Sensing a Change for Bridge Monitoring

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Falling prices, improved performance now making it feasible to put sensor modules on questionable bridges.

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Mouser Electronics.

Bridge collapses and numerous studies about the poor state of America’s bridges have prompted a lot of interest in sensors that monitor the health of bridges, but implementations have remained rare. Adoption of the technology seems likely to increase, accelerated in part by the deadly bridge collapse in Genoa, Italy, which shined a spotlight on the decreasing cost and improved performance of sensors and bridge health monitoring modules.

In the U.S., interest in monitoring bridges soared after the 2007 I-35W bridge collapse in Minneapolis. Sensor makers stepped up with modules they said would help civil engineers quantify bridge quality, but high deployment costs and other parameters didn’t meet most user’s requirements.

Over the past few years, sensor suppliers have improved performance on a number of different levels, saying that enhancements and lower costs make bridge monitoring more effective now. Some state experts feel that the time’s right for using sensors to pinpoint structural weaknesses before a failure occurs.

“In five years’ time, we plan to have about a half-dozen of these systems on bridges,” said Behrooz Far, State Bridge Engineer for the Colorado Dept. of Transportation. “A cost-saving benefit is that the technology is portable. If after a year or so our analysis confirms that a bridge is structurally sound, we can move the sensors to another location, another bridge.”

Oregon too is looking to leverage the newest technology to monitor bridges. In June, the Oregon Department of Transportation released an RFP soliciting bids for the design, installation, operation and maintenance of a structural monitoring system. Similarly, South Carolina DOT officials recently announced that acoustical monitoring and instrumentation will be added to the inside of the Wando Bridge, which would allow inspectors to hear a cable snap. In 2010, duct tape was found in use in several places on the bridge.

Far cited improvements in size, reliability, power and communications as reasons he’s bullish about getting sensing systems onto bridges in the near future. Communications is one of the key factors. Getting data on the structural health of a bridge out to sites where it can be analyzed has been expensive up until now, but improvements in wireless communications technologies is helping to take costs down.

“Hardwired signal lines in any applications are expensive, time-consuming to install, and have no flexibility when structural changes are being implemented,” said Pete Smith, Manager of Sensor Product Knowledge at TE Connectivity, a module supplier. “Wireless communications, on the other hand, address all these issues. IoT is driving the development of dedicated wireless RF channels for sensor communications. 5G networks will also be able to handle sensor communications at high speeds, which will tend to improve latency performance.”

Bridge engineers look forward to using sensor modules to check the health of bridges. (Source: STMicroelectronics)

The changeover to solid state semiconductor sensors is another key driving force. Improvements in semiconductor processing are allowing  design teams to shrink device and module sizes, making it simpler to install sensors with less worry about maintenance and upkeep. Equally important, prices are falling as micro-electromechanical (MEMS) sensor production matures.

“Module prices are definitely coming down,” said Edoardo Gallizio, Product Marketing Manager at STMicroelectronics, a leading sensor supplier. “We have a reference design, which includes multiple sensors, processor, a wireless radio, a chargeable battery and an SD card for storing data, that costs under $10.”

Lower cost is not the only benefit as silicon devices ramp up in production; Improved device reliability and longer lifetimes are also now the norm. For example, TE Connectivity recently started shipping load cell sensors built with silicon strain gauge elements. Using solid state silicon parts brought a significant increase in operating lifetimes compared to sensors built with metal foil elements.

“Silicon devices exhibit an infinite cycle life, as compared to bonded foil strain gauge which will have a finite cycle life in some cases,” Smith said. “The result is solid state sensors that provide years of service without failure.”

Once wires are removed, however, designers have to get creative about conserving power. Fortunately, silicon parts consume low power and can often be put in sleep modes to further prolong battery runtime. Some strategies call for adding compact energy generation modules.

“We understand that low power is critical, and we’ve designed modules with all the technologies – sensors, processor and connectivity components – that can run on a coin cell battery for 10 years,” Gallizio said. “If end users want, they can add energy harvesting capabilities or add a little photovoltaic panel to provide most of the power.”

Power and pricing aren’t the only improvements of the past few years. Component manufacturers have extended the capabilities of a number of sensing technologies, with a goal of giving bridge engineers the ability to gather more information about the bridge structure.

“A wireless combo sensor–with altimeter, temperature, and relative humidity–provides for simple installation, and a variety of sensor inputs,” Smith said. “In addition, the relative humidity reading and temperature reading can be used to calculate the dew point, so this module’s three sensors provides four different parameters to the user. This is ‘pushing intelligence to the edge.’”

While the proliferation of technologies brings many benefits, it also adds a fair amount of complexity. It may be prudent for civil engineers to call in unbiased intermediaries to help them analyze the nuances of different types of MEMS sensors. In Colorado, for example, Far tapped consultants for help in determining how to gather the most useful information at the lowest cost.

“We’re working with consultants because there are so many technologies on the market,” Far said. “Those technologies are all improving, sensors are getting smaller, cheaper and more reliable, methods for powering them are improving and cellular technology makes it easier to collect data. We can now outfit an entire bridge, using half a dozen sensors, for $150,000.”

Sensors can also act as remote policemen. Drivers who take a single large load over a bridge can ravage the structure, causing more harm than literally thousands of cars would inflict. Sensors can immediately measure these overloads and activate cameras, working much like the red-light cameras deployed on many urban intersections.

“On bridges that are sensitive or are close to their end-of-life phase, we can use sensors to identify when overloads cross a bridge,” Far said. “Sensors can be tied to cameras that take a picture of the license plate and the truck. If we can catch them with that picture, people will learn that Colorado catches people who damage bridges.”

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