The response of a fiber optic sensor [linearly chirped fiber Bragg grating (LCFBG)] to a linear thermal gradient applied on its sensing length (i.e., 1.5 cm) has been investigated. After these bench tests, we assessed their feasibility for temperature monitoring during thermal tumor treatment. In particular, we performed experiments during ex vivo laser ablation (LA) in pig liver and in vivo thermal ablation in animal models (pigs). We investigated the following: (i) the relationship between the full width at half maximum of the LCFBG spectrum and the temperature difference among the extremities of the LCFBG and (ii) the relationship between the mean spectrum wavelength and the mean temperature acting on the LCFBG sensing area. These relationships showed a linear trend during both bench tests and LA in animal models. Thermal sensitivity was significant although different values were found with regards to bench tests and animal experiments. The linear trend and significant sensitivity allow hypothesizing a future use of this kind of sensor to monitor both temperature gradient and mean temperature within a tissue undergoing thermal treatment.
An Aluminum coated FBG was heated to 650 °C in order to study the regeneration process at low temperature. When compared to the standard regeneration process (at 800 °C -1000 °C) the regeneration occurred at longer times, the onset of the process took around 150 days to appear. The reflected signal after regeneration also is weaker than that obtained in the usual process, although it presents a good stability after the recovery phase – measured for a time span of ~200 days.
In this work we tested use of thin film aluminium coating to enhance the temperature sensing features of FBG sensors. Thin film coating was evaporated on the Bragg grating after removal of the original acrylate recoating, using a modified vacuum evaporation system that allows uniform radial deposition around the fibre. Results show that recoating thickness up to 0.5 μm do not produce appreciable effects, whereas thickness of 2.3 μm appreciably increases temperature sensitivity without increasing time response constant.
The paper points out the progress in the exploitation of high, fast and precise non-contact velocimetry for robot
applications. A technique for the precise measurement of the speed between two sliding surfaces has been developed
during the research project for the realisation of an autonomous robot. The robot is devoted to the scouting of dangerous
sites and to the execution of measurements in these places for the exploration of an extreme Antarctica environment
(RAS project). This technique is based on the precise calculation of the common movement of a laser speckle field. This
approach allows the realisation of a velocimeter suitable for use in extreme conditions. A description of the adopted
methodology and the obtained results are the main topic of our work.
The very low thermal expansion coefficient of silica at cryogenic temperature prevents the use of Fibre Bragg Grating
(FBG) sensors for high resolution temperature monitoring in cryogenic environments involving liquid gases or space
applications. To overcome such limitations sensors have been coated with different metals to improve the measurement
sensitivity in the very low temperature region, i.e. 4.2-35 K. Various coatings have been deposited by electrowinning on
the external fibre surface after aluminium pre-coating. Full characterization of this new type of sensor is described in the
Here we present our approach to apply the direct Radon transform for the analysis of interferometric images, in particular for the laser speckle velocymetry, and for the ESPI measurements. A technique for the precise velocity measurement is based on the precise calculation of the common movement of a laser speckle field. This approach allows realising a velocimeter suitable for use in extreme conditions. The latest results of our investigations are presented in this report. We also present the method of image analysis for the automatic set up of an Electronic Speckle Pattern Interferometry (ESPI). The idea of our method is to make a direct Radon-like transformation for each pixel (x0, y0) a 2D field of an image brightness B(x,y):, and than we calculate the rms spatial deviation by s, the maximum of which determines two values: (i) immediate maxφ(σs), and (ii) the corresponded angle φ((σs)max). So, from 2D function B(x,y) we have two functions depended from same 2D field, but gives a clear defect location. Our investigations show a perspective of our approach. The submitted results have both methodological and applied significance for the pattern analysis.
Techniques able to perform weighing of road vehicles not requiring any lowering of their cruise speed are of great interest for a large amount of applications. Many of such applications are traditionally related to determining custom duties, toll-way fares and cost of paying freight, but new applications often concerned with high-speed travelling vehicles are arising, as for instance the smart management of highway lorry traffic. In this paper we present preliminary results for the development of a weigh-in-motion technique based on Fibre Bragg Grating sensors. The proposed technique is intended for the production of a weigh-in-motion station suitable for high-speed road vehicle and high load resolution.
In this paper the design and testing of a novel displacement sensor, based on optical in-fibre Bragg Grating sensing (FBGs), is described. The principle of operation is based on the conversion of the relative displacement into optical fibre strains by means of an elastic spring: its design allows to measure linear displacements with high accuracy and extended linear range. Owing to the key features of fibre optic sensors, as high sensitivity and immunity to electro-magnetic noise, this packaged sensor has been devoted for monitoring Pixel Vertex Detectors linear displacements of BTeV, an High Energy Physics experiment to be run at the Fermilab Tevatron.
Fibre Bragg Grating (FBG) sensors, can be used for temperature monitoring but the low thermal expansion coefficient of silica limits its sensitivity in cryogenic environment. However it will be shown that the use of a lead coating, electrodeposited on an aluminum pre-coated senosr, will enhance FBG sensing properties down to 5 K. Slight variations on the optical response of the coated FBG sensor are interpreted with the micrographic observation of the deposit.
An aluminum prototype of the AMICA (Astro Mapper for Instrument Check of Attitude) Star Tracker Support (ASTS) of the AMS_02 (Alpha Magnetic Spectrometer) space experiment has been instrumented with Fiber Bragg Gratings (FBGs). In this work the use of FBGs to perform dynamic tests on the ASTS prototype is reported. The excitation has been provided by an instrumented impact hammer, the mechanical response of the structure has been obtained by bonded FBGs and accelerometers. All time histories have been recorded, transformed in the frequency domain to retrieve Frequency Response Functions (FRFs)-accelerometer responses- and Strain Frequency Response Functions (SFRFs) -FBG responses-, both providing resonant frequencies and displacements (strain) shapes of the ASTS. Numerical simulations of this structure have been performed to predict its resonant frequencies and vibrational displacement (strain) shapes. Experimental results demonstrate the capability of FBGs to perform in situ experimental modal analysis as confirmed by the comparison of the optical response with the accelerometers one and the good agreement with the numerical analysis.
This paper describes an application of Fiber Bragg Grating sensors devoted to both health monitoring of road bridge structures and traffic load monitoring. The ultimate aim of the application is the remote continuous monitoring of the structures, with real time acquisition of the dynamic and quasi-static deformations inferred by both the road traffic and the daily and seasonal thermal variations. A Fiber Bragg Grating network composed of 24 sensors has been installed on the bridge on the Po river of the 'A21 Torino Brescia' Italian Highway. The bridge is a concrete structure, and sensors are applied on various rebar components. The sensors were installed directly on the rebars, adopting a special technique specifically developed for permanent concrete embedding. An extensive data acquisition program is in progress with the aim of both health monitoring and in transit lorry weighing. Results are discussed and planned future work is presented.
In this paper, we present an application devoted to measure the dynamic response of a prototype of a support of a star tracker. Four different techniques have been used and compared with each other. Specifically Electronic Speckle Pattern Interferometry and Speckle Shearography have been used as full field modal appropriation techniques, impact hammer and accelerometer as a broad band conventional technique and finally broad band with impact hammer and FBG sensor as a newly proposed methodology. Modal parameters have been retrieved using all the four methodologies described above and compared. We have demonstrated that FBG sensors can be used to retrieve modal parameters useful as indicators of the state of health of a structural component. When embedded the FBG can be used actively also during the qualification process of the smart component as well as in service for shape monitoring. This latter purpose is important for precision pointing instruments such as the star tracker under investigation.
This work is relevant to the development of a precision velocimeter for a self-powered robotic vehicle designed for operation in the Antarctic Regions. This is a crucial problem because of the difficulty to measure the velocity with appropriate accuracy under slip conditions and in the absence of reliable reference points over vast snow or ice fields. Speckle velocimetry is expected to provide fairly accurate measurements under Antarctic conditions. The results of early theoretical and experimental studies on the problem are very encouraging.
Re-entry in planetary atmospheres is one of the most challenging environments to be faced by an aerospace structure. Presently space agencies are studying and developing programs to reduce launch costs by developing a new generation reusable launch vehicles. In fact a significant portion of the launch cost, for those vehicles, is represented by maintenance, non destructive testing and personnel involved in ground operations. For instance NASA and Lockeed Martin are leading the VentureStar program, where the real time health monitoring is considered an important aspect, while ESA has now finished a preliminary analysis for different reusable launch vehicle configurations. Fiber optic sensors which can be embedded into structural components can provide an efficient means for fast and reliable structural health monitoring. In this paper the possibility of embedding fiber optic sensors into materials subjected to particularly critical thermal treatments is verified. Several specimens of metal alloys and carbide based powders with embedded optical fibers have been prepared by the high pressure high velocity oxy fuel technique. The tests have proven the feasibility of the embedding with the above mentioned technology which exposes the fibers to quite a severe environment during the deposition. Micrographic analysis and optical transmission tests have been carried out on the sprayed specimens.
The aim of the present paper is to describe the tests performed on the zinc-aluminum specimens cast with optical fibers inserted in it. Aspect concerning the wettability of the fiber matrix interface and the buoyancy of the fiber into the melt are considered. Special care has been devoted to the realization of the chill for obtaining a slender specimen suitable for vibration tests. Metallographic studies have been carried out in order to obtain information on the state of the optical fiber. One arm of a Mach-Zehnder interferometer has been set with the specimen described above. Optimal correlation has been found between the excitation applied on the specimen and the response obtained by the fringe pattern variation.
Different laser induced diagnostics, originally developed for different purposes including material characterization and environmental monitoring, have been applied in the field of Cultural Inheritance preservation with the aim to facilitate successive conservation and restoration actions. In this paper results relevant to three different techniques are reviewed. The use of topologic laser and 3D sensor in checking small artifacts and large surfaces is discussed, the application of Speckle interferometry to defect analysis of ceramic artwork is represented, a demonstration of the capabilities of a time resolved LIF system in the characterization of surface composition of ancient ceramics and frescoes is finally given.
Modal analysis of a metallic specimen has been performed by use of Double Pulsed Holographic Interferometry. The specimen was excited by an electromagnetic shaking platform operating in an industrial-like environment, in day light and with severe floor vibration. A piezoelectric accelerometer was used to detect the specimen natural frequencies, Double Pulsed Holographic Interferometry was used to detect vibration modes. For each identified natural frequency, various double pulsed interferograms were made in order to obtain a sequence of fringe patterns showing a `stroboscopic' video-recording of the specimen displacement. Numerical modal analysis of the specimen was performed by use of Finite Elements Model analysis (ABAQUS s/w). Comparison between numerical and experimental results showed that the numerical model originally assumed with `no degree of freedom' at the edge fixed on the shaking platform, had to be corrected to improve the description of the real constrain. After the correction was made, very good agreement between numerical and experimental modal shapes was obtained, thus demonstrating that effective validation of numerical model can be obtained by holographic experimental measurements. The highly reliable validation obtained by use of Holographic Interferometry, is compared to the validation obtainable by classic methods based on comparison between numerical and experimental natural frequencies.
A new experimental procedure devoted to analyse deformation patterns due to the contemporary excitation of different vibration modes (mode coupling) is proposed. The proposed procedure, based on use of holographic interferometry techniques, is particularly appropriate whenever coupled modes are expected according to FEM computation and an experimental modal analysis is required for FEM model validation. This work tests the proposed procedure by verifying the reliability of the experimental results worked out in analysing two simple specimens whose modal behavior can be confidently predicted by analytical and numerical computation.
Within the planning stage devoted to realize an innovative turbine for an aircraft engine, an experimental prototype has been made. Several measurements have been carried out to experimentally verify the expected structural and dynamic features of such a prototype. Expected properties were worked out by finite elements method, using the well-known Nastran software package. Natural frequencies and vibration modes of the designed prototype were computed assuming the turbine being in both `dynamic condition' (rotating turbine at running speed and temperature), and in `static condition' (still turbine at room temperature). We present the experimental modal analysis carried out by time average holographic interferometry, being the prototype in `static condition;' results show the modal behavior of the prototype. Experimental and computed modal features are compared to evaluate the reliability of the finite elements model of the turbine used for computation by the Nastran package; reliability of the finite elements model must be checked to validate results computed assuming the turbine blade is in hostile environments, such as `dynamic condition,' which could hardly be tested by experimental measurements. A piezoelectric transducer was used to excite the turbine blade by sine variable pressure. To better estimate the natural vibration modes, two holographic interferograms have been made for each identified natural frequency, being the sensitivity vector directions of the two interferograms perpendicular to each other. The first ten lower natural frequencies and vibration modes of the blade have been analyzed; experimental and computed results are compared and discussed. Experimental and computed values of natural frequencies are in good agrement between each other. Several differences are present between experimental and computed modal patterns; a possible cause of such discrepancies is identified in wrong structural constraints imposed at nodes of the finite elements model. Slight structural differences among different blades of the same prototype turbine are pointed out and discussed.
Holographic and Speckle Interferometry provide full-field inspection of surfaces with high accuracy: these techniques offer some useful advantages (e.g. speed, high-precision,...) over other optical nonintrusive inspection techniques. Although the performance and precision of Holographic and Speckle Interferometry are affected by ambient conditions (temperature gradients, air turbulences, ...), the development of special experimental methods allows the use of these techniques directly in-field. An application of these interferometric methods can be foreseen, for example, in traditional testing laboratories or for a direct industrial production control. A great effort is currently in progress at ENEA-Frascati and at JRC-Ispra in order to develop new methods and computerized optoelectronic systems aiming at the application of Holographic and Speckle Interferometry to structural testing. Our most recent experimental results using time-average holographic interferometry and pulsed holographic interferometry for dynamic displacement measurements are presented. A theoretical background of the time-average ESPI procedure is described and some experimental results are shown.
Use of CW holographic interferometry in order to experimentally analyze structural behavior of an aircraft turbine blade prototype is presented. Results have been used to verify expected turbine specifications worked out by finite element method.
The work done in order to perform a vibrational analysis of an aircraft winglet is presented. The analysis was intended to acquire general purpose structural information, and to define the best location for accelerometers to be used in subsequent vibration fatigue tests. The winglet was mounted on a platform driven by an electromagnetic shaker and thus excited in order to simulate on-flight stressing; induced deformations were detected by pulsed holographic interferometry. Different stress conditions were tested while the shaker was vibrating at frequencies in the 30 - 1500 Hz range and with accelerations in the 0.5 - 2.0 g range.