Abstract:
A radar device has a wave projecting part for projecting pulsed electromagnetic waves to a target object in a specified direction at specified time intervals, a wave receiving part for receiving reflected waves reflected by the target object, an intensity measuring part for measuring the intensity of the reflected waves, a distance measuring part for measuring the distance to the target object based on elapsed time from when the pulsed electromagnetic waves are projected until when the reflected waves are received and also serving to determine a maximum reflection point at which the intensity of the received reflected waves is the largest, and a scanning part for varying the direction of the projected electromagnetic waves and the received reflected waves in horizontal and vertical directions respectively within a specified angular scan range. A calculating part calculates a relative speed of the target object based on the measured distance and the elapsed time and a displacement quantity between the vertical scan direction of the maximum reflection point and a vertical reference direction of the scan range by determining a time constant of a filtering calculation based on the distance to the maximum reflection point and the relative speed and carrying out the filtering calculation by using this determined time constant. A correcting part moves the scan range vertically according to this calculated displacement quantity.

Description:
This application claims priority on Japanese Patent Application 2006-124308 filed Apr. 27, 2006. 
     BACKGROUND OF THE INVENTION 
     This invention relates to a radar device for scanning the frontal direction with electromagnetic waves such as laser light and millimetric waves and thereby measuring the distance to an object in front based on the reflected waves therefrom. 
     Radar devices of the kind for scanning the frontal direction of a vehicle with laser light or the like to determine the presence or absence of an object and to measure its distance and direction have been known. Such devices are likely to erroneously detect an object due to its faulty attachment to the vehicle or the slope of the detection area caused by the condition of the load on the vehicle. 
     If the radar device makes an error in the detection of an object, this may cause an error in the vehicle control device connected on the downstream side. Such a vehicle control device may be carrying on an adaptive cruise control (ACC), controlling the vehicle to follow another vehicle traveling in front (the front-going vehicle) at a constant distance based on its position data measured by the radar device. If the radar device makes an error in detecting objects, the vehicle control device may decelerate or stop the vehicle when the erroneously detected distance to an object becomes less than a specified safety threshold distance. 
     In view of the above, Japanese Patent Publication Tokkai 2000-329853 (Reference 1) proposed a device for correcting the displacement of an optical axis based on the position of the area where the level of reflected waves becomes greater than a specified intensity and a reference direction of the scanned area. Japanese Patent Publication Tokkai 11-64489 (Reference 2) disclosed a device for calculating the displacement based on the area where the level of reflected waves becomes a maximum. 
     These devices, however, are adapted to make corrections while the vehicle is at rest, not while the vehicle is in motion. A device for making a correction while the vehicle is in motion is disclosed in Japanese Patent Publication Tokkai 11-142520 (Reference 3). This device is adapted to obtain a plurality of (position) data items on the front-going vehicle while it is traveling straight forward, judge its average center position (correction position) and make a correction such that the center position of the optical axis will come to coincide with this correction position. 
     While in motion, however, the positional relationship between one&#39;s own vehicle and its front-going vehicle keeps changing, depending on the conditions of the travel. For example, the orientation of the own vehicle would change as it accelerates and decelerates and the positional relationship of the own vehicle and the front vehicle in the vertical direction would change according the change in the condition of the road surface such as its slope. 
     Japanese Patent Publication Tokkai 2004-125739 (Reference 4) discloses a technology of dynamically controlling the optical axis of laser light vertically in response to changes in the vertical positional relationship while in motion. According to this technology, a scan is executed vertically in a plurality of steps and a displacement in the vertical direction is corrected by calculating the quantity of correction of displacement in the vertical direction based on the relationship between the vertical direction at which the level of reflected waves is the largest and the direction of the center of the maximum range of the scan. Since a device according to this technology relaxes an instantaneous displacement of the optical axis when a gap on the road surface causes an instantaneous vertical oscillation, a filtering process is carried out by a moving average of the calculated quantities of displacement correction. 
     With a device according to Reference 3, a displacement of the optical axis can be corrected even while the vehicle is in motion. With a device according to Reference 4, the correction is possible even if there is a change in the positional relationship between the own vehicle and the front-going vehicle. With these devices, however, the correction of the optical axis cannot be effected quickly enough and the front-going vehicle is lost out of sight if there is a sudden change in the distance between the own vehicle and the front-going vehicle. 
     If the own vehicle approached the front-going vehicle, for example, the reflector plate on the front-going vehicle comes to a position with a relatively large angle of elevation with respect to the radar device on the own vehicle such that its optical axis is moved to an upward corrected direction. If the front-going vehicle has accelerated suddenly in this situation, the laser light may come to miss the front going vehicle since its optical axis is pointing upward. With a device according to Reference 4, the process of adjusting the direction of the optical axis downward will be carried out if the laser light is on the front-going vehicle but since the calculated quantity for the correction is subjected to a filtering process in order to reduce the variations in the direction of the optical axis due to instantaneous external disturbances, the correction of the optical axis becomes delayed. If the time constant of the filter is shortened, however, there arises the problem that the device may react too sensitively to the vertical vibrations of the own vehicle. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide radar device capable of correcting its optical axis so as not to lost sight of the front-going vehicle even if its distance from the own vehicle changes suddenly. 
     A radar device of this invention is comprised of a wave projecting part for projecting pulsed electromagnetic waves to a target object in a specified direction at specified time intervals, a wave receiving part for receiving reflected waves reflected by the target object in the direction of projection of the pulsed electromagnetic waves, an intensity measuring part for measuring the intensity of the reflected waves received by the wave receiving part, a distance measuring part for measuring the distance to the target object based on elapsed time from when the pulsed electromagnetic waves are projected until when the reflected waves are received and also serving to determine a maximum reflection point at which the intensity of the received reflected waves is the largest, a scanning part for varying the direction of the projected electromagnetic waves and the received reflected waves in horizontal and vertical directions respectively within a specified angular scan range, a calculating part for calculating a relative speed of the target object based on the measured distance and the elapsed time and a displacement quantity between the vertical scan direction of the maximum reflection point and a vertical reference direction of the scan range and a correcting part for moving the scan range vertically according to the calculated displacement quantity. The calculating part according to this invention calculates the displacement quantity by determining a time constant of a filtering calculation based on the distance to the maximum reflection point and the relative speed and carrying out the filtering calculation by using this determined time constant and recording the calculated displacement quantity obtained by this filtering calculation. 
     Thus, if the distance to the front-going vehicle as the target object is small and its relative speed is large, the filter coefficient for the filtering calculation is increased such that the displacement value obtained by the current (more recent) measurement becomes more influential and the response characteristic is improved. In other words, the optical axis can be corrected more quickly and the probability of losing sight of the front-going vehicle is reduced. 
     The calculating part of this invention may be further characterized as increasing the time constant by specified steps if the distance to the maximum reflecting point is less than a certain specified value and the relative speed is less than another certain specified value, based on a decrease in the distance to the maximum reflecting point and an increase in the relative speed. In other words, the filter coefficient is increased in steps as the distance to the front-going vehicle is reduced or its relative speed increases. A table may be preliminarily prepared for determining the filter coefficient according to the distance and the relative speed and the change in the filter coefficient may be effected according to such a table. 
     The scanning part of this invention may be adapted to move the direction of projecting the pulsed electromagnetic waves and receiving the reflected waves horizontally for each of vertical directions separated by a specified angle. In other words, the direction of projecting and receiving electromagnetic waves may be changed in steps. 
     In summary, the present invention has the merit of reducing the probability of losing sight of the front-going vehicle by quickly adjusting the direction of the optical axis even when the distance to the front-going vehicle changes suddenly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a laser radar device embodying this invention. 
         FIG. 2  is a structural diagram of the parts of the scanner supporting the light projecting and receiving lenses. 
         FIG. 3  is a diagram for showing the optical paths of light through the light projecting and receiving lenses. 
         FIG. 4  shows an example of scan range in the horizontal direction when the laser radar device on the own vehicle projects laser light towards a front-going vehicle. 
         FIG. 5  shows an example of scan range in the vertical direction when the laser radar device on the own vehicle projects laser light towards a front-going vehicle. 
         FIG. 6  shows the direction of scan by the scanner over the scan ranges shown in  FIGS. 4 and 5 . 
         FIG. 7  shows the order in which main and sub scans are carried out to scan the entire range of scan area. 
         FIGS. 8 and 9  are a flowchart for the process of correcting the optical axis vertically. 
         FIG. 10  shows an example of histogram stored in the memory. 
         FIG. 11  shows an example of vertical position of the scan range set by the scanner when the correction is zero degrees (when the optical axis is not corrected). 
         FIG. 12  shows an example of vertical position of the scan range set by the scanner when the correction is +1 degree (when the optical axis is corrected upward by 1 degree). 
         FIG. 13  shows an example of vertical position of the scan range set by the scanner when the correction is −2 degrees (when the optical axis is corrected downward by 2 degrees). 
         FIG. 14  shows an example of the quantity of light obtained from the light receiving circuit. 
         FIG. 15  is a drawing for showing how targets are grouped. 
         FIG. 16  is a flowchart of the process for determining the quantity of correction for a short-term displacement of the optical axis. 
         FIG. 17  is a flowchart of the filtering process. 
         FIG. 18  shows a table for determining the filter coefficient. 
         FIG. 19  is a flowchart of the clipping process. 
         FIG. 20  is a list of conditions for adding to the histogram. 
         FIG. 21  is a flowchart of the process for determining the quantity of correction for a long-term displacement of the optical axis. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a block diagram of a laser radar device (or a distance-measuring device for a vehicle) embodying this invention. Emission of light from a laser diode (LD)  12  is controlled by a LD driver circuit  10  based on a drive signal generated by a control circuit  11 . The laser light generated by the LD  12  is moved by a scanner  13  within a specified scanning range based on a control by the control circuit  11 . The laser light emitted from the scanner  13  is projected forward in the direction of travel of the own vehicle  1  ( FIG. 4 ) through a light projecting lens. A vertical scan position detector  14  and a horizontal scan position detector  15  detect respectively the vertical and horizontal scan positions of the laser light at the scanner  13  and output them to the control circuit  11 . 
     Reflected light, which is the laser light from the scanner  13  reflected by an object (such as a vehicle) in front regarded as an object of detection, is collected by a light receiving lens and received by a photodiode (PD)  16  which outputs to a light receiving circuit  17  a signal corresponding to the received light level. The light receiving circuit  17  converts the received light level into a numerical signal and outputs it to the control circuit  11 . The numerical value thus received by the control circuit  11  is stored in a memory  18  corresponding to the scan position inputted from the vertical and horizontal scan position detectors  14  and  15 . The memory  18  stores also other data such as the correction of displacement of the optical axis (or the angle of projection of the light, to be set initially along the central axis of the own vehicle) and a histogram (to be explained below). The control circuit  11  is connected to a speed sensor  19 , a yaw rate sensor  20  and a global positioning system (GPS)  21 . The speed sensor  19  is for detecting the speed of the own vehicle. The yaw rate sensor  20  is for detecting the yaw rate of the own vehicle in the horizontal direction. The GPS is for detecting the position of the own vehicle. 
     The control circuit  11  calculates the distance between an object (such as the front-going vehicle) and the own vehicle based on the quantity of received light stored in the memory and the time from when the laser light was emitted until when its reflected light is received. The control circuit  11  also calculates the relative speed of the object by measuring its distance continuously in time. These calculated values are recorded in the memory  18 . The control circuit  11  also serves to judge whether the detected object is a front-going vehicle or not based on its distance and relative speed. Throughout herein, the front-going vehicle means a single vehicle immediately in front, traveling in the same lane as the own vehicle. 
       FIG. 2  shows the structure of the parts of the scanner  12  supporting the light projecting and receiving lenses. A control signal from the control circuit  11  is inputted to a driver circuit  30 . The driver circuit  30  supplies driving currents to driver coils  31  and  32  for horizontal and vertical directions based on the inputted control signal. The driver coils  31  and  32  serve to move respectively in the horizontal and vertical directions supporting member (not shown) which supports the light projecting and receiving lenses  35  and  36  integrally. The supporting member is supported by plate springs  33  and  34  so as to be able to move respectively in the horizontal and vertical directions. Thus, the supporting member (and hence the light projecting and receiving lenses  35  and  16 ) not only moves to and comes to rest at the horizontal position where the attractive or repulsive force between the magnetic field generated in the driver coil  31  for the horizontal direction by the driving current and a permanent magnet (not shown) balances with the reaction force generated in the plate spring  33  in the horizontal direction but also moves to and comes to rest at the vertical position where the attractive or repulsive force between the magnetic field generated in the driver coil  32  for the vertical direction and another permanent magnet (not shown) balances with the reaction force generated in the plate spring  34  in the vertical direction and the gravitational force. The position of each of the lenses is detected by a sensor (not shown). Outputs from this sensor are inputted to the driver circuit  30  to form a servo mechanism. Thus, the light projecting and receiving lenses  35  and  36  can be moved to specified horizontal and vertical positions. 
       FIG. 3  shows the optical paths of the light projecting and receiving lenses  35  and  36  projected from the scanner  13 . The light projecting lens  35  is in front of the LD  12  and the light receiving lens  36  is in front of the PD  16 . The laser light from the LD  12  is collected by the light projecting lens  35 . If the light projecting lens  35  is at the center position for the scan, the laser light is projected to the front as shown by solid lines in  FIG. 3 . The projected light is reflected by an object (such as a vehicle) in front and is received by the light receiving lens  36  as shown by solid lines and by the PD  16 . 
     If the scanner  13  moves the light projecting lens  35  upward (with respect to the figure), the laser light is projected in an upward direction (also with respect to the figure) as shown by dotted lines. If the projected light is reflected by an object in that upward direction, the reflected light is received by the light receiving lens  36  as shown by dotted lines and received by the PD  16 . 
     Thus, the scanner  13  moves the light projecting and receiving lenses  35  and  36  together in the horizontal direction to specified positions, thereby making a horizontal scan with the laser light. The scanner  13  similarly moves the light projecting and receiving lenses  35  and  36  together in the vertical direction to make a vertical scan with the laser light. 
       FIGS. 4 and 5  show an example of horizontal and vertical scan ranges as a laser radar device  100  of the own vehicle  1  projects laser light towards a front-going vehicle  2 . As shown in  FIG. 4 , the scan range in the horizontal direction is divided into seven areas  61 - 67 . Similarly, as shown in  FIG. 5 , the scan range in the vertical direction is divided into five areas  81 - 85 . In real applications, the scan ranges are actually divided into larger numbers of areas. The examples in  FIGS. 4 and 5  are for simplifying the explanation. 
     The scanner  13  may cause laser light to scan the scan ranges shown in  FIGS. 4 and 5  as shown in  FIG. 6 . Vertical area  83  is the center area of the five vertically divided areas. In this area, the horizontal scan is carried out from area  61  to area  67  (referred to as the main scan  1 ). In vertical areas  81  and  82  above the center vertical area  83  and in vertical areas  84  and  85  below the center vertical area  83 , the horizontal scan is carried out from area  67  to area  61  (referred to as the sub scans  1 ,  2 ,  3  and  4 ). 
       FIG. 7  shows the order in which the main and sub scans are carried out to scan the entire scan range. In Step  1 , the scanner  13  carries out main scan  1  to scan the central vertical area  83  from area  61  to area  67 . In Step S 2 , the scanner carries out sub scan  1  to scan vertical scan area  81  from area  67  to area  61 . In Step  3 , the scanner  13  carries out main scan  1  again and in Step  4 , the scanner  12  carries out sub scan  2  to scan vertical scan area  82  from area  67  to area  61 . In Step  5 , the scanner  13  carries out main scan  1  again and in Step  6 , the scanner  12  carries out sub scan  3  to scan vertical scan area  84  from area  67  to area  61 . In Step  7 , the scanner  13  carries out main scan  1  again and in Step  8 , the scanner  12  carries out sub scan  4  to scan vertical scan area  85  from area  67  to area  61 . The time required for each scan is 50 ms. The total scan including aforementioned Steps  1 - 8  is referred to as one set of scans. 
     According to this invention, displacements of the optical axis of the laser radar can be corrected not only on a short-term basis but also on a long-term basis. Displacements of the optical axis on a short-term basis means displacements due to vertical vibrations of the own vehicle when it is accelerating or decelerating and the displacements of a few seconds in duration when a front-going vehicle is traveling on a sloped road surface. Long-term displacements of the optical axis include displacements due to a tilting caused by a change in the load on the own vehicle as well as displacements as a result of a minor collision. The optical axis becomes displaced also instantaneously in the vertical direction as the own vehicle goes over a gap on the road surface but such instantaneous displacements of the optical axis are not corrected according to this invention in order to avoid erroneous detection. 
     Next,  FIGS. 8-21  are referenced to explain specific operations of the laser radar.  FIGS. 8 and 9  are a flowchart for the process of vertical correction of the optical axis. To start, the control circuit  11  initializes the histogram stored in the memory  18  (Step S 21 ).  FIG. 10  shows an example of histogram stored in the memory  18 , showing the number of times of correction on the vertical axis and the quantity of correction of the horizontal axis. In the example of  FIG. 10 , the frequency of correction by −2 degrees is 8 times and that by −1 degree is 40 times. No correction is made (or correction by zero degree) most frequently, or 100 times. The frequency of correction by 1 degree is 70 times. It was least frequent (twice) to make a correction by 2 degrees. The frequency values are all set to zero in Step S 21 . 
     Next, the control circuit  11  sets a reference value preliminarily determined at the time of shipment from the factory for the correction of the optical axis (Step S 22 ). This reference value is also stored in the memory  18 . Next, the control circuit  11  controls the scanner  13  to correct the optical axis according to the quantity that has been set (Step S 23 ). This is done by supplying a control signal to the driver circuit  30  of the scanner  13  as shown in  FIG. 2 , supplying the correction coil  32  for the vertical direction with a current with the intensity corresponding to the control signal. The plate spring  34  in the vertical direction connected to the supporting member supporting the light projecting and receiving lenses  35  and  36  is moved by the current to the position balancing the force generated in the coil with the reaction force generated in the plate spring and the gravitational force. As a result, the vertical position of the scan range is set to the position at the time of shipment from the factory. 
     Examples of set position of scan range at the time of shipment from the factory are explained next with reference to  FIGS. 11-13 . In these examples, the movable vertical range of the scanner  13  is 20 degrees, of which a range of 4 degrees is defined as the scan range. 
       FIG. 11  shows an example of vertical position of the scan range set by the scanner  13  when the correction is zero degrees (when the optical axis is not corrected). In this example, main scan  1  is set at an angular position 4 degrees above the lower limit of the vertical range of scan (approximately horizontal direction). Sub scan  2  is set 1 degree above main scan  1  and sub scan  1  is set further 1 degree above (a total of 2 degrees above main scan  1 ). Sub scan  3  is set one degree below main scan  1  and sub scan  4  is set further 1 degree below (a total of 2 degrees below main scan  1 ). There is a margin of 14 degrees above and 2 degrees below. 
       FIG. 12  shows an example of vertical position of the scan range set by the scanner  13  when the correction is +1 degree (when the optical is corrected upward by 1 degree). In this example, main scan  1  is set at an angular position 5 degrees above the lower limit of the vertical range of scan (approximately horizontal direction). Sub scan  2  is set 1 degree above main scan  1  as in the example of  FIG. 11  and sub scan  1  is set further 1 degree above (a total of 2 degrees above main scan  1 ). Sub scan  3  is set one degree below main scan  1  and sub scan  4  is set further 1 degree below (a total of 2 degrees below main scan  1 ). There is a margin of 13 degrees above and 1 degree below. 
       FIG. 13  shows an example of vertical position of the scan range set by the scanner  13  when the correction is −2 degrees (when the optical axis is corrected downward by 2 degrees). In this example, main scan  1  is set at an angular position 2 degrees above the lower limit of the vertical range of scan (approximately horizontal direction). Sub scan  2  is set 1 degree above main scan  1  as in the example of  FIG. 11  and sub scan  1  is set further 1 degree above (a total of 2 degrees above main scan  1 ). Sub scan  3  is set one degree below main scan  1  and sub scan  4  is set further 1 degree below (a total of 2 degrees below main scan  1 ). There is a margin of 16 degrees above and 0 degree below. 
     In Step S 24  of the flowchart of  FIG. 8 , the control circuit  11  controls the scanner  13  to carry out one set of scanning of the set scan range in the order and directions shown in  FIGS. 6 and 7 . The laser light is reflected by an object and received by the PD  16 . The laser light received by the PD  16  is converted into an electrical signal and is further converted into a numerical signal by the light receiving circuit  17 . The control circuit  11  receives data on the quantity of received light from the light receiving circuit  17  (Step S 25 ). 
       FIG. 14  is an example of the data thus received by the control circuit  11  in Step S 25  from the light receiving circuit  17 . In  FIG. 14 , no entry means that the quantity of received light was zero. 
     In the case of the example shown in  FIG. 14 , the quantity of light received from horizontal area  64  of vertical area  81  is 10. In vertical area  82 , the quantity received from horizontal area  63  is 20, that from horizontal area  64  is 100 and that from horizontal area  65  is 25. In vertical area  83 , the quantity received from horizontal area  63  is 90, that from horizontal area  64  is 150 and that from horizontal area  65  is 100. In vertical area  84 , the quantity received from horizontal area  63  is 150, that from horizontal area  64  is 200 and that from horizontal area  65  is 160. In vertical area  85 , the quantity received from horizontal area  63  is 80, that from horizontal area  64  is 180 and that from horizontal area  65  is 75. 
     In Step S 26  of the flowchart of  FIG. 8 , the control circuit  11  determines whether a front-moving vehicle has been recognized or not. This determination is done as follows. 
     When the quantity of received light is found to be larger than a preliminarily set reference value, the control circuit  11  judges that a target object of detection is present. The detection of the target is repeated for a plural number of times successively such that the speed of motion of this target and its direction (or a displacement vector) can be obtained. Mutually adjacent targets having the same displacement vector are grouped together. 
     The process of grouping is explained next with reference to  FIG. 15 . In  FIG. 15 , the horizontal axis represents the detected position of each target in the horizontal direction and the vertical axis represents the distance to each detected target. Although not illustrated, the detected position and the distance to each target are compared also in vertical direction. In  FIG. 15 , each circle represents a detected target and each arrow indicates a displacement vector. Each displacement vector represents the distance and direction of motion of each target during the time of one scan and may be calculated from the distance between the current detection position and the previous detection position. The time of one scan may be 100 msec. 
     The control circuit  11  calculates the displacement vector of each target and compares them. As a result, those targets of which the displacement vectors and the distances are judged to be the same (or similar) are grouped together as belonging to the same object.  FIG. 15  shows targets  101 A- 101 H as having been detected, of which targets  101 A- 101 E are considered to have approximately the same distance and displacement vector and hence grouped together as belonging to the same object. Target  101 F is at about the same distance but since its displacement vector is different, pointing in the opposite direction, it is judged as not being the same object. Target  101 G has nearly the same displacement vector as targets  101 A- 101 E but since its distance is significantly different, it is judged as not belonging to the same object. Neither is target  101 H judged as belonging to the same objects it has a different displacement vector and its distance is different. 
     From an assembly of targets thus grouped together, it is possible to calculate the size (or the width in the horizontal direction) of the corresponding object. From such information, the control circuit  11  judges whether the detected targets are a human, a vehicle, a roadside structure, etc., thereby carrying out the process of recognizing the kind of the object. 
     The control circuit  11  also serves to estimate the shape of the road in front of the own vehicle and recognizes a front-going vehicle among the objects judged to be a vehicle. The control circuit  11  estimates the shape of the road ahead based on data detected by the vehicle speed sensor  19 , the yaw rate sensor  20  and the GPS  21 . Any known method of estimating the road shape may be used for this purpose. If the position data of the own vehicle detected by the GPS  21  are to be used, navigation map data may also be used in a coordinated way. Alternatively, a camera may be used to obtain an image of the forward direction to estimate the shape of the road ahead. The control circuit  11  identifies the vehicle traveling immediately in front of the own vehicle in the same traffic lane as the front-going vehicle. 
     After a front-going vehicle has been identified as described above, the control circuit  11  determines the quantity of correction of the short-term displacement of the optical axis (Step S 27 ). The flowchart of  FIG. 16  is referenced next to explain this process. 
     To start, the control circuit  11  detects the vertical area where the quantity of received light is the largest (Step S 5   1 ). In the case of the example of  FIG. 14 , vertical area  84  with the largest received quantity  200  is identified. The control circuit  11  calculates the offset angle of this vertical area (with the largest quantity of received light) as the displacement of the optical axis from the horizontal projection angle (4 degrees above the lowest limit of the scan range) of the laser of the scanner  13  (Step S 52 ). In the case of the example of  FIG. 14 , since main scan  1  is set to vertical area  83 , the horizontal angle of projection of the laser is vertical area  83  and its angle from the vertical area  84  with the largest quantity of received light (3 degrees above the lower limit of the scan range) is 1 degree. 
     Next, the control circuit  11  carries out the filtering process (Step S 53 ).  FIG. 17  will be referenced to explain the filtering process of the control circuit  11  in detail. 
     To start, the control circuit  11  reads out the distance to the front-going vehicle and its relative speed from the memory  18  (Step S 71 ) and then calculates the filter coefficient based on the distance and the relative speed that have been read out (Step S 72 ). The filter coefficient is determined based on a table, which will be explained below with reference to  FIG. 18 . 
     The control circuit  11  determines the filter coefficient as below based on the relative speed and the distance between the vehicles read out from the memory  18 : 
     (1) When the relative speed is −1 km/h-1 km/h, the filter coefficient F is set equal to 0.2; 
     (2) When the relative speed is −3 km/h-−1 km/h or 1 km/h-3 km/h, the filter coefficient is set equal to 0.3 if the distance between the vehicles is 0-3 m, 3-5 m or 5-10 m, and to 0.2 if the distance between the vehicles is 10-20 m or over 20 m; 
     (3) When the relative speed is −5 km/h-−3 km/h or 3 km/h-5 km/h, the filter coefficient is set equal to 0.4 if the distance between the vehicles is 0-3 m or 3-5 m; to 0.3 if the distance between the vehicles is 5-10 m or 10-20 m, and to 0.2 if the distance between the vehicles is over 20 m; 
     (4) When the relative speed is −10 km/h-−5 km/h or 5 km/h-10 km/h, the filter coefficient is set equal to 0.5 if the distance between the vehicles is 0-3 m or 3-5 m; to 0.4 if the distance between the vehicles is 5-10 m, and to 0.3 if the distance between the vehicles is 10-20 m or over 20 m; 
     (5) When the relative speed is −20 km/h-−10 km/h or 10 km/h-20 km/h, the filter coefficient is set equal to 0.5 if the distance between the vehicles is 0-3 m, 3-5 m or 5-10 m, and to 0.3 if the distance between the vehicles is 10-20 m or over 20 m; and 
     (6) When the relative speed is −20 km/h or less or 20 km/h or more, the filter coefficient is set equal to 0.5 if the distance between the vehicles is 0-3 m, 3-5 m or 5-10 m; to 0.4 if the distance between the vehicles is 10-20 m, and to 0.3 if the distance between the vehicles is over 20 m. 
     This is how the control circuit  11  sets the filter coefficient F although this method is not intended to limit the scope of the invention. A function with the distance between the vehicles and the relative speed as independent variables may be preliminarily defined and the filter coefficient may be set based of such a function. 
     Next, the control unit  11  reads out the quantity of previous correction of short-term displacement of the optical axis from the memory  18  (Step S 73 ) and calculates the sum of its product with multiplicative factor (1-F) and the quantity of the current correction of short-term displacement multiplied by factor F to set as the quantity of correction of short-tern displace of the optical axis (Step S 74 ). If the quantity of previous correction was 1 degree and that of the current correction is 4 degree and if the filter coefficient is 0.4, the value of 1.6 degrees is set as the quantity of correction of short-term displacement. For the same situation, if the filter coefficient F is 0.5, the value of 2.5 degrees is set. In other words, if the filter coefficient is larger, the effect of the quantity of the current displacement becomes greater and the response characteristic increases. Thus, when the distance between the vehicles has become small or the relative speed has increased (due to a sudden approach or a sudden separation), the optical axis can be corrected quickly and the probability of losing sight of the front-going vehicle is reduced. 
     After Step S 74 , the control unit  11  proceeds to carry on the clipping process. The flowchart of  FIG. 19  is referenced next to explain this process. 
     To start, the control unit  11  reads out from the memory  18  the quantity of correction of the previous short-term displacement of the optical axis (the quantity stored in Step S 55  to be explained below) (Step S 91 ) and then judges whether or not the absolute value of the difference between the quantity of correction of the current short-term displacement of the optical axis (the quantity set in Step S 74  of  FIG. 17 ) and the quantity of correction of the previous short-term displacement of the optical axis is 1 degree or larger (Step S 92 ). If this difference is judged to be 1 degree or larger (YES in Step S 92 ), the latter quantity is replaced by the sum of the former and their difference (Step S 93 ). The numbers below the decimal point may be cut off or rounded off in Step S 93 . If the updated value exceeds the allowable range of motion, the updated value may preferably be adjusted to a value within the allowable range. The speed of adjusting the optical direction should preferably be the highest within the range allowable by the mechanical system such as the driver circuit  30  and the coil  32  for the displacement in the vertical direction. 
     If the difference between the quantities of correction of the current and previous short-term displacements of the optical axis is judged to be less than 1 degree (NO in Step S 92 ), the quantity of correction is not changed and assumes the previous value. 
     After Step S 93  or after the judgment in Step S 92  turned out to be NO, the control unit  11  returns to Step S 31  of  FIG. 9 , reading out the quantity of correction stored in the memory  18  and it is set as the quantity of correction. Next, the control unit  11  judges whether or not this quantity of correction satisfies statistical conditions for adding to the histogram (Step S 32 ). As the process returns from Step S 40  to Step S 23 , the vertical direction of the scan range is corrected based on this newly set quantity. 
       FIG. 20  shows an example of these statistical conditions. The first condition is that the distance to the front-going vehicle should be less than 100 m. This is because the own vehicle may be considered to be following the front-going vehicle at a constant speed if the distance of separation satisfies this condition. The second condition is that the absolute value of the quantity of correction of the optical axis is less than 2 degrees. This is for the purpose of eliminating the effects of instantaneous displacements of the optical axis. The third condition is that the speed of the own vehicle is 60 km/h or more. This is because the time for displacing the optical axis (say, due to a slope) is longer when the own vehicle is traveling relatively slowly and it is to minimize the effects of short-term displacements of the optical axis. The main purpose of the invention is to be used on roads with few slopes or on high-speed roadways. 
     If the conditions as shown in  FIG. 20  are all satisfied (YES in Step S 32 ), the quantity thus set in Step S 31  is added to the histogram such as shown in  FIG. 10  stored in the memory  18  (Step S 33 ). If the quantity of correction was −1 degree, for example, the correction frequency corresponding to −1 degree is changed from 40 to 41. 
     If any of the required statistical conditions is not satisfied, Step S 33  is skipped and the quantity of correction is not added to the histogram. Thus, unwanted effects based on short-term displacements of the optical axis are prevented from being included. 
     If it is judged in Step S 26  that the front-going vehicle cannot be recognized, it is judged whether or not the sample number of the histogram is less than 1000 (Step S 34 ). If the sample number of the histogram is found to be less than 1000 (YES in Step S 34 ), the control circuit  11  judges whether or not a quantity of correction of long-term displacement of the optical axis (stored in Step  113  of  FIG. 21  to be explained below) has been stored in the memory  18  (Step S 35 ). If it is not stored in the memory  18  (NO in Step S 35 ), a preliminarily selected value stored in the memory  18  is set as the quantity of correction of the optical displacement (Step S 36 ). 
     If the sample number of the histogram is found to be not less than 1000 (NO in Step S 34 ), the control unit  11  calculates a quantity of correction for long-term displacement of optical axis (Step S 37 ). This process is explained next with reference to the flowchart of  FIG. 21 . 
     With reference to  FIG. 21 , the control unit  11  detects the quantity of correction with the largest correction frequency from the histogram (Step S 111 ). In the case of the histogram shown in  FIG. 10 , the quantity of correction of zero with the largest correction frequency of 100 is detected. This quantity of correction (zero in this example) is set as the quantity of correction of long-term displacement of the optical axis (Step S 112 ) and is stored in the memory  18  (Step S 113 ). 
     Thereafter, the control unit  11  reduces the sample number of the histogram to one half (Step S 38  of  FIG. 9 ). For example, the frequency value of each quantity of correction of the displacement of the optical axis is halved. After Step S 38  or after the quantity of correction of long-term displacement of the optical axis is found to be stored in the memory  18 , the control unit  11  sets the quantity of correction of long-term displacement of the optical axis (Step S 39 ). Thus, the quantity currently set in Step S 112  or the quantity previously stored in the memory  18  in Step S 113  is set as the quantity of correction of the displacement of the optical axis. 
     Thus, if the front-going vehicle is not recognized and the sample number of the histogram is 1000 or larger, the control circuit  11  determines a new quantity of correction for long-term displacement of the optical axis, and sets it as the quantity of correction. 
     If it is found that the conditions for adding to the histogram are not satisfied (NO in Step S 32 ), the control circuit  11  judges whether the processing of the laser radar should be terminated according to a command from the user (Step S 40 ) after Step S 33 , S 36  or S 39 . If it is judged not to terminate (NO in Step S 40 ), the optical axis is corrected according to the set quantity (Step S 23 ). In other words, if the front-going vehicle is detected, the correction is based on the quantity of correction of short-term displacement. If the front-going vehicle is not detected and the sample number of the histogram is 1000 or larger, the optical axis is corrected according to the quantity of correction of long-term displacement. If the front-going vehicle is not detected and the sample number of the histogram is less than 1000, the correction is made according to a reference value in the case where the quantity of correction of previous displacement is not stored in the memory  18 . In the case where the quantity of correction of previous long-term displacement is stored in the memory  18 , this quantity is used to correct the optical axis. The above is repeated until a command to terminate the processing of the laser radar is given. 
     If the processing of the laser radar is judged to be terminated (YES in Step S 40 ), the control circuit  11  terminates the processing. 
     In summary, the displacement of the optical axis is corrected such that the vertical area with the largest quantity of received light will become the center of the vertical scan range. Thus, the optical axis can be set at the optimum position even if the optical axis has been displace instantaneously due to the vibrations of the own vehicle or the vertical motion of the front-going vehicle going over a slope such that the distance to the target object can be measured accurately. Since the filtering and clipping processes are carried out in obtaining the quantity of correction, the effect on the quantity of received light due to instantaneous displacements of the optical axis can be minimized. The probability of losing sight of the front-going vehicle can be reduced by increasing the filter coefficient (by reducing the time constant of the filter) when the front-going vehicle approaches suddenly. 
     Even in the absence of a target object to be detected, the optical axis can be corrected based on the quantity of the previous correction of the optical axis and the histogram of the frequency of corrections such that long-term displacements (say, due to a minor collision) can be corrected.