Structural health monitoring with wireless smart sensors

Using smart sensing technology to monitor the integrity of civil infrastructure cuts inspection costs and increases safety for the public.

4Automating tasks that help ensure safety is a great job for a wireless sensor network, which can collect data continuously and warn of unusual conditions when they arise. This application story shows how bridges can be monitored more effectively than intermittent inspections.

Investment in civil infrastructure in the United States alone is estimated to be $20 trillion, and annual costs amount to between 8-15 percent of the GDP for most industrialized nations. In recent years, much attention has been focused on the declining state of the aging infrastructure in the United States such as the nation’s bridges, highways, and buildings. The ability to continuously monitor the integrity of civil infrastructure in real time offers the opportunity to reduce maintenance and inspection costs while providing increased safety to the public.

Bridges account for a large part of the capital investment in constructing road networks and represent a key element in terms of safety and functionality of the entire highway system. Informed bridge management is important to provide the public with timely transportation while maintaining a high level of safety. Manual inspection of bridges costs millions of dollars, is relatively unreliable, and can only be carried out sporadically. Tremendous attention has been directed toward the development of structural health monitoring strategies, with a primary goal of enhanced safety and reliability, along with reduced maintenance and inspection costs.

Smart sensors form framework

The Illinois Structural Health Monitoring Project (ISHMP), based at the University of Illinois at Urbana-Champaign (UIUC), is working to develop an inexpensive means for continuous and reliable Structural Health Monitoring (SHM) using dense arrays of smart sensors. Researchers have designed, developed, and tested sensors to produce the high-fidelity data required for SHM, as well as a customizable software framework that simplifies the development of SHM applications for smart sensor platforms. In combination, the sensors and software create an integrated framework that can be utilized easily by most civil engineers without requiring an extensive background in computer science.

ISHMP researchers have focused on MEMSIC’s Imote2 wireless sensor platform because they believe it can meet the high data throughput demands of SHM applications. The Imote2’s powerful processor and onboard memory enable its use for the high-frequency sampling required in dynamic structural monitoring. The platform has a low-power Marvell XScale processor with variable processing speed to optimize power consumption. Moreover, it has 256 KB of integrated RAM, 32 MB of external SDRAM, and 32 MB of flash memory.

The Structural Health Monitoring Accelerometer (ISM400) sensor board was designed to meet the specific needs of SHM. The board provides flexible and accurate user-selectable sampling rates and anti-aliasing filtering capabilities to account for the local nature of structural damage, where higher mode responses of the structure are often required (up to 500 Hz) in addition to low-frequency signals (DC to 20 Hz). The board interfaces with the Imote2 via SPI and I2C I/O and has a general input and a three-axis analog accelerometer for vibration measurement, as well as digital temperature, humidity, and light sensors. The temperature sensor provides onboard temperature compensation to the acceleration data. The ISM400 board (shown mounted on an Imote 2 in Figure 1) provides the enabling technology that was previously unavailable to perform SHM using the Imote2.

Figure 1: The Structural Health Monitoring Accelerometer sensor board mounted on the Imote2 wireless sensor platform provides accurate sampling rates and anti-aliasing filtering capabilities.
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The customizable software framework greatly reduces both the complexity and time for development of SHM applications for smart sensor platforms. The ISHMP Services Toolsuite provides an open source software library of customizable services and examples of SHM applications utilizing (WSNs). The ISHMP provides users with a collection of guides to help those who are new to the Imote2 platform or its operating system and programming languages.

The ISHMP Services Toolsuite employs a Service-Oriented Architecture (SOA) that lends itself to further expansion, customization, and development of WSN applications for SHM. It provides complete applications that facilitate common tasks throughout the design, testing, deployment, and monitoring of the SHM system, while utilities offer a set of basic testing and debugging commands to be included with existing applications. The SHM Services Toolsuite includes utilities for remotely resetting nodes, listing the nodes within the local node’s communication range, testing radio communication performance, and changing the radio channel and power for local and remote nodes. Specific services and tools include the following:

·    Foundation services: Provide commonly used wireless sensor functionalities that are required to support higher-level applications, including basic communication and functionalities.

·    Application services: Provide the numerical algorithms (some shown in Figure 2) necessary to be used independently and implement SHM applications on the Imote2.

·    Tools and utilities: Provide network testing and debugging capabilities that are necessary in any large-scale or long-term WSN deployment. These tools facilitate evaluation of the structure’s network conditions to determine appropriate values of adjustable system parameters and assess power consumption and longevity issues.

·    Continuous and autonomous monitoring services: Provide for continuous and autonomous WSN operation while maintaining power efficiency.

Figure 2: The ISHMP Services Toolsuite provides the numerical algorithms needed to implement SHM applications on the Imote2.
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The framework allows researchers and ultimately application engineers to design and implement successful SHM systems without requiring knowledge of how the underlying middleware and numerical services are implemented.

After much testing and lab work, ISHMP researchers launched full-scale monitoring of the Jindo Bridge in South Korea in June 2009. The Jindo Bridge is Korea’s first cable-stayed girder bridge. A collaborative effort led by UIUC, KAIST in Korea, and the University of Tokyo in Japan, this deployment constitutes the first dense deployment of a WSN on a cable-stayed bridge. Installed with 110 nodes (with examples shown in Figure 3), the deployment is the largest of its kind for any civil infrastructure to date.

Figure 3: The Jindo Bridge includes 110 nodes that provide data for maintaining the bridge’s structural health.
(Click graphic to zoom by 1.7x)

Prior to deploying the smart sensor network on the Jindo Bridge, the Imote2 and sensor board stacks were housed in enclosures that provided protection against environmental conditions such as rain, wind, and dust. The enclosures had a gasketed lid and could be mounted to the structure. Several nodes were designed to accommodate larger battery sources, while some were designed to utilize integrated solar panels.

Changing traditional inspection

Smart sensing technology for SHM is an emerging field that combines civil engineering knowledge with developments in sensor technology, information management, and networking technologies to develop a solution that is a robust, significantly lower-cost alternative to traditional structure inspection techniques. This process of implementing a damage-detection and -characterization strategy for engineering structures has changed the concept of SHM.

Giri Baleri is currently the director of product management at MEMSIC, Inc., where he is responsible for new product strategies, market analysis, product positioning, and go-to-market strategy. He previously worked as applications engineering manager for the Wireless Sensor Network business unit at Crossbow Technology, Inc., which was acquired by MEMSIC. Giri obtained his Master’s degree from the University of British Columbia, where his thesis research focused on simulation and modeling of stick slip-induced vibrations on large, uneven contact loads.