It can detect frequencies more than double that of traditional fibre-optic accelerometers, making them suitable for monitoring wheel-rail interactions.
The team from Hong Kong Polytechnic University (HKPU) said the sensors are durable and include no moving parts and therefore work well in the noisy and high-voltage environments found in railway applications.
The fibre-optic accelerometers used in condition-monitoring systems typically cannot detect vibrations higher than 500Hz.
Although this is adequate for most railway applications it can’t be used to measure the wheel-rail interaction, which is an important source of monitoring track wear.
To overcome this problem, the researchers designed a new fibre-optic accelerometer that uses a ‘polarisation-maintaining photonic crystal fibre’ that is coiled into the shape of a disc only 15 millimetres in diameter.
The coiled fibre is glued between a stainless-steel substrate and a cylindrical mass block. When a vibration occurs, the mass block will press on the coiled fibre at a frequency matching the vibration. This external force causes the wavelength of light in the fibre to shift in a measurable way.
“Each year, train accidents lead to severe injuries and even deaths,” said research team leader Hwa-yaw Tam.
“Our fibre accelerometers could be used for real-time monitoring of defects in the railway track or the train to pinpoint problems before an accident occurs.”
Researcher Zhengyong Liu said that in addition to railway monitoring, these new accelerometers “can be utilised in other vibration monitoring applications, for example, structural health monitoring for buildings and bridges and vibration measurements of aircraft wings.”
“An all-optical sensing network has many advantages as it is immune to electromagnetic interference, has long transmission distance and the sensors don’t require electricity.
“Installing these accelerometers on the undercarriage of an in-service train allows them to monitor vibrations that would indicate defects in the track. They can also be used to detect problems in overhead lines used to power trains.”
After thoroughly testing prototypes of the accelerometer in the laboratory, the researchers carried out a field test by installing the device on an in-service train. They also installed an FBG-based accelerometer and a piezoelectric accelerometer for comparison.
They found that the new fibre accelerometer detected acceleration in a manner similar to the piezoelectric accelerometer.
However, piezoelectric sensors require expensive shielded cables to reduce the effects of electromagnetic interference noise. Because the FBG-based accelerometer can’t operate well at high frequencies, noise concealed some of the useful vibration information.
“Our results showed that our new accelerometers perform considerably better than existing accelerometers used for monitoring acceleration in trains,” Liu said.
Last month The Department for Transport awarded £7.8m to a series of projects designed to modernise the UK’s railway infrastructure including aids to reduce noise pollution and leaf-proof tracks.