be a piezo type for detecting an output of a gauge generated by pressure. Furthermore, the sensor may include a combination of these different type reception devices. 
         [0073]    While the invention has been described with reference to preferred embodiments thereof, it is to be understood that the invention is not limited to the preferred embodiments and constructions. The invention is intended to cover various modification and equivalent arrangements. In addition, while the various combinations and configurations, which are preferred, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.