In late 2008, a fiber optic structural health monitoring (SHM) system was installed on the Chiapas Bridge in Mexico. This installation represents the first SHM instrumented bridge in Mexico using optical fiber sensors and SHM techniques. The project was conceived and managed by the Structures Department within the Institute of Engineering at the National Autonomous University (UNAM) in Mexico, under contract from the Mexican Transportation Undersecretary. The system was designed by MCH Engineering LLC, equipment and sensors provided by Micron Optics Inc. and the system installed by Chandler Monitoring Systems Inc. (www.chandlermonitoring.net).

Figure 1: Aerial view of the Chiapas Bridge in Southwest Mexico

The Chiapas bridge was opened to traffic in 2003 and is the most important structure of an entire highway going through the state of Chiapas, in Southwest Mexico, and part of the Transamerican highway system. The superstructure is a continuous grade 50 steel orthotropic box of constant height comprising 8 spans, with a total length of 1208m. Piers heights range from 27 to 89 m measured from the bottom of the dam. Steel jackets of the offshore type, were used as the main elements of the piers.

Figure 2: Aspect and topology of the installed sensor network inside the bridge

During construction of the superstructure, a basic monitoring system was initially implemented to monitor and assess the structural behavior of the bridge. However, advances in SHM techniques and the importance of the Chiapas Bridge in the region, prompted the need for a new and advanced instrumentation system. The new solution is based on a multi-point, multi-sensor monitoring system based on optical fiber Bragg grating sensors and opto-electronic interrogators. The system is self contained, works as a stand-alone equipment and allows for the in-situ, real-time monitoring of the bridge as well as its long-term condition. A total of 82 fiber optic strain and temperature sensors were installed in key bridge locations. Local electric power is supplied by a solar panel system.

The monitoring system instrumentation is composed of a single optical interrogator (Micron Optics model sm130-500) with 4 optical channels, scanning at 1khz; a 4×16 channel sensor multiplexer (model sm041-416) and a sp130 controller and data acquisition module (Figure 3). The system can be configured to record data at any specific interval and to any threshold level.

Figure 3: Micron Optics Interrogation System on Chiapas Bridge

Admin edited to add images 4/6/09

I’ve written about why many applications must use FBG sensors, but how do users deal with the data? Conventional electronic gages deliver an analog signal that’s proportional to the strain or temperature change. Optical gages deliver a digital signal that reports an absolute wavelength value indicative of the strain, temperature, displacement, pressure, etc.

Converting from wavelength to engineering units requires some basic arithmetic. For example, the gage factor for an FBG strain sensor might be 1.2 picometers per microstrain (gage factors are provided by the strain gages manufacturers — just like electrical strain gages). So, for example, if the measured wavelength shift is 120 picometers, the strain sensor is actually measuring a change of 100 microstrain.

Some calibrated FBG temperature gages may use a third-order polynomial fit to fully characterize the gage factor, but still it’s just a matter of doing the arithmetic to make the conversions from wavelength to temperature.

Up to now, most users have been on their own to make these calculations. Micron Optics has always provided a basic LabVIEW example user interface that customers modify to convert, store and display sensor data. But now Micron Optics is providing a new tool called ENLIGHT^Pro.

ENLIGHT^Pro provides an all-in-one solution to configuring sensors connected to Micron Optics instruments, converting wavelengths to engineering units for hundreds or thousands of sensors, displaying data in charts, graphs or images, setting alarm limits and sending alerts, and saving data. A free download of ENLIGHT^Pro Beta release is available at http://micronoptics.com/sensing_software.php

The release of ENLIGHT^Pro represents yet another milestone for making fiber optic sensing solutions more accessible and easy to use. Along with improved sensor packages, sensor installation kits, and simplified instrumentation choices, this software tool allows the user to quickly move beyond optical setup details to actually using and analyzing the data to get the answers that they need.

New applications for FOS come about in many ways. Starting more than ten years ago university researchers led the way, but, in recent years, more commercial entities are solving problems using FOS. One example is Durham Geo Slope Indicator (DGSI) in Stone Mountain, Georgia. They’ve earned a solid reputation for providing vibrating wire (and many other technologies) for geotechnical measurements. But for some of their customers with buried pipelines, high EMI conditions made the vibrating wire technology less useful. They recognized that fiber optic strain gages would work well in this environment.

So they coupled their know how for installing and protecting electrical lead wires and sensors to the FOS sensors and have made several installations. The results are impressive. The pipeline owners can resolve much smaller movements in the pipes than ever before, expected lifetime is improved by at least a factor of ten, and installation is about four times faster.

They’ll be exhibiting their new applications at the ASCE Pipeline Division International Conference on July 23-24 in Atlanta. Learn more at the ASCE websiteand Durham Geo’s website

Over the past decade use of fiber optic sensors has moved from university labs to widespread industrial applications.  Many packaged sensor and instrument products are well developed and field proven.  They are not science projects any more.

 

Our message, and the purpose of this blog, is to help users and potential users to find answers to fundamental questions about how they can better use fiber optic sensing technologies to solve their measurement problems.

 

Today, emphasis is shifting from proving fiber optic sensing capabilities (these are widely accepted now) to refining the nuts and bolts of fielding applications.  Simplifying installation.  Selecting cable types and junction boxes. Choosing connectors or splices.  Operating in harsh environments.  Qualifying lower cost sensors.

 

This trend is apparent in how Micron Optics serves the fiber optic sensing market.  Our customers who purchased a few instruments years ago for trials and studies are now deploying, in volume, focused solutions to their customers.  Some use custom interrogators from Micron Optics with their own sensor designs.  Others rely on both our standard interrogators and sensors.  A few work with Micron Optics optical modules to develop their own solutions.

 

We hope you’ll find this blog useful and that you’ll ask us to answer your specific questions.

 

Tom Graver

Director, Optical Sensing

twgraver@micronoptics.com