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Wireless sensor networks offer engineers the chance to monitor ageing infrastructure and extend their lifetimes

Across the developed world, one of the greatest challenges facing civil engineers is the maintenance of ageing infrastructure. Nowhere is this more apparent than in the bridges, tunnels and pipes in cities around the world.

Much of that infrastructure was built 50 to 100 years ago and, as a result, there is growing evidence of deterioration. In many cases, new construction has impacted on existing infrastructure, whether through the use of deep excavation or tunnel construction.

Rapid urbanisation means cities are becoming increasingly crowded and complicated and safety issues arise when infrastructure ages or requires maintenance, hence an increasing demand for monitoring solutions to ensure structural integrity and safety.

The scale of the problem was illustrated by the US Department of Transportation (DOT), which found that, in 2014, 24% of the US’ 611,000 public bridges were considered either structurally or functionally deficient.

Managing underground structures – tunnels or pipes – is often problematic; inspection is difficult as they are in constant use. Their long term structural performance tends not to be understood through lack of available historical data and, as a result, the number of research projects investigating the impact of the damage to which these structures are susceptible is growing.

Much of the deterioration is as a result of the wear and tear generated by increased traffic loads, much of which is beyond the original design specification. As a result, infrastructure is ageing far more rapidly than originally anticipated.

Whether monitoring structures above or below ground, there is a growing role and demand for wireless sensor networks in the management of infrastructure networks and the mitigation of damage.

Raghu Das, CEO of IDTechEx, said: “Developments in sensor and energy harvesting technology are helping to drive this market. The growing deployment of sensors on structures to monitor problems associated with vibration and strain, in the case of bridges and tunnels for example, is growing rapidly.

“Many of these devices tend to be deployed where structures may need to be repaired, but engineers are unsure. For example, with bridges supporting two or three lanes of traffic, up to 300 sensors can be deployed to monitor the structure over a period of three to six months. Once the data has been collected and processed, an informed decision can then be made as to whether maintenance or further action is required.”

Monitoring systems based on wireless networks are relatively simple today, but are expected in the future to comprise of autonomous modes capable of integrating specific sensing capabilities with communication, data processing and power management.

These networks of sensors could be scattered across engineering systems to not only monitor damage from daily usage, but also to extend their lifetime.

Structural damage tends not to be obvious and may only become apparent when the structure fails.

Practical solutions

Wireless monitoring systems offer a practical structural health monitoring (SHM) solution, avoiding the expense of wired systems and enabling simpler placement in existing infrastructure. But it is the use of energy harvesting techniques to power these sensors that is helping to avoid the safety and costs associated with maintenance especially where devices are battery powered.

Powered by a variety of environmental sources – solar, thermal and vibration – the choice of SHM wireless sensor nodes is determined less by technical considerations and more by the logistics, cost and maintenance requirements associated with a specific structure.

“Bridges and roads that support large amount of traffic tend to deploy sensors powered by solar energy harvesting,“ explains Raghu, “while in other situations, we are seeing the use of thermoelectrical generators, where power is generated across the structure by variations in temperature.”

Vibrational energy is another ready source of power for roads and bridges as it is neither reliant on sunlight nor large temperature changes. Using piezoelectric devices to generate power from the vibrations of passing vehicles allows multiple wireless sensors to be placed across a road or bridge. Embedding them is relatively easy; whether in a new construction or an existing piece of infrastructure.

Piezoelectric devices, typically fixed on one end to form a cantilever, will then deliver an voltage proportional to the deformation of its crystalline structure. These devices produce AC voltage output when allowed to flex both above and below its resting plane.

In operation, they will produce a maximum voltage output at a natural frequency determined by a combination of the device’s characteristics and how it has been attached to the structure. Using devices possessing a natural frequency close to that of the predominant ambient vibrational source is preferred, but the natural frequency can be tuned by adding mass.

“Linear Technology’s LTC3588-1 combines an on-chip full-wave bridge rectifier, buck converter and power management circuitry that has been specifically designed to maximise energy harvesting from the piezoelectric devices described above. It includes an under voltage lockout capability and can accumulate charge on an input capacitor until the buck converter can transfer a portion of the stored charge to the output,” says Richard Miron, technical content engineer at Digi-Key Electronics.

Wireless sensor nodes powered by environmental factors typically combine a low voltage microcontroller, an RF transceiver and an energy-harvesting subsystem for power conditioning and management. For vibrational energy, the harvesting subsystem relies on a full bridge to convert the AC output of the piezoelectric into a useable voltage.

Power management ICs will be deployed to monitor the harvested energy, regulate the voltage supplied to the load and route any excess energy to a storage device such as a rechargeable battery.

While the energy harvesting subsystem provides power, the functional capability of a wireless sensor node relies on sensors, processors and communications capabilities. In a typical SHM application, vibration, moisture and temperature sensors tend to be combined.

“The MCU and wireless transceiver in these systems will dominate the power budget and require the selection of ultra low-power devices,” explains Miron. “MCUs, such as the AVR ATiny 8bit MCU from Atmel, can operate from supplies as low as 1.8V while consuming 200µA/MHz. The C8051F9xx 8bit MCU family from Silicon Laboratories can operate with a supply as low as 0.9V. For more demanding applications, designers can use TI’s MSP430 MCU family (160µA/MHz). In fact, TI’s MSP430FR5969 MCU takes advantage of the low power requirements of its FRAM based on chip memory to achieve a power consumption level of only100µA/MHz.”

For wireless communications, transceivers capable of operating at sub GHz frequencies can achieve an optimum blend of low power and extended range, with RF transmitters that require about 5mA to support serial data rates of up to 10kbit/s over ranges of up to 1km.

Structural health monitoring is becoming an ever more critical requirement for ensuring the safety of public infrastructure.

The ability to monitoring the health of roads and bridges over time requires wireless sensor nodes that can extract power from a number of energy sources, but provide a continuous sensing capability while removing the need to replace batteries.

Author
Neil Tyler

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