Sensors for Structural Health Monitoring

Public bridges and roads require their structural health to be monitored, and engineers use sensors for continuous measurement. To power these embedded sensors, they exploit several sources of ambient energy. This can include vibrational energy obtained from vehicular traffic, which can generate adequate power for sensor nodes that engineers have built into the infrastructure. Off-the-shelf devices make it easier for engineers to design structural monitoring devices. Many manufacturers now provide such sensors.

Drivers are rather well-acquainted with potholes on the bridges and roads on which they frequently travel. However, apart from the surface damage, there are more insidious structural damages that may be less obvious. One of them is stress corrosion cracks in structural components that may lead to a bridge collapse.

Therefore, engineers are rightly concerned about existing infrastructure developing similar defects. The rise in vehicular traffic over bridges and roads, often going beyond the original design specifications, together with rapid aging from the stress, can lead to their continual wear and tear and deterioration. Engineers use Structural Health Monitoring or SHM based on continuous monitoring of infrastructure. This is critical for identifying structures at risk.

Monitoring the system through wireless means is more practical, as this avoids the expenses of using wired system monitoring. Wireless monitoring also leads to the simpler placement of sensors within the existing infrastructure. Powering the wireless sensors with energy harvesting techniques further enables avoiding the cost and maintenance concerns related to using batteries and their periodic replacement.

Engineers use various ambient sources for powering the nodes of SHM wireless sensors. This includes vibrational, thermal, and solar sources. Ultimately, the optimum choice depends less on the technical requirements but rather on the logistics, cost, and maintenance requirements related to the target structure. For instance, noise barriers may be necessary for roads in urban areas with heavy traffic. These noise barriers may double as solar panels for energy harvesting.

Some situations may offer alternative sources of energy for powering sensors. These could be thermoelectric generators or TECs, which generate power based on the temperature differential across them. Such differentials often exist between the subgrade layers and the pavement surface of a road. Although using TECs in new constructions may be quite effective, retrofitting in existing roads may involve prohibitive costs.

Engineers often use a heavier tip mass to augment the mechanical loading of a piezoelectric device. Such loading helps to reduce the natural frequency of the device, bringing it closer to the predominant frequencies from the ambient vibrational energy source, enabling maximization of power generation.

In some cases, the ambient vibrational energy source may have frequencies well below the tunable range of the piezoelectric devices available. Engineers then turn to alternative low-frequency vibrational energy transducers like electromagnetic generators. The low-frequency vibrations cause a spring-mounted magnetic core to move through a coil, thereby converting the energy of vibrations to a current following Faraday’s law of induction.

Ambient-powered wireless sensors also require power conditioning and management. Power management circuits monitor the energy harvested, regulate the voltage applied to the load, and use the excess energy to charge external energy storage devices like a rechargeable battery or a supercapacitor.