This paper resumes the nine year long experience of our group in the installation of fiber optic sensors in the most diverse structure types, including bridges, tunnels, dams, piles, anchors, historical monuments, nuclear power plants and many others. To date, we have installed about 1'500 sensors in almost 70 different applications. Statistics show that, by proper handling, it is possible to achieve 95 - 100% survivability during installation and for many years afterward.

We have used the electrostatic self-assembly process to grow nanostructured thin films, molecular layer-by-molecular layer, on the ends of silica optical fibers. Such thin films effectively form optical cavities that allow the measurement of temperature by low reflectivity optical Fizeau interferometry.

In this study a new algorithm for modulating a PZT stack using a sine-wave signal instead of a saw-tooth signal was developed in order to resolve the flyback problem in a PZT driver. An improved synthetic heterodyne demodulator circuit was designed and arranged to interrogate the output signal from an interferometric sensor with a sine-wave modulation (phase-generated carrier). A depth of modulation or a gain amplifier to the circuit design was adjusted such that the standard heterodyne signal could be formed. The result is a conventional PM modulated carrier that can be demodulated using standard techniques, followed by a lock-in amplifier or a phase meter, to recover the phase shift in the sensing information.

A distributed fibre optical temperature sensing technique for different monitoring tasks, especially for leakage detection in oil and gas facilities, pipelines, underground storage sites, water constructions sites, mining and environmental industries is presented.

This paper presents a device to measure the dynamic horizontal shear strain of the ground during earthquake. The proposed device consists of a bronze plate with fiber Bragg grating sensors attached on it. The device is vertically installed in the ground, and horizontal shear strain of the ground is measured as deflection angle of the plate. Employment of optical fiber sensors makes the proposed device simple in mechanism and highly durable, which makes it easy to install our device in the ground. We conducted shaking table tests using ground model to verify applicability of the proposed device.

A distributed fiber optic sensor for the detection and location of hydrocarbon fuel spills is presented. The sensor is designed such that liquid swelling polymers transducer their swelling into a microbend force on an optical fiber when exposed to hydrocarbon fuels. Interrogation of the sensor using standard Optical Time Domain Reflectometry techniques provides the possibility of rapidly detecting and locating target hydrocarbon fuels and chemicals at multiple positions along the sensor length. Events can typically be located to a precision of 2 m over a 10 km sensor length. Sensor response time on exposure to the hydrocarbon fuel is within 30 seconds. A detailed explanation of the operational characteristics of the sensor and the underlying technology utilized in its operation is given. Experimental tests using prototype sensors to simultaneously detect three separate 50 centimeter-long events are described. The characteristics of the sensor response in a range of hydrocarbon fuels under varying environmental conditions were investigated. Some of the safety advantages in using the sensor and its practical implementation in continuous monitoring of pipelines or fuel containment vessels are discussed.

All evidence indicates that new physics, and answers to some of the most profound scientific questions of our time, lie at energies around 1 TeV. To look for this new physics, the next research instrument in Europe's particle physics armory is the Large Hadron Collider (LHC). This challenging machine will use the most advanced superconducting magnet and accelerator technologies ever employed. LHC experiments are being designed to look for theoretically predicted phenomena.

A CMOS imager-based spectrometer is used to interrogate a network containing a large number of Bragg grating sensors on multiple fibers as part of a proprietary structural health monitoring system. The spectrometer uses a Prism-Grating-Prism (PGP) to spectrally separate serially multiplexed Bragg reflections on a single fiber. As a result, each Bragg grating produces a discrete spot on the CMOS imager that shifts horizontally as the Bragg grating experiences changes in strain or temperature. The reflected wavelength of the Bragg grating can be determined by finding the center of the spot produced. The use of a random addressing CMOS imager enables a flexible sampling rate. Some fibers can be interrogated at a high sampling rate while others can be interrogated at a lower sampling rate. However, the use of a CMOS camera brings several specific problems in terms of signal processing. These include a logarithmic pixel response, a low signal-to-noise ratio, the long pixel time constant, obtaining sufficient process priority for the control program, and proper selection of the window of interest. In this paper we investigate computer algorithms and hardware solutions to address these problems. We also present experimental data to validate these solutions including calibration data and initial field-testing data with 24 sensors on 4 fibers.

Structural health monitoring with optical fiber sensors requires the embedded or surface bonded optical fibers and sensors to remain functional for the lifetime of the structure being monitored, as repairs are generally impossible. Thus, the feasibility of an embedded optical fiber monitoring concept depends heavily on the durability of the optical fiber. Processes that degrade the mechanical properties of these fibers are therefore of great concern. It is well known that silica optical fibers are vulnerable to moisture degradation, which in combination with applied loads will make flaws grow and eventually lead to fiber failure. Fabrication of optical fiber sensors often involve removal of the protective coating locally, thus exposing the glass surface to moisture and handling loads. Static fatigue experiments lasting up to one year were conducted on coated and uncoated optical fibers to determine the durability of the fibers in terms of time-to-failure as a function of applied loading, environment, and handling. Both an acrylate-coated and a polyimide-coated fiber were studied. Uncoated fiber, representing the worst-case condition, was obtained by chemical removal of the protective coating. A mechanism-based model was introduced to model the data. Results show that the static fatigue effect is very significant with the time-to-failure decreasing with applied load. For the uncoated fiber, it was found that the static fatigue life depended appreciably on how the fiber was handled, with the durability decreasing significantly by the slightest mechanical contact. The results indicate that strain levels of 0.3% should be survivable for years even in the worst case tested, but that above this level the static fatigue durability may become a critical issue.

Global Positioning System technology can provide position information with accuracy to a few millimetres in near real-time. Thanks to this level of precision, the movement of structures can be monitored. The falling tendency of GPS receiver pricing and their miniaturisation, along with their modest power requirements spurred a research and development project at SUPSI which resulted in a prototype system, deemed interesting for a number of applications. It is shown that using carrier phase differential GPS, a network of receiver modules can be installed in order to perform monitoring and surveillance operations for small movements. Typical applications for this type of sensor network include monitoring of structures such as buildings, dams, bridges, as well as measuring the movement of landslides and rock formations. The system consists of a number of small receivers installed on the object to be monitored. A radio-linked base station provides for data collection, post-processing, and monitoring for correct operation of the network. Ancillary sensors may be added to the single receiver units and their measurements synchronized to the positional measurements. The base station may be programmed to initiate a warning or alarm action when absolute position differences or the velocity of movement exceed a pre-set limit.

This is an overview paper that discusses the concept of an embeddable structural health monitoring system for use in composite and heterogeneous material systems. The sensor system is formed by integrating groups of autonomous unit cells into a structure, much like neurons in biological systems. Each unit cell consists of an embedded processor and a group of distributed sensors that gives the structure the ability to sense damage. In addition, each unit cell periodically updates a central processor on the status of health in its neighborhood. This micro-architectured synthetic nervous system has an advanced sensing capability based on new continuous sensor technology. This technology uses a plurality of serially connected piezoceramic nodes to form a distributed sensor capable of measuring waves generated in structures by damage events, including impact and crack propagation. Simulations show that the neural system can detect faint acoustic waves in large plates. An experiment demonstrates the use of a simple neural system that was able to measure simulated acoustic emissions that were not clearly recognizable by a single conventional piezoceramic sensor.

Fiber optic Bragg gratings have been used for years to measure quasi-static phenomena. In aircraft engine applications there is a need to measure dynamic signals such as variable pressures. In order to monitor these pressures a detection system with broad dynamic range is needed. This paper describes an interferometric demodulator that was developed and optimized for this particular application. The signal to noise ratio was maximized through temporal coherence analysis. The demodulator was incorporated in a laboratory system that simulates conditions to be measured. Several pressure sensor configurations incorporating a fiber optic Bragg grating were also explored. The results of the experiments are reported in this paper.

A fiber Bragg grating (FBG) sensor network has been developed for structural health monitoring on board of the X-38 spacecraft, the prototype of a crew return vehicle for the International Space Station. The FBG sensors will monitor mechanical and thermal load profiles of a frame component during launch and re-entry of the spacecraft. Four sensor pads are interrogated which contain three Bragg gratings each for 2D orthogonal strain and temperature measurements. The requirements for mechanical and thermal stability under the demanding conditions of vibration and shock loads, at temperatures within -40..+200 degree(s)C, have been fulfilled applying appropriate fiber passivation and Ormocer re-coating techniques, and by the development of a robust polychromator based signal processing unit.

In the present study, embedded Bragg grating sensors were arranged in a rosette configuration to simultaneously measure the in-plane strain components and the temperature change. The rosette consisted of four optical sensors. The sensors were embedded in neighbouring plies to realise a point-like measurement. Assuming linear thermoelastic behaviour, the relationship between the optical response from each sensor and the strains in the laminate was established. A finite element model was then used to quantify numerically the expected accuracy of the measured strain under various loading conditions.

Strain and temperature measurement using one Fiber Bragg Grating transducer is demonstrated. The device is constructed in a hydrogenated standard telecommunication optic fiber with one-step optimized UV writing process. A complete strain and temperature characterization is reported to assess the viability of this device.

In future aircraft, integrated sensors for monitoring structural performance and structural integrity will become increasingly important. For integration into carbon-fiber- reinforced composite structures, fiber-optic Bragg grating sensors represent a very promising approach. In order to characterize the behavior of integrated sensors for different mechanical load cases theoretical and experimental investigations were conducted.

The spectral response changing of a Fiber Bragg grating due to the interactions with an ultrasound wave have been numerically analyzed by a transfer matrix approach has been used, taking into account the geometrical and elasto-optic effects. Numerical analysis show that it's possible to characterize the ultrasound wave, only if the ratio between the ultrasonic wavelength and the length of the used grating exceeds an established value, which depends on both the characteristics of the Bragg grating and the ultrasound amplitude itself.

Ultrasonic Lamb waves have been proved to be a potentially desirable method for quick interrogation of large plate structures. Integrating fiber optic sensors offer great potential to monitor large areas due to their geometric flexibility. To date this potential has not been widely exploited and there have been only a few reports of non-destructive testing with integrating fiber optic sensors. In this report we present our preliminary measurements for the interrogation of hole defects in plate-like materials with surface-bonded integrating fiber optic sensors. Due to the signal integrating characteristics of fiber sensors very complicated signals were obtained. Therefore signal processing is needed to interpret waveforms for flaw detection.

This paper summarizes the continued CH-46 in-flight data analysis for the rotor acoustic monitoring systems (RAMS). This ambitious 38-month proof-of-concept effort, which was a part of the Naval Surface Warfare Center Air Vehicle Diagnostics System program, culminated in a successful three-week flight test of the RAMS system at Patuxent River Flight Test Center in September 1997. This paper presents the results of additional signal data analysis of the flight-test dataset over analysis results previously reported in 1998 and 1999.

The present work is aimed at elucidation of the specific features in propagation of ultrasound waves in an electrostructurized medium, of their dependence on an applied electric field and implementation of the effects obtained in practice. In experiments, the use was made of electrosensitive liquid interlayers, which manifest the essential dependence of viscoelastic-plastic properties on a magnitude of an electric field applied to shear macroflows. The basis of a homogeneous suspension was the transformer oil, to which the finely divided silica powder, the corresponding surfactant and activator were added. To determine the velocity of ultrasound propagation in liquid media, the time of ultrasound signal propagation was compared with the delay time of the reference signal in the standard delay line.

In smart composite structures, piezoelectric patches are usually bonded to surfaces or embedded in structures for strain measurement and active control. This paper investigates applications of these patches for acousto-ultrasonic(AU) inspection of structure without special design or additional change of their shapes and configurations. By using these multi-functional built-in transducers, the smart composite has the potential to monitor a large area on the structure with AU method. Experiments were conducted on a fiber-epoxy composite beam. A PZT transmitter pasted on the beam was stimulated by a nanosecond pulse generator, reflected stress wave caused by edges of the beam was clearly observed. Artificial slots on the beam can be well located from the arriving time of the reflected waves, their size can be estimated from the shape and peak value of the waves reflected by or propagating through slot. Wavelet analysis was used to extract features from the measured signals. Local maxima of coefficients of Mexico hat wavelet decomposition were employed as features to describe arriving time and slope of signals. Features in different damage conditions were compared, results show these features can represent both the location and extent of the simulated damages.

This paper presents a brief description and testing results of Ionic Polymer-Metal Composites (IPMC's) as dynamic sensors. As previously noted a strip of IPMC can exhibit large dynamic deformation if placed in a time varying electric field. Conversely, dynamic deformation of such ionic polymers produces dynamic electric fields. The underlying principle of such a mechanoelectric effect in IPMC can be explained by the linear irreversible thermodynamics in which ion and solvent transport are the fluxes and electric field and solvent pressure gradient are the forces. Important parameters include the material conductance and the permeability.

In the last decade, in light of their superior mechanical properties advanced polymer matrix composites have been indicated as the most suitable candidates as Smart Materials and Structures. However, their final properties are strongly dependent on the processing stage and key points to improve the quality and the reliability of these materials that have been identified in the cure monitoring and the optimization of the manufacturing process. Based on this line of argument, an integrated fiber optic sensing system for simultaneous refractive index and temperature measurements has been designed and developed in order to monitor the curing process of thermoset based composites. A fiber optic refractometer has been designed by using the free end of the sensing optical fiber. A theoretical model has been developed for converting refractive index changes in detailed information on the extent of the curing. Its validation has been proved by comparison with calorimetric characterization. In addition, integrated fiber Bragg gratings has been used for local temperature measurements. The interrogation of the sensing Bragg grating has been implemented by using a different fiber Bragg grating able to convert the resonance wavelength shift in intensity changes. Preliminary results are presented.

We have studied health-monitoring systems using embedded optical fiber sensors for damage detection in FRP (Fiber Reinforced Plastics) composite laminates. For sensing of strain or temperature changes in these composite materials, we developed the FBG (Fiber Bragg Grating) sensors using the polyimide-coated small-diameter optical fiber with cladding diameter of 40 micrometers , which can be embedded in FRP composite laminates without inducing any structural defects. The FBG was subjected to high-temperature heat treatment at 300 degree(s)C in order to suppress the optical characteristics change of the FBG over a long period of time. Even after such high temperature treatment, the FBG retained sufficient mechanical strength and reflectivity. We embedded the FBG into FRP composite laminates and measured the optical characteristics against temperature and tensile strain. These experimental results show this type of the embedded FBG is very promising for practical health-monitoring systems. In addition, we have investigated various multipoint measuring systems and propose the time division multiplexing system using these FBG sensors.

Small-diameter fiber Bragg grating (FBG) sensors have been developed by Hitachi Cable Ltd. and the authors. Since the outside diameter of polyimide coating is 52 micrometers , embedding of the sensors into carbon fiber reinforced plastic (CFRP) composites prepregs of 125 micrometers in thickness does not deteriorate the mechanical properties of the composite laminates. In this research, the small-diameter FBG sensor was applied for the detection of transverse cracks in CFRP composites. The FBG sensor was embedded in 0 degree(s) ply of a CFRP cross-ply laminate.

A wavelet-based crack detection algorithm is being developed which combines active detection by means of Fiber Bragg Grating sensors and passive detection by means of wavelet- based crack detection of composite-embedded optical fibers.

The electrical properties of fiber reinforced plastics (FRP) have been investigated in order to develop structural materials with a damage diagnosis function. Electrical conductivity was achieved by adding carbon particles or carbon fiber as a conductive phase into the FRP. The composites containing carbon particles connected by a percolation structure were found to have advantages in terms of response of conductivity to small strains and the size of the detectable strain range, compared to composites containing carbon fiber. A part of the resistance change in the elongated composites containing carbon particles remained after unloading despite deformation being predominantly elastic. This residual resistance was found to depend largely on morphology of the carbon particles and orientation of the glass fiber. A distinct residual resistance was observed in composites containing spherical carbon particles (carbon black) and glass fibers aligned at an angle of 0 degrees with respect to the tensile direction. Electrical time domain reflectometry (ETDR) was used to locate the damaged region in multilayer composites containing CFRP and GFRP. The position of local damage in the multilayer composites was clearly located to a precision of within 20 mm.

The increase in electrical resistance of Carbon Fiber Reinforced Plastic (CFRP) composites caused by mechanical damage are studied experimentally and analytically. Considering the contacts of misaligned conducting carbon fibers of CFRP, we proposed a discrete network model as an equivalent electrical circuit for CFRP. This model incorporates both an electrical ineffective length, over which broken fibers carry no current, and the Weibull- Poisson statistics of fiber breakage. The model is also used to explain experimental data showing that the resistance change does not depend on the gage length of specimen and that the resistance and electrical ineffective length are controlled by the fiber volume fraction.

Advancements in atomic force microscopy have led to the development of new measurement techniques that take advantage of the different vibration modes of the cantilevers. Each vibration mode has a different sensitivity to the variations in surface stiffness. The cantilever interacts with the sample surface through the tip of the cantilever. This interaction is approximated as a linear spring such that linear vibration theory may be used for analysis. This simplification restricts the results to experiments involving low amplitude excitations. For imaging, a single vibration mode is selected for feedback control. The image contrast is directly controlled by the modal sensitivity of the cantilever. Low-stiffness cantilevers have typically been unusable for imaging of stiff materials because of the lack of sensitivity of the first flexural mode. In this article, a closed form solution of the modal sensitivity for flexural vibration modes is derived for cantilevers with constant cross-sections. For cantilevers with other shapes, an approximate solution is developed using the Rayleigh-Ritz method. For given nominal values of surface and AFM cantilever properties, the appropriate mode for highest contrast may be predicted.

A novel electronic device capable of sensing and monitoring the myoelectric, polarization wave and electromagnetic activities of the biological systems and in particular the human body is presented. It is known that all the physical and chemical processes within biological systems are associated with polarization, depolarization waves from the brain, neural signals and myoelectric processes that manifest themselves in ionic and dipole motion. The technology developed in our laboratory is based on certain charge motion sensitive electronics. The electronic system developed is capable of sensing the electromagnetic activities of biological systems. The information obtained is then processed by specialized software in order to interpret it from physical and chemical point of view.

(gamma) -Fe₂O₃ is a n-type semiconductor oxide and it has been extensively studied as a gas sensing material. It is usually obtained by oxidizing Fe₃O₄ at 250 degree(s)C, temperature confirmed by the DTA curve. (gamma) -Fe₂O₃ has a spinel type crystal structure with a lattice parameter of 8,3 angstroms.

This paper describes a quasi-distributed optical fiber strain sensor whose principle of operation is based on the coherent frequency-modulated continuous-wave reflectometry technique. The sensing system is basically composed of a laser diode, which is frequency-swept by a triangle waveform injection current, and an unbalanced two-beam interferometer.

In this paper, a multi-parameter optical sensor system for vibration, temperature and strain monitoring is presented. It is based on a hybrid singlemode-multimode fiber optic transducer, where the vibration is detected by means of a fiber cantilever beam and extrinsic coupling of light, while two fiber Bragg gratings measure the temperature and strain. Experimental results show good performance for the monitoring of large electric machines.

The quality of the steel, among others, depends on the temperature profile during the last cooling process. Because of that, the capability of real-time temperature measurements at several points of the steel bars during the aforementioned production process is highly desirable in order to build a smart structure. To measure temperatures in the range of 800 - 1300 degrees centigrade, the sensor system proposed, is doted of new fiber optic transducers. They are based on the measurement of the optical radiation generated by the hot steel bars through a geometric cavity which is properly designed and fabricated to maximize its accuracy.

A systematic study of acoustic emission detection using fiber Bragg grating sensors has been carried out over the last year. In this, we attempt to use the fiber Bragg grating to sense the dynamic strain created by a passing ultrasonic wave signal. Our goal here is to see if such a sensor is possible, and if so, what the detection sensitivity and limitations will be. To answer these questions, we carried out several experiments involving the detection of simulated acoustic emission events. In the first experiment, we attach fiber Bragg grating to the surface of a piezoceramic resonator which is driven by a signal generator. We were able to detect the resulting surface vibration of the resonator up to 2.1 MHz. In the second experiment, we attach a fiber Bragg grating to the surface of an aluminum plate. We excite an acoustic wave using an ultrasonic transducer located at various positions of the aluminum plate. In this way, we demonstrated that the fiber Bragg Grating sensor is capable of picking up the signal coming from a distance (up to 30 cm) for up to 2.5 MHz. In a third experiment, we use the same fiber Bragg grating on aluminum plate set up, but set up an acoustic signal by either a gentle knock on the plate by a pin, or by breaking a pencil lead on the plate. We were able to detection acoustic emission set up by pencil lead breaking up to a frequency of 30 kHz. Higher frequency components were not detected mainly due to the limitation of available electronic equipment at this time (higher frequency band-pass filters and amplifiers. In all the above-mentioned experiments we use a match Bragg grating to demodulate the detected optical signal and use a dual channel scheme for electronic data acquisition and processing (a signal channel and a reference channel).

Distributed optical fiber sensors are effective in the monitoring of large structures because of their large sensing range. In this research, the methods of strain and deflection measurement for bending beams using Brillouin distributed optical fiber sensors were presented. The deflection monitoring of beams makes possible the damage detection of large structures such as bridges. The measured strain with Brillouin distributed optical fiber sensors is expressed as the averaged value within its spatial resolution. Due to the characteristic of the spatial resolution, strain near the ends of a measurement range cannot be correctly measured with respect to the measuring position. Therefore, its compensation is required. The strain measurement method suitable to the sensor characteristic was proposed, and it was verified by the analysis of aluminum-tube beams. The beam deflection could be monitored using the distributed measurement more simply and effectively than using general point-measurement.

A fiber optic distributed sensing system has been used for in-situ measurement of in-plane strain and temperature variation during the curing of AS4-3501 composite. The distributed sensing system based on Brillouin scattering has a spatial resolution of 15 cm. In this paper, we present preliminary experimental results on the Brillouin frequency shift measured by an optical fiber embedded within a 16-ply composite panel during the heat-up, isothermal hold and cool-down stages of the cure process. By deducting the temperature effects, the average strain profile along the mid-plane of the composite panel at various stages of the cure process can be seen. The distributed sensor can detect the reaction advancement by measuring the cure shrinkage at the gelation and vitrification stages. Shrinkage is then correlated with the degree of cure data from Differential Scanning Calorimetry (DSC) obtained for the same cure cycle. The thermal response of the solidified composite during cooling is also profiled. Details of the data processing the Brillouin frequency shift data to obtain strain as a function of cure temperature, time and location have been explained. The measurement accuracy is discussed. The ultimate goal of this research is to detect in real time the evolution of process-induced strains within these materials.

We present experimental results on testing a twin grating fiber optic sensor for measurement of static strain. The sensor is built with two identical Bragg gratings closely spaced in a single mode fiber. It produces a reflection spectrum modulated due to interference. Similar to traditional FBG sensors, strain of the fiber leads to the wavelength shift of the reflection spectrum. This shift can be determined precisely measuring phase changes for corresponding components of Fourier transform of the reflection spectrum. Such an approach allows an absolute strain measurement with an interferometric sensitivity. Resolution of 0.02 degree(s)C and 0.2(mu) (epsilon) was demonstrated experimentally for temperature and static strain measurements using a pair of 0.7mm long Bragg gratings with reflectivity of 1% each.

The paper presents distributed fiber optic bending sensor for petroleum hydrocarbon detection based on COFDR technique. Sensitive polymer, which effectively swells under hydrocarbon influence, was employed in order to introduce bending losses in hydrocarbon presence. In this work we used lumped reflectors, namely fiber Bragg gratings, placed between distributed sensitive elements. Proposed design of the sensor utilises the principle of truly distributed detection with discrete localisation of perturbation. We have demonstrated that the COFDR technique with bending based chemical sensor is capable to detect hydrocarbon presence within a few minutes for 20-cm perturbation-length with spatial resolution up to 0.5 meters.

We present a novel miniaturized sensor configuration based on fiber Bragg gratings for simultaneous measurement of curvature and temperature. Due to the particular geometry of the sensing head it is possible to measure not only the curvature radius, but also to determine the plane of curvature. This is achieved by arranging three Bragg gratings in the vertices of the smallest equilateral triangle that can be defined by the cross sections of the fibers. The set is then inserted in a glue-filled capillary stainless-steel tube to provide both a suitable protection for the Bragg sensors and rotational symmetry to the sensing head. This tube also ensures isolation from axial-strain, allowing the additional determination of temperature. The proposed sensing head is particularly well suited for applications in smart structures because it can be embedded along any layer of a composite material (including the neutral line) without special concerns on the relative orientation of the Bragg gratings arrangement and the composite layers. This sensing configuration may also be used to implement more sophisticated sensors dedicated to the measurement of multi-axial acceleration or flow and temperature.