Magneto-Inductive Six Degree of Freedom Smart Sensors (MiSixthSense) for Structural and Ground Health Monitoring

Catastrophic failure of large civil structures like bridges, dams, embankments and buildings can result in fatal, costly and environmentally detrimental consequences. However, failures can also occcur in the surrounding groundwork, for example landslides and subsidence (sinkholes). Structural collapse during construction also poses high risk to people working on construction sites. There is a strong need for a sensing technology that is able to measure the performance of a structure over its entire lifetime, as well as its associated foundations and the surrounding soil and rock supporting the structure. This will help to provide early warning of impending failure, inform repair operations and optimize building methods.

The current gold-standard for monitoring structural stress and failure are distributed fibre optic sensors, which use the change in the properties of a thin fibre-optic cable to measure aspects such as strain. However, fibre-optic sensors are essentially wired into the structure, require deployment effort and provide a point of ingress, weakening the integrity of the structure. More importantly though, fibre-optic sensors can only measure strain along the fibre axis, meaning that the three-dimensional shape deformation of the structure cannot be directly measured. Additionally, it is time-consuming and costly to install fibre-optic sensors within the foundations and surrounding soil/rock, limiting their use to high risk projects.

This ambitious project seeks to develop a low-cost, wireless, embeddable sensing technology that can measure structural deformations in 3-D from deep within a structure, its foundations and surrounding ground, that are small enough to add to the concrete mix or injected into rock. Not only can each sensor measure changes in its position, it can also measure changes in orientation, yielding a full six degree of freedom sensor. Key to this is the use of low frequency magnetic fields that are able to penetrate rock, soil, concrete and water with minimal loss of signal, a marked advantage over current wireless technology based on high frequency radio that cannot penetrate even a few cm of concrete.

These cm-scale, low cost sensors are mixed in with the concrete pour, instantly forming a self-organizing and healing communication network. These devices start monitoring from the moment the element (e.g. a pillar or a beam) is poured, providing information over the entire lifetime of a particular structural element, from the concrete curing process to loading to monitoring cracks and corrosion. When structural elements are placed next to each other, the network will automatically extend to form a larger, merged communication system.

The sensors can measure their precise position and orientation within the structure and how this changes over time. With a number of these sensors the actual shape of the structural element and how it is bending or twisting with loads can be sensed. This is currently impossible to achieve using any other distributed sensing technology, a key advantage of low frequency vector fields, having both magnitude and direction in 3-D.

One of the issues of embedding sensors within a structure is maintaining operation over the lifetime of the structure, which can be many decades. The sensors use the same low frequency magnetic fields to harvest energy, which is either collected from ambient magnetic fields, such as mains wiring, or directly injected into the metallic reinforcement of the structure. This allows for battery-free, indefinite operation.

This technology has the potential to make buildings and large structures truly smart, using low cost, easy to deploy sensors that can operate from within the structure and the surrounding groundwork. This will enable real-time monitoring of key indicators of potential failure over the lifetime of the structure, providing early warning of impending disaster, with potentially life-saving results.

As a technology that can provide early warning of catastrophic failure, the potential lifesaving impact of this project is immense. By making structures safer, by not only monitoring them, but also their surrounding groundwork, the risk in terms of death and injury can be greatly reduced, both during and after construction. In addition, instant notification of damage to structures and their foundations will aid maintenance, extending lifetimes of structures through targeted repairs.

One of the barriers to adoption of structural health monitoring is deployment effort. As a wireless technology that can be added to the concrete mix, structural elements can be assembled offsite and then when placed together, the sensors will form a network across the entire structure, without any additional effort. The devices will know their position within the structure, yielding a truly self-organizing sensor system. This low-effort approach to making structural elements smart is likely to lead to great impact in actually installing sensors.

This low cost technology will benefit the construction industry by providing real-time information on structural shape deformation. This can be used to optimize building methods, as CAD and FEM models can be directly compared with sensor data to assess how well the two agree. This feedback can drive informed choices about integrating new materials into the construction process, improving efficiency and making buildings "greener", providing both economic and environmental impact.

This technology could be used to monitor sinkholes in areas of risk, providing instant updates on relative movement of soil and rock. This could not only act as a lifesaving measure, it could also provide information that would aid deeper understanding of sinkhole formation. The sensors could also be directly applied to making roads and pavements smarter, providing accurate information about traffic movements, wear and damage, by adding them to the tarmac aggregate. These could replace wired sensors with cost savings in terms of deployment and repair effort.

Although this proposal is focussed on making structures smarter, the potential impact of a small, injectable sensor that can form a network and measure relative position and orientation in 3-D is very broad. Some particular industries that could benefit from this include the oil and gas industry as these sensors could be used in downhole monitoring and could even be pumped into fractures to provide information about pressure and temperature thousands of metres beneath the surface. They can also be used for pipeline monitoring, from within the pipe itself. The mining industry would also greatly benefit from a small, self-powered device that could provide indications of potential collapse of shafts and be integrated into rockbolts to measure relative changes in position. Industrial process control is another potential beneficiary, as these sensors could be added to fluids or slurries that are challenging to monitor with conventional sensors. As MI fields propagate through water as well, there are a number of applications such as wastewater treatment, monitoring of erosion and scour with "smart-rocks" and underwater sensor networks that could directly benefit from this research. Thus, there are many associated applications with commercial importance that could be readily spun-out from the core technology developed in this proposal.

This technology can also be used by science discovery centres, universities and schools as an educational aid, as it provides direct indications of how a structure is deforming under a load, and in what manner. There is also likely to be broad interest from the general public through popular media about how sensing technology can be used to make structures safer.