SHIFTING ballast beds or objects which infringe the clearance profile pose severe hazards to rail traffic. Railways mostly still rely on conventional video-based or even tactile measurement techniques to check the clearance profile of their tracks. However, novel laser measurement systems offer an efficient and cost-effective alternative, because they allow an automated evaluation of the measurement results, which significantly reduces the processing time.

Fraunhofer IPM's laser-based clearance profile scanner (CPS) provides three-dimensional measurement data in real time. Rails are detected automatically which allows an automatic compensation of the vehicle's movements independent of other measurement systems.

CPS has been successfully running on the Singapore metro since March 2003, and another one has recently been installed in Serbia. A successor to CPS called the high-speed profiler (HSP) is currently being developed. With the HSR the CPS's original measurement frequency of 250kHz has been quadrupled. In future, the effective frequency will be expanded to 6MHz.

Laser-based measuring systems have several advantages over video-based systems. They work almost independently from ambient illumination conditions and measure distances quickly. This allows for automatic evaluation, because in contrast with image-based systems no prior knowledge of the measured objects is required. The major limitation for laser-based systems is the minimum size that an object must have to be reliably detected at a certain driving velocity. Technically, the future version of the HSP will be able to detect objects as small as 15mm at speeds of up to 100km/h. This means that measurements can be carried out within the normal timetable thereby avoiding costly night work. The higher speed will also solve the problem of measuring the clearance profile of high speed lines which, up to now, could not be checked as low-speed inspection cars are not allowed on them.
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Laser-based profile scanning guarantees an exact gauging of the clearance profile, detecting all objects within a distance of up to 10m at every single rotation. Our system relies on laser distance measurement, using the track's centre line as a reference.

A 360[degrees] laser scanner is at the core of the system. High-frequency modulated laser beams are projected on the clearance profile by rotating mirrors. The laser beams are reflected by the surrounding objects and re-enter the measuring system.

The travelling time of the laser signal is used to deduce the distance to the object, from a fraction of a nanosecond. The distance values and the scanning mirror's angle of rotation determine the exact position of each measurement point. A two-dimensional profile is derived from the mirror's rotational movement.

As the vehicle moves forward, the laser beam describes a helix with a pitch proportional to the travel speed. This pitch, to put it simply, can be imagined as the distance between two measurement profiles in the driving direction.

Small objects such as insulators, pipes, signs, and cables can only be detected reliably if the density of measurement points is sufficient. The cross-section of a sign, for example, may be missed by the system, if the spacing between adjacent measurement points exceeds a few centimetres. Therefore, an important objective is to increase the scanning speed.

Thanks to optimised electronic data processing, the measurement frequency of the HSP could be enhanced significantly. Due to novel scanning mechanics, the scanning frequency of the HSP will in future reach 1800 scan profiles per second, with up to 3300 distance values per profile. This produces the effective measurement frequency of 6MHz.