Sensors weave safety protection for bridge safety
in many laboratories in the United States, engineers are improving the embedded sensor network, which can report defects in key structures to maintenance personnel in advance, so as to avoid catastrophic accidents, such as the collapse of I-35W bridge in Minneapolis not long ago
structural health monitoring (SHM) is an active defense method based on sensors, which can make up for the deficiencies of the monitoring system based on visual inspection and subsequent testing in the structures (bridges, buildings, aircraft and others) whose safety performance is very important at present. However, SHM sensor system has not been deployed on a large scale in the United States. Due to its short history, this method has not been added to the safety regulations at present
at present, companies such as materialtechnologies can complete on-site structural tests when needed, but because they are not monitored continuously, the ability of these tests to prevent disasters is limited. In contrast, once the sensor based SHM system is successfully deployed, it can theoretically maintain the security of the structure all the time
"the era of SHM has arrived." Dennisroach, head of the SHM feasibility study group at Sandia National Laboratory, believes that "the safety warning issued by the embedded sensor cannot be achieved by periodic inspection. Using the sensor for SHM is a reliable and inexpensive method, which can detect the defect at the first time, and has good elasticity and vibration damping performance."
in addition to bridges, buildings and aircraft, SHM can also be used to monitor the structural health of spacecraft, weapons, rail vehicles, oil extraction equipment, pipelines, armored vehicles, ships, wind turbines, nuclear power plants, and even hydrogen powered vehicle fuel tanks. At present, SHM is only used routinely in Asia, and some Asian governments have introduced sensor networks to monitor seismic activity
"the most important thing we should do is to place the sensor network in bridges, buildings and aircraft, especially in the construction process, because in this way we can place the sensor inside the structure." Jeromelynch, an electrical engineer at Stanford University and now a professor at the University of Michigan, said, "the most instrumented bridge in the world is the Tsing Ma Bridge in Hong Kong; in Chinese Mainland, at least six bridges under construction will be embedded in sensor arrays from the beginning. But in the United States, no one has done so."
how much does it cost
at present, the inspection of bridges in the United States almost completely depends on vision: only when the naked eye sees corrosion or cracks, can cochlear flow, ultrasound or penetratingdye be used for further testing. Under the warning of the collapse of I-35W bridge, some laboratories in the United States suggest using wireless nodes to continuously transmit structural health information to maintenance personnel. Because the sensor can detect tiny cracks invisible to the naked eye, we can use the data provided by the sensor to repair the problem in the "embryonic" stage
the University of Michigan has developed a "nanotube coating", which can convert the information of any surface to a sensorpatch and present the microscopic defect image in a two-dimensional manner. In addition, Los Alamos National Laboratory is using patchlike piezoelectric converters to detect defects through a novel inspection system based on unmanned helicopters, using the method of "sending out sound waves and monitoring responses". Sandia National Laboratory said that its scheme of comparative vacuum monitoring (CVM) can detect molecular size cracks (even if the cracks are in inaccessible places), and the cost of each sensor chip is only $1, so it will bring greater advantages to buildings and bridges
the solution of the University of Michigan is to spray a sensing "skin" on the structure. Lynch claims that nanotube coatings can transform the health of any surface covered under them into a two-dimensional image. Lynch once designed an explosion-proof structure for the Central Intelligence Agency (CIA) and installed a wireless SHM system on the geumdang bridge in Lichuan, South Korea. By spraying paint and installing wireless transmitters, Lynch pointed out that maintenance personnel can realize remote monitoring of the bridge, and the cost of doing so is low enough to allow us to deploy it as a standard equipment in new projects, or to adopt it when transforming existing structures
the reason why this method can keep the cost low is that as long as the monitoring sensor is placed around the spraying area, it is OK. This method can obtain the image of any position on the surface of the structure, and the corroded and cracked areas show high resistance. Lynch said that by adjusting the density and type of nanotubes used, this technology can meet the requirements that attention should be paid to strengthening the construction of product safety departments for different applications. The University of Michigan is looking for commercial partners
at the same time, losalamos National Laboratory also confirmed that the implementation cost of its piezoelectric sensor scheme is about US $1 per node. The implementation method is as follows: firstly, the sheet piezoelectric converter emits a beam of ultrasonic wave in the driver mode; Then, the system quickly switches to the sensor mode, and uses the same piezoelectric element to record and analyze the ultrasonic reflection. This step is to convert the bridge vibration caused by ultrasonic into electrical signals; Using pattern recognition software, the computer can analyze the ultrasonic reflection, so as to sense the structural changes of the bridge
"our system is a bit like active sonar." Chuckfarrar, a losalamos laboratory engineer who cooperates with the University of California San Diego research group, said, "we send a high-energy elastic wave with a frequency of kHz to the architecture; then, we compare the response we receive with the last test results."
more than a month ago, a bridge in New Mexico in the southern United States became the first bridge in the United States to install piezoelectric sensors and drivers. "By eliminating the power supply system, this wireless sensor node realizes cost reduction. Instead, these sensors are powered by the microwave beam sent by the small unmanned helicopter. The microwave beam charges a capacitor on the sensor circuit board, so that it has enough capacity to read data and send the results back to the helicopter wirelessly. On the helicopter, a single chip microcomputer will record all these data." Lynch said
the sensor scheme advocated by Sandia laboratory has ultra-low cost but ultra-high sensitivity. The CVM sensor contains a grooved belt that can be bonded to the structure to be monitored. The air in the groove is evacuated, making the grooved belt a very sensitive monitoring device. Because even if there are molecular size cracks on the surface on which it is attached, the vacuum state of the grooved band will be destroyed. This self 5. Limit protection: it has two-level limit protection functions of mechanical and program control; The thickness of adhesive thin rubber sheets is between coins and credit cards, and the cost of each is only one dollar. Moreover, a vacuum line can be used to monitor any number of CVMs
"once cracks are formed, they will destroy the vacuum seal and be detected by the instrument." Roach said, "this is the first kind of on-site sensor that can monitor structural fatigue, and its price is so cheap that we can embed it into the structure and let them stay there for continuous monitoring."
a CVM manufactured by structuralmonitoringsystems has been used in commercial routes as a technical test since a few years ago. Boeing recently confirmed the effectiveness of this approachSandia laboratory is also testing a bridge repair system based on reinforced composite materials. The company recently repaired a fatigue crack on a bridge in New Mexico with this new composite material, and installed an eddy current monitoring sensor with wireless sensor, which can remotely monitor the health status of the structure after maintenance
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