This article presents an integrated formulation for the calculation of the spectral response of a fiber Bragg grating sensor embedded in a host material system, as a function of the loading applied to the host structure. In particular, the calculation of the transverse strain sensitivity of a fiber Bragg grating sensor through the calculation of the change in effective index (or indices) of refraction of the fiber cross-section due to the applied load is presented in detail. For the calculation of the fiber propagation constants, a two-step finite element formulation is used moeling the optical, geometric and material properties of the cross-section. Once the propagation constants and principle optical axes are known along the fiber, a modified transfer matrix method is applied to calculate the spectral response of the FBG. It is shown that the inclusion of the change in index of refraction throughout the cross-section yields close agreement with previous methods. However the current method provides the potential to evaluate the effects of high strain gradients across the optical fiber core for some loading applications.

A carbon composite strain sensing system using an embedded array of polymide-coated fiber Bragg grating sensors, which are interrogated by a tunable laser, is described. The system is controlled from a personal computer using a customized program. The system was tested using a composite panel consisting of 28 plies of carbon fiber. Consistent strain measurements were achieved using a laser scan resolution of 0.02 nm, which corresponds to a strain resolution of approximately 15 me. Simulations of the panel were carried out using the Marc/Mentat finite element program for advanced engineering analysis. The experimental results compared well to those obtained from the simulations.

A two-wave mixing (TWM) wavelength demodulator using InP:Fe photorefractive crystal (PRC) in the C-band (1530-1570nm) is demonstrated. The system can be used as a wavelength demodulator for use with Fiber Bragg Grating (FBG) sensors to monitor dynamic strains. In this configuration, the FBG is illuminated with a broadband source, and any strain in the FBG is encoded as a wavelength shift of the light reflected by the FBG. The reflected light from the FBG is spilt into two unbalanced paths and both beams (pump and signal) mix in the PRC. Any wavelength shift of the reflected light results in an equivalent phase shift between the pump and signal beams as they travel unbalanced path lengths. Since TWM is an adaptive process, the two interfering beams are naturally in quadrature and remain in quadrature even in the presence of large quasi-static strains. We demonstrate that FBG demodulation using TWM has the ability to selectively monitor dynamic strains without the need for active compensation of large quasi-static strains that otherwise would cause the FBG sensor to drift. As TWM interferometers can be readily multiplexed at relatively low cost; the proposed technique can be used to demodulate signals from a network of FBG sensors for use in structural health monitoring.

We present a new type of fiber Bragg grating (FBG) in which we etch the grating into the flat surface of a D-shaped optical fiber. Instead of being written in the core of the fiber, as are standard FBGs, these surface relief fiber Bragg gratings (SR-FBGs) are placed in the cladding above the core. These gratings are a viable alternative to standard FBGs for sensing applications. In this work we describe the fabrication process for etching Bragg gratings into the surface of D-fibers and demonstrate their performance as temperature sensors. We show that SR-FBGs resist much higher temperatures than standard FBGs by demonstrating their operation up to 1100 degrees Celsius.

Fiber Bragg gratings (FBG) have become preferred sensory structures in fiber optic sensing system. High sensitivity, embedability, and multiplexing capabilities make FBGs superior to other sensor configurations. The main feature of FBGs is that they respond in the wavelength domain with the wavelength of the returned signal as the indicator of the measured parameter. The wavelength is then converted to optical intensity by a photodetector to detect corresponding changes in intensity. This wavelength-to-intensity conversion is a crucial part in any FBG-based sensing system. Among the various types of wavelength-to-intensity converters, unbalanced interferometers are especially attractive be-cause of their small weight and volume, lack of moving parts, easy integration, and good stability. In this paper we investigate the applicability of unbalanced interferometers to analyze signals reflected from Bragg gratings. Analytical and experimental data are presented.

In this paper, we report the development of a new bonding agent and method for the surface mounting of optical fiber Bragg grating strain and temperature sensors for use in harsh environments. The compound is based on a combination of ceramic fillers with an epoxy binder that is applied with a brush technique. Samples of optical fiber Bragg gratings were successfully encapsulated and mounted on metal shims. The packaged sensors were tested for strain (+/- 1000µε) and temperature (-20 to +120 °C) response. The encapsulated sensors display a linear response with an increase in the temperature sensitivity of the FBG, with a factor of 24.37pm/°C, and a strain gauge factor of 1.25pm/με.

We have been studying optical sensing technologies that use fiber Bragg gratings (FBGs) for health monitoring of aircraft structures made of carbon fiber reinforced plastic (CFRP) composite materials. The sensing system is composed of a piezoelectric transducer (PZT) actuator, which generates an elastic wave of several hundred kHz, and FBG sensors that receive the elastic wave. When some damage occurs in the composite materials, the elastic wave that propagates through those materials changes. Therefore the damage can be detected by analyzing the elastic waveform to be received by FBG sensors. For detecting this wave, we developed a high-speed optical wavelength interrogator for FBG sensors, and FBG sensor modules that can be embedded in the composite materials. In this interrogator, we employed an arrayed waveguide grating (AWG) as an optical filter that can convert the wavelength shift of the FBG sensors into optical power change. Using this interrogator and FBG sensor modules, we detected elastic waves of 300 kHz in frequency. We determined the required characteristics of FBG sensor both through simulation and experiments for improving the sensitivity of this health monitoring system.

An optical fiber sensor to simultaneously measure strain and temperature was designed and embedded into an adaptive composite laminate which exhibits a shape change upon thermal activation. The sensor is formed by two fiber Bragg gratings, which are written in optical fibers with different core dopants. The two gratings were spliced close to each other and a sensing element resulted with Bragg gratings of similar strain sensitivity but different response to temperature. This is due to the dependence of the fiber thermo-optic coefficient on core dopants and relative concentrations. The sensor was tested on an adaptive composite laminate made of unidirectional Kevlar-epoxy pre-preg plies. Several 150μm diameter pre-strained NiTiCu shape memory alloy wires were embedded in the composite laminate together with one fiber sensor. Simultaneous monitoring of strain and temperature during the curing process and activation in an oven was demonstrated.

Long-gauge SOFO sensors have been in use for the last 10 years for the monitoring of civil, geotechnical, oil & Fiber optic sensing systems are increasingly recognized as a very attractive choice for structural health monitoring. Moving form demonstration project to industrial applications requires an integrated approach where the most appropriate technologies are combined to meet the user's requirements. In this context it is often necessary and desirable to combine different sensing technologies in the same project. A bridge-monitoring project might for example require long-gauge interferometric sensors to monitor the concrete deck, interferometric inclinometers for the piles and fiber Bragg grating sensors for the monitoring of the strains in the steel beams and for measuring temperatures. Although fiber optic sensors relying on different technologies can easily be combined at the packaging and cable levels, they often require dedicated instruments to be demodulated. A unified demodulation system would therefore be very attractive. This paper describes a technique relying on the analysis of reflected spectra and allowing the demodulation of interferometric (Michelson or Faby-Perot) sensors and fiber Bragg grating sensors with a single measurement system. It also compares the obtained performance in terms of resolution and dynamic range with the available dedicated systems.

Fiber Bragg gratings (FBGs) can provide extremely sensitive strain measurements for various materials and structures. The main functionality of the Bragg grating is along the fiber's main axis, where changes in the grating's spacing can be converted into strain measurements. Previous work from a number of researchers has identified bifurcation and broadening of the Bragg signal under transverse loading. The work presented in this paper highlights efforts to relate transverse loading to changes in index of refraction in the fiber core cross section, and then ultimately to predicted changes in Bragg signals. The modeling work completed provides a useful tool in predicting effects on FBGs of potential transverse loading scenarios, whether these effects are undesirable, or sought after. Analytical and numerical analyses are presented as well as a background of interest in loading effects and birefringence with respect to Bragg grating applications.

Arrays of multi-axis fiber grating strain sensors have been integrated into a composite pressure vessel test article, and are used to monitor changes in the transverse and axial strain fields during curing and pressure cycling near cut tow and Teflon tape defects. These changes in the multi-axis strain due to four pressure cycles and repeated impacts are measured and compared to ultrasonic and eddy current scans. Examples of the remote detection of damage using transverse strain and transverse strain gradients is given as well as data showing the ability of the system to distinguish broken tow and delamination defects.

This paper presents a part of the study conducted for developing a damage diagnostic system for an advanced composite material that can be utilized in next-generation aircraft structure. The authors have been working on a detection of elastic wave which can be launched from the PZT actuators, using small- and normal-diameter FBG optical fiber sensors that are bonded to the surface of the CFRP laminate under different conditions. Based on the results, it was verified that it is possible to achieve a high-accuracy detection of elastic wave by using FBG sensors bonded to the surface of the CFRP laminate. It was also verified that the damages generated on the inside of the composite material may be detected by the waveform analysis of the received elastic wave. In this study, the authors succeeded in the embedment of small-diameter FBG optical fiber sensors into the bonding surface of the double-lap type coupon specimen, which simulates the bonding structure of the CFRP composite structure. In this study, we also clarified several issues pertaining to the conditions, methods, and techniques involved in fiber embedding. An optical loss was observed during the embedment process, which may result in the loss of both accuracy and reliability. Based on these observations, the authors developed embedding techniques for optical fiber sensors that can reduce this optical loss. Additionally, the possibility of detecting an elastic wave, which was launched from the PZT actuators bonded to the surface of the coupon and directed to the host material, was verified using double-lap type coupon specimen having embedded small-diameter FBG optical fiber sensors at the bonding surface. Therefore, this specimen has provided an artificial defect that simulates the delamination generated at the bonding interface. Based on the measurements of the elastic wave, it was verified that the change in the elastic wave depends on the damage length, which is caused by the artificial defect. Moreover, based on the analysis of the received elastic wave, the possibility of damage detection was confirmed. The successful development of this damage monitoring system would ease the implementation of structural health monitoring system in aircraft structures in the near future.

In this research, the authors developed various detection techniques for particular damages, such as debonding and impact damage, in sandwich panels consisting of composite face-sheets and aluminum honeycomb core with small-diameter optical fiber sensors. First, two methods for debonding detection were established taking advantage of the behavior of fillets formed on the adhesive layer between the core and the skin. One method uses the fracture of optical fibers, and the other one uses the shape recovery of the reflection spectrum from a fiber Bragg grating (FBG) sensor because of the release of thermal residual stress in the fillets. Secondly, as for impact damages, chirped FBG sensors were applied to monitor the change in strain distribution of the face-sheet due to the dent caused by the impact loadings. Furthermore, a newly developed MEMS-optical spectrum analyzer (MEMS-OSA) was introduced to identification of impact points and damages. This system could measure the reflection spectrum at very high speed. The change in the form of the reflection spectrum during the impact loading was found to be different depending on the impact energy and the impact location, and this tendency was confirmed by theoretical simulations using the change in the strain distribution obtained by foil strain gauges. These results show that the high speed measurement of the reflection spectrum by MEMS-OSA has a potential to identify the impact location and damage magnitude through the comparison with theoretical simulations.

A sensor network consisting of unified fiber grating based temperature and strain sensor pads has been developed for integration in carbon/aluminum composite current collector strips, and has been tested in electrical trains on commercial railways. The fiber optic sensor network measures value and position of both con-tact forces and impacts under real-time conditions, immediately at the high voltage location of the interface between overhead contact line and current collector.

Optical Fibre Bragg Grating (FBG) sensors are excellent non-destructive tools for internal strain characterization of composite materials and structures. They can be embedded at selected locations during material preparation to provide accurate in-situ measurements. In this study, long-gauge-FBGs are introduced in cylindrical specimens of epoxy. This configuration is particularly attractive because it simplifies the study of some relevant phenomena in micromechanics of composites, for instance residual stresses and fracture of the fibre-matrix interface. Because the matrix epoxy shrinks during the polymerisation process, the optical sensor undergoes substantial non-uniform strain along the fibre. The response of a FBG to a non-uniform strain distribution is investigated using a new Optical Low-Coherence Reflectometry (OLCR) technique developed at EPFL. This method provides a direct reconstruction of the optical period and the corresponding strain distribution along the grating without any a priori assumption about the strain field. Considering the non-uniform residual strain as a reference state, new Bragg wavelength distributions are obtained for two configurations. First, a new Bragg wavelength distribution is measured as a function of the depth of circular cracks machined in the radial direction. These measurements lead to the knowledge of (a) the zone of perturbation of the reinforcing fibre on the residual stresses and (b) the effect of the presence of the mechanically induced crack on the residual stress state in the specimen. A finite element modelling of the residual stress field based on an equivalent thermo-elastic approach is also proposed, showing a very good agreement with experimental data. Second, an interface crack (debonding) between the epoxy and the fibre is introduced by fatigue and monitored using a specifically designed video acquisition system. The induced variations in the FBG response are measured when the fibre is unloaded and then subjected to an axial static load. As preliminary results, a debonding length comparable to the one observed by the video system is found from the Bragg wavelength distribution. Moreover, for the two cases, the measurements clearly indicate that the fibre is in tension in the debonding region while compressive stresses (due to the matrix shrinkage) prevail in the intact part of the specimen.

The authors are constructing a damage detection system using ultrasonic waves. In this system, a piezo-ceramic actuator generates Lamb waves in a CFRP laminate. After the waves propagate in the laminate, transmitted waves are received by a fiber Bragg grating (FBG) sensor attached on the laminate using a newly developed high-speed optical wavelength interrogation system. At first, the optimal gauge length of the FBG to detect ultrasonic waves was investigated through theoretical simulations and experiments. Then, the directional sensitivity of the FBG to ultrasonic waves was evaluated experimentally. On the basis of the above results, the 1mm FBG sensors were applied to the detection of Lamb waves propagated in carbon fiber reinforced plastic (CFRP) cross-ply laminates. The piezo-actuator was put on the laminate about 50mm away from the FBG sensor glued on the laminate, and three-cycle sine waves of 300kHz were excited repeatedly. The waveforms obtained by the FBG showed that S0 and A0 modes could be detected appropriately. Then, artificial delamination was made in the laminate by removing of a Teflon sheet embedded in the 0/90 interface after the manufacturing. When the Lamb waves passed through the delamination, the amplitude decreased and a new wave mode appeared. These phenomena could be well simulated using a finite element method. Furthermore, since the amplitude and the velocity of the new mode increased with an increase in the delamination length, this system has a potential to evaluate the interlaminar delamination length quantitatively.

The necessity for Airplane structural health-monitoring technology has been increasing because of improvement of reliability and cost saving. Optical fiber sensor system is an attractive method for structural health monitoring, because of its lightweight, non-electromagnetic interference, and to be embeddable to composite structures. Especially the distributed optical fiber sensor fits the health monitoring for large-sized structures. However, the distributed optical fiber measurement system using the pulse light represented by BOTDR has low spatial resolution and long measurement interval. These performances have been the obstacle of application to airplane structure health monitoring system. Then, the authors have proposed the Brillouin optical frequency modulation method for improvement of the spatial resolution and shortening of measurement intervals. In this work, we conducted basic approach in order to develop Brillouin Optical Frequency Domain Analysis (BOFDA) measurement system, such as pump power property and frequency modulation property for Brillouin stimulated light. We confirmed ability to measure stimulated Brillouin Scattering light in 50mm section. Moreover, We considered the optical fiber sensor installation issue on the airplane structure. The issue is optical fiber sensor birefringence under asymmetric load and durability of installation method. We conducted two confirmatory tests for the issues. The proposed installation method has adequate performance. From these results, it was confirmed that BOFDA system has potential to be applied to an airplane structure health monitoring system.

The paper discusses the instrumentation of leak detection and location on underground water supply pipelines based on adaptive algorithms. The equipment is designed to be self-contained and portably mobile with sensors, signal conditioning and data acquisition units and a laptop PC operating signal processing, information analysis and instrument control. The vibrational acoustic signals collected on tubular pipelines are used for pinpointing a leak or leaks in buried pipelines. Because of the complexity and the heavy corruption with ambient noises, it is essential to use an appropriate signal model and the scheme of analysis and synthesis in order to extract leak signatures and specify leak locations. In addition, the features of vibrational acoustic signals vary with the materials, the sizes and the inbuilt conditions of tubular pipes. It is difficult to pre-determine the knowledge of signals, such as the spectral knowledge. Here, an adaptive detection and estimation strategy is proposed based on LMS adaptive filtering and modified Wavelet denoising. The availability for leak detection and pinpointing estimation is automatically analyzed by the signal processing procedure without any prerequisite on signals and the procedure may adaptively find the way to get better estimate.

With the augmented use of high performance composite materials in critical structures, it has become increasingly important for 'smart' systems to monitor these materials and provide rapid evaluation. Using fiber Bragg gratings embedded into the weave structure of carbon fiber epoxy composites allow the capability to monitor these composites during manufacture, cure, general aging, and damage. Fiber optic sensors allow greater insight into damage progression and can be used to verify analytical models. This paper emphasizes the results of recent work in which multiple arrays of Bragg gratings were wound into composite vessels and monitored while the part was damaged. Based on the response of these sensors, algorithms were developed to identify the location of damage impacts. Results were verified against eddy current and ultrasonic NDE methods.

In this work we present a successful non-contact ultrasound laser generation and detection system for the extraction of the structural properties on mechanical structures. The system uses a Q-switch Nd:YAG short pulse high power laser to generate a broadband source of Lamb waves that propagate along the plate, interacting with the structure’s entire thickness. A modified Michelson surface displacement optical fibre interferometer is used for the detection of the stress and strain waves. In order to extract the structural information stored in the generated and detected waves we present two completely different signal processing tools; the reassigned spectrogram as a time-frequency analysis and the two dimensional Fourier transform. We compare these two techniques and extract interesting conclusions of their properties. Finally we apply these two techniques and the developed system to temperature change sensitivity and damage detection applications.

This paper presents a new method for identifying impact events using a built-in diagnostic technique. The identification includes the impact location and force time history. The proposed technique does not rely on a mathematical model to predict the behavior of a structure: instead system transfer functions are obtained from experimental tests. An experimental system identification technique was used to establish system transfer functions capable of reproducing the system's physical behavior. This technique simplifies the monitoring procedure not only because there is no need for a complex structure model, but also because there is no need of the evaluating procedure for sensors to get accurate strain/stress values. This paper summarizes the monitoring technique and presents examples from several impact tests on complicated structures including stiffened panels.

NASA's recent space programs require the use of many ultra-lightweight deployable/inflatable structures, but there are many modeling, analysis and testing problems. A total-Lagrangian finite element code GESA (Geometrically Exact Structural Analysis) is being developed by the authors by implementing geometrically exact structural theories for the modeling, analysis, and fast prototyping of Highly Flexible Structures (HFSs). However, it is very challenging to experimentally verify the large static and dynamic deformations of such lightweight HFSs by using conventional sensors. This paper presents the use of an EAGLE-500 real-time motion analysis system to measure large static and dynamic deformations of HFSs in order to verify the accuracy of numerical solutions obtained from GESA. Experimental results obtained from several highly flexible structures are used to show the capabilities of this system in characterizing nonlinear statics and dynamics of HFSs.

The work presented relates to the design and construction of an inexpensive distributive load sensor. It is to be used for impact tests on samples which are in the form of flexible or deformable beams of a considerable length. The sensor compromises of forty fingers made of steel, arranged next to each other and covering a total length of about a meter. Both ends of each finger are clamped. PZT ceramic patches, which are bonded to the bottom surfaces of each finger, are used to convert the impact response into an electrical signal. An amplifier and filter were designed for each finger. The frequency range over which each finger operates is extended by the use of a Butterworth filter. An amplifier box was built containing the charge amplifier circuitry and filter of each of the forty fingers comprising the distributive sensor. Tests are performed on the distributive sensor in order to show that this simple and inexpensive distributive sensor is effective. The results are presented and discussed.

Phase modulated fiber-optic sensors based on Fabry-Perot interferometers have been widely used in many applications. In this paper, a novel digital phase-stepping demodulation technique is developed, which is based on combining the principle of low coherence fiber-optic interferometry (LCFOI) and phase-measurement interferometry (PMI). An integrated optical circuit (IOC) phase modulator that enables a high frequency modulation is used to acquire multi-step phase shifts that are applicable to each sensing interferometer in a spatial division multiplexing system. The phase retrieval algorithm along with phase unwrapping technique is detailed and phase error associated with this technique is discussed. A multichannel acoustic measurement system was demonstrated using this technique. Experimental results show that this technique is especially useful for Fabry-Perot sensors with sensor cavity length in micrometer range and sensor bandwidth requirements in kHz to MHz range.

Fiber optic sensor technology offers the possibility of implementing low weight, high performance and cost effective health and damage assessment for infrastructure elements. Common fiber sensors are based on the effect of external action on the spectral response of a Fabry-Perot or a Bragg grating section, or on the modal dynamics in multimode (MM) fiber. In the latter case, the fiber itself acts as the sensor, giving it the potential for large range coverage. We were interested in this type of sensor because of its cost advantage in monitoring structural health. In the course of the research, a new type of a rugged modal filter device, based on off-center splicing, was developed. This device, in combination with a MM fiber, was found to be a potential single point-pressure sensing device. Additionally, by translating the pressing point along a MM sensing fiber with a constant load and speed, a sinusoidal intensity modulation was observed. This harmonic behavior, during load translation, is explained by the theory of mode coupling and dispersion. The oscillation period, L~0.43. mm, obtained at 980 nm in a Corning SMF-28 fiber, corresponds to the wavevector difference, Db, between the two-coupled modes, by L = 2p/Db. An additional outcome of the present research is the observation that the response of the loaded MM fiber is strongly dependent on the polarization state of the light traveling along the MM fiber due to different response of the modes to polarization active elements. Our main conclusions are that in MM fiber optic sensor design, special cautions need to be taken in order to stabilize the system, and that the sensitivity along a MM fiber sensor is periodic with a period of ~ 0.4 - 0.5 mm, depending on various fiber parameters and excited modes.

Geotechnical measurements are producing various data which are used for interpretation and e.g. safety calculation. The reliability of such data is of most importance as every decision on civil engineering action needed is based on this data. Known and also unknown influences changing data are a basic demand for deeper investigation and research. In present time we have limited tools only to minimize perturbing influences. One of these demands-the long-term development of data also called long-term stability-is described in this paper. The paper describes a sensor head for long-term high-precision measurements of very small deflections of a diaphragm used for pressure gauges. High precision deformation measurement is assured by using a fiber Fabry-Perot interferometer sensor; identification of zero-point changes, and thus, long-term stable measurement is achieved by a specially designed absolute interferometer sensor. Several fiber optic solutions based on fiber Fabry-Perot technique have been investigated to find out a reliable sensor design. The presented sensor design has reached prototype status and allows to measure unambiguously static deformations with high precision. In order to evaluate repeatability and possible changes of zero-point reference if the head has been disconnected, validation of the described pressure gauge has been started. This validation work includes calibration and enables to evaluate possible drift effects, and to identify mechanical or thermal hysteresis. Thus, the highlight in this paper is the observation and measurement of zero-point development over time.

We report the fabrication of a diffraction grating cast into a responsive hydrogel using a silicone rubber intermediate cast from an engraved glass master grating. The aim to investigate if changes in the swelling of this gel in response to changes in the concentration of a specific analyte led to changes in the line spacing, and hence diffraction pattern, of the grating. The protocol for casting gratings was initially developed using a composite carboxymethyl dextran/bovine serum albumin gel produced using carbodiimide chemistry to assess the optimum gel properties required. Examination under a light microscope showed that, formed under appropriate synthesis conditions, CM-dextran-BSA hydrogels retained the grating structure and appeared to have similar optical properties to the silicon rubber sub-master used for casting. For a facile initial evaluation of the detection principle a number of similar gels were produced using cross-linked alginic acid. In this case excess carboxylic groups remaining after cross-linking were able to form additional ionic cross-links in the presence of divalent cations (Ca²⁺). Test experiments with these gels showed that both the size and position of the reflected and refracted spots obtained from illumination with a Helium-Neon laser changed as gel swelling changed with calcium ion concentration i.e. the size of both diffraction and reflection spots increased as the alginate hydrogel shrank in response to changes in environmental Ca²⁺. The utility of the alginate based gels for the detection of cations, together with evidence that dextran protein gels can retain grating structures, suggest that this assay procedure should applicable to any hydrogel where the response is based on protein-ligand interactions. The key requirement is that the cross-linking interactions constraining gel swelling can be quantitatively displaced by the analyte acting as a specific competitor.

ENSCO, Inc has identified a need for airborne in-situ measurements of meteorological parameters such as temperature, humidity, pressure, wind velocity, as well as biological and chemical agents. In this paper, we describe one approach for the development of such a probe using advances in materials, microelectromechanical systems, and nanotechnology. We present the technology roadmap for creating an Observational Roving Body (ORB) and discuss power generation / storage, communications, networking, sensing, and aerodynamic design associated with development of airborne probes for in-situ measurements.

Corrosion in steel-reinforced concrete structures is a critical issue. Corrosion appears if the pH value of the concrete matrix decreases due to deterioration of the calcium hydroxide layer on the steel surface. At present, several reliable systems for determination of chemical parameters in aggressive environments are available on the market, but can not be used for long-term monitoring of pH in concrete structures. This paper describes the development of a fiber optic chemical sensor for this purpose. Particular attention is paid to the requirements on such a sensing system. Usually applied methods of fiber optical chemical sensing were investigated and compared by using several pH-sensitive materials. Based on these results, a functional pH sensor has been configured. It shows good response behavior and works under strongly alkaline conditions for one year. Therefore, it represents a promising sensor type for in-situ long-term monitoring in concrete structures. Further work is in progress to test such sensors on-site under real application conditions, e.g. in ground anchors.

Wearable sensing networks have been the focus of the robotics and biotechnology industry for a number of years. While there has been quite a bit of work on sensor technologies, the physical integration of the electronic components with the human body has not received much attention. We have created a body area network that seeks to address this issue by relying on two innovations; the use of conductive fabrics, and the use of DC powerline communication. By combining these innovations, we have created a truly wearable network that allows full generality of sensor location, spatial distribution of the medium to reduce overall bulk, and maintains sufficiently low line impedance for simultaneous power and data delivery over a single conductor. We have created a method for analysis of the transmission properties of conductive fabric garments that takes into account the unique geometry of the human body. We will provide a verification of our analysis method experimental results.

A sensing approach, based on resonance frequency shifts of an oscillating micro- or nano- cantilever, can potentially provide ultimate sensitivity for detection of a single molecule. However, implementation of this sensing technology on a micro-scale has intrinsic limitations: The quality, Q, of an oscillating microcantilever vibrating in air is approximately in the 30-100 range and this value dramatically drops in a liquid environment. Feedback control of the oscillations can improve the quality of the system but multiple challenges are encountered with the sensing and actuation. Traditional data acquisition approaches, which include optical, piezoresistance, piezoelectric and capacitance methods, have very limited application in signal transduction from micro- or nano- cantilever beams. In addition, electrostatic and thermal actuations are not appropriate for liquid environments. A novel approach, utilizing the self-sensing and self-actuation response of electroactive materials is proposed for control of cantilever beam vibration. As far as sensing is concerned, we exploit the fact that any dielectric material exhibits dielectrostriction effect; this is defined as variation of dielectric properties of the material with deformation. Similarly, on the actuation side electrostriction response can also be used. In this work, control challenges and approaches for such nonlinear systems with self-sensing and self-actuation capabilities will be discussed.

Musca domestica, the common house fly, has a simple yet powerful and accessible vision system. Cajal indicated in 1885 the fly's vision system is the same as in the human retina. The house fly has some intriguing vision system features such as fast, analog, parallel operation. Furthermore, it has the ability to detect movement and objects at far better resolution than predicted by photoreceptor spacing, termed hyperacuity. We are investigating the mechanisms behind these features and incorporating them into next generation vision systems. We have developed a prototype sensor that employs a fly inspired arrangement of photodetectors sharing a common lens. The Gaussian shaped acceptance profile of each sensor coupled with overlapped sensor field of views provide the necessary configuration for obtaining hyperacuity data. The sensor is able to detect object movement with far greater resolution than that predicted by photoreceptor spacing. We have exhaustively tested and characterized the sensor to determine its practical resolution limit. Our tests coupled with theory from Bucklew and Saleh (1985) indicate that the limit to the hyperacuity response may only be related to target contrast. We have also implemented an array of these prototype sensors which will allow for two - dimensional position location. These high resolution, low contrast capable sensors are being developed for use as a vision system for an autonomous robot and the next generation of smart wheel chairs. However, they are easily adapted for biological endoscopy, downhole monitoring in oil wells, and other applications.

Previous experiences during earthquake events emphasize the need for new technologies for real-time monitoring and assessment of facilities with high value nonstructural elements such as equipment or other contents. Moreover, there are substantial limitations to our ability to rapidly evaluate and identify potential hazard zones within a structure, exposing rescue workers, society and the environment to unnecessary risks. A real-time monitoring system, integrated with critical warning systems, would allow for improved channeling of resources. Ideally such a system would acquire all relevant data non-intrusively, at high rates and resolution and disseminate it with low latency over a trusted network to a central repository. This repository can then be used by the building owner and rescue workers to make informed decisions. In recognition of these issues, in this paper, we describe a methodology for image-based tracking of seismically induced motions. The methodology includes calibration, acquisition, processing, and analysis tools geared towards seismic assessment. We present sample waveforms extracted considering pixel-based algorithms applied to images collected from an array of high speed, high-resolution charged-couple-device (CCD) cameras. This work includes use of a unique hardware and software design involving a multi-threaded process, which bypasses conventional hardware frame grabbers and uses a software-based approach to acquire, synchronize and time stamp image data.

We report recent improvements of Metal RubberTM strain sensors formed by electrostatic self-assembly (ESA) processing. The sensors may be used to measure strains from approximately 1 microstrain to several hundred percent strain, over gauge lengths ranging from approximately 1 millimeter to several tens of centimeters.

Nondestructive damage sensing and load transfer mechanisms of thermal treated carbon nanotube (CNT) and nanofiber (CNF)/epoxy composites were investigated using electro-micromechanical technique. Carbon black (CB) was used only for the comparison. Electro-micromechanical techniques were applied to obtain the fiber damage and stress transferring effect of carbon nanocomposites with their contents. Thermal treatment and temperature affected on apparent modulus and electrical properties on nanocomposites due to enhanced inherent properties of each CNMs. Coefficient of variation (COV) of volumetric electrical resistance can be used to obtain the dispersion degree indirectly for various CNMs. Dispersion and surface modification are very important parameters to obtain improved mechanical and electrical properties of CNMs for multifunctional applications. Further optimized functionalization and dispersion conditions will be investigated for the following work continuously.

A simple and effective method to measure an electromagnetic wave polarization plane's weak rotations in various media is proposed. The specific features of the polarization plane’s rotation amplification (PPRA) for the light reflection and transmission through the absorbing and amplifying anisotropic layers are calculated. The amplification effect for an isotropic and cholesteric liquid crystal (CLC) layers are also considered. In conclusion the question of probable choice of the amplifier's noise/signal ratio is discussed.

A ferroelectric based multiple radiation source is being developed and tested at JPL. This device has the potential to emit five radiation types enabling a new generation of compact, low power, low mass in-situ analysis instruments. These radiation types include visible light, ultraviolet, X-ray, as well as electron and ion beams. These types of emitted radiation can support multiple instruments that may potentially be used in future NASA missions to detect water, perform mineralogical/chemistry analysis and identify biological signatures. The source consists of a ferroelectric wafer having a continuous ground electrode on one side and a grid-shaped cathode on the other side. This source is placed in a vacuum tube and is used to generate plasma by switching high voltage pulses. A series of experiments were performed to evaluate the characteristics of the generated radiation and the results are described and discussed in this paper.

There are numerous engineering design problems where the use of wires to transfer power and communicate data thru the walls of a structure is prohibitive or significantly difficult that it may require a complex design. Such systems may be concerned with the leakage of chemicals or gasses, loss of pressure or vacuum, as well as difficulties in providing adequate thermal or electrical insulation. Moreover, feeding wires thru a wall of a structure reduces the strength of the structure and makes the structure susceptibility to cracking due to fatigue that can result from cyclic loading. Two areas have already been identified to require a wireless alternative capability and they include (a) the container of the Mars Sample Return Mission will need the use of wireless sensors to sense pressure leak and to avoid potential contamination; and (b) the Navy is seeking the capability to communicate with the crew or the instrumentation inside marine structures without the use of wires that will weaken the structure. The idea of using elastic or acoustic waves to transfer power was suggested recently by Y. Hu, et al.1. However, the disclosed model was developed directly from the wave equation and the linear equations of piezoelectricity. This model restricted by an inability to incorporate head and tail mass and account for loss in all the mechanisms. In addition there is no mechanism for connecting the model to actual power processing circuitry (diode bridge, capacitors, rectifiers etc.). An alternative approach which is to be presented is a network equivalent circuit that can easily be modified to account for additional acoustic elements and connected directly to other networks or circuits. All the possible loss mechanisms of the disclosed solution can be accounted for and introduced into the model. The circuit model allows for both power and data transmission in the forward and reverse directions through acoustic signals at the harmonic and higher order resonances. This system allows or the avoidance of cabling or wiring. The technology is applicable to the transfer of power for actuation, sensing and other tasks inside sealed containers and vacuum/pressure vessels.

In this paper, a smart wireless MEMS-based accelerometer(MA) system has been designed and experimented for smart monitoring system of civil structures. In order to estimate the performance of a smart wireless MA system(SWMAS), dynamic characteristics of our model structure need to be identified. This system thus employed a high-performance AVR microcontroller, a wireless modem, and MA for multiplex communication capability and real time duplex communication. Various performance and experimental tests have been carried out to evaluate whether this system is suitable for monitoring system of civil structures. First, we examined its sensitivity, resolution, and noise, specifically to evaluate the performance of the smart wireless MA system. The results of experiments enabled us to estimate performance of the MA in SWMAS in comparison to the value of data sheet from MA. Second, characteristics of model structure were analyzed by the ambient vibration test based on the NExT combined with ERA. Finally, this analysis was compared to the one that was made by FE results, and the comparison proved that a smart wireless MA system was fitted in smart monitoring system effectively.

This paper introduces a novel application of fiber microbending sensor to monitor the highway vehicles, i.e. overtime pull-over vehicles. Precise locations and durations of the overtime pull-over vehicles can be detected and alarms can be sent to highway administrators. Highway administrators can use these data to maintain the traffic order, secure the passengers and enforce the law. The sensor is designed based on fiber microbending effect and optical time-domain reflectometry method is utilized to generate, collect and process the optical signals. The experiment is designed to simulate the highway shoulder with vehicle parking on it. Different vehicle weight-induced fiber microbending losses are detected and measured. By the optical time domain reflectometry technique, the precise locations of pull-over vehicles have been obtained.

With increased interest in the use of compressed gas as a vehicle fuel, attention has been focussed on the safety issues surrounding the tanks used to store the fuel. Currently it is necessary to remove the tanks from the vehicle in order to inspect them, which entails a considerable cost in manpower and takes the vehicle being out of service. We have been developing a sensor scheme that can provide in situ monitoring of the tanks condition. This entails bonding optical fibre sensors to the tank and using them to measure the strains experienced by the tank during pressurisation. If the tank is significantly damaged, then the tank will expand in a distorted manner. We therefore measure the strain characteristics of a healthy tank and use them as a reference for future measurements. The method of strain measurement is the well established rf subcarrier phase detection technique, however in this application the changes in optical power caused by microbending of the fibres during pressurisation produces inaccuracies. In order to overcome this problem we use both in-phase and quadrature mixing and then take the ratio of the outputs to obtain a value of arctangent that is independent of amplitude.

Reliability is an important aspect of any sensor, and especially in terms of long term monitoring of structures. Some issues pertaining to the reliability of optical fiber sensors in civil structures are discussed in this article. The strength and fatigue properties of optical fibers influence their performance, and life span. Lessons learnt from the reliability of optical fibers in the telecommunication industry are useful for assessment of reliability in optical fiber sensors. However, optical fiber sensors go through additional manufacturing steps, handling processes, and in general operate under environmental conditions and stress levels different from the telecommunication lines. In general, optical fiber sensors in structures are subjected to fatigue loading under high stresses. Other reliability concerns pertain to the effects of the packaging, installation issues at the construction site. These issues along with some of the results acquired from fatigue tests on fiber optic Bragg gratings and long gauge interferometric sensors are discussed in this article.

Establishing and testing the performance of a monitoring system designed for long-term applications presents some challenges that do not always apply to other measurement instruments. The evolution of the main measurement parameters must be evaluated over long periods of time and the reliability and failure rate must be estimated in a statistically sound procedure. When establishing testing procedures for any system it is important to achieve three goal: the procedures must be based on traceable international standards, the procedures must be reproducible by the system users or third-parties testing authorities and the procedure should be applicable to different measurement systems to allow a fair comparison of the respective performances. This contribution describes the main testing procedures used to test the compliance of SMARTEC's SOFO system components to the published specifications. It also offers a general overview of some issues related to the testing of long-term performances.

This paper provides an overview of considerations associated with placement and operation of fiber optic sensors placed in composite materials. Issues that are discussed include coatings placed on optical fibers and their relationship to the composite structure, orientation of optical fibers in the composite parts, methods of providing strain relief, and terminations. Examples are given associated with a series of examples from aerospace and civil structure applications.

We shortly review general reliability definitions and methods used for optical fiber components for telecommunication applications and attempt to discuss in how far these can be applied to fiber sensor components.

Full qualification for commercial photonic parts as defined by the Military specification system in the past, is not feasible. Due to changes in the photonic components industry and the Military specification system that NASA had relied upon so heavily in the past, an approach to technology validation of commercial off the shelf parts had to be devised. This approach involves knowledge of system requirements, environmental requirements and failure modes of the particular components under consideration. Synthesizing the criteria together with the major known failure modes to formulate a test plan is an effective way of establishing knowledge based "qualification". Although this does not provide the type of reliability assurance that the Military specification system did in the past, it is an approach that allows for increased risk mitigation. The information presented will introduce the audience to the technology validation approach that is currently applied at NASA for the usage of commercial-off-the-shelf (COTS) fiber optic components for space flight environments. The focus will be on how to establish technology validation criteria for commercial fiber products such that continued reliable performance is assured under the harsh environmental conditions of typical missions. The goal of this presentation is to provide the audience with an approach to formulating a COTS qualification test plan for these devices. Examples from past NASA missions will be discussed.

This article presents the use of Bragg reflection and cladding mode measurements to independently measure axial strain and the integrity of a Bragg grating sensor. While the Bragg reflection is known to be sensitive to applied strain, the cladding modes are shown to be sensitive to expected damage within the sensor such as microcracking and debonding from the host structure. This phenomenon allows the intelligent self-testing of the Bragg grating sensor without additional instrumentation and permits the separate identification of sensor failure from the failure of the host structure. The growth of cladding modes during degradation of a Bragg grating is experimentally demonstrated in controlled tension tests with different fiber-host interface conditions.

With the development of modernized construction industry, constructions are more and more complicated enormous, and need more sensors to obtain the structural message, so traditional health and diagnosis technology can not take on the task of damage identification and multi-sensor data fusion technology is beginning to be used in this field. Firstly, this paper simply reviews the necessity of the appearance and development of the structural health monitoring and damage identification and multi-sensor data fusion. Secondly, the framework of structural health monitoring and damage identification system is introduced. Thirdly, the three levels of multi-sensor data fusion, which are pixels-level, feature-level and decision-level fusion, are analyzed in details, and the fusion methods and their applications of each data fusion level are also discussed. Lastly, we discuss a new two-level data fusion and two-level neural network architecture model for structural damage identification. A data fusion method of neural network combined with wavelet analysis is researched in this paper.

This paper introduces a novel coding resonant SAW passive wireless sensing system. A composite sensor structure with a resonator, a delay line and matching circuit is designed and the sensor system is constructed. It analyses sensor impulse response and response to sinusoidal burst signal input. The frequency of the response signal is related to sensing parameter. The delay time of the response signal is used to encode sensor elements. The duration of input signal has a significant effect to waveform and amplitude of response. An optimal duration of exciting burst signal can be selected. Because the composite structure can combine both sensitivity of high Q SAW resonator and massive coding ability of delay line, this coding resonant sensor can be used in long distance and large-scale distributed measurement as well.

In weigh-in-motion (WIM) system, the gross weight or the axle weight of the passing vehicle can be measured dynamically by the sensors installed in or on the pavement. One of the WIM sensors is piezoelectric sensor. Although piezoelectric sensor has limited measurement accuracy, it is widely deployed for its low cost and easy installation. To use the piezoelectric sensor, several factors have to be considered for the WIM site itself to ensure the vehicle passing over the sensor with a relative stable state. In addition, the piezoelectric sensor is seldom used for low speed measurement because of the piezoelectric material's performance. Traditional measurement method just uses the interactions between the sensor and the vehicle's tires that make the measurement inaccurate because the sensor cannot cover the whole tire patch along the driving direction. In this paper, the pavement deflection by the vehicle under measurement is introduced. New weighing method is developed for embedded piezoelectric sensors. Field tests are performed and the measurement errors are calculated based on the static weights measured from a static scale. Comparing to the traditional method, the proposed method is proved to have a higher accuracy and require less from installation site and vehicles under measurement. Furthermore, the method shows a better performance at low vehicle speed.

Fiber Bragg gratings are use to monitor the structural properties of composite pressure vessels. These gratings optically inscribed into the core of a single mode fiber are used as a tool to monitor the stress strain relation in laminate structure. The fiber Bragg sensors are both embedded within the composite laminates and bonded to the surface of the vessel with varying orientations with respect to the carbon fiber in the epoxy matrix. The response of these fiber-optic sensors is investigated by pressurizing the cylinder up to its burst pressure of around 2800 psi. This is done at both ambient and cryogenic temperatures using water and liquid nitrogen. The recorded response is compared with the response from conventional strain gauge also present on the vessel. Additionally, several vessels were tested that had been damaged to simulate different type of events, such as cut tow, delimitation and impact damage.