Abstract:
A radar sensor for motor vehicles includes a printed circuit board which carries the mass and antenna structures of the radar sensor, and includes a housing accommodating the printed circuit board, the housing being formed on a transmit and receive side of the radar sensor by a radome which is transparent to radar radiation, characterized in that the radome has an essentially plane wall oriented obliquely to the printed circuit board.

Description:
RELATED APPLICATION INFORMATION 
     The present application claims priority to and the benefit of German patent application no. 10 2013 220 259.8, which was filed in Germany on Oct. 8, 2013, the disclosure of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a radar sensor for motor vehicles, including a printed circuit board which carries the mass and antenna structures of the radar sensor, and including a housing accommodating the printed circuit board, the housing being formed on a transmit and receive side of the radar sensor by a radome which is transparent to radar radiation. The object of the present invention is moreover a motor vehicle having such a radar sensor. 
     BACKGROUND INFORMATION 
     Radar sensors are used in motor vehicles, for example, for measuring the distances and relative velocities of preceding vehicles, so that, for example, a collision warning and/or an automatic distance regulation is made possible. In the case of these applications, the radar sensor is installed in the front area of the vehicle body. The radome has the purpose of protecting the sensitive electronic components of the radar sensor against mechanical impact and weather conditions. In several known radar sensors, the radome is formed as a radar lens, which is used to simultaneously achieve collimation and beam forming of the radar beam. In the case of other known radar sensors the beam forming is solely carried out by the geometry and activation of the antenna elements, for example, according to the principle of a phased array antenna. In this case, the radome may be formed simply by a plane wall made of plastic. 
     Since the bumpers of vehicles today are usually made of plastic and are consequently transparent to radar radiation, it is frequently desired to install the radar sensor protected and concealed behind the vehicle&#39;s bumper. However, a problem exists that while the bumper made of plastic is largely transparent to the radar radiation, it does have a certain reflective capacity. Since, on the other hand, the incoming radar waves are also reflected on the mass structures of the printed circuit board after having passed through the bumper, multiple reflections may occur between the printed circuit board and the vehicle&#39;s bumper. These multiple reflections represent an undesirable interfering signal when the radar signal is evaluated. 
     Such multiple reflections are particularly interfering in the case of angle-resolving radar sensors, in which multiple antenna elements are situated side by side across the width of the vehicle, so that it is possible to measure or at least estimate the azimuth angle of the located object by evaluating the amplitude and phase relations between the radar echoes received by the different antenna elements. The multiple reflections may falsify the amplitudes and phases so severely that the accuracy of the angle measurement is considerably adversely affected. 
     A possible countermeasure is to tilt the bumper in such a way that its reflective surface is no longer parallel to the plane of the printed circuit board and consequently the radar waves no longer strike the antenna structures after one or repeated reflections. However, such a tilt of the bumper limits the constructional flexibility in designing the bumper and in installing the radar sensor. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a radar sensor, which is less sensitive to multiple reflections on the vehicle&#39;s bumper. 
     According to the present invention, this objective is achieved in that the radome has an essentially plane wall oriented obliquely to the printed circuit board. 
     Before reaching the printed circuit board, the incoming radar waves, which have passed through the vehicle&#39;s bumper, must also pass through the radome, a small component of the radiation energy being reflected. After being reflected on the printed circuit board, the radar waves are again reflected on the radome. Due to the oblique position of the radome, the reflected waves are deflected at an angle which is double the angle between the plane of the radome and the plane of the printed circuit board. An adequate oblique position of the radome makes it possible to achieve that the reflected waves no longer strike the antenna structures as early as after the first reflection, but no later than after the second or third reflection. 
     A certain component of the waves that return from the printed circuit board to the radome pass through the radome and are again reflected on the bumper. A certain component of these waves are again reflected and deflected when they again pass through the radome. Due to the additional reflection losses on the radome, the multiple reflections between the printed circuit board and the bumper are generally significantly more severely attenuated than in the case of a radar sensor whose radome is oriented in parallel to the printed circuit board. This makes it possible to obtain a relatively interference-free signal even when the bumper is not tilted or is tilted only slightly. 
     Advantageous refinements and embodiments of the present invention are described herein. 
     Exemplary embodiments are explained in greater detail below with reference to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic diagram of a radar sensor without a radome, which is installed behind a tilted bumper. 
         FIG. 2  shows a similar schematic diagram for a radar sensor including a radome according to the present invention. 
         FIG. 3  shows an additional exemplary embodiment which differs in the geometry of the radome. 
         FIG. 4  shows an additional exemplary embodiment which differs in the geometry of the radome. 
         FIG. 5  shows an additional exemplary embodiment which differs in the geometry of the radome. 
     
    
    
     DETAILED DESCRIPTION 
     A side view of a printed circuit board  10  in the form of a plane plate of a radar sensor of a motor vehicle is shown schematically in  FIG. 1 , the printed circuit board being mounted in a vertical orientation in the front area of the motor vehicle, which is not shown. Printed circuit board  10  carries mass structures  12 , which in the example shown have the form of a metallization on the rear side of the printed circuit board. Furthermore, on its front side, printed circuit board  10  carries antenna structures  14 , for example, in the form of multiple vertical gaps  16  of antenna elements  18 . A single one of these gaps  16  is shown schematically also in layout in  FIG. 1 . 
     Antenna elements  18  of each gap are connected in series to a high-frequency signal of an oscillator, which is not shown. The dimensions of antenna elements  18  and the distances between them are selected in such a way that the resonance vibrations excited in individual antenna elements  18  have phase and amplitude relationships, which result in a certain beam forming in elevation, more precisely, a bundling of the emitted radar radiation forward in the direction of travel of the motor vehicle (to the right in  FIG. 1 ) in the case of a front radar. 
     As an example, it should be assumed that the radar sensor is a sensor including a monostatic antenna system. This means that antenna elements  18  which are used for emitting the radar signal are also used for receiving the radar echoes reflected by the located objects. The received radar echoes are evaluated separately in a known manner for each gap  16 . 
     If a radar echo arrives from an object whose line of sight is not exactly oriented at a right angle to the plane of printed circuit board  10 , i.e., from an object having an azimuth angle different from 0°, due to the different signal propagation delays, this results in characteristic amplitude and phase differences between the signals which are received in the different antenna gaps situated at a distance from one another in the transverse direction of the vehicle. Based on these characteristic differences, it is possible to determine the azimuth angle of the located object, at least approximately. 
     In the example shown in  FIG. 1 , the radar sensor is installed concealed behind a bumper  20  of the motor vehicle. Of bumper  20 , only a certain section of a wall of the bumper is shown schematically in cross section in  FIG. 1 . This wall of the bumper is formed by one or multiple layers of plastic materials and in the example shown, it is tilted about a certain angle α in relation to the vertical and consequently also in relation to the plane of printed circuit board  10 . 
     In  FIG. 1 , an incoming radar beam  22  is shown schematically, which (after reflection on an object which is not shown) passes through bumper  20  and strikes the outermost lower edge of printed circuit board  10 . The angle of elevation of radar beam  22  amounts to 0°, i.e., the beam strikes printed circuit board  10  in elevation at a right angle. Radar beam  22  is nearly completely reflected on printed circuit board  10 , in particular on its mass structures  12 , so that a reflected beam  24  returns to bumper  20  and strikes the inner surface of bumper  20  under an angle of incidence α, which is equal to the tilt of the bumper. A certain component of the radiation is again reflected here and forms a multiply reflected first-order beam  26 , which again strikes printed circuit board  10  at a certain vertical offset x. Here the beam must be reflected again, so that a reflected second-order beam  28  returns again to bumper  20 . This process may in principle be repeated multiple times, so that a cascade of multiple reflections is obtained, the intensity of which, however, decays exponentially due to losses occurring in each reflection. 
     When the multiply reflected first-order and higher order beams strike in the zone on printed circuit board  10 , in which gaps  16  of antenna elements  18  are located, the multiply reflected beams are received by the antenna elements, and they form an interference signal which falsifies the amplitude and phase relations between the signals received in different gaps  16  and thus makes it more difficult to measure the azimuth angle. 
     In the example shown in  FIG. 1 , the tilt of bumper  20  about angle α has the result that only multiply reflected first order beam  26  strikes antenna structures  14 , while multiply reflected second-order and higher order beams are already deflected far enough upwards that they no longer strike the antenna structures. 
     When bumper  20  is tilted about a larger angle α, this makes it possible for multiply reflected first-order beam  26  to also no longer strike antenna structures  14  (not even when incoming beam  22  strikes printed circuit board  10  on its outermost lower edge as in  FIG. 1 ). 
     In  FIG. 1 , “a” denotes the vertical distance between the lower edge of printed circuit board  10  and the lower edge of antenna structures  14 , “b” denotes the height of antenna structures  14 , and “c” indicates the distance between the upper edge of antenna structures  14  and the upper edge of printed circuit board  10 . The condition that multiply reflected first-order beam  26  should also not strike antenna structures  14 , may then be expressed as inequality
 
 x&gt;a+b   (1)
 
Incoming beam  22 , multiply reflected beam  26  and the section of printed circuit board  10  which has the height x form a right triangle. Based on the law of reflection (angle of incidence equals angle of reflection), the angle which is diametrically opposed to the side of the triangle having length x has the value 2α. Therefore:
 
tan(2α)= x/h   (2)
 
h being the distance between printed circuit board  10  and bumper  20  at the height of incoming beam  22 .
 
     Based on these relationships, it is possible to determine angle α on which bumper  20  should at least be tilted in order to avoid interfering multiple reflections. In practical specific embodiments, angle α amounts to at least 18° or more. However, there are installation situations and vehicle designs in which such a large tilt angle of the bumper is undesired. 
     In a diagram similar to  FIG. 1 ,  FIG. 2  illustrates a radar sensor according to the present invention in which the interfering multiple reflections may be largely suppressed independently of the tilt angle of the bumper. 
     In the case of this radar sensor, printed circuit board  10  is (as is customary per se) accommodated in a protective housing  30 , which is limited on the side toward which the radar radiation is emitted by a wall made of plastic, which is transparent to radar radiation, a so-called radome  32 . The particularity in this case is that radome  32  is tilted in relation to printed circuit board  10  about a certain angle β. 
     Incoming beam  22  must now not only pass through bumper  20  but also radome  32 , a certain reflection loss again occurring (beam  34 ), which, however, does not significantly reduce the sensitivity of the radar sensor. 
     Reflected beam  24  must also pass through radome  32 , with the consequence that a certain radiation component (beam  36 ) is already reflected on the radome. In the example shown, angle β is selected in such a way that multiply reflected first-order beam  36  does not strike antenna structures  14 . Angle β of the tilt of the radome required for this may be calculated using equations (1) and (2) provided above for angle α, by substituting β for α and the distance between printed circuit board  10  and radome  32  for h. 
     The larger share of reflected beam  24  will pass through radome  32  and strike bumper  20 , resulting in another multiply reflected beam  38 . In the case of unhindered propagation, this beam  38  could strike antenna structures  14 ; however, it must again pass through radome  32 , a certain component of the radiation again being reflected (beam  40 ). Reflected beam  40  is in this case directed away from antenna structures  14 . The component of multiply reflected first order beam  38 , which ultimately strikes the antenna structures, is for that reason significantly attenuated by two additional reflections on radome  32  in addition to the reflection on bumper  20 , so that the useful signal is less falsified and interfered with. In the case of multiple higher order reflections, the additional reflection losses result in a significantly faster decay of the beam intensity, so that the interfering influence of the multiple reflections may ultimately be effectively suppressed even when angle α, about which bumper  20  is tilted, is relatively small or even 0°. 
     The effect of radome  32  may, if necessary, be increased in that the rear side of radome  32 , which faces printed circuit board  10 , is “mirrored” by a suitable coating, so that it has a particularly high reflective capacity for the radar radiation having the frequency of the radar sensor and having angle of incidence β, and consequently the intensity of reflected beam  36  is increased at the expense of beam  24  which has passed through. 
     While in the example shown in  FIG. 2 , radome  32  is tilted in such a way that its lower edge is closer to printed circuit board  10  than its upper edge, specific embodiments are of course also possible in which radome  32  is tilted in the opposite direction. For determining angle β, in which multiply reflected first-order beam  36  no longer strikes antenna structures  14 , variable “c” must be substituted for variable “a” in above inequality (1). A radome  32 ′ configured in this way is shown in  FIG. 3 . 
     Optionally, the radar sensor may also have a roof-shaped radome  32 ″, as shown in  FIG. 4 . This radome has two asymmetric roof surfaces  42 ,  44 . Such a system may, for example, be advantageous when antenna structures  14  on printed circuit board  10  are also situated asymmetrically with regard to the edges of the printed circuit board (a≠c). 
     As another example,  FIG. 5  finally shows a radome  32 ′″ having a symmetrical roof form. This system is advantageous in particular when the bumper has only a relatively small tilt or no tilt, and makes it possible to reduce the overall height of the radome, so that the distance between the radar sensor and bumper  20  may also be correspondingly short.