System and method for determining track occupation

Abstract

 A system for determining track occupation of a track section (B), comprises a first and a second tag (1a, 1b, 2a, 2b), the first tag (1a, 1b) being mounted at the front end (3) and the second tag (2a, 2b) being mounted and the rear end (4) of a vehicle (5); a first and a second tag counter unit (8', 9'), the first tag counter (8') unit being mounted at one end and the second tag counter unit (9') being mounted at the other end of the track section (B), each tag counter unit (8', 9') comprising two tag readers (10a, 10b, 11a, 11b) and a counting device (12a, 12b, 13a, 13b) for incrementing or decrementing a counter value; and an axle counter evaluator (ACE) connected to the counting devices (12a, 12b, 13a, 13b) of both tag counter units (8', 9') for evaluating whether the track section (B) is free or occupied in dependence of the counter values of the counting devices (12a, 12b, 13a, 13b).

Description

Background of the invention

[0001] The invention relates to a system for determining track occupation of a track section within a track, the system comprising a first tag and a second tag, each tag comprising a transponder, e.g. an RFID transponder, the first tag being mounted at the front end of a vehicle and the second tag being mounted and the rear end of the vehicle.

[0002] Such a system is known from US 2011/0130899 A1.

[0003] Usually an axle counter system is used for determining track occupation for guided vehicles, e.g. for trains. Such an axle counter system is disclosed in http://www.thalesgroup.com/Portfolio/Documents/FieldTrac 6315 Az LM/ and comprises an axle counter evaluator and detection points with two independent sensors at each end of a track section. The mechanism of the detection point is based on the influence of an electromagnetic field by an iron wheel. The detection point can detect the direction of the train by the temporal sequence in which the axles passed the sensor. In dependence of the moving direction a counter is decremented or incremented. The occupation of the track is determined by a safety critical computer (axle counter evaluator) which compares the counter values of subsequent detection points. The existing AzLM-Thales axle counter system enables a safe detection of a track occupation. Yet, this system only works for vehicles with ferromagnetic wheels.

[0004] US 2011/0130899 A1 discloses a system for determining movement properties of a guided vehicle independent of the material of the wheels. The vehicle includes at least four on-board transponders that are arranged in pairs. At each end of a track section a transponder reader is provided. A ground calculator communicates with the transponder readers and determines a driving direction and an orientation of the vehicle. The system known from US 2011/0130899 A1 is also suitable for monorails with rubber wheels and vehicles which are not moving on classical iron wheel rail tracks. However the described system is complex and costly since new evaluation software has to be developed and authorized in order to achieve the required safety integrity level (SIL 4 or better).

Object of the invention

[0005] It is therefore an object of the invention to provide a system and a method for determining track occupation which can be used with a variety of different vehicles, including monorails, and which is involved with less effort concerning development of hardware and software.

Description of the invention

[0006] This object is achieved by a system according to claim 1 and a method according to claim 10.

[0007] The inventive system comprises a first tag counter unit and a second tag counter unit, the first tag counter unit being mounted at one end of the track section, the second tag counter unit being mounted at the other end of the track section. Each tag counter unit comprises at least two tag readers for detecting transponder signals, the tag readers being separated from each other along the track direction. Each tag counter unit further comprises at least one counting device for incrementing or decrementing a counter value in dependence of the detected transponder signals. According to the invention the system further comprises an axle counter evaluator which is connected to the counting devices of both tag counter units for evaluating whether the track section is free or occupied in dependence of the counter values of the counting devices.

[0008] The invention combines tag detection with a common axle counter evaluator thereby enabling the implementation of a proved axle counter evaluation procedure and proved axle counter evaluation hardware in a system for vehicles without axles (e.g. suspension railways). This is achieved by the inventive tag counter unit which mimics a counter detection point of a standard axle counter system.

[0009] Axle counter evaluators are known from existing axle counter systems, e.g. AzLM of Thales, and are already proved and configured for high security integrity levels (SIL). The invention makes use of an existing working component thereby reducing cost concerning development and purchase. The axle counter evaluator is a safe computer system, in particular a SIL 4 system, or a vital computing system. The axle counter evaluator receives counter values from the tag counter unit. It is constructed for determining whether a track section is occupied or not in dependence of transmitted counter values for subsequent track sections. In case the counter values of tag counter units of subsequent track sections correspond the axle counter evaluator indicates the track as "free". In case the counter values of tag counter units of subsequent track sections differ from each other the axle counter evaluator indicates the track as "occupied". In case of a failure (e.g. no continuous counting in or out throughout subsequent track sections) the axle counter evaluator identifies the corresponding track section as "disturbed".

[0010] In contrast to the system known from US 2011/0130899 A1 no identification information of the tag is needed to determine the status of a track section. Since the axle counter unit only increments or decrements counter values a simple system design is sufficient.

[0011] In a preferred embodiment of the invention each tag reader of a tag counter unit is provided with a separate counting device and the axle counter evaluator is connected to both counting devices of each tag counter unit.

[0012] Preferably each tag reader is connected with both counting devices for reporting the detection of a tag at both tag readers to both counting devices. As soon as one of the tag readers detects a tag, both tag counting devices of the corresponding tag counter unit are informed. As soon as the tag has been detected by both tag readers of a tag counter unit, the tag is counted, i.e. the counter value is incremented or decremented depending on the moving direction of the tag. The resulting counter value is then transmitted to the axle counter evaluator. The tag counting devices preferably transmit the information via separate links (e.g. Ethernet links) to the axle counter evaluator for redundancy reasons. Within the axle counter evaluator a consistency check may be carried out.

[0013] The tag readers are separated from each other along the track, i.e. along a moving direction of a vehicle which moves on said track, but are preferably in close proximity to each other, i.e. several meters or less, in particular less than 2 m, preferably between 60 cm and 100cm. The separation of the tag readers along the moving direction of the vehicle ensures a time shifted detection of the passing tag. Thereby it can be determines whether the tag enters or leave a specified track section. The minimum distance between both tag readers is defined by the read lobe of the tag readers (area in which the tag reader is able to detect a tag). The read lobes and the distance of the both tag readers have to be chosen such that the read lobes don't overlap completely, i.e. both tag reader mustn't read a tag in at the same point of time. It should be noted that the read lobe can be influenced by external effects, e.g. weather conditions, which has to be taken into account for determining the distance of the tag readers. The overlap of the read lobes of the first and second tag reader should be as small as possible in order to ensure resolution of the detected signals, i.e. distinguishing which tag reader detected the tag first. The distance between both tag readers can be fixed in a way that one tag of the vehicle passing both tag readers is detected in the following way:

• the first tag reader will detect the tag while the second tag reader will not detect the tag;
• the second tag reader will detect the tag while the first tag reader still detects the tag (overlap);
• the second tag reader will still detect the tag while the first tag reader has already lost the tag.

[0014] Via this mechanism the direction of the moving tag (left to right or vice versa) can be assigned. The direction defines if the counter will be decremented or incremented. E.g. for a vehicle with a maximum speed of 60 km/h a distance of 0,5 m between the tag readers within a tag counter unit is appropriate.

[0015] However an overlap is not necessarily required if each tag is provided with an individual code (ID) which could be checked by the tag counting device. It enables the tag counting device to determine the moving direction of the tags. Thus, in a special embodiment of the invention an individual code, e.g. an individual number, is assigned to each tag. The code can be used for registration and tracking of the tags.

[0016] It is preferred that the counting device comprises means for checking whether a tag is valid. E.g. a tag can be classified as a "valid tag" if it had been registered. Undesired detection signals arising from external disturbances, e.g. signals induced by a non-system tag which is transported within the vehicle, can be identified and ignored for the further processing. Only valid tags are counted.

[0017] In a special embodiment of the inventive system, exactly one tag is mounted at the front end and exactly one tag is mounted at the rear end of the vehicle. However, for redundancy reasons a further (redundant) tag can be provided at each end of the vehicle. Nevertheless the present invention also works with only one tag at each end of the vehicle.

[0018] It is preferred that the tags are mounted on the same side of the vehicle. Thereby the read lobe of the tag readers can be kept small. Thus, also the tag counter unit can be kept compact.

[0019] Preferably the read lobe of each tag reader is smaller than 5m, in particular smaller than 1m. By using small read lobes it is possible to place tag readers in close proximity to each other without running the risk of losing resolution of the detected signals (no complete overlap of the read lobes). I.e. although the tag readers of one tag counter unit are placed in close proximity, it can be determined which tag reader detects the tag first.

[0020] Preferably the read lobe is chosen such that only tags mounted at the side of the vehicle that is facing the tag reader are detected.

[0021] In a preferred embodiment the tag counter units are mounted on that side of a track which is facing away from a neighboring track. Thereby it is ensured that transponder signals of one track are not detected by the tag readers of the neighboring track.

[0022] Guided vehicles often move within routes A "route" comprises a number of successive track sections which have been rated (e.g. by the interlocking module) as being" free" before allowing a vehicle to enter a route. Prior to entering a route it is advisable to check, whether the tags are working properly. Therefore a preferred embodiment of the inventive system comprises at least three test counter units, which limit two consecutive test track sections, wherein the distance between consecutive test counter units is smaller than the distance of the first tag and the second tag. The test counter units are tag counter units as described above which are used for testing the successful operation of the tags. Due to the limitation to the distance of the test counter units the tags at the rear end and the tag at the front end can be checked separately at the same time.

[0023] The invention also refers to a method for determining track occupation of a track section within a track using the system according to one of the preceding claims. The inventive method comprises:

• detection of a transponder signal of a tag at a first end of the track section by a first tag reader and by a second tag reader, the first and the second tag reader being spaced by one another within a first tag counter unit,
• determining the temporal sequence of the detected transponder signals by means of a counting device of the first tag counter unit,
• incrementing or decrementing a counter value in dependence of the order in time of the detected transponder signals by means of the counting device,
• transmitting the incremented or decremented counter value to an axle counter evaluator, and
• evaluating whether the track section is occupied or free in dependence of the transmitted counter value of the first tag counter unit mounted at the first end of the track section and a further counter value of a second tag counter unit mounted at the other end of the track section.

[0024] The further counter value is obtained at a further tag counter unit at the other end of the track section analogous to the counter value of the first tag counter unit.

[0025] The inventive method allows evaluation whether a track section is free or occupied without determining the orientation of the vehicle (in contrast to the system known from US 2011/0130899 A1 comprising only one reader at each end of the section, but two pairs of tags). The present invention merely determines in which direction a tag moves (into or out of a track section) independent of the orientation of the vehicle. Depending on which tag reader detects the tag first, the counter is incremented or decremented. In case of both tag reader would detect a tag at the same time (complete overlap of the read lobes) an error is reported.

[0026] The transmission of the counter value to the axle counter evaluator can be carried out each time the counter is incremented or decremented (triggered transmission). Alternatively the counter value can be transmitted periodically, e.g. every 200 ms.

[0027] If the counter value of the first tag counter unit and the further counter value of the second tag counter unit are consistent (same number of tags have been counted into a specified track section and out of the same track section) the axle counter evaluator outputs a "track section is free" information. If the counter for a specified track section is greater than the counter of the subsequent (relating to the moving direction of the vehicle) track section (number of tags that have been counted into the specified track section is greater than the number of tags that have been counted out of the same track section), the axle counter evaluator outputs a "track section is occupied" information. If the counter for a specified track section is less than the counter of the subsequent track section, the axle counter evaluator outputs a "track section is disturbed" information for safety reasons.

[0028] In a preferred variant of the inventive method a verification procedure is carried out for determining whether a tag is valid and that an invalid tag is ignored for the further processing.

[0029] Advantageously, for the evaluation whether the track section is occupied or free only the detected tag signals of exactly one first tag and exactly one second tag of the vehicle can be considered. Though in principle tag signals of only one tag at each end of the vehicle are required for the inventive method, for redundancy reasons several tags may be provided at each end of the vehicle.

[0030] The inventive system and method are designed to use existing axel counter evaluator hardware and software to a maximum extend in order to minimize research and development expenses for the electronic interlocking system.

[0031] Further advantages can be extracted from the description and the enclosed drawing. The features mentioned above and below can be used in accordance with the invention either individually or collectively in any combination. The embodiments mentioned are not to be understood as exhaustive enumeration but rather have exemplary character for the description of the invention.

Drawings

[0032] The invention is shown in the drawings.

FIG. 1

shows an embodiment of the inventive system;

FIG. 2

shows another embodiment of the inventive system with redundant counting devices;

FIG. 3

shows an embodiment of the inventive system with two test track sections for testing the tags prior to entering a route.

[0033] Fig. 1 shows a simple embodiment of the inventive system, comprising a first tag 1 and a second tag 2 which are mounted at the front end 3 and the rear end 4 of a vehicle 5 respectively. In the figures the vehicle 5 is an overhead monorail moving on a track 6 along a track direction 7, thus the tags 1, 2 are mounted at the top of the vehicle 5. It should be noted that the inventive system and method can be implemented also for vehicles running on tracks comprising two rails, as well as for any other guided vehicle. In case of a train moving on top of a track the tags are preferably mounted at the bottom or at the sides of the train.

[0034] Beside the track 6 several tag counter units 8, 9 are mounted, each tag counter unit 8, 9 limiting one end of a track section A, B, C. Each tag counter unit 8, 9 comprises two tag readers 10a, 10b, 11a, 11b, which are separated from each other at a distance along the track direction 7. When the vehicle 5 passes one of the tag counter units 8, 9 the tags 1, 2 are detected successively, i.e. first by the first tag reader 10a, 11a followed by the second tag reader 10b, 11b of the respective tag counter unit 8, 9. Detection information is sent to a tag counting device 12, 13 of the corresponding tag counter unit 8. Depending on the temporal sequence of the detected transponder signals the tag counting device increments or decrements a counter value. Via a data link 18, e.g. IP/UDP, the tag counting device 12, 13 is connected with an axle counter evaluator ACE, which evaluates whether a track section is free or occupied on the basis of the counter values of the different tag counter units.

[0035] In fig. 1 both tags 1, 2 of the vehicle 5 passed the first tag counter unit 8 and therefore left section A and entered section B. Both tags have been detected first by the first tag reader 10a followed by the second tag reader 10b. The tag counting device 12 accordingly increments the counter value twice for section B (alternatively decrements the counter value for section A), indicating that that both tags have entered section B and transmits the counter value to an axle counter evaluator ACE. The transmission from tag counting device 12 to the axle counter evaluator ACE can be done, e.g. via IP/UDP within a closed network. Since the two adjacent sections A/B are separated by the tag counter unit 8, the counter value is considered as a "count in" value for section B and as a "count out" value for section A.

[0036] In order to evaluate the occupation status of track section B the axle counter evaluator ACE compares the counter values of the first tag counter unit 8 and the second tag counter unit 9, which limit track section B. For the example shown in fig. 1 the tags, 1, 2 haven't been detected by the tag readers 11a, 11b of the second tag counter unit 13. Thus, the counter values of the first tag counter unit 12 and the second tag counter unit 13 differ from each other and the axle counter evaluator ACE sends an occupied-message to the interlocking module IM.

[0037] Fig. 2 shows a preferred embodiment of the inventive system with tag counter unit 8', 9' comprising two counting devices 12a, 12b, 13a, 13b each. Additionally the vehicle is provided with redundant tags 1a, 1b, 2a, 2b, i.e. at each end 3, 4 of the vehicle two tags 1a, 1b, 2a, 2b are mounted for redundancy reasons. Thus, despite of a lost or defect tag the entering of the vehicle 5 into a track section A, B, C can be detected.

[0038] The detection of the tags 1a, 1b, 2a, 2b is carried as analogue to fig. 1. As soon as a tag 1a, 1b, 2a, 2b is detected by the first tag reader 10a of the first tag counter unit 8' a detection signal is transmitted to both counting devices 12a, 12b of the first tag counter unit 8'. Both counting devices 12a, 12b determine whether to increment or decrement a counter value (i.e. each tag is counted twice at each tag counter unit). For this purpose both tag counter modules (tag reader and counting device) communicate via an internal link 14. Accordingly two counter values are created and transmitted to the axle counter evaluator ACE. The transmission is carried out via two independent data links 18a, 18b. The axle counter evaluator ACE compares the received counter values of the tag counter unit 8' against each other with regard to consistency. In case of an inconsistency the tag counter unit is handled as faulty and is set to the status "occupied" or "disturbed". The section will then be locked for further traffic. The same procedure is carried out for the second tag counter unit 9'.

[0039] The hardware and software of the tag based tag counter units 8, 8', 9, 9' can be realized on safety integrity level SIL 0, whereas the axle counter evaluator ACE and the interlocking module IM have to comply with SIL 4 requirements. Axle counter evaluators and the interlocking modules complying with SIL 4 requirements are already known from axle counter systems and can therefore be reused for the inventive tag-based system.

[0040] Since the tag counter units are implemented as SIL 0 modules, systematic errors should to be taken into account. Advantageously the following error treatments are carried out:

• Wrong tag counter unit address (ID) used
  Each tag counter unit is provided with a unique ID which is transmitted to the axle counter evaluator ACE together with the counter values. In case due to an error a tag counter unit uses the ID of another tag counter unit, a wrong track occupation status is assigned. In order to prevent this, the IDs of the tag counter units can be encrypted, e.g. at CAE-time. In case a corrupted tag counter unit ID is received by the axle counter evaluator, a fixed small number of retries could be allowed before setting the tag counter unit into fail-state.
• Wrong counting of tags
  A wrong counting of tags can happen if a tag is temporarily inactive or a tag counter unit doesn't work correctly. Every tag which is count by tag counter unit N but is not count by tag counting unit N+1 will lead to a difference of counted tags. I.e. the track section limited by tag counter units N and N+1 stays in state "occupied". If the next tag counter unit N+2 in turn will detect the full number of tags the axle counter evaluator ACE will preferably set track section N+1 to state "disturbed" since more tags have been counted into track section n+2 than have been counted into track section n+1. As a result the track sections limited by tag counter units N and N+2 will stay in state "occupied/disturbed". It will lead to a special operator action followed by a vehicle movement in order to check the proper counting again.
• Lost tag
  Another failure scenario could be that a tag is lost. This would lead to a remaining occupation of one track section (assumed that at least once all tags have been read). The operator will identify such a case by recognizing that only one track section stays occupied while the following track sections get free with the movement of the vehicle. The still occupied track can be only released by a special procedure, e.g. resetting the internal counter and performing a test vehicle movement.
• Non valid tag read
  The tags will be mounted on the vehicle close to the tag reader. In order to prevent detection of non-system tags (e.g. within the vehicle), the physics of the tag readers (in particular the read lobe) can be chosen such, that that only tags within a distance corresponding to the minimum distance between the tag and the tag reader are read.
  Alternatively the tags can be provided with a unique code. The tag counter unit is then provided for verifying whether the detected tag is within a list of valid tag codes. If this is true, the tag will be handled (counted); if this is not true the tag will be ignored and the tag counter unit will preferably enter a fail mode which will lead to a disturbance of the track sections which are separated by the corresponding tag counter unit. No "route" running these track sections is allowed to be released.

[0041] Fig. 3 shows the usage of the inventive system for testing the technical condition of the tags. Prior to entering a route R, e.g. if the vehicle 5 leaves a holding track H it should be ensured that the tags 1a, 1b, 2a, 2b are working properly. Therefore it is proposed to provide at least two consecutive test track sections T1, T2 on which the vehicle 5 runs in order to enter route R. Each test track section T1, T2 is limited by test counter units 15, 16, 17. Prior to entering route R vehicle 5 stops on a signal/balise 18. The length of the test track sections T1, T2 is configured such that the front end 3 and the rear end 4 of the vehicle 5 cannot be positioned within the same test track section T1, T2 at the same time (in particular the length of each test track section T1, T2 is smaller than the distance between a front end tag 1a, 1b and a rear end tag 2a, 2b). The operator (or the interlocking module IM) can now check whether the axle counter evaluator ACE reports both test track sections T1, T2 as being occupied. If this is true, vehicle 5 can start to move into route R. In case of no test track section T1, T2 or only one test track section T1, T2 is reported as being occupied the vehicle 5 must not enter route R. The entering of route R is only allowed if both test track sections T1, T2 are occupied. In order to determine whether the test track sections T1, T2 are occupied, it is sufficient to detect one tag 1a, 1b at the front end 3 and one tag 2a, 2b at the rear end 4 of the vehicle 5.

List of reference signs

[0042] 

A, B, C

track section

ACE

axle counter evaluator

IM

interlocking module

T1, T2

test track section

1, 1a, 1b

first tag

2, 2a, 2b

second tag

3

front end

4

rear end

5

vehicle

6

track

7

track direction

8, 8'

tag counter units

9, 9'

tag counter units

10a, 11a

first tag readers

10b, 11b

second tag readers

12, 12a, 12b

tag counting devices

13, 13a, 13b

tag counting devices

14

internal link

16, 16, 17

test counter units

18, 18a, 18b

data link

Claims

1. A system for determining track occupation of a track section (B) within a track (6), the system comprising:

• a first tag (1, 1a, 1b) and a second tag (2, 2a, 2b), each tag (1, 1a, 1b, 2, 2a, 2b) comprising a transponder, the first tag (1, 1a, 1b) being mounted at the front end (3) of a vehicle (5) and the second tag (2, 2a, 2b) being mounted and the rear end (4) of the vehicle (5);

• a first tag counter unit (8, 8') and a second tag counter unit (9, 9'), the first tag counter (8, 8') unit being mounted at one end of the track section (B), the second tag counter unit (9, 9') being mounted at the other end of the track section (B),
each tag counter unit (8, 8', 9, 9') comprising at least two tag readers (10a, 10b, 11a, 11b) for detecting transponder signals, the tag readers (10a, 10b, 11a, 11b) being separated from each other along the track direction (7),
each tag counter unit (8, 8', 9, 9') comprising at least one counting device (12, 12a, 12b, 13, 13a, 13b) for incrementing or decrementing a counter value in dependence of the temporal sequence of the detected transponder signals; and

• an axle counter evaluator (ACE) which is connected to the counting devices (12, 12a, 12b, 13, 13a, 13b) of both tag counter units (8, 8', 9, 9') for evaluating whether the track section (B) is free or occupied in dependence of the counter values of the counting devices (12, 12a, 12b, 13, 13a, 13b).

2. System according to claim 1, characterized in that each tag reader (10a, 10b, 11a, 11b) of a tag counter unit (8', 9') is provided with a separate counting device (12a, 12b, 13a, 13b), and that the axle counter evaluator (ACE) being connected to both counting devices (12a, 12b, 13a, 13b) of each tag counter unit (8', 9').

3. System according to claim 2, characterized in that each tag reader (10a, 10b, 11a, 11b) is connected with both counting devices (12a, 12b, 13a, 13b) for reporting the detection of a tag (1, 1a, 1b, 2, 2a, 2b) at both tag readers (10a, 10b, 11a, 11b) to both counting devices (12a, 12b, 13a, 13b).

4. System according to any of the preceding claims, characterized that each tag (1, 1a, 1b, 2, 2a, 2b) has been assigned an individual code.

5. System according to any of the preceding claims, characterized in that the counting device (12, 12a, 12b, 13, 13a, 13b) comprises means for checking whether a tag (1, 1a, 1b, 2, 2a, 2b) is valid.

6. System according to any of the preceding claims, characterized in that exactly one tag (1) is mounted upstream and exactly one tag (2) is mounted downstream of the vehicle (5).

7. System according to any of the preceding claims, characterized in that the tags (1, 1a, 1b, 2, 2a, 2b) are mounted on the same side of the vehicle (5).

8. System according to any of the preceding claims, characterized in that the read lobe of each tag reader (10a, 10b, 11a, 11b) is smaller than 5m, in particular smaller than 1m.

9. System according to any of the preceding claims, characterized in that the tag counter units (8, 8', 9, 9') are mounted on that side of a track (6) which is facing away from a neighboring track.

10. System according to any of the preceding claims, characterized in that the system comprises at least three test counter units, which limit two consecutive test track sections, wherein the distance between consecutive test counter units is smaller than the distance of the first tag and the second tag.

11. Method for determining track occupation of a track section (B) within a track (6) using the system according to one of the preceding claims, the method comprising:

• detection of a transponder signal of a tag (1, 1a, 1b, 2, 2a, 2b) at a first end of the track section (B) by a first tag reader (10a) and by a second tag reader (10b), the first and the second tag reader (10a, 10b) being spaced by one another within a first tag counter unit (8, 8'),

• determining the temporal sequence of the detected transponder signals by means of a counting device (12, 12a, 12b) of the first tag counter unit (8, 8'),

• incrementing or decrementing a counter value in dependence of the temporal sequence of the detected transponder signals by means of the counting device (12, 12a, 12b),

• transmitting the incremented or decremented counter value to an axle counter evaluator (ACE), and

• evaluating whether the track section (B) is occupied or free in dependence of the transmitted counter value of the first tag counter unit mounted at the first end of the track section and a further counter value of a second tag counter unit (9, 9') mounted at the other end of the track section (B).

12. Method according to claim 11, characterized in that a verification procedure is carried out for determining whether a tag (1, 1a, 1b, 2, 2a, 2b) is valid and that an invalid tag is ignored for the further processing.

13. Method according to claim 11 through 12, characterized in that for the evaluation whether the track section (B) is occupied or free only the detected tag signals of exactly one first tag (1, 1a, 1b) and exactly one second tag (2, 2a, 2b) of the vehicle (5) is considered.

Drawing