Abstract:
A radar system for use in a vehicle includes a transmission function for transmitting a transmission wave at a predetermined interval, a redirection function having a plurality of redirection planes for redirecting the transmission wave at least twice in a successive manner in a same direction, a reception function for outputting a reception signal based on a reception of a reflected wave that corresponds to the transmission wave, and an integration function for outputting an integration signal upon integrating a plurality of the reception signals that correspond to the transmission waves redirected in the same direction.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is based on and claims the benefit of priority of Japanese Patent Application No. 2005-43380 filed on Feb. 21, 2005, the disclosure of which is incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention generally relates to a laser radar for detecting an object. 
   BACKGROUND OF THE INVENTION 
   Conventionally, a radar system in a vehicle detects preceding vehicle by transmitting a laser beam or the like. The transmitted laser beam is reflected from a reflective surface on the preceding vehicle, and is returned to be captured by the radar system. In this manner, the radar system can detect the preceding vehicle at a distance of more than 100 meters. However, the signal noise ratio (S/N ratio) of the laser beam reflected from the reflective surface decreases when the reflection from the preceding vehicle is deteriorated. That is, a signal factor in the reflected laser beam from the preceding vehicle and a noise factor in the reflected laser beam from other objects are indistinguishable. As a result, a detection range of the radar system decreases 
   Various methods for increasing the S/N ratio of the signal used in the radar system have been proposed. That is, for example, a radar system having a signal integration function for an improvement of the S/N ratio of the signal from a detected object is disclosed in Japanese patent document JP-A-2004-177350. The radar system in this disclosure uses a polygon mirror that rotates at a constant speed for reflectively transmitting a transmitted wave. Each of the transmitted waves transmitted in a different direction scans a predetermined range of angles for detecting an object. The radar system integrates the signals reflected from the detected object for an improvement of the S/N ratio. 
   However, a problem is experienced in the radar system disclosed in Japanese patent document JP-A-2004-177350 because the transmitted wave cannot be directed in the same direction more than once due to the use of the rotating mirror having a constant rotation speed. That is, for example, the transmitted wave cannot be projected toward the same part of the preceding vehicle plural times within a predetermined interval. Further, the transmitted wave in a horizontal direction is scattered over an angle that is much greater than a required range for object detection because of the mechanical characteristics of the rotating polygon mirror. Therefore, the radar system has to cancel transmission of the transmitted waves toward unnecessary angle ranges. As a result, time for transmission of the transmitted waves for unnecessary angle ranges is wasted. In other words, reduction of wasted time is desirable for improvement of S/N ratio. 
   SUMMARY OF THE INVENTION 
   In view of the above-described and other problems, the present invention provides a radar system having an improved S/N ratio for both of a transmitted wave and a received wave. 
   The radar system for a vehicular use in the present invention is characterized by a transmission unit for transmitting a transmission wave at a predetermined interval, a redirection unit having a plurality of redirection planes for redirecting the transmission wave at least twice in a successive manner in a same direction, a reception unit for outputting a reception signal based on a reception of a reflected wave that corresponds to the transmission wave, and an integration unit for outputting an integration signal upon integrating a plurality of the reception signals that correspond to the transmission waves redirected in the same direction. 
   The redirection unit may be combined with a drive unit that for a movement of the redirection unit relative to the transmission unit. 
   Further, the redirection unit may include a plurality of prisms having the redirection plane, and the respective prism has a different refraction angle. The prisms may be substituted by mirrors. 
   Furthermore, the redirection unit may be a disc that is formed by a circular arrangement of the plurality of redirection planes. The redirection unit may be a board that is formed by a linear arrangement of the plurality of redirection planes. 
   Furthermore, the plurality of the redirection planes in the redirection unit may have respectively different refractive indexes. In this manner, the radar system can detect an object in a three-dimensional space. 
   Furthermore, the plurality of the redirection planes may have respectively different sizes. In this manner, the radar system can send an increased number of transmission waves in a specific direction, thereby having an improved S/N ratio of reception signals for the specific direction of importance such as a front space of a vehicle or the like. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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 which: 
       FIG. 1  shows a block diagram of a radar system in a first embodiment of the present invention; 
       FIG. 2  shows an illustration of components in the radar system in the first embodiment; 
       FIG. 3  shows an illustration of relationship between a horizontal projection angle and slant angle of refraction plane in the first embodiment; 
       FIG. 4  shows a diagram of relationship between a rotation angle of a reflector and the horizontal projection angle in the first embodiment; 
       FIG. 5  shows a time chart of laser beam projection in combination with the horizontal projection angle in the first embodiment; 
       FIG. 6A  shows a diagram of reception signals before an integration process for an improvement of an S/N ratio in the first embodiment; 
       FIG. 6B  shows a diagram of an integrated reception signal in the second embodiment; 
       FIG. 7  shows a block diagram of the radar system in a second embodiment; 
       FIG. 8A  shows a first illustration of a laser diode in combination with a reflector in the second embodiment; 
       FIG. 8B  shows a second illustration of a laser diode in combination with a reflector in the second embodiment; 
       FIGS. 9A and 9B  show an illustration of cross sections of prisms relative to projection angles of the laser beam; 
       FIG. 10A  shows a top view of a relationship between the horizontal projection angle relative to a vehicle in the second embodiment; and 
       FIG. 10B  shows a side view of a relationship between the vertical projection angle relative to the vehicle in the second embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A first embodiment and a second embodiment of the present invention are described with reference to the drawings. The radar system of the present invention is intended for use in a subject vehicle such as an automobile or the like. 
   First Embodiment 
   The first embodiment of the present invention is described with reference to the drawings in  FIGS. 1 to 6B . 
     FIG. 1  shows a block diagram of the radar system in a first embodiment of the present invention. The radar system includes four groups of components. That is, a transmission unit  101 , a rotating refraction unit  104 , a reception unit  110 , and a calculation unit  115 . These components are disposed in a front bumper in a front section of a vehicle. The transmission unit  101  includes a laser diode  103  and a diode drive circuit  102 . The diode drive circuit  102  outputs an emission command to the laser diode  103  based on an output from the diode control unit  119  in the calculation unit  115 . The laser diode  103  emits a laser beam at very short intervals when it receives an emission command from the diode drive circuit  102 . 
   The rotating refraction unit  104  includes a refraction disc  105 , a motor  106  for rotating the refraction disc  105 , an encoder  107  for detecting a rotation angle of the refraction disc  105 , and an emission lens  108 . The radar system of the present invention is mainly used to detect a preceding vehicle by transmitting a transmission wave  109  horizontally toward a front space of a subject vehicle without scanning vertically. 
   The reception unit  110  includes a reception lens  112  for receiving a reflected wave  111  that comes from a detecting object as a reflection of the transmission wave  109  transmitted from the transmission unit  101 , a photo diode  113  for converting reflected wave  111  to reception signal in an analog format, and an amplifier  114  for amplifying the reception signal outputted from the photo diode  113 . 
   The calculation unit  115  includes an A/D converter  116  for converting the reception signal amplified by the amplifier  114  in the analog format to a digital format, an integration unit  117  for integrating information on the rotation angle of the refraction disc  105  derived from the encoder  107  and the reception signal in the digital format, a comparison unit  118  for outputting a detection signal  120  based on a integration signal outputted from the integration unit  117 , and diode control unit  119  for outputting a circuit control command to the diode drive circuit  102  based on the information on the rotation angle of the refraction disc  105 . 
     FIG. 2  shows an illustration of components in the radar system in the first embodiment. 
   The refraction unit  105   a  corresponds to the refraction disc  105  in  FIG. 1 . The drive unit  201  corresponds to the motor  106  and the encoder  107 . The refraction unit  105   a  in combination with the emission lens  108  and the drive unit  201  corresponds to the rotating refraction unit  104  in  FIG. 1 . The refraction unit  105   a  includes j pieces of prisms  200  in a fan shape, each of the fan shape having a same center angle of 360/j degree. A slant angle θn (n=1 to j) described later is different for each prism  200 . The refraction unit  105   a  has the motor  106  on its axis for rotating the refraction unit  105   a  at a constant speed. The encoder  107  is attached in a position between the motor  106  and the refraction disc  105  for detecting the rotation angle of the axis of the refraction disc  105  in real time. The laser diode  103  is disposed to emit the laser beam to be projected in parallel with the axis of the refraction unit  105   a  toward the refraction disc  105 . The laser beam emitted from the laser diode  103  is refracted by the prism  200  in the refraction unit  105   a , and is projected toward the detecting object as the transmission wave  109  after being formed as parallel light by the emission lens  108 . The reflected wave  111  is received by a receiver  110   a  that corresponds to the reception unit  110  in  FIG. 1 , and is integrated by an integrator  117   a  as the reception signal. The integrator  117   a  corresponds to the integration unit  117  in  FIG. 1 . 
     FIG. 3  shows an illustration of relationship between a horizontal projection angle and slant angle of the refraction plane in the first embodiment. 
   A slant angle of a refraction plane  200   b  of the prism  200  is defined as the angle θn between the refraction plane  200   b  and a prism base  200   a . The laser beam is injected into the prism  200  from the prism base  200   a  and is projected from the refraction plane  200   b . A horizontal projection angle εn is defined as an angle between a perpendicular line of the refraction plane and an axis of the laser beam. A size of the prism base  200   a  is larger than a projection area of the laser beam emitted from the laser diode  103 . Therefore, the rotation angle of the refraction disc  105  between time Tn−1 and Tn is between αTn−1 and αTn. That is, the laser beam is projected through the n−1th prism  200 . The horizontal projection angle εn is constant during the time between Tn−1 and Tn because the angle between the refraction plane  200   b  and the prism base  200   a  is constant regardless of an injection position of the laser beam in the prism base  200   a . The horizontal projection angle εn is set to increase by an angle of φ when the refraction disc  105  is rotated to bring the n+1th prism  200  in a path of the laser beam. That is, the slant angle θn+1 of the n+1th prism  200  is set so that the horizontal projection angle εn+1 becomes greater than the angle εn by the angle of φ. 
   The relationship of the rotation angle of the refraction disc  105  and the horizontal projection angle is shown in  FIG. 4 . 
     FIG. 5  shows a time chart of laser beam projection from the laser diode  103  relative to the horizontal projection angle εn in the first embodiment. This time chart shows that the transmission wave  109  is repeatedly projected in a same direction for k times at a constant interval of t 3  while the nth prism  200  passes the path of the laser beam emitted from the laser diode  103 . The laser beams are projected in the same direction because the horizontal projection angle εn is constant while the nth prism  200  is passing in front of the laser diode  103 . In this case, the laser diode  103  emits the laser beam toward only one prism  200  for time t 1 , and the laser diode  103  emits the laser beam transitionally toward two adjacent prisms  200  for time t 2 . That is, the projection area of the laser beam includes a boundary of the nth prism  200  and the n+1th prism  200  for time t 2 . 
   The prism  200  is switched at an interval of (t 1 +t 2 ) because the rotation of the refraction disc  105  is at a constant speed. The diode control unit  119  and the integration unit  117  determine whether the position of the laser diode  103  is under the boundary of the two prisms  200  based on the information of the rotation angle of the refraction disc  105  derived from the encoder  107 . 
     FIGS. 6A and 6B  are used to illustrate integration method in the integration unit  117 . The integration unit  117  determines a switch timing of the prism  200  receiving the laser beam based on the circuit control command from the diode control unit  119  and the information on the rotation angle of the refraction disc  105 , and integrates the reception signals of the laser beams refracted by the same prism  200  as shown in  FIG. 6A . A signal factor Sm in the reception signal of the reflected wave  111  from a same object appears at a same time t 4  from transmission of the transmission wave  109  in all of the k counts of the reception signals. Therefore, the integrated signal factor S 0  in the integration signal is k time amplification of the signal factor Sm in the reception signal. On the other hand, an integrated noise factor N 0  derived from integration of reception signals of k counts is only amplified by √k times because of the randomness of the cause of the noise. In this manner, the integrated signal factor S 0  can be easily distinguished from the integrated noise factor N 0  in the integrated reception signal integrated by the integration unit  117  even when the signal factor Sm in each of the reception signal is weak to be distinguished from a noise factor Nm. 
   The comparison unit  118  is used to output a detection signal  120  after detecting a distance from the radar system to a detecting object based on time difference between a start time of the transmission of the laser beam upon switching the prisms  200  and an end time when the integrated signal factor of the reception signals exceeds a threshold. 
   The radar system of the present invention uses the transmission unit  101  disposed to emit the laser beam in parallel with the axis of rotation of the refraction disc  105  for eliminating wasted time caused by the refraction of the laser beam toward unnecessary directions owing to the use of the polygon mirror. Further, the S/N ratio of the reception signal is increased by transmitting the transmission waves  109  for plural times in the same direction to receive the reflected waves for integration of the signal factors. 
   Second Embodiment 
   A second embodiment is described in comparison with the first embodiment with reference to the drawings. 
   Difference between the second embodiment and the first embodiment exists in two points. That is, the rotating refraction unit  104  is changed to translational refraction unit  700 , and the radar system in the second embodiment scans the three-dimensional manner instead of two-dimensional scan in the horizontal direction in the second embodiment. In the following description, like numerals corresponds to like parts or functions in the first embodiment. 
     FIG. 7  shows a block diagram of the radar system in the second embodiment. The radar system includes the translational refraction unit  700 , the transmission unit  101 , the reception unit  110 , and the calculation unit  115 . 
   Details of the translational refraction unit  700  are shown in  FIGS. 8A and 8B . The translational refraction unit  700  includes a row of the prisms  200  by the number of j, each of the prism  200  projects the laser beam toward a horizontal projection angle of εn and a vertical projection angle of τn as a refraction board  701 , a rack gear  702  for translational movement of the refraction board  701 , a gear  703  that engages the rack gear  702 , the motor  106  for driving the gear  703 , and the encoder  107  for determining the rotation angle of the motor  106 . The translational movement of the refraction board  701  is in a constant speed because of the constant rotation speed of the motor  106 . The laser beams injected in one of the prisms  200  are projected in the same direction as are in the first embodiment. The space in front of the subject vehicle, for example, can be scanned in the three-dimensional manner when the horizontal projection angle of εn and the vertical projection angle of τn of the prisms  200  are changed in the three dimension as shown in  FIG. 9B .  FIG. 8A  shows an illustration of the refraction board  701  seen from the projection lens  108  side, which is perpendicular to a face of an illustration in  FIG. 8B . A line IXB-IXB in  FIG. 9A  shows a trace of the laser beam on the prisms  200 , and cross sections of the prisms  200  along the line IXB-IXB are shown in  FIG. 9B . In  FIG. 9B , the horizontal projection angle εn of the prism  200  is set to x 1 , x 2  or x 3 , and the vertical projection angle τn of the prism is set to y 1 , y 2 , or y 3 .  FIG. 10A  shows a relationship between the horizontal projection angle εn relative to the subject vehicle in the second embodiment (a top view of the subject vehicle) and  FIG. 10B  shows a relationship between the vertical projection angle τn relative to the subject vehicle in the second embodiment (a side view of the subject vehicle). 
   Operation of the radar system is described with an initial condition that the laser diode  103  is under the left most prism  200  in  FIG. 9A . The laser beam is refracted by a first prism  200  having the horizontal projection angle ε 1  of x 1  and the vertical projection angle τ 1  of y 1 . Then, a second prism  200  having the horizontal projection angle ε 2  of x 2  and the vertical projection angle τ 2  of y 1  comes to refract the laser beam. Then a third prism  200  having the horizontal projection angle ε 3  of x 3  and the vertical projection angle τ 3  of y 1  refracts the laser beam. Then, a fourth prism  200  and after have the horizontal projection angles ε 4  of x 1 , ε 5  of x 2 , ε 6  of x 3 , and the vertical projection angles τ 4  of y 2 , τ 5  of y 2 , τ 6  of y 2 . This means that the transmission wave  109  from the fourth prism  200  is projected right above the area that is scanned by the transmission wave  109  from the first prism  200 . The transmission wave from the fifth and sixth prisms  200  are projected likewise above the scanned areas by the wave  109  from the second and the third prisms  200 . The area scanned by wave  109  from the fifth prism  200  is horizontally adjacent to the area scanned by the wave  109  from the fourth prism  200 . In this manner, a three-dimensional space can be scanned by the radar system described in the second embodiment. 
   The translational movement of the refraction board  701  makes it necessary to have a motor control unit  704  in the calculation unit  115 . The motor control unit  704  uses the information on the rotation angle from the encoder  107  for inversing the direction of rotation of the motor  106  after the laser beam from the laser diode  103  is projected from the left/right most prisms  200  (i.e., the first and the jth prisms). The number of the prism being used to project the laser beam from the laser diode  103  can be determined by calculating the rotation angle of the motor  106  derived from the encoder  107  and gear ratio of the rack gear  702  and the gear  703 . 
   This concludes the description that the radar system in the second embodiment of the present invention can scan a three-dimensional space, that is vertically in addition to horizontally (effect of the invention described in the first embodiment), when put in use in a vehicle. 
   Although the present invention has been fully described in connection with the preferred embodiment thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications-will become apparent to those skilled in the art. 
   For example, the refraction disc  105  in the first embodiment can only project horizontally diversified laser beams for a two-dimensional scan, the refraction disc  105  can be modified to have the vertically diversified laser beam for the three-dimensional scanning by adding the prisms  200  with varying vertical projection angles τn as described in the second embodiment. 
   The embodiments described above include the prisms  200  whose slant angles θn are respectively different. However, the rotating refraction unit  104 , the translational refraction unit  700 , and the refraction unit  105   a  may have a plurality of the prisms having the same slant angle θn. For example, the front space of the subject vehicle may be selectively scanned when the number of the prisms  200  having the projection angles toward the front space of the subject vehicle is increased. 
   The embodiments described above include the prisms  200  having the same area of bases. That is, the prisms  200  that refract the laser beam toward the front space of the subject vehicle may have increased area of bases for selectively scanning the front space in order to have an improved S/N ratio. 
   Further, the rotation speed of the motor  106  may be varied (e.g., decreased) for selectively scanning the front space of the subject vehicle. In this manner, the signal factor of the reflected wave  111  may further be amplified for an improvement of the S/N ratio. 
   Further, the transmission unit  101  may be rotated or may be translationally moved instead of rotating/moving the rotating refraction unit  104 , the translational refraction unit  700 , and the refraction unit  105   a . For example, a laser diode may be rotationally moved relative to the refraction disc  105  in a circle that has a same axis and a smaller diameter than the refraction disc  105 . 
   Further, the refraction unit may be a mirror or the like for refracting the laser beam. Furthermore, the transmission unit  101  may not necessarily be limited to the laser diode  103 . 
   Further, the position of the refraction disc  105  and the refraction board  701  may not necessarily be determined by the encoder  107 . For example, a potentiometer or the like may be used for determination. Furthermore, the determination of the position may be skipped. For example, the rotation of the motor  106  rotating in a constant speed may be reversed when the laser diode  103  proceeds to an edge of the refraction board  701  to turn on a touch sensor. 
   Such changes and modifications are to be understood as being within the scope of the present invention as defined by the appended claims.