Detecting defective rails and wheels

Researchers at the University of Illinois are fabricating fibre-optic sensors that can improve train safety by detecting flaws in rails and wheels.

In projects sponsored by the Association of American Railroads and the Transportation Research Board at the National Academy of Science, Shun-Lien Chuang, a UI professor of electrical and computer engineering and his research assistants are developing different sensor designs for specific applications.

‘Our sensors are based upon optical signal transmission through sensitive optical fibres that are firmly attached to the rails with epoxy and tape,’ Chuang. ‘We use fibre 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.’

The research on these sensors will, according to the researchers, help protect both freight and passenger trains from derailment, regardless of the speed they are travelling.

In one sensor design, the weight of a passing train causes strain in the rail, which is transferred to the attached fibre. The intensity of light that is transmitted through the fibre 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 fibre as a function of distance using a time-gated pulse detection technique,’ said Chuang. ‘A moving train creates perturbations in the fibre’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.

‘Fibre optics operate on total internal reflection – so when the fibre is bent, some of the light leaks out,’ said Chuang ‘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 fibre, the researchers say they can measure any additional pressure, including the weight of passing rail cars.

The palm-sized sensor is also said to offer 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,’ said Chuang. ‘By measuring the impact force between wheel and rail as a train passes over the sensor, defective wheels can be readily identified.’

The sensors have been field-tested in co-operation with the Canadian National Illinois Central Railroad, and are currently being tested at the AAR’s Transportation Technology Centre in Pueblo, Colorado.