Abstract:
The invention relates to a radar sensor ( 10 ) for use with automobiles. Said radar sensor emits pulsed radiation. The radar sensor is characterized in that it comprises an antenna with at least one layer-structured block ( 34 ) consisting of metal layers ( 36, 38, 40, 42 ) which are arranged according to the Yagi principle and which are respectively separated from each other by a dielectric intermediary layer ( 46, 48, 50 ). At least one of the metal layers ( 36, 38, 40, 42 ) is excited by a supply system ( 18 ) with a radar frequency.

Description:
This application is the national stage of PCT/EP2004/003433 filed on Apr. 1, 2004 and also claims Paris Convention priority of DE 103 22 371.1 filed on May 13, 2003. 
   BACKGROUND OF THE INVENTION 
   The invention concerns a radar sensor that emits pulsed radiation for automobile applications. The invention also concerns a method for the production of such a radar sensor. Radar sensors of this kind are known in the art. Automobile applications such as radar sensors are usually used to assistant parking, to monitor blind spots, to anticipate accidents (pre-crashed sensing), for starting/stopping operation or during driving with distance monitoring, and/or to regulate separations (cruise control support). 
   Towards this end, differing sensors are normally used for monitoring the environment of the vehicle and for detection of remote objects, with these different sensors operating at different radar frequencies. For near field observations, high spatial resolution is important (with respect to separation as well as angle), whereas the angular information is less important for large separations. 
   For the monitoring of separations at large range, radar sensors are conventionally used having a frequency of approximately 76 Gigahertz. These frequencies have, however, the associated disadvantages that the short wave lengths in the microwave region cannot be used together with conventional components. 
   In contrast thereto, so-called ISM frequencies of approximately 24 Gigahertz are used for near field monitoring. These frequencies can be irradiated in a wide band fashion. A wide band signal is desirable, since the spatial resolution of reflected objects, i.e. the smallest possible separation with which two separate objects can be recognized as being separate, is improved with increasing band width. In order to further improve the bandwidth, the conventional radar sensors are generally operated in a pulse manner, since the signal bandwidth increases with shorter pulse width. 
   The conventional radar sensor has a planer, slot-coupled patch antenna, which can be excited via an associated aperture in a metallic, ground surface and via a dielectric dispose between the ground surface and the radiation surface. Excitation via a feed network to the radar sensor, results in the irradiation of electromagnetic waves. The conventional radar sensors have a length and width of several centimeters and a depth of approximately up to 3 centimeters, so that they can be integrated into conventional bumpers of motor vehicles. 
   With the assistance of a plurality of radar sensors, it is possible to detect objects throughout a wide angular region using the triangulation procedure. The directional characteristics of the radiating and receiving antennas are thereby adjusted in a geometric fashion with the assistance of the interference principal and also with the assistance of signal phase differences or with signal travel time differences using approximately 4 to 6 patches (radiation surfaces) at 3 db in an angular range from approximately 15 to 25 degrees in one direction and approximately 7 degrees in the other direction. 
   An advantage of the flat antenna structures compared to conventional antennas is that they are more economical to produce and they also result in a compact and light weight construction which can be built using standard components and which is easily integrated into circuits having micro-strip leads. 
   This economical principle is sufficient for small object detection ranges. 
   However, for larger separations of approximately 40 meters from the object, a relatively highly focused beam must be used, since ambient influences are otherwise excessive. Realization of such a strongly focused beam using a conventional planer antenna technology on the basis of the conventional interference principle would require a plurality of patches and are therefore a large amount of space. The antenna surfaces would then determine the size of the sensor and the size of future sensors would greatly exceed the size of current radar sensors. 
   Departing from this prior art, it is the object of the present invention to produce an economical radar sensor for automobile applications which, in addition to the detection of proximate objects, can also detect further removed objects while utilizing standard components and with one single radar sensor whose geometrical size is not substantially larger than the sizes of current conventional radar sensors. 
   SUMMARY OF THE INVENTION 
   This purpose is achieved in accordance with the radar sensor of the above mentioned kind in that the radar sensor has an antenna with at least one layered structured block which has metal layers disposed in accordance with the Yagi principle, each of which are separated from the other by means of a dielectric intermediate layer, wherein at least one of the metallic layers is excited by a feed network at a radar frequency. 
   Moreover, this purpose is achieved with a method of the above mentioned kind in an antenna of a radar sensor produced with at least one layered structured block having metal layers structured according to the Yagi principle each of which is separated from each other by an intermediate dielectric layer, wherein at least one of the metal layers is coupled to a feed network. 
   With these elements, the purpose of the invention is achieved. The radar sensor in accordance with the invention can be constructed for automobile applications in an economical fashion and can simultaneously detect the proximate range as well distant objects. Its size is comparable to that of conventional sensors. A conventional Yagi antenna is a longitudinal irradiator comprising a plurality of dipoles that effect the desired directionality. It is exited by radiative coupling. By replacing the metal patches used in conventional radar sensors with one or multi-layered block structures, an increased directionality is achieved. 
   The intermediate dielectric layers are preferentially made from ceramic having a dielectric constant between 5 and 50. 
   The relatively high dielectric constant values of the ceramic decrease the wavelengths so that a relatively large number of layers can contribute to the Yagi principle without having an excessively high block height. In this manner, even one single layered structured block having metal layers disposed in accordance with the Yagi principle leads to substantial directionality. 
   It is furthermore preferred when at least two of the metal layers are excited by the feed network in a phase coupled manner via separate structures. 
   It is furthermore advantageous when individual layers of the layered structured block have at least regions that are trough shaped. 
   It is furthermore preferred when the layered structured blocks taper with increasing separation from the feed network coupling. 
   These configurations further increase the directionality of a layered structured block having metal layers disposed in accordance with the Yagi principle. This is the case for each individual element as well as for the combination of these elements. 
   It is furthermore preferred when the metal surfaces, which are disposed proximate to the radio frequency electromagnetic energy feed, are stacked in a denser manner than those, which are further removed. 
   This has led to improved excitation of the further removed layers. This feature can also be preferentially combined with the above-mentioned ones. 
   A further preferred embodiment is distinguished in that a plurality of the layered structured blocks are combined into a common structure, wherein the individual blocks are excited in a phase coupled fashion. 
   Such a phase directed feed of the radio frequency energy RF (radio frequency energy) into a plurality of blocks can further increase the directionality of a LTCC Yagi block configuration. 
   Furthermore, the radar sensor preferably has a plurality of the above-mentioned configurations, excited in a phase-coupled fashion. 
   This configuration also increases the directionality of the antenna. It is also preferred when the network has a radar frequency of 24 Gigahertz. This feature permits combination of the conventional wide band, near field observation in the ISM frequency region with the detection of further removed objects as achieved in accordance with the invention by means of directed irradiation. 
   With regard to the method, it is preferred when the dielectric intermediate layers having embedded metal layers are produced as LTCC layers (low temperature co-fired ceramics). 
   LTCC technology is suitable for the production of monolithic ceramic multi-layer systems in which electrically conducting material, such a metallic layers, can be integrated. 
   Further advantages can be extracted from the description and the associated figures. 
   Clearly, the above mentioned features and those to be described more closely below can be used not only in the particular combination given but also in other combinations or individually, without departing from the framework of the instant invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments in the invention are described more closely below, illustrated in the drawings, and described in detail in the following description. 
       FIG. 1  shows a schematic overall view of a radar sensor for motor vehicle applications; 
       FIG. 2  shows a schematic cut representation of the radar sensor according to  FIG. 1  with an inner construction known in prior art; 
       FIG. 3  shows a schematic cut representation of a layered structured block embodiment of the invention that replaces the radiation layers (patches) of  FIG. 2 ; 
       FIG. 4  shows an embodiment of a method in accordance with the invention; 
       FIG. 5  shows a schematic cut representation of a layered structured block according to a further embodiment of the invention; 
       FIG. 6  shows a schematic representation of a layered structured block having various layer separations, and 
       FIG. 7  shows a schematic representation of a layered structured block having a cross-section that tapers in the longitudinal direction. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Reference symbol  10  of  FIG. 1  designates a schematic overall view of a radar sensor having a housing  12 , which is sealed by a lid  14 . The dash lines  15  indicate the direction of orientation of the radiation elements within the housing  12 . Reference symbol  16  designates a connecting element by means of which the radar sensor  10  can receive e.g. a power supply voltage and/or by means of which the radar sensor  10  can send or receive signals to and from a controlling apparatus of a motor vehicle. The arrow designated with reference symbol  17  indicates the direction of the longitudinal axis of the motor vehicle. 
   The orientation of the radar sensor  10  relative to the direction  17  of the longitudinal axis represents the typical assembled position of the radar sensor  10  in a motor vehicle application. However, the invention is clearly not limited to such a relative direction between the radar sensor  10  and the direction  17  of the longitudinal axis of the motor vehicle. 
     FIG. 2  shows a conventional internal construction of the radar sensor  10  according to  FIG. 1 , in partial section. Reference symbol  18  of  FIG. 2  represents a feed network which is connected to the connecting elements  16  of  FIG. 1  and which is disposed on a first side  20  of a radio frequency substrate  22 . A metallic ground surface  24  is disposed on a second side  26  of the radio frequency substrate  22 . The radar sensor  10  also has at least one radiative surface  28  (patch), which is excited by an input network  18  via an opening  30  in the metallic ground surface  34  and via a dielectric  32  disposed between the ground surface  24  and the radiation surface  28  to irradiate electromagnetic waves. In the conventional radar sensor, the radiation surface is supported and borne by the dielectric  32 . The dielectric  32  of the conventional radar sensor is generally made from hardened foam. 
     FIG. 3  shows a first configuration of a radiative device having the features of the inventive radar sensor. Reference symbol  34  designates a layered structured block, which has metal layers  36 ,  38 ,  40  and  42  disposed in accordance with the Yagi principle. The metal layers  36 ,  38 ,  40  and  42  are embedded in dielectric layers  44 ,  46 ,  48 ,  50  and  52 . The configuration of the metal layers  36 ,  38 ,  40  and  42  thereby represents a Yagi configuration. 
   A first metal layer  36  is coupled to a feed network  18  by means of a first dielectric layer  24 , an opening  30 , a metallic ground surface  24 , and the radio frequency substrate  22 . The feed network  18  generates electrical oscillations in the first metallic surface  36  which excite the additional metallic surfaces  38 ,  40  and  42  via the additional dielectric layers  46 ,  48  and  50 . The Yagi configuration amplifies the directional properties of the electromagnetic waves irradiated from the individual metallic surfaces. Arrow  54  indicates the principal irradiation direction. 
   Complementary, additional metallic layers can be coupled to the feed network  18 . Through proper phased input of radio frequency electromagnetic energy to the metallic surfaces  36 ,  38  that are stacked in the radiation direction, the directional properties of a block are increased.  FIG. 3  shows such a multiple input having a network connection  58  and a connection  56  to a further metallic surface  38  in addition to the coupling by means of the aperture  30  and the associated electrically conducting connection of a network connection  58 . Reference symbol  60  indicates a phase displaceable element, e.g. a capacitance and/or an inductance and/or an optionally controllable network of capacitances and/or inductive elements. Such an additional connection is optional: the object of the invention is also realized with the coupling of only one metallic surface  36  to the feed network  18 . 
   As shown in  FIG. 3 , the metal layers are embedded in an equidistant fashion in ceramic. This configuration is, however, not absolutely necessary, as will be described more closely below. 
   Ceramic material is preferred for the dielectric layers  44 ,  46 ,  48 ,  50  and  52 , since the higher dielectric constant of ceramic leads to a large decrease in the wavelengths of the electromagnetic waves, which are transmitted inside the block  34 . As a result thereof, a relative large number of metal layers can be stacked together with alternating ceramic layers without having the height of the block  34  be excessively large. In this manner, an improved directionality is achieved even with one single block  34 . 
   In order to further improve the directionality, a plurality of such Yagi blocks  34  can be coupled to each other in groups. With proper phase driving of the individual blocks  34  within the group, a constructive interference among the electromagnetic waves emanating from each individual Yagi block  34  can be achieved, leading to an improved directionality. Towards this end, six or groups of six Yagi blocks can be directed in a linear fashion or in a plurality of lines  15  (see  FIG. 1 ). 
   The directionality can be further improved when a plurality of such phased coupled groups produce mutual constructive interference. Towards this end, groups of patches can be combined with groups of Yagi blocks. In this fashion, a three-by-six configuration with two outer rows of patches and a middle row of six Yagi blocks can be combined. 
   Each Yagi block  34  is preferentially produced using LTCC technology (low temperature co-fired ceramic). 
   This technology is particularly good for the production of monolithic structures made from ceramic and having integrated metallic layers. Within the framework of the LTCC technology, a raw, glass ceramic foil is initially produced having an organic binder. Glass ceramic consists essentially of a ceramic material and a glass material. Openings are subsequently produced in the raw ceramic foil and filled with the subsequent metal layers. The raw ceramic foils are then stacked together with the metallic layers and laminated into a composite. The composite is then sintered into a block having monolithic, multi-layer construction. 
   Following the sintering process, the metallic layers  36 ,  38 ,  40  and  42  of the antenna are embedded in the ceramic of the multi-layered structure of the block  34 . The dimensions of the antenna depend on the effective dielectric constants of the ceramic. The higher the ceramic dielectric constant, the smaller is the height of the block  34 . 
     FIG. 4  illustrates a method for the production of a radar sensor in accordance with the invention. Towards this end, in a first step  62 , at least one layered structured block  34  is manufactured having metal layers  36 ,  38 ,  40  and  42  disposed in accordance with the Yagi principle, each layer being separated from the next by means of a dielectric intermediate layer  46 ,  48  and  50 . The number of metal and separating, ceramic intermediate layers is not confined to a particular number. The larger the number of layers, the better the directionality. The number of layers could be limited by specifications for a maximum constructional height of the radar sensor  10 . 
   The step  56  is preferentially followed by the above-mentioned LTCC technology. In a second step  64 , the block is coupled to a feed network  18  and, in a third step  66  is embedded in a housing. The coupling to the feed network  18  can be effected via an aperture  30  in one of the ground surfaces  24  which is disposed on one side of a radio frequency substrate  22 , opposite to a feed network  18 . The coupling can, however, also be effected in other ways e.g. by means of a galvanic coupling between the first metal layer  36  and the feed network  18 . 
     FIG. 5  shows a schematic section of a layered structured block  34  according to an additional embodiment of the invention. In this embodiment, the metal surfaces  36 ,  38 ,  40  and  42  as well as the associated ceramic layers  44 ,  46 ,  48 ,  50  and  52  are trough shaped (concave). This configuration leads to improved directionality. 
     FIG. 6  shows a layered structured block  34  having metal layers  36 ,  38 ,  40  and  42  which are not embedded in the ceramic at equal separations. The densely stacked metal layers proximate the input of the radio frequency electromagnetic energy via the aperture  30 , form a transitional zone for improved excitation of the more distant outer metal surfaces layers. 
     FIG. 7  shows a schematic representation of a layered structured block  34  having a cross-section that tapers in a longitudinal direction. This embodiment also leads to improved directionality of the block  34 .