Abstract:
A radar sensor for motor vehicles has a transmitting antenna in the form of a planar array antenna having a plurality of juxtaposed antenna elements, and a supply network for supplying microwave power to the antenna elements, wherein the supply network is developed to supply the antenna elements with the microwave power having a phase shift increasing at constant increments from one end of the row to the other.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is the national stage entry of International Patent Application No. PCT/EP2011/062195, filed on Jul. 18, 2011, which claims priority to Application No. DE 10 2010 040 692.9, filed in the Federal Republic of Germany on Sep. 14, 2010. 
     FIELD OF INVENTION 
     The present invention relates to a radar sensor for motor vehicles, having a transmitting antenna in the form of a planar array antenna having a plurality of juxtaposed antenna elements, and having a supply network for supplying microwave power to the antenna elements. 
     BACKGROUND INFORMATION 
     Antennas for radar sensors, which are provided for use in motor vehicles, are frequently designed as patch antennas on an HF substrate. This permits a cost-effective construction of the radar sensor. When array antennas are used, the desired directional characteristic of the radar sensor is able to be achieved in azimuth and/or in elevation, without requiring a radar lens. Separate antennas are frequently used for the radiation of the radar signal and for receiving the reflected signal. The desired directional characteristic of the transmitting antenna in azimuth is achievable by supplying the microwave power to the plurality of antenna elements juxtaposed on the substrate in the same phase. A radar lobe is then created by interference, whose main radiation direction is oriented at a right angle to the plane of the substrate, and which covers an azimuth angle range of about −45° to about +45°. On the receiving side also a plurality of juxtaposed antenna elements or patches are used, which belong, however, to different receiving channels, so that, based on the phase differences between the signals received by the different antenna elements, one is able to draw a conclusion on the azimuth angle of the object. 
     SUMMARY 
     The present invention relates particularly to a rear area radar sensor system for motor vehicles, for instance, in an LCA system, (lane change aid), which supports the driver during a lane change by warning of vehicles that are approaching from behind in one&#39;s own lane or the passing lane. In this case, the radar sensor system has to have a big range in the backward direction, so that even rapid vehicles are able to be detected in time, and, on the other hand, it has to be in a position to locate vehicles that are located at a small distance or nearly at the same level on the passing lane, and are consequently at a blind spot for the driver. 
     It is an object of the present invention to create a simply constructed and cost-effective radar sensor system, which permits fulfilling the above-mentioned requirements. 
     According to the present invention, this object is attained by a radar sensor of the type named at the outset, in which the supply network is developed to supply to the antenna elements the microwave power at a phase shift that increases at constant increments from one end of the row to the other. 
     By interference between the radar waves radiated by the various antenna elements, there then develops an asymmetrical antenna diagram, so that a large part of the microwave power is radiated at high intensity in a certain direction, and at the same time a smaller part of the microwave power is radiated to one side at a high azimuth angle. In this way, it is possible to detect the following traffic in one&#39;s own lane and in the passing lane, all the way into the blind spot, using a single radar sensor. 
     In one preferred exemplary embodiment, the supply network is developed so that the amplitude of the emitted microwaves also varies from antenna element to antenna element, for example, that it decreases from one end of the row of antenna elements to the opposite end. The power distribution of the emitted radar radiation is thereby equalized over the azimuth angle, so that position-finding gaps between the main lobe and the side lobes are closed to a great extent. 
     In the following, exemplary embodiments of the present invention are explained in detail with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a plurality of antenna elements situated in a horizontal line on a substrate that is not shown, with an indication of an example of the phase and amplitude configuration of the individual antenna elements. 
         FIG. 2  is an antenna diagram for the antenna system and the phase and amplitude configuration according to  FIG. 1 . 
         FIG. 3  is a block diagram of a radar sensor according to an exemplary embodiment of the present invention. 
         FIG. 4  is a locating diagram of an LCA radar sensor according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows four antenna elements  10 ,  12 ,  14 ,  16 , which are situated in a horizontal row at uniform distances on an HF substrate that is not shown. The antenna elements are shown here as single patches. Via a supply network that will be described in greater detail below, the antenna elements obtain a microwave signal, which is then supposed to be radiated as radar radiation. The average distance d of the antenna elements, in the example shown, amounts to one-half of a wavelength of the microwave radiation (d=λ/2). 
     The phase and amplitude configuration of antenna elements  10 ,  12 ,  14 ,  16  are also given in  FIG. 1 . With reference to antenna element  10  at the left end of the row (phase=0°), second antenna element  12  has a phase shift of 60°, third antenna element  14  has a phase shift of 120° and fourth antenna element  16  has a phase shift of 180°. The phase shift thus increases at the same increments (60°), and antenna elements  10  and  16  at the opposite ends of the row obtain signals that are 180 degrees out of phase. 
     The amplitude of the signals decreases linearly over the row of antenna elements from left to right. If the amplitude of the outermost left antenna element  10  is normalized to 1.0, the amplitude decreases to the right, from antenna element to antenna element. In the example shown, the amplitude decreases degressively to 0.7 for antenna element  12 , 0.5 for antenna element  14 , and finally 0.35 for antenna element  16 . 
       FIG. 2  shows the antenna diagram that results from the phase and amplitude configuration shown in  FIG. 1 . Curve  18  in  FIG. 2  gives the relative power of radar radiation emitted by antenna elements  10 ,  12 ,  14 ,  16  as a function of the azimuth angle. Because of interference between the radiation proportions emitted by the individual antenna elements, a clear maximum occurs at an azimuth angle of about 20°. For larger azimuth angles, the power drops off. By contrast, in the range of +20° to −90°, there are some side maxima, so that the power remains at a relatively high level until in the range of about −60°. Because of the nonuniform amplitude configuration according to  FIG. 1 , it is achieved that the minima in the antenna diagram are marked fairly weakly. 
       FIG. 3  shows a detailed circuit diagram of the essential components of a radar sensor having a transmitting antenna system according to  FIG. 1 . 
     The four antenna elements  10 ,  12 ,  14 ,  16  together form a transmitting antenna Tx. Three additional antenna elements  20  are situated at irregular lateral distances and together form a receiving antenna Rx. Antenna elements  10 - 16  and  20  are each made up of a column of patches  22 , into which the microwave signals are connected in phase. Therefore, by interference, in elevation one obtains a directional characteristic having a marked main maximum at elevation angle 0° (at right angles to the substrate). This main maximum extends over an angular range of about −45° to about +45°. By contrast, side lobes are developed only weakly. 
     By contrast, in azimuth, the directional characteristic of transmitting antenna Tx corresponds to the antenna diagram according to  FIG. 2 , so that overall one obtains a radar beam that is vertically bundled but horizontally asymmetrically fanned out, without using a radar lens. 
     The microwave power for transmitting antenna Tx is generated by an oscillator  24  and is supplied to the individual antenna elements  10 ,  12 ,  14 ,  16  via a parallel supply network  26 . This network branches from the output of oscillator  20  first into two branches  26 , which in their length differ by λ/3, as a third of wavelength λ. Each branch  26  then branches again into two branches  30  and  32  having a difference in length of λ/6 each. In this way, the phase configuration shown in  FIG. 1  is achieved. In order to set the amplitude configuration, in each case one of branches  28 ,  30  and  32  includes a so-called impedance transformer  34 , using which the power passed on to the respective antenna elements are adjusted by the desired quantity. 
     The three antenna elements  20  of receiving antenna Rx are connected to a three-channel mixer  36 , which mixes the signal received from each individual antenna element  20  with the transmitted signal supplied by oscillator  24 . At outputs  38  of three-channel mixers  36 , one obtains, as mixed products, the intermediate frequency signals, whose frequency corresponds to the frequency difference between the radiation emitted by transmitting antenna Tx and the radiation received at the same time from respective antenna element  20  of receiving antenna Rx. Since the frequency of oscillator  24  is ramp modulated, (FMCW radar: frequency-modulated continuous wave), the frequency of the intermediate frequency signals is a function both of the signal running time, and thus of the distance of the located object, and of the Doppler shift, and thus of the relative speed of the object. The phase differences between the intermediate frequency signals represent corresponding phase differences between the radar echos which are received from the various antenna elements  20 . These phase differences are functions of the different length of the signal paths to juxtaposed antenna elements  20 , and therefore give insight into the azimuth angle of the located object. 
     The evaluation of the intermediate frequency signals is known per se and will not be further discussed here. 
     Antenna elements  10 ,  12 ,  14 ,  16  of the transmitting antenna and antenna elements  20  of the receiving antenna, as well as supply network  26  may be formed on a common substrate in microstrip technology, which also accommodates three channel mixer  36  and oscillator  24 , as well as possibly further components of the radar sensor. 
       FIG. 4  shows a locating field  40  of a radar sensor  42  according to the present invention. Radar sensor  42  is installed in the rear of a motor vehicle  44  in such a way that azimuth angle 0° of the y′ axis corresponds to an orthogonal coordinate system (x′, y′), which is somewhat rotated compared to a vehicle coordinate system (x, y) (the y axis corresponds to the backwards travel direction of the vehicle). 
     In the example shown, radar sensor  42  is a rear area radar sensor, which is a part of an LCA system which warns the driver of the following traffic during an intentional lane change. In this example, the following traffic is made up of vehicles  46 ,  48 , which are approaching on the passing lane (in this case, for left-hand driving). The radar sensor is oriented so that its far-reaching main lobe (by an azimuth angle of +20°) covers the passing lane and a large part of the own lane of vehicle  44 . Thus, for example, vehicle  46  is able to be detected early. Vehicle  48  has just started to pass, and is located in a blind spot, as far as the driver of vehicle  44  is concerned. Because of the asymmetrical form of the locating field, however, even vehicle  48  is still able to be detected.