Sandra LLoret

In-line coherence multiplexing of displacement sensors: a fiber optic extensometer

Civil smart structures often require displacement sensors with measurement bases between a few centimeters and a few meters with a precision of the order of 1/100 mm. Low-coherence interferometry offers these performances even for long-term measurements. Being a non- incremental setup, it does not require an uninterrupted monitoring. The main drawback of this technique resides in the fact that a separate sensor is required for each section to be measured. A typical civil structure such as a bridge requires up to 50 sensors for each span, so the complexity for this type of instrumentation and the number of connections often limit its large- scale application. It would be interesting to subdivide the fiber sensor in domains that can be measured separately but have a single lead-out connection. This contribution presents an in- line multiplexing scheme for displacement sensors based on low-coherence interferometry and partial reflectors installed in pairs along the sensing fibers. The multiplexing of up to ten displacement sensors along the same fiber line is demonstrated theoretically and experimentally. Different types of partial reflectors are also compared. The special case of structures that are constructed in sections is also analyzed.

Special sensors for deformation measurements of different construction materials and structures

SOFO is a fiber optic sensor system that allows the monitoring of micrometer deformations over measurement bases up to a few meters. It is particularly adapted to measure civil structures built with conventional civil engineering materials (concrete, steel and timber). It has been successfully tested in different types of structures such as bridges, tunnels and piles. The application of the system is however limited in some case when unusual materials are used in the construction and in other cases by the dimensions of standard SOFO sensors. To extend the domain of application of the current system, special sensors have been developed. In this paper we present four special SOFO sensors: long, membrane, thin and stiff sensors. The long sensor has a measurement basis of several tenths of meters and its purpose is the measurement of deformations in massive and large structures (dames, tunnels). The membrane sensor is for use on laminated materials (e.g. membrane roofing) and it is easy to install by simply gluing it to the structure to be monitored. Since standard sensors can not be used for thin mortar layers because of their cross- section, a thin sensor has been developed, too. Finally, the aim of the stiff sensor is to determine the hardening (solidification) time of concrete. This time is determined by comparing the deformations of a stiff and a standard sensor, closely placed in the concrete at the very early age. The design of these sensors is presented along with significant application examples.

Measurement of dynamic deformations using a path-unbalance Michelson-interferometer-based optical fiber sensing device

A novel demodulation technique for performing dynamic de- formation measurements using a path-unbalanced Michelson interferom- eter is reported. The method is based on the rf amplitude modulation of a low-coherence source, and demodulation is achieved by tracking in the frequency domain the position of the minimum of the detected intensity. This technique is particularly suitable for deformation measurements in civil engineering structures where deformations of the order of few milli- meters over the sensor length are expected. The method features a measurement range of at least 10 mm, sensitivity better than 10 mm, and dynamic deformation measurements with a bandwidth up to 100 Hz.

Optical setup development for the monitoring of structural dynamic behavior using SOFO sensors

Fiber optic sensors have already demonstrated to be suitable for the monitoring of full-scale structures. Usually, most effort has been concentrated in improving the different sensing techniques and it remains often the problem of adapting the sensor to the harsh and dusty environment of the building site. In this context, the SOFO deformation sensors have successfully been tested in different types of structures. The current measurement system, based on low- coherence interferometry, is specially conceived for the long- term assessment of structures, where high precision, resolution and stability is required. Since each measurement takes a few seconds, the set-up is not adequate to monitor the dynamic behavior of structures, where measurement frequencies of up to 1 KHz are necessary. In order to take advantage of the good performance of the current SOFO sensors, a new technique to demodulate at a high frequency the signal from these sensors has been investigated. Based in the radio- frequency amplitude modulation of a low-coherence source, this intensity-based method offers the required dynamic range of several mm to measure the deformations of the structure under dynamic loads. In this paper we present the theoretical background together with the experimental verification of the principle.

Amplitude Modulation of a Low-Coherence Source, Applications to Distance and Dynamic Displacement Sensing

Abstract The possibilities of the radio-frequency amplitude modulation of a low-coherence source for distance and displacement measurements are discussed. This intensity-based method, used within a Michelson interferometer, is a novel alternative to perform dynamic deformation measurements. The theoretical background is presented, together with the experimental verification of the principle. Besides, the results of the first quasi-dynamic tests are shown and the perspectives of the technique discussed. This method is specially useful for the dynamic monitoring of civil structures, where large measurements bases are needed. Furthermore, the application of the amplitude modulation for distance measurements is demonstrated experimentally.

Static and dynamic bridge monitoring with fiber optic sensors

In many concrete bridges, deformations due to static and dynamic loading are the most relevant parameters to be monitored in both the short and long term. Strain monitoring give only local information about the material behavior and too many of such sensor are therefore necessary to gain a complete understanding of the bridge evolution. Recent advances in measurement technology have demonstrated that optical fiber sensors are suitable for monitoring full-scale structures. A network of such sensors installed inside a bridges enables the measurement of parameters such as internal deformation and temperature. In the past four years, our laboratory has installed hundreds of fiber optic deformation sensors of varying seizes in concrete and composite steel-concrete bridges. These sensors give useful information during the construction phases and about the long-term geometrical deformations of a bridge under static load. Recently it has been found that these sensors can also be used to measure the quasi-static part of the dynamic deformation of a bridge under traffic load. The measurement technique relies on low-coherent interferometry and it guarantees high resolution, high precision ad long-term stability. Nevertheless each measurement takes a few seconds, thus it is inadequate for monitoring the dynamic behavior of bridges, where measurement frequencies of up to 1 kHz are necessary. This paper also presents a new technique to demodulate at a high frequency of up to 1 kHz are necessary. This paper also presents a new technique to demodulate at a high frequency the signal for the same sensors used for long-term monitoring. With its large dynamic range of several mm it allows for the monitoring of bridges under dynamic loads while maintaining the configuration of the original fiber optic sensor that has been proven to be reliable.

Long-term monitoring of a concrete bridge with 100+ fiber optic long-gauge sensors

In 1996, a concrete highway bridge near Geneva (Switzerland) was instrumented with more than 100 low-coherence fiber optic deformation sensors. The Versoix Bridge is a classical concrete bridge consisting in two parallel pre-stressed concrete beams supporting a 30-cm concrete deck and two overhangs. To enlarge the bridge, the beams were widened and the overhang extended. In order to increase the knowledge on the interaction between the old and the new concrete, we choose low-coherence fiber optic sensors to measure the displacements of the fresh concrete during the setting phase and to monitor the long term deformations of the bridge. The aim is to retrieve the spatial displacements of the bridge in an earth-bound coordinate system by monitoring its internal deformations. The vertical and horizontal curvatures of the bridge are measured locally at multiple locations along the bridge span by installing sensors at different positions in the girder cross-section. By taking the double integral of the curvature and respecting the boundary conditions, it is then possible to retrieve the deformations of the bridge. This paper presents the sensor network design and the measurements that were performed during the construction phases, during the bridge operation since it was reopened and under a recent static-loading test.

An optical fibre sensor for dynamic deformation measurements based on the intensity modulation of a low-coherence source

An optical fibre sensing technique for the measurement of dynamic deformations using a Michelson interferometer is reported. The method applied to interferometers with an initial path unbalance of around 1 cm is based on the rf intensity modulation of a low-coherence source. A large measurement range of 1 cm is intrinsic to the method and with an adequate demodulation process sensitivity better than 10 μm is obtained. This novel approach allows for measuring dynamic deformations with a bandwidth up to 100 Hz.

RANGING OF REFLECTIVE MARKERS IN OPTICAL FIBER SENSORS BY DOUBLE-PASS PHASE MODULATION

A novel demodulation scheme for the ranging of reflective markers is presented. This method is useful for implementing in-line mul- tiplexing of displacement and temperature sensors based on low- coherence interferometry. It allows the discrimination of reflective mark- ers such as air-gap connectors or other partial reflectors spaced by as little as 0.1 m and the measurement of deformations occurring between two successive reflectors with a resolution of 2 mm. The multiplexing of two displacement sensors is demonstrated experimentally. This type of sensor is especially useful for the long-time geometrical monitoring of large structures.