Development of LoRa-based Wireless Sensing Technologies for Structural Health Monitoring

Zhang, Hui
Beskhyroun, Sherif
Li, Xuejun
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Doctor of Philosophy
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Auckland University of Technology

As vital components of national service facilities, civil infrastructures must meet design standards for safety, sustainability, and longevity. However, the performance of struc- tures in long-term service can be compromised or even destroyed by ageing and natural calamities. Therefore, monitoring the integrity of these structures enables the owner, users, and other stakeholders to have a better understanding of the safety state, allowing them to better maintain and extend the service lives of these structures.

Visual inspection is currently the most used non-destructive testing method, although it is unreliable, especially when monitoring huge structures and difficult-to-access locations. The Structural Health Monitoring (SHM) system can be a useful instrument for monitoring and evaluating structural performance. It refers to the process of adopting a strategy for structural damage detection. A typical SHM system operating procedure includes the monitoring of dynamic response via a sensory system, the extraction of damaged physical features, and the analysis of gathered data to determine the condition of the monitored structure. Strain is a metric that has a direct relationship with stress and deflection. In other words, aberrant structural symptoms, such as damage and degeneration, typically manifest as strain field anomalies.

Hence, the objective of this research is to build a new strain-based wireless sensory system for an integrated SHM system. Due to the strain gauge’s extensibility and high sensitivity, it is feasible to monitor the progression of local damage across a small area at an early stage. Besides, by incorporating strain-based sensors into the wireless sensor network design model, the SHM system’s integration and management scope will be expanded to include the underlying sensor equipment and the user- graphical interface for monitoring data visualization, thereby achieving the integration of real-time monitoring and rapid evaluation of the structure. The strain-based flexible sensing system developed in this study not only fulfils the requirements for low power consumption but also exhibits excellent sensitivity, allowing it to accurately monitor minor strain changes in localised locations. The wireless sensor system is composed of three parts: a flexible strain-sensing plate, a wireless transmission communication system based on LoRa, and a visual real-time monitoring system.

The aim of this system is to overcome the problem of local damage detection in structural health monitoring (SHM) systems. Thus, it can be utilised as part of an intelligent SHM system to monitor, collect, and transmit strain changes in essential structural components. This research’s primary objective is to reduce development and installation costs and the system’s power consumption in order to enable long-term monitoring. Because the developed highly flexible strain sensing sheets can be easily mounted on the surface of the structure, as well as having the properties of being waterproof and heat and low temperature resistant, the installation and maintenance costs of the sensor are substantially reduced.

A series of experimental experiments were conducted on a full-scale concrete frame construction in order to evaluate the various properties of the flexible strain gauge. The performance of a flexible strain sensing device was investigated in terms of sensitivity, strain monitoring, and crack damage. The system was also designed to utilise LoRa wireless communication technology for low power consumption and long distance transmission in order to solve the need to install a large number of sensors to cover a larger area in huge constructions.

The second component of the research involved the development of a hardware system for the continuous gathering of data for two strain measuring configurations: bend- ing strain and axial strain. Utilising multiplixers to achieve seamless configuration switching. Additionally, LoRa technology is employed to enable long-distance wireless transmission. The strain data has a low transmission rate need and is well fitted to LoRa technology, allowing the designed sensing system to have extremely low power consumption and reducing the cost of battery replacement and system maintenance by a significant amount.

The third component of the research is developing an user interface for data management and visualisation for the integrated SHM system. Node-RED is used to create a dashboard for data visualisation, which is connected to The Thing Network (TTN) and Clould to enable real-time data transfer and demonstrate. It is entirely compatible with the deceloped flexible strain sensor system, allowing users to simultaneously monitor the minimum and maximum values of bending and axial strain. The Dashboard also includes the Timechart function to assist users in comprehending the pattern of strain data over time and detecting outliers. The visualisation tool additionally gives information on the number of sensors and Gateways in use, including their operational state and installation locations. Moreover, the original data is kept to permit in-depth analysis and extraction.

The experimental results presented in this thesis demonstrate that the developed system is practicable and possesses the qualities of high sensitivity, low power consumption, and low cost. It is conducive to the continuous real-time monitoring of multiple strains on the structure’s main components and allows the flexible wireless sensor system to identify damage in a wide variety of structures.

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