Recent applications of fiber optic sensors to health monitoring in civil engineering

Structural Health Monitoring (SHM) can be understood as the integration of sensing and intelligence to enable the structure loading and damage-provoking conditions to be recorded, analyzed, localized, and predicted in such a way that nondestructive testing becomes an integral part of them. In addition, SHM systems can include actuation devices to take proper reaction or correction actions. SHM sensing requirements are very well suited for the application of optical fiber sensors (OFS), in particular, to provide integrated, quasi-distributed or fully distributed technologies. In this tutorial, after a brief introduction of the basic SHM concepts, the main fiber optic techniques available for this application are reviewed, emphasizing the four most successful ones. Then, several examples of the use of OFS in real structures are also addressed, including those from the renewable energy, transportation, civil engineering and the oil and gas industry sectors. Finally, the most relevant current technical challenges and the key sector markets are identified. This paper provides a tutorial introduction, a comprehensive background on this subject and also a forecast of the future of OFS for SHM. In addition, some of the challenges to be faced in the near future are addressed.

Fiber Optic Sensors in Structural Health Monitoring

This paper reviews the development of energy harvesting for low-power embedded structural health monitoring (SHM) sensing systems. A statistical pattern recognition paradigm for SHM is first presented and the concept of energy harvesting for embedded sensing systems is addressed with respect to the data acquisition portion of this paradigm. Next, various existing and emerging sensing modalities used for SHM and their respective power requirements are summarized followed by a discussion of SHM sensor network paradigms, power requirements for these networks, and power optimization strategies. Various approaches to energy harvesting and energy storage are discussed and limitations associated with the current technology are addressed. The paper concludes by defining some future research directions that are aimed at transitioning the concept of energy harvesting for embedded SHM sensing systems from laboratory research to field-deployed engineering prototypes. Finally, it is noted that many of the technologies discussed herein are applicable to powering any type of low-power embedded sensing system regardless of the application.

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Energy Harvesting for Structural Health Monitoring Sensor Networks

Sensing via fiber optics has occupied R&D groups for over 40 years, and some important transitions into the commercial sector have been achieved. We look at the fundamental concepts involved in the various sensing approaches, and the differentiators which have led to commercial impact. We also look to the future of fiber-optic sensors.

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Fiber-Optic Sensing: A Historical Perspective

Fiber optic sensor technology has wide-ranging potential in sensing and measurement and the aim of this paper is to explore the factors that determine this potential. The basic principles of fiber sensing are well established and share much with fiber communications. However, the implementation technologies can be radically different and the market characteristics contrast totally with the communications sector. We explore these issues in the context of applications in structural monitoring, electromagnetic measurements, and environmental sensing. We also present very brief accounts of underlying principles and a short discussion concerning evolving technologies for fiber sensing.

Optical fiber sensor technologies: opportunities and-perhaps-pitfalls

This work describes a new technique for fibre Bragg grating (FBG) sensor interrogation and multiplexing. The technique combines a scanning Fabry-Perot (SFP) bandpass filter used to wavelength-multiplex multiple gratings in a single fibre, and an unbalanced Mach-Zehnder fibre interferometer made with a 3×3 coupler to detect strain-induced wavelength shifts. A passive technique for interferometer drift compensation using non-sensing FBGs is included in the system. A prototype complete system interrogates four gratings in a single fibre at a Nyquist sampling rate up to 10 kHz, with a noise floor measured near 4 n Hz-1/2 above 0.1 Hz. The inclusion of the interferometer drift compensation technique is shown to make quasi-static measurements feasible.

https://iopscience.iop.org/article/10.1088/0957-0233/12/7/303/pdf

A novel Bragg grating sensor interrogation system utilizing a scanning filter, a Mach-Zehnder interferometer and a 3×3 coupler

A new optical sensing device containing fiber Bragg gratings (26), a scanning bandpass filter (28), an interferometer (40) and multiple photodetectors (44, 46, 48) is disclosed. The present invention also describes a new system and method for fibre Bragg grating sensor interrogation and multiplexing. The new system combines a scanning Fabry-Perot bandpass filter used to wavelength-multiplex multiple gratings in a single fiber, and an unbalanced Mach-Zehnder fibre interferometer made with a 3x3 coupler to detect strain-induced wavelength shifts.

Optical sensing device containing fiber bragg gratings

We present a new technique for fiber Bragg grating (FBG) sensor interrogation and multiplexing. The technique combines a scanning bandpass filter used to multiplex by wavelength multiple gratings in a single fiber, and an unbalanced Mach-Zehnder fiber interferometer made with a 3/spl times/3 coupler to detect strain-induced wavelength shifts. A demonstration system interrogates four gratings in a single fiber at a sampling rate up to 20 kHz, with a noise floor measured at less than 10 n/spl epsiv///spl radic/(Hz) above 0.1 Hz.

Fiber Bragg grating interrogation and multiplexing with a 3/spl times/3 coupler and a scanning filter

The invention includes a method and apparatus for monitoring wavelength of an optical channel. A tunable optical bandpass filter is used to correlate a signal from an optical transmission line with a signal from a reference light source. The signal from the reference light source passes through a reference gas cell. The absorption lines of the reference gas provide a known calibration standard against which measured values may be compared. The invention finds applications in optical transmission systems, including wavelength division multiplexed and dense wavelength division multiplexed (WDM and DWDM) optical communication systems.

Optical channel monitor with continuous gas cell calibration

Introduction: An interesting and increasingly popular method of building steel and concrete composite bridges involves the use of a large steel based structure called a box-girder. These massive structures, which have dimensions on the order of 5m x 5m x 150m with wall thickness sometimes exceeding 5 cm, are assembled by welding together many shorter (e.g. 5m x 5m x 5 m) box-girder pieces near a bridge building site. The massive and long box-girder structure thus created, is then ‘pushed’ from one end of the bridge pier to the other. A concrete deck is eventually built on top of the box-girder to complete the bridge structure. This bridge building approach apparently holds advantages in the sense that it requires far fewer on-site workers and also allows for rapid building of large scale bridges over difficult terrain. One of the more interesting (and potentially dangerous) phases during the construction process of a box-girder bridges is the so-called ‘push’ phase. During this phase, the long box-girder structure, which prior to the push rests on a pier closest to land, is literally pushed from one bridge pile-on to the next, requiring it to travel a distance of over 100 meters. The far pile-on typically is built in a ravine or a deep valley over which the bridge is being built. During the push, the steel box-girder structure is subjected to static strains at various locations, depending upon the position of the box-girder at any given time. For instance, the far end of the box girder would be subject to cantilever motion, which may be driven into oscillations due to winds in the ravine. Monitoring the strain at various locations in the box-girder during the push phase of the bridge construction has potential benefits in future infra-structure modeling efforts as well as in determining construction safety factors. In addition, utilizing enabling technologies (e.g. fiber Bragg grating strain sensors) for strain monitoring during an actual construction phase of a real world structure allows determination of the maturity of: (i) the instrumentation system, (ii) sensor packaging, and (iii) installation approaches [1]. This work reports the results of a 32 element fiber Bragg grating (FBG) array used to monitor strain at various locations in box-girder vaux viaduct being constructed near Lausanne, Switzerland. The entire push phase of the box-girder bridge, which took about a day, was monitored using several FBGs whose responses were interrogated using the scanning FabryPerot (FP) method [2]. Preliminary quasi-static strain sensing data shows fine as well as gross details of the push phase. The sensors are planned to operate during many phases of the boxgirder bridge construction including the push, the concrete deck pouring, and eventually traffic monitoring. We report here on the push phase only.

Bryan Althouse

Papers

A novel Bragg grating sensor interrogation system utilizing a scanning filter, a Mach-Zehnder interferometer and a 3×3 coupler

Optical sensing device containing fiber bragg gratings

Fiber Bragg grating interrogation and multiplexing with a 3/spl times/3 coupler and a scanning filter

Optical channel monitor with continuous gas cell calibration

Quasi-static strain monitoring during the push phase of a box-girder bridge using fiber Bragg grating sensors