GB2213972A - A railway track circuit - Google Patents

Abstract

A first railway track circuit (B) includes a rail track section limited at each end by a set of three dipoles (8, 5, 9 or 6, 3, 7) each connected between two rails (1) and spaced a short distance from each other. The central dipole (5 or 3) of each set has an impedance which is high for the frequency of the current supplying a further track circuit (A or C) adjacent the respective end, a transmitter (2) of the first track circuit and a receiver (10) of the further track circuit (C) adjacent one end of the first track circuit are connected to the terminals of the central dipole (3) located at said one end of the first track circuit, and a receiver (4) of the first track circuit and a tranmitter (11) of the further track circuit (A) adjacent the other end of the first track circuit are connected with the terminals of the central dipole (5) located at said other end of the first track circuit. <IMAGE>

Description

A RAILWAY TRACK CIRCUIT The present invention relates to a railway track circuit.

.A track circuit for a railway typically comprises two rails of a railway block, a generator (transmitter) of alternating current connected with an extremity of the track circuit to produce in the circuit an alternating signal of a specific frequency and a receiver operating a track relay at the other extremity. Such track circuits are presently widely used to ensure the safety of trains running on railway tracks. They enable a knowledge of whether a train is on a specific section of the track when a short circuit is produced in the track circuit between two rails by one or several axles.

In the case of railways using welded track, the length limitation of track circuits is obtained by creating an electric filter (or 'electric joint') at each end of a track circuit, consisting of a short track section, self-inductance coils and capacitors, enabling a limit on the propagation of the electric signal of the track circuit to the section of the track between these two filters, the track circuits on opposite sides of the track circuit being supplied by currents of different frequencies.

Generally, such track circuits are satisfactory as long as the shunt consisting of the axles of a train has an electric resistance which is sufficiently low and that it comprises at least two axles sufficiently distant to ensure the continuity of occupancy of two adjacent (adjoining) track circuits.

However, improvements made in track circuits of this type in order to increase their length by using either low supply frequencies or capacitors placed between the rails show a shuntage discontinuity between two adjacent track circuits and a short working length .(engine running light, line inspection car, a wagon by itself) may not be detected if it is in this critical area.

An attempt to improve this by superposing a short length track circuit at a high frequency on this critical area is costly, complicated and difficult to use when one wants to get the repetition of signals on board a railway vehicle.

According to the present invention, there is provided a first railway track circuit, including a rail track section limited at each end by a set of three dipoles which are connected between two rails and spaced a short distant from each other, the central dipole of each set having an impedance which is high for the frequency of the current supplying a further track circuit adjacent the respective end, a transmitter of the first track circuit and a receiver of the further track circuit adjacent one end of the first track circuit being connected to the terminals of the central dipole located at said one end of the first track circuit and a receiver of the first track circuit-and a transmitter of the further track circuit adjacent the other end of the first track circuit being connected with the terminals of the central dipole located at said other end of the first track circuit.

The present invention will now be described, by way of example, with reference to the accompanying drawing, in which:- Fig. 1 is a drawing of a track circuit (B) according to an embodiment of the invention and two adjacent track circuits (A and C); -Fig. 2 is a graph showing the voltage variation at the terminals of the receiver of track circuit B in Fig. 1 when a pinpoint shunt moves along track circuit A in Fig. 1 towards track circuit C in Fig. 1, chain-dash lines in Fig. 1 enabling location of the position of the shunt for the voltage indicated by the graph of Fig. 2; Fig. 3 shows various possible methods to make dipoles for a first construction, couplings 3a and 3b being respectively equivalent to couplings 3c and 3d; Fig. 4 shows a dipole type which may be used for track circuits supplied by alternate frequencies;; Fig. 5 shows the dipole type which must be substituted for capacitors 3 and 5 in Fig. 1 when supplying track circuits with alternate frequencies; and Fig. 6 shows the impedance variations according to frequency of this last .dipole connected to a short track section.

According to preferred embodiments of the invention a railway track circuit is characterised by the fact that: a filter placed near a transmitter consists of a capacitor (or capacitance) connected between both rails of a short track section, such track section being limited on each side of the capacitor (or capacitance) by a dipole; the transmitter of the track circuit and a receiver of a track circuit adjacent this transmitter are connected to the terminals of the capacitor (or capacitance) ; a filter placed near a receiver of the track circuit also consists of a second capacitor (or capacitance) connected between both rails of a short track section limited an each side of this capacitor -(or capacitance) by a dipole; and the receiver of the track circuit and a transmitter of a track circuit adjacent this receiver are connected to the terminals of the second capacitor (or capacitance) in such a way that a single axle placed on this capacitor (or capacitance) short circuits every time simultaneously the transmitter of the track circuit and the receiver of the adjacent track circuit or the receiver of the track circuit and the transmitter of the adjacent track circuit, the dipoles enabling control of the propagation of the currents of the track circuit and of the adjacent track circuits. As a result of this, one may have a long track circuit without any insulating gaskets and having substantially no bad detection zone at each of its extremities.

In a first construction, dipoles consisting of the elements marked 6, 7, 8 and 9 of the filters or electric joints represented in Figure 1 each only comprises three series and parallel connected components, i.e. a capacitor and two inductance coils or two capacitors and an inductance coil (see for example Figure 3). This arrangement enables dipoles 6 and 8 to obtain a very low impedance (a few milliohms) for the frequency of the currents of track circuits A and C and a high impedance (several ohms) for the frequency of the current of track circuit B. Dipoles 7 and 9 have a very low impedance for the frequency of the current of track circuit B and a high impedance for the frequency of the currents of track circuits A and C.

In a second construction method which can be used when the track circuit is supplied alternately by two currents of slightly different frequency (a frequency modulation system), one can use dipoles each consisting .of nine components as shown in Figure 4. These components are grouped in series-parallel, i.e. four series circuits each comprising an inductance coil and a capacitor and a fifth capacitor in parallel with the four series circuits described above. The choice of the values for inductances and capacitors enables one to obtain low and high impedance for the alternate frequencies which have been chosen. In this second construction method, capacitors 3 and 5 of Figure 1 must be replaced as shown in Figure 5 by capacitor 12 and inductance 13 assembled in series and connected in parallel with capacitor 14.

In accordance with the construction shown in Figure 1, a track circuit B shown with continuous lines comprises a track section where both rails 1 act as electric conductors for the transmission of electric current from a transmitter 2 connected to the terminals of a capacitor 3 and received by a receiver 4 connected to the terminals of a capacitor 5. Receiver 4 is responsive only to the current frequency of transmitter 2. Track circuit B is situated between two track circuits A and C shown in broken lines. Both .these track circuits comprise dipoles 6 and 7 which are common to track circuits B and C and dipoles 8 and 9 which are common to track circuits A and B. Receiver 10 shown in broken lines is the receiver of track circuit C and is responsive exclusively to the current frequency of the transmitter of track circuit C which is not shown in Figure 1. Likewise, transmitter 11 shown in broken lines supplies track circuit A with current of a frequency which is different from the frequency of track circuit B.

The role of dipole 7 is to create a short circuit for the frequency of track circuit B between both rails of -the short track section used to make the filter (or electric joint). This short circuit limits the propagation of the current from transmitter 2 of track circuit B, which enables the creation of a resonant circuit for the frequency of track circuit B with the self-inductance of the track section between dipole 7 and capacitor 3. The impedance at the terminals of capacitor 3 is equal to several ohms. Likewise, for the frequency of track circuit C, the same dipole 7 has a high impedance which reduces slightly the transmission of the current emitted by the transmitter of track circuit C to receiver 10 of track circuit C.

Dipole 6 situated at the other end of the track section making the 'electric joint', just referred to, plays a similar role for currents of track circuits B and C.

It creates a short circuit for the current of track circuit C and does not allow the propagation of such current beyond the connection points of the said dipole to the rails towards dipole 8. Such short circuit for the frequency of the current of track circuit C enables the creation, with the self-inductance of the track section between dipole 6 and capacitor 3, of a resonant circuit at the frequency of track circuit C. The impedance at the terminals of capacitor 3 is equal to several ohms. The impedance at the terminals of capacitor 3 is high for the frequency of track circuit B and the impedance is also high for the frequency of track circuit C.

The same phenomena re-appear at the other end of track circuit B with dipole 8, capacitor 5 and dipole 9.

With the provisions described above, it is possible to make a track circuit which complies with Figure 1 measuring 2500 metres long and to secure a minimum shunt of 0.25 ohm on the total length of this track circuit as well as on the adjacent track circuits.

This track circuit uses a 1700 Hz frequency current and the adjacent track circuits use 2300 Hz frequency current. The track circuits mentioned use compensating capacitors as usually used in these circumstances and the value of this shunt for any track insulation varying from 1.5 ohm per km to 1000 ohms per km is secured. It is to be kept in mind that with the devices known presently, it would be impossible to secure a pinpoint shunt of 0.25 ohm at a distance of at least 15 metres around capacitor 3 or 5.

It is to be noted that the dipoles are constructed for given resonance frequencies and that the positions of the dipoles on respective sides of the capacitor 3 or 5 as the case may be, are determined according to the overlapping zone desired.

With the same dipoles, it is therefore easy to make different overlappings by giving capacitors 3 and 5 the values which correspond with the desired lengths of the overlapping zones. It is important to note that the characteristics of the dipoles are independent of the lengths of the track sections used to make the filters or electrical joints.

Claims (6)

1. A first railway track circuit, including a rail track section limited at each end by a set of three dipoles which are connected between two rails and spaced a short distant from each other, the central dipole of each set having an impedance which is high for the frequency of the current supplying a further track circuit adjacent the respective end, a transmitter of the first track circuit and a receiver of the further track circuit adjacent one end of the first track circuit being connected to the terminals of the central dipole located at said one end of the first track circuit and a receiver of the first track circuit and a transmitter of the further track circuit adjacent the other end of the first track circuit being connected with the terminals of the central dipole located at said other end of the first track circuit.

2. A track circuit according to Claim 1, wherein the central dipole of each three dipole set which limits the track circuit at each end comprises capacitance means.

3. A track circuit according to Claim 2, wherein each three dipole set comprises an inductance coil and two capacitors for the dipole located on one side of the capacitance means and two inductance coils and a capacitor for the dipole located on the other side of the capacitance means, dipoles consisting of the same elements being located between the two said capacitance means.

4. A track circuit according to Claim 1, wherein the central dipole of each three dipole set comprises an inductance coil connected in series with a capacitor, these being connected in parallel with a second capacitor.

5. A track circuit according to Claim 4, wherein the dipoles of each three dipole set on respective sides of the central dipole thereof each includes four inductance coils and five capacitors which form five branches of a parallel circuit, four branches of which circuit comprise each a series circuit including an inductance coil and a capacitor and the fifth branch of which circuit is a capacitor.

6. A track circuit, substantially as hereinbefore described with reference to the accompanying drawings.