EP1473208B1 - Process for detection of track occupation - Google Patents

Abstract

A broadband, information- (S) containing signal (SB) generated in a unit (50) modulates the transmitter (5). Signals (SB') registered by the receiver (6) are sent to a unit (60) for information recovery. Transmitter and receiver are coupled. Difference in received signal level is indicative of rail icing.

Description

The present invention relates to a method for detection a track occupancy according to the preamble of the claim 1.

From the state of railway safety technology is known, the Occupancy of a track with a track circuit to detect. These are the two rails of a track section isolated from each other. When driving through a Railway vehicle, the two rails are electrically shorted and thereby the occupancy can be reliably detected become. Likewise, track vacancy reporting means the axle counting - mechanically or electronically - known. For example, in [1] an axle detector is disclosed in which by means of an inductance passing through a conductor loop and The rails are formed in a limited section that Pre-crossing of a wagon or axle by short-circuiting the aforementioned inductance is detected. An application of an axle counting sensor is presented in [2]. Electronic detection systems and in particular electronically working axle counters have the disadvantage that they turn on the relevant working frequencies, such as 33kHz, 43kHz or 850 kHz or 1.2 MHz exposed to interference and therefore have limited availability. The interference can be caused, for example are powered by electric eddy current brakes and inverter locomotives. As stated in [3], a relocation of the Operating frequencies in supposedly interference-free areas, bring lasting success. Especially with very broadband Sources of interference can actually no working frequency which are outside the frequency band of the whole the sources of interference. This is suggested in [3], an interference field through a first coreless coil to detect and provide a second coreless coil to the Compensate interference field. This application can not full compensation can be achieved as these are only for homogeneous interference fields acts. Because the field distribution radially around the rail profile goes, are inhomogeneous interference always available. The compensation acts at the resonance of a Circle around a working frequency point therefore not complete, since the compensation is frequency-dependent. An overlay strong transient currents can cause dimming signals or no compensation of the interference field to have.

In [4] a wheel sensor is proposed which has two independent, having galvanically isolated Radsensorsysteme. These detect the wheel flange of a railway wheel. The operating principle based on an electromagnetic alternating field deprived of energy by damping with metal. This System requires monitoring of correct assembly.

The present invention is therefore based on the object to provide a method for detecting the lane occupation, that as far as possible immune to electromagnetic interference in a wide frequency range and no settings required during installation.

This object is achieved by the in the claim 1 specified method solved.

The inventive method opens up the possibility existing installations on the track, be it Axle counter or track circuits, continue to use. By the Coupling of transmitting unit and receiving unit requires this inventive method no calibration and no level adjustments during installation. Through the spectral broadband, information containing signal is ensures that occurring interference fields, for example by magnetic rail brakes or eddy current brakes caused, does not affect the safety-related Detecting a track occupancy.

The spectrally wideband signal containing an information can be coded in various ways, for example through the Direct Sequence Spread Spectrum technique (DSSS) or by repetitive sending of a rising or falling Frequency curve. Because with this procedure always a receiver is present, this receiver can also be used for detection use of static or transient interference fields and thereby, in the sense of a learning system, it can be sent out Frequency course, if necessary outside or on Edge of an interference field to be created and safety be further improved. But this learning behavior is not absolutely necessary for the process according to the invention.

Further advantageous embodiments of the invention are in further claims.

Figur 1

Prinzipdarstellung eines Gleisstromkreises bei Anwendung einer ersten Ausführungsform der vorliegenden Erfindung;

Figur 2

Anordnung von Sende- und Empfangsspulen am Gleis;

Figur 3

Aufbau eines Radsensors in montiertem Zustand an einer Schiene;

Figur 4

Prinzipdarstellung der Spreizung eines Signals für die DSSS-Technik;

Figur 5

Darstellung eines gestörten Signals S_B';

Figur 6a

Darstellung des zeitlich linearen FSK-Verfahrens

Figur 6b

Darstellung des zeitlich logarithmischen FSK-Verfahrens;

Figur 6c

Darstellung des Verlaufes des Sende- und Empfangspegels über ein Frequenzband beim FSK-Verfahren;

Figur 7

Prinzipschema zur Raddetektion für FSK-Verfahren;

Figur 8

Prinzipschema zur Raddetektion für FSK-Verfahren für digitalen Signalprozessoren.

The invention will be explained in more detail with reference to a drawing, for example. Showing:

FIG. 1

Schematic representation of a track circuit when using a first embodiment of the present invention;

FIG. 2

Arrangement of transmitting and receiving coils on the track;

FIG. 3

Structure of a wheel sensor in the assembled state on a rail;

FIG. 4

Schematic representation of the spread of a signal for the DSSS technique;

FIG. 5

Representation of a disturbed signal S _B ';

FIG. 6a

Representation of the temporally linear FSK method

FIG. 6b

Representation of the time-logarithmic FSK method;

FIG. 6c

Representation of the course of the transmission and reception level over a frequency band in the FSK method;

FIG. 7

Schematic diagram for wheel detection for FSK methods;

FIG. 8

Schematic diagram for wheel detection for FSK methods for digital signal processors.

1. Detektion mit der Direct Sequence Spread Spectrum Technik, im folgenden DSSS-Technik genannt;

2. Detektion mittels Frequency Sweep Keying Technik; im folgenden FSK-Technik genannt. Bei dieser FSK-Technik ist ein steigender oder fallender Frequenzverlauf innerhalb eines vorgegebenen Frequenzbandes vorgesehen: "überstreichen" = sweep des Frequenzbandes.

The invention will be explained with reference to two coding methods using a broadband frequency range for the detection of a track occupancy:

1. Detection with the Direct Sequence Spread Spectrum technique, called DSSS technique in the following;

2. Detection by frequency sweep keying technique; in the following called FSK technique. In this FSK technique, an increasing or decreasing frequency response within a given frequency band is provided: "sweep" = sweep of the frequency band.

In this document, the frequency sweep keying technique is different from the otherwise commonly used meaning labeled with FSK technology. It may therefore not with the so-called Frequency Shift Keying Technique (also called: Frequency shift keying method) for which also the acronym FSK is in use.

A1 "Galvanische" Detektion einer Geleisebelegung über einen Gleisstromkreis durch "Kurzschliessen" zweier einen Geleiseabschnitt bildende Schienen;

A2 magnetische Detektion eines Rades und Anwendung einer Rad- bzw. Achszählmethode.

The above two techniques are applicable to:

A1 "Galvanic" detection of a track occupancy via a track circuit by "short-circuiting" two rails forming a track section;

A2 magnetic detection of a wheel and application of a wheel or axle counting method.

For the magnetic detection, an explanation will be given Figures 2 and 3. The magnetic or galvanic detection are independent of the aforementioned two coding methods.

Ein Information S in binärer Darstellung der Breite n Bit wird gemäss der DSSS-Technik mit einer sogenannten "Chipping Sequence" B - auch Barker Code B genannt - exklusiv-oder verknüpft. Diese Verknüpfung wird meist mit XOR bezeichnet.

The first embodiment of the present invention is explained with reference to the "galvanic" implementation form according to FIG. 1 and includes the following steps:

An information S in binary representation of the width n bits is exclusive-or linked according to the DSSS technique with a so-called "chipping sequence" B - also called Barker code B. This link is usually called XOR.

The linkage XOR is shown in FIG. 4, where for the information S only a width of n = 2 bits has been assumed, the result of the linkage XOR with the barker code B is denoted by S _B. The width m of the Barker code B is m = 11 bits in this representation. An information S _B spread in this way is modulated to a carrier frequency.

Fig. 1 shows two separated by a rail insulation 12 Track sections. For a track section is in the distance d from a transmitter / transmitter 5 a receiving circuit with a Receiver / transformer 6 connected. A typical area for such a distance d lies in the interval 25 m .. 2500 m. As stated above, the thus modulated Carrier frequency when used in a track circuit as shown in FIG. 1 on the transmitting coil 51 as Part of a transformer 5 switched. The secondary transmission coil 52 as part of the transformer 5 is connected to the two rails. 3 a track section connected. The receiver / transmitter 6 has a receiving coil 62 and in the primary circuit Secondary circuit on a receiving coil 61. The principal Function of such a track circuit in DC or AC technology is the following: Responds to a quiet track circuit a receiving device at a free track and the Rail-mounted relay or electronic detection "picks up". If a wagon is on the track section, it will change the electrical conditions. It creates a shunt, so that by the receiving device only a small Residual current flows. This will determine that the section is occupied, since the jelly relay then no longer "attracts". This construction has the consequence that all frequently occurring Errors such as interruptions of the circuit (for example by a Line break or defective fuses) to one alleged lead occupied track section. This procedure in DC or AC technique has the disadvantage that reverse currents in the rails can simulate an occupancy, the do not represent any occupancy by an axle or vehicle.

This also applies to a DC circuit, since, due to the high currents, only a few parts per thousand of a DC component cause the aforementioned relays to respond or not to respond. This disadvantage is remedied in this first embodiment of the present invention in that the spread signal modulated onto a carrier frequency band is broadband. The specified upper limit of the frequency band can be explained by the fact that above this frequency emission limits must be observed. The level of the signal S _B 'received by the receiver 6 in the receiver coil 62 is dependent upon the presence of a wheel 1 in the immediate vicinity of the transmitter / receiver located on the rail. In a receiving device (not shown in the figures), this signal S _B 'must be despread. This is generally much more complex than the spread. However, in the case of the present invention, it is particularly advantageous that the transmission clock is directly present in the reception device, because transmission and reception devices can be realized directly next to one another or as an integrated device. This essential requirement is generally not the case for DSSS messaging. Thus, no special effort for synchronization is provided in the receiving device. The received signal S _B 'is in turn XORed to the same Barker code B. The received signal S _B 'has according to the figure 5 at the point i a curve S _i . Of course, a non-ideal course also occurs in the other places, but this is not shown in FIG. 5, but is shown merely by way of example for location i. This signal S _i deviates from the mean signal strength S _Avg by D _i . The deviation D _i is then multiplied by -1 if the value of the Barker code at the position i is equal to 1. The result R _{i is} calculated from this value added to the average signal strength S _Avg . An integrator sums the values obtained for each position i = 1, 2,... N of the Barker code of length n. If the result is greater than the mean signal strength S _Avg , a comparator decides on this bit (the useful information S) to be 1 , otherwise to 0. In this way it can now be decided whether or not there is a wheel at the relevant point of the rail.

The particular advantage of the aforementioned coding with the DSSS technique is that there is an eleven-fold redundancy. This is given by the Barker code width of m = 11. In a narrow-band interference at different locations in the frequency band, the transmitted spread S _{B can} nevertheless be regenerated as information S. For example, the following frequencies or a frequency band from the frequency band listed below are used for the transmission:
10 kHz .. 30 MHz.

In a particularly advantageous further development of the present invention, the following can be provided by using the existing infrastructure such as the above-mentioned track circuit for increasing the detection reliability: The information S is spread with another Barker code B ₂ , this code comprising m ₂ digits. The spread signal S _B2 is modulated on a carrier frequency band disjoint with respect to the first carrier frequency band. For the Barker Code B ₂ no special conditions apply, in particular the lengths m and m _{2 may be} different. In this way, a redundant two-channel system for detecting a track occupancy is created, which is largely immune to interference on certain frequency bands and thereby optimally meets security requirements. Despite the redundancy, no second installation of facilities on the track is required. Such a second channel with another Barker code B ₂ can be implemented with relatively little effort.

The second embodiment of the present invention will be based on the "magnetic" implementation according to the Fig. 2 and 3 explained.

f₁ = 10 kHz

f₂ = 10 MHz

T_rep = 10 .. 2000 µs

T_ab = 2 .. 500 µs

FIG. 2 shows the arrangement of a transmitter 5 and a receiver 6 to the left and right of a rail 3 of a track 11. Transmitter 5 and receiver 6 are connected via a connecting line 8 to a connection terminal. It should be noted that the receiver 6 has at least two receiving coils 9a and 9b (not shown in Figs. 2 and 3), typically arranged at a distance of about 10 cm (-0 cm; + 10 cm). In Figure 3, the structural arrangement of the transmitter 5 and the receiver 6 is shown in a wheel sensor 9. In a housing protected transmitter 5 and receiver 6 resp. Reception coils 9a, 9b, .. housed and connected to a fastening means 7, such as a screw, fixed to the rail 3. The connection lines 8 are seen from the track ago outwards to an unillustrated terminal 10 out. Also not shown is the attachment of the rail at the thresholds represented by the upper edge 4. Between transmitter 5 and receiver 6 leading field lines 13 are shown on the top rail. The course of the field lines 13 is influenced by a passing wheel 1. To illustrate the orientation of the wheel 1 is shown with a rim 2. The transmitter 5 resp. the transmitting coil extend over the at least two receiving coils 9a, 9b. However, it could be provided even more receiving coil 9c and 9d. The transmission coil 5 is acted upon in a first variant with a signal according to the illustration of Fig. 6a. This signal, ie the frequency curve f _a is emitted repetitively in a grid of T _rep . This frequency curve f _a is linear within T _rep over the time from a first lower frequency f ₁ to an upper frequency f ₂ . In technical terms, such a linear or otherwise increasing or decreasing frequency response is referred to as a "sweep". In a time interval of T _from a subsequent course is f _b switched to the transmit coil. 5 The time interval T _ab can be used as a coding of a specific information S. Typical values for the aforementioned variables are:

f ₁ = 10 kHz

f ₂ = 10 MHz

T _rep = 10 .. 2000 μs

T _ab = 2 .. 500 μs

The receiving coils 9a and 9b detect over the field lines 13, the spectrum emitted by the transmitting coil 5. The level This spectrum is dependent on the presence of a Rades or an object made of a magnetic material between transmitting coil 5 and receiving coil 9a and 9b.

FIG. 6 c shows a constant level P _s over the frequency range, which is defined by the lower frequency f ₁ and the upper frequency f ₂ . The transmitting coil 5 is supplied via the aforementioned frequency range [f ₁ .. f ₁₂ ] this level. In the same diagram, a reception level P _S , is shown in a momentary level distance P _D , it is assumed that an interference field D _{f is} present in a small subrange around a frequency f. The term "momentary" mentioned above means for the relevant frequency f at a certain time. This reception level occurs at a reception coil 9a or 9b. This interference field is of course part of the received signal S _B , with the level P s _,. For clarification, however, this is designated in FIG. 6c particularly by D _f . Due to a self-calibration can be determined whether the respective level difference P _D between P _S and P _S , the presence of a wheel or not. This level difference P _D is "falsified" by the aforementioned interference field. However, this is irrelevant for determining the presence or absence of a wheel, as in discrete intervals over the frequency P _D range [f ₁ .. f ₁₂ ] detected the respective distance P _D , continuously summed and finally averaged. Thus, outliers caused by an interference field have no influence on the aforementioned determination of the presence or absence of a wheel. In addition, the interference fields are highly transient, so that with the given repetition T _REP in the range of 10-2000 μs, the instantaneous state with respect to the presence / absence of a wheel on a sensor is determined several times.

For further evaluation of the received spectrum is on Figures 7 and 8, reference is made.

Zählerstand = 0 : Geleiseabschnitt ist frei;

Zählerstand ≠ 0 : Geleiseabschnitt ist belegt.

FIG. 7 shows a first schematic diagram for wheel deflection by means of the FSK method. With the reference numerals 5 _a , 5 _b , etc. Synchronized transmitting devices for the frequency response f _a , f _b , etc. are shown. The signals generated by the transmitting devices 5 _a , 5 _b , etc. are fed to the transmitting coil 5. Depending on the presence of a wheel in the region of the transmitting coil 5, an induction takes place in the receiving coils 9a, 9b, etc. These receiving coils 9a, 9b, etc. are each connected to a receiving device 6a, 6b, etc., which in turn are connected to a generator for generating the repetition time T _REP . In this way, in the receiving devices 6a, 6b, etc., the received signal can be correlated with the signal supplied to the transmitting coil and occurring level differences for the presence of a wheel can be used. In a minimal embodiment, that is to say in a single-channel embodiment, two receiving devices 6a and 6b are to be provided. These are each preceded by a synchronized bandpass filter. The synchronization extends over the respective frequency curve f _a , f _b . In this way, disturbances in a specific frequency range can be filtered out at a certain point in time, without the signal induced in the receiving coils 9a, 9b being thereby corrupted, thus correlating with the transmitted signal, even in the case of spectrally and moderately massive interference Therefore, in a single-channel design, two receiver coils 9a, 9b and two receiver devices 6a and 6b are required in order to detect the direction of the wheel movement For the track section in question, a counter is clocked up or down by 1. Detection of a track occupancy itself takes place on the basis of the counter reading for the relevant track section:

Meter reading = 0: track section is free;

Counter reading ≠ 0: Track section is occupied.

FIG. 8 shows an evaluation of the frequency characteristics f _a and f _b generated according to FIGS. 6 _a in an implementation with digital signal processors. The transmitting device 50 has a voltage-controlled oscillator 55 (VCO voltage controlled oscillator). A controller 56 generates a corresponding voltage curve for the desired frequency profile. Not shown in FIG. 9 is a possibly provided feedback in order to avoid transient or quasi-static interference fields detected in the receiver coils 9a and 9b. For determining the respective distance T _{ab from} the two frequency curves f _a , f _b as well as for the repetition T _REP , a transmission clock generator 54 is provided. The receiver coils 9a and 9b are each connected to a broadband filter 71. The transmit clock generator is also connected to bandwidth attenuated filters 71 to supply the received level of the respective frequency to the A / D converters 72 in the respective time frame. The digital values of the levels at discrete time intervals are levels at specific frequencies, see FIG. 6c. Although the frequency is indicated on the abscissa, in a method with, for example, a linearly increasing frequency, this abscissa is also underlaid by a time axis. These digital levels are subjected to a reference storage 64 over time in order to be able to compensate for any aging phenomena. The time constant here is orders of magnitude of weeks or months. The above-mentioned digital signals are, as explained earlier on FIGS. 6a and 6c, used for wheel or axle recovery. The result is a value ± 1. The sign contains the direction of travel. The long-term return of the stored reference values has the consequence that this embodiment of the present invention requires no external calibration. The aforementioned value ± 1 is further evaluated in a known manner in the Achszählelektronik 80.

The above-mentioned evaluations / analyzes for a linear frequency characteristic f _a , f _b according to FIG. 6 a can be used to increase the robustness of the method according to the invention also for a frequency curve f _a and f _b according to FIG. 6 _b . FIG. 6b shows a logarithmic progression, the scale f ₁ to f _{2 being} linear. A representation with a logarithmic scale would also have been possible, the graphical representation then appearing as "linear". The advantage of a logarithmic progression lies in the fact that the residence time in an assumed interference band f _d1 and f _{d2 is} shorter than in the linear time profile according to FIG. 6a. In this way, a statically known interference band can be additionally avoided. Since in the receiving device 6a, 6b, etc., the respective emitted frequency response f _a , f _b , etc. is known, it would also be possible to determine by a detection and analysis of the disturbance field "the frequency response to be emitted so that the current, quasi-static interference field 6a and 6b, the frequency response over time is strictly monotonically increasing, but it is also possible to provide a frequency response that is either monotonically increasing, monotonically decreasing, or severe is monotonically decreasing.

In a further embodiment, the first course f _a can also be ascending and the second course f _{b formed} falling.

The method described above can also be carried out with two channels. In this case, independent transmitting devices 6c and 6d are provided for a second channel, which generate a frequency curve f _c and f _{d as shown} in FIG. 6a. Regardless of the course f _a and f _b , the corresponding courses f _c and f _{d can be} registered in the receiver coils 9c and 9d and analyzed or correlated in the receiver units 6c and 6b. This dual-channel design enables the required safety requirement with regard to redundancy and independence to be met. The transmitting coil 5 is a static element. A possible impairment of the function of the transmitting coil 5 can be determined from the transmitter side and is thus independent of signals of the receiving circuits.

List of reference numbers used

1

wheel

2

rim

3

rail

4

Threshold, top edge threshold

5

Transmitter, transformer

5a

1. First channel transmitter with synchronized carrier of frequency f _a

5b

Second transmitting means first channel with synchronized carrier of the frequency f _b

5c

3. Second channel transmitter with synchronized carrier of frequency f _c

5d

4. Second channel transmitter with synchronized carrier of frequency f _d

6

Receiver, transformer

6a

1. receiving device for first channel with synchronized Bandpass filter

6b

2. Receiving device for first channel with synchronized Bandpass filter

6c

1. receiving device for second channel with synchronized Bandpass filter

6d

2. Receiving device for second channel with synchronized Bandpass filter

7

Fastener for transmitter / receiver

8th

connecting cables

9

wheel sensor

9a

1. receiver / receiver coil for first channel

9b

2. Receiver / receiver coil for first channel

9c

1. receiver / receiver coil for second channel

9d

2. Receiver / receiver coil for second channel

10

terminal

11

track

12

rail insulation

13

Presentation of progress field lines

20

Generator for repetition time

50

transmitting device

51

Transmitting coil primary circuit, transformer

52

Transmitter coil secondary circuit, transformer

53

amplifier

54

send clock

55

Voltage controlled oscillator, VCO Voltage controlled oscilltor

56

Control for frequency or voltage curve

60

receiver

61

Receiver coil secondary circuit, transformer

62

Receiver coil primary circuit, transformer

63

Axle recovery, ± 1

64

Reference value storage over time

70

Analysis filtering database

71

bandwidth-damped filter

72

Analog / digital converter

80

Achszählelektronik

B, B ₂

Barker code

D _f

Interference field around a frequency f

d

distance

D _i

deviation

f

frequency

f _a , f _b , f _c , f _d

Frequency characteristics for FSK procedures

f _d1 , f _d2

Lower, upper frequency of an interference band

f ₁ , f ₂

Lower, upper frequency for FSK method

i

current index identifying a body of the Barker codes, i = 1, .., m

m, m ₂

Width of the Barker code in bits, number of digits of the Barker code

n

Width of the information S in bits

p

level

P _s , P _s ,

Transmission level, reception level

P _D

Level difference between P _s , P _s

S

information

S _Avg

mean signal strength

S _B

spread signal, useful signal

S _B '

received signal in spread representation

S _i

Received signal at the position i

T _rep

Repetionszeit

T _off

Follow-up time between two frequency curves f _a and f _b

List of acronyms used

DC

Direct Current, DC

AC

Alternating Current

GFM

Train detection system

DSSS

Direct Sequence Spread Spectrum

Rated

Frequency Sweep Keying

Bibliography

[1] DE 196 47 737 A1, Siemens AG, axle detector

[2] SIGNAL + WIRE (95) 3/2003 p. 20-23

[3] DE 101 37 519 A1, Siemens AG, wheel sensor

[4] SIGNAL + WIRE (95) 4/2003, p. 15 - 17

Claims (15)

1. Method for establishing track occupancy by:

   A1

   a wheel sensor (9) attached to a rail (3) containing a transmitter (5) and receiver (6) which registers a change in the magnetic field as a result of a train wheel (1) passing over the rail (3),

   or

   A2

   a direct current circuit formed by two rails (3) featuring a transmitter (5) and receiver (6) which is short circuited when a railway vehicle passes over it, with the short circuit being able to be registered in the receiver;

   characterized by the following steps:

   B

   a spectral broadband signal (S_B) containing information (S) generated in a transmit unit (50; 5a, 5b, ..) is applied to the transmitter (5);

   C

   signals (S_B') registered by the receiver are routed to a receive unit (60, 6a, 6b, ..) for recovering the information (S) transmitted in step B, with transmit unit (50; 5a, 5b, ..)and receive unit (60; Ga, 6b, ..) being coupled;

   D

   different levels of the registered signal (S_B') arising are included for determining the track occupancy.

2. Method according to claim 1,
   characterized in that
   in step B in the transmit unit (50; 5a, 5b, ..) the information (S) to be transmitted is injected in Direct Sequence Spread Spectrum technology with a code (B) and is modulated on a carrier frequency band.
3. Method according to claim 2,
   characterized in that
   in step C for coupling transmit unit (50; 5a, 5b, ..) and receive unit (60; 6a, 6b, ..) the transmit clock (54; 20) is routed to the receive unit (60; 6a, 6b, ..).
4. Method according to claim 2 or 3
   characterized in that
   in stpe B the information (S) to be transmitted is injected with two different codes (B, B2) and modulated on disjunctive carrier frequency bands.
5. Method according to one of the claims 2 to 4,
   characterized in that
   the carrier frequency band features a range of 10 kHz to 30 MHz.
6. Method according to claim 1,
   characterized in that
   in step B in the transmit unit (50; 5a, 5b, ..) the information (S) to be transmitted is encoded by transmitting two consecutive frequency curves (T_ab, f_a, f_b, ..), with the two curves being transmitted repetitively (T_Rep).
7. Method according to claim 6,
   characterized in that
   in step B in the transmit unit (50; 5a, 5b, ..) the information (S) to be transmitted is encoded by transmitting four consecutive frequency curves (T_ab, f_a, f_b, f_c, f_d), with the four curves being transmitted repetitively (T_Rep).
8. Method according to claim 6 or 7,
   characterized in that
   a generator (20) is provided for repetitive transmission and that the generator to the transmit unit (5a, 5b, ..) and to the receive unit (6a, 6b, ..).
9. Method according to one of the claims 6 to 8,
   characterized in that
   the frequency curve (f_a, f_b,..) rises and falls in a linear way over time.
10. Method according to one of the claims 6 to 8,
    characterized in that
    the frequency curve (f_a, f_b,..) is a curve which rises or falls logarithmically over time.
11. Method according to one of the claims 6 to 8,
    characterized in that
    the frequency curve (f_a, f_b,..) is a curve which rises or falls monotonously over time.
12. Method according to one of the claims 6 to 11,
    characterized in that
    Before the execution of step B a disturbance field recorded by the receiver is anayzed and that in step B the frequency curve (f_a, f_b,..) is defined outside or at the edeg of the disturbance field.
13. Method according to one of the claims 6 to 12,
    characterized in that
    the frequency curves (f_a, f_b,..) are arranged within a band from 10 kHz to 10 MHz.
14. Method according to one of the claims 1 to 13,
    characterized in that
    the wheel sensor (9) features two receive coils (9a, 9b) for detection of the direction of travel.
15. Method according to one of the claims 1 to 13,
    characterized in that
    the wheel sensor (9) features four receive coils (9a, 9b, 9c, 9d) for detection of the direction of travel.

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