Health is an issue not just for humans but for structures built by us, too. The periodic monitoring of the health of bridges is crucial, and if you can do this with a wireless system and remotely, it’s even better. A collaboration of computer scientists and civil engineers from IIT Madras has developed exactly such a wireless health monitoring system suited to observe the fitness of railway bridges.
Usually the health of bridges is assessed visually, by actually examining whether any component has given way. However, it is known that vibration-based damage detection systems are much better than mere visual examination at assessing the propensity of bridges to fail. For instance, measuring the strain, or expansion of the limbs of the bridge when a train passes over it, can help in an early detection of flaws in the limb. In this case, the sensor measures the strain and the acceleration on the limb to which it is attached, even as the train passes over the bridge.
There are wired systems in use. But these are difficult to maintain, expensive and also take many days to set up. This is where the importance of the wireless health monitoring systems comes in.
The system consists of three units: a mote, which is responsible for collecting, processing and transmitting data (the measured acceleration and strain); a head node, which is a computer close to the mote (about eight metres away), which receives this data and transmits it in bunches to a remote monitoring station and a remote server, which receives the data and analyses it. In practice, there can be multiple motes placed on the railway bridge and they can all transmit their data to the head node. By looking at the correlation of strains from spatially separated motes, the observer sitting at a remote location, even a hundred kilometres away, can predict whether the bridge is within its utility period.
The system was deployed at Nagari bridge in Chittoor district of Andhra Pradesh. When asked what are the types of failure the system would pick up, U. Saravanan of the Department of Civil Engineering, IIT Madras, who mathematically modelled the bridge for the data analysis, cites fatigue life, which correlates with crack propagation and failure, bearing seizure, loosening of rivets, etc.
To maximise the usability of the wireless motes, they are designed to go to sleep when they are not in use. They are “woken up” by vibrations from trains whose speeds exceed 10 kmph. This is tied to the reason why Nagari bridge was chosen for the deployment. “Trains pass over this bridge at high speeds, such as 100 kmph, and all these trains would ‘wake up’ the mote,” says Saravanan. About 2,500 train passes have been observed so far, and the researchers want to record data from 5,000 train passes to validate the system.
With 50-100 sensors being needed to cover one such bridge, the cost of scaling up this would be huge if the wired systems, which cost Rs 50,000 a piece, were to be used. The cost per piece is reduced to less than 10 per cent of this with the wireless motes. The deployment of the system did come with major challenges.
“I have taught students for nine years, but even so, actually deploying this system taught me much more than a classroom lesson could have,” says V. Kamakoti, Department of Computer Science, IIT Madras, who is responsible for developing the 12,000 lines of embedded code in the system. The team plans to make this software freely available for all.
