Abstract:
A radar sensor for motor vehicles, having an optical system which subdivides a radar beam generated by a single antenna element into a plurality of beam components that are radiated in different directions, wherein the optical system has a diffraction grating.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a radar sensor for motor vehicles, having an optical system which subdivides a radar beam, generated by an antenna element, into several beam components radiated in different directions.  
       BACKGROUND INFORMATION  
       [0002]     Radar sensors are used in motor vehicles for locating other vehicles or other obstacles in the surroundings of one&#39;s own vehicle, and for measuring their distances and relative speeds, so that, for instance, an automatic distance control (ACC; adaptive cruise control) is made possible.  
         [0003]     A radar sensor for this purpose is described in German Patent No. DE 102 07 437, which has several antenna elements situated next to one another, which, in connection with an optical system in the form of a plano-convex lens, generates several slightly divergent radar lobes. By using several radar lobes whose radar echoes are able to be received by the antenna elements and separately evaluated, a certain angular resolution capability of the radar sensor is achieved. Furthermore, in this known radar sensor, the lens is simultaneously developed at a part of its cross section as a prism by which, from each radar beam generated by a single antenna element, once more a beam component is coupled out that is directed slantwise downwards onto the roadway. This beam component, which hits the roadway surface already at a relatively short distance from the vehicle, is reflected from unevennesses in the roadway, and the radar echoes thus received may, for example, be used to measure the speed of the vehicle over the ground, independently of the signals of the wheel rotary speed sensors. By measuring the distance of the point of impingement of this beam component on the roadway surface, it is also possible to monitor the vertical calibration of the radar sensor. Since the radar echo from the roadway surface is present even when there are no vehicles in the locating range of the main beam of the radar sensor, a loss of sight of the radar sensor may also be detected in the light of the absence of these radar echoes, that may be caused, for instance, by snow or ice in front of the lens of the radar sensor.  
       SUMMARY OF THE INVENTION  
       [0004]     The present invention provides a radar sensor in which the geometry of the beam components may be better adapted to the respective requirements.  
         [0005]     According to the present invention, this is achieved by the optical system&#39;s having a diffraction grating.  
         [0006]     A diffraction grating generates a diffraction pattern which has at least one secondary maximum besides a main maximum. The radar beam generated by the antenna element is thus subdivided into a number of beam components which correspond to the main maximum and to the secondary maxima of the diffraction pattern. The angle between the main maximum and the first secondary maximum is a function of the grating constant of the diffraction grating. Thus, using a diffraction grating having a sufficiently low grating constant, even in the case of thin lenses, into which a prism having sufficient refractive power could hardly be integrated, a large angle between the beam components may be achieved.  
         [0007]     The diffraction grating may simply be placed before the lens as transmission grating or it may be developed in the flat surface of the lens, and, in this context, it may extend over the entire cross section of the lens. This has the advantage that the lens has the same numerical aperture for the deflected beam component corresponding to the first secondary maximum as for the main beam, so that the geometry of the deflected beam is not falsified so strongly by diffraction effects of the lens. An additional substantial advantage, especially for the loss of sight detection, is that the deflected beam, just as the main beam, also starts from the entire cross sectional area of the lens. Deposits of snow, ice or slush, that lead to a loss of sight of the sensor, consequently have the same effect on the deflected beam component, which is used for loss of sight detection, as on the non-deflected main beam.  
         [0008]     By a suitable embodiment of the diffraction grating, for example, as a so-called blaze grating, an asymmetrical diffraction pattern may be achieved, in which all secondary maxima, except for the first secondary maximum, are extensively suppressed on one side of the principal maximum, so that the beam, similar to the case of a prism, is subdivided into only two beam components, namely a non-deflected main beam and a deflected secondary beam. Then, in the same way as with the radar sensor described in German Patent No. DE 102 07 437, the deflected beam may be directed slantwise onto the roadway surface, and used for measuring the absolute speed, for calibration control and/or for the detection of loss of sight. However, since, for the beam deflected with the aid of the diffraction grating, the entire numerical aperture of the lens is available, this beam is easier to focus.  
         [0009]     Optionally, the diffraction grating may also be oriented vertically, so that the secondary beam is deflected to the side. In this case, the secondary beam may be used, for example, for the early detection of a driver&#39;s swinging in or for locating objects on side lanes or at the edge of the roadway. Then the diffraction grating may also be embodied in such a way that exactly two secondary beams are generated symmetrically to the main beam that correspond to the two first diffraction maxima. In this way, in the very short range of the vehicle, monitoring both side lanes or of the left side lane and the right roadway edge is made possible. By evaluating the radar echoes that are generated by the side lanes, for example, at left and right guardrails or guardrail posts, it is also possible to detect in which lane of the roadway one&#39;s own vehicle is located. A differentiation between radar echoes that are generated by the main beam on the one hand, and by secondary beams on the other hand, is, for instance, made possible in that objects that are detected by the secondary beams will have a very small distance from one&#39;s own vehicle, already at their first detection. In the case of standing objects or objects that are slower than one&#39;s own vehicle, a differentiation between echos of the left and the right secondary beam, in the case of angle-resolving radar sensors, is made possible in that the azimuth angles of the located object change in the opposite direction.  
         [0010]     According to one refinement of the present invention, it is also possible to vary or modulate the grating constant of the diffraction grating dynamically. This may be done, for instance, by producing the diffraction grating using electronically controllable elements, for example, using piezoelectric elements or using photon crystal technology in a suitable substrate, so that the grating parameters are able to be controlled electronically. This also opens up the possibility of enlarging the locating angle range of the radar sensor, or to improve the angular resolution capability, by “scanning” the environment by periodic change in the angle of deflection of the secondary beams. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  shows a schematic section through a radar sensor according to the present invention.  
         [0012]      FIG. 2  shows a vehicle having a radar sensor according to another exemplary embodiment, in a top view.  
         [0013]      FIG. 3  shows a schematic section through the radar sensor as in  FIG. 2 . 
     
    
     DETAILED DESCRIPTION  
       [0014]     In  FIG. 1 a  radar sensor  10  is shown schematically in a vertical section which, for instance, is installed in the front of a motor vehicle. A housing  12  of the radar sensor is closed at the front end by a plano-convex lens  14 , which is made of a plastic material having a suitable refractive index that is transparent to microwaves. On the inside of housing  12 , approximately in the focal plane of lens  14 , there are several antenna elements  16  of which only one is seen in the drawing, and they are situated side by side in such a way that, in connection with lens  14 , they generate several radar beams  18  that are slightly angularly offset against one another, which lie in a horizontal plane that goes through the optical axis of lens  14  and is perpendicular to the plane of the drawing in  FIG. 1 . By evaluating the intensity differences and phase differences between the radar echoes received by the different antenna elements  16 , the angle of azimuth of the located objects may be determined.  
         [0015]     However, even radar beam  18 , generated by a single antenna element  16 , is here once more subdivided into two beam components  20 ,  22 , namely a main beam running approximately horizontally, and a secondary beam  22  directed slantwise at the roadway surface. This beam subdivision is achieved by a diffraction grating  24  which, in the example shown, is formed by a sequence of grooves  26  in the planar surface of lens  14 . The diffraction effect comes about because the empty space in the grooves  26  has a lower optical density than the ribs of the material of lens  14  that have been left standing, so that phase differences in the radiation emitted by antenna element  16  are brought about. Instead, one might optionally use protruding ribs instead of grooves  26 , or strips embedded in the lens made of a material having an optical density deviating from that of the material of lens  14 . In principle, lattice rods made of a material that is opaque to microwave radiations would also be conceivable, but this would lead to losses in the overall intensity of radar beam  18 .  
         [0016]     The angle between main beam  20  and secondary beam  22  is a function of the grating constant of diffraction grating  24 , that is, the distance between grooves  26 , and this distance is approximately of an order of magnitude of the wavelength of the microwaves, which amounts to approximately 3.9 mm for a 77 kHz radar.  
         [0017]     Secondary beam  22  corresponds to the first diffraction maximum of diffraction grating  24 . In the example shown, diffraction grating  24  is a transmission grating, and especially a so-called blaze grating, in which all diffraction maxima, with the exception of the first secondary maximum, are suppressed on one side of the main maximum. As is known from the theory of diffraction gratings (blazed gratings), this may be achieved by the grooves  26  having an asymmetrical prism profile that is so tuned to the grating constant and the index of refraction of the material that the undesired secondary maxima are removed by destructive interference. Consequently, the entire radiation energy concentrates on the two beam components  20 ,  22 .  
         [0018]     The intensity ratio of main beam  20  and secondary beam  22  is a function of the ratio of the width of grooves  26  and the width of the ribs standing between the grooves, and may be adjusted as needed.  
         [0019]     Optionally, instead of diffraction grating  24  that is integrated into lens  14 , one could also use a separate diffraction grating. However, the design shown in  FIG. 1  has the advantage of permitting simple manufacturing and assembly, and of ensuring that, if lens  14  is correctly calibrated, diffraction grating  24  is, at the same time, correctly calibrated.  
         [0020]     Secondary beam  22 , for example, may be utilized for a direct measurement of the speed of the vehicle over the ground. Since the point where secondary beam  22  hits roadway  28  is at a relatively short distance from the vehicle, a sufficiently strong echo of reflection centers P is received, which are formed by surface unevennesses in roadway  28 . The relative speed of the reflection centers P, that may be determined by the Doppler effect, directly gives the initial speed of the vehicle. Besides that, measuring the distance between radar sensor  10  and reflection centers P, that is, the point where secondary beam  22  hits the roadway, makes it possible to control the vertical calibration of radar sensor  10 .  
         [0021]     Furthermore, secondary beam  22  may be used to detect a possible loss of sight of radar sensor  10 . If, for example, the upper half of lens  14  is encrusted with snow, ice or slush, and if therefore the intensity of main beam  20  is weakened to the extent that no reliable locating of obstacles is possible any more, the intensity of the secondary beam will also be weakened in the same manner and, in the light of the intensity of the echo of reflection centers P, which, with the aid of the known distance is identifiable, the extent of the loss of sight may be determined, in particular even if main beam  20  is not locating any preceding vehicles.  
         [0022]      FIG. 2  shows the outline of a motor vehicle  30 , having a radar sensor  32  that is constructed according to the same principle as radar sensor  10  as in  FIG. 1 . However, in radar sensor  32  the diffraction grating is oriented vertically and is symmetrically constructed, and is, in this context, designed in such a way that, besides main beam  20 , two secondary beams  22 ,  22 ′ are generated, which correspond to the two secondary maxima. Secondary beams  22 ,  22 ′ may be used in this case, for instance, for locating objects in the immediate vicinity of motor vehicle  30 , to the left and the right of the vehicle. This makes possible, in particular, the early detection of a driver swinging in.  
         [0023]      FIG. 3  shows a conceivable construction of radar sensor  32 . Diffraction grating  24 ′ is, in this case, developed separately from lens  14  and is formed by an elastic diaphragm  34  that is equipped with a sequence of grating strips  36 . Both diaphragm  34  and grating strips  36  are transparent to the microwave radiation, but grating strips  36  have a higher index of refraction than air. At one end, diaphragm  34  is firmly clamped in a holder  38 , and at the other end it is deflected by a deflecting roller  40  and fastened to an electromechanical adjusting drive  42 . With the aid of actuating drive  42 , diaphragm  34  may be stretched, so that the distance between grating strips is increased, and thereby also the grating constant is increased, with the result that the angle between secondary beams  22 ,  22 ′ and main beam  20  decreases. In this manner, the radiation angle of secondary beams  22 ,  22 ′ may be varied in a stepless manner during the operation of the radar sensor.  
         [0024]     Finally, a combination of vertical grating  24 ′, as in  FIG. 3 , with horizontal grating  24 , as in  FIG. 1 , is also conceivable, so that, in addition to main beam  20  and lateral secondary beams  22 ,  22 ′, as in  FIG. 2 , one obtains in each case an additional secondary beam that is directed to the roadway in a slantwise manner.