Traditional methods of structural health monitoring (SHM) typically involve slow static measurements, the use of basic sensors such as vibrating wires, and simple turnkey data loggers or standalone boxed instruments. While standalone traditional instruments are simple and can be relatively easy to set up, they suffer from limited functionality and performance, and also cannot be easily extended or customized.
During the last decade have become larger and more complex, incorporate a higher mix of materials, and are more optimized for their intended purpose. Aging buildings and infrastructures require more frequent visual inspection, a method that is expensive and often insufficient for proper diagnosis. And the times of overdesigned and overbuilt robust structures are long gone; engineers of new structures must strongly consider the increasing cost of raw materials. These trends put pressure on engineers to provide the necessary measurement data to better understand structures and help optimize designs, often involving higher channel count, longer distances, higher-speed measurements, a high mix of sensor types, and the need for advanced data management, analysis, and visualization.
Meanwhile, PC and commercial technologies have advanced rapidly, providing improvements such as multi-core processors, the PCI Express bus, USB 2.0, GPS, and alternatives to wired electrical communications such as wireless and fiber optic. These new technologies are ideal for the growing needs and increasing complexity of the civil engineering profession. This is where the concept of virtual instrumentation can help bridge the gap.
Civionics and virtual instrumentation
Civionics is the synergistic combination of civil engineering, electrical engineering, computer engineering, photonics, and other disciplines for SHM, with the ultimate goal of optimizing design techniques and understanding structural performance.
At the core of civionics is the concept of virtual instrumentation, the notion of user-defined software running on a computer coupled with instrumentation hardware to create an intelligent system that meets the exact requirements of a specific application, based on commercially available off-the-shelf hardware and software tools. PC buses and synchronization are combined with various technologies to provide many configuration options, ranging from simple PC-controlled USB-based measurement devices to high-performance modular hardware architectures.
With civionics and virtual instrumentation, the application requirements ultimately dictate the shape and form of the structural monitoring system, the converse of legacy approaches typically found with traditional systems. Virtual instrumentation has become the standard of modern consumer electronics and semiconductor tests and is used in a wide variety of applications including machine condition monitoring and automotive tests.
Flexible software and modular hardware are at the foundation of virtual instrumentation in civionics applications. Various software development environments exist, ranging from low-level, text-based programming languages to high-level graphical development environments such as National Instruments’ (NI) LabVIEW.
Graphical system design with graphical programming (also known as visual programming) is the ideal choice for civil engineers because it abstracts most of the hardware complexity and provides built-in tools for the acquisition of measurements, control of tests, results-driven data analysis, management, and visualization. Graphical programs are called virtual instruments (VI), and each VI has two components: a block diagram and a front panel. The structure of the block diagram determines the execution in graphical programming, as can be seen in Figure 1. The front panel provides the controls and indicators that give an operator the ability to input or extract data from a running VI.
The Donghai Bridge and 2008 Beijing Olympic venues are examples of virtual instrumentation applied to SHM applications. Both used graphical system design and took advantage of the latest commercially available technologies.
The Donghai Bridge, China’s first sea-crossing bridge, opened in 2005 and cost $1.2 billion to complete. The six-lane bridge is a challenging structure to monitor, with a full length of 20 miles and an “S” shape to provide a safer driving route in typhoons and high waves known to affect the region.
The Donghai Bridge requires a synchronized, real-time system to monitor its structural integrity and response to environmental factors, including typhoons, earthquakes, and corrosive saltwater. Monitoring the Donghai Bridge requires hundreds of accelerometers and fiber Bragg grating (FBG) optical sensors across each segment of the bridge to acquire the frequency response and strain from environmental stimuli. The bridge is also equipped with anemometers and load cells to record environmental conditions associated with the sensor measurements.
Each segment of the bridge contains a PC-based PXI data acquisition station, where data is collected from surrounding sensors. Additionally, a synchronization module, counter module, and controller are used to synchronize the measurements precisely. Each PXI chassis is connected to a GPS unit using pulse per second and IRIG-B timing signals for signal synchronization and time stamping, respectively. This allows the acquisition modules to acquire data with less than 100 ns jitter across all channels on the bridge.
Engineers selected the LabVIEW software development environment to integrate measurement and synchronization modules seamlessly, transfer data between system components, analyze the acquired data, and provide the appropriate notifications, reporting, and visualizations.
By using graphical system design, a team of only two engineers developed the entire Donghai Bridge monitoring system — including sensor configuration, data acquisition, signal processing, and user interface — in just three months.
2008 Beijing Olympic venues
In 2004, the China Earthquake Administration (CEA), the governmental body managing the country’s earthquake preparedness and disaster mitigation, selected seven newly constructed megastructures as the test bed for SHM technology. These landmark structures included the 2008 Summer Olympic venues in Beijing, including both the Beijing National Stadium and the Beijing National Aquatics Center; the 104-story World Financial Center in Shanghai; the 66-story Park Hyatt Hotel complex in Beijing; the 787-foot concrete arch dam in Ertan; the 5-mile cable-stayed bridge in Shantou; and the base-isolated CEA data center in Beijing.
The main objective of this major civil engineering project was to develop a state-of-the-art solution to monitor structural health characteristics, including stability, reliability, and livability, in real time by using the latest computing, sensor, and communication technology.
The SHM system was designed to capture the vibration signatures of a structure and detect any sudden shifts of structural characteristics. Similar to the way cardiologists diagnose human heart disease by measuring pulse and blood pressure, structural engineers determine structural performance by continually monitoring the natural frequency, damping ratio, and hysteresis diagram derived from the acceleration time history that accelerometers measure.
Two key requirements for the system were continuous and real-time SHM. Because most disasters strike in an abrupt and unpredictable manner, emergency management and effective reactions to sudden disasters had to be based on real-time knowledge of how a structure performs during and immediately after adverse events. Additionally, because the health of structures gradually degrades over time, by performing continuous monitoring and capturing early symptoms of health decay, engineers could compare key health indicators against previously recorded levels.
Nine 64-channel and two 36-channel systems in a client-server architecture encapsulated in a rugged NEMA enclosure were deployed at critical points in the six selected sites throughout China.
The nine 64-channel units each consist of three rugged and embedded NI CompactRIO systems, while the two 36-channel devices each contain two. Each device also incorporates multiple accelerometers for vibration measurements as well as a GPS receiver for real-time synchronization. GPS-disciplined clocks are used to achieve real-time, intrachassis synchronization within ±10µs. In areas where GPS signals are not available, engineers synchronized the systems using a computer clock.
Using graphical system design, the engineers designed, prototyped, and deployed a high-channel-count SHM system with GPS synchronization in less than one year.
Overall, virtual instrumentation offers more complete, more capable, and lower cost structural test and monitoring systems. It can help provide engineers with the feedback necessary for optimizing designs and aid them in better understanding infrastructure performance, allowing them to assist with intelligent maintenance and repair of structures.
Nathan Yang is the product manager – structural test and monitoring at National Instruments. For more in-depth information on the monitoring system including data analysis, visit www.ni.com