Patent Publication Number: US-2023141051-A1

Title: Sensor Device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and all the benefits of German Patent Application No. 102021128982.3, filed on Nov. 8, 2021, which is hereby expressly incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a sensor device that has a sensor concealed by a cover on a motor vehicle. 
     2. Description of the Related Art 
     A radiator grill for motor vehicles can contain a cover for sensors. In the field of radar system sensors these covers are referred to as radomes. DE 10 2015 003 207 A1 discloses such a radome. This relates to a radar distance measuring device in an adaptive cruise control or automatic parking system placed behind a decorative device on a motor vehicle. The decorative device is located in the middle of the radiator grill. The decorative device comprises a lighting unit that comprises a light emitting diode. The decorative device also comprises a decorative element made of metal. 
     WO 2019/038107 A1 discloses a backlit radome. The radome contains a decorative coating. This coating is produced in a hot stamping process or by applying a metallized film or chrome paint, or through physical vapor deposition (PVD) of metals such as indium, gold or tin. 
     A disadvantage with metallic applications is that these metals have a negative effect on the radar performance. Furthermore, the efficiency of the lighting of the metallic layer is relatively poor due to the low transmission thereof. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to create a sensor device with a sensor cover, at least part of which exhibits a polished metallic surface, the functioning of which is impaired as little as possible. 
     In the case of a sensor device according to the invention, which has a sensor concealed by a cover for a motor vehicle, and which detects incoming electromagnetic waves, the cover has a mount through which the incoming waves can pass, on at least part of which a dichroic, nonmetallic, and reflective layer is placed. 
     The dichroic, reflective layer forms a color filter that is not only perceived as being reflective, but also only reflects certain wavelengths of the visible light spectrum. Other wavelengths can pass through this layer. 
     The functioning of the dichroic layer is based on light wave interference. The dichroic layer is nonmetallic and is composed of numerous thin layers of dielectric material, which have a significantly lower effect on radar beams, blocking them much less than metallic layers. The dichroic layer can be applied directly to the mount. The mount can be made of plastic, in particular. The radar waves can consequently pass through both the dichroic layer and the mount, such that they can then be detected by the sensor. 
     The contours or shape of the cover can be clearly illuminated, and the cover can still have an attractive appearance when it is not lit. This means that the lighting can be such that it is not apparent, even after it is shut off. For this, visible light can exit the mount toward the dichroic layer, such that this light distribution propagates in the dichroic layer, and then exits the dichroic layer. 
     In order to sharply focus the contours of the illuminated dichroic layer, such that it is clearly distinguished from the rest of the cover, an absorbing layer can be placed on the mount adjacent to the dichroic layer, which absorbs visible light exiting the mount, and transmits the incoming waves. 
     To ensure that at least 5% of the visible light from the light source is still transmitted by the dichroic layer, the dichroic layer can exhibit a reflection R that is greater than 80% and less than 95% in the visible light spectrum. 
     To obtain an inexpensive, uniform lighting through the mount, the mount can comprise a substantially planar waveguide plate, into the front surface of which the visible light can be projected by a light source, part of which is reflected at the boundary surfaces of the waveguide plate and within the waveguide plate, and part of which exits and enters one side of the dichroic layer, and then exits the other side of the dichroic layer. 
     In order to keep the light away from the part of the cover plate that is not to be lit, the mount can be composed of at least two parts and comprise two waveguide plates, the second of which is located between the first waveguide plate and the dichroic layer, with gaps being formed between the two waveguide plates, at least in portions of the area therebetween, such that less light enters the second waveguide plate from the first waveguide plate where there is a gap than if there were no gap. 
     To make the cover plate lighting more efficient, at least one waveguide web can be placed next to the gap, through which the visible light is transmitted with a greater transmission T between the two waveguide plates than through the gap. 
     To further increase the efficiency of the cover plate lighting, and focus the light more accurately onto the dichroic layer, it may be the case that the dichroic layer is located in the region of the waveguide web, and/or the region where the gap is located is covered by the absorbing layer on an outer surface of the second waveguide plate, facing away from the first waveguide plate. 
     To obtain a homogenous illumination of the cover plate, the spacing formed by the gap between the two waveguide plates can be uniform, such that the light distribution from the first waveguide plate to the second waveguide plate is homogenous. The gap can be filled with a medium that has a lower refractive index than the medium from which the waveguide plates are made. The relative permittivities ε r  of the two media can be approximately the same. The gap can be filled with air or a transparent film. 
     To extract the light from the light source exiting the cover plate in a targeted manner, the dichroic layer can comprise a band-pass filter, which has a greater transmission or lower reflection for wavelengths of the light that can be emitted by the light source than for wavelengths that are greater or smaller than the wavelengths that can be emitted by the light source. 
     To keep the spectral range of the reduced reflection with a band-pass filter as narrow as possible, and thus keep the range of relatively strong reflection broad, the light source can be a laser diode. 
     To be able to mix the light that can be perceived from the outside coming from light emitting diodes of different colors, or to make the cover appear to have different colors, the band-pass filter can have a greater transmission in a first range and second range of visible light wavelengths than in a third range, the wavelengths of which lie between the wavelengths of the first and second ranges. 
     The cover of the present invention can be employed particularly advantageously as a radome for a radar sensor. The sensor can also be a lidar sensor or an infrared sensor, or a “thermal-imaging camera,” which functions outside the visible light spectrum. 
     Further features, application possibilities, and advantages of the invention can be derived from the following description of exemplary embodiments of the invention, which shall be explained in reference to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows a schematic illustration of two vehicles from above, one behind the other, each of which has a cover for a sensor; 
         FIG.  2    shows the cover from  FIG.  1   , from above; 
         FIG.  3    shows the cover cut along the line III-III in  FIG.  2   , with a dichroic layer on the cover; 
         FIG.  4    shows an alternative embodiment of the cover in an illustration analogous to  FIG.  3   ; and 
         FIG.  5    shows a graph for a dichroic layer with a band-pass filter. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG.  1    shows a motor vehicle  2  with a radar system at the front that has a transmitter  4  and a sensor device. The sensor device comprises a receiver, or sensor  6 , which is concealed by a cover  8  on the motor vehicle. The decorative cover  8  may include a trademark from the vehicle manufacturer, for example, and is located in front of the sensor  6 . The cover  8  can be integrated in particular in a front trim panel made of plastic or in a bumper. With motor vehicles that have internal combustion engines in particular, the cover  8  can be integrated in a radiator grill. 
     The cover  8  is a radome, which allows radar waves  9  to be transmitted by the transmitter  4 , which have a frequency of 75 to 78 GHz in motor vehicles. The radar waves reflected back by a metallic object in front of the vehicle, in particular another vehicle  10 , are detected with the sensor  6 . Incident waves  11  are also detected by the sensor  6 . The distance to the metallic object is determined from the transit time of the respective radar waves. The dimensions of the object can be determined with focused radar waves. 
       FIG.  2    shows the cover from  FIG.  1    from above, which can be formed to resemble a trademark or emblem of the vehicle manufacturer in particular. The cover  8  has contours  12 ,  14 , which delimit at least one highly reflective surface  16  from at least one less reflective surface  18 . The highly reflective surface  16  can have a polished metallic appearance. The less reflective surface  18  does not have a polished metallic appearance. Instead, this surface  18  can be the same color as the vehicle, or it can be black. This surface  18  can also be the color of the manufacturer’s trademark. This colored, black, or white surface  18  can be matte or glossy. 
       FIG.  3    shows the cover with the sensor  6  behind it in the direction of travel, from the side, cut along line III-III in  FIG.  2   . The highly reflective surface  16  is formed on a nonmetallic, reflective and dichroic layer  20 . The dichroic layer  20  comprises numerous dielectric layers applied to the mount  24 , which have a relatively low permittivity ε r  compared to metallized layers. The refractive index for a medium n is derived as follows from the square root of the relative permittivity ε r : 
     
       
         
           
              n= 
             
               
                 
                   ε 
                   r 
                 
               
             
           
         
       
     
     The relative permittivity ε r  of the dielectric layers therefore lies within the range of 1 to 4. Metallic layers normally have a relative permittivity of ε r  &gt; 5. The tangent of the dielectric loss angle δ in the dielectric layers is also relatively low compared to that of the metallic layers. The dielectric loss angle δ is a measure for the dielectric and ohmic losses in a medium. The tangent of the dielectric loss angle δ in dielectric layers is therefore defined as tan δ &lt; 0.01. 
     With regard to the transparency for radar systems, low values of ε r  and tan δ tend to be more advantageous. Because the dichroic layer  20  eliminates the need for a metallic layer, this results in a reduction in the interferences for the sensor located behind it, in particular for radar waves. 
     The less reflective surface  18  is formed on an absorbing layer  22 , which is in the form of a film or a coating. The two layers  20 ,  22  are applied or glued to a mount  24 . The mount  24  comprises a substantially planar waveguide plate  26  with a slight three dimensional spatial form. The waveguide plate  26 , or mount  24  is arched or bowed in particular. Alternatively, the waveguide plate  26  can also be planar. Visible light can enter the front surface  28  of the waveguide plate  26  from a light source  30 , indicated by a light beam  32  in the drawing. Part of the light beam  32  is reflected on boundary surfaces  34 ,  36  of the waveguide plate  26  and within the transparent waveguide plate  26 , and part of the light exits and enters one side  37  of the dichroic layer  20 . The light beam  32  then exits the other side  39  of the dichroic layer  20 . Furthermore, a light distribution of visible light can exit the mount  24  toward the dichroic layer  20 , such that the light distribution propagates in the dichroic layer  20 , and then exits the dichroic layer  20 . The light source  30  can be switched on and off. This makes it possible to create the emblem shown in  FIG.  2   , such that the surface  16  generates a metallic reflective appearance when it is not lit, while its less reflective surface  18  remains dark. When the light source  30  is switched on, the surface  16  appears to be lit. 
     The two layers  20 ,  22  shown in  FIG.  3    abut one another on the waveguide plate  26 . In an alternative embodiment, not shown, the two layers  20 ,  22  overlap slightly, such that the absorbing layer  22  then preferably overlaps the dichroic layer  20 , because the material that the mount  24  or the waveguide plate  26  is made from is selected such that the dichroic layer  20  adheres particularly well to this surface. 
       FIG.  4    shows another alternative embodiment in which the mount  4  is made from two parts, and therefore comprises two waveguide plates  26 ,  36 . The second waveguide plate  36  is placed between the first waveguide plate  26  and the dichroic layer  20 . Gaps  38 ,  40  are formed in portions of the area between the two waveguide plates  26 ,  36 , where less light exits the first waveguide plate  26  into the second waveguide plate  36  than if there were no gap  38 ,  40 . 
     There are waveguide webs  42 ,  44 ,  46  next to the gaps  38 ,  40 , through which visible light can be transmitted between the two waveguide plates  26 ,  36  with a higher transmission T than through the gaps  38 ,  40  that are surrounded by the waveguide webs  42 ,  44 ,  46 . The waveguide webs  42 ,  44 ,  46  are formed as an integral part of the second waveguide plate  36  and come in contact with the first waveguide plate  26 , such that the gaps  38 ,  40  are formed by recesses in the second waveguide plate  36 . The shape of the second waveguide plate  36  corresponds to the shape of the first waveguide plate  26  and is also spatially curved in this regard. 
     The gaps  38 ,  40 , or recesses, are filled with air or a transparent film. The mount can also be made of polycarbonate (PC) and the filler can be poly(methyl methacrylate) (PMMA). 
     As in  FIG.  2   , the dichroic layer  20  is placed on the mount  24 . This dichroic layer  20  is placed on an outer surface  41  of the second waveguide plate  36  facing away from the first waveguide plate  26 , in the region of the waveguide webs  42 ,  44 ,  46 . The gaps  38 ,  40  are also concealed on this outer surface  41  by the absorbing layer  22 . 
     In the exemplary embodiment in  FIG.  4   , the efficiency of the lighting by the light source  30  is higher than in the exemplary embodiment in  FIG.  3   . A large portion of the light from the light source  30 , which is converted to heat by the absorbing layer  22  in the exemplary embodiment in  FIG.  3    is still available for lighting purposes in the exemplary embodiment shown in  FIG.  4   . 
     In order to illuminate the cover plate  8  homogenously, instead of two gaps  38 ,  40 , there can be a single gap with a uniform depth, such that the waveguide plates are spaced apart at a uniform distance. In this exemplary embodiment, not shown, the gap prevents direct contact between the two waveguide plates. The gap can then be filled with air or some other filler, such as a transparent film, as is the case in  FIG.  4   . The filler has a refractive index in this case that is lower than that for the medium from which the waveguide plates are made. 
     The efficiency of the lighting can be improved when the dichroic layer  20  in all of the embodiments described above has a band-pass filter, which has a higher transmission level for those wavelengths, or spectral range, of the light source  30  than for the wavelengths lying outside this spectral range. 
       FIG.  5    shows a schematic graph for such a band-pass filter, in which the reflection R of the dichroic layer with a band-pass filter is plotted over the wavelengths. The reflection R is given in percentages. 
     The broken line  48  indicates a continuous reflection R of 85%, which is obtained without a band-pass filter on the dichroic layer. The reflection R = 85% therefore lies within the preferred range of a reflection R from 80% to 95%, with which it is ensured that at least 5% of the visible light is still transmitted by the dichroic layer  20 . 
     The broken line  48  is largely covered in the graph by a continuous line  50 , which indicates the reflection with a band-pass filter, the reflection of which is reduced in a wavelength range of 420 nm to 480 nm, and has a minimum at a wavelength of X = 450 nm. In this case, the band-pass filter is adjusted to the emission spectrum of a blue light emitting diode, the spectral range, or wavelengths of which are indicated by the curve  52  in the graph. 
     The wider and deeper the reflection of the band-pass filter falls within the spectral range of the light emitting diode, the more the color impression of the overall reflection is displaced for the observer toward complementary colors of the light emitting diode. In the present exemplary embodiment, the color impression of the overall reflection is displaced to the complementary color for blue, which is greenish-yellow. The dichroic layer  20  in this case would be perceived as a yellowish-green reflection outside the motor vehicle  2 . 
     To increase the efficiency of the lighting even more, a light source  30  can be selected that has a particularly narrow spectral range. A laser diode is such a light source  30 , for example. The spectral range or curve  52  with such a laser diode extends over only a few nanometers. With a blue laser diode, the spectral range does not extend over a spectral range of 100 nm (400 nm to 500 nm) over the curve  52 , but instead exhibits a smaller spectral range of 45 nm, by way of example. 
     The reflection of the dichroic layer  20  is as low as possible over a narrow spectral range in this narrow spectral range, or over precisely these wavelengths. As a result, the visible light from the light source  30  is transmitted very efficiently, while much of the external light in the visible light spectrum, in particular sunlight, is reflected by the dichroic layer. 
     In an alternative embodiment, a multi-band-pass filter is used. With this multi-band-pass filter, the reflection takes place along the x-axis in two separate spectral ranges. As a result, the multi-band-pass filter is optimized for two different light sources or two different spectral ranges. 
     By way of example, the reflection can decrease not only in the spectral range of the blue laser or light emitting diode, but also in the spectral range of a yellow laser or light emitting diode, such that by combining the blue and yellow light, a white light is obtained. 
     The cover  8  does not have to be a radome. The cover  8  can also be used with other sensors  6  that detect electromagnetic waves, which are not supposed to be affected, at all or very much, by the dichroic layer  20 . Examples of such sensors are lidar sensors or infrared cameras, or “thermal-imaging cameras,” which function outside the visible spectrum. 
     The invention has been described in an illustrative manner. It is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation. Many modifications and variations of the invention are possible in light of the above teachings. Therefore, within the scope of the appended claims, the invention may be practiced other than as specifically described.