Jul 2 2020
Transport networks are crucial infrastructure and are the lifelines of contemporary society. Roads, bridges, tunnels, and railway tracks can be damaged by adverse weather conditions.
3D laser scanner contained in a housing and mounted on the roof of a vehicle. Image Credit: © Fraunhofer IPM.
An innovative 3D laser scanner recently developed by the Fraunhofer Institute for Physical Measurement Techniques IPM can be employed to closely track the transport infrastructure and schedule maintenance activities in a well-timed manner.
The new multispectral sensor system not only quantifies surface structures but also measures the surface moisture present on objects, and all these are done in a single inspection procedure.
Roads and railway networks can be considerably damaged by floods, heavy rainfall, and winter storms. Hence, it is important to identify cracks and other defects that may occur in tunnel walls, road surfaces, and other critical infrastructure.
One way to achieve this is by employing surveying vehicles that come with high-precision, non-contact mobile laser scanners. These scanners are capable of mapping their environments in three dimensions.
At Fraunhofer IPM based in Freiburg, scientists created a tunnel inspection system (TIS) that function with a pair of different laser wavelengths. In addition to recording the geometry of the bridge, tunnel, or other similar structures, the TIS is also renowned for its special ability to quantify surface moisture.
For instance, the TIS can find out whether the internal wall of a certain tunnel is wet or dry and thus offers crucial clues about the structural health of the tunnel. The TIS is different from camera-based techniques and operates in unfavorable lighting conditions.
Apart from assessing tunnels, the scanner can also evaluate the structural health of railways and roads. It produces georeferenced three-dimensional (3D) data that can be automatically assessed.
Fastest Laser Scanner in the World
Defects down to a few millimeters can be detected by the TIS.
Mounted on the surveying vehicle, the scanner scans the structure at a speed of up to 80 kilometers an hour, measuring its overall geometry and—during subsequent inspections—any changes in this geometry.
Alexander Reiterer, Professor and Scientist, Fraunhofer Institute for Physical Measurement Techniques IPM
The novel system has been designed to record two million measurement points per second, that is, the measuring beam spans the distance between the TIS and the object it quantifies—for example, a wall—two million times per second.
By using a rotating mirror, the measurement beam is deflected in a 360-degree radius 200 times per second, guaranteeing blanket coverage of the object being analyzed. This aspect makes this scanner the fastest instrument of its kind in the world.
The scanner can quantify distances of up to 80 m, which means it is more than sufficient for its purpose. The outcome of this scanning procedure is a 3D model of the setting in the form of a point cloud. The TIS has been developed to operate even in adverse conditions. It has excellent resistance to cold and heat, and can work at temperatures ranging from −50 °C to 50 °C.
The operation of the laser scanners is usually based on the principle of time-of-flight measurement. To put this in simpler terms, the laser scanners quantify the time taken by light to pass from the emitter to the object and back again to the detector, and compute the distance on the basis of the speed of light.
The TIS adopts a completely different method by applying the more complex phase shift technique.
This involves high-frequency modulation of the intensity of the emitted signal. The time it takes light to travel to the target and back is calculated from the phase shift between the emitted and received signal.
Alexander Reiterer, Professor and Scientist, Fraunhofer Institute for Physical Measurement Techniques IPM
Two laser beams of varying wavelengths (1450 and 1320 nm) are used to quantify the surface moisture. These wavelengths, which are produced collinearly, are absorbed by water to varying, but highly specific, degrees. The power of the quantified signals indicates the quantity of moisture present on the surface of the tunnel wall.
Infrared light is strongly absorbed by water, and that physical effect is something we can take advantage of. We use two very closely adjacent wavelengths, one of which is absorbed strongly and the other much less strongly. The difference allows us to calculate the amount of moisture.
Alexander Reiterer, Professor and Scientist, Fraunhofer Institute for Physical Measurement Techniques IPM
Machine Learning Makes Analysis More Efficient
The scanner generates georeferenced, high-resolution data that comes in a digital format. For long-term infrastructure monitoring, digital measurement data is an essential prerequisite. The following investigation is based on machine learning techniques.
With the help of uniquely designed algorithms, the system automatically identifies the types of objects that exist in the area being analyzed. The objects may be anything from a crack in the wall to a light pole. Following this, the system assigns more data to every data point denoting the object to which it belongs.
This offers the basis to automatically extract in-depth map materials. However, before the algorithms can deduce the quantified information, they should be trained first.
“The big challenge is building up a database that is suitable for training,” added Reiterer.
As such, Fraunhofer IPM has already obtained data for an array of applications that can be used for application-specific and customized training.
Compact and Maintenance-Free
The TIS is still at the prototype phase but it has already been assessed in a Swiss test tunnel, where it effectively carried out a preliminary set of measurements. The objective of the scientists is to make the ultimate product highly compact, measuring only 30 × 30 × 30 cm3.
Another aspect that makes the scanner stand out is that the whole system comes in a self-contained, fully enclosed product. The rotating parts deflecting the laser beam are accommodated in a glass cylinder.
“Encapsulating the system in this way makes it robust, long-life and maintenance-free,” Reiterer added. The subsequent step is to test the system on railways and roads under real-life situations.
“Much of Europe’s transport infrastructure is in a poor state of repair. It is vital that we start monitoring it more frequently and in much greater detail. The only way to do that is by developing systems that make these kind of inspections more efficient,” Reiterer further stated.
“For the first time, the TIS offers us a multimodal system that can simultaneously measure multiple parameters, namely geometry (3D data), structure (cracks) and moisture. This is a big step forward in terms of costs, speed and efficiency,” concluded Reiterer.