Blue Road Research and the University of California at San Diego have been collaborating over the past four years to develop a system employing fiber Bragg grating strain sensors and modal analysis to provide real-time, quantitative information on a bridge’s response to a dynamic input (such as a seismic event), and a fast prediction of the structure’s integrity. This research, being funded by the National Science Foundation, has several publications showing its strong progress. In the latter part of 2004, this system will be installed on the Broadway Bridge in downtown Portland, Oregon, USA. In preparation for this deployment, the system is undergoing testing from the sensors through the readout unit and bridge modeling to ensure its performance once installed. The steps of preparation and testing are discussed here.
In a quest for fiber optic sensors that could monitor soil moisture, Blue Road Research implemented fiber Bragg grating sensors in such a way that they could detect humidity, soil moisture evaporation rates, and pressure changes from soil weight. These were then used to monitor soil in controlled flood tests to determine the moisture levels in a soil test bed. Fiber optic sensors seem well suited for humidity and soil moisture monitoring since they can easily be multiplexed with many sensors on one fiber line, and they have distinct longevity advantages that enable their use in applications involving wet environments, remote locations or long distances, electromagnetic interference, flammability, or other harsh environmental conditions that may degrade ordinary electronic sensors or their measurements. This paper describes the workings of a highly accurate optical humidity sensor that can be multiplexed on a fiber optic strand to monitor humidity that may lead to corrosion, soil moisture levels and changes, weather conditions, etc. as well as means to record such data.
It is desirable to make dynamic strain measurements in multiple directions at a single point. In particular this technology could be useful in monitoring multidimensional deflections of ship hulls measuring both axial and transverse strain fields. Conventional strain gages are unable to deliver this type of performance due to the size requirements and lack of available technology. Fiber grating sensors have been shown to deliver accurate strain measurements in two orthogonal directions, but have been limited by the speed at which the signal can be demodulated. As it is desirable to make multi-directional dynamic measurements, a system is required that can deliver a high speed demodulation of the signal from a dual-axis fiber grating sensor. Using available optical components a system was developed to read out orthogonal directions of strain at a point at up to 100 kHz. The resolution in one direction is +/- 40 microstrain and in the other +/- 180 microstrain. The system was initially tested over 4000 microstrain in tension along the higher sensitivity axis.
Fiber optic sensors may be used to monitor strain and temperature in composite materials. These measurements can be useful in determining rate and degree of cure of composite. Multi-dimensional strain
measurements enabled by fiber gratings written onto polarization maintaining optical fiber enable monitoring changes in transverse strain, transverse strain gradients, and shear strain internal to composites and adhesive joints. This paper provides a brief historical overview of the usage of fiber sensors to provide strain measurements in composite parts, leading eventually to multi-axis strain sensing.
Polyimide coated fiber Bragg gratings have a linear response to changes in relative humidity and temperature. Blue Road Research is using this technology to monitor relative humidity and using acrylate-coated gratings to monitor temperature. This paper describes some of the sensors and readout systems for simple and multiplexed relative
humidity and temperature sensors. Additionally, an indirect method for monitoring soil moisture is described.
Over the past few years Blue Road Research and the University of California at San Diego have been collaborating to develop a bridge health monitoring system using long gage length fiber optic strain sensors and modal analysis. Two programs supporting this effort have been funded by the National Science Foundation and from this work
several papers have been published showing its strong progress1-5. In 2002, the Federal Highway Administration and Caltrans performed a full-scale test on some of the components that will be used for the planned I-5/Gilman Advanced technology Bridge in California, USA. As a part of this test Blue Road Research used its developmental system to validate the use of this damage detection technique and to compare the results with conventional modal analysis tools.
Fiber Bragg grating sensors have attracted considerable attention for measurement applications due to their greatly reduced size, low weight, and immunity to electromagnetic interference in comparison with traditional sensing methods. Dynamic measurement of industrial machine tools is useful for gauging surface accuracy, monitoring tool condition, and predicting process stability, but requires a robust sensing scheme. The small size and high natural frequencies of micro
machining tools coupled with a harsh manufacturing environment can render traditional sensors ineffective. This work presents a new method for measuring tool motion with fiber Bragg grating strain sensors. The feasibility of the sensing scheme is first demonstrated with a simple bench-top cantilever beam experiment. Then, a method for potting the sensors in the through coolant holes of a 1/8” carbide end mill with a high-viscosity gap-filling cyanoacrylate is
demonstrated. Comparative structural analysis tests demonstrate the effectiveness of the sensors. Measurements of tool motion during cutting are presented. Finally, methods of noise reduction and improving signal accuracy are discussed.
Blue Road Research and the University of California have been collaborating over the past three years to develop a system employing fiber Bragg grating strain sensors and modal analysis to provide real-time, quantitative information on the structure's response to a dynamic input (such as a seismic event), and a fast prediction of the structure's integrity. This research, being funded by the National Science Foundation, has several publications showing its strong progress. This year marks a significant step forward in this effort, with the successful completion of a full-scale test performed on a longitudinal carbon shell girder being tested as part of the planned I-5/Gilman Advanced technology Bridge in California, USA.
Blue Road Research is developing moisture-sensing technology that is capable of precision measurements of relative humidity and soil moisture content. Based on optical fiber sensor technology, the sensors are highly corrosion and EMI resistant and can be installed in locations where it would be impractical to insert a bulky conventional sensor. While the sensors are currently in the research and development phase, Blue Road Research has demonstrated the performance of the technology and is moving toward a commercially available product.