Forth Road Bridge
“We may be approaching the point at which a vibration powered wireless sensor network, with no need to change batteries, becomes a reality”
Extending the life of a vital link
The Forth Road Bridge provides a vital link between Edinburgh and Fife, but now carries far more traffic than it was originally designed for. About 25 million vehicles now cross the bridge each year, nearly ten times the number it carried when it opened in 1964.
The increased strain that the additional traffic load has had on the structure became apparent during a routine inspection of the bridge’s cables in 2004, when extensive corrosion was discovered in the strands making up the main suspension cables. This was illustrated painfully when the bridge was closed for an extended period in December 2015 when a truss end was found to have cracked.
After damage was identified in 2004, the decision was made to build a new road bridge alongside the Forth Road Bridge and Forth Rail Bridge. The Queensferry Crossing is due to open in May 2017, and traffic on the Forth Road Bridge will then be limited to buses, taxis, cyclists and pedestrians. Although traffic load will be reduced significantly when the new bridge opens, there will still be a requirement for ongoing monitoring of the older structure.
Wireless monitoring – without replacing batteries
Professor Campbell Middleton and the CSIC bridges team believe the work they are undertaking to develop and demonstrate wireless sensor network (WSN) technology may have a role to play in monitoring various key structural elements on the bridge. However, one of the problems holding back the adoption of WSN, particularly in difficult-to-reach areas like underneath a busy bridge, is the need to change batteries on a regular basis. The issue is not the cost of the batteries themselves, but rather the cost of the human power to replace the batteries.
In a bid to solve this issue, Dr Yu Jia and Dr Ashwin Seshia have developed a new vibration energy harvester based on a phenomenon known as parametric resonance, which amplifies vibrations. “As vibration energy harvesting improves and the amount of energy available to power sensors increases, new radio technologies are emerging with lower power requirements,” said Professor Middleton. “We may be approaching the point at which a vibration powered wireless sensor network, with no need to change batteries, becomes a reality. This would be a world first.”
The low cost, wireless, battery-free sensors will enable CSIC to measure the behaviour of key structural elements on this critical piece of infrastructure, giving its owners a far greater understanding of the actual capacity and level of safety of the bridge.
Professor Middleton continued: “The Forth Road Bridge offers a fantastic opportunity to test this innovative technology which will provide key information to the bridge owners and managers, leading to knowledge and reassurance of its on-going safety performance, which could see the Forth Road Bridge surviving a further 50 years or more.”
Designed by CSIC’s Dr Yu Jia, Dr Ashwin Seshia and the Sensors and Data Collection team, the patented low-cost, wireless, battery-free energy harvesting device was trialled at the live site in April 2015 where it was deployed for part of a day at a number of locations under the deck of the Forth Road Bridge.
Conventional resonant approaches to scavenge kinetic or vibration energy are typically confined to narrow and single-band frequencies or bulk motion only seen in a limited range of applications. This vibration energy harvesting device combines both direct resonance and parametric resonance in order to enhance the power responsiveness towards more efficient harnessing of real-world ambient vibration, opening up many new uses of the technology.
In the field-site, the packaged electromagnetic harvester designed to operate in both of these resonant regimes, with an operational volume of ~126 cm3, was capable of recovering in excess of 1 mW average raw AC power from the traffic and wind-induced vibrations in the lateral bracing structures underneath the bridge deck. The harvester was integrated off-board with a power conditioning circuit and a wireless mote. The level of power harvested in this trial is sufficient to perform structural condition measurements and duty cycled wireless transmissions entirely from the power generated by vibration, avoiding the need for batteries to be replaced.
“This limited duration field test provides the initial validation for realising vibration-powered wireless structural health monitoring systems in real world infrastructure, where the vibration profile is both broadband and intermittent” said Dr Jia.
The results of the Forth Road Bridge trial, which demonstrated that the technology works in live conditions, were presented at the world’s premier academic conference on micro and nanotechnology for power and energy applications, PowerMEMS 2015, in Cambridge (MA, USA). “The paper was well received at the conference and was one of only a few academic contributions that has demonstrated the operation of the technology in a real environment,” said Dr Jia.
A paper detailing the prototype and field trial, titled A vibration powered wireless mote on the Forth Road Bridge, has been published in the Journal of Physics: Conference Series.
Development of the technology
We now plan to work on enhancing the robustness of the harvester prototype, improving the efficiency of the power conditioning circuitry, further minimising the power requirement of the WSN mote and incorporating sensor systems onto the vibration powered mote in order to realise long term deployment trials at field-sites in the near future.
8power has been formed to commercialise this and related technologies. Robert Trezona, from IP Group plc, a venture capital organisation for British technology companies, said: “The new vibration harvesting energy technology developed by CSIC is a world-class innovation with several large potential markets.”
Text source: University of Cambridge and Cambridge Centre for Smart Infrastructure and Construction under a Creative Commons Licence. Image source: Bermrunner/Shutterstock (c).