The Development of a Wireless Modular Health Monitoring System for Civil Structures

Current structural monitoring systems employ conventional cables to allow sensors to communicate their measurements to a central processing unit. Cabled based sensing systems for structures have high installation costs and leave wires vulnerable to ambient signal noise corruption. To address these disadvantages, a research effort has been initiated towards the development of a wireless modular monitoring system. The developed wireless modular monitoring system (WiMMS) would have lower capital and installation costs as well as ensure more reliability in the communication of sensor measurements. Some key areas of innovations emphasized are the use of a wireless communication system for inter-sensor communication, the utilization of micro-electro mechanical sensing elements, and the use of a microprocessor for advanced damage detection methods. Introduction There exists a need for a rational and economical method of monitoring the performance of civil structures over their lifespans. Monitoring systems are currently playing a dominant role in applications such as nonlinear model validation, structural health monitoring and structural control. Current design practice analytically determines a structure’s nonlinear response based upon nonlinear models of the key load carrying elements. A monitoring system can provide invaluable insight into the accuracy of these nonlinear models and can assist engineers in refining them. For example, short and long span bridges in California are being instrumented by the California Department of Transportation (Caltran) to monitor the nonlinear response of the bridges during extreme seismic events [1]. Just as important is the need of a rapid assessment of the performance and safety of civil structures. Using a monitoring system to measure structural responses, a damage detection strategy is then employed to diagnose possible short and long-term damage in a structure. Last but not least, in the structural control field, one key component of a control system is an integrated monitoring system that can provide feedback of real time measurements of structural response. With the rapid advancement of sensing, microprocessor, wireless and other technologies, one of the research challenges is to assess the benefits gained from the application of such technologies in the structural engineering field. Our research efforts have identified wireless communication technology, micro-electro mechanical (MEM) devices, microprocessors and digital signal processors, as key areas of innovation that can be used to develop a novel wireless monitoring system for civil structures. Traditional Structural Monitoring Systems The origin of commercially available structural measurement systems is from those regularly used in enclosed, laboratory settings. As a result, the systems are characterized as being of the hub-spoke architecture with accelerometers remotely placed throughout the structure but wired back to a single centralized data acquisition unit. Among the key problems inherent in these systems are the installation time and cost. From experience, the installation time of a complete measurement system for bridges and buildings, can potentially consume over 75% of the total testing time. Installation labor costs can approach well over 25% of the total system cost. Caltran reported that it costs over $300,000 per toll bridge to install a measurement system comprised of 60 to 90 accelerometers. To isolate the wires from the bridge’s harsh environment, a wire conduit is installed at a cost of $10 per linear foot [1]. Within buildings, wires are susceptible to tearing, rodent nibbling and measurement corruption through signal noise. Wireless Structural Monitoring System Our primary goal is to change the practice of using extensive cabling and high cost labor as is typical of the traditional monitoring systems to a system of inexpensive wireless embedded systems that can be installed, maintained and operated with ease (see Figure 1). Centralized Data Acquisition Sensors Cabling Sensors Cables Data Acq. Unix Box Bus Sensors Sensors M icroProcessor W ireless M odem Sensors A-to-D Batteries W ireless M odem PC Centralized Data Storage SU