GB2208449A

GB2208449A - Track indicator apparatus

Info

Publication number: GB2208449A
Application number: GB8818636A
Authority: GB
Inventors: Christopher Bartram
Original assignee: M L ENGINEERING
Current assignee: M L ENGINEERING
Priority date: 1987-08-05
Filing date: 1988-08-05
Publication date: 1989-03-30
Anticipated expiration: 2008-08-05
Also published as: GB8818636D0; GB2208449B; GB8718519D0

Abstract

A railway track indicator system for indicating the absence or presence of a train on sections of track has a plurality of receiver/transmitter units (20, 22) each coupled to the track via a single broadband track-coupler (24) located at the junction between adjacent track sections. Each unit (20, 22) transmits a signal comprising carrier signal bursts having a predetermined pattern of carrier frequencies characteristic of the track section associated with the transmitter, the signal being received by the receiver of the unit coupled to other end of the relevant track section. The carrier frequencies are in the region of 100kHz and the track sections are substantially non-resonant, cross-talk between the sections being largely avoided due to the attenuation of signals of this frequency along the track. This avoids the expense of large tuning inductances in the track-couplers (24) capable of carrying traction currents. <IMAGE>

Description

TRACK INDICATOR APPARATUS This invention relates to track indicator apparatus having the ability to detect the presence of a train in a predetermined section of railway track.

It is well known to divide a continuous railway track into a number of sections known as track circuits without inserting physical breaks in the rails.

Functional separation of adjacent track circuits is achieved using a.c. track circuit signals of different frequencies (generally audio frequencies) and by having transmitting and receiving units tuned to the different frequencies for applying signals to and picking signals from up the rails. Such track circuits can be operated in the presence of a.c. or d.c. traction currents in the rails.

A disadvantage of these known track circuits is their unsuitability for city transport networks (particularly underground railways) due to the requirements for short track circuits, typically 30 to 300 metres in length, the unsuitability arising from the fact that the electrical overlap region between adjacent track circuits is a significant proportion of the track circuit length resulting in significant "dead" zones.

Secondly, the tuning units coupled to the track tend to be costly due particularly to the need to provide substantial inductance values in components which are capable of carrying traction currents and therefore require large copper windings. Signal attenuation along the track and the presence of electrical traction noise and traction current harmonics demand that the track signals be transmitted at power levels of the order of 10 to 100 watts with a consequent danger of high frequency harmonics being transmitted to the surroundings to cause interference with radio frequency communications. A further difficulty with conventional jointless track circuits occurs when earthed tracts due structures such as traction power line supports are connected at intervals to one of the rails.In such a case, the a.c. track circuit signals can, in certain condit4ons, be conducted through the earth connections from structure to structure rather than through the rail, with the result that the presence of a train between the relevant structures may not be detected.

The present invention provides a means of detecting trans or other track status information on a predetermined sect'on of railway track which overcomes at least some of the disadvantages referred to above by transmitting along at least one of the rails cf tie section an alternating signal having a frequency of lOkHz or greater, and preferably of 20kHz or greater.

In the case of the transmitting signal being applied across a pair of rails at or adjacent one end of the track section, coupling of a transmitter to the rails may be performed by a coupler device without the use of costly resonant circuits capable of carrying high currents. In other words, the track section Is not a tuned area as in the prior art. Similarly, at the other end of the track section a receiver can be coupled to the rails without the use of high current resonant cIrcuits. Several such transmitter/receiver pairs may be coupled to the ends of neighbouring track sections along the length of the track, each pair preferably using a transmission frequency which is different from the transmission frequencies of the immediately adjacent track sectIons.The ability to dispense with the resonant high current isolating circuits of the prior art arises from the Applicants' discovery that at frequencies in excess of lOkHz the track acts as a lossy transmission line rather than as an unpredlcatable reactIve impedance. The losses are of a level such that the transmitted signal suffers considerable attenuation outside the section in question to the extent that a short circuit across the track outside the section has a relatively minor effect on the level of the received signal. Additionally, providing suitably spaced frequencies are used for adjacent track sections, interference between sections is negligible. The transmission line effect holds true in most circumstances up to transmission frequencies in excess of 50MHz.

The applicants have also found that at frequencies above the specified lower limit, interference due to electric traction currents is considerably less than at the frequencies employed by conventional track indicator apparatus to the extent that comparatively low transmitter power levels of the order of 50mW or less may be used with consequent benefits in apparatus power consumption, radiated interference levels, and component costs.

Typically, apparatus in accordance with the invention comprises a crystal controlled transmitter locked to a frequency or group of frequencies in the region of 100 kHz and coupled across the rails at one end of the track section via a broadband balanced transformer, and a crystal controlled receiver locked to the same frequency or frequencies and coupled in a similar manner across the rails at the other end of the track section, the receiver including detector means arranged preferably to sense the level of the signal received from the transmitter. In particular the preferred detectormeans is. capable of distinguishing, by computing the rate of change of a characteristic of the receiver signal, the entry and departure of a train into and from the track section on the one hand fom effects due to changes in ambient conditions on the other hand.

For example, the detector means may be arranged to produce a track occupied indication only when the received signal level decreases by a predetermined amount in less than 10 seconds. Weather effects such as rain or changes in temperature tend to produce changes in received signal levels which occur slowly over a period of, for example, one minute or more.

Preferably the detector means uses a combination of absolute level sensing combined with rate of change sensing.

By sensing level changes in adjacent track sections simultaneously, the speed and direction of travel of a train can be determined. Track breaks also cause a detectable change in received signal level.

Since the means for coupling the transmitter and the receiver of each track section to the track comprise broadband elements, it is perfectly feasible within the scope of the invention for the transmitter of one track section to share a single coupling element with the receiver of an adjacent track section. This largely eliminates the dead zone effect of the prior art apparatus and further reduces cost. A diplexer connected to each coupling element and having low current, high-Q tuned circuits tuned to the respective track section frequencies avoids significant interference of the receiver by the adjacent transmitter and minimises the sensitivity of the receiver to traction current and other interference, the frequencies being chosed to avoid traction current and other harmonics.

A further benefit of operation at higher than conventional frequencies is the ease with which bonds between rails and tracks and between the tracks and trackside structures can be made high impedances. In many cases, a ferrite ring around the conductor forming the bond is sufficient.

The invention will now be described by way of example with reference to the drawings fn which: Figure 1 is a diagram of track indicator apparatus in accordance with the invention; Figure 2 is a signal attenuation graph; Figure 3 is a circuit diagram of a track coupler suitable for use in the apparatus of Figure 1; Figure 4 is a block diagram of a transmitter of the apparatus of Figure 1; Figure 5 is a waveform diagram illustrating the transmitter output; Figure 6 is a block diagram of a receiver of the apparatus of Figure 1; Figure 7 is a block diagram of processor circuitry forming part of the apparatus of Figure 1; Figure 8 is a sketch of an impedance bond between the rails of a track; and Figure 9 is a sketch of an impedance bond between a rail and a trackside structure.

Referring to Figure 1 of the drawings, a preferred track indicator system in accordance with the invention involves the division of a railway track 10 into a number of track sections 12,14,16,18, etc.. Each track section has associated with it a transmitter 20 and a receiver 22 coupled to respective opposite ends of the section by means of a track coupler 24. Due to the nature of the signals transmitted by each transmitter 20 along its respective track section, it is possible to connect the receiver 22 associated with one track section to the track by means of the same coupler 24 as the transmitter 22 of the adjacent track section. Both the received signal and the transmitted signal are conducted through the same pair of rail connections and the same coupler 24.

Interference between the transmitter of one track section and the receiver of the adjacent section is largely eliminated by arranging for their operating frequencies to be different and by connecting the transmitter and receiver to the common coupler via a high-Q tuned diplexer 26. Typically, track section 14 operates with pulsed signals at carrier frequencies which are switched according to a first pattern PA, while track sections 12 and 16 on either side operate with pulsed signals of difference frequency patterns Pz and Pg respectively.

At frequencies of the order of 100kHz, the track 10 acts as a true lossy transmission line, the attenuation of the transmitted signal of each section being such that a mismatch of the transmission line located at a distance of more than one track section length from the transmission location has a much smaller effect on the level of the signal at the receiver location than a mismatch within the section. This is illustrated by the graph of Figure 2, showing the received signal level at 100 kHz for a varying short circuit position (representing a train on the track) both within and outside an 80 metre track section. Note particularly the steep rise in signal level just oustide the boundaries 28 and 30 of the track section (the receiver and transmitter locations respectively).In this way, the need for resonant lengths of track is avoided, and dead zones between track sections (zones in which an overlap is required for a train to be detectable) are substantially eliminated.

The avoidance of tuned circuits on the track allows the use of relatively inexpensive non-adjustable broadband track couplers 24, such as that shown in Figure 3. The coupler 24 comprises a balanced ferritecored transformer having a centre-tapped secondary 32 (which may be earthed as shown), a primary 34, and a pair of series isolating capacitors 34,36 coupled between respective ends of the secondary 32 and the respective rails 38,40 to provide traction voltage isolation of the coupler 24 and apparatus connected to it.Since the diplexers 26 (Figure 1) are narrowband devices, and since interference from traction current harmonics and transients at the relatively high frequencies employed is comparatively low, the transmitted signal can be at a level of tens of milliwatts. For this reason, and because the track coupler 24 does not need to be able to carry traction currents, the couplers 24 can be relatively low current devices. A common connection for both transmission and reception is provided across the primary winding 34 in the form of a coaxial connector 42.

Each diplexer 26 comprises narrowband bandpass filters tuned to the transmitter and receiver frequencies respectively, and a mixing bandpass filter containing both frequencies within its passband. This device is the only major device that must be selected at each track connection location, the track couplers 24 being broadband and the transmitters and receivers being tunable under microprocessor control by means of processors 48 (shown in Figure 1).

To allow microprocessor tuning, each transmitter 20 has a frequency synthesiser 48 as shown in Figure 5.

Associated with the synthesiser 48 is a voltage controlled oscillator 50, coupled in a phase locked loop, the loop being locked to an internal crystal controlled reference oscillator. Control of the synthesiser frequency is performed by one of the microprocessors 48 (Figure 1) via an interface 51 associated with a plurality of links 52 so that the frequency is switched rapidly between different frequencies of a predetermined band of frequencies allocated to the particular track section with which the transmitter is associated, the frequency being selected by the links 52. A pulse modulator 53, also controlled by the microprocessor, applies 100% amplitude modulation to the synthesiser output, and the resulting signal is a succession of carrier bursts each 7 ms in length and separated by a zero-signal period of 2 ms, as shown in Figure 5.Referring to Figure 5, each burst has a respective one of four carrier frequencies fa, fb, fc or fd, characteristic of the track section. If there are four frequencies, as shown, these are repeated successively in each group of four carrier bursts, so that the groups of carrier bursts form a track identity code.

The frequencies may be selected on the basis of a four band (i.e. one band for each of the group of four bursts) ten channel per band spectrum yielding approximately 1000 possible combinations Each track section is allocated frequencies within a band different from the bands allocated to the neighbouring track sections to minimise interference from signals from the neighbouring sections. Each diplexer is thus tuned to accept transmission signals within one frequency band and reception signals within another frequency band.

The modulator output is amplified in amplifier 54 and filtered in a filter 55 of a sufficiently wide passband to accommodate all possible transmission frequencies of the apparatus to avoid the need for tuning.

Referring to Figure 6, the preferred receiver is also synthesiser based, having a frequency synthesiser 56 with an associated voltage controlled oscillator 58 and crystal reference 60. The synthesiser tuning is microprocessor controlled. For reasons which will be described below, the receiver has a controllable input attenuator 62. This feeds two mixers 64 and 66 each fed with a signal of a frequency which is one quarter of the frequency of the synthesiser output frequency, the two signals being quadrature phase-related and obtained from a digital phase shifter 68 coupled to the output of the synthesiser.

By appropriate control of the synthesiser 56 using the microprocessor for switching between frequencies selected by links (not shown), the mixers are fed with signals having switched frequencies according to a pattern corresponding to the pattern of carrier signals produced by the transmitter of the transmitter/receiver pair. A control loop (not shown) fed with the demodulated pulse waveform locks the rate of switching of the synthesiser 56 with that of the incoming signal.

Typically, one of the frequencies of the local oscillator signals fed to the mixers is 99 kHz, yielding quadrature intermediate frequency (i.f.) signals at 1kHz with an input frequency of 98kHz. Connected to the output of the mixers 64,66 are respective bandpass filters 70,72 centred on the 1kHz i.f. to reject interference from adjacent channel signals, and a differential phase shifter and summing stage comprising individual phase shifter circuits 74,76 and a summing amplifier 78. This stage has the function of combining the quadrature i.f. signals to yield a signal corresponding to the wanted 98kHz received signal and to reject the image signal at 100kHz which, when fed to the mixers, also provides 1kHz i.f. signals. The phase shifters 74,76 in effect shift the phase of the i.f.

signals 90" relative to each other such that signals derived from a 98kHz input, for example, are fed in phase to the summing amplifier 78, whereas any signals derived from the image at 100kHz appear 1800 out of phase and are thus cancelled out by the summing amplifier 78. Accordingly, the need for a complex narrowband filter upstream of the mixers 64,66 is avoided, with the result that the receiver can be tuned easily to receive different input frequency signals merely by controlling the synthesiser 56.

Final filtering of the i.f. signal available at the output of the summing amplifier 78 is performed by a chopping filter 80 driven by a 100kHz clock signal. At this point the signal is rectified in a precision rectifier 82 to detect the pulse modulation applied at the transmitter, a comparator 84 being used to provide a clean squared pulse signal the level of which is used to evaluate the track status. The rectifier output is also fed via a filter 86 to a window comparator 88 which compares the d.c. level of the received signal with two thresholds to produce outputs indicative of whether the level is within the window defined by the thresholds, or whether it is above or below that window. It is at this point that the purpose of the input attenuator 62 becomes apparent.By microprocessor means (not shown) the outputs of the window comparator 88 are used to adjust the attenuator 62 to bring the d.c. level at the window comparator input to a value between the two thresholds. The microprocessor means periodically adjusts the attenuator value over a range to ascertain threshold levels for a 'clear' track. The digital control signal for the attenuator may be used as a convenient digital measure of the received signal strength for train detection.

The receiver microprocessor monitors the level of the signal applied to the mixers 64,66. By controlling the attenuator 62, the microprocessor is able to maintain the signal level fed to the mixers at an optimum level determined by prevailing ambient conditions.

As is apparent from Figure 1, each transmitter unit coupled to the track is controlled by a processor 48, the main functions of which are to select the transmission frequencies and modulation parameters.

The receiver microprocessor system, shown in Figure 9, comprises two interconnected microprocessors 90,92 operated by failsafe software. In addition to selecting receiver frequencies, the system adjusts the attenuator 62 (Figure 6) and monitors the correct transmitted signal. In order to adjust the attenuator of the receiver, the processor has an input port 94 which not only senses the output of the receiver window comparator, but also senses the presence or lack of a received signal. The receiver attenuator is controlledin the manner described above via a first output port 96. Receive frequencies are set up prior to or during installation of the unit, the control signals for the frequency synthesisers being provided at a second port 100.

Coded information including the sensed level of the received signal is available at a fourth output port 102 which may be connected to a remote data processor.

A separate connection 104 to the receiver allows the injection of a test signal into the receiver to check the attenuator operation. Parallel with the output port 102 is an error logging port 106.

To provide for direct, failsafe control of signalling equipment, a relay 108, controllable by both microprocessors 90,92 via drive circuit 110 or 112, forms part of the processor circuitry. The relay coil is protected by a fuse and fuse blowing circuitry 114. The fuse allows disabling of the relay should a fault be detected by either processor circuitry.

The software of the receiver microprocessor allows adjustment of the receiver sensitivity in the event that the changes in the receiver signal are of a slow rate.

Rapid changes however, cause a track occupied signal to be fed to the output port 102. Since train detection is dependent on changes in the received signal level, differences in track section attenuation between different track sections can be accommodated automatically without receiver adjustment on installation. Thus, when the apparatus is first powered, the receiver is automatically adjusted by the processor to suit the particular track section to which it is connected, as well as the prevailing ambient conditions.

The pulse modulation and transmitted signal frequency identify the originating transmitter, the receiver processor circuitry being programmed to check that the received signal carries the correct transmitter identity code.

Referring to Figures 10 and 11, since the track circuit signals of the apparatus are of a relatively high frequency, blocking of the signals at an impedance bond 116 and a track/structure connection 118 is easily performed by ferrite clamp rings 120,122 encircling the conductor as shown.

Claims

CLAIMS:

1. A railway track indicator system comprising at least one transmitter and receiver pair operable to transmit an electrical signal along a section of railway track and to monitor the said signal after transmission along the track section thereby to detect the presence or absence of a train on the track section, the electrical signal being an alternating signal having a carrier frequency greater than or equal to 10kHz.

2. A system according to claim 1 having a plurality of transmitter/receiver pairs for monitoring a plurality of consecutive sections of a length of railway track, wherein the transmitters and receivers of the transmitter/receiver pairs are linked to the track by a plurality of track couplers, each coupler being connected to the transmitter of one transmitter/receiver pair and the receiver of another transmitter/receiver pair.

3. A system according to claim 2, wherein each coupler comprises a broadband transformer arrangement having a primary winding coupled to the said transmitter and the said receiver, and a secondary winding coupled to the track.

4. A system according to claim 3, wherein the secondary winding is connected to the track via an isolating capacitance.

5. A system according to any preceding claim, operable to sense the rate of change of a characteristic of the signal received by the or each receiver.

6. A system according to any preceding claim, operable to sense changes in the voltage amplitude of the signal received by the or each receiver.

7. A system according to any preceding claim, wherein the or each receiver includes an input attenuator operable automatically in response to slow changes in the level of the received signal to yield a signal datum level between predetermined thresholds.

8. A system according to claim 7, operable to indicate a change in track section status when the receiver signal level changes by more than a predetermined period of less than 10 seconds.

9. A system according to any preceding claim, wherein the transmitter and receiver of the or each transmitter/receiver pair each have means for selecting carrier frequencies from a range of predetermined carrier frequencies.

10. A system according to any preceding claim, wherein the transmitter of the or each transmitter/receiver pair is operable to transmit a pulsed signal comprising a succession of carrier bursts, the carrier bursts being arranged in packets with each packet containing bursts of different predetermined carrier frequencies in a predetermined order, the pattern of the frequencies being selected to be characteristic of the track section to which the transmitter/receiver pair is coupled.

11. A system according to claim 10, wherein the said transmitter has a frequency synthesiser for generating a carrier signal, the synthesiser being controllable to alter the carrier frequency successively according to the respective pattern of the relevant track section, and wherein the receiver of the same transmitter/receiver pair also has a frequency synthesiser for generating a local oscillator signal, the receiver synthesiser being controllable to execute a frequency hopping pattern corresponding to that of the transmitter.

12. A system according to any preceding claim, wherein the carrier frequency or frequencies are in the region of 100kHz.

13. A system according to any preceding claim , wherein the or each transmitter has an output power of less than 50 mW.

14. Apparatus for a railway track indicator system, comprising a combined receiver and transmitter unit for coupling to a railway track at the junction of two neighbouring track sections, the receiver and the transmitter both being operable to receive and transmit electrical signals from or to respective track sections at carrier frequencies greater than or equal to 10kHz for the purpose of monitoring the presence or absence of a train on the track sections.

15. Apparatus according to claim 14, including a track coupler for linking the receiver and transmitter unit to the track.

16. Apparatus according to claim 15, wherein the track coupler is arranged to link the receiver and the transmitter to the track via common connections.

17. Apparatus according to claim 16, wherein the coupler comprises a non-resonant transformer arrangement having a primary winding connectible to the transmitter and the receiver and a secondary winding connectible to the track.

18. Apparatus according to claim 17, wherein the coupler includes one or more isolating capacitances for connection between the secondary winding and the track.

19. Apparatus according to any of claims 14 to 18, wherein the receiver is operable to sense the rate of change of a characteristic of a signal received from the track.

20. Apparatus according to any of claims 14 to 19, wherein the receiver is operable to sense changes in the voltage amplitude of the signal received from the track.

21. Apparatus according to any of claims 14 to 20, wherein the receiver includes an input attenuator operable automatically in response to slow changes in the level of the received signal to yield a signal datum level between predetermined thresholds.

22. Apparatus according to claim 21, wherein the receiver is operable to indicate a change in track section status when the received signal level changes by more than a predetermined amount within a predetermined period of less than 10 seconds.

23. Apparatus according to any of claims 14 to 22, wherein the unit has means for selecting transmission and reception carrier frequencies from a range of predetermined carrier frequencies.

24. Apparatus according to any of claims 14 to 23, wherein the transmitter is operable to transmit a pulsed signal comprising a succession of carrier bursts. the carrier bursts being arranged in packets with each packet containing bursts of different predetermined carrier frequencies in a predetermined order and with the pattern of carrier frequencies being selected to be characteristic of the track section to which the transmitter is to be connected.

25. Apparatus according to claim 24, wherein the transmitter has a frequency synthesiser for generating a carrier signal, the synthesiser being controllable to alter the carrier frequency successively according to the pattern characteristic of the said track section, and wherein the receiver also has a frequency synthesiser for generating a local oscillator signal and controllable to execute a frequcney hopping pattern corresponding to the pattern of the neighbouring track section to which the unit is to be connected.

26. Apparatus according to any of claims 14 to 25, wherein the carrier frequency or frequencies are in the region of 100kHz.

27. Apparatus for a railway track indicator system comprising a transmitter for tranmsitting alternating signals over a first track section, a receiver for receiving signals over a neighbouring track section, and a non-resonant track coupler linked to the transmitter and the receiver for coupling both transmitter and receiver to the track by means of a common connection at the junction of the track.

28. A railway track indicator system constructed and arranged substantially as herein described and shown in the drawings.

29. Apparatus for a railway track indicator system, the apparatus being constructed and arranged substantially as herein described and shown in the drawings.