Raising the profile

A laser scanner under development by German engineers could lead to improved rail safety by monitoring the whole track area around a train as it moves.

As rail travel gets faster the potential for foreign objects causing accidents dramatically increases, and careful monitoring of the area around the tracks becomes more and more crucial.

Operators currently employ a variety of different techniques to do this. Some systems are video-based, others use lasers, the vast majority are deployed on separate track inspection vehicles and most only analyse the track geometry and not the entire space through which the train is travelling.

But now a team from Germany’s Fraunhofer Institute is working with Austrian rail company Plasser & Theurer to develop a high-speed system that can be fitted to the front of passenger trains and monitor not only the track profile, but also the space above and around the train.

The chief component of the scanner, explained project leader Dr Heinrich Hofler, is a sensor that attaches to the front of a train and emits a high-frequency-modulated laser beam via a rotating mirror. Light reflected by surrounding objects is collected by a lens and sent to a detector via optical fibre.

The system compares the phase shift of the outgoing and incoming beams and uses this information to determine the distance between, for instance, the vehicle and the roof of a tunnel. From this distance and the current orientation of the mirror, the position of individual pixels is calculated to form a two-dimensional graphic. The addition of data describing the train’s forward movement creates a 3D spiral that elongates as the vehicle’s speed increases.

A key advantage of laser scanners is that the measurement data is delivered in real time, making it instantly available – in contrast to other systems that must first convert video images of the clearance profile before computing the distance. Laser scanners are also less sensitive to light interference.

An early version of the system – the Clearance Profile Scanner (CPS) – has actually been used on the Singapore Metro since March 2003, but the new scanner will be far more powerful. On the existing system the mirror can turn at up to 100 revolutions per second, generating as many as 100 profiles. It also has a data rate of 250KHz, meaning that it can generate 250,000 distance measurements per second.

Hofler said that the new system will be based on the same operating principles but the team is aiming for a mirror that revolves at up to 600 times per second and a system that will take two million distance measurements per second (2MHz). The advantages of a faster system are simple: small objects may not show up on a 50Hz rotating mirror if you’re going at 100kph – people want trains to go faster and to pick up smaller objects.

Hofler said that the nearest competitor to the device is Epesem Stoebner’s Observer, a laser-based system used by SNCF across most of the French rail network. But Observer, although highly accurate, only checks the track profile and is mounted on a separate measurement vehicle.

In the UK, Network Rail recently introduced a laser-based track geometry measurement system that fits on the bogie of a passenger train. Developed by Imagemap in the US, it has been on trial on the Chiltern line.

Hofler said that his team has already developed the distance measurement device and the data acquisition system but not the mirror mechanism. He added that depending on the amount of funding the project receives, a complete system could be ready for trials within months.

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