Part 1 Hardware
Investment in civil infrastructure in the United States alone is estimated to be $20 trillion. Moreover, annual costs amount to between 8-15% of the GDP for most industrialized nations, and this investment is only likely to increase. Much attention has been focused in recent years on the declining state of the aging infrastructure in the U.S., 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. As such, 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. Recent years have seen tremendous attention directed toward development of structural health monitoring strategies, with the primary outcome sought being enhanced safety and reliability, with reduced maintenance and inspection costs. Many of the recently constructed bridges have in-depth monitoring systems; for example, the Stonecutters Bridge in Hong Kong which has over 1200 sensors. However, the enormous expense of installing traditional monitoring systems has significantly limited deployment. For example, the total system cost of the monitoring system (including installation) on the Bill Emerson Memorial Bridge in Cape Girardeau, Missouri, USA is approximately $1.3M for 86 accelerometers, which makes the average installed cost per sensor a little over $15,000. Average per node cost for other wired bridge monitoring installations are of a similar magnitude.
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 SHM (Structural Health Monitoring) using dense arrays of wireless smart sensors. Researchers have designed, developed, and tested sensors to produce the high-fidelity data required for SHM that can be manufactured very cheaply. Their research has also produced a customizable software framework that simplifies the development of SHM applications for smart sensor platforms. In combination, their sensors and software create an integrated framework that can easily be utilized by most civil engineers without requiring an extensive background in computer science!
ISHMP researchers have focused on Crossbow’s Imote2, because they believe that it to be the only commercially available wireless sensor platform that can meet the high-data throughput demands of SHM applications. The powerful processor and the onboard memory of the Imote2 are the features that set it apart from other wireless sensor platforms and allow its use for the high-frequency sampling required for dynamic structural monitoring. The Imote2 has a low-power X-scale processor (PXA27x) 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 (SHM-A) sensor board was designed to meet the specific needs of SHM by Prof. Jennifer A. Rice from Texas Tech University as part of her PhD research at UIUC. 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). To avoid potential signal errors, especially in the higher frequency range, minimizing sample-rate fluctuation (jitter) is critical. While simply interfacing an analog accelerometer with a high-quality ADC could address the sampling rate issues, a programmable signal conditioner was chosen because of the flexibility it provides the user in terms of anti-aliasing and signal processing. The key component of the SHM-A board is the Quickfilter QF4A512, a versatile, 4-channel ADC and programmable signal conditioner with user-selectable sampling rates and programmable digital filters. The board interfaces with the Imote2 via SPI and I2C I/O and has a 3-axis analog accelerometer for vibration measurement, one general analog input, as well as digital temperature, humidity and light sensors. One purpose of the temperature sensor is to provide onboard temperature compensation to the acceleration data. The SHM-A board provides the enabling technology that had been previously lacking to perform structural health monitoring using the Imote2.
Full-scale Implementation
After much testing and lab work, ISHMP researchers moved forward on the full-scale monitoring of the Jindo Bridge in South Korea in June. The Jindo Bridge is Korea's first cable-stayed girder bridge. A joint project between University of Illinois at Urbana-Champaign, 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 70 nodes, the deployment is largest of its kind for civil infrastructure to date.
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 the ability to mount to the structure. Several nodes were designed to accommodate larger battery sources, while some were designed to utilize integrated solar panels.
Smart sensing technology for structural health monitoring is an emerging field that, by combining civil engineering knowledge with developments in sensor technology, information management, and networking technologies presents a solution that is a robust, significantly lower-cost (~$500/node), alternative to traditional structure inspection techniques. This process of implementing a damage detection and characterization strategy for engineering structures has changed the idea of SHM. The SHM process involves the observation of a system over time using periodically sampled dynamic response measurements from an array of sensors, the extraction of damage-sensitive features from these measurements, and the statistical analysis of these features to determine the current state of system health.
For more details on the Imote2 platform visit Crossbow's site here.
For more information on the SHM sensor board developed at UIUC visit the ISHMP site here and view a detailed report on the development and implementation here.
Stay tuned for Part 2 of the Jindo Bridge Deployment related to the software strategy implemented...
