Feb. 7, 2001 Champaign, IL -- Broken rails or damaged wheels can cause train accidents with potential loss of life, injury or property damage. Researchers at the University of Illinois are fabricating fiber-optic sensors that can improve train safety by detecting flaws in rails and wheels. “Our sensors are based upon optical signal transmission through sensitive optical fibers that are firmly attached to the rails with epoxy and tape,” said Shun-Lien Chuang, a UI professor of electrical and computer engineering. “We use fiber optics to sense an environmental change – such as the weight of a passing train or the strain created by a cracked, broken or buckled rail.”
In projects sponsored by the Association of American Railroads and the Transportation Research Board at the National Academy of Science, Chuang and his research assistants are developing different sensor designs for specific applications. The research on these sensors will help protect both freight and passenger trains from derailment, no matter what speed they are traveling.
In one sensor design, the weight of a passing train causes strain in the rail, which is transferred to the attached fiber. The intensity of light that is transmitted through the fiber will depend upon the condition of the rail and the amount of induced strain. In addition to detecting damaged rails, this sensor also can be used for detecting a train’s position and speed.
“The device uses an optical time domain reflectometry system, which measures the signal loss in the optical fiber as a function of distance using a time-gated pulse detection technique,” Chuang said. “A moving train creates perturbations in the fiber’s optical transmission, so the system takes several scans and measures the distance to the perturbations in order to pinpoint the train’s location and speed.”
Another sensor design is based on the “micro-bending” effect. “Fiber optics operate on total internal reflection – so when the fiber is bent, some of the light leaks out,” Chuang said. “We can calibrate the intensity of the optical transmission as a function of the applied bending pressure.” By introducing a certain amount of micro bending into the fiber, the researchers can measure any additional pressure, including the weight of passing rail cars. The palm-sized sensor also offers a fast and cost-effective method to detect deformities – particularly flat spots – in rail-car wheels. “Wheels can develop flat spots in service, which can damage the rail due to the severe dynamic loads they cause,” Chuang said. “By measuring the impact force between wheel and rail as a train passes over the sensor, defective wheels can be readily identified.”
The telecommunications market has driven down the cost of optical fibers and lasers, making the fiber-optic sensors less expensive than conventional track circuitry or strain gauges, Chuang said. “Our sensors also can operate 24 hours a day unattended and are immune to electromagnetic interference.” The sensors were field-tested locally in cooperation with the Canadian National Illinois Central Railroad. They are currently being tested at the AAR’s Transportation Technology Center in Pueblo, Colo.
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