Construction Corner: Sensor technology evolving to monitor large structures

Sometimes it’s hard to know which came first.

Are the advances in sensor technology driven by the need to build safer, greener, more sustainable structures? Or is the demand for those structures driving sensor technology?

As with robotics, things are happening so quickly in the field of sensor technology that it’s hard to keep up.

Now we read there is great interest in using distributed sensors to continually monitor the structural health of large structures like dams, bridges or tunnels. And one of the most promising is using long fibre-optic strands with many sensing points along them.

A research team from the University of Alcala in Spain and the Swiss Federal Institute of Technology has developed a fibre-optic sensor with a million sensing points. The sensors detect both strain and temperature changes.

Fibre-optic sensors are thought to be ideal for monitoring infrastructure because they can be used in harsh environments and in areas that lack a nearby power supply. If a single fibre is placed along the length of a bridge, for example, changes in the structure at any of the sensing points along the fibre will cause detectable changes in the light travelling along the fibre.

Alejandro Dominguez-Lopez, a member of the research team, says that "with fibre-based sensors it is possible to precisely detect corrosion or cracking before a dam fails, for example.

"Earlier detection of a problem means it might be possible to prevent it from getting worse or could provide more time for evacuation."

In most applications, sensors are spotted at key points in a structure, but this means there could be gaps where something is happening but there’s no sensor to detect it. This new fibre-optic device has sensing points that are distributed along its length and only about one centimetre apart.

With so many sensing points, speed becomes a problem. The new sensor uses an approach that requires both continuous and pulsed laser signals to interact. By tweaking the way the laser signal is generated, researchers were able to increase the speed with which a problem is reported.

The result is that their device is capable of sensing strain and temperature changes from a million sensing points along a 10 kilometre optical fibre in less than 20 minutes.

That makes the new device about 4.5 times faster than previous sensors with so many sensing points. Using their new approach, the researchers demonstrated that they could accurately measure the temperature of a hotspot from the end of a fibre 10 kilometres long, but it took the signal just under 10 minutes to reach the datalogger.

What the researchers have done, says Dominguez-Lopez, is identify what had been a major limitation of the sensing technique, then demonstrated a way to overcome that limitation.

"The new sensor could enable improved structural monitoring and help move the sensing technology into exciting new research areas and applications," he said.

The researchers are now working to make the sensor even faster by looking for ways to further reduce the data acquisition time. They also want to increase the density of sensing points to more than one per centimetre. This could allow the technology to expand into completely new areas such as avionics and biomedical applications.

In avionics, for example, Dominguez-Lopez notes it’s important to know what is happening in every centimetre of an airplane wing.

The device could also be used in textiles where sensors might help to monitor a person’s health or screen for disease.

Although the popularity of fibre-optic distributed sensors is growing, their main use so far has been to detect leaks in oil pipelines and to monitor for landslides along railways.

The research was published in a recent issue of Optics Letters, which is published by the Optical Society of America.

Korky Koroluk is an Ottawa-based freelance writer. Send comments to editor@dailycommercialnews.com.