An axle counter is a device on a railway that detects the passing of a train in lieu of the more common track circuit. A counting head (or ‘detection point’) is installed at each end of the section, and as each axle passes ahead at the start of the section, counter increments. As the train passes a similar counting head at the end of the section, that counter decrements. If the net count is evaluated as zero, the section is presumed to be clear for a second train.
This is carried out by safety critical computers called ‘evaluators’ which are centrally located with the detection points located at the required sites in the field. These detection points are either connected to the evaluator via dedicated copper cable or via a telecommunications transmission system. This allows the detection points to be located significant distances from the evaluator. This is useful when using centralized interlocking equipment but less so when signaling equipment is distributed at the line side in equipment cabinets.
Axle counters are used in places such as wet tunnels, like the Severn Tunnel, where ordinary track circuits are unreliable. Axle counters are also useful where there are uninsulated steel sleepers which prevent the operation of track circuits. Axle counters are also useful on long sections where several intermediate track circuits may be saved.
A set of track inductors fitted at the entry and exit end of the track which COUNTS IN and COUNTS OUT the number of axles passing over. These are transmitting coils which generate magnetic flux on the flow of high-frequency current. These coils are set opposite to each other on each side of the rail. When the wheel passes through, it cuts the magnetic flux and the induced voltage in the receiving coil is reduced, these dips are evaluated in the evaluator and counted. An evaluator registers the counts and if the number at each end is same then the section is CLEAR otherwise OCCUPIED.
The basic components of an axle counter are the following :
- Two or more track devices which ensure the entry and the exit points of a portion of a portion of the track is checked for the numbers of axles entering and the number of axles going out.
- Trackside interface which buffers the signals of the track devices and amplifies them for counting at the centrally located counter where the input from both the ends come.
- A counter which is connected to all the trackside buffers and counts the axles coming in and the axles going out.
- A transmitter unit kept at all trackside locations to feed signal to the trackside equipment for the purpose of detection of axles.
Specifications of analog axle counter (Typical) :
- Operating Voltage: 24 V
- Frequency used for monitoring: 5 KHz
- Distance from the trackside transmitter/ Buffer to Track: 15 m
- Method used for detection of wheels: Electromagnetic
- Length of track which can be monitored: Up to 15 Km
- Medium of Telecommunication between track devices and the counter unit: Copper shielded cable 0.9 mm
- No. of conductors used for the operation: 2 pairs for each track devices.
How it works:
Double wheel detectors mounted on one of the rails detect the axles of the passing vehicles. As they pass, the wheels change the alternating electromagnetic field between the transmitter and receiver of each detector and hence the voltage induced in the receiving coil of the detector.
The changes in amplitude and their chronological sequence are evaluated. The offset arrangement of transmitter and receiver permits direction-of-travel identification. The signals required for counting and determining the direction of travel are transferred to the evaluation computer in the form of frequency- and amplitude modulated signals.
The evaluation computer is the central processing and detection unit of the system. It establishes an overall result of the detection information proceeding from the counting heads and generates a track clear or track occupied indication, taking account of the operating state. The track clear or track occupied indication is output via two channels using floating relay contacts with opposite or the same normal condition. The plug-in printed circuit boards of the evaluation computer are inserted into a single-tier 19″ rack. All inputs and outputs, including the power supply, plug into the front/rear of the rack.
The evaluation computer is fitted with LEDs and measuring sockets. The LEDs are used for fault diagnosis and for the output of statistical information. Usually, a board of the evaluation computer or counting head can be determined to be the cause of a fault. The measuring sockets are used in conjunction with some of the LEDs for adjusting and checking the system.
Axle counters may forget how many axles are in a section for various reasons such as a power failure. A manual override is, therefore, necessary to reset the system. This manual override introduces the human element which may be unreliable too. An accident occurred in the Severn Tunnel due to the improper resetting of the axle counters.
There are three methods of securing the reset and restoration of axle counters into service:
Co-operative reset requires both the technician and signaler to co-operate to reset and then restore the section into service. This directly manages the cause of the Severn Tunnel accident.Preparatory reset uses the internal logic of the axle counter system to enforce that a train must proceed through a reset section at slow speed, by holding its output as ‘occupied’ until the train is successfully detected as passing through the section. This logically proves the section free of obstruction and therefore allows the section to change its output to ‘clear’.
Conditional reset (with aspect restriction) has the section reset only if the last count was in the outward direction. This at least shows that any trains in the section at the time of reset were at least moving out. The signal protecting the reset section is held in danger by signaling logic outside of the axle counter evaluator to enforce a low speed ‘sweep’ of the section prior to restoration to service.