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Improvements in Railway Signalling Systems
This invention relates to railway signalling systems and specifically to the detection of a train within a defined section of track by means of track circuit apparatus utilising the rails within the section as part of the track circuit.
Such track circuits detect the presence of trains in a track section by detecting the change in the shunt impedance between the running rails of the track circuit when a train enters the sections and the wheels and axles of the train present a low impedance, henceforth called the train shunt impedance, between the rails and in parallel with the existing shunt resistance formed by the track ballast, henceforth called the ballast resistance. This is illustrated in the accompanying Figures la and lb.
Figure la shows a track section defined by insulated joints X at the ends of rails 1 and 2. A source of electricity diagrammatically represented by battery 3 is connected across the rails 1 and 2 at one end of the section and a detector 4 is connected across the rails at the other end of the section to complete the track circuit through the rails. In the condition shown in Figure la. the detector 4 would register the voltage applied across the rails, less any losses along the track section through ballast resistance, hence indicating that there is no train in the section.
When a wheel/axle set 5 enters the track section as shown in Figure lb and provided that the wheel/axle set 5 makes σood electrical contact with the rails 1 and 2, the

SUBSTITUTE SHEET voltage at the detector 4 falls nearly to zero as a result of the low train shunt impedance. This condition of the track circuit is interpreted as track section occupied by the signalling system.
5 Present day railway signalling depends on reliable detection of the occupancy of track sections. Decisions made by signalmen or signalling equipment as a result of faulty detection could lead to unsafe situations developing.
The reliable operation of the track circuit depends 0 upon good electrical contact between the wheels and rails and good electrical conductivity of the wheel/axle set so that the train shunt resistance is low enough to provide in effect a short circuit between the rails . Under certain conditions however the rail-wheel-axle-wheel-rail circuit is

■>c not as good an electrical conductor as is required for reliable operation of the train detection circuitry. This is mainly caused by the growth of surface films, for example, rust films on the rails from place to place along the rails . This is particularly noticeable in the case of

_n modern designs of multiple unit rolling stock.
The presence of a contaminant such as a rust film between the wheels and rails will form a layer of insulating and/or semiconducting material. To overcome the electrical barrier so formed there are two mechanisms which may be considered, namely:
1. Breaking down the insulating layer by application of a sufficiently high voltage, and
2. Minimising the effect of the semiconducting property by biasing the wheel/rail contacts to a working level where

SUBSTITUTE the electrical resistance is sufficiently low to permit the track circuit to operate satisfactorily.
It is already known to provide a so-called shunt assist circuit in order to overcome the aforesaid electrical barrier. One such circuit is described in an article entitled "Lightweight Vehicle Track Shunting" by Thomas . Dyer, Inc. published in April 1981 by the U.S. Department of Commerce National Technical Information Service. In this article is described a shunt assist circuit comprising an excitation circuit which circulates a relatively high amperage 400 cps current from wheel to rail and back to wheel of a train unit. It is stated that this breaks down the rail-wheel resistance and improves the shunting by a wheel/axle set very effectively. The excitation circuit consists basically of a transformer, the turns of the transformer primary being wound around a first axle of a truck or bogie. The power is supplied to the transformer primary from a small on-board alternator. The secon ary of the transformer comprises a single turn formed by said first axle a second axle of the train unit spaced from said first axle and the two rails between said first and second axles .
In order to provide the secondary winding in this way it is necessary to insulate said first axle from the rest of the bogie. This is not only disadvantageous from the manufacturing point of view but also for retrofitting the shunt assist system to existing vehicles.
The object of the invention is to provide a shunt assist circuit which does not suffer from the aforesaid disadvantages .

SUBSTITUTE SHEET According to the present invention a shunt assist circuit is characterised in that an inductive loop aerial is provided on a railway vehicle so that it is closely coupled, inductively, with the rails whereby when the loop aerial is 5 energised from an alternating source a current is induced in the required wheel-rail-axle circuit.
Thus there is a transformer coupling between the inductive loop aerial constituting the primary and the single turn secondary comprising two spaced wheel/axle sets •JQ and the rails extending between these two sets. However, because the aerial inductively couples directly with the rails there is no need to insulate the two wheel/axle sets from the rest of the bogie or vehicle.
Shunt assist circuits in accordance with the ,- invention will now be described by way of example with reference to Figures 2 to 4 of the accompanying diagrammatic drawings, in which:
Figure 2 shows a first shunt assist circuit,
Figure 3 shows a εeccnd shunt assist circuit, and
Figure 4 is an explanatory diagram.
Figure 2 of the drawings shows a vehicle bogie having two wheel/axle sets 10 and 11 running on the rails 1 and 2. Mounted on the bogie is a loop aerial 12 which inductively couples with the rails 1 and 2 and also with the wheel/axle sets 10 and 11. The loop aerial 12 is powered

from a drive unit 13 comprising an oscillator 14 and an amplifier 15 which produces for example a fixed frequency of 165 kHz output. The aerial 12 tuned in situ with a parallel connected tuning capacitor 16 to resonate at the energising

SUBSTITUTE SHEET frequency. It is known for efficient power transfer into a reactive load (the loop aerial 12) that the inductive reactance must be compensated by an equivalent capacitive reactance, i.e. the network becomes resonant at the energising frequency. Thus the loop aerial 12 is tuned to provide high current at low power dissipation in the drive unit 13.
In practice, the magnitude of the inductive reactance of an installed loop aerial is not constant and is also subject to production and installation tolerances. To accommodate this variability the value of the tuning capacitor 16 can be adjusted for individual installations but this may prove inconvenient for large scale implementation. An advantageous alternative is to make the energising frequency adjustable and employ an optimising circuit which seeks the frequency at which the system is resonant. This is conveniently implemented with Phase Locked Loop (PLL) techniques which are commonly understood. There are many benefits with this solution; production, installation and ageing variations are automatically accommodated.
A shunt assist circuit utilising adjustable frequency is shown in Figure 3. Referring to Figure 3, the same reference numerals as in Figure 2 have been used to designate corresponding items. Thus loop aerial 12 is powered from a drive unit 13 comprising an oscillator 14 and an amplifier 15. A tuning capacitor 16 is connected in parallel with the loop aerial 12 so that the loop aerial resonates at the energising frequency.

SUBSTIT In order to compensate for changes in the inductive reactance of the loop aerial and so substantially avoid a loss of resonance, an automatic control circuit is provided for controlling the output frequency of the oscillator 12 5 which in this case is advantageously a voltage controlled oscillator. The automatic control circuit comprises a phase comparator 18, which receives as a reference signal the output voltage from the oscillator 14. The phase comparator 18 receives as its comparison signal a voltage signal from ,g the loop aerial 12.
Since the amplifier 15 inherently has a finite output impedance (resistance), when the loop aerial 12 goes off tune, the phase of the amplifier output voltage changes with respect to its input voltage, because of the change in . „ current flowing in the loop aerial circuit. This change in phase appears in the comparison signal fed to the comparator 18.
The output from the comparator 18 is fed via a low pass filter 19 to control the output frequency of the oscillator 14. Thus a change in the phase of the comparison

signal will change the frequency of the output from the oscillator 14 in the sense to vary the comparison signal and so restore the phase of the output voltage from the amplifier 15 to its original relationship with the reference signal. Thus the output frequency of the oscillator is

adjusted to maintain a substantially zero phase difference between the reference and comparison s-ignals .
In bcth the embodiments of Figures 2 and 3, the loop aerial 12 forms a single turn primary winding of a transformer, the single turn secondary of which comprises the loop formed by the two wheel/axle sets and the lengths of rail 1 and 2 between the two wheel/axle sets . The area of the primary is made as large as possible within the 5 constraints put upon it by the physical design of the vehicle and is for example approximately equal to 50% of the secondary loop. Thus the loop aerial 12 inductively couples directly with the rails 1 and 2.
In the case of a bad wheel to rail contact a voltage i of say 10V is generated in the secondary winding at the wheel-rail contact area and it has been found that this is adequate to break down any barrier to current flow in the wheel-rail contact and hence in the shunt path provided by the wheel/axle set within the track circuit.
,.. The efficacy of the shunt assist circuit is shown in

Figure 4 of the drawings which is a graph of voltage V at the track circuit detector with time, the shunt assist circuit being turned "on" and "off" periodically, i.e. the loop aerial 12 energised and de-energised periodically at say one second intervals. These results were obtained from

tests carried out on a section of line which had a history of bad-detection problems. To exacerbate the poor performance of the line a rust film was artificially introduced by application of moisture prior to the tests. With the shunt assist circuit turned "on" the voltage at the

detector drops very nearly to zero thereby indicating very low train shunt impedance and hence that the track section is occupied by at least one wheel/axle set.
Figure 4 shows an upper "unoccupied" line above which it is guaranteed that the track section should be unoccupied. It also shows a lower "occupied" line below which it should be guaranteed that the track section is occupied. The space between these lines represents the 5 hysteresis of the track circuit detector which is typically in the form of an electromagnetic relay.
The results clearly show the improved performance of the track circuit when the shunt assist circuit is "on" and the non-detection of the train where the shunt assist 0 circuit is "off".
The above described shunt assist circuit has further possible variations or alternative applications, namely:
1. Use of the shunt assist circuit to characterise track electrical performance by monitoring the electrical

,- impedance of the wheel-rail circuit formed by the shunt assist circuit.
2. Use of the inductive loop aerial to convey data messages from train to trackside equipment in addition to the shunt assist function.
3. Use of the inductive loop aerial to monitor the

condition of track. (Making use of dynamic performance of wheel-rail contacts, noting effect on effective shunt impedance of the wheel-rail-vehicle circuit).
It will be appreciated that with the above described shunt assist circuit there is no need to insulate the

wheel/axle sets from the bogie frames in order to direct current flow around the shunt assist circuit.