Abstract:
A method is described for detecting a rotating wheel of a vehicle that is travelling on a roadway in a travel direction, the wheels of which are at least partially exposed laterally, the method comprising: emitting an electromagnetic measurement beam having a known temporal progression of its frequency onto a first section above the roadway in a direction in a slant with respect to the vertical and normally or at a slant with respect to the travel direction; receiving a reflected measurement beam and recording the temporal progression of its frequencies, relative to the known progression, as a reception frequency mixture progression; and detecting a frequency band, which is continuously inclining or declining over a period of time, in the reception frequency mixture progression as a wheel. A device for conducting the method is also described.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a U.S. national phase of International Application No. PCT/EP2012/061645 filed Jun. 19, 2012, which designated the U.S. and claims priority to European Patent Application No. 11 450 079.6 and European Patent Application No. 11 450 080.4, the disclosures of which are herein incorporated by reference in their entireties. 
     BACKGROUND 
     Technical Field 
     The present application relates to a method and an apparatus for detecting a rotating wheel of a vehicle that is travelling on a roadway in a travel direction, the wheels of which are at least partially exposed laterally. 
     Detecting vehicle wheels is of interest for numerous applications. Thus it is possible to infer with certainty from the recognition of wheels that a given traffic area is being driven on in order to, for example, monitor borders or to initiate certain actions such as triggering an alarm, switching on lighting, opening a barrier, taking a picture for monitoring purposes, etc. Modern traffic fee systems also frequently base the calculation of fees on the number of axles of a vehicles, such that the detection of wheels (wheel axles) can also be an important basis for charging or controlling road tolls, especially by means of mobile control vehicles, which are to control the number of axles of vehicles subject to road tolls while overtaking or in oncoming traffic. 
     From DE 10 2008 037 233 A1 it is known to detect the wheels of a moving vehicle based on the horizontal component of the tangential velocity, which differs from the remainder of the vehicle and brings about a corresponding Doppler frequency shift of a radar measuring beam. For this purpose, a radar speed measuring unit is used which irradiates the lower area of passing vehicles with a radar beam lobe and, from the returning frequency mixture, determines a single speed measurement signal that has signal maxima at the locations of the wheels. Gaps between a traction vehicle and its trailer can falsely indicate signal minima and intermediate “false” maxima, which lead to an erroneous wheel detection. 
     BRIEF SUMMARY 
     An aim of the present application is to create a method and an apparatus for detecting wheels which enable a safer detection than the known solutions. 
     This aim is achieved in a first aspect with a method, which is characterised by the steps of 
     emitting an electromagnetic measurement beam having a known temporal progression of its frequency onto a first section above the roadway in a direction in a slant with respect to the vertical and normal or at a slant with respect to the travel direction; 
     receiving a reflected measurement beam and recording the temporal progression of its frequencies relative to the known progression as a reception frequency mixture progression; and 
     detecting a band of frequencies which is continuously inclining or declining within a period of time in the reception frequency mixture progression as a wheel. 
     An embodiment is based on a novel approach of detecting a wheel passing substantially horizontally past a Doppler-sensor by an inclining (e.g. if the Doppler-sensor lies above the axle of the wheel, is pointed downwards and is moving towards the wheel) or declining (e.g. if the Doppler-sensor lies below the axle of the wheel, is pointed upwards and is moving towards the wheel) reception frequency mixture progression during the passage. Unlike the known state of the art (DE 10 2008 037 233 A1), not just a signal maximum per wheel is evaluated, but the signal progression during the passage of the wheel. 
     In the ideal case of a line-like measuring beam which strikes the wheel from above or at a slant from the side and normal to the travel direction, the progression of the frequency shift of the reflected measuring beam caused by the Doppler effect is line-like inclining or declining. If the measuring beam is not normal to, but at a slant with respect to the driving direction, a horizontal component of the tangential velocity of the wheel caused by the Doppler shift is added to this progression, which leads to an additional offset of the progression; however, this does not change the criterion of the detection of an inclining or declining reception frequency progression during the passage of the wheel. 
     Furthermore, in reality the cross section of a measuring beam is never ideally point-like but always expanded, e.g. to an area of incidence on the vehicle is the range of a few centimeters or some tens of centimeters. Thereby the reception frequencies are broadened or spread from the described line-like progression to a “mixture” or rather “band” of reception frequencies: On varying height or width positions in the area of incidence of the measuring beam the rotating wheel has varying vertical and horizontal components of the tangential velocity and thereby creates different Doppler frequency shifts which lead to a “splitting” or “spreading”, respectively, of the sending frequency of the measuring beam to a plurality of simultaneously reflected reception frequencies, a “reception frequency mixture”; viewed over time, the reception frequency mixture progresses as a band in the frequency/time plane with the described inclining or declining progression. 
     This spreading effect caused by the velocity of the wheel is superposed by a second parasitic frequency spreading effect which can be attributed to the different projection angles of the vertical and horizontal components of the tangential velocity onto the direction to the receiver: This projection angle varies according to the respective place of reflection in the area of incidence. The second spreading effect is independent of whether the vehicle body or the rotating wheel is passing the receiver at that moment and is solely determined by the geometrical constraints of the measurement setup. Both effects superpose to the mentioned band-like reception frequency mixture progression over time. 
     In a first embodiment said detecting can be carried out by evaluating the progression of the frequency average of the band, which frequency average shows the described incline or decline during the passage of the wheel. 
     In a second embodiment said detecting can be carried out by checking if the band falls into a given contour in the frequency/time plane. The contour constitutes the maximal boundaries in which the reception frequency progression for different sampling progressions can occur, and if all of the measurement data of the Doppler reception frequencies over time fall into said contour, there is a continuously inclining or declining band of frequencies in the reception frequency mixture progression, which indicates a wheel. 
     According to an embodiment, the measurement beam is emitted normally with respect to the travel direction at a slant downwards. Thereby a shadowing of the wheels can be minimised and the gap between a trailer and a traction vehicle can safely be detected on the one hand, and—with exception of the spreading effects mentioned above—the horizontal components of the velocity of the rotating wheel as well as the velocity component of the vehicle are ignored on the other hand, which eases the detection of said inclining and declining bands in the reception frequency mixture progression. 
     For further improvement of the band detection, in an optional embodiment the method can comprise following steps: 
     measuring the velocity of the body of the vehicle relative to the location of emission of the measurement beam and reception of the reflected measurement beam; and 
     compensating the reception frequency mixture progression by those frequency parts which are caused by the velocity of the vehicle body, before said detecting of the band is conducted. 
     For the same reasons the method can also comprise the following steps: 
     detecting the presence of a part of the body of the vehicle in a second section which lies above the first section, in the temporal progression as a passage time window; 
     wherein detecting the wheel in the reception frequency mixture progression is only conducted during said passage time window. 
     In knowledge of the passage time window of the vehicle the reception frequency mixture progression can be further processed to ease the detection of the band therein, namely by the steps 
     determining an interfering signal fraction in a section of the reception frequency mixture progression immediately preceding the passage time window; and 
     compensating the reception frequency mixture progression in the passage time window by the interfering signal fraction, before said step of detecting the band is conducted. 
     In another further embodiment of the method wheels, which are detected during the same passage time window, are assigned to the very same vehicle. The number of wheels of a vehicle can be used as a basis for e.g. a road-toll charging dependent on the number of axles. 
     To further keep said parasitic spreading effects low and to obtain a distinct inclining or declining progression of the reception frequency mixture, the area of incidence of the measuring beam on the vehicle may be minimised. The measuring beam has an area of incidence whose diameter is less than a wheel which is to be detected, such as less than 10 cm, and especially less than 5 cm. 
     In a variation, a concentrated laser beam can be used for this purpose, or, in an alternative variant, the measuring beam is a radar beam emitted by a directional antenna, such as in a frequency range above 70 GHz. With such high frequencies the wavelength is very small and the antennas can thereby be mechanically realised very small with a high antenna gain, e.g. in form of horn antennas or antenna arrays. 
     In a second aspect an apparatus is created for detecting a rotating wheel of a vehicle that is travelling on a roadway in a travel direction, the wheels of which are at least partially exposed laterally, the apparatus being characterised by 
     a Doppler-lidar device or a Doppler-radar device which emits an electromagnetic measurement beam having a known temporal progression of its frequency onto a target above the roadway in a direction in a slant with respect to the vertical and normal or in a slant with respect to the travel direction; and 
     which records the temporal progression of the frequencies of the measurement beam reflected by the target, relative to the known progression, as a reception frequency mixture progression; and 
     a subsequent evaluation device configured to detect a band of frequencies which is continuously inclining or declining over a period of time in the reception frequency mixture progression, as a wheel. 
     With regard to the advantages of the apparatus it is referred to the teachings stated above for the method. 
     The measuring beam of the Doppler-lidar device or the Doppler-radar device may be oriented normally with respect to the travel direction and at a slant downwards. 
     It is especially favourable if the apparatus has a sensor connected to the evaluation device for measuring the velocity of the body of the vehicle, wherein the evaluation device compensates the reception frequency mixture progression by those frequency parts which are caused by the velocity of the vehicle body. 
     According to another feature the apparatus comprises a sensor connected to the evaluation device which detects the presence of a part of the body of the vehicle above that section onto which the measurement beam is directed in the temporal progression as a passage time window, wherein the evaluation device detects a wheel in the reception frequency mixture progression only during said passage time window. In this case, the evaluation device can optionally be configured to determine an interfering signal fraction in a section of the reception frequency mixture progression immediately preceding said passage time window and to compensate the reception frequency mixture progression in the passage time window by said interfering signal fraction. 
     In case of a Doppler-radar device, its measuring beam may be a radar beam emitted by a directional antenna, especially in a frequency range above 70 GHz; in case of a Doppler-lidar device the measuring beam may be a concentrated laser beam. 
     The apparatus is suited for both a stationary as well as a transportable, especially a mobile use. In the first case the apparatus can—if it works with a Doppler-radar device—be designed especially as to be assembled with the radio beacons of an already existing radio-road infrastructure, like WLAN (wireless local area network), WAVE (wireless access in a vehicle environment) or DSRC (dedicated short range communication). In a practicable embodiment the Doppler-radar device is designed as a roadside WLAN, WAVE or DSRC radio beacon. In the second case the Doppler-lidar device or the Doppler-radar device is mounted on a mobile platform, such as a control vehicle, to permit the control of vehicles on different road lane or in the oncoming traffic. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
       Further features and advantages of the method and of the apparatus will become apparent from the subsequent description of an embodiment with regard to the enclosed drawings, in which: 
         FIGS. 1 and 2  show the apparatus mounted on a control vehicle in combination with a vehicle controlled by it in a top view ( FIG. 1 ) and viewed in the travel direction ( FIG. 2 ); 
         FIG. 3  shows the velocity relations in a rotating wheel in detail; 
         FIG. 4  shows an exemplary reception frequency mixture progression in the frequency/time plane during measurement of the wheel of  FIG. 3  laterally from above and normally to the direction of travel in oncoming traffic; 
         FIG. 5  shows the derivation of a detection contour for detecting an inclining and declining band in the reception frequency mixture progression in the frequency/time plane; 
         FIG. 6  shows the geometrical relations in a real expanded measurement beam for exemplification of the frequency spreading effects caused by velocity and by geometry; 
         FIGS. 7 a  to 7 g    show exemplary idealised reception frequency progression and the frequency averages, respectively, of reception frequency mixture progressions at different angular positions of the Doppler measurement beam with respect to the wheel; 
         FIG. 8  shows the effect of the frequency spreading caused by geometry during the passage of a vehicle in the temporal progression; and 
         FIG. 9  shows the implications of the effects of the frequency spreading of the reception frequency mixture progression caused by velocity and geometry during the passage of a vehicle with two exemplary wheels, wherein in the left and in the right half of  FIG. 9  two different cross sections of the measuring beam are used. 
     
    
    
     In  FIGS. 1 and 2  a vehicle  1  is moving on a roadway  2 , more precisely on a lane  2 ′ of the roadway  2 , in a travel direction  3 ′. The vehicle  1  has wheels  4  which protrude downwards above the body  5  of the vehicle  2  and are thereby exposed—at least partially—on the sides of the vehicle body  5  in recesses thereof, i.e. they can be seen from the side. 
     DETAILED DESCRIPTION 
     On a second lane  2 ″ of the roadway  2  a control vehicle  6  is moving in an opposite travel direction  3 ″. The travel directions  3 ′,  3 ″ may be anti-parallel, but could also be parallel, i.e. the control vehicle  6  could overtake the vehicle  1  or vice versa. The control vehicle  6  could also be stationary and the travel directions  3 ′,  3 ″ could be non-parallel; in the following the relative movement direction of the vehicle  1  with respect to the control vehicle  6  is denoted as the travel direction  3  of the vehicle  1 . For simplicity it is also assumed that the travel direction  3  is approximately normal to the axles  4 ′ of the wheels  4  and is approximately horizontal, although this is not compulsory and deviations thereof are merely reflected in correspondingly changed projection angles of the velocity components considered in the following. 
     The control vehicle  6  carries a measuring apparatus  7  with a Doppler-lidar or Doppler-radar device  8  which emits an electromagnetic measuring beam  9 , in this case a lidar or radar measuring beam, onto the vehicle  1  or its wheels  4 , respectively, during the passage to detect the wheels  4  of the vehicle  1 . The measuring beam  9  is oriented in an angle β to the vertical V and in an angle γ to the travel direction  3 . The angle β is 0≦β&lt;90° or 90°&lt;β≦180°, in any case ≠90°, i.e. the measuring beam  9  runs at a slant to the vertical V, such as at a slant downwards as shown, e.g. in an angle of β=100° to 170°, including β=120° to 150°. In an alternative (not shown) embodiment the measuring beam  9  could also be directed at a slant upwards, e.g. β=10° to 80°, including β=30° to 60°, if the Doppler lidar/radar device  8  is mounted close to the ground, e.g. stationary on the side of the road  2 , and aims at the vehicle  1  and its wheels  4  at a slant from below. 
     The angle γ may be 90°, i.e. the measuring beam  8  is oriented normally to the travel direction  3 . In alternative variants the angle γ can also be ≠90°, e.g. at a slant forwards or backwards, as viewed from the control vehicle  6 . 
     In a manner known in the art, the Doppler lidar/radar device  8  evaluates the reception frequency of the measurement beam  9  reflected by the vehicle  1  or its wheels  4 , wherein the (projected) component v p  of the relative vehicle velocity v of the vehicle  1 , or the tangential velocity v t  of the wheel  4  at the respective point P of the incidence area of the measurement beam  9  (see  FIGS. 3 and 5 ), respectively, lying in the direction of the measurement beam  9 , can be determined e.g. from the Doppler effect induced frequency shift between emitted and reflected measurement beams  9 . The wheels  4  of the vehicle  1  can then be detected from this information, as will be described in greater detail below. 
     The Doppler lidar/radar device  8  itself can be of any type known in the art, whether with a continuous, modulated, or pulsed measurement beam  9 . For a continuous measurement beam  9  a Doppler frequency shift between the natural frequencies (“carrier frequencies”) of the emitted and reflected measurement beam  9  can be determined by interference measurement. For a pulsed or modulated measurement beam, a Doppler shift between the pulse rates or modulation frequencies of the emitted and the reflected measurement beams  9  can be measured. The terms “sending frequency” of the measuring beam  9  and “reception frequency” of the reflected measurement beam  9  used herein are understood to mean all such natural, carrier, pulse, or modulation frequencies of the measurement beam  7 , i.e., the term reception frequency comprises any type of frequency of the measurement beam  9  which can be influenced by the Doppler effect. 
     As shown in  FIG. 2 , the measuring apparatus  7  further comprises a velocity sensor  10  to measure the (relative) movement v of the vehicle  1  with respect to the control vehicle  6 , as well as a presence sensor  11  to detect the presence of a part of the vehicle body  5  during the passage of the vehicle  1  at the control vehicle  6 . The presence sensor  11  “sees” and detects the vehicle body  5  in a section in which the measuring beam  9  is directed onto the vehicle  1  during the vehicle passage, whereby a passage time window T F  of the vehicle  1  can be determined with respect to the lidar/radar device  8 , as will be described in greater detail below. The presence sensor  11  and its line of sight  12  may be arranged above the measuring beam  9  of the lidar/radar device  8 —or in a known geometrical relation thereto—to obtain a temporal relation between the passage time window T F  and the measurement signals of the lidar/radar device  8 . From the passage time window T F  and in knowledge of the velocity v measured by the sensor  10  the length L of the vehicle  1  can also be calculated according to L=v·T. 
     The lidar/radar device  8  and the velocity and presence sensors  10 ,  11  are connected to an evaluation unit  14  of the device  7 , which performs the evaluation calculations illustrated hereinafter. 
       FIG. 3  shows different embodiments of the measuring beam  9  with respect to its concentration or expansion, respectively, by means of several exemplary areas of incidence  16 ,  16 ′,  16 ″ with varying size on a wheel  4 . In a first variant the measurement beam  9  is strongly concentrated, so that its area of incidence  16  on the vehicle body  5  or the wheel  4  has a small diameter in the range of several centimeters, such as &lt;2 cm. Defined requirements are placed on the concentration of the measurement beam  9 , depending on the distance of the device  8  from the vehicle  1 : In the ideal case, the measurement beam  9  is a bundle of nearly parallel light or radar rays that can be obtained with a laser. But even with a radar measurement beam, a corresponding concentration can be achieved by using radar waves with a very high frequency, such as above 70 GHz, which have nearly the properties of light and can be concentrated e.g. by radar lenses. The use of directional antennas, e.g. horn antennas, antenna arrays and patch antennas, with the most parallel, small-diameter radiation characteristic possible, also generates an appropriate radar measurement beam. Especially suited are radar devices from the automotive field, such as those used in vehicles as collision and distance warning devices. Such concentrated measurement beams  9  have a concentration or a diversion or expansion range (aperture angle) of less than 1° (which corresponds to a solid angle of less than approximately 0.00024 sr). 
     In a second embodiment the measuring beam  9  is expanded wider, e.g. scattered or expanded in a plane or cone, in the manner of a “measuring beam lobe” with a substantially larger area of incidence  16 ′. Such an area of incidence  16 ′ can be achieved in a lidar device e.g. by a disperging lens placed in front thereof, or appears with radar devices whose concentration is not exact. 
     In the case of radar, a widened measurement beam  9  is characterised by the aperture angle of the radar antenna being used. The aperture angle (or the half-value width) of a directional antenna refers to the points where the power has declined to half (−3 dB) relative to the maximum. As known to those skilled in the art, the gain of the antenna in its main radiation direction can be estimated with the following formula from knowledge of the respective aperture angle: 
     
       
         
           
             g 
             = 
             
               10 
               ⁢ 
               lg 
               ⁢ 
               
                 27.000 
                 ΔφΔϑ 
               
             
           
         
       
     
     where 
     g=gain [dBi] 
     Δφ=horizontal aperture angle (in degrees) 
     Δθ=vertical aperture angle (in degrees) 
     The aperture angle of the radar antenna of the device  8  should allow for a good separation of the individual wheels  4  in the measurement signal of the vehicle  1  to be detected. Thus, it is e.g. favorable if the incidence area  16 ′ of the measurement beam lobe  15  does not exceed half the diameter of the wheel  4  of the vehicle  1 . The optimal area of incidence  16 ′ results from the measuring distance from the vehicle  1  and therefore the selection of the radar antenna depends on the geometry of the overall arrangement. In general, antennas with a gain g of more than 10 dB are especially suitable, depending on the arrangement and frequency of the radar device  8 . 
     Directional antennas usually have an antenna gain g of more than 20 dB (which corresponds to an aperture angle Δφ=Δθ=approx. 16°). Thus, an area  16 ′ that is 28 cm in diameter can be illuminated 1 meter away from the vehicle  1  with an antenna gain of 20 dB. An antenna gain g of 30 dB can be necessary for more distant vehicles  1  in order to achieve an aperture angle Δφ=Δθ=approx. 5°, which implies an illumination area  16 ′ of approx. 30 cm in size at a distance of 3 m. 
     In a third variant the size of the area of incidence  16 ″ of the measuring beam  9  on a wheel  4  is between the size of the variants  16  and  16 ′, e.g. in a range of 2-10 cm, such as 2-5 cm. 
       FIG. 3  shows the movement of the area of incidence  16 ,  16 ′,  16 ″ during the mutual passage of the vehicle  1  and the control vehicle  6  along a sampling line  17  which crosses the wheel  4  about in the middle of its upper half in this example. The tangential velocity v t  or v t (P) occurring on a point P of the sampling line  17  on a radius r of the wheel  4  rotating in the rotation direction U can be divided into a horizontal component v t,h (P) and a vertical component v t,v (P). The horizontal component v t,h (P) stays substantially constant on a given horizontal sampling line  17 , whereas the vertical component v t,v (P) changes from a negative maximum value v t,v (A) on a point A on the circumference of the wheel to the value 0 at a point B on the axis  4 ′ of the wheel up to a positive maximum value v t,v (C) at a point C on the other circumference of the wheel. 
     In detail, the tangential velocity v t (r) on a radius r is proportional to this radius r, namely 
     
       
         
           
             
               
                 
                   
                     
                       v 
                       t 
                     
                     ⁡ 
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       r 
                       R 
                     
                     ⁢ 
                     
                       v 
                       t 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     The vertical component v t,v (r) of the tangential velocity v t (r) at an angle α is a cosine projection corresponding to 
     
       
         
           
             
               
                 
                   
                     
                       v 
                       
                         t 
                         , 
                         v 
                       
                     
                     ⁡ 
                     
                       ( 
                       r 
                       ) 
                     
                   
                   = 
                   
                     
                       r 
                       R 
                     
                     ⁢ 
                     
                       v 
                       t 
                     
                     ⁢ 
                     cos 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     α 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     With 
                     cos   ⁢           ⁢   α     =     g   r             (   3   )               
the horizontal component v t,v (r) of the tangential velocity results to
 
                       v     t   ,   v       ⁡     (   r   )       =       v   t     ⁢     g   R               (   4   )               
where g is the horizontal distance to the center of the wheel and thereby—when sampling with a constant velocity v—proportional to the time t, which describes a linear incline or decline.
 
     If the measuring beam  9  is directed normally to the travel direction (γ=90°) and e.g. at a slant from above (90°&lt;&lt;β&lt;180°), the lidar/radar device  8  measures a frequency shift Δf due to the Doppler effect, which corresponds exactly to this vertical component v t,v (P). The frequency shift Δf is depicted in  FIG. 4  over the time t as a reception frequency progression  18 . The Doppler shift Δf of the reception frequency with respect to the sending frequency is proportional to the vertical velocity component v t,v  of the corresponding sampled parts (points P) of the vehicle  1  or wheel  4 , respectively; the reception frequency progression  18  depicted in  FIG. 4  is therefore equivalent to a vertical velocity progression. 
     The reception frequency progression  18  of  FIG. 4  is an idealised progression for an idealised measuring beam  9  with a point-like cross section of the beam. The progression  18  shows a linear incline from v t,v (A) to v t,v (B) crossing the point of origin during a time segment T R  which corresponds to the sampling of the wheel along the sampling line  17  with the velocity v. Would the measuring beam  9  be directed at a slant from below onto the wheel  4  (β&gt;90°) or be moved in the opposite direction along the sampling line  17  (e.g. control vehicle  6  overtakes vehicle  1 ), then the reception frequency progression  18  shows a decline, i.e. it is mirrored about the time axis t of  FIG. 4 . 
     Because of the expansion of the area of incidence  16 ,  16 ′ or  16 ″, respectively, of a real, non-idealised measuring beam  9 , for each sending frequency emitted at a specific point in time t not only one reception frequency, which is shifted by the Doppler effect, is received, but a slightly differing reception frequency from each different point in the area of incidence  16 ,  16 ′,  16 ″. On one hand this is due to the fact that on a height h 1  differing from the height h of the sampling progression  17  the vertical component v t,v  (and also the horizontal component v t,h ) of the tangential velocity v t  each has a slightly differing value, such that the reception frequencies originating from different points of incidence P in the areas  16 ,  16 ′,  16 ″—compare the exemplary sampling progression  17 ′ in  FIG. 3 —superpose to a mixture of differing reception frequencies or velocities, respectively, see  FIG. 4 . 
     In other words, the reception frequency f splits or spreads to a mixture F of reception frequencies (or velocities) caused by the Doppler effect, respectively, during the passage T f  of a vehicle  1  when a wheel  4  occurs, which leads to a reception frequency mixture  20  over time t. 
     The frequency spread effect caused by the velocity of the wheel is superposed parasitically by a second frequency spreading effect which is caused by the geometry of a measuring beam  9  flared in a cone shape. As can be seen from  FIG. 6 , the radar/lidar device  8  observes, from a position P 1 , different points P 2 ′, P 2 ″ in the area of incidence  16 ′ of the measuring beam  9  each under a different spatial direction  21 ′,  21 ″, which each enclose a different solid angle with the vertical and horizontal components v t,v  and v t,h  of the tangential velocity v t  of the wheel  4  or the velocity v of the vehicle body  5 , respectively. The projection of the velocity v t,v  or v t,h , respectively, onto the measuring beam direction  21 ′,  21 ″ et cet. in the measuring beam  9  thereby leads to a splittering or spreading, respectively, caused by the geometry in the areas  16 ,  16 ′,  16 ″. 
     The spread caused by the velocities of the rotating wheel ( FIG. 3 ) superposes with the spread caused by the geometry ( FIG. 6 ) to the “real” reception frequency mixture progression  20  with the frequency spread F varying over time t. 
     As can be seen from  FIG. 4 , the reception frequency mixture progression  20  therefore shows for a measuring beam  9 , which is directed at a slant from above or at a slant from below (0&lt;β&lt;180°) and approximately normally to the travel direction  3  (γ=90°), a continuously inclining or—depending on the viewing direction—declining band  22  during the passage time T F  of the wheel  4 , which can be used as a criterion for the occurrence of a wheel and therefore for the detection of the wheel  4 . For example, the band  22  can be detected by signal analytical means by averaging the occurring reception frequency mixture F, i.e. by analysis of the frequency average (which again substantially corresponds to the idealised progression  18 ). 
       FIG. 5  shows an alternative way of the detection of the occurrence of an inclining or declining band  22 , namely by checking if the reception frequency mixture progression  22  falls into a given contour  22 ′, which constitutes the maximum boundaries in which reception frequency progressions  18   0 ,  18   1 ,  18   2 , . . . , generally  18   i , for different sampling progressions  17   0 ,  17   1 ,  17   2 , . . . , on different heights h 0 , h 1 , h 2 , . . . , can occur. The superposition of all possible reception frequency progressions  18   i  for a certain area of incidence  16  provides the given contour  22 ′ in the frequency/time plane of  FIG. 4 or 5 , respectively, into which a band  22  falls in any case. 
     Although the size and form of the contour  22 ′ indeed depends on the size of the area of incidence  16 , the global progression of the contour  22 ′ over time t is always inclining or declining. By checking if all (or at least the predominant part, i.e. except for a few statistical “outliers”) reception frequency measurements of the reception frequency mixture progression  20  lie within the contour  22 ′, the occurrence of a band  22  continually inclining or declining over a period of time can again be detected. 
     If the measuring beam  9  is not directed normally to the travel direction  3  but at a slant (γ≠90°) thereto onto the vehicle  1  or the wheels  4 , respectively, due to the projection of the horizontal components v t,h  of the tangential velocity v t  of the wheel  4  onto the direction of the measuring beam an additional horizontal velocity component is measured which is constant for a certain height h, h 1  of the sampling line  17  and weighs in as an offset on the idealised reception frequency progression  18  or real reception frequency mixture progression  20  of  FIG. 4 . In  FIG. 7  this is shown for the idealised reception frequency progression  18  of  FIG. 4 , and the following Table 1 depicts the values of β and γ for the examples of  FIGS. 7 a  to 7 g   : 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 FIG. 7 
                 β 
                 γ 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 a) 
                 135° 
                 90° 
               
               
                   
                 b) 
                 135° 
                 135° 
               
               
                   
                 c) 
                 135° 
                 45° 
               
               
                   
                 d) 
                 90° 
                 45°/135° 
               
               
                   
                 e) 
                 45° 
                 90° 
               
               
                   
                 f) 
                 45° 
                 135° 
               
               
                   
                 g) 
                 45° 
                 45° 
               
               
                   
                   
               
             
          
         
       
     
     In knowledge of the velocity v, which e.g. is measured by the velocity sensor  10  or by the device  8  itself, the reception frequency progressions  18  or reception frequency mixture progressions  20  can be corrected or compensated, respectively, by the respective parts v t,h  caused by the velocity, which correspond to an offset compensation of  FIGS. 7 a   ) to  7   g ) and again leads back to the exemplary reception frequency mixture progression shown in  FIG. 4  or to a progression mirrored about the time axis t. 
       FIG. 8  shows the measurement of a passage time slot T F  for the passage of a whole vehicle  1  with respect to the device  8  or measurement beam  9 , respectively, such as by means of a separate presence sensor  11 . For example, the presence sensor  11  can again be a radar or lidar device, which emits a radar or lidar measurement beam  12  onto the passing vehicle  1  to measure the duration T F  of the vehicle passage and to reference the recorded reception frequency mixture progression  20  thereto. 
     In  FIG. 8  the measurement beam  9  was exclusively directed onto the vehicle body  5  for means of comparison, namely under an angle of γ≠90°, i.e. at a slant to the travel direction  3 , such that the relative velocity v of the vehicle  1  during the vehicle passage T F  can be measured as a rectangular frequency shift, which is spread to a reception frequency mixture F in a band  23 , which is caused exclusively by the spread caused by the geometry of an conically flared measuring beam  9  according to  FIG. 6 . 
     An interfering signal fraction in the reception signal of the lidar/radar device  8  which is occurring outside of the vehicle passage T F  is denoted by  24 . In knowledge of the passage time window T F , a section  25  immediately preceding the passage time window T F , or a section  26  immediately succeeding the passage window slot T F  can be extracted from the reception frequency mixture progression  20  and the interfering signal fraction  24  can be determined therein; this interfering signal fraction  24  can be used to compensate the reception frequency mixture progression  20  for this interfering signal fraction  24 . For example, a frequency analysis of the reception frequencies occurring in the sections  25 ,  26  could be performed and these could be deleted or subtracted, respectively, from the reception frequency mixtures F during the vehicle passage T F . 
     To this end the section  25  preceding the vehicle passage T F  may be used, because the vehicle  1  could, for example, have a trailer which could mistakenly be used as an interfering signal in the succeeding section  26 . 
     Furthermore the determination of the passage time window T F  can be used to assign all those wheels which are detected during the passage time window T F  to this very same vehicle  1 , which can be calculated accordingly from the evaluation unit  14  of the apparatus  7 . 
     The passage time window T F  of the vehicle passage could also be directly determined from the radar/lidar device  8  instead of the separate presence sensor  11 , i.e. with the very same measuring beam  9 . If the measuring beam  9  is directed under an angle of γ≠90° (as in  FIG. 8 ) onto the vehicle  1 , the passage time slot T F  could be determined e.g. on the basis of the frequency shifts on leaps  27 ,  28  of the band  23 , and/or from the occurrence of the frequency spread caused by the geometry in the reception frequency mixture progression  20 . 
     The determination of the relative velocity v of the vehicle  1  could also be conducted by e.g. the lidar/radar-device  8  itself, e.g. by means of the size of the frequency leaps  27 ,  28  of the band  23 , instead of the separate velocity sensor  10 . 
       FIG. 9  shows two exemplary reception frequency mixture progressions  20 , after these have been corrected by the components due to the velocity v of the vehicle  1  on the one hand and by the interfering signal fractures  24  that were determined in the preceding section  25  on the other hand. In the left half of  FIG. 9  the occurrence of a continuously inclining band  22  in the reception frequency mixture  20  is apparent, which indicates a wheel  4 , in the case of a small area of incidence  16 . In the right half of  FIG. 9  the same situation is depicted when the area of incidence  16 ′ of the measuring beam  9  on the wheel  4  is larger than half the wheel diameter, such that the measuring beam  9  simultaneously measures significant positive and negative vertical components v t,v  of the wheel  4  at certain points in time. This leads to a closer “merging” of the beginning and ending spikes of the reception frequency mixture  20 , i.e. to a steeper incline or decline  18 . 
     The device  7  can both be realised in mobile form, e.g. mounted on the vehicle  6 , and in stationary form, e.g. using existing wireless infrastructure of a roadway, e.g., using WAVE or DSRC radio beacons of a road toll system or WLAN radio beacons of a roadside Internet infrastructure. Thereby already existing transmitter components of the WLAN, WAVE, or DSRC radio beacons can be used as transmission components of the Doppler radar device  8 ; receiver sections of the radio beacons can likewise be used as the receiver components of the Doppler radar device  8 , or can at least be integrated into the receiver components of the radio beacons. The apparatus and the method can be implemented in this manner as a software application running a conventional mobile or stationary WLAN, WAVE, or DSRC radio control device or beacon, for example. 
     It has been assumed that the transmission frequency of the radar/lidar device  8  or the measurement beam  9  is constant, i.e., its progression over time (temporal progression) is a constant progression. However, it is also possible that the device  8  could emit a measurement beam  9  with a temporally non-constant transmission frequency progression, e.g., as in frequency hopping methods in which the frequency changes constantly according to a predetermined or known pattern. The recorded reception frequency (mixture) progressions  18 ,  20  are recorded relative to the known temporal progression of the transmission frequency of the measurement beam  9 —whether constant or varying, i.e., referenced or standardized thereto, so that the effect of known transmission frequency progressions can be compensated. 
     CONCLUSION 
     The invention is thus not restricted to the described embodiments, but also encompasses all variations and modifications which fall under the scope of the enclosed claims.