The important task of inspecting cables on bridges, elevators, ski lifts and cable cars for signs of strain, wear and corrosion is commonly carried out by a device that clasps around the cable and exposes it to a magnetic field, looking for disruptions in the field. The problem is that the diameter of the cables and their jackets can vary considerably, limiting the use of such devices. Researchers at the Fraunhofer Institute for Nondestructive Testing have come up with a one-size-fits-all approach in the form of a robot they’ve dubbed the FluxCrawler.
Instead of clasping around a cable, the FluxCrawler revolves around the cable with a permanent magnet used to keep it from falling off. As well as keeping the robot attached to the cable, the u-shaped permanent magnet also provides the magnetization of the cable that is required for a magnetic flux leakage test. This test involves looking for magnetic field “leaks” from the steel cable that indicate defects.
A line of sensors running between the two ends of the u-shaped magnet rapidly scan the surface of the cable as the robot moves around it and can detect not only the presence of a defect, but also the exact angle of the defect and on which side of the cable the defect lies.
Measuring around 70 cm (27.5 in) long, the Fluxcrawler can inspect cables from four to 20 cm (1.6 to 7.9 in) in diameter. The robot is powered by batteries and controlled via Bluetooth by a computer that displays a graphical representation of the magnetic field along the cable’s entire surface. Any suspicious spots are highlighted in high resolution on the screen.
The robot is not without it’s limitations, however. It cannot recognize defects in covered areas, such as the area where the cable is anchored, for example. In such cases, the researchers use another non-destructive monitoring method that involves placing a transducer directly on the cable to create a guided ultrasonic wave that penetrates the cable and reflects back when it hits a flaw. These signals are used to create an image that is analyzed by a computer to identify physical changes in the cable.
Fraunhofer has already patented the robot, which the researchers say has successfully been tested on cables in the lab. They will now conduct further tests as a cable testing facility at DMT GmbH in Bochum, Germany and have already received interest from industry, giving the team confidence the FluxCrawler will soon be crawling cables and hunting for defects on bridges, elevators and cable cars.
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