GB2134681A

GB2134681A - Railway track circuit termination

Info

Publication number: GB2134681A
Application number: GB08302627A
Authority: GB
Inventors: Damian Mounsey Poole
Original assignee: GEC General Signal Ltd
Current assignee: GEC General Signal Ltd
Priority date: 1983-01-31
Filing date: 1983-01-31
Publication date: 1984-08-15
Also published as: GB2134681B; GB8302627D0

Abstract

A railway track circuit for indicating the presence of a train within a track section by means of a transmitter and a receiver Rx connected across the track at opposite ends of the section. The transmitter 'reach' is limited to the location of the receiver by means of a negative feedback circuit in which a voltage V2 is imposed in series with the rail downstream of the receiver, the voltage being equal and opposite to the voltage V1 across the track at the receiver termination. The imposed voltage is derived as the output of an amplifier 13 the input of which is proportional to the signal current in the rail. The feedback circuit thus simulates a high impedance in the rail. <IMAGE>

Description

SPECIFICATION Railway track circuit termination This invention relates to railway track circuit terminations. Railway signalling track circuits provide a means for detecting the position of a train on a track in order to control its movement and thereby prevent accidents. The track is conventionally divided into sections by insulated joints in one rail or in both. A transmitter is connected between the rails at one end of a section and a receiver at the other end. The receiver, for example a relay, is normally energised from the transmitter by way of the rails. In the presence of a train in the section the track circuit is short-circuited so de-energising the relay which drops out to indicate the train presence.

This arrangement works reasonably well but the insulated rail joint is a source of mechanical weakness and maintenance cost is significant.

The object of the invention is to provide an electronic equivalent of the insulated joint without interrupting the mechanical weakness.

According to the present invention, a railway track circuit comprises signal transmitting means coupled to the track and arranged to transmit signals to remote receiving means coupled to the track, and negative feed-back means coupled to the track in such manner as to tend to suppress signal current in the rails downstream of the receiving means.

The negative feedback means may comprise an amplifier having input and output coupled to one rail at a location downstream of the receiving means.

The transmitting and receiving means preferably operate on an alternating current signal, and at least the input ot the amplifier is coupled to the rail inductively.

A railway track circuit in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 is a diagram of a conventional insulated joint track circuit; Figure 2 is a circuit diagram of a portion of track coupled to means for imposing a voltage in series in the rail: and Figure 3 is a diagram of a track circuit termination in accordance with the invention.

Referring to Figure 1 a known track circuit comprises one continuous rail 1 and one rail 2 having insulated joints 3 at intervals which define track sections. The length of such a section is chosen to provide adequate information of the position of a train, i.e. whether the section is occupied or not. A transmitter 5 is positioned just within one end of the section and is shown diagrammatically to comprise a battery and impedance, connected between the rails. The interrupted rail 2 is insulated from earth but the continuous rail is earthed. A receiver 7 is positioned at the other end of the section, again connected between the rails and comprising a relay having a contact 9. With the particular section unoccupied, i.e. no train present, the receiver 7 is energised from the transmitter by way of the rails 1 and 2 and the contact 9 closes to indicate the absence of a train.

When a train (indicated by the axle and wheels 11) enters the section, the transmitter signal is short circuited by the train, the receiver is no longer energised and the relay contact drops out to indicate 'section occupied'.

The transmitter is shown as a D.C. source but can equally be A.C.

It will be apparent that the use of insulated joints in rail 2 prevents any receiver in the next or any other section being actuated by the transmitter in the section shown.

It has been appreciated in this invention that an insulated joint can be simulated electronically by imposing a voltage on the track circuit in series with the rails which voltage is in opposition to the transmitter signal voltage across the rails at that location. This may equally be seen as imposing a voltage in series with the rails which opposes and tends to cancel the existing signal current. The result is thus to produce, in effect, a very high impedance in the track circuit at that location so preventing the passage of signal current.

Figure 2 shows a power amplifier 13 having its output connected to spaced points A and B on the rail 15, which is continuous. If the amplifier input and gain are suitably selected, the voltage imposed between A and B can be made qual to the voltage between the rails at that location but in opposition so that the net voltage between the rails after this location is zero.

The necessary input to the power amplifier is derived, as shown in Figure 3, from the rail by way of a pick-up coil 1 7. This coil is fixedly mounted adjacent the rail and is inductively coupled to it.

The transmitter (not shown in this Figure) is an A.C. transmitter and the receiver 7 responds accordingly. The amplifier output is connected in such a way as to impose a voltage V2 between A and B which is in opposition to the current in the rail on which the voltage V2 depends. A negative feedback loop is thus formed. With a sufficiently high gain amplifier, the current in the rail 1 5 is substantially suppressed 'downstream' of the receiver 7, the effect of the pick-up coil, amplifier and output coupling being equivalent to a high impedance in the line.

It may be seen that to suppress current in the rail, the amplifier output voltage V2 must be equal and opposite to the track voltage V1 and will automatically correct itself to this value. The extent of the signal suppression achieved will be affected by the leakage resistance between the line 1 5 and earth downstream of the termination.

The position of the pick-up coil 1 7 can be varied substantially: it may be placed anywhere in the 'zero-current' portion of the rail 1 5, and in fact, may be coupled to the rail 1 if that rail is isolated from earth downstream of the track circuit termination in question. Thus the input and output of the amplifier 1 3 may be coupled to the track at any two positions in the loop formed by the isolated portions of the rail(s) downstream of the receiver.

Again, it would be possible to share the suppression between pick-up coils and amplifiers in each rail.

It would be possible to operate the arrangement in a D.C. track circuit but a resistive coupling to the rail 15 would then be necessary.

An A.C. system would, in general, be more practical than the D.C. version.

In a further variation of the arrangement of Figure 3, the output of the amplifier could, and would preferably, be coupled inductively to the rail 15.

Claims

1. A railway track circuit comprising signal transmitting means coupled to the track and arranged to transmit signals to remote receiving means coupled to the track, and negative feedback means coupled to the track in such manner as to tend to suppress signal current in the rails downstream of said receiving means.

2. A railway track circuit according to Claim 1, wherein said negative feedback means comprises an amplifier having input and output coupled to one rail at a location downstream of said receiving means.

3. A railway track circuit according to Claim 1, wherein said negative feedback means comprises an amplifier having input and output coupled to different rails at locations downstream of said receiving means.

4. A railway track circuit according to Claim 1, wherein said negative feedback means comprises, in respect of each rail, an amplifier having input and output coupled to that rail at a location downstream of said receiving means, each amplifier contributing to the suppression of the signal current.

5. A railway track circuit according to Claim 1 or Claim 2, wherein said transmitting and receiving means operate on an alternating current signal.

6. A railway track circuit according to Claim 5 and any of Claims 1 to 4, wherein at least the input to the amplifier is coupled to the rail inductively.

7. A railway track circuit substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

8. A method of terminating a railway track circuit section, in which signal current in a rail is detected and a corresponding voltage is imposed in series in the rail loop downstream of the termination location in such manner as to oppose and substantially balance signal voltage across the track at the termination.