Abstract:
A multi-beam radar sensor has a plurality of antenna elements disposed next to each other, a collective lens situated at a distance in front of the antenna elements, and an additional preliminary focusing lens disposed in such a way that it affects only a portion of the radar radiation transmitted from, and/or received by, the antenna elements.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a multi-beam radar sensor having a plurality of antenna elements situated next to each other, a collective lens disposed with distance in front of the antenna element, and an additional preliminary focusing lens, and the present invention also relates to a radar sensor having angular resolution for use in motor vehicles. 
         [0003]    2. Description of Related Art 
         [0004]    The radar radiation emitted or received by the plurality of antenna elements disposed next to each other passes through the collective lens shared by all antenna elements and is bundled in this manner. However, because of the offset of the individual antenna elements in the lateral direction, the radar beams generated and/or received thereby are likewise angularly offset with respect to each other. If a reflective radar object is hit by multiple beams, then it is possible to determine, or at least estimate, the azimuth angle of the radar object on the basis of the amplitude and/or phase relation between the signals received by the various antenna elements. 
         [0005]    The transmission characteristic of such a multi-beam radar sensor is a function of the geometry of the individual antenna elements and the optical characteristics of the collective lens, but is also affected by interference effects between the signals emitted by the different antenna elements. 
         [0006]    From published Germany patent document DE 197 55 607, a radar sensor of the type mentioned in the introduction is known, in which a preliminary focusing lens is disposed in relatively close proximity in front of the antenna element(s). This preliminary focusing lens is dimensioned such that all of the radar radiation that also passes through the actual collective lens traverses it. The preliminary focusing lens is provided to enable a more compact design of the radar sensor. 
         [0007]    In the case of a multi-beam sensor, it has also already been proposed to place a cylindrical lens on the planar inner side of the plano-convex collective lens, through which greater bundling than in the azimuth is obtained in the elevation. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    It is an object of the present invention to provide a multi-beam radar sensor having improved angular detection capabilities. 
         [0009]    According to the present invention, this object is achieved in that, in a multi-beam radar sensor of the type described in the introduction, the preliminary focusing lens is disposed in such a way that it affects only a portion of the radiation transmitted and/or received by the antenna elements. 
         [0010]    In particular, the preliminary focusing lens has the effect that a portion of the beams emitted by the four antenna elements is focused before striking the main lens. This brings about a change in the aperture occupancy behind the lens such that a broader principal maximum occurs in the azimuth diagram, so that the precision of the determination of the azimuth angle is improved. In a conventional radar sensor of the same design but without preliminary focusing lens, the principal maximum in the azimuth diagram has a width of only approximately 20° (zero positions at ±10°), so that greater variance and thus reduced precision results in the angle estimation at azimuth angles in the vicinity of ±10°. On the other hand, in the radar sensor according to the present invention the first zero positions are shifted outwardly to a considerable extent, i.e., to higher angles, thereby allowing the determination of the azimuth angle with a significantly lower variance. 
         [0011]    The preliminary focusing lens may advantageously be disposed directly on the inner surface of the collective lens, e.g., a plano-convex lens. This allows a simple and cost-effective production. 
         [0012]    Since basically only the azimuth diagram of the radar sensor is relevant for the angle measurement, the preliminary focusing lens may be designed as vertically oriented cylindrical lens. 
     
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWING 
         [0013]      FIG. 1  shows a schematic representation of a radar sensor according to the present invention. 
           [0014]      FIG. 2  shows the radar sensor according to  FIG. 1  in a schematic side view. 
           [0015]      FIG. 3  shows an azimuth transmission diagram of the radar sensor according to the present invention. 
           [0016]      FIG. 4  shows an elevation transmission diagram of the radar sensor. 
           [0017]      FIG. 5  shows an azimuth receiving diagram for one of the beams of the radar sensor according to  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The radar sensor shown in  FIG. 1  has four antenna elements (patches)  12 ,  14 ,  16 ,  18  mounted on a shared mounting plate  10 . Spaced at a distance to mounting plate  10  is a collective lens  20  such as an aplanatic plano-convex lens whose planar side faces mounting place  10 , collective lens  20  having a refractive effect for radar radiation. The radiation emitted by all four antenna elements passes through collective lens  20  and is focused by it. 
         [0019]    In the radar sensor examined in this instance by way of example, a monostatic antenna principle is realized, i.e., each antenna element  12  acts both as transmit antenna and as receiving antenna. As a consequence, the radiation reflected by a radar sensor, which radiation was emitted by a specific antenna element, is also refocused by collective lens  20  in the direction of the particular antenna element, where it is received. 
         [0020]    Since antenna elements  12 ,  14 ,  16 ,  18  are uniformly offset with respect to each other in the horizontal direction, their principal radiation and sensitivity directions are angularly offset with respect to each other. As an example, the principal radiation and sensitivity direction of antenna element  14  in  FIG. 1  is indicated by a straight line A, which passes through antenna element  14  and through the center of collective lens  20 . It can be seen that straight line A forms a specific angle with the optical axis of collective lens  20 . This applies analogously to the other antenna elements as well. 
         [0021]    In addition, for antenna element  14 , the angular range that is spanned by collective lens  20  is marked by two straight lines a 1 . The radiation emitted into this angular range therefore passes through collective lens  20  and is focused by it. 
         [0022]    A plano-convex preliminary focusing lens (cylindrical lens)  22  is fixed in place on the planar inner side of collective lens  20  or is integrally formed with collective lens  20 . This preliminary focusing lens  22  has a vertical orientation so that its profile is visible in the plan view of  FIG. 1 . The width of cylindrical lens  22  is considerably smaller than the diameter of collective lens  20 , in particular less than one half of this diameter. Lines a 2  mark an angular range for antenna element  14  spanned by preliminary focusing lens  22 . This angular range is considerably smaller and lies completely within the angular range spanned by collective lens  20 . As a result, only a portion of the radar radiation emitted by antenna elements  12  through  18  passes through cylindrical lens  22  while another portion of this radar radiation passes through via the regions of collective lens  20  that lie outside of cylindrical lens  22  and thus remains unaffected by the cylindrical lens. 
         [0023]    As shown in  FIG. 2 , the height of preliminary focusing lens  22  in the vertical direction is also considerably smaller than the diameter of collective lens  20 , in particular less than one half of this diameter. 
         [0024]    Preliminary focusing lens  22  causes preliminary focusing of the radar beams for each antenna element  12  through  18 , for the particular central angular range a 2 . 
         [0025]    The azimuthal angular distribution of the radar radiation jointly emitted by all four antenna elements, generally known as azimuth diagram, is affected by interference between the signals that are emitted by the individual antenna elements.  FIG. 3  illustrates the manner in which preliminary focusing lens  22  changes the azimuth transmission diagram. 
         [0026]    Curve  24  in  FIG. 3 , represented as thinner lines, reproduces the azimuthal angular distribution of the transmission gain of all four antenna elements together, in those instances where no preliminary focusing lens is present. Because of interference between the various antenna elements, one obtains only a relatively narrow principal maximum, which is restricted by zero positions  26  at approximately ±10°. As a result, only a relatively weak signal is obtained for objects at an azimuth angle in the vicinity of ±10°, which makes a precise determination of the azimuth angle more difficult. 
         [0027]    On the other hand, curve  28  in  FIG. 3 , drawn as a bolder line, represents the angular distribution of the transmission gain for the radar sensor according to  FIG. 1 , which has preliminary focusing lens  22  in addition to collective lens  20 . It can be seen that in this instance the principal maximum is considerably broadened and that first zero positions  30  first occur at approximately ±15°. As a result, more precise angular measurements in a greater angular range are possible. 
         [0028]    Since preliminary focusing lens  22  is a cylindrical lens in the example illustrated, it predominantly affects the azimuth transmission diagram and to a lesser extent the elevation transmission diagram, which is shown in  FIG. 4 . Here as well, curve  32  shown as thinner line represents the radar sensor without preliminary focusing lens, and curve  34  represents the radar sensor including the preliminary focusing lens. 
         [0029]      FIG. 5  illustrates the receiving diagram for an individual antenna element, i.e., antenna element  14  in  FIG. 1 . Curve  36  shows the receiving diagram for the radar sensor without preliminary focusing lens, and curve  38  shows the diagram for the sensor including preliminary focusing lens  22 . Here, too, it can be seen that the first secondary minima  40  in the diagram with the preliminary focusing lens are less pronounced than in the diagram without preliminary focusing lens and that the first zero positions are located at a clearly greater distance from the main sensitivity direction.