SHM is used to identify damage in structures to improve safety and reliability of mechanical systems. Monitoring has been a manual process in the majority of the cases and requires experienced people. Since lack of information cannot be avoided without increasing costs and time, a common trend has been to adopt objective protocols of damage detection, run in an automated manner, in place of subjective inspection run by a human operator according to a time-based maintenance schedule. The adoption of WSNs, capable to collect data such as temperature, light intensity, humidity, proximity, etc. and transmit them wirelessly to a central hub for remote analysis represent an effective solution to the demand of high technology industries for managing their products over life cycles. WSNs can perform local analysis and control for functions such as data logging, event detection, alarming, short message service notifications, and web-based network data visualisation. Therefore, WSNs can allow easy access to information for better decision-making in the maintenance process of high technology products and, furthermore, do not add weight or burdens to structures as running wires or cabling to main.
Most prior wireless structural monitoring systems have relied on continuous power supplied by batteries [27] but periodic replacing or recharging represents an additional maintenance task to be performed. A few studies on purely passive systems (which do not need any external power source) for SHM have been carried out. For instance, in [28], Guyomar et al. proposed a method of impact location estimation for SHM based on resistively shunted piezoelectric inserts. This method relies on the comparison of the electrical energy extracted by closely laid piezoelectric elements, which are considered as local electromechanical dampers and give insight of the energy flow. However, such a passive detection technique is based on testing upon an infinite lossless beam and it does not take into account the effects of simultaneous multiple impacts, which possibly lead to interferences of waves and cancellation of the energy at the location of a piezoelectric element. On the contrary, wireless sensors can constantly monitor through direct measurements and automatically report on the required information so as to identify early warning signs of structural failure [29].
Wireless sensor applications for SHM are thus interesting applications for the deployment of EH and vibration EH, in particular, being vibrations almost ubiquitous around us. Therefore, PEH can be an optimal solution through developments of compact and light-weight self-powered wireless sensor systems capable to convert strain energy into electrical energy, measure, record, and transmit useful data in a completely energy-autonomous manner. For example, electrical energy can be freely generated by passing cars on bridges to detect their health status via piezoelectric inserts as shown in Figure 1-6.
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Figure 1-6 Bridge monitoring by mean of PEH powered wireless sensors (Figure courtesy of Fraunhofer-Gesellschaft Inc.)
Also buildings progressively accumulate damages during their operational lifetime due to seismic events, unforeseen foundation settlement, material aging, design error, etc. Harvesting pads can collect energy from the vibrations of trains passing through the railway by using piezoelectric architectures such as shown in Figure 1-7.
Figure 1-7 Pads for PEH from trains (Figure courtesy of Innowattech Ltd)
The pads can harvest mechanical energy and transduce it into electrical energy for counting the number, weight, diameter and position of wheels, or for detecting speed. PEH sensors can track damages on commercial and military vehicles including ships, aircrafts, and helicopters. Particularly, they can determine dynamic and static loads on rotating components without sleep rings and batteries by use of PEH powered wireless nodes as shown in Figure 1-8.
Figure 1-8 PEH powered wireless sensors aboard a helicopter (Figure courtesy of MicroStrain Inc.)
SHM military applications of PEH powered wireless sensor systems may also include perimeter defense such as oil pipeline protection. In collaboration with Intel, for example, BP has run a project for preventive maintenance on an oil tanker in the North Sea by use of sensor networks to support preventive maintenance onboard the ship. More in general, environmental/habitat monitoring offer plenty of case applications for energy-autonomous WSNs in relation to the integrity of the soil, water conditions, air regulations with regards to temperature and humidity. For instance, the FireBug wildfire instrumentation system developed by the University of California (Berkeley – CA) is a GPS- enabled wireless thermal sensor network based on TinyOS that self-organise itself for collecting real-time data in wild fire environments in order to allow predictive analysis of the evolving fire behavior [30]. In addition, energy- autonomous WSNs carry out the promise to improve streamline business practices by supplying real-time data. Even people‟s location tracking sensors and body area networks can be powered by limb movements during breathing, heart beating, normal walking, hand cranking or lifting. Medical applications for vital sign monitoring or accident recognition are thus attracting a lot of interest as a potential use of PEH powered wearable sensing nodes. The common denominator in the aforementioned scenarios are the sensor networks composed of low-cost, low-power miniaturised sensor nodes that communicate untethered across short distances to carry out their collective task. For these
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communication protocols must be developed to allow large amounts of nodes to collect, process, and disseminate data. While wireless sensors have the advantage of eliminating wiring installation expense and weight, as well as connector reliability problems, they still require a considerable amount of power in order to operate. Indeed, if power outages occur, critical data may be lost.