Abstract:
A radar sensor for motor vehicles includes a plurality of transmission and receiving antennas, which differ in their azimuthal directivity characteristic and to which a separate mixer is assigned, which mixes a transmitted signal with a received signal, at least one of the mixers being a transfer mixer, and at least one other of the mixers having a lower transfer output, wherein the assignment of the transmission and receiving antennas is asymmetrical with respect to the mixers differing in their transfer output.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a radar sensor for motor vehicles, having a plurality of transmission and receiving antennas which differ in their azimuthal directivity characteristics and to which a separate mixer is assigned, which mixes a transmitted signal with a received signal, at least one of the mixers being a transfer mixer and at least one other mixer having a lower transfer power. 
     BACKGROUND INFORMATION 
     Radar sensors are used in motor vehicles in connection with driver assistance systems for the purpose of locating objects in the environment of the vehicle, in particular vehicles driving ahead, and for the purpose of measuring their distances and relative speeds. On the basis of the location data of such a radar sensor, an ACC system (adaptive cruise control), for example, not only is able to automatically regulate the speed of the own vehicle to a specific desired speed, but if a preceding vehicle has been located, also to automatically adapt the own vehicle to the speed of the vehicle driving ahead, so that this vehicle is followed at a suitable safety distance. Another application example for radar sensors in motor vehicles is a predictive safety system (PSS), which detects a looming collision as early as possible and automatically initiates measures by which the collision is averted, if possible, or by which the consequences of the collision are mitigated to the greatest extent possible. 
     Radar sensors in which one and the same antenna element is used both for transmitting the radar signal and also for receiving the signal reflected at an object are referred to as monostatic radar sensors. An associated mixer mixes a portion of the transmitted signal with the received signal and thereby generates an intermediate frequency signal whose frequency corresponds to the frequency difference between the transmitted and the received signal. Because of the Doppler effect, this frequency difference is a function of the relative speed of the reflecting object. In the case of a radar system in which the frequency of the transmitted signal is modulated continuously, e.g., in an FMCW radar (frequency modulated continuous wave), the frequency difference is also dependent on the propagation time of the signal, so that the use of known evaluation technologies makes it possible to derive from the location signal of an object both its distance and the relative speed. 
     Moreover, a multi-beam radar sensor having a plurality of transmission and receiving antennas that differ in their azimuthal directivity characteristic allows a determination of the azimuth angle of the located objects. To influence the directivity characteristic, for instance, an optical lens which refracts radar waves may be provided and/or it is possible to utilize diffraction and interference effects of the involved antenna elements. In a radar sensor having an optical lens, for example, the plurality of transmission and receiving antennas is situated along a horizontal line at a slight offset relative to the optical axis of the lens, so that their main radiation directions and, consequently, their main sensitivity directions, differ slightly from each other. By comparing the amplitudes and/or phases of the signals received from the different paths (i.e., from the different transmission and receiving antennas), it is then possible to determine the azimuth angle of the object, i.e., the directional angle under which the object is seen from the direction of the sensor. 
     In the case of advanced radar systems for motor vehicles, the system must respond not only to moving objects, i.e., in particular to other moving vehicles, but also to stationary objects such as stopped vehicles or other obstacles on the road. While moving vehicles are relatively easy to detect based on their own motion and the corresponding difference between the measured relative speed and the driving speed of the own vehicle, it is much more difficult to evaluate the relevance of the objects to the particular task of the driver assistance system in the case of stationary objects. In an environment rich in structures, e.g., in city driving, it must also be taken into account that a considerable number of stationary objects will generally be located within the viewing range of the radar sensor, which complicates the evaluation of the multitude of signals considerably. 
     For these reasons it is desirable, if possible, to set up the visual range or the field of view of the radar sensor in such a way that from the outset, if possible, only signals from objects are received that are actually also relevant to the task to be performed by the driver assistance system. In particular, the field of view should therefore be set up in such a way that, if possible, no radar echoes are received from objects located far beyond the traffic lane. On the other hand, a radar sensor (LRR: long range radar) for an automatic cruise control system (ACC), for instance, should have the largest possible range, on the order of magnitude of approximately 200 m or more. Because of the unavoidable divergences of the radar beams, at greater distances it will then be nearly impossible to restrict the field of view to the immediate road lane area. 
     In the case of a multi-beam radar, e.g., in a radar sensor having four transmission and receiving antennas, it is described in DE 10 2004 030 755 to symmetrically reduce the transmission power of the two outer antennas in comparison with the transmission power of the two inner antennas. In this manner a large range is able to be achieved by the two inner antennas, and a still relatively small field of view for large distances, while the weaker margin radiation ensures a sufficient width of the field of view at shorter distances. 
     The transmission power of the different beams is able to be set via the configuration of the associated mixers, for instance. Radar sensors of the type examined here typically use transfer mixers in which a portion of the transmitted signal used for the mixing with the received signal is transferred to the line leading to the antenna and therefore is emitted via the antenna. On the other hand, so-called isolation mixers are known where virtually no transfer of the transmitted signal to the antenna line takes place. The antenna to which such a mixer is assigned is therefore only able to receive signals transmitted from other antennas, and these received signals are then mixed with the transmitted signal in the mixer. Any possible graduation between pure transfer mixers and pure isolation mixers is possible in this context. The relative strength of the signal transferred onto the antenna line will be called the “transfer output” here. 
     SUMMARY 
     Example embodiments of the present invention provide a radar sensor for motor vehicles, by which a shape of the field of view is able to be achieved that is easily adaptable to the particular application. 
     In example embodiments of the present invention, the assignment of the transmission and receiving antennas is asymmetrical with respect to the mixers that differ in their transfer output. 
     As a result, there are at least two transmission and receiving antennas in the radar sensor, which are situated symmetrically relative to one another with respect to the center axis of the sensors, and in which the associated mixers differ in their transfer output. The result is an asymmetrical field of view. 
     Example embodiments of the present invention therefore make it possible to produce in a simple manner radar sensors having an asymmetrical field of view, in particular using the same components from which conventional radar sensors having a symmetrical field of view are made up as well, so that the field of view is optimally adaptable to the particular task of the individual sensor. 
     A particular use for example embodiments of the present invention includes fitting a motor vehicle with two radar sensors, which are situated on the left and right side of the vehicle and have asymmetrical fields of view, e.g., mutually mirror-inverted fields of view. Superpositioning two fields of view provides an overall broader field of view without losses in range. This considerably reduces the dead angle in the near range. 
     Moreover, it is especially advantageous that the overlap of the fields of view of the two radar sensors results in high redundancy in the range especially relevant for the cruise control, for example in the region of the azimuth angle of zero and at large and medium distances, which facilitates so-called tracking of the objects from measuring cycle to measuring cycle. Furthermore, by comparing the azimuth angles obtained from the different radar sensors for the same object, the precise location of the object is able to be determined more accurately. 
     Example embodiments of the present invention may be used for sensors having any number of transmission and receiving antennas. In the same manner, any possible combination of intermediate forms between pure transfer mixers and pure isolation mixers are possible for the mixers. 
     Exemplary embodiments of the present invention are depicted in the drawing and described in greater detail in the description below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an ideal transfer mixer; 
         FIG. 2  is a block diagram of an ideal isolation mixer; 
         FIG. 3  is a block diagram of a radar sensor according to an example embodiment of the present invention; 
         FIG. 4  is an antenna diagram of the radar sensor according to  FIG. 3 ; 
         FIG. 5  is a diagram (not true to scale) of the field of view of the radar sensor according to  FIG. 3 ; and 
         FIG. 6  is a schematic layout of a motor vehicle having two radar sensors according to an example embodiment of the present invention and associated fields of view. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a simplified circuit diagram of an antenna  10  and an associated mixer  12  of a radar sensor. Via a supply line  14 , mixer  12  receives a transmitted signal S from an oscillator  16  shown in  FIG. 3 , e.g., a signal having a frequency on the order of 76 GHz, for example. Via an antenna line  18 , which connects mixer  12  to antenna  10 , the mixer receives a signal E received by antenna  10 , and by mixing these two signals the mixer generates an intermediate frequency signal Z, which is output via an output line  20  to an evaluation circuit (not shown). 
     In practice, mixer  12  is an integrated component (MMIC: monolithic microwave integrated circuit), which is configured as transfer mixer in this instance. This means that a portion of the output of transmitted signal S is transferred to antenna line  18  and then emitted via antenna  10 , so that antenna  10  is operating as transmission and receiving antenna. The corresponding signal flows are symbolized by arrows in  FIG. 1 . It can be seen that two signal flows, in opposite directions, occur on antenna line  18 . Accordingly, a double arrow A symbolizes that antenna  10  transmits as well as receives radar radiation. 
       FIG. 2  shows a corresponding circuit diagram for an antenna  10 ′ and an associated mixer  12 ′. The difference from the circuit diagram from  FIG. 1  consists only of the fact that mixer  12 ′ is configured as isolation mixer. This means that no output is transferred from supply line  14  to antenna line  18  (in the ideal case). Antenna  10 ′ thus operates as pure receiving antenna in this idealized case, which is symbolized by a single arrow B. As a consequence, there is also only one signal flow, in one direction, on antenna line  18 , i.e., in the direction toward mixer  12 ′. 
     In practice, it is also possible for mixer  12 ′ according to  FIG. 2  to transfer a small portion of the output of transmitted signal S to antenna line  18 , so that antenna  10 ′ also transmits a weak signal. In general, MMICs are able to be configured such that any intermediate form between a pure transfer mixer and a pure isolation mixer is realized, i.e., the transfer output, that is to say, the portion of the output that is transferred onto antenna line  18 , is able to be adjusted according to the requirements. 
       FIG. 3  shows a block diagram of a radar sensor  22  according to an example embodiment of the present invention. A total of four antennas  10 ,  10 ′ is situated on a mounting plate  24  such that they lie at regular intervals on a line that is horizontal in the installation position. Situated at a distance from antennas  10  is a shared lens  26  such that the antennas lie approximately in the focal plane of the lens. Lens  26  defines an optical axis  28  of the radar sensor. Since antennas  10  are offset at different distances with respect to optical axis  28 , lens  26  bundles their radiation into four beams  30   a - 30   b , which are irradiated at slightly different azimuth angles. 
     A separate mixer  12 , which operates as transfer mixer, is assigned to the two upper antennas  10  in  FIG. 3 , which generate right beams  30   a  and  30   b  in the emission direction. In contrast, a separate mixer  12 ′, which has a considerably lower transfer output and therefore is referred to as isolation mixer in abbreviated form here, is assigned to the two other antennas  10 ′. Double arrows or arrows in  FIG. 3  symbolize that antennas  10  are operating as transmission and receiving antennas, while antennas  10 ′ operate predominantly as receiving antennas and in so doing, receive the echo of signals transmitted by the two other antennas  10 . 
     The special feature of radar sensor  22  described here consists of the fact that the placement of antennas  10  and  10 ′, which differ in the transfer output of associated mixers  12 ,  12 ′, is asymmetrical relative to optical axis  28  of the sensor. In the illustrated example two antennas  10  having a strong transmission power lie on one side of the optical axis, while two antennas  10 ′ having a weak transmission power or no transmission power at all are situated on the other side of the optical axis. 
     The associated antenna diagram is illustrated in  FIG. 4 . Curve  32  in  FIG. 4  indicates the transmission gain of radar sensor  22  as a whole as a function of the azimuth angle. Since the transmitted radar radiation stems only from the two antennas  10 , which have an offset to the same side relative to optical axis  28 , curve  32  has its maximum not at the azimuth angle of 0°, but at a particular azimuth angle α 0 , which amounts to approximately +6° in the example illustrated. With regard to main radiation angle α 0 , the transmission gain indicated by curve  32  is substantially symmetrical, inasmuch as practically only the two antennas  10  whose mixers  12  have an identical configuration and transfer signals of approximately equal strength to the antennas contribute to this gain. 
     Curves  34   a - 34   d  in  FIG. 4  indicate the two-way gain (transmission and receiving gain) for each individual beam  30   a - 30   d . Curves  34   a  and  34   b , which are associated with the transmitting antennas  10 , have the highest main maxima, and these maxima lie symmetrically with respect to main radiation angle α 0 . The reason for this is that these antennas have their greatest sensitivity in the direction in which their transmission gain is greatest as well. 
     On the other hand, curves  34   c  and  34   d , which are associated with non-transmitting antennas  10 ′, have weaker main maxima, which in addition are asymmetrical with respect to main radiation angle α 0 . Furthermore, in curve  34   d  the main maximum is smaller than in curve  34   c , because the antenna associated with curve  34   d  has its greatest sensitivity for a direction into which only relatively little output is radiated by the two transmitting antennas  10 . 
     To make it possible for an object situated at a specified distance from radar sensor  22  to be located by the radar sensor in the first place, the two-way gain of at least one of the four antennas must lie above a specific detection threshold. One example of such a detection threshold is shown in  FIG. 4  in the form of a horizontal line  36 . The points of intersection of this detection threshold and curves  34   a  and  34   d , which are associated with the two edge beams  30   a  and  30   d  of the radar sensor, define interval limits α 1 , α 2  of a viewing range that radar sensor  22  has for this specified distance. It can be seen that this viewing range is asymmetrical not only with respect to azimuth angle 0°, but with respect to main radiation angle α 0  as well. Left limit al of the viewing range has an angular distance of approximately 18° from α 0 , while right limit α 2  has a distance of only approximately 8° from α 0 . 
     If a viewing threshold that corresponds to viewing threshold  36  is determined for each conceivable object distance and the associated limits of the viewing range, then the overall result is a field of view  38  as it is illustrated for the sensor described here in  FIG. 5 . Here, field of view  38  is shown in a coordinate system whose X-axis, denoted by “distance”, extends parallel to the longitudinal vehicle axis, while the Y-axis, denoted by “lateral offset”, corresponds to the transverse axis of the vehicle. In this example it is assumed that radar sensor  22  is installed in the vehicle such that the longitudinal vehicle axis (Y-axis) does not correspond to the azimuth angle 0° but instead corresponds to main radiation angle α 0 . In other words, the angular deviation of the main radiation direction caused by the offset of the two transmitting antennas  10  is largely compensated by the corresponding installation of the radar sensor. 
     Not compensated, however, is the asymmetry resulting from the different assignments of the antennas to mixers  12  and  12 ′, which leads to the asymmetrical appearance of curves  34   a - 34   d  in  FIG. 4 . This asymmetry can also be seen in the field of view in  FIG. 5 . In particular, it is noticeable that the field of view on the right side extends only up to lateral offsets of approximately 15 m, while on the left side, it extends up to lateral offsets of approximately 25 m. On the left side, the field of view has two “protuberances”  38   c ,  38   d , which correspond to the main maxima of curves  34   c  and  34   d  in  FIG. 4  and are missing on the right side. In the vicinity of the main radiation direction, field of view  38  has two main lobes  38   a ,  38   b , which correspond to the main maxima of curves  34   a ,  34   b  in  FIG. 4 . 
       FIG. 6  schematically shows the layout of a motor vehicle  40 , in which two radar sensors  22 L and  22 R are installed in the region of the front bumper, one each on the left and right side of the vehicle. These two radar sensors  22 L and  22 R basically have the same design as radar sensor  22  shown in  FIG. 3 , but they are configured as mirror-images of each other, with the result that their fields of view  38 L,  38 R are mirror-inverted relative to each other as well. Moreover, the two fields of view  38 L and  38 R are laterally offset from one another according to the offset of radar sensors  22 L and  22 R. Seen by itself, each individual field of view  38 L and  38 R has the same asymmetry as field of view  38  in  FIG. 5 . However, an overall symmetrical field of view for the locating system made up of two sensors is achieved overall nevertheless because of the mirror-inverted placement of the sensors. 
     The superpositioning of two asymmetrical and substantially mutually mirror-inverted fields of view yields a number of advantages. 
     For one, in this manner it is possible to ensure that the overall field of view has a relatively constant width across a relatively large distance range. This allows sufficient field depth to be achieved, without more and more irrelevant objects from beyond the road appearing in the visual field with increasing distance; on the other hand, virtually continuous monitoring of the area in front is made possible at short distances, without any significant occurrences of dead angles. 
     Using at least two antennas, an object is able to be located in a relatively large core region of each field of view  38 L and  38 R, so that the azimuth angle of the object is able to be determined by evaluating the amplitudes and phases. Moreover, in the range where fields of view  38 L and  38 R overlap, the measured distances and relative speeds and also the azimuth angles (the latter by taking the offset between radar sensors  22 L and  22 R into account) can be adjusted to each other, so that higher measuring precision and reliability are obtained. For control purposes, the measured azimuth angles of the objects may additionally also be calculated by triangulation. 
     In this manner a possible maladjustment of one of the two radar sensors  22 L,  22 R is able to be detected as well and compensated arithmetically. 
     The exemplary embodiment described may be modified in a multitude of manners. For example, radar sensors utilizing different numbers of beams, e.g., three or six beams, may be used. In the case of a radar sensor having four beams, for example, it is also possible to combine three transfer mixers and one isolation mixer with each other. Accordingly, different combinations of mixer types are also possible for sensors having a different number of beams. Furthermore, three or more different types of mixers may be provided such as one pure transfer mixer, one pure isolation mixer, and one mixer having an average transfer output, and to then assign these mixers asymmetrically to the various antennas.