In this report we will present some preliminary results on a RF sub-carrier based fibre strain sensing system for on-line monitoring of composite high pressure storage tanks for natural gas or hydrogen. By employing a temperature compensation scheme to the system developed has shown stable performance with better than 20μm length resolution and comparable long-term drift. The system uses readily available components and could be used for low-cost high volume applications such as on-line continuous monitoring.

Fiber optic sensors have the capability to simultaneously measure multi-scale data (e.g. strain, strain gradients, and integrated strains) for the purpose of structural health monitoring of structures. Presented here is a technique that can detect and localize damage in a structure suitable for the fusion of such multi-scale data. The flexibility method has been previously proposed for damage localization using changes in natural frequencies and mode shapes. However, such changes are difficult to apply in a real world application due to the extreme accuracy required for the input excitation frequencies. In addition, computer modeling of the undamaged structure to compare with the damaged structure can lead to significant errors due to imperfections. The flexibility method for solving indeterminate structures using static conditions presented here avoids these difficulties. The method follows a procedure of applying a known load to determine the flexibility matrices of the structure in pre- and post- damaged states. The introduction of fiber optic strain and displacement measurements permits the calculation of these matrices from the static loading conditions. These matrices are subtracted and the resulting null space calculated. When reapplied to the original system this null space locates the damage. To demonstrate the validity of this process numerical models are created for a simple truss structure and a plate instrumented with several fiber optic sensors. The examples show that damage can be located in both cases.

Ultrasound has been demonstrated to be a perfect tool for NDT. There are several detectors that can be applied in NDT, for example fibre Bragg grating, interferometry, etc. Here we concentrate in polarimetric optical fibre detection. In this paper we develop a simple but realistic analysis of the ultrasonic wavefront integration technique along an optical fibre for acoustic detection. Our model considers the perturbation caused by the acoustic wave as an isotropic change in the effective refractive index of the sensing fibre used as the detection system and neglects the polarization modulation. Also we assume the stress homoegeneous through the section of the fibre. The theoretical analysis has been simulated in MATLAB. In this program we have analyzed the relation between the length of the sensing fibre, its distance to the ultrasound source and its sensitivity to ultrasound detection, for different orientations of the source with respect to the sensing fibre. The results indicate that optimum ultrasonic detection may be achieved through careful positioning and orientation of the optical fibre. These results may be applied, for example in NDT, where scattered ultrasound from defects introduces new effective sources that may be characterized by arrays of these integrating sensors.

Luna Innovations has developed a prototype 8-channel fiber optic sensor system to demonstrate fiber optic sensor operation in flight environments. As an intial flight demonstration, long period grating (LPG) relative humidity sensors along with extrinsic Fabry-Perot interferometric (EFPI) pressure and temperature sensors were installed in an aging Delta 767-300ER jet. The fiber optic signal-conditioning system is a multi-purpose platform that can also be used to operate other types of fiber optic LPG and EFPI sensors, including strain gages, metal-ion corrosion sensors, and fiber Bragg grating (FBG) sensors. The system configuration and operation is described.

To perform the real-time health monitoring of the smart composite structures, two fiber optic sensor systems are proposed, that can measure the strain and detect the moment of fracture simultaneously. The types of the coherent sources used for fracture signal detection classify the systems--EDFA with FBG and EDFA with Fabry-Perot filter, and these systems were applied to extrinsic Fabry-Perot interferometer sensors imbedded in composite specimens to monitor the tensile tests. To understand the characteristics of matrix cracking signals, at first, we performed tensile tests using surface attached PZT sensors. This paper describes the implementation of time-frequency analysis such as short time Fourier-transform for the quantitative evaluation of the fracture signals like matrix cracking. From the test of tensile load monitoring using optical fiber sensor systems, measured strain agreed with the value of electric strain gage and the fracture detection system could detect the moment of damage with high sensitivity to recognize the onset of micro-crack fracture signals.

As part of a program to measure and model vertical strain in the West Antarctic ice Sheet, we developed a new sensor to accurately and stably record displacements. The sensors consist of optical fibers, encased in thin-walled stainless steel tubes, frozen into hot-water drilled boreholes, and stretched from the surface to various depths in the ice sheet ranging to 1000 m. An EDM (electronic distance meter) connected annually to the fibers read out their absolute lengths with a precision of about 2 mm. An initial elongation of about 0.15% of the optical fibers allowed them to follow an ice thinning rate of 300 ppm per year for up to five years. Two sets of five sensors were installed in the 1997-1998 field season: one set was near the Siple Dome core hole (an ice divide) and a second set was on the flank 7 km to the north (the ice thickness at both sites is approximately 1000 m).

We describe a novel but simple method of detecting Lamb waves through the measurement of the changes in the polarimetric state of light propagating through an optical fibre which has been either embedded into or bonded onto the plate to be tested. The directional properties of the sensor are described, as is its ability to detect a hole produced in the plate. We also show how the relative sizes of the detected signal amplitudes from the source PZT and from the hole vary according to the alignment of the source with the sensor axis.

The performance of sensor systems in smart structures is subject to orientation, installation, and environmental effects. The capability for initial and maintenance calibrations can provide added confidence in sensor information and facilitate investigations of long-term sensor behavior. The use of Extrinsic Fabry-Perot Interferometric (EFPI) fiber-optic sensors was examined for calibration of companion strain sensors. The output of the EFPI sensor is periodic with strain and the signal behavior with periodic strain displays well-defined harmonic content. In particular, the strains giving maximums and minimums in the harmonics can be calculated from the excitation wavelength and the EFPI gage length. A Polyvinylidene Fluoride (PVDF) piezoelectric strain sensor was surface mounted on a cantilever beam and its voltage-to-strain ratio was precisely calibrated using the accuracy of a co-located EFPI sensor. The experimental responses of both sensors were obtained for a periodic actuation. The PVDF output was calibrated using a linear-fit of the strains obtained from four points in the harmonic response of the reference EFPI sensor. The selected maximum and minimum points of the EFPI harmonics were directly observed with a spectrum analyzer. This fast, efficient approach was performed under resonant conditions with relatively inexpensive demodulation requirements. The results of the calibration compared well with the expected PVDF response.

In-situ planetary sample analysis is a major goal in current and future NASA exploration missions. In general in-situ analysis experiments are designed to investigate chemical, biological or geological markers or properties to determine the complex history of the body being studied or for use as a pre-screening measurement to increase the scientific value of samples selected for sample return. In order to expand the number of applicable sensor schemes and the available capability an investigation into piezoelectric bulk acoustic wave (BAW) and surface acoustic wave (SAW) resonators has been initiated with emphasis on applications to future NASA missions. In general, BAW and SAW sensors can be configured to directly measure mass, acoustic impedance, density and elastic property changes. Indirectly they can be designed to measure or monitor pressure, temperature, dew/melting point, curing, adsorption/desorption, and viscosity and be configured with the appropriate reaction layers as chemical sensors or as Immunosensors. The various models used to describe these sensors will be presented and the measurand sensitivity and importance of cross sensitivities will be discussed. Recent advances in passive wireless RF interrogated SAW technology has increased the scope of these sensor systems to remote sensing (10m) and to applications which may have been deemed previously inaccessible. Examples include SAW stress sensors buried in large structures that once assembled are inaccessible for measurement that can be interrogated with wireless RF signals to determine the health of the structure. In addition, this technology has recently been coupled with other sensor technology allowing for an expansion of the possibilities for remote sensing. On the basis of the cost, range, versatility and ease of array fabrication of these sensors offer significant potential for future NASA missions.

Modal actuators and sensors may be used to excite or measure either single modes or combinations of modes. In beam structures they may be implemented using either discrete transducers or continuous, distributed transducers. Continuous transducers require less processing and thus can employ simple controllers. For beam structures the width of the transducer may be approximated using the underlying finite element model shape functions. The transducers may then be designed using a discrete model, and the shape recovered by using the shape functions. The side constraint of minimizing the curvature of the transducer shape is introduced to ensure that the resulting shape is as simple as possible. Assuming that the thickness of the transducer may be varied (for example using printing techniques) then this procedure may be extended to plate structures. The alternative is to determine the shape of a constant thickness modal transducer. One approach outlined in this paper is to use a fine finite element mesh and determine which elements should be covered with a transducer. The possibility of using a continuous shape definition will also be explored. The approaches are tested on various beam and plate structures to demonstrate their effectiveness and also to demonstrate the errors introduced for non-proportionally damped structures.

Piezoelectric materials can be used as mechanisms to transfer ambient vibrations into electrical energy that can be stored and used to power other devices. With the recent surge of micro scale devices, Piezoelectric power generation can provide a conventional alternative to traditional power sources used to operate certain types of sensors/actuators, telemetry, and MEMS devices. In this paper, two types of piezoelectric materials were experimentally investigated for use as power harvesting devices. The two types being the commonly used monolithic piezoelectric (PZT) and Macro Fiber Composites (MFC), which were recently developed at the NASA Langley Center. Our experimental results estimate the efficiency of these devices and identify the feasibility of their use in real world applications. In general the power produced by the vibration of a piezoelectric device is on the order of a few milliwatts which is far too little to power for most applications. Therefore, each the transducer is used to charge nickel metal hydride batteries of varying sizes to compare their performance and ability of to store electrical power. The results presented in this paper show the potential of piezoelectric materials for use in power harvesting applications.

Accurately knowing the bulk modulus of a fluid is very important in many hydraulic applications. The absolute bulk modulus has a major effect on the position, power delivery, response time and stability of virtually all hydraulic systems. Within this work a novel sensing technique to determine the bulk modulus of a fluid or hydraulic system is proposed. The constitutive equations of a piezoelectric actuator are coupled to a fluidic system that contains entrained air and mechanical compliance. The model can be used to extract the bulk modulus of the fluid system in real time. The results indicate that matching the stiffness of the actuator to the stiffness of the fluidic system is critical in obtaining a high sensitivity to the bulk modulus measurement.

We describe a novel fiber-optic system that is able to detect both ultrasonic Lamb waves and the location of their source. The aim of the system is to detect damage in structures such as those found in aerospace applications. Our system involves the use of fiber Bragg gratings, which may be either bonded to the surface of the material or embedded within it in order to detect the linear strain component produced by the acoustic waves. Interrogation of the Bragg gratings is carried out using a laser, which is tuned to the wavelength that gives the maximum sensitivity on the grating response curve. An amplitude modulated signal is produced by the interaction of the Lamb wave with the grating. The well defined directional properties of the Bragg grating (compared to the isotropic response of the more commonly used piezoceramic disc transducers) are used to determine the direction of propagation of the acoustic waves by mounting three of the gratings in a rosette configuration. Two suitably spaced rosettes are used to locate the source of the ultrasound by taking the intersection of the directions given by each rosette. This will become important when we extend the technique to include the study of the use of changes in the propagation properties of Lamb waves as a method of damage detection. We will present both theoretical and practical results on the interaction of the Lamb waves with the grating and the extraction of directional information from the response of the rosettes.

The wood support is an essential element of the works of art and is highly sensitive to the environmental climate modification. Wood deformations may have irreversible destructive effects on the work of ar t. The use of fiber Bragg grating (FBG) sensors for the quasi-distributed in-situ measurement and continuous monitoring of the painted wood panel deformations is proposed. FBG sensors have high resolution low invasivity and intrinsic safety. The results of a set of measurement on a wood panel in different climate conditions are presented. The applicability of fiber Bragg grating sensors to the cultural heritage is demonstrated.

A Bragg grating based acoustic emission crack detection (AECD) sensor system is developed. The ultimate goal of the sensor system is to provide structural health monitoring of composite structures. Various aspects of the sensor system including the parameters involving the optical filter using a matched fiber Bragg grating have been analyzed. The prototype sensor system was tested using pencil lead break tests and Open-Hole-Tension (OHT) specimens (ASTM D 5766). The fiber Bragg grating sensor was embedded into the composite tension specimen at the mid-plane and outside of the region under strain so as to reduce the effects of unwanted structural strains in the sensor response. Impact tests with the surface mounted sensor system determined the sensitivity of the sensor to the direction of the stress wave. The tests demonstrate the ability of the AECD sensor system to detect a pencil lead break event and actual AE events from a composite specimen both in the near surface and embedded configurations.

We have been studying optical sensing technology using embedded fiber Bragg grating (FBG) sensors. The FBG is inscribed in a small-diameter optical fiber with the cladding diameter of 40 μm. This technology is very promising for health monitoring in aviation components, because the diameter of the sensor optical fiber is so small that embedding of the sensor does not deteriorate mechanical properties of the composite materials. For practical use, we have also studied high reliable fused-splicing method between the small-diameter optical fiber and an ordinary optical fiber in order to improve handling. The embedded FBG sensors are useful for vibration or impact detection as well as static strain detection. For the purpose of detecting dynamic phenomena, we have developed a high speed wavelength detection unit for the FBG sensors which uses wavelength division multiplexing (WDM) coupler based on planar lightwave circuit (PLC) technique. WDM coupler converts wavelength of the light reflected from the FBG sensor into output powers. Since there is no mechanical moving part, this type of wavelength detection technique is suitable for high speed detection.

Fiber Bragg grating (FBG) sensor systems are being widely used as temperature measurement and strain measurement systems for aerospace structures, civil structures, and high rise buildings. However, the systems are rather complicated because the wavelength change must be measured using several kinds of optical filters such as Fabry-Perot filters, edge filters, and bandwidth filters. In this paper, FBG sensor without filter system is proposed. The system consists of SLD (Super Luminescence Diode), diode driver, FBG sensors and the photo diode as a detector. Neither Fabry-Perot filter nor edge filter is applied. SLD has its own intensity slope according to various wavelengths. The slope is very linear at certain wave length range. In the beam experiments, the 1525 nm center wavelength FBG is employed as a sensor and the grating shows good linear responses to the dynamic loads. The data compare to those of electric strain gauges. The system also has a potential to be multiplexed by the pulse modulations in the time domain. If the FBGs have different wavelengths, they can be placed in the same fiber and if the FBGs have the same wavelength, they must be in the separate optical fiber and connected with couplers in order to be multiplexed.

The goal of a structural health monitoring system is to detect, locate, and identify damages in a structure during its lifetime. The concept of structural health monitoring is particularly important for fiber reinforced composites due to the complexity of the possible failure mechanisms. The goal of this work is to simulate the response of optical fiber Bragg grating sensors to multi-component loading for their implementation in structural health monitoring algorithms for composites. A simulation method is presented to determine the effects of axial, bending and shear loading on an embedded optical fiber Bragg grating sensor. The effect of fiber bending on the Bragg grating sensor is experimentally verified by embedding the sensor in a solid cone, clamped at the base and subjected to a point load at the apex. Next, a numerically efficient method to calculate the response of sensors embedded in a unidirectional composite is developed using both finite element analysis and optimal shear-lag theory and taking into account the above effects. The limitations of the optimal shear-lag theory are derived through comparison with the finite element results. The application of this method is demonstrated through a numerical example, simulating the response of sensors embedded in one fiber layer to a transverse crack.

Chirped fiber Bragg (FBG) sensors were applied for the detection of delamination in carbon fiber reinforced plastics (CFRP) cross-ply laminates. Reflection spectra from the embedded chirped FBG sensor were measured at various delamination lengths under static four-point bending test. The sensor were embedded into two different positions for the investigation of the relation between the spectrum and the direction of delamination propagation. The spectrum changed sensitively depending on both the delamination length and the direction of the delamination propagation. For confirmation of the measured results, the spectrum was simulated considering the strain distribution in the chirped FBG sensor theoretically. The change in the form of the measured spectrum was consistent with that of the calculated spectrum. Moreover, the area ratio of the two peaks in the spectrum and the spectrum width were proposed as effective indicators for the identification of the local delamination.

Thermoplastic and thermoset fiber-reinforced composite materials are well established in aerospace engineering, but also more and more used in the oil and gas industry as well as in civil engineering. In these applications they are mainly used to reinfoce, repair or straighten existing structures, but recently full-composite structures have also been built. Independently from the domain of the use, there is a need for these composite structures to be monitored. Since the composite materials are usually applied in the form of thin tapes or sheets, sensors have to be embedded within the structure, depending on structural layer that has to be monitored. Embedding the sensors may have as a consequence a significant decrease of the mechanical properties of the composite material due to the dimensions of the sensor. The solution presented in this paper is integration of a fiber optic sensor directly into the main composite component, i.e. into the composite tape. In this paper we present the development of a thermoplastic fiber-reinforced composite tape with integrated long-gage fiber-optic sensors. The fiber-optic sensors are selected due to small transversal dimension and good compatibility with the plastic materials. The tape with integrated optical fiber can be used for tape winding of a structural element, embedded between different layers, but also as a separate sensor - a sensing tape. The optical and mechanical properties of the tapes with sensor are tested. The sensing tape is then installed onto the rail along with standard long-gage fiber optic sensor, additional tests are performed and performance of both sensor compared. The integration of optical fiber into the composite tape, the results of the tests as well as the performances of the tape with integrated optical fiber are presented in this paper.

Multiple Fiber Bragg-gratings are embedded in carbon-epoxy laminates as well as in composite wound pressure vessel. Structural properties of such composites are investigated. The measurements include stress-strain relation in laminates and Poisson’s ratio in several specimens with varying orientation of the optical fiber Bragg-sensor with respect to the carbon fiber in an epoxy matrix. Additionally, fiber Bragg gratings are bonded on the surface of these laminates and cylinders fabricated out of carbon-epoxy composites and multiple points are monitored and compared for strain measurements at several locations.

A temperature sensor network basing on 20 multiplexed fiber Bragg gratings has been developed to perform gas temperature distribution monitoring within the 60-m diameter helium gas fill of a novel cargo airship CL75 for loads of up to 75 metric tons providing a long-term repeatability of temperature measuring results of 0.15 K. The fiber optic temperature sensors overcome problems with lightning protection in the all-plastic airship and allow to measure true gas temperatures through the whole balloon volume. The gas temperature data of the sensor network are used in airship lifting control to improve the flight height stabilization.

We describe an optical system to monitor small long-term changes in the shape of a surface by using a network of optical fibre Bragg grating strain gauges, for applications in which space does not permit the use of techniques such as photogrammetry or structured light methods. Gratings are bonded to copper beryllium strips held under tension in contact with the test surface. The copper beryllium strips enable sufficient force to be transferred to the optical fibre from the compliant surface. Shape changes are revealed as strain changes in the sensor strips, inferred from wavelength shifts in the Bragg peaks. The optical signals are obtained in reflection by illuminating the sensor fibres with a broadband source and using a scanning Fabry-Perot filter to generate the spectrum with a wavelength resolution of 0.3 pm over the range 1530 to 1570 nm. Laboratory tests show that a strain resolution of 8 microstrain can be achieved with temperature compensation over the range 20 to 50 C, with a multiplexing capability of between 11 and 16 temperature - strain sensor pairs, depending on temperature gradients on the test surface. We present experimental measurements on a cylindrical test object subject to diametral loading, and show a comparison with a finite element model.

Deformation induced variations in dielectric properties of an elastic material is called electrostriction. This effect can be detected using capacitance measurements and employed for sensing shear and normal strains. Almost any solid dielectric can be used as a sensing medium, but electroactive polymeric composites present a unique advantage. A composite’s structure can be locally micro-tailored and optimized for a given sensing application by exposing liquid polymeric suspensions to an electric field and curing the obtained structure. A single plane sensor configuration for shear and normal loads is analyzed. Such a design has no moving electrodes, can be implemented using surface micro-machining and supplied with on-chip electronics. Advantages of electroactive polymeric sensors include: intrinsic sensitivity to shear strain, robustness and flexibility, high tolerance to overloads, a large selection of candidate material for specific applications, simplicity in the manufacture, and operation and potential for miniaturization.

A recently developed class of magnetic field sensors is based on the action of a magnetostrictive material on a piezoelectric material. Described here is a significant improvement on this class, a passive magnetic field sensor made of layers of Terfenol-D {Fe₂(Dy_0.7Tb_0.3)} magnetostrictive material and ceramic PZT-5. These devices show a large magnetic field sensitivity of order 6 kV/T. Application of this device to harvesting vibration energy indicates that more than 10 mW of electrical power can be harvested using a Terfenol-D/PZT/ Terfenol-D sandwich (volume = 1 cm³) from 30 Hz vibrations having an acceleration of 0.5 g.

Filtering analog signals is certainly not a new topic. It is essential in a variety of applications ranging from communication systems, such as radio, telephone and paging devices, to navigation equipment and system control. It is a subject matter that is more familiar when using strictly electrical components. However, it is known that electromechanical systems, specifically mechanical filters, can also be used to filter signals. Existing mechanical filter technology could be considered to be macro-scale, which encompasses dimensions of centimeters and larger. There is interest in developing micro-scale filters that are well-integrated with electronics. Filter design literature is also quite spread out, making for a design process that is neither precise, nor easily grasped. The objective is to investigate and build a more structured design procedure for developing mechanical filters. This paper presents a design procedure for a mechanical filter. The procedure is carried out, resulting in a three-resonator piezoelectric filter (with 1500 Hz center frequency) that is built and tested. The results are presented, indicating how closely the design specifications are met. The desire is to use this general process to design and fabricate a micro-scaled device in the future.

The work in this study develops a methodology for using Polyvinylidene Fluoride (PVDF) film for gathering impact information from a structure. The method can be employed on sensors that are deployed over rigid structures or those that are placed on top of compliant/viscoelastic structures. This method utilizes spatially shaded (etched) electrodes, which allow for selective charge collection and signal processing. This means that impact information, such as impact location and impact angle, can be directly related to the time varying measured output voltage. The basic equations describing charge collection are developed followed by the development of an experimental technique that can be used for charge collection. Results were validated on an experimental test stand and show acceptable performance with error ranging between 0.025 cm (0.01 in) and 0.635 cm (0.25 in).

Single-axis rate sensors (rate gyroscopes) that utilize Coriolis coupling between a single pair of modes of a simple vibrating structure have been widely researched and are being used in increasing numbers in a range of aerospace, automotive and other applications. To meet market demands for such sensors, there are now significant pressures to reduce cost and size and to improve basic performance. Size and cost reduction will be achieved via MEMS technology, but size reduction and performance improvement are often conflicting requirements. This paper examines some smart alternatives to simple size reduction. The presented concepts allow more information to be measured by a single sensor structure by using more of the available spectrum of vibration modes available in the sensor structure. The concepts are illustrated in the form of a multi-channel rate sensor based on a vibrating cylinder and a multi-axis rate sensor based on a vibrating ring. Test results from prototype sensors are presented.

Ultrasonic inspection techniques using magnetostrictive transducers have received much attention in recent years as non-contact, non-destructive means of inspecting ferromagnetic materials. By the selection of a desired mode and thus the rejection of the unwanted modes among propagating waves in a waveguide, different types of flaws existing in a cylindrical ferromagnetic waveguide can be effectively detected. However, desired mode selection methods have not been fully developed yet. The purpose of this research is to present a mangetostrictive sensor based technique for the selection of either the bending or longitudinal waves alone in a ferromagnetic waveguide. To achieve this goal, new bias magnet configurations, particularly for bending mode selection are suggested. Several experimental results are conducted to verify the effectiveness of the suggested magnetostrictive sensors.

The purpose of this work is to suggest a new non-contact diagnostic method applicable to rotating shafts. The presence and the location of any damage in rotating shafts are assessed by means of longitudinal elastic waves propagating along the shafts. These waves are measured by magnetostrictive sensors that make use of the coupling between strains and magnetic induction in ferromagnetic shafts. These sensors have been successfully applied in pipes and others, but it appears that the present application of them to rotating shafts is made here for the first time. Several issues appearing in the magnetostrictive sensor application to rotating shafts are carefully investigated. The usefulness of the present method is verified by several experimental results

A fiber Bragg grating (FBG) sensor network has been installed into a large diameter concrete pile on a real construction site. The intention was to monitor its deformation behavior during several quasi-static loading cycles. The skin friction between pile and subsoil affecting the ultimate bearing capacity of the pile as well as the settlement behavior of the structure under investigation has been derived from our measurements. A comparison between the results of the fiber Bragg grating sensors and conventional concrete strain gages (CSG) has shown excellent correspondence.

This paper presents an investigation of the application of "long gage" fiber optic sensors (FOS) to monitor the behaviour and integrity of pipelines. A description of the long gage sensor technology is provided, together with the sensor system developed for structural applications. Tests were conducted on pipe sections under a variety of load conditions, including internal pressure, axial compression, bending and local buckling. Long gage sensors were boneded to the pipes and displacements measured using a FOX-TEK FTI 3300 instrument that employs an interferometric technique to obtain displacements to an acuracy of 20 microns. Results obtained showed that the FOS could track changes in loads, detect prebuckling deformations, and measure post-buckling plastic strains. The long gage sensors were then applied to a tailings pipeline in northern Alberta (Canada) to monitor continuously the pipe wall thinning due to erosion/corosion. Employing the FTI 3300 with a PC containing an Aircard for wireless transmission, test data were monitored remotely through internet access. Using analytical models in combination with real-time measurements of the pipe's response, predictions of the operational lifetime for the pipe were made.

Most civil engineering structures have been built in the 50's and 60's and reach similar level of degradation accelerated by loading conditions and corrosion. In Europe, National Authorities and the European Commission promote Health Monitoring concepts, instrumentation of existing structures and help in the design of new durable structures of higher performance. In this context, the CEA-List has achieved a non-exclusive industrial transfer of its Bragg grating sensing technology for civil engineering applications to Hydrolog (French SME), supported by the European Community and the french ministry of Industry. In order to check the reliability and user-friendliness of this instrumentation, eleven spectrally-multiplexed Bragg grating-based extensometers, four FBG temperature sensors and an acquisition unit have been installed into the Saint-Jean bridge in Bordeaux, France with the help of the Infrastructure Regional Direction (DRE-Aquitaine) and the Bordeaux Authority (Communaute Urbaine de Bordeaux). A standardized loading of the bridge has been performed on October 29, 1001, with the purpose of correlating its mechanical reaction to loading conditions. Moreover, the equipment has been operating for one year to take into account the winter-summer cycle.

Road monitoring instrumentation is becoming an important part of traffic management systems. Axle detectors are part of this instrumentation and have been subject to considerable practical investigation in recent years. This paper presents a new fiber optic vehicle axle detector for roadways. It is based on a fiber optic Michelson interferometer that is mounted directly into the road surface. The system is based on a minimum number of standard, cost effective, telecommunication fiber optic components. The detector is placed beneath the road surface and is not subjected to the wear and tear as in teh case of current commercially available axle detectors. The detector operation was tracked for a period of one full year deployed in a freeway with an average frequency of 20,000 vehicles per day. During this period we did not observe any degradation of performance or malfunction of the proposed system. In addition, a fully dielectric design allowed for remote operation of the sensor via a long section of optical fiber.

This paper describes a fibre-optic interrogation device based on a pulsed time-of-flight (TOF) technique for the measurement of integral strain. The precision of the measurement system is 100 μm (1 ps) and it has a spatial resolution of less than 0.50 m (5 ns), achieved by the use of ultra-short probe pulses of about 500 ps, a GHz band receiver channel and a custom-made time-to-digital converter (TDC) implemented in a standard CMOS process. The TDC can simultaneously measure the distance to 9 reflectors (e.g., Bragg gratings) in the fibre core using the same optical pulse. Combined with a common receiver channel and an ultra-fast timing discriminator, this capability makes the system fast and stable, thus enabling both long-term and dynamic measurements. Potential application areas of the system include measurement of integral strain and its derivatives, especially in large civil engineering structures and composite materials. Pull tests with bare optical fibres have demonstrated that the obtained results are in good agreement with those of a reference sensor.

As the development of intelligent robots, more and more sensors of higher-technique are required, and tactile sensing technology gets extensively attention. It is the three-axis force that is working when the robot is grasping or walking, but it is quite difficult to measure the three-axis force directly in the numerous tactile sensors. To get the contact-alike nonlinear solution in FEA(Finite Element Analysis), an advanced analysis method of ANSYS - APDL(Advanced Program Description Language) is employed, with which the miscellaneous and time-consuming process is automatically completed in an intelligent way. This paper introduces a series of simulation experiments about an innovative optical wave-guided three-axis tactile sensing system and brings forward the corresponding mathematical model to calculate the three-axis force. A special sensing system is designed for the experiments, and the results considerably conform to the theoretical analysis. Thus, a new method comes into being for tactile sensing of intelligent robots.

Laser interferometry is one of the most sensitive methods for small displacement displacement. This technique was successfully used in several fields of physics, giving very good performances also due to the large availability of optical components and high quality and relatively low cost laser sources. At the same time this technique is enough flexible to be effectively used in very different applications. In particular, in this paper, we present a laser interferometric system used to read the position of the sensitive element of a standard seismic accelerometer. The working mechanism of seismic accelerometers is based on a control system that acts on the sensible element to stabilize its position. By looking at the force needed to perform this feedback it is possible to obtain the acceleration. Usually the feedback system of these instruments is based on magnets and coils, or on piezoelectric actuators, while the signal transducer is usually a capacitive device. The performances of the interferometric system were analyzed in comparison with standard one. The result are encouraging also if some problem, mainly connected with the dynamic of the read-out system have to be better studied.

An approach has been developed for real-time vibration monitoring of a composite cantilever beam. The fiber Bragg grating (FBG) sensor has an advantage to be embedded or bonded to the structure compared with other sensors such as piezoelectric sensor or strain gage, thus allowing the measurement of parameters like strain and temperature. In this paper the vibration sensor system with a FBG embedded in a composite smart structure is proposed. This system can assort vibration direction and sensing vibration amplitude just by measuring output voltage. The sensing resolution is decided by the slope of the filter used in the system.

We report the use of a fiber-optic distributed sensing system to monitor crack growth on aircraft panels. The system utilizes optical frequency domain reflectometry to demodulate the reflected signals from up to thousands of weakly reflecting gratings photoetched along a single optical fiber. In our experiment, data from a regular array of sensors attached to an aircraft panel were recorded as the panel was subjected to increasing loads. Strain contour maps generated from these data enable clear visualization of the crack growth over time. A similar experiment was also performed using fiber-optic strain sensors embedded in aircraft composite repair patches. The results of these experiments demonstrate the viability of distributed fiber-optic sensing for crack growth monitoring.

We report the use of a fiber-optic distributed sensing system to monitor structural fatigue on an aircraft undergoing a full scale fatigue test. This technique involves using optical frequency domain reflectometry to demodulate the reflected signals from multiplexed Bragg gratings that have been photoetched in the core of an optical fiber. The optical fibers, containing a high density of Bragg gratings, were applied along the surface of a Lockheed Martin P-3C Orion fatigue test article to assess the suitability of this technique for long-term structural damage detection and monitoring. Preliminary results indicate good agreement with quasi-collocated foil strain gauges and demonstrate great potential for supplementing or replacing conventional non-destructive evaluation techniques.