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External-Grade 3D Scanners

All 3D scanners have had one thing in common: You place the object inside the scanner, which performs a scan either around the object or through the object. In all cases, the scanner surrounds the item to be scanned. However, that is of course completely useless if the item to be scanned is outside, or a building or section of train tracks. You cannot build a huge scanner to put a skyscraper inside.

However, we do use scanner-like techniques as part of navigation in the outside world. SLAM (Simultaneous Location And Mapping) for example, simultaneously scans the environment around ta robot whilst determining an appropriate navigational path. So, since scanning methods can be used outside a scanner – albeit with lower resolution and only partial scans – perhaps an actual scanner can be constructed that scans items outside the scanner itself.

It sounds like an impossible task, but perhaps not. By using a sensor web of multiple scanners it might be possible to scan large outside objects with a high degree of accuracy, with each scanner compensating for the parts occluded from the others.

That is essentially, the idea behind a laser scanner being developed for railway use by Dr. Heinrich Höfler and Dipl.-Ing. Harald Wölfelschneider from the Fraunhofer Institute for Physical Measurement Techniques in Freiburg, Germany. They have developed a Three-dimensional laser scanner which scans objects at remote locations relative to itself.

It can be used outdoors without hesitation. Extremely fast and precise, it is able to spatially measure and monitor the position of the contact wire or the track from a train travelling at up to 100 kilometres (62 mph) per hour. If the scanner is stationary, it can capture passing trains and check for loads that might have slipped.

It's not a true 3D scanner in the usual sense of the word, as it can only capture one side of a passing train, but the system is designed so that two such scanners, one either side of the track, can capture the whole train, or continuously monitor the state of the tracks themselves and determine the size and shape of something appearing suddenly and blocking them – such as a drunk person or a fallen tree.

The system is fairly simple and straightforward. As you might expect they send off a laser beam and wait until it returns. The time in-between sending and receipt is measured to determine how far away an object is.

The difficult part is capturing the returning beam. Often, only very little light comes back and what‘s more, the transmitted light beam is back in an extremely short space of time. The solution: A kind of slow motion. The laser beam is very rapidly switched on and off – modulated,  as scientists would put it. The time shift of this modulation wave can be determined more quickly and precisely than is possible with a single laser pulse.

The two inventors and a working non-prototype version of their 3D scanner, ready for deployment.

The system measures, by default, one million times per second. “For Deutsche Bundesbahn (German Railway), we equipped a measurement train that scans the surroundings of the test track, using several laser beams and  which delivers, taking four million measurements per second, a 3D image of what it scans”, explains Harald Wölfelschneider. That allows even small obstacles and constrictions to be detected, or we can plan the route via which a heavy load can best be transported to its destination.

Another field of application is the measuring of passing trains. This requires the scanner to be permanently mounted, which, however, does increase the chance of someone looking into the laser beam for a longer period. To make the scanner safe for the human eye, the researchers had to develop a new wavelength range: infra-red, which is harmless for our eyes. The consequence being that the entire system had to be fully reconfigured.

If we examine railways carefully, it makes sense that we then also examine other traffic routes, such as roads. The team at IPM has developed a 3D scanner, safe for the human eye, which is mounted onto a moving car and which scans the road from a height of about three meters. “We can now detect height differences of even 0.2 millimetres on the road, even at speeds of 80 kilometres per hour (approx. 50 mph)“, says Höfler. This is the first scanner approved for this purpose by the Federal Highway Research Institute. It is to detect lane grooves, potholes and water drainage potentials.

Not only fast and precise, this system is also highly robust. Dr. Heinrich Höfler and Dipl.-Ing. Harald Wölfelschneider will receive one of the 2012 Joseph-von-Fraunhofer awards for this eye-safe 3D laser scanner.

References

Laser scan at full speed

Staff Comments

 


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