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CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This is a continuation application, under 35 U.S.C. §120, of copending international application No. PCT/EP2015/079718, filed Dec. 15, 2015, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of German patent application No. 10 2014 018 924.4, filed Dec. 22, 2014; the prior applications are herewith incorporated by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0002]    The invention concerns a device for the contactless actuation of an adjustable vehicle part that contains a body cladding part or an outer cladding part that is or can be mounted on the vehicle and a proximity sensor that is indirectly or directly attached thereto. The invention further concerns a vehicle containing the device. 
         [0003]    Modern vehicles are often fitted with sensors that enable the contactless displacement of vehicle parts. For example, a user of a vehicle enables the contactless actuation (i.e. opening or closing) of a vehicle door by such a sensor. Such sensors are in particular provided for the motorized tailgates of motor vehicles, so that the user of a vehicle can cause the opening or closing of the tailgate by a suitable leg movement without having to put down a load being held in the hands (“Hands-free Access”). Such a contactless tailgate switch is known from published, non-prosecuted German patent application DE 10 2010 049 400 A1, for example. 
         [0004]    Capacitively operating proximity sensors are often used as sensors for the detection of an actuation command. Typically, a capacitive proximity sensor contains two sensor electrodes and an electronics unit, each of which is connected to the sensor electrodes via corresponding feed lines. The proximity sensor is often indirectly or directly attached to the inside of a rear bumper of the vehicle. In this case, the proximity sensor is sometimes disposed in a longitudinal side section of the bumper relative to the vehicle. The side section of the bumper often contains a lower edge extending at an angle relative to the horizontal. It has been shown that the reliability of the proximity sensor is often adversely affected with the proximity sensor in a side section of such a design in the installation position. 
       SUMMARY OF THE INVENTION 
       [0005]    It is the object of the invention to provide a device of the aforementioned type for the contactless actuation of an adjustable vehicle part that is particularly reliable to operate (i.e. fail-safe). 
         [0006]    The device according to the invention for the contactless actuation of an adjustable vehicle part contains at least one capacitive proximity sensor that for its part contains at least one elongated sensor electrode for the detection of an actuation command produced by proximity to the proximity sensor. The sensor electrode is formed here by an elongated electrical conductor, for example in the form of a round conductor or a flat conductor. In particular, in an advantageous embodiment the sensor electrode is formed by the outer conductor of a coaxial cable or of a similar cable without an inner conductor. The proximity sensor preferably comprises two elongated sensor electrodes, wherein particularly preferably one of the sensor electrodes is formed by a flat conductor (i.e. a conductor with a band-shaped flat conductor geometry), whereas the second sensor electrode is formed by a round conductor. It is however also possible within the scope of the invention that both sensor electrodes are of the same construction. 
         [0007]    Furthermore, the device contains a body cladding part or an outer cladding part that is or can be mounted in a defined installation position on the vehicle. The proximity sensor is indirectly or directly attached to the body cladding part or outer cladding part (referred to in brief below as a “supporting part”). The proximity sensor can thus either be directly mounted on the supporting part, or alternatively the proximity sensor can also be attached to an interposed mounting part (intermediate support), which for its part is in turn attached to the supporting part. 
         [0008]    In one advantageous embodiment of the invention, the supporting part is formed by a—in particular rear—(vehicle) bumper. The proximity sensor is in this case preferably attached to the inside of the bumper (again indirectly or directly). However, within the scope of the invention the supporting part can for example also be formed by a door sill or another part of outer vehicle cladding. 
         [0009]    According to the invention, with the proximity sensor in the installed state on the supporting part, the sensor electrode extends at an angle to a lower edge of the supporting part. The oblique position is dimensioned here so that with the supporting part in the installation position on the vehicle, the sensor electrode is oriented exactly or at least approximately horizontally regarding the longitudinal extent thereof. In other words, owing to the oblique arrangement on the supporting part, the sensor electrode extends at least approximately parallel to the (flat) ground on which the vehicle is standing. If the proximity sensor contains two or more sensor electrodes, then the electrodes are in particular mounted on the supporting part so as to be spaced apart and parallel to each other. Accordingly, each of the sensor electrodes is oriented at an angle to the lower edge of the supporting part or parallel to the ground. In particular, an edge of the supporting part that faces the ground on which the vehicle is standing with the supporting part in the intended installation position is referred to as the lower edge. 
         [0010]    The invention is based on the consideration that in modern vehicles supporting parts to which sensor electrodes are attached sometimes comprise lower edges that are inclined relative to the horizontal. This is particularly true for the side sections of bumpers. Conventionally, a sensor electrode is regularly placed on the supporting part parallel to the lower edge. Disadvantageously however, it has been found that such an arrangement often results in proximity sensor detection errors. For example, it can happen that an incidence of an approach to a first end of the sensor electrodes is interpreted as an actuation command, whereas an incidence of an approach to an opposite end of the sensor electrodes erroneously does not trigger an actuation command. The invention removes this disadvantage by providing that the sensor electrode is deliberately laid out of parallel to the lower edge of the supporting part, so that the sensor electrode (with the supporting part in the intended installation position) is at least approximately at the same distance from a (flat) surface on which the vehicle is standing over the entire length thereof. The result of this on the other hand is that a body part (in particular a foot of the user of a vehicle), which is moved towards the proximity sensor as intended to produce an actuation command, is always at approximately the same distance from the sensor electrode regardless of the position along the length (at which point) of the sensor electrode such an approach occurs. It is thus advantageously ensured that similar approach events at different points along the longitudinal extent of the sensor electrode result in the same sensor signals, whereby the risk of detection errors is advantageously reduced. 
         [0011]    The sensor electrode with suitable dimensions is in particular referred to as “oriented approximately horizontally” if the distance from the ground varies by no more than 20 mm over the length of the sensor electrode. For an at least approximately linear sensor electrode, the result of the dimensioning is a maximum permissible angle of attack (maximum angle of attack) relative to the horizontal depending on the length of the sensor electrode. It is thus preferably provided that the sensor electrode is inclined relative to the horizontal by no more than an angle that is smaller than the maximum angle of attack. 
         [0012]    In a preferred embodiment of the invention, the supporting part is formed by a bumper, wherein the proximity sensor is attached to a side section of the bumper. In this case, the device preferably contains two proximity sensors, wherein the sensors are disposed on the two opposing side sections of the bumper. 
         [0013]    The adjustable vehicle part is preferably a tailgate of the vehicle. It is however generally also conceivable that the adjustable vehicle part is a different vehicle door, for example a side door, in particular a sliding door, or an engine hood. 
         [0014]    Other features which are considered as characteristic for the invention are set forth in the appended claims. 
         [0015]    Although the invention is illustrated and described herein as embodied in a device for the contactless actuation of an adjustable vehicle part, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
         [0016]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0017]      FIG. 1  is a basic schematic representation a motor vehicle with a bumper to which a proximity sensor for a contactless actuation of a tailgate of the motor vehicle is attached according to the invention; 
           [0018]      FIG. 2  is a diagrammatic, perspective view of a body assembly that contains the bumper according to  FIG. 1 ; 
           [0019]      FIG. 3  is a schematic individual representation a side section of the bumper according to  FIG. 2 , on the inside of which two sensor electrodes of the proximity sensor are installed; 
           [0020]      FIG. 4  is a graph showing a time profile of signals produced by the sensor electrodes for an incidence of an approach to a first location according to  FIG. 3 ; 
           [0021]      FIG. 5  is a graph showing a time profile of the signals produced by the sensor electrodes for an incidence of an approach to a second location according to  FIG. 3 ; and 
           [0022]      FIG. 6  is a schematic view of the sensor electrodes according to  FIG. 3  which represents a maximum permissible angle of attack of a sensor electrode relative to the horizontal. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    Mutually corresponding parts and variables are always provided with the same reference characters in all figures. 
         [0024]    Referring now to the figures of the drawings in detail and first, particularly to  FIG. 1  thereof, there is shown a rear part of a (motor) vehicle  1  that is represented in a side view. The vehicle  1  contains a device  2  for the contactless actuation of a motorized vehicle part, in this case a tailgate  3  thereof. The device  2  contains capacitive proximity sensors  6 . Furthermore, the device  2  contains a control and analysis unit  7 . A rear bumper  8  is attached to the vehicle  1 . The proximity sensors  6  are each disposed in a side section  10  on the bumper  8  that bounds the bumper  8  in the lateral direction of the vehicle. The bumper  8  thus forms (in the sense of the nomenclature introduced above) a supporting part of the device according to the invention. The position of the proximity sensors  6  is represented here for one of the side sections  10  and thereby only in a rough simplified schematic form. 
         [0025]    An operating demand or actuation command detected by one of the proximity sensors  6  is output to the control and analysis unit  7 , whereupon the control and analysis unit  7  actuates a flap drive  11  and/or a door lock  12  of the vehicle  1  to open or close the tailgate  3  according to the operating demand. 
         [0026]    A Cartesian vehicle coordinate system  15  for orientation is indicated in  FIG. 1 . The X-axis of the coordinate system denotes the longitudinal direction of the vehicle X here and is oriented parallel to the ground  16  on which the vehicle  1  is standing. The Y direction of the vehicle coordinate system  15  indicates the lateral direction of the vehicle Y and is likewise oriented parallel to the ground  16 . In the representation according to  FIG. 1 , the Y-axis points out of the plane of the drawing. The longitudinal direction of the vehicle X and the lateral direction of the vehicle Y thus span a horizontal plane. Finally, the Z-axis of the coordinate system points in the direction of a vertical Z of the vehicle and is thus erected perpendicularly on the ground  16  and the parallel horizontal plane. The directions introduced above are also used below to characterize the spatial orientation of components in the intended installation position thereof in the vehicle  1 . 
         [0027]      FIG. 2  shows a body assembly  20 . The body assembly  20  contains a plurality of body parts, in this case specifically on the one hand the bumper  8  according to  FIG. 1 , and on the other hand a wheel arch  21 . The bumper  8  is in turn divided into the two side sections  10  and an interposed rear section  22 . The body assembly  20  is shown with the direction of view approximately in the longitudinal direction of the vehicle X according to  FIG. 1 . Accordingly, in  FIG. 2  an inner side  23  of the bumper  8  facing the vehicle  1  is to be seen in the intended installed state of the body assembly  20  or of the bumper  8  on the vehicle  1 . The bumper  8  is formed by a thin-walled, three-dimensional molded plastic part. In the installed state, the bumper  8  partly envelops the vehicle  1  to the rear and to the side. In  FIG. 2  it is shown that the proximity sensors  6  are disposed in the two mutually opposite side sections  10  of the bumper  8 . One of the proximity sensors  6  is shown here purely as an example, wherein the opposite (not shown) proximity sensor is correspondingly implemented diametrically opposite. The proximity sensors  6  are mounted directly on an inner side  23  of the bumper here. The bumper  8  is thus used in the exemplary embodiment as a supporting component on which the proximity sensors  6  are mounted. In an alternative embodiment, it is provided by contrast that the proximity sensors  6  are placed on a supporting component that is separate from the bumper  8 , wherein for its part the supporting component—for example formed by a flat molded part—is attached to the inner side  23  of the bumper. 
         [0028]    As can be seen from the representation, the proximity sensor  6  contains a first elongated (sensor) electrode  30 , a second elongated (sensor) electrode  31  and an electronics unit  32 . Connecting lines that connect the two electrodes  30 ,  31  to the electronics unit  32  are conventionally implemented and are not shown here for clarity. In an alternative embodiment of the invention, the function of the control and analysis unit  7  is integrated within the electronics unit  32 . In this case, the control and analysis unit  7  is a component of the proximity sensor  6 . 
         [0029]    In the exemplary embodiment shown, the first (upper) electrode  30  is formed by a flat conductor with a flat strip-shaped conductor track, whereas the second (lower) electrode  31  is formed by a cylindrical, flexible round electrode in the form of an insulated round conductor. The round conductor is in particular a solid wire, a braided conductor or the outer conductor of a coaxial cable. The inner conductor usually provided with a coaxial cable is in this case optionally omitted or replaced by a core of non-conducting material, in particular plastic. 
         [0030]    The electronics unit  32  includes analysis electronics (not shown explicitly) as an essential component that preferably in turn contains a microcontroller. The analysis electronics are accommodated in a housing  35 . Plug connectors that are not represented further are formed on the housing  35 . The plug connectors are used on the one hand to contact the connecting lines and on the other hand for connecting a control line on the vehicle side, via which the electronics unit  32  communicates with the control and analysis unit  7 , and via which the electrical energy required for operation is delivered to the electronics unit  32 . 
         [0031]    In the exemplary embodiment represented here, the electronics unit  32  is bolted to the inner side  22  of the bumper by lugs formed on the housing  35 . It can however, also be attached to the bumper  8  in a different way, for example by gluing, riveting or welding. 
         [0032]    Each of the electrodes  30 ,  31  contains on each longitudinal end a plug connection that is used to contact the connecting line. The electrodes  30 ,  31 , as well as the connecting lines, are clipped onto the inner side  22  of the bumper using further latching elements that are not represented here. Alternatively, the electrodes  30 ,  31  can even be glued to the inner side  22  of the bumper. 
         [0033]    The arrangement of the electrodes  30 ,  31  according to the invention relative to the side section  10  is described in detail below using  FIG. 3 .  FIG. 3  shows the side section  10  according to  FIG. 2  with a view of the inner side  23  of the bumper, wherein the wheel arch  21  (not shown) connects on the left according to the representation, whereas the rear section  21  (also not shown) connects on the right. The side section  10  contains a lower edge  40  that is inclined to the horizontal plane or to the ground  16  at an angle α of about 13.5° in this case. In this case, the rear end of the lower edge  40  on the vehicle side is disposed further from the ground  16  than the front end. 
         [0034]    As can be seen from  FIG. 3 , the two electrodes  30 ,  31  are disposed spaced apart and parallel to each other on the inner side  23  of the bumper. In the exemplary embodiment, both electrodes  30 ,  31  have about the same length L. Each of the electrodes  30 ,  31  is at least approximately installed linearly (i.e. not curved). Here each of the electrodes  30 ,  31  encloses an angle β with the lower edge  40  of the side section  10 . The angle β is in this case dimensioned such that each of the electrodes  30 ,  31  extends exactly or at least approximately parallel to the ground  16  if the bumper  8  is mounted as intended on the vehicle  1 . The angle β thus coincides in magnitude at least approximately with the angle α. 
         [0035]    In  FIG. 3 , in addition a first location  50  and a second location  51  are each characterized by an arrow. For each location  50 ,  51 , a time profile of signals is shown in  FIG. 4  or  FIG. 5  in each case, such as is produced on the electrodes  30 ,  31  in the event of an intended foot movement at the first location  50  or the second location  51 . During the intended foot movement, a foot of a user of a vehicle is extended in a typically short “kicking motion” below the bumper  8 , wherein the foot penetrates both into a detection space of the first electrode  30  and into a detection space of the second electrode  31 . 
         [0036]    In the diagrams according to  FIG. 4  and  FIG. 5 , each one of the measures of capacitance C detected at the respective sensor electrodes  30 ,  31  is represented as a function of time t. A first curve represents a proximity signal S 1  produced at the first electrode  30  in each case here, whereas a second curve represents a proximity signal S 2  produced by the second electrode  31  in each case. It can be seen from the diagrams that the individual signal profiles are at least approximately identical, both in the comparison of the two electrodes  30 ,  31  with each other (signal profile S 1  compared to signal profile S 2 ), and also in the comparison of the two locations  50 ,  51  with each other ( FIG. 4  compared to  FIG. 5 ). In particular, a change of the same magnitude of the measure of capacitance C is detected in signal pulses  60  in the respective proximity signals S 1 , S 2 . It is clear from this that owing to the arrangement according to the invention of the electrodes  30 ,  31 , in the event of the proximity of a body part to the electrodes  30 ,  31  an at least approximately equal attenuation of the proximity sensor  6  is always achieved at each location of the electrodes  30 ,  31 . A different attenuation over the length of the electrodes  30 ,  31 , as is detected in the case of a conventional manner of installation of the electrodes  30 ,  31  parallel to the lower edge  40 , is advantageously prevented owing to the arrangement according to the invention of the electrodes. 
         [0037]    In  FIG. 6 , the electrode  30  is shown once in a horizontal installation position (shown in solid) and once in a position that is inclined to the maximum extent relative to the horizontal installation position (shown dashed). With suitable dimensioning, the electrode  30  is installed such that in the installed state a distance A of the electrode  30  from the ground  16  varies over the length L of the electrodes  30  by no more than a difference in distance a of 20 mm here. As can be seen from  FIG. 6 , a maximum angle of attack γ, at which the electrode  30  should deviate most from the horizontal plane, at least approximately results from the maximum difference in distance a. The angle of attack γ is calculated for a predetermined length L of the electrode  30  according to equation. 1. To derive equation 1, an angle bisector  61  relative to the angle of attack γ is additionally plotted in  FIG. 6 . 
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         [0038]    Expressed in another way, the invention is in particular used if the angle α at which the lower edge  40  is inclined relative to the horizontal (see  FIG. 3 ) exceeds the maximum permissible angle of attack γ. 
         [0039]    The subject matter of the invention is not limited to the previously described exemplary embodiments. Rather, further embodiments of the invention can be derived from the previous description by the person skilled in the art. In particular, the described individual features of the invention and the design variants thereof can also be combined with each other in other ways. 
         [0040]    The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention:
     1  (motor) vehicle     2  device     3  tailgate     6  proximity sensor     7  control and analysis unit     8  bumper     10  side section     11  flap drive     12  door lock     15  vehicle coordinate system     16  ground     20  body assembly     21  wheel arch     22  rear section     23  inner side of bumper     30  (sensor) electrode     31  (sensor) electrode     32  electronics unit     35  housing     40  lower edge     50  location     51  location     60  signal pulse     61  angle bisector   A distance   a difference in distance   C measure of capacitance   L length   S 1 , S 2  proximity signal   X longitudinal direction of the vehicle   Y lateral direction of the vehicle   Z vehicle vertical   α angle   β angle   γ angle of attack

Summary:
A device for the contactless actuation of an adjustable vehicle part. The device has a capacitive proximity sensor with at least one elongated sensor electrode, and a body part to which the proximity sensor is indirectly or directly fastened. The body part in turn is or can be mounted on the vehicle in a specific mounting position. The sensor electrode extends obliquely relative to a lower edge of the body part in such a way that it is in an exact or at least approximately horizontal position with respect to its longitudinal extension when in the mounting position.