Used with an SIL 4 automaton, the SIL4 Axle counter which integers DRF sensors detects trains and movement in a signaling installation.
The axle counting system is certified SIL4 for multi zone management and detection function is similar as track circuits.
Benefits of axle counters over track circuits
Presentation of track circuits
A track circuit is a system based on an electrical circuit, using the rails (conductors) of a railway track, to detect the presence of a train in a given track section called area. This movement detection device can be used together or alone for automatically controlling the signals. These signals may be stop signals such as those used for the protection of turnouts (switches); they may also be spacing signals. In so-called automatic block system (BAL) this device is used to regulate trains spacing, it is called cantonment.
The principle of the track circuit is: car axles shunt the circuit (in green) and the absence of current in the downstream part of the circuit causes a fallback state of the signal power relay (by default red).
Disadvantages of the track circuit:
- The quality of the electrical signal transmitted by the rail is dependent on the insulation resistance of the ties and the ballast. Causes leakage currents (if insufficient).
- This resistance is a limiting factor for the maximum length of the track circuit.
- This resistance may depend on hydrometeorological circumstances and on condition of the top layer (ties and mud). A simple rotten tie soaking in water or mud can reduce resistance and to leak too much current so that the receiver is no longer excited and declares a false occupation.
- The track circuit function also depends on the quality of the conductivity of the axle. If rusty, the function is not guaranteed.
- The traction return current can influence the track circuit if, for example, the upper harmonics are similar to the frequency of the main signal, which complicates signal detection. This is amplified if there is a tension with thyristor.
- The track circuits require insulated rail joints. (Even if jointless track circuits are available, in the punctual areas, is less reliable)
- Track circuits require more wiring and connections which increase the cost of installation and maintenance as well as the cases of failure.
- Track circuits are facing problems when the rail head is contaminated, for example with rust or accumulated leaves.
- Track circuits are unreliable under wet conditions, so they cannot be used to detect a train in a tunnel.
- Track circuits may be unreliable on the steel structures (e.g. steel sleepers).
Presentation of axle counters
An axle counter is a rail device for detecting the presence of traffic on a section by counting axle crossing detectors bordering this section. A counting head (or “detection point”) is installed at each end of the section, and each time an axle passes over the head at the beginning of the section, a counter increments. A detection point includes two independent sensors (present in one case), so the device can detect the direction of a train according to the order in which the sensors are activated. When the train passes over the counting head at the end of the section, the counter decrements. If the final number is zero, the section is assumed to be free for a second train.
Axle counters advantages:
- Virtually unlimited length of the section.
- No insulating joints between sections which means better reliability and reduced maintenance.
- Independence of isolated conditions with the track which means better reliability.
- Independent from the quality of axles of the train shunt which means improved safety.
- Works perfectly where track circuits cannot work because of the ballast poor condition, steel sleepers, bridges, metal, wet tunnels.
- Applications on long blocks lines where track circuits are not a cost effective solution.
- Can operate in addition to track circuits to make temporary areas or increase the reliability of a section.
- No connection constraint and less wiring. The axle counters are generally less expensive to install and maintain.
- Very useful on steel structures that prevent normal operation of track circuits.
- Very useful on long sections on which several intermediate track circuits can be recorded.
- Some analyzes and experience with axle counters in continental Europe show that they regularly reach up to five times the reliability of track circuits with the same function.
- Immediate improvement in service reliability as a track circuit failure is often the most important cause of late trains.
- May be readjusted in the event of functional modification of a section or addition of a disconnection for the extension of a track while the cost of a track cicuit embedded in the concrete is lost.