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A novel and highly sensitive liquid level sensor based on a polymer optical fiber Bragg grating (POFBG) is experimentally demonstrated. Two different configurations are studied and both configurations show the potential to interrogate liquid level by measuring the strain induced in a POFBG embedded in a silicone rubber diaphragm, which deforms due to hydrostatic pressure variations. The sensor exhibits a highly linear response over the sensing range and a good repeatability. For comparison, a similar sensor using a FBG inscribed in silica fiber is fabricated, which displays a sensitivity that is a factor of 5 smaller than the POFBG. The temperature sensitivity is studied and a novel multi-sensor arrangement proposed which has the potential to provide level readings independent of temperature and the liquid density.
© 2015 Optical Society of America
1. Introduction
Polymer optical fibers (POFs) are increasingly considered as a potential alternative to silica fibers in certain sensing applications [1–4]. POFs have a number of potential advantages, such as excellent flexibility, higher mechanical resistance to impacts and vibrations, low cost, and ruggedness. They can survive higher strain than silica and they possess a much lower Young’s modulus [2,5]. In view of these qualities, POF has been used as a transducer or signal transmission medium in the measurement of temperature, humidity, strain, refractive index, acceleration and so on [4–8].
Research into liquid-level sensing technologies is of great importance because such measurements are crucial to industrial applications, such as fuel storage, for providing flood warning and in the biochemical industry. Traditional liquid level sensors are based on electromechanical techniques; however they suffer from intrinsic safety concerns in explosive environments. Given that optical fiber sensors have many well-known advantages such as high accuracy, compact size, cost-effectiveness and ease of multiplexing [9], a number of optical fiber liquid level sensors based on different operating principles have been investigated in recent years [10–13]. Several of these rely on coupling light out of the fiber, either by thinning the cladding of the fiber [10], by removing several zones of the cladding [11] by side-polishing the cladding and a portion of core [12] or by using a spiral side-emitting optical fiber [13]. Many types of liquid level sensors using silica fiber gratings have been demonstrated [14–19]. Two examples are based on the fiber Bragg grating (FBG) [14,15]. The first of these uses an etched FBG, however the device became fragile due to the use of hydrofluoric acid [14]. The second one is based on the bending of a cantilever beam and the device displayed some instability [15]. Another level sensor is based on long period gratings but the fabrication procedure is complicated [16]. Tilted FBGs have also been used [17–19], however the measurement range is limited to the tilted FBG length [17,18]. All these sensors exhibit some drawbacks such as low sensitivity [11,12], limited range [14,18,19], long-term instability [13], limited resolution [10,11], high cost [10] and complicated manufacturing [10,14,15,17].
In this paper, a simple and highly sensitive liquid level sensor using a polymer optical fiber Bragg grating (POFBG) is investigated. The key element of the sensor is the POFBG embedded in a silicone rubber diaphragm. The performance of this sensor is compared with a similar sensor incorporating an FBG inscribed in silica fiber and exhibits a factor of 5 improvement in sensitivity, resulting from the much lower elastic modulus of POF compared to silica fiber. The temperature behavior and measurement resolution were also studied in detail. Compared with other liquid level sensors [20], the proposed configuration displays much greater sensitivity, highly linear response, high resolution and good repeatability. Furthermore, a novel configuration involving multiple pressure sensors is proposed that offers advantages over the single sensor, particularly an insensitivity to the density of the liquid being monitored.
2. Operation principle
The sensor takes the form of a diaphragm sealed over a cavity containing air at atmospheric pressure. When submerged in a liquid, the side of the diaphragm exposed to the liquid experiences a greater pressure than the air facing side. If the displacement of the diaphragm is sufficiently small, such that the pressure in the cavity remains approximately constant, then the pressure difference across the diaphragm, Δp, is given by
where ρ is the liquid density (kg/m3), g is the gravitational acceleration (m/s2) and h is the height of the liquid.A diaphragm disk of diameter 2r and thickness t is deflected when there is an external pressure increase owing to increasing liquid level. This in turn causes strain to appear across the diaphragm disk, and with an FBG attached to, or embedded in the disk, this strain can be measured.
For a clamped circular diaphragm, the center deflection δc, is given by the following equation as long as the material remains within the elastic region [21]:
where υ is Poisson’s ratio, r is the radius of the diaphragm, and E is the Young’s modulus of the diaphragm. The maximum strain εmax, at the center of the diaphragm is a linear function of Δp, and is given as:The resultant deformation of the diaphragm will change the physical fiber dimensions resulting in a change in the Bragg wavelength λB. The wavelength shift ΔλB, caused by this deformation is given as [22]:
where λB is the initial Bragg wavelength and is the photo-elastic coefficient of the fiber. By monitoring the wavelength shift of the FBG, the level of liquid can be inferred.If we take typical values for silicone rubber (υ = 0.47, r = 9.5 mm, t = 1.1 mm, and E = 0.0016 GPa), δc calculated from Eq. (2) is 4.1 mm for a water level of 75 cm. From Eq. (4) we can estimate the wavelength shift, ΔλB, caused by this deformation. For the poly(methyl methacrylate) (PMMA) material, taking the Pockels coefficients to be p11 = 0.3 and p12 = 0.297, υ = 0.34, and neff = 1.49, we calculate ΔλB as 29 nm.
3. Design of sensor
3.1 Fabrication of POFBGs
Several identical FBGs were inscribed in single-mode POF fabricated from PMMA - for details of the fabrication see [23]. Most polymer fibers in use today are based on PMMA, although it should be noted that other materials may have advantages for sensing applications [1]. The single-mode fiber has a core diameter of ~8 μm and an outer diameter of ~125 μm. 10 cm long POF sections were laid in a v-groove and taped down using polyimide tape to prevent them moving during inscription, which was carried out by illuminating from above a phase mask placed on top of the POF using 325 nm UV light from a helium-cadmium (HeCd) laser with a power output of 30 mW. The HeCd laser beam was focused vertically downward using a 10 cm focal length cylindrical lens, through the phase mask, and onto the fiber. A butt-coupled connection was made between one arm of a 1550 nm single-mode silica coupler and the POF using a fiber connector/angled physical contact (FC/APC) connector on the silica fiber. A small amount of index matching gel was used in order to reduce Fresnel reflections, lowering the background noise. The inscription process was monitored using a broadband light source (provided by Thorlabs ASE-FL7002-C4 centered at 1560 nm), and an optical spectrum analyzer (OSA) connected to the coupler. The optimum inscription time for this fiber is between 35 and 45 min. Following grating inscription, the POF sections containing FBGs were UV-glued (Norland 78) to one 8° angled silica fiber pigtail.
3.2 Diaphragm preparation
The silicone rubber used for the diaphragm was prepared by mixing homogeneously liquids of silicone rubber (SILASTIC® T-4 base from Dow Corning Corporation) and curing agent (SILASTIC® T-4 catalyst) in a ratio of 100:10 by volume. Hand mixing was carried out for a short period of time and done slowly to avoid the generation of air bubbles. To ensure thorough mixing of base and curing agent mixing was carried out in small quantities. The mixture was then placed in a vacuum chamber to remove the entrapped air. After 2-3 minutes under controlled vacuum, the mix was inspected to check if it was free of air bubbles. The de-gassed silicone rubber solution was poured in a circular plastic container 50 mm in diameter and 1.1 mm in height, in which was also placed the POF containing the FBG. Figure 1 shows the design of the plastic container used for the diaphragm fabrication. Additional care was taken to ensure the POFBG was at the center of the diaphragm. To guarantee a reasonable uniformity of the diaphragm, a metal piece was placed on top of the container to exert a slight load. Furthermore, the viscous liquid nature of the mixture before curing helps ensure the uniformity of the diaphragm. With regard to uniformity, diaphragms were obtained with thickness between 1.04 mm and 1.10 mm. The mold was kept undisturbed for 24 hours at room temperature to allow the silicone rubber to set. Later, the excess silicone rubber was cut in order to give the diaphragm a specific size according with each liquid level sensor configuration. It should be noted that regardless of the configuration used, a slight strain was applied to the diaphragm when it was sealed in its housing to avoid hysteresis effects. The silicone rubber is generally nonreactive, stable, resistant to liquids, translucent in appearance, hard but flexible, resistant to extreme environments and temperatures from −50°C to + 250°C, while still maintaining its useful properties [24].
3.3 Single POFBG sensor
The sensor configuration is presented in Fig. 2. It is based on an aluminum gasket, which houses the silicone rubber diaphragm with a POFBG embedded directly in it. Figures 2(a) and 2(b) are the SolidWorks image and technical designs of the aluminum gasket and retaining ring. The square aluminum gasket piece has a 50 mm width and 10 mm height, containing a central cavity 15 mm in diameter and with a depth of 5 mm. Eight screw holes were marked off and drilled around the cavity, separated by 45°. The retaining ring had a 40 mm diameter and the central hole a 19 mm diameter. The retaining ring also had eight holes to suit M3 screws. To fix the diaphragm in position, silicone sealant was placed around the rim of the diaphragm on both sides, with the diaphragm then sandwiched between the base and retaining ring. Eight screws were used to hold the base and retaining ring together producing a seal once the sealant had cured for 24 hours. To complete the seal, a thin layer of silicone sealant was brushed around the external joint where the retaining ring meets the base. In this way, an air cavity was formed underneath the diaphragm. Figure 2(c) is a photograph of the assembled sensing unit and Fig. 2(d) is an appropriate zoom showing the POFBG embedded in the silicone rubber.
Fig. 2 Design of the sensor system using a single POFBG: (a) sensor base design; (b) complete sensor with diaphragm and retaining ring. (c) Photograph of the assembled sensor unit. (d) Zoom showing the POFBG embedded in the silicone rubber.
3.4 Multi-sensor based system
This is a new development that builds on the idea of determining liquid level by measuring the pressure at the bottom of the liquid container; but with some critical advantages. The system features several FBG based pressure sensors as described above placed at different depths. Any sensors that are above the surface of the liquid will all read the same ambient pressure. Sensors below the surface of the liquid will read pressures that increase linearly with depth. The position of the liquid surface can therefore be approximately identified as lying between the first sensor to read an above-ambient pressure and the next higher sensor. This level of precision would not in general be sufficient for most liquid level monitoring applications; however a much more precise determination of liquid level can be made by linear regression to the pressure readings from the sub-surface sensors, as shown in Fig. 3(a). There are several remarkable advantages to this multi-sensor approach: firstly, common mode temperature induced wavelength shift in the individual sensors is automatically compensated. Secondly, temperature induced changes in the sensor pressure sensitivity are also compensated. Thirdly, the approach provides the possibility to detect and compensate for malfunctioning sensors. Finally, the system is immune to changes in the density of the monitored fluid and even to changes in the effective force of gravity, as might be obtained in an aerospace application. The design of the prototype multiple sensor configuration consists of a circular acrylic tube (80 cm length, 46 mm outer diameter and 38 mm inner diameter), with windows drilled at equidistant positions along it as shown in Fig. 3(b). It contains five sensors positioned over 15 mm diameter holes spatially separated by 150 mm (with the first hole separated by 50 mm from the tube base). The sensors were then placed at positions aligned with the window positions such that the FBG center aligned with the window center. As in the single sensor configuration, a silicone sealant was used to seal the sensing area. Figure 3(c) is a photograph of the system, displaying three of the five sensors used.
Fig. 3 (a) Left: five discrete pressure sensors, with three submerged in liquid; right: determination of liquid level using linear regression. (b) Diagram of the acrylic tube sensor arrangement using multi-POFBGs. (c) Photograph of the multi-POFBG sensor.
With the tube sealed at the bottom, and open at the top, the atmospheric pressure inside the tube remains relatively constant. The system relies on increasing hydrostatic pressure deforming the diaphragms causing the fiber to become elongated, which results in a positive shift in the Bragg wavelength. When not immersed in any liquid, the internal pressure matches the external pressure; therefore the diaphragm is not deformed.
4. Experimental results and discussion
The experimental setup for evaluation of the liquid level sensitivity of the sensor is depicted in Fig. 3. Sensors were installed in a liquid container of 80 cm height with an inner diameter of 94 mm. The experimental setup consists of a broadband light source (provided by Thorlabs ASE-FL7002-C4 centered at 1560 nm), a coupler, and a high-resolution OSA.
The sensor performance was tested within a liquid level range of 0 to 75 cm and with a liquid level increment step of 5 cm. Cyclic testing was performed to investigate the increasing and decreasing of liquid level. To guarantee the stability of the sensor, a constant liquid level was maintained for a period of 5 minutes at each step before the reading was taken. Less than 3 pm shift of the central wavelength was observed over each such step period.
4.1 Single POFBG sensor configuration
The results of the configuration based on a single POFBG (Fig. 4(a)) are shown in Fig. 5. The results demonstrate that the sensor gives a highly linear response over the entire measurement region. The wavelength shift induced over the 75 cm measurement region was around 4.3 nm, leading to a mean sensitivity of 57.2 ± 0.4 pm/cm. At the end of section 2 some theoretical predictions of sensor behavior were provided. The calculated deflection caused by 75 cm of water was 4.1 mm, which was similar to the deflection in the real device (4.0 ± 0.5 mm). The calculated wavelength shift for the same depth however was 29 nm, which is considerably greater than the 4.3 nm observed experimentally. We attribute this difference to the modulus of the PMMA (around 3 GPa) being much greater than that of the silicone rubber (0.0016 GPa). The much stiffer fiber will locally reinforce the more elastic diaphragm, restricting its elongation and reducing the strain in the region of the diaphragm close to the fiber.
Fig. 4 Experimental setup for liquid level measurement using (a) a single diaphragm/POFBG sensor and (b) multiple diaphragm/POFBG sensors.
Fig. 5 Response of the Bragg wavelength shift versus liquid level using a single diaphragm/POFBG sensor.
Cyclic testing was also performed to investigate the repeatability of the sensor. Three experiments using different diaphragm/POFBG sensors were carried out, with five full cycles each. Each cycle involved the sensor initially starting at the container base, being raised in 5 cm intervals until it reached the liquid surface, then being lowered in 5 cm intervals until it finally reached the container base. The thickness of each diaphragm for experiments 1, 2 and 3 was 1.10 mm, 1.07 mm and 1.09 mm. It should be noted that we tested other thicknesses of silicone rubber diaphragm (3.0 mm and 3.5 mm) however, a thickness around 1 mm was found to be the thinnest that could be easily fabricated and provided a better sensitivity than the thicker devices. The sensitivity values for each increasing and decreasing depth response were extracted for the three different experiments and are listed in Table 1. They demonstrate good repeatability. The overall sensitivity was found to be 57.3 ± 0.4 pm/cm.
Table 1. Single POFBG sensor system – wavelength / sensitivity analysis.
For comparison purposes, a similar sensor was fabricated using an FBG inscribed in 9/125 μm silica optical fiber. Five full cycles were performed to compare the sensitivity range. Figure 6 shows the first cycle demonstrating that the sensor gives a much more repeatable response than the POFBG over the measurement range however with less sensitivity: 10.22 ± 0.05 pm/cm. The results of the five cycles, summarized in Table 1, demonstrate a consistent sensitivity with a mean value of 10.21 ± 0.09 pm/cm. These results indicate that the sensitivity using POFBGs is increased by more than 5 times, compared to the FBGs inscribed in silica fiber, though it should be noted that the relative errors in the sensitivities are comparable at about 1%. Furthermore, it is considerably larger than that of the other previously published studies mentioned in the introduction [9–19]. A similar study was performed in [25] using a silica FBG embedded in a 0.42 mm-thick Kapton diaphragm, however this provided a sensitivity around 9 pm/cm, similar to the silica sensor investigated in this work.
Fig. 6 Response of the wavelength shift versus liquid level using a single diaphragm/FBG sensor based on a silica fiber.
4.2 Multi-POFBGs sensor configuration
The possibility of using a multi-POFBG based sensor was also explored using the experimental setup of Fig. 4(b). The experimental procedure was similar to the previous case. The sensor positions relative to the container base are shown in Fig. 3. Here, we first observed the behavior of each sensor when the liquid level was increased and decreased. Five full cycles were carried out. A set of optical fiber couplers was used in order to obtain the data from all the sensors simultaneously; this was possible because the gratings operated in different wavelength ranges.
Figure 7 shows the first cycle of each of sensors 1, 2, 3 and 4. Sensor 1 measures the liquid level range from 0 cm to 75 cm (considering the 5 cm position shown in Fig. 2 as the beginning of the measurement range); sensor 2 from 20 cm to 75 cm; sensor 3 from 35 cm to 75 cm; and sensor 4 measures from 50 cm to 75 cm. The wavelength shift was extracted and the sensitivity of each sensor was calculated, showing similar values for the increasing and decreasing of liquid level and a good linearity. The results of the five cycles of each sensor are summarized in Table 2, showing a sensitivity with a mean value of 54.5 ± 0.5 pm/cm (sensor 1), 54.0 ± 0.7 pm/cm (sensor 2), 54.9 ± 1.0 pm/cm (sensor 3) and 53.7 ± 1.9 pm/cm (sensor 4).
Fig. 7 Responses of the wavelength shift versus liquid level using multiple diaphragm/POFBG sensor: (a) sensor 1, (b) sensor 2, (c) sensor 3 and (d) sensor 4.
Table 2. Multi-POFBG sensor system – wavelength / sensitivity analysis.
The first three sensors were submerged to allow each to respond to hydrostatic pressure changes. The aim of this experiment was to study the behavior of three sensors submerged as shown in Fig. 8(a). Thus, the liquid depth was varied between 40 cm and 75 cm, giving a 35cm measurement region (see Fig. 8(a)). Figures 8 (b)-8(d) show the responses from sensor 1, sensor 2 and sensor 3, respectively. Table 4 summarizes these results.
Fig. 8 (a) Diagram of the acrylic tube sensor arrangement showing the sensors submerged and the measurement range. (b-d) Wavelength change versus liquid level for each submerged sensor.
Table 4. Measurement range and resolution of several liquid level sensors.
Comparing these results with the single POFBG sensor configuration in terms of sensitivity variation between each experiment (experiment 1, 2 and 3 – Table 1), one can see that there is a greater discrepancy for the multi-sensor case (see Table 3). There are two factors contributing to the increased variation between sensors. The first is most likely to be attributed to the amount of silicone sealant used. Ideally, all sensors would be bonded in the exact same position using the exact same amount of silicone sealant. In practice, the positioning and bonding process was not controlled well enough to guarantee this and therefore it is highly probable that significant discrepancies in grating positioning and silicone sealant quantity exist. Second, the slight strain applied to the POFBG/diaphragms, when they are sealed to the prototype, may be different for each of the multi-POFBG sensors. These are limitations of the manual assembly process and could be significantly reduced in a proper manufacturing process.
Table 3. Wavelength / sensitivity analysis of three submerged sensors.
In addition, experiments were carried out to investigate the temperature response of the sensors. For this experiment four sensors (sensor 1, 2, 3 and 4) were submerged and sensor 5 was kept out of the liquid to compare the temperature behavior. The entire prototype was then placed in an environmental chamber (Sanyo Gallenkamp) under varying temperatures to study its response. The temperature was increased with steps of 5°C from 18°C up to 43°C. In each step, the temperature was kept constant over 4 hours to ensure thermal equilibrium was achieved. Figure 9 shows the measured wavelength shift of each sensor at different temperatures. It can be seen that the measured results show a typical POFBG temperature behavior [26]. From Fig. 9, the change in Bragg wavelength over the 25 temperature variation was obtained for sensors 1 to 5 giving −1.34 nm, −1.35 nm, −1.33 nm, −1.30 nm and −1.15 nm, respectively. It may be seen that the sensor out of the water (sensor 5) shows significantly less sensitivity to temperature when compared to the submerged sensors. We propose the following explanation for the difference. The sensors 1, 2, 3 and 4 can be compared to pre-strained POFBGs since the submerged sensors are under pressure-induced strain. Since the POFBG is under pressure-induced strain, the length of the PMMA optical fiber does not undergo significant thermally induced expansion (given that the applied strain was larger than any temperature induced fiber expansion). In this case the POFBG temperature sensitivity relies mainly on the thermo-optic effect of the fiber. According to [27], restricting the thermal expansion contribution to the grating wavelength change by pre-straining the fiber can increase the sensitivity to temperature and improve the response linearity. For standard silica fiber the volume change due to the thermal effect is very small. However, in PMMA optical fiber the thermal expansion is much larger and the refractive index change is significantly altered by the change of volume and polarizability [28]. In this particular case, where the POFBG is confined to a silicone rubber diaphragm and both are pre-strained by the liquid pressure, the optical fiber cannot expand and contract freely, its thermal expansion is restrained and its density change is reduced [29]. Consequently, the sensor out of the liquid shows a smaller temperature coefficient since it is not under pressure-induced strain.
Fig. 9 Measured wavelength shift versus temperature variation for submerged sensors (sensor 1, sensor 2, sensor 3, and sensor 4) and the sensor above the liquid (sensor 5).
4.3 Determination of liquid level using linear regression
From Fig. 3(a), the determination of liquid level can be made by linear regression to the wavelength shift from sub-surface sensors at different depths. Two cases are presented for liquid surface level analysis using linear regression: first, when the liquid surface level, Lsurface, is at 60 cm and secondly when the Lsurface is at 43 cm. Initially the container was emptied of liquid and kept undisturbed during 30 minutes at room temperature, after which the Bragg wavelength of each sensor was recorded. Next liquid was added up to a specific level and kept undisturbed for a time of 30 minutes at room temperature. Then, the Bragg wavelength of each sensor was again recorded to determine the wavelength shift caused by the hydrostatic pressure. The results are depicted in Fig. 10, showing for each case the equations of linear fit. For each equation, the position of the Lsurface will be estimated through the interception of the linear fit with the y-axis. From the first equation presented in Fig. 10(a), we achieved an intercept value of 58.07 ± 1.55 cm, being a value close to the real value – 60 cm of liquid level. For the equation shown in Fig. 10(b), we achieved a value of 41.23 ± 2.17 cm.
Fig. 10 Determination of liquid level using linear regression for a position of the liquid surface at (a) 60 cm and (b) 43 cm.
The temperature response has an impact on the accuracy of the system. Figure 9 shows that a temperature rise of 10 °C induces a wavelength shift of approximately 0.5 nm. In the case of a single sensor, Fig. 7 shows that such a wavelength shift is equivalent to a level change of about 9 cm and so this represents the error that would result from a 10°C rise in temperature. In the case of the multiple sensor system employing linear regression, the slightly different temperature sensitivities of the submerged sensors compared to those above the liquid mean that temperature changes still cause an inaccuracy in the recovered level, but in this case a 10 °C rise is equivalent to an error of just under 2 cm in the liquid level. If we can achieve sensors where the thermal sensitivity is independent of whether the sensor is submerged then the thermally induced error would be completely removed. This is the subject of our current research.
4.4 Resolution of the sensors
The measurement resolution of the individual sensor fabricated in this work was also analyzed. A crude method which is nevertheless used in the literature is to observe the short-term fluctuations in the Bragg wavelength reading from the OSA and directly relate this to a liquid depth resolution using the measured sensitivity. In the case of the POFBG sensors the 3-10 pm variation corresponds roughly to a 2 mm resolution. We believe that a better, more rigorous, approach to the determination of resolution is to calculate the root mean square deviation of the data points from the line of the best fit in Fig. 8, which leads to a resolution just under 10 mm. Nevertheless, in Table 4 where we compare the resolution of our system some previously published studies [10–12,14,18], we use the better figure of 2 mm since it was obtained using a similar method to those employed in the earlier studies and hence offers a fairer comparison. In this table, the sensors with larger measurement range have a lower resolution, while the sensors with higher resolution have a smaller range. Our results can be seen to compare very favorably in terms of range-to-resolution, which represents the number of effective measurement points.
5. Conclusion
In conclusion, a highly sensitive liquid level monitoring sensor based on a POFBG embedded in a silicone rubber diaphragm has been proposed and its performance characterized. The simple structural configuration not only eases the fabrication requirements but also improves the compactness of the device. The experimental results show that the proposed sensor has a high sensitivity to liquid level, great repeatability when the liquid level increases and decreases and exhibits a highly linear response. This POFBG sensor, when compared with a similar sensor based on a silica fiber grating, exhibits 5 times sensitivity, due to the fiber’s much lower elastic modulus. Furthermore, studies of the behavior at different temperatures of the sensors submerged in liquid were performed and compared with sensors in air, revealing a smaller temperature coefficient for the sensor out of the liquid, likely due to the sensor not being under pressure-induced strain. A multi-sensor level monitoring system is proposed to enable operation insensitive to temperature, liquid density and even effective gravitational force. Finally, the results suggest the new configurations can be useful in different applications such as aircraft fuel monitoring and biochemical and environmental sensing, where accuracy, stability and performance are crucial.
Acknowledgment
This work was supported by a Marie Curie Intra European Fellowship included in the 7th Framework Program of the European Union (POSSIBLE PIEF-GA-2013-628604 project).
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