Abstract:
A radar sensor for motor vehicles, having an antenna system that can be controlled by a control device so that it has a temporally varying directional characteristic, and having an evaluation device for evaluating the radar echoes received by the antenna system and for the location of objects using angular resolution, wherein the antenna system has at least two groups of antenna elements that differ in elevation in their effective direction, and the control device is fashioned to activate and deactivate the two groups in periodically alternating fashion, and the evaluation device is configured to estimate the elevation angle of the objects on the basis of a contrast between the radar echoes received by the various groups.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a radar sensor for motor vehicles, having an antenna system that can be controlled by a control device in such a way that it has a temporally varying directional characteristic, and having an evaluation device for evaluating the radar echoes received by the antenna system and for the location of objects using angular resolution. 
     BACKGROUND INFORMATION 
     Radar sensors for motor vehicles are used to acquire the surrounding traffic conditions in the context of driver assistance systems, for example for radar-supported distance regulation (ACC; Adaptive Cruise Control). A driver assistance system of this type is discussed for example in the publication “Adaptive Fahrgeschwindigkeitsregelung ACC (Adaptive Speed Regulation ACC),” Robert Bosch GmbH, Gelbe Reihe series, 2002 ed., Technische Unterrichtung. In addition to distance and relative speed, an important measurement quantity of the radar sensor is also the angle of the located objects. 
     Here, both the horizontal angle (azimuth angle) and the vertical angle (elevation angle) are important. The azimuth angle is used to estimate the transverse offset, and is thus used for lane assignment. The elevation angle makes it possible to distinguish between objects that can be driven under or driven over and objects that are genuine obstacles. Thus, in particular in safety applications (PSS; Predictive Safety Systems), false alarms due to metallic objects such as manhole covers, metal cans on the road surface, and the like can be avoided. 
     The azimuthal angular resolution capacity is in most cases achieved in that a plurality of radar lobes are produced having an angular offset from one another, in which the radar echoes are evaluated in separate channels. Scanning radar systems are also known in which the radar lobe is pivoted in the horizontal direction. An estimation of the elevation angle is possible for example through mechanical pivoting of the radar sensor in the vertical direction. For reasons of cost, however, the elevation angle is usually determined only indirectly, via a temporal evaluation of the back-scatter characteristic of objects. 
     For use in radar sensors for motor vehicles, so-called planar antenna devices or patch antennas are particularly suitable, because, due to their flat configuration, they can be produced easily and at low cost, for example using an etching method. Such an antenna device is typically a planar configuration of radiating resonators on an RF substrate, each resonator being assigned a particular amplitude and phase. The directional characteristic of the antenna system then results through superposition of the radiation diagrams of the individual patch elements. 
     German patent document DE 102 56 524 A1 discusses a device for measuring angular positions using radar pulses and overlapping radiation characteristics of at least two antenna elements. 
     SUMMARY OF THE INVENTION 
     An object of the exemplary embodiments and/or exemplary methods of the present invention is to create a radar sensor for motor vehicles that, with a relatively simple configuration, makes possible an estimation of the elevation angle of the located objects. 
     According to the exemplary embodiments and/or exemplary methods of the present invention, this object may be achieved in that the antenna system has at least two groups of antenna elements that differ in elevation in their effective direction, and that the control device is fashioned to activate and deactivate the two groups in a periodic exchange, and that the evaluation device is configured to estimate the elevation angle of the objects on the basis of a contrast between the radar echoes received by the various groups. 
     Due to the alternating activation of the two groups of antenna elements, the radar lobe is periodically pivoted vertically, so that a larger elevation angular area can be covered without loss of sensitivity and range. In general, the angular deviation between the two radar lobes can here be less than the angular expansion of a single radar lobe in its elevation, so that an object situated in front of the vehicle always remains in the field of view of the radar sensor, independent of which of the two groups of antenna elements is active at the moment. Consequently, the development of the distance, relative speed, and azimuth angle of the object can take place with a high degree of temporal resolution, and the simultaneous estimation of the elevation angle (with lower temporal resolution) supplies important additional information about the object, such as whether the object can be driven over or not, whether it is a truck or passenger vehicle, and the like. Changes in the inclination of the road surface, for example driving over a bump or driving through a dip, can in this way be recognized on the basis of the change in elevation angle of a vehicle traveling in front. 
     The estimation of the elevation angle can easily be achieved through the alternating activation and deactivation of two groups of antenna elements, and thus requires neither mechanical pivoting of the radar sensor nor the use of expensive phase shifting elements to change the phase relationships. 
     Advantageous embodiments of the present invention are indicated in the further descriptions herein. 
     Although in principle each “group” of antenna elements can also be made up of a single antenna element, each group may be made up of a plurality of antenna elements that are connected to a common supply network whose configuration determines the phase relationship between the individual antenna elements and thus determines the directional characteristic of the group. Optionally or in addition, however, lens elements may also be used to influence the directional characteristic. 
     In an advantageous specific embodiment, a bistatic antenna configuration is realized, i.e. separate antenna elements are provided for the transmission of the radar signal and the reception of the radar echo. For example, in order to receive the radar echo a plurality of antenna elements can be provided that are offset horizontally relative to the optical axis of a lens, so that a plurality of angularly offset radar lobes are produced that permit a determination of the azimuth angle of the objects. 
     In the following, an exemplary embodiment is explained in more detail on the basis of the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a block diagram of a radar sensor according to the present invention. 
         FIG. 2  shows a drawing explaining the functioning of the radar sensor in the estimation of the elevation angle of an object. 
         FIG. 3  shows elevation diagrams for two groups of antenna elements in the radar sensor according to  FIG. 1 . 
         FIG. 4  shows a contrast curve that permits an estimation of the elevation angle. 
     
    
    
     DETAILED DESCRIPTION 
     Radar sensor RS shown in  FIG. 1  has an antenna system having three groups Rx, Tx1, Tx2 of antenna elements  10 ,  12 ,  14 . Antenna elements  12 ,  14  of groups Tx1 and Tx2 are used to transmit, in alternating fashion, a radar signal produced by a local oscillator  16 . Antenna elements  12  of group Tx1 are configured in a plurality of columns that are oriented vertically and that each includes a plurality of antenna elements situated at uniform distances from one another. Via a supply network  18 , the radar signal that is to be transmitted is distributed to the individual columns, and is then fed into the individual antenna elements  12  serially within each column. The columns have uniform horizontal distances from one another. Supply network  18  is configured such that all antenna elements  12  of group Tx1 are controlled with the same phase, so that the superposition of the radiation emitted by the individual antenna elements results in a bundling effect both in the azimuth and in the elevation. The main direction of radiation is perpendicular to the plane in which antenna elements  10 ,  12 ,  14  are situated, for example on a common radio-frequency substrate. So that the individual antenna elements  12  of a column are excited with the same phase, spacing d1 between two adjacent antenna elements  12  within a column agrees with wavelength λ on the radio-frequency substrate (or is a whole-number multiple thereof). 
     The configuration of antenna elements  14  in group Tx2 is in principle the same as in group Tx1, but with the difference that spacing d2 between adjacent antenna elements  14  within a column differs from wavelength λ. In the depicted example, it is larger than this wavelength. This has the consequence that successive antenna elements  14  within each column have a particular phase shift from one another, so that superposition results in a radar lobe K 2  ( FIG. 2 ) whose main direction of radiation is pivoted in its elevation by a particular angle relative to the main direction of radiation of radar lobe K 1  produced by group Tx1. The number of antenna elements  14  per column and the number of columns can here be the same as for antenna elements  12  of group Tx1, so that the bundling of radar lobe K 2  in elevation essentially agrees with the bundling of radar lobe K 1 , and both radar lobes are also essentially equally strongly bundled in the azimuth. In addition, in group Tx2 the individual columns of antenna elements  14  are controlled with the same phase via a supply network  20 , so that the main direction of radiation of radar lobe K 2  in the azimuth is perpendicular to the substrate. 
     If the radar signal emitted either by group Tx1 or group Tx2 impinges on an object  22  ( FIG. 2 ), for example a vehicle traveling in front, a part of the radar radiation is reflected and undergoes a Doppler shift that is a function of the relative speed of the object, and the reflected signal is then received by antenna elements  10  of group Rx. Antenna elements  10  of this group Rx are configured in four columns and are connected to one another in series within each column. Each column forms a receive channel and is connected to an input of a four-channel mixer  24 . Oscillator  16  supplies the same signal to a different input of this four-channel mixer  24  that is also communicated to supply network  18  or  20 . The signal received by each antenna column is mixed with the signal of local oscillator  16 . Four-channel mixer  24  thus supplies, as a mixed product, four intermediate frequency signals Z1-Z4 whose frequency corresponds in each case to the frequency difference between the received signal and the signal of local oscillator  16 . 
     Corresponding to the configuration of an FMCW (Frequency Modulated Continuous Wave) radar unit, the frequency of oscillator  16  is modulated with a ramp shape (distance d1 between antenna elements  12  therefore corresponds, strictly speaking, to the average wavelength of the transmitted signal). The frequency of the radar echo received by antenna elements  10  therefore differs from the signal of the local oscillator by an amount that is a function on the one hand of the signal runtime from the radar sensor to the object and back, and on the other hand, due to the Doppler effect, of the relative speed of the object. Correspondingly, intermediate frequency signals Z1-Z4 also contain information about the distance and relative speed of the object. In the frequency modulation, rising and falling frequency ramps alternate, and by once adding and once subtracting the intermediate frequency signals on the rising ramp and on the falling ramp, the portions that are a function of distance and the portions that are a function of speed can be separated from one another, so that values are obtained for distance D and relative speed V of each located object. 
     Intermediate frequency signals Z1-Z4 are supplied to an evaluation device  26 , and are there recorded channel-by-channel, in each case over the duration of a frequency ramp, and are analyzed to form a spectrum using fast Fourier transformation. In this spectrum, each object is identified by a peak at the frequency determined by the respective object distance and relative speed. 
     The radar echoes received by the various columns of group Rx have a phase shift from one another that is a function of the respective azimuth angle φ of the object. Due to the bundling of the signal sent by group Tx1 or Tx2, the amplitude of the received radar echo is also a function of the azimuth angle of the object. Through comparison of the amplitude and phase differences with a corresponding antenna diagram, it is therefore also possible for azimuth angle φ to be determined in evaluation device  26 . 
     An electronic control device  28  controls not only the frequency modulation of oscillator  16 , but also causes the oscillator to send the signal that is to be transmitted to group Tx1 and to group Tx2 in alternating fashion. The active and inactive phases of groups Tx1 and Tx2 thus alternate periodically, for example with a period that corresponds to a complete cycle of the rising and falling frequency ramps of oscillator  16 . The signal of control device  28 , which brings about the changeover between supply networks  18  and  20 , is also supplied to a contrast calculation unit  30  that moreover receives a signal P from evaluation device  26 . For each located object, signal P indicates the strength (power) of the radar echo, for example averaged over all four channels. In the periods in which group Tx1 is used to send the radar signal, a power P1 is obtained in this way for a particular object, and in the periods in which group Tx2 is used to send the radar signal a power P2 is obtained for the same object. In contrast calculating unit  30 , a contrast K is now calculated using the following equation:
 
 K =( P 1− P 2)/( P 1+ P 2)
 
     On the basis of contrast K calculated in this way, in an elevation angle estimating unit  32  an estimated value can then be calculated for elevation angle α of the object, as is explained below with reference to  FIGS. 2 through 4 . 
     In  FIG. 2 , it is assumed that radar sensor RS is installed in a vehicle in such a way that the substrate on which antenna elements  10 ,  12 ,  14  are situated is oriented vertically. The main direction of radiation of radar lobe K 1  produced by group Tx1 is then horizontal (corresponding to elevation angle α=0). In  FIG. 3 , curve K 1 ′ shows the corresponding angular distribution of the intensity of radar lobe K 1 . It will be seen that the maximum is at elevation angle 0°. 
     In contrast, due to the phase shift between antenna elements  14  of each column, group Tx2 produces radar lobe K 2  whose main direction of radiation is inclined upward by a particular angle. The corresponding angular distribution of the intensity is shown by curve K 2 ′ in  FIG. 3 . It will be seen that here the maximum is at an elevation angle of 5°. 
     In the example shown in  FIG. 2 , object  22  is situated at an elevation angle α of approximately 4°. The consequence is that in the periods in which group Tx2 is active a relatively strong radar echo is obtained, because object  22  is situated approximately in the center of radar lobe K 2 , whereas in periods in which group Tx1 is active, a significantly weaker signal is obtained, because object  22  is situated more at the edge of corresponding radar lobe K 1 . Therefore, contrast K, calculated according to the above-indicated equation, is negative in this example. 
     According to the elevation diagrams shown in  FIG. 3 , for each elevation angle an associated value of contrast K can be calculated. The relation between contrast K and elevation angle α is indicated in  FIG. 4  by curve E. On the basis of this curve, elevation angle α of the located object can then be determined in elevation angle estimating unit  32 . 
     In  FIG. 1 , contrast calculating unit  30  and elevation angle estimating unit  32  are shown as separate units. In practice, however, these units are usually formed by software modules of an electronic data processing system that also carries out the functions of evaluating device  26  and of control device  28 .