Patent Publication Number: US-2002008876-A1

Title: Stray light barrier structure of reflection measuring apparatus

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates generally to an improved structure of a reflection measuring apparatus such as a radar designed to scan a beam to detect an object present in a scan range, and more particularly to a stray light barrier structure of a reflection measuring apparatus which blocks emission of the stray light to avoid the leakage thereof outside the reflection measuring apparatus.  
       [0003] 2. Background Art  
       [0004] Optical reflection measuring apparatuses are known which are designed to emit a light beam over a given scan range and receive a return of the beam to obtain information (e.g., an angular direction) about a reflective object present in the scan range. Such optical reflection measuring apparatuses are employed, for example, in automotive alarm systems working to detect a sign post, a preceding vehicle, or a white line on a road and sound an alarm when a system vehicle is approaching the sign post, coming into a collision with the preceding vehicle, or going outside the white line.  
       [0005] Optical reflection measuring apparatuses of the above type usually emit stray light such as light scattered or reflected on a surface of an optical element or on inner structural elements of the apparatus in addition to a light beam used to detect an object. Stray light minimizing measures are already used, however, with an increasing need for the improvement of optical sensitivity, an error in detecting an object existing out of an optical axis caused by the stray light will increase.  
       [0006] For instance, if the stray light, as shown in FIG. 7, is emitted from a reflecting measuring apparatus in a direction shifted from a main beam, it may cause the reflecting measuring apparatus to receive as an optical noise a return of the stray light from an object existing at a distance from the main beam. This will result in an error in an operation of, for example, an alarm system equipped with such a reflection measuring apparatus. Such a problem is encountered, especially when an object on which the stray light hits has a higher reflectivity. For example, typical automotive vehicles have a reflector installed on a rear end thereof which reflects the stray light at a stronger level which will give rise to the above problem.  
       SUMMARY OF THE INVENTION  
       [0007] It is therefore a principal object of the invention to avoid the disadvantages of the prior art.  
       [0008] It is another object of the invention to provide a reflection measuring apparatus which is designed to have a stray light barrier structure working to avoid leakage of the stray light outside the apparatus.  
       [0009] According to one aspect of the invention, there is provided a reflection measuring apparatus such as a laser radar. The reflection measuring apparatus comprises: (a) a light source producing a light beam; (b) a scanner directing the light beam produced by the light source to a given scan range to detect an object therein; (c) an optical receiver receiving a reflection of the light beam from the object within the scan range to obtain preselected information on the object; and (d) a stray light blocking member having a beam exit opening formed on an optical path along which the light beam travels from the light source to the scanner. The stray light blocking member has a barrier wall defined around the beam exit opening which works to interrupt traveling of stray light produced optically upstream of the beam exist opening to the scanner. The barrier wall has a surface which tapers off in cross section thereof to the beam exist opening.  
       [0010] In the preferred mode of the invention, the tapered surface of the barrier wall faces the scanner.  
       [0011] An angle which the tapered surface makes with the optical path of the light beam extending through the beam exit opening is greater than an angle which an outer periphery of the light beam makes with the optical path.  
       [0012] A structural element holding member is further provided which holds a given structural element of the reflection measuring apparatus. The structural element holding member has a beam exit opening through which the light beam produced by the light source passes. The beam exit opening of the structural element holding member is formed optically upstream of the beam exit opening of the stray light blocking member.  
       [0013] According to another aspect of the invention, there is provided a reflection measuring apparatus which comprise: (a) a light source producing a light beam; (b) a scanner directing the light beam produced by the light source to a given scan range to detect an object therein; (c) an optical receiver receiving a reflection of the light beam from the object within the scan range to obtain preselected information on the object; and (d) a stray light blocking member having a beam exit opening formed on an optical path along which the light beam travels from the light source to the scanner. The stray light blocking member has a barrier wall defined around the beam exit opening which works to interrupt traveling of stray light produced optically upstream of the beam exist opening to the scanner. The barrier wall spreads in cross section in an advancing direction of the light beam at an angle greater than a spread angle of the light beam. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0014] The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only.  
     [0015] In the drawings:  
     [0016]FIG. 1 is an exploded view which shows a reflection measuring apparatus according to the present invention;  
     [0017]FIG. 2 is a sectional illustration as viewed from the top of the reflection measuring apparatus of FIG. 1;  
     [0018]FIG. 3 is a sectional illustration as viewed from the right side of the reflection measuring apparatus of FIG. 1;  
     [0019]FIG. 4( a ) is a sectional view which shows a stray light barrier structure;  
     [0020]FIG. 4( b ) is a sectional view which shows a relation between a beam exit opening and a laser beam;  
     [0021]FIG. 4( c ) is a sectional view which shows a modification of a stray light barrier structure;  
     [0022]FIG. 5 is a sectional view which shows a comparative example of an optical system;  
     [0023]FIG. 6 is an explanatory view which shows stray light blocking effects provided by a structure of the invention; and  
     [0024]FIG. 7 is an illustration for explanation of a stray light-caused measurement error encountered in a conventional automotive radar system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0025] Referring to the drawings, wherein like reference numbers refer to like parts in several views, particularly to FIGS.  1  to  3 , there is shown a reflection measuring apparatus  2  according to the present invention which will be discussed below as an example as used in a radar system of an automotive vehicle which emits a laser beam in an advancing direction of the vehicle and receives a return of the laser beam from an object to obtain information on the object such as an angular direction thereof.  
     [0026] The reflection measuring apparatus  2  includes an optical system consisting of a lens unit  10 , a reflecting mirror  20 , a polygon mirror  30 , and a light-receiving lens  40 . The lens unit  10  works to produce a parallel laser beam to the reflecting mirror  20 . The reflecting mirror  20  directs the laser beam from the lens unit  10  to the polygon mirror  30 . The polygon mirror  30  works as a scan mechanism which radiates the laser beam over a given scan range. The light-receiving lens  40  captures a return of the laser beam from the scan range.  
     [0027] The lens unit  10  includes, as shown in FIGS. 2 and 3, a laser diode  12 , a diode driver  13 , a collimating lens  14 , a unit casing  16 . The diode driver  13  supplies the power to the laser diode  12  to emit a laser beam in an infrared region in the form of a pulse. The collimating lens  14  receives the laser beam from the laser diode  12  and outputs a parallel beam. The unit casing  16  retains therein the laser diode  12 , the diode driver  13 , and the collimating lens  14  so as to direct the laser beam from the laser diode  12  to the collimating lens  14 .  
     [0028] The reflection measuring apparatus  2  also includes, as shown in FIG. 1, a motor driver  60 , an optical processing circuit  70 , a control circuit  80 , and a power supply circuit  90 . The motor driver  60  actuates a polygon scanner motor  32 , as shown in FIG. 3, to turn the polygon mirror  30 . The optical signal converter  70  includes a photo diode  50  which converts an input light into an electric signal and adjusts the electric signal in amplitude and waveform to provide an optical signal. The control circuit  80  actuates electronic components of the diode driver  13  and motor driver  60  to perform a scanning operation of the optical system and analyzes the optical signal from the optical signal converter  70  to determine the position and relative speed of an object present in the scan range (i.e., a radar detectable range). The power supply circuit  90  supplies the power to electronic components installed in the diode driver  13 , the motor driver  60 , and the control circuit  80 .  
     [0029] The reflecting measuring apparatus  2  also has a physical structure for retaining and protecting the above component parts which consists of a front casing  100 , an inner casing  110 , and a rear plate  120 . The inner casing  110  is disposed in the front casing  100 . The rear plate  120  is installed on the front casing  100  to close an opening thereof.  
     [0030] The front casing  100  has an exit window  102  and an entrance window  104  formed in a front wall thereof. The laser beam from the polygon mirror  30  is radiated from the exit window  102 . A return of the laser beam enters the entrance window  104 . A protective plate  130  is installed on an inner front wall of the front casing  100  through a sealing member  140  to close the exit and entrance windows  102  and  104  for avoiding the ingress of dust and water into the front casing  100 . The protective plate  130  is made of, for example, a transparent glass or resin without interrupting light.  
     [0031] The sealing member  140  has O-rings  142  and  144  formed integrally therewith which are attached to the periphery of the exit and entrance windows  102  and  104 , respectively. The O-rings  142  and  144  are made of rubber and have substantially the same shapes as those of the exit and entrance windows  102  and  104 , respectively. The O-rings  142  and  144  are pressed by the front of the inner casing  110  against the inner front wall of the front casing  100  to establish a hermetic seal between the protective plate  130  and the exit and entrance windows  102  and  104 .  
     [0032] Disposed between the front casing  100  and the rear plate  120  is an O-ring  146  to seal a gap between the front casing  100  and the rear plate  120  hermetically for avoiding the ingress of water. A connector  150  is installed on an outside wall of the rear plate  120  which provides a connection of the reflecting measuring apparatus  2  with an external device for establishing transmission of a signal therebetween and supply of power to the reflecting measuring apparatus  2 . The connector  150  has connection pins  150   a.  The connection pins  150   a  pass through holes  122  formed in the rear plate  120  and enter inside the reflecting measuring apparatus  2 . An O-ring  148  is disposed between the connector  150  and the rear plate  120  to seal a gap between the rear plate  120  and the connector  150  hermetically for avoiding the ingress of water into the reflection measuring apparatus  2  through the holes  122 .  
     [0033] The lens unit  10 , the reflecting mirror  20 , the polygon mirror  30 , the light-receiving lens  40 , the diode driver  13 , the motor driver  60 , the optical signal converter  70 , the control circuit  80 , and the power supply circuit  90  are retained by the inner casing  110  within the reflection measuring apparatus  2 . The lens unit  10 , the reflecting mirror  20 , and the polygon mirror  30  are, as clearly shown in FIG. 2, arranged at the right side of an inner space of the reflection measuring apparatus  2 .  
     [0034] Specifically, the right inner space of the reflecting measuring apparatus  2  is, as shown in FIG. 3, separated by a partition wall  112  provided on the inner casing  110  into upper and lower chambers. Within the upper chamber, the lens unit  10  and the reflecting mirror  20  are disposed on the partition wall  112 . Within the lower chamber, the polygon mirror  30  is disposed. The partition wall  112  also serves as a barrier, as will be described later in detail, which shuts the stray light in the upper chamber to avoid leakage of the stray light outside the reflection measuring apparatus  2 .  
     [0035] The lens unit  10  is secured on an upper surface of the partition wall  112  using a tapping screw  114  screwed into the unit casing  16  through the partition wall  112 . The reflecting mirror  20  is located ahead of the lens unit  10  and retained on the inner casing  110  through a mirror holder  22 . The inner casing  110  has a mirror mount wall  116 , as shown in FIGS. 2 and 3, to which the mirror holder  22  is attached. The mirror holder  22  has a spring  22   a  which is fitted in the mirror mount wall  116  to nip the reflecting mirror  20  between itself and the mirror mount wall  116 .  
     [0036] The mirror mount wall  116  has formed therein a beam exit opening  117  through which the laser beam passes. The polygon mirror  30  is installed on an output shaft of the polygon scanner motor  32  secured on a metallic base (i.e., a substrate) of the motor driver  60 . The base of the motor driver  60  is secured on the inner casing  110  using screws  62 .  
     [0037] The partition wall  112  has formed therein, as shown in FIGS. 2 and 3, a beam exit opening  118  through which the laser beam reflected from the reflecting mirror  20  travels toward the polygon mirror  30 . The periphery of the beam exit opening  118 , as clearly shown in FIG. 4( a ), is wedge-shaped to have a sharp edge  118   a.  Specifically, the partition wall  112  has a tapered surface  118   b  which faces the polygon mirror  30  and serves as a stray light barrier. The angle α 1 , as shown in FIG. 4( b ), which a line extending parallel to the surface  118   b  of the partition wall  118  makes with an optical axis along which the laser beam reflected from the reflecting mirror  20  travels toward the polygon mirror  30  is set greater than the angle α 2  which the outer periphery of the laser beam makes with the optical axis (i.e., half a spread angle of the laser beam). Note that FIGS.  4 ( a ) and  4 ( b ) are cross sectional views taken by a plane containing the optical axis extending between the reflecting mirror  20  and the polygon mirror  30 .  
     [0038] If a beam exit opening is, as shown in FIG. 5, defined by a flat inner surface formed in a partition wall substantially in parallel to the optical axis along which a laser beam emitted from a laser diode travels, the stray light arising from reflection or scattering of the laser beam on a surface of a structural element disposed between the laser diode and a lens may reflect on the inner surface of the partition wall so that it is outputted as a noise beam. Additionally, the main beam (i.e., a parallel laser beam emerging from the collimating lens  14 ) may also reflect on the inner surface of the partition wall so that it is outputted outside the partition wall as the stray light. The partition wall  112  of this embodiment, as described above, has the tapered surface  118   b  oriented outward (i.e., in an advancing direction of the laser beam). The stray light emerging from the lens unit  10  is, thus, reflected, as indicated by broken lines illustrated in an upper portion of FIG. 6 by the inner surface of the partition wall  112  without being reflected outward by the surface  118   b  of the partition wall  112 , thereby avoiding the leakage of the stray light outside the reflection measuring apparatus  2 .  
     [0039] The surface  118   b  of the partition wall  112  may alternatively be oriented toward the reflecting mirror  20 . In this case, however, a portion of the laser beam traveling from the reflecting mirror  20  may reflect on the surface  118   b  to produce the stray light. It is, thus, advisable that the tapered surface  118   b  of the partition wall  118  be oriented outward to minimize a hit of the laser beam on the tapered surface  118   b.    
     [0040] The angle α 1  which the tapered surface  118   b  of the partition wall  118  makes with the optical axis is, as described above, greater than the angle α 2  which the outer periphery of the laser beam makes with the optical axis. Specifically, the surface  118   b  of the partition wall  118  spreads at an angle greater than the spread angle of the laser beam in an advancing direction of the laser beam. This minimizes hits of the laser beam on the surface  118   b,  thus avoiding emission of the stray light substantially in the same direction as that in which the laser beam outputted from the reflection measuring apparatus  2 .  
     [0041] In operation, when the reflection measuring apparatus  2  is turned on, the laser diode  12  emits a parallel laser beam through the collimating lens  14  in the form of a pulse. The laser beam travels to the reflecting mirror  20  through the beam exit opening  117  formed on the mirror mount wall  116 . The laser beam reflected by the reflecting mirror  20  reenters the beam exit opening  117  and travels to the polygon mirror  30  through the beam exit opening  118  formed in the partition wall  118 . The laser beam falling on the polygon mirror  30  is reflected by one of facets thereof and outputted from the exit window  102  of the front casing  100  for scanning the scan range defined ahead of the system vehicle equipped with the reflection measuring apparatus  2 . If there is an object within the scan range, the laser beam is reflected thereby, so that a return of the laser beam enters the reflection measuring apparatus  2  through the entrance window  104 . The laser return then passes through the protector plate  130  and travels to the photo diode  50  through the light-receiving lens  40 . The photo diode  50  converts the input beam into an electric signal and outputs it as an optical signal. The laser beam emitted from the lens unit  10  is, as described above, a pulse beam. The positional information on the object (i.e., the distance between the object and the reflection measuring apparatus  2 , the angular direction of the object, etc.) may, thus, be determined based on an emission timing of the laser beam and the time interval between transmission of the laser beam and reception of the echo.  
     [0042] The partition wall  112  is, as described above, provided on the optical path along which the laser beam emitted from the lens unit  10  travels to the polygon mirror  30 . The partition wall  112  has formed therein the beam exit opening  118  through which the laser beam passes. The periphery of the beam exit opening  118  tapers off to the beam exit opening  118  so as to form the surface  118   b  oriented toward the polygon mirror  30 , thereby preventing the stray light from being reflected on the polygon mirror  30  outside the reflection measuring apparatus  2 , thus resulting in a decrease in error in detecting an object.  
     [0043] The mirror mount wall  116  has formed therein the beam exit opening  117  through which the laser beam produced by the lens unit  10  passes twice. There is, thus, a high possibility that the stray light will be produced when the laser beam passes through the beam exit opening  117 . However, the partition wall  118  is located optically downstream of the mirror mount wall  116  and thus shields the polygon mirror  30  against the stray light effectively.  
     [0044] While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments witch can be embodied without departing from the principle of the invention as set forth in the appended claims. For example, the partition wall  112  is formed integrally with the mirror mount wall  1   16 , but they may be provided separately. The lens unit  10  is carried on the partition wall  112 , but may alternatively be retained by another structural member. The tapered surface  118   b  of the partition wall  112  has, as shown in FIG. 4( a ), the sharp edge  118   a,  but may have a small flat end  118   c,  as shown in FIG. 4( c ), extending parallel to the optical axis.