CH21 | AXLE COUNTERS-SECONDARY TRAIN DETECTION SYSTEMS
Signalling
- Introduction
- Principle of Operation
- General Description of Equipment
- Wheel Detection
- Transmission
- Indoor Equipment
- Failure of Equipment
- Disadvantages of Axle Counter System Vs Track Circuit
- Type of Axle counters
- Australian Standard
1 ) Introduction
For many years, the normal means of train detection has been carried out by Track Circuits. A wide variety of track equipment is available to cater for different types of Tracks and traction equipment.
However, there remain some situations where the track circuit cannot be used or unreliable in operation. Some of the situations are listed below: -
A) Ballast conditions, the ground conditions or sometimes the structure upon which the track is laid, may be electrically unsuitable for a track circuit
Eg: Track directly fixed to a steel bridge, use of steel sleepers, ground conditions varying between totally dry and waterlogged making satisfactory adjustments impossible. Environmental conditions could impact the Rail To Rail and Track to Earth resistance. During wet conditions or poor insulation on the track bed to rail, Track to Earth resistance could go below the required value of certain track circuits ( Eg: FS3000 track circuit expect resistance not less than 7.5 ohm-km )
B) Length of Track Section. For very short track circuits, there is a minimum length below which some vehicles may not be detected. It may therefore be impossible to provide track circuits sufficiently short in length through some crossings (Eg: Large angle between tracks of multiple track lines crossing each other). Conversely, there is also maximum length over which any track circuit will operate. Multi-section track circuits can be very expensive to provide
C) Access to Line. A track circuits line will need access at regular intervals for maintenance. In long tunnels and in areas where the terrain makes safe access difficult, its undesirable to locate any equipment which requires regular maintenance
D) Power Supplies. On long sections of rural railway, its expensive and therefore undesirable to provide line side power supplies.
E) Some audio frequency track circuits such as Siemens FS2550, FS3000 have null zone around the boundary of track circuits where train will not be detected. Sometimes this undetected gap can go above 2m.
F) Track circuits requires (depends on configuration and type) IRJ’s which is expensive to install and maintain.
G) Track circuits on electrified lines requires traction bonding & impedance bond.
H) Rusted sparingly used tracks can go trains undetected, also flooding, contaminated ballast, or other problems with insulation between the rails can bypass the circuit;
I)crushed leaves or ice on the rails can form an insulating layer that makes the circuit unreliable.
J) Sometimes a train sprinkles anti-slip sand to aid deceleration while braking, but the sand contaminates the rail, and the track circuit stops working
Axle counter can assist in solving these problems.
2) Principle of Operation
As its name suggests, the axle counter operates by counting the number of axles entering and leaving a track section, as opposed to a track circuit which proves that an entire track section is clear of rail vehicles. Provided there is a reliable, fail-safe communication link between the ends of the track section and the interlocking, its length may be as small or as large as required.
Eg: An axle counter can go up to around 10,000 metres (33,000 ft) from the evaluation unit when connected directly. However, with the addition of an Ethernet network, the distance is limited by the transmission system.
However latest development is inclusion of optical fiber network which makes larger distance possible.
It is not constrained by minimum vehicle lengths or the electrical characteristics of the track bed.
When the section is clear, the count of axles in the section must be zero. A train runs into the section, each axle is counted in and added to the total. When the train leaves the section, each axle is counted out and subtracted from total. A zero total is therefore equivalent to a track circuit clear, a non-zero total is equivalent to a track circuit occupied.
The detection and counting equipment must be capable of bidirectional operation. Even if its not required for normal traffic, but the absence of this facility will prevent return to normal use without manual intervention after an emergency or irregular train movement.
3) General Description of Equipment
German manufacturer ,SEL(Standard Elektrizitätsgesellschaft) was one of the earlier popular axle counter equipment .SEL later merged with Alcatel and now its part of Thales. I thought it would be good to discuss with original SEL system for understanding the Axle Counter Concept. The following description relates to their systems. Current version of Thales Axle counter and Frauscher axle counter will be mentioned later stage of this article, as these are proprietary products, and the owner of the system provides manuals and are also widely available on internet. Refer Figure 1 for a SEL Axle counter general arrangement
The equipment can be divided into three basic parts:-
- The trackside (Including Track Mounted) equipment which detects the passage of the wheels of the train. This in turn controls a.c signals to the interlocking or other suitable central location
- A transmission link from the trackside to a suitable location or equipment room. The equipment has the facility to transmit to two separate destinations to enable one set of track equipment to serve the boundary between track sections in different interlockings.
- The “Indoor” equipment which processes the signals from the various trackside detection points and convert them into “track clear” or ‘Track Occupied” data in a form that can be used by the interlocking and Train control system (or ATS Mimic/HMI) to display the status. It also monitors the trackside equipment for correct operation.
4) Wheel Detection
The track mounted equipment must reliably detect the passage of each axle as it passes the detection point. It must be able to detect the direction of travel of the train. It must also operate over the full possible range of train speeds.
An electronic system is being used. A detector consists of a transmitter on the outside of each rail and a receiver on the inner (running) edge of the rail. Two of these detectors are mounted in close proximity (170-200mm) measured along the track). To detect the direction of the movement, the two detectors are staggered .The stagger distance must be large enough to detect the direction of movement at high speed but small enough to ensure detection of only one axle at a time .Older systems were mounted on each rail ,newer systems are constructed so that all track equipment can be mounted together on one rail ,simplifying the cable connections .The distance is normally set about 170mm .Both detectors can therefore be mounted in the same sleeper bay .
Both the transmitter and receiver consist of coils wound on to a magnetic (ferrite) core. The transmitter coil is adjustable to suit different magnetic characteristics of various rail profiles. Its continuously fed with an a.c signal. Refer Figure 2 for a Rail Mounted Transmitter and Receiver.
Refer Figure 3 for SEL Axle Counter Track Equipment Diagram
Although the magnetic field surrounding the transmitter ,receiver and rail is complex ,the simplified diagram assumes two components of flux Φ1 and Φ2 linking the two coils .With no wheel present Φ1 is greater than Φ2.The wheel and flange have greater effect on Φ1 reducing it almost to the level of Φ2 in older systems and due to different frequencies used ,below the level of Φ2 in later systems .The induced voltage therefore reduces almost to zero (older systems) or reverse phase (later systems) .By processing the outputs from the two receivers the number of axles passing the detection pint ,and their direction can be determined.
Alongside the two rail mounted detectors is a junction box containing the electronics and power supply. It is connected to the location case or equipment room by a two-core balanced cable (four wire system was used in first versions).
As the wheels pass over the detector, each receiver experiences a reduction or phase reversal of voltage in each receiver. On earlier systems the outputs were each amplified and transmitted to the indoor equipment.
Later system perform more processing at trackside .The Tx/Rx board for Tx/Rx1 produces a continuous 30KhZ signal .TX2 7 Rx2 operate at 29kHz.The receiver signal is then compared with the transmitted signal .As the wheel passes ,the phase of the receiver signal will reverse .This causes the d.c output of each Tx/Rx board to switch between a high voltage (logic 1 -no wheel present) to a low voltage (logic 0-wheel present) and back again .These two pulses are modulated on to 5060Hz and 4150 Hz signals respectively. These are then transmitted together with a supervisory signal at 2530Hz over a single cable pair to evaluator.
Referring to Figure 4 shown above, if we assume that the track section is to the right of the detectors, a vehicle passing from left to right (into the track section will produce a pulse on Rx1 first .A vehicle travelling from right to left will produce a pulse from Rx2 first.
5) Transmission
This consists essentially of line matching at each end and a twisted cable pair between the trackside and the location or equipment room .The length of the transmission circuit is limited by the attenuation of the audio frequency signal which imposes a practical limit of about 20km .A lower transmission frequency can be used to increase this distance if required
If the d.c supply to the junction box is fed via transmission line (to avoid a separate power cable ) the maximum circuit length for 0.9mm diameter conductors is about 4km .It can therefore be seen that there is potential capability for axle counters to cover much longer track sections than track circuits
6) Indoor Equipment
To operate the equivalent of a track circuit, data is required from two (plain line) or more (points & crossings) detection points. More complex track layouts can be provided for by the use of additional detectors. By provision of appropriate wiring between the inputs and counters, overlapping track sections could be catered if required. The evaluator requires an input for each detection point and a counter for each track section.
The received a.c voltages are filtered and amplified and converted to a d.c pulse for counting. Axles can now be counted “in” and “out” with respect to a nominated reference direction (equivalent to up or down traffic). An “in” count for example may add to or subtract from the count for a section depending on whether the detector is at the up or down end.
Pulse 1 is used as a gate pulse which must be present for any count to register. Pulse 2 count in or out depending on whether the falling or rising edge falls within pulse 1
Each counter is a binary counter capable of counting up to 511,1023 or more axles as appropriate to the traffic needs. If the count is zero, a ‘track clear’ output is produced, if a non-zero, a ‘track occupied’ output is produced. On a plain line track section, in pulses from one input will add to the count while out pulses from the same input will subtract from it. The converse will apply for the inputs from another detector.
To ensure fail safe operation the equipment also includes several checking and monitoring circuits at each stage of the counting process. There will not be opportunity to cover these in detail. In essence, however, a correct count ‘in’ or ‘out’ will only be registered by the combined signal from both rails. Any irregular signal from one or the other of the detectors will invalidate the count, raise an alarm and show the equivalent of a ‘track occupied’ indication.
7) Failure of Equipment
The outputs from the counters are in the form of ‘track occupied’ (count non-zero),’track clear’ and ‘alarm’. To correctly give a track clear indication to the interlocking, the track clear output must be present and the other two absent. All other conditions will produce a track occupied indication to the interlocking
Circuits are provided to monitor all counts into a track section. Any failure in counting out will automatically result in a track occupied indication because the resultant axle total will not be zero.
A significant change in the gain of any amplifiers in the system will result in the count in not being valid, due to change in received voltage. Any change in the frequency of the oscillators and /or filters will result in the absence of an input a.c signal thus initiating an alarm.
If a failure occurs, the counter can be reset to zero by the maintenance technician after remedial action has been taken. Great care must be taken to ensure that the section is clear of all trains when this is done .
Axle counters can therefore be used as an effective substitute for track circuits where track circuits would be either impractical or unreliable.
8) Disadvantages of Axle Counter System Vs Track Circuit
Even though axle counter eliminates many of the track circuit disadvantages, there are disadvantages for axle counter compared to Track circuits. Track circuits can recognise rail breaks under certain circumstances where as axle counter cannot detect the breaks in rails.
For various reasons, such as a power failure, axle counters may 'forget' how many axles are in a section. A manual override is therefore necessary to reset the system. This manual override introduces the human element which may be unreliable. An accident which occurred in the Severn Tunnel is thought to have been due to the improper restoration of an axle counter. That was not proven during the subsequent inquiry, however. In older installations, the evaluators may use 8-bit logic, causing numeric overflow when a train with 256 axles passes the axle counter. As a result, that train would not be detected. That imposes a length limit of 255 axles on each train. More modern systems are not restricted by train wheel numbers.
Where there are interlocked turnouts, an axle counter unit needs to be provided for each leg of that turnout. On lines with non-interlocked/hand-operated switches, detection of the switch points would have to be monitored separately, whereas on track-circuited lines misaligned points can be set to automatically break the track circuit.
Axle counters have problems maintaining correct counts when train wheels stop directly on the counter mechanism. That is known as 'wheel rock', and can prove problematic at stations or other areas where cars are shunted, joined and divided. Also, where main lines have switches to siding, spur or loop tracks, extra counters will need to be deployed to detect trains entering or exiting the line, whereas the same infrastructure using track circuits needs no special attention.
Magnetic brakes are used on high speed higher speed trains with a maximum speed greater than 160 kilometres per hour (100 mph). These are physically large pieces of metal mounted on the bogie of the vehicle, only a few centimetres above the track. They can sometimes be mistakenly detected by axle counters as another axle. This can happen at only one end a track block, because of magnetic field curvature, defects of track geometry, or other issues, leading the signalling system to become confused, and also requiring reset of the detection memory. Modern axle counters are 'eddy current' brake-proof and the magnetic effect of the braking system as described above is overcome, with count information remaining stable even when a vehicle fitted with magnetic brakes is braking whilst traversing the detection point.
9) Type of Axle counters
Thales and Frauscher are the two major manufactures of axle counter system .Below is the list of some manufactures .If you know any other manufactures and wish to include in this list .Comment below
9.1 Frauscher(Austria) -ACS2000(Fail Safe Relay interface suitable for relay based interlocking
9.2 Frauscher(Austria) - FAdC (Fail Safe Serial Inetrface ) can do relay interface as well
9.3 Frauscher(Austria)-FAdCi-Failsafe Serial interface
9.4 Thales(France) - AzLM (Fieldtrac 6315)
9.5 Thales(France) -AzLS
9.6 Voestalpine(Austria)- UniAC[1]
9.7 Voest Alpine (Austria)-UNIAC[2]
9.8 Central Electronics Limited (India)-SSDAC-710P/HA SSDAC-720P/MSDAC-730P
10) Australian Standard
Refer AS 7651:2020 (Rail Industry Safety And Standards Board )
Rollingstock compatibilty ,reset procedures (Train Sweep / Manual Reset) are operational specific to individual railways and shall be performed according to your railway operating procedures .
Note :- Purpose for this article is to brief the Axle counter system .You selected suppliers will provide Installation Manual ,Operation & Maintenance manual .There for all these topics are out of scope for the article
To continue reading Register Now or Login