At the IRSE 2017 Aspect Conference in Singapore, two speakers updated delegates on the progress being made with fibre-optic technology to sense occurrences at the trackside that can be used to detect both predicted and unpredicted events.

Rail Engineer has looked at such systems in the past – in issue 89 (March 2012) as a means of detecting rock fall onto the track and in issue 114 (April 2014), in conjunction with the company Optasense, to use fibre cable to detect intrusion and theft. Both these initiatives seem not to have progressed into everyday applications and have thus slipped below the horizon as to general awareness.

Fibre-optic axle counting

With axle counters now in the ascendancy over track circuits for reliable detection of trains, modern processor-based devices give much improved reliability. Further, by clamping instead of bolting to the rail, the need for rails to be drilled is avoided. However, problems remain with EMC interference, the threat of copper thieving and the ever-present risk of damage by tamping and grinding machines.

Thales has been in the axle counter business for many years through the acquisition of Alcatel’s rail signalling interests. Joachim Janle from Thales, Germany, explained how a fibre-optic-based counter has been developed that eliminates these deficiencies.

The technology is based on a Fibre Bragg Grating concept. The sheer strains inside the rail web caused by a passing wheel lead to a shift in wavelength of the reflected light inside the optical sensor. This wavelength shift is then translated into electrical signals by powerful opto-electrical chips. These electrical signals are used by a counting unit, within the axle counter system, to count the number of passing wheels and thus the occupancy status of a track section.

These counting units can be connected to an existing evaluator if axle counters are already in use.

The resulting rail-mounted device is small in size, has no electronic components, is glued to the rail web and is cheap to produce. It provides for maintenance-free operation as no adjustment of the sensor is needed and furthermore, tampers and rail grinders can operate without removing the sensor from the rail.

Trials have been underway since 2015 on a mixture of different type rails and situations – high speed lines, slab tracks, tram lines, on bridges – at five locations in Germany, Switzerland, Luxembourg and the Stuttgart tram network. Results are impressive, with a near-perfect count being achieved in millions of passes.

When questioned about the resilience of the glue, this is a high-impact substance that makes removal of the sensor virtually impossible. If the rail has to be changed, then a new sensor and fibre connection is provided, thus making them ‘throw away’ items. It is anticipated that a safety integrity rating of SIL 4 will be achieved as part of the general approval process.

Infrastructure health: a real time view

The Austrian company Frauscher (also in the axle counter business) is investigating and trialling the use of spare ‘dark’ fibres in existing cables to detect train presence and faults. Interestingly, the cables can be some small distance from the rail and can be in troughs, wall mounted or buried. Investigation has shown that the fibre becomes a ‘virtual microphone’, capable of detecting any sound waves or local vibration that translate into a disturbance or movement.

Known as Distributed Acoustic Sensing (DAS, and not to be confused with Driver Advisory Systems), Martin Rosenberger explained that tests have shown how systems based around fibre technology are capable of tracking trains and monitoring the condition of assets, including security encroachment through the detection of footsteps. As such, items such as loose track fixings, ballast washout, sleeper damage including bouncing and loose installations, rail web cracks and rail-head shelling can all be detected.

The principles are based around laser pulses being sent into an optical fibre with millions of ‘scatter sites’ sending back a small reflection to the emission point. These backscatter reflections are constantly compared to the original reference, with any change being detected and interpreted in the form of ‘acoustic signatures’.

In several trials, 80km of track is covered, 40km in either direction from a central light source and receiver. Even this ‘short’ distance can create over two terabytes per hour of data, which creates a data management challenge. The use of artificial intelligence to identify patterns and information is part of the process of differentiating between the various types of disturbance, such as people walking and trains moving.

Claims that train movement can also be detected prompted the question as to how far up the safety ladder this technology might progress? The answer is that, currently, the system has no SIL rating but, as with Frauscher’s axle counter products, all of which are SIL4 rated, the possibility exists that DAS technology could eventually be linked in some way to the axle-counting process.

For the present, however, it is very much seen as a track maintenance aid but, since it can detect train movement, this might lead to an interface with lineside warning systems to help protect track workers. Watch this space…

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