EP0570289B1 - Device for the detection of the passage of a vehicle using a passive transponder - Google Patents

Description

The present invention relates to devices for detecting the passage of a mobile at a determined point in its guided movement along a track and it finds a particularly important, although not exclusive, application in collective transport installations with automatic control. .

Detection devices of the type already known comprising an interrogation unit and a passive responder (that is to say deprived of any clean electrical supply, either by battery or accumulator, or by connection to a supply network) , carried one by the mobile and the other by the track, in which:
the interrogation unit has on the one hand a low frequency transmitter and a medium frequency transmitter intended to operate continuously, having respective antennas transmitting towards a determined area with respect to the antennas and crossed by the answering machine during movement and on the other hand, a unit sensitive to the characteristics of the answering machine when it is in the area, said interrogation assembly being designed to be connected to a supply of electrical energy,
and the transponder comprises a medium frequency signal reception circuit and a low frequency signal reception circuit provided for controlling the medium frequency circuit.

In general, the passive answering machine is carried by the track although it can be mounted on the mobile; it is often referred to by the term "tag". Due to the fact that it does not require an electrical supply and that it is of a low price, transponders can be distributed in large numbers along a track and constitute a non-repairable one-piece assembly.

Document US-A-4,038,653 also describes, very succinctly, a device for detecting the passage of a vehicle. It includes an interrogation unit carried by the vehicle and an answering machine placed on the ground. The interrogation assembly comprises a low frequency transmitter which cooperates with a circuit for receiving the low frequency signal provided in the answering machine. Respective tuned resonant circuits are associated one with the low frequency signal, the other with a higher frequency signal (radar frequency). There is no short-circuiting of a resonant circuit of the responder at the rate of the low frequency.

The document GB-A-2 187 916 describes a device for detecting a vehicle, of the type comprising an interrogation assembly and a passive responder, in which the interrogation assembly has on the one hand a medium frequency transmitter ( 20 to 160 KHz), the signal of which being modulated by a low frequency signal (1 KHz), and an antenna and on the other hand a unit sensitive to the characteristics of the answering machine when it is in the area, said set of interrogation being provided to be connected to an electrical energy supply. The transponder includes a resonant circuit for receiving the medium frequency signal and a circuit for short-circuiting the resonant medium frequency circuit of the transponder at the rate of the low frequency. The unit of the interrogation unit is sensitive to disturbances of the medium frequency transmitter caused by the short-circuiting of the tuned medium frequency radiating circuit of the responder. With this, a coded digital signal from the answering machine can be detected by the interrogation unit. The transponder also includes a rectifier network which creates, from the medium frequency power induced in the transponder antenna, the necessary supplies.

The invention aims to provide a device making it possible to reduce the cost and the complexity of the answering machines, compared to those which exist.

For this purpose, the invention provides a device according to claim 1. The answering machine can easily be formed therein to be programmable in situ.

In a particularly simple embodiment, the responder can be considered analog: the low frequency circuit short-circuits the medium frequency resonant circuit as long as it receives a low frequency magnetic field of sufficient level from the interrogation unit , to the rhythm of the low frequency. The processing unit of the interrogation unit recognizes that the medium frequency resonant circuit has short-circuited (that is, the active state or, on the contrary, inhibited), which modifies the characteristics of the transmitter. medium frequency due to the magnetic coupling between the medium frequency antenna of the whole interrogation and the medium frequency tuned circuit of the answering machine. In in practice, the processing unit will simply determine whether the average transmission frequency current is above or below a threshold.

In a more elaborate embodiment, the answering machine can be considered as digital. It also includes a logic block intended to supply, when energized, a serial digital message modulated at the rate of the low frequency and this logic block short-circuits the resonant medium frequency circuit of the responder only for a determined value of a digital message consisting of bits modulated in phase at the rate of the low frequency constituting the clock. This message can be different for each answering machine. It is reconstituted by processing in the sensitive unit of the interrogation assembly. The electrical power necessary for the operation of the logic block is generated by rectification of the low frequency signal induced in the resonant circuit tuned in low frequency.

In such an answering machine, which can be described as digital, digital messages can be transmitted in phase jump modulation (+ π / 4, - π / 4), at the rate of the induced low frequency.

The use of a low frequency magnetic field link, which we know how to control the propagation and the energy balance, is an intrinsic safety factor, compared to arrangements in which the beacon would be sensitive to an electromagnetic field, many more disturbed and, at least at high frequency, subject to parasitic excitations, in particular due to reflections.

• la figure 1 est un schéma de principe du dispositif ;
• la figure 2 est un schéma de réseau passif d'amplification d'écart de phase, utilisable dans l'ensemble d'interrogation du dispositif de la figure 1 ;
• la figure 3 montre une variante de la figure 2 ;
• la figure 4 est un schéma de principe montrant le mode de mise en court-circuit du résonateur d'antenne moyenne fréquence du répondeur dans le dispositif selon la figure 1 ;
• la figure 5 montre une constitution d'antenne basse fréquence (BF) permettant une mesure de vitesse ;
• la figure 6 est un synoptique d'un dispositif selon un mode particulier de réalisation de l'invention, à répondeur numérique ;
• les figures 7 et 8 montrent l'allure des signaux qui interviennent respectivement dans l'ensemble d'interrogation et dans le répondeur.

The invention will be better understood on reading the following description of particular embodiments, given by way of nonlimiting examples. The description refers to the accompanying drawings, in which:

• Figure 1 is a block diagram of the device;
• FIG. 2 is a diagram of a passive network for amplification of phase difference, usable as a whole interrogation of the device of Figure 1;
• Figure 3 shows a variant of Figure 2;
• Figure 4 is a block diagram showing the short-circuiting mode of the medium frequency antenna resonator of the responder in the device according to Figure 1;
• FIG. 5 shows a constitution of low frequency antenna (LF) allowing a speed measurement;
• Figure 6 is a block diagram of a device according to a particular embodiment of the invention, with digital answering machine;
• Figures 7 and 8 show the appearance of the signals which intervene respectively in the interrogation unit and in the answering machine.

The embodiments of the invention which will now be described are in particular applicable to rail transport installations in which each answering machine constitutes a fixed beacon relative to the track. They can be used in co-operation with safe automatisms for driving transport vehicles of the kind already implemented in the VAL system and which will therefore not be necessary to describe again.

The interrogation assembly can be divided into two parts, on the one hand the antennas placed under the mobile so as to affect the antennas of the answering machine and on the other hand all of the electronic circuits, transferred to a distance which can be a few meters, in a protected area. As will be seen below, the presence of a connecting cable between the antennas and the electronics can be used to amplify the phase difference used for detection.

• une liaison à basse fréquence, par exemple à 128 KHz, qui fournit l'énergie nécessaire à l'alimentation du répondeur 12 et fixe la fréquence de modulation de la réponse (ainsi que l'horloge de synchronisation du message en retour dans le cas d'un répondeur numérique) ;
• une liaison à fréquence plus élevée, dans le domaine généralement qualifié de moyenne fréquence (MF), par exemple entre 5 et 80 MHz, que l'on peut considérer comme constituant la voie retour vers l'ensemble d'interrogation.

Whatever the embodiment, the detection device has the basic principle shown in FIG. 1. It includes an interrogation unit 10 and an answering machine 12 between which two links by magnetic field are established during operation:

• a low frequency link, for example at 128 KHz, which supplies the energy necessary to supply the responder 12 and sets the frequency of modulation of the response (as well as the clock for synchronization of the return message in the case of '' a digital answering machine);
• a higher frequency link, in the field generally qualified as medium frequency (MF), for example between 5 and 80 MHz, which can be considered as constituting the return path to the interrogation unit.

The operating principle of the device is then as follows: by permanently analyzing the impedance of an antenna resonator 14 tuned to the medium frequency and connected to a medium frequency oscillator 16 with permanent operation, it recognizes the short-circuiting circuit at the low frequency of a resonant circuit 18, tuned to the medium frequency, when the low frequency signal is received by a LF receiver 20 belonging to the responder.

• soit du déphasage du courant MF par rapport à une référence insensible à l'environnement (telle qu'une tension de sortie d'amplificateur),
• soit de l'amplitude du courant d'antenne, qui se modifie au rythme des mises en court-circuit du circuit résonant 18, c'est-à-dire au rythme de la basse fréquence (et d'un message d'identification contenu dans une mémoire morte 24 dans le cas d'un répondeur numérique).

When the interrogation unit is placed above a responder and the MF resonant circuit 18 is not short-circuited, the antenna resonator 14 is detuned by magnetic coupling, resulting in a variation in the current MF in the antenna resonator 14. A unit 22 of the interrogation assembly makes it possible to detect this condition by monitoring:

• either of the phase shift of the current MF relative to a reference insensitive to the environment (such as an amplifier output voltage),
• either of the amplitude of the antenna current, which changes at the rate of the short-circuits of the resonant circuit 18, that is to say at the rate of the low frequency (and of an identification message contained in a read-only memory 24 in the case of a digital answering machine).

The low frequency link to the LF receiver 20 is provided from the interrogation unit by a LF oscillator 26 operating continuously during use and by a BF 28 antenna.

The detection of the phase or amplitude difference caused by the passage over a responder 12, the resonant circuit 18 of which is short-circuited at the rate of the low frequency, advantageously uses a passive network of amplification of the phase or amplitude difference presented periodically by the MF current emitted by the antenna resonator 14. This passive network can in particular use the properties of mismatched transmission lines and creating standing waves. Such a network has a sufficient line length for a mismatch at the end of the line, even a slight one, to create an exploitable source defect. This defect is revealed by a modification of the complex impedance, as seen from the MF 16 oscillator. It results in a voltage / current phase shift or in a variation in amplitude of the current.

In practice, the line will have a length equal to (2k + 1) λ / 2, where k is an integer greater than 1 and λ is the wavelength of the medium frequency signal.

In the embodiment shown in FIG. 2, the line 30 consists of a section, of length l , of cable of known characteristic impedance. The current I in the cable is taken using a current transformer, shown diagrammatically by a loop 32.

In the case illustrated in FIG. 3, a line of length l is synthesized by cascading elementary quadrupoles 34 (R, L, C, G). A sufficient number of quadrupoles is used to represent a line length greater than the wavelength of the medium frequency signal and which is an odd multiple of half a wavelength. The current I is also taken from one of the conductors of the line and the voltage V is taken between the conductors, at the input of the unit 22, seen from the antenna. It is not necessary to describe the BF 26 oscillator, the construction of which may be conventional. The antennas 14 and 28 can both consist of flat coils and be superimposed, or even formed on the same insulating support.

où kT/q est le potentiel thermo-dynamique, d'environ 25 mV à 20°.The receiver 20 of the responder, intended to control the short-circuiting of the resonant circuit MF 18, has a different constitution depending on whether the responder is analog or digital. The control mode can in all cases be based on the modification of the dynamic impedance of a diode, constituted for example by a PN junction, according to the current which crosses it. This dynamic impedance can be considered as a resistance rd which varies, as a function of the low frequency current Ie which crosses it, according to the relationship:

$\text{rd = (kT / q). (1 / Ie)}$

where kT / q is the thermodynamic potential, around 25 mV at 20 °.

The medium frequency link must give rise to a current Ie, the extreme values of which, when the interrogation assembly passes over the responder, are sufficient to cause a significant variation in rd , authorizing the detection of the responder.

The use of the dynamic impedance of a diode as a switching element provides an important advantage from the point of view of intrinsic safety. On this subject, it should be remembered that security requires the absence of excitation of an answering machine by a mobile or another source other than the mobile which circulates above the answering machine (for example to avoid incorrectly detecting the output a vehicle from a railroad canton). On the other hand, the failure to locate the passage over an answering machine generally has much less significant consequences so that security must go in the direction of untimely non-detection.

As shown above, the resistance rd of the diode is an inverse function of the current flowing through it. Any reduction in the current flowing through the diode below a threshold will result in a resistance of the diode which remains high enough so that there is no finding of a short-circuiting. As a result, the magnetic coupling to the medium frequency resonant circuit 36 is degraded.

In the case of an analog responder, the control mode can be that shown diagrammatically in FIG. 4: a diode 38 is mounted in shunt on the output of the antenna 36 of the receiver 20. This output is connected to the resonator MF 18 of the responder by a filter 40, intended to eliminate the transients and a fraction of the noise.

In the case of a digital beacon, the receiver also includes a programmable read-only memory 24 intended to control the current Ie so as to constitute a serial message, which can be decoded by the unit 22.

An embodiment of the invention, making it possible to measure the speed of movement of the mobile under intrinsic safety conditions, uses a low frequency antenna 28 comprising three components offset in the direction of movement and supplied in a different manner. In practice, the low frequency antenna can then consist of three coils 28a, 28b, 28c supplied in phase opposition (0, π, 0) by the oscillator BF 26 (Figure 5).

As in the previous case, the unit 22 can detect variations in current by detecting an envelope on the current MF, in intrinsic safety. But this time, the phase inversions, when the interrogation unit circulates over the answering machine, are noted, at the same time as the instants at which they intervene, which makes it possible to calculate the speed. In the case of a digital answering machine, the demodulation of the digital signal representative of the identification message must take account of the phase inversions (0, π) of the low frequency signal.

As noted above, treatment with safety of phase inversions of the low frequency signal can be carried out by implementing safety electronics of known type, such as that of the on-board automations of the VAL transport system, for which the same function is necessary to detect crossings of the lines d control of the transmission belt placed on the track.

It is also possible to establish, when necessary, a low speed communication link between the interrogation unit and the beacon. For this, the low frequency signal can be modulated in phase, with a low modulation rate, of around 1 Kb / s. This modulation can in particular constitute a return channel, advantageous for certain locations of answering machines, for example in a station. This channel is produced without the need for an additional antenna on the interrogation unit.

Responders can be used with a length, in the direction of the track, which differs according to their location. It will generally be desirable to have short responders in the running part of the channel. On the other hand, it may be desirable to use transponders of considerable length, for example with tuned circuits having a length of 1 to 3 m, in stations. Indeed, security reasons often lead to authorizing the opening of the vehicle doors only when the vehicle interrogation unit is placed above a beacon. However, the stopping precision of vehicles would often not allow this condition to be fulfilled in the case of transponders having the acceptable short length in the running part of the track.

Such transponders are compatible, like transponders in the main part, with a transmission at low speed of a signal on the low frequency channel.

We will now describe, by way of a particular embodiment, a detection device with a digital answering machine authorizing speed measurement.

The overall constitution of this device is given in FIG. 6, where the elements corresponding to those of the preceding figures are designated by the same reference number.

The interrogation assembly 10 comprises, as indicated above, an oscillator BF 26 which attacks the antenna BF 28 via a tuning circuit 42. A second output of the oscillator BF 26 attacks a demodulator 52 and a safe phase comparator 44 belonging to the processing unit 22, which will be described later.

The FM transmitter 16 in turn attacks the MF antenna 14 via a cable compensation network 46 and a tuning circuit 48. To limit the space requirement, the antennas can be constituted by concentric flat radiating coils. The LF antenna overvoltage coefficient must be sufficient to ensure a coupling creating a significant signal in the transponder 12. It is possible, for a railway application, to use an LF 26 oscillator at 128 KHz, providing a sinusoidal signal having an effective power 10 W at the BF 28 antenna. The tuning circuit can be connected to the electronics by a cable with characteristic impedance close to 50 Ω.

The MF oscillator can supply an effective power of the order of 1 W to the antenna via a characteristic impedance cable of 50 Ω for a frequency of 10 MHz. The compensation network 46 is such that the link has a length sufficient for the MF current at the output of the oscillator 16 to be sensitive to the detuning caused by the presence of the medium frequency resonant circuit of the responder.

The unit 22 represented by way of example recovers the low frequency component of modulation of the MF current at the output of the oscillator 16. The unit must have sufficient bandwidth so as not to distort the digital message supplied by the read only memory programmable beacon 24. For a low frequency of 128 kHz, you can allow a bandwidth of around 300 KHz.

The unit 22 has a functional channel, the presence of which is necessary, and a security channel, which is simply optional. The functional channel comprises an envelope detector 50 for recovering the LF signal and a phase demodulator 52 for recovering the digital message modulated by phase jump. The detector 50 may conventionally include a diode rectifier.

The security envelope detector 54 of the security channel operates on the same principle as the detector 50, with a narrow bandwidth. The security channel does not have to recover a message, but simply to identify the line at 128 KHz and the phase inversions when going from the coil 28a to the coil 28b, and from the latter to the coil 28c. The assembly of FIG. 6 also makes it possible to carry out speed measurements in safety. The function of the detector 54 is to recover the low frequency emitted by the interrogation unit and to make it possible to recognize the phase of the signal (0, π), which depends on that of the coils (FIG. 5) which is located under the BF antenna 28. The secure phase comparator 44 determines the phase rotations (0, π) of the low frequency and can be constituted in a secure manner, as in the case of the circuits used in the VAL system.

The responder 12 also advantageously comprises antennas made up of concentric coils, the resonant circuit MF having a sufficient overvoltage coefficient for the effect of magnetic coupling with the antenna resonator 14 to cause a detectable disturbance.

The responder further includes a rectifier network 54 which creates, from the low frequency power induced in the antenna 56 of the low frequency receiver 20, the necessary supplies. A second circuit 58 extracts a clock signal from the low frequency signal. The rectified signal is applied to a logic block 60 connected to the read-only memory 24. This logic block also incorporates a phase modulator, allowing the digital signal coming from the read-only memory to be modulated by phase jump, at the rate of the low frequency supplied by the clock circuit 58.

The MF resonant circuit 18 is short-circuited by a diode 38 current-controlled by a control circuit 62, the switching element of which can be a bipolar transistor. A network 64 makes it possible to adapt the impedance of the control circuit to that of the resonant circuit MF 18.

FIG. 7 shows, by way of example, the appearance of the signals in the interrogation assembly, at the points designated, in FIG. 6, by letters corresponding to the lines of FIG. 7. The time interval 66 corresponds to operation during the period when the interrogator does not fly over an answering machine. The time interval 68 corresponds to the passages of the first coil of an antenna of the kind shown in FIG. 5 above the answering machine. The instant t corresponds to a phase inversion, when it is the second coil which is coupled with the responder. At the end of a second step 70, there is a third step, not shown, for which the phase is the same as during the first step.

FIG. 8 shows the appearance of the signals in the answering machine 12, at the points indicated by letters in FIG. 6, in the case of a digital beacon (line M) and an analog beacon (line N).

The signal K is only produced in the case of a digital answering machine. In the case of a digital answering machine, the signal M corresponds to the opening and short-circuiting of the resonant circuit MF 18. In the case of an analog answering machine, there is simply opening and closing of a circuit resonant at the cadence of the low frequency. The signal J shows, due to the rectification, a magnifying glass effect.

Claims (9)

1. Device for detecting the passage of a mobile at a predetermined point during a guided displacement thereof along a track, comprising an interrogation assembly and a passive responder, one of which is associated with the mobile and the other of which with the track, wherein
      the interrogation assembly (10) includes; on one hand a low frequency transmitter (26) and a medium frequency transmitter (16) operating continuously, having respective antennas (28, 14) which transmit towards a zone having a predetermined location relative to the antennas and traversed by the responder during displacement, and
      on the other hand, a unit (22) responsive to the characteristics of the responder when the latter is in the zone,
      said interrogation assembly being connectable to an electrical power supply; and
      the responder (12) comprises: a circuit for receiving the low frequency signal, arranged to control the medium frequency circuit,
      said low frequency circuit and said medium frequency circuit comprising respective tuned radiating circuits (56, 18) associated in such a manner that the low frequency signal, which is induced in the tuned resonant low frequency circuit short-circuits the medium frequency tuned resonance circuit at the low frequency rate, and
      said unit (22) of the interrogation assembly being responsive to disturbances of the medium frequency transmitter caused by the short-circuiting of the medium frequency tuned radiating circuit of the responder.
2. Device according to claim 1, and in that the circuit for receiving the low frequency signal (18) short-circuits the medium frequency tuned circuit at the low frequency as long as it is subjected to a low frequency magnetic field having a sufficient level.
3. Device according to claim 1, characterized in that the responder (12) further includes a logic unit, which delivers, when enabled, a serial digital message modulated at the low frequency for short-circuiting the medium frequency resonant circuit (18) only for a predetermined value of a digital message constituted by bits which are phase-modulated at the low frequency and which constitute a clock signal.
4. Device according to claim 3, characterized in that the serial digital messages are (+ π/4, - π/4) PSK encoded.
5. Device according to any one of the preceding claims, characterized in that the low frequency antenna (28) of the interrogation unit (10) comprises a plurality of coils, two successive coils being fed with a 180° phase shift by the low frequency transmitter (26).
6. Device according to any one of the preceding claims, characterized by low-rate phase modulation means for modulating the output signal of the low frequency transmitter (26).
7. Device according to any one of the preceding claims, characterized in that the unit (22) of the interrogation assembly comprises a passive network for amplifying the phase shift of the medium frequency current, comprising a line having a length higher than the wave length of the medium frequency signal and equal to an odd multiple of half the wave length, or a cascade of cells synthetizing such a line.
8. Device according to any one of the preceding claims, characterized in that said unit (22) of the interrogation assembly comprises a functional channel and a safety channel, said functional channel having an envelope detector (50) and a phase demodulator (52) while the safety channel comprises an envelope detector (54) and a phase comparator (44) the envelope detector of the safety channel having a pass band which is narrow as compared with the envelope detector of the functional channel.
9. Device according to any one of the preceding claims, characterized in that the low frequency tuned circuit of said responder drive a current in a diode (38) which is in parallel with the medium frequency tuned circuit, whereby achieving safety of the short-circuiting of the medium frequency tuned resonant circuit due to the intrinsic safety provided by said diode and the sizable energy exchange in the low frequency link between interrogation unit (10) and the responder (12).

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