Sensors and Actuators A: Physical Liquid-level monitoring sensor ...
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<strong>Sensors</strong> <strong>and</strong> <strong>Actuators</strong> A 152 (2009) 248–251<br />
Contents lists available at ScienceDirect<br />
<strong>Sensors</strong> <strong>and</strong> <strong>Actuators</strong> A: <strong>Physical</strong><br />
journal homepage: www.elsevier.com/locate/sna<br />
<strong>Liquid</strong>-<strong>level</strong> <strong>monitoring</strong> <strong>sensor</strong> systems using fiber Bragg grating<br />
embedded in cantilever<br />
Kyung-Rak Sohn ∗ , Joon-Hwan Shim<br />
Department of Electronics <strong>and</strong> Communications Engineering, Korea Maritime University, 1 Dongsan-Dong, Youngdo-Gu, Busan 606-791, South Korea<br />
article info<br />
Article history:<br />
Received 25 November 2008<br />
Received in revised form 30 March 2009<br />
Accepted 3 April 2009<br />
Available online 10 April 2009<br />
Keywords:<br />
Fiber Bragg grating (FBG)<br />
<strong>Liquid</strong>-<strong>level</strong> sensing<br />
Intensity demodulation<br />
Interrogation<br />
Thermo-optic effect<br />
1. Introduction<br />
abstract<br />
Sensing of liquid-<strong>level</strong>s in vessels is important in the <strong>monitoring</strong><br />
of liquid in fuel containers, oil tanks, <strong>and</strong> ballast water tanks [1–4].A<br />
wide range of liquid-<strong>level</strong> <strong>sensor</strong> systems have been reported <strong>and</strong><br />
partially commercialized. For example, systems based on electric<br />
pressure, radar beam, magnetic floating, air purging type, <strong>and</strong> electric<br />
pneumatics have already developed for oil tanker applications.<br />
In particular, electrical liquid-<strong>level</strong> <strong>sensor</strong>s are widely employed,<br />
but their applicability is restricted if the environment is explosive<br />
or the liquid to be sensed is conductive. In comparison, liquid-<strong>level</strong><br />
<strong>sensor</strong>s based on optical fibers have well-established advantages.<br />
First, optical fiber <strong>sensor</strong>s are dielectric, electrically passive, <strong>and</strong><br />
inherently nonflammable. In the case that the <strong>sensor</strong> has to run<br />
along to a long path, optical fibers can be easily installed because<br />
they are smaller, lighter, <strong>and</strong> more flexible than copper-based wires.<br />
Several optical fiber <strong>sensor</strong>s for detecting a liquid-<strong>level</strong> have been<br />
proposed. Yang et al. [2] reported a multiplexed array of fiber optic<br />
liquid-<strong>level</strong> point <strong>sensor</strong>s in a cryogenic environment that uses<br />
rectangular prisms as optical <strong>sensor</strong>s. Lomer et al. [3] presented<br />
lateral side-polished fiber optic liquid-<strong>level</strong> <strong>sensor</strong>s. These two <strong>sensor</strong><br />
designs are based on the serial connection of liquid probes.<br />
However, they cannot continuously indicate the liquid-<strong>level</strong>. Guo<br />
et al. [4] proposed fiber Bragg grating liquid-<strong>level</strong> <strong>sensor</strong>s based<br />
on a cantilever beam. The spectral width at the Bragg wavelength<br />
∗ Corresponding author. Tel.: +82 51 410 4312; fax: +82 51 404 3986.<br />
E-mail address: krsohn@hhu.ac.kr (K.-R. Sohn).<br />
0924-4247/$ – see front matter © 2009 Elsevier B.V. All rights reserved.<br />
doi:10.1016/j.sna.2009.04.003<br />
A fiber-optic liquid-<strong>level</strong> <strong>sensor</strong> based on the bending of a fiber Bragg grating (FBG) is proposed. The<br />
FBG embedded in a cantilever rod such that the elongation <strong>and</strong> contraction of the Bragg grating indicate<br />
a liquid-<strong>level</strong> change. The shift in the Bragg wavelength has a good linear response to the bending of<br />
the cantilever. An optical b<strong>and</strong>pass filter is used to determine the Bragg wavelength by converting the<br />
wavelength shift to an intensity demodulation. The relative power measured by a photo-detector is also<br />
linearly proportional to the liquid-<strong>level</strong> variation. The sensitivity is approximately 0.1 dB/cm. To eliminate<br />
the temperature dependence of the FBG <strong>sensor</strong>, we suggest that the tuning of the optical b<strong>and</strong>pass filter<br />
should take into account the temperature variation. The operating principle is described in detail.<br />
© 2009 Elsevier B.V. All rights reserved.<br />
changes with the bending of the cantilever beam. The liquid-<strong>level</strong><br />
is monitored by optical detection via pin photo-detector. However,<br />
it is difficult to determine the direction of the liquid-<strong>level</strong><br />
variation.<br />
In this study, we propose <strong>and</strong> demonstrate a novel liquid-<strong>level</strong><br />
<strong>monitoring</strong> systems, which is based on wavelength-dependent loss,<br />
using an FBG embedded in a cantilever rod <strong>and</strong> an optical b<strong>and</strong>pass<br />
filter (OBPF). For high-resolution wavelength-shift detection, interferometric<br />
wavelength discrimination <strong>and</strong> a tunable, narrowb<strong>and</strong><br />
filter can be used for interrogation. In this experiment where high<br />
resolution is not paramount, only simple wavelength-shift detection<br />
is required. The performance of the OBPF as a wavelength<br />
demodulation element is described <strong>and</strong> the characteristics of the<br />
cantilever rod as a liquid-<strong>level</strong> <strong>sensor</strong> are investigated.<br />
2. Operating principle <strong>and</strong> <strong>sensor</strong> system design<br />
The basic principle of the <strong>sensor</strong> is the Bragg wavelength shift<br />
that results from the bending of the cantilever installed with a FBG<br />
during liquid-<strong>level</strong> change [5–7]. A schematic diagram of the proposed<br />
fiber-optic liquid-<strong>level</strong> <strong>sensor</strong> is shown in Fig. 1. The <strong>sensor</strong><br />
consists of a water tank, polypropylene rod as a cantilever, FBG, <strong>and</strong><br />
properly designed float. The FBG (Fiberpia Inc., Korea) centered at<br />
1530.8 nm wavelength, has a length of 15 mm, a full width at half<br />
maximum (FWHM) 3-dB b<strong>and</strong>width of 0.5 nm, <strong>and</strong> a reflectivity<br />
of 20 dB. To embed the FBG in the rod, a groove of about 1.5 mm<br />
width <strong>and</strong> 2 mm depth was cut into the center of the rod. The FBG<br />
was glued into the groove with ultraviolet epoxy resin as shown in<br />
Fig. 1(a).
Fig. 1. Schematic diagram of a FBG-based liquid-<strong>level</strong> <strong>sensor</strong> system; (a) basic structure<br />
of the cantilever rod <strong>and</strong> (b) experimental setup.<br />
The cantilever with a length of 800 mm is attached to the support<br />
beam by a pivot. The surface of the groove with the FBG initially<br />
points downward. When the liquid <strong>level</strong> rises or falls, the cantilever<br />
rod accordingly bends upwards or downwards, resulting in a Bragg<br />
wavelength shift of the FBG. The pivot is designed for vertical movement<br />
of the float. To prevent free rotation of the float, guardrails<br />
are installed in the tank as shown in Fig. 1(b). The float, which has<br />
a roller to minimize friction with the rails, slides up <strong>and</strong> down the<br />
rails as the liquid <strong>level</strong> changes.<br />
For the FBG, the shift in Bragg wavelength BG is expressed as<br />
a function of strain <strong>and</strong> temperature [8,9]:<br />
BG<br />
= (1 − Pe)εx + (˛c + ςf)T (1)<br />
BG<br />
where εx is the axially applied strain, Pe is the photoelectric coefficient<br />
of the fiber, BG is the Bragg wavelength, ˛c is the coefficient of<br />
the thermal expansion of the cantilever material, f is the thermooptic<br />
coefficient of the fiber material, <strong>and</strong> T is the ambient<br />
temperature change. Using material mechanics theory, the axially<br />
applied strain can be represented as<br />
εx = 3<br />
2L3 ( c + f) · (L − xf) · Dy (2)<br />
where L is the length of the cantilever. c <strong>and</strong> f are the diameters<br />
of the cantilever <strong>and</strong> optical fiber, respectively. xf is the distance<br />
to the center of the FBG <strong>and</strong> Dy is the vertical displacement at the<br />
end of the cantilever rod. Thus, the shift in the Bragg wavelength is<br />
given by<br />
BG<br />
=<br />
BG<br />
3<br />
2L3 (1−Pe)( c+ f) · (L − xf) · Dy + [ςf + (1 − Pe)˛c]T (3)<br />
The relative change in the Bragg wavelength is linearly proportional<br />
to the vertical displacement of the cantilever <strong>and</strong><br />
temperature.<br />
K.-R. Sohn, J.-H. Shim / <strong>Sensors</strong> <strong>and</strong> <strong>Actuators</strong> A 152 (2009) 248–251 249<br />
Fig. 2. Reflection spectra of FBG; (a) liquid-<strong>level</strong> rise <strong>and</strong> (b) liquid-<strong>level</strong> fall.<br />
3. Experiment <strong>and</strong> discussion<br />
Fig. 1(b) shows the experimental setup for measuring the spectral<br />
response <strong>and</strong> wavelength-encoded strain change of the FBG<br />
corresponding to the water-<strong>level</strong> variation. If broadb<strong>and</strong> light travels<br />
in the core of the fiber via a 3-dB coupler, an incident beam<br />
with a resonant wavelength will be reflected back to the 3-dB coupler.<br />
When the water <strong>level</strong> rises, the cantilever rod moves upwards,<br />
causing the Bragg wavelength to shift to a longer wavelength. An<br />
optical spectrum analyzer (OSA) is used to measure the Bragg wavelength<br />
shift by <strong>monitoring</strong> the reflection spectrum of the FBG. The<br />
reflection spectra are measured in 6-cm steps of the rise or fall<br />
of the liquid <strong>level</strong> in the tank, as shown in Fig. 2. When the liquid<br />
<strong>level</strong> falls, the Bragg wavelength shifts to a shorter wavelength.<br />
In spite of there being a continuous strain applied to the FBG, the<br />
shape <strong>and</strong> spectral b<strong>and</strong>width of the reflection spectrum are almost<br />
unchanged because the length of the FBG is much shorter than the<br />
length of cantilever on which the FBG is fixed. During the liquid<strong>level</strong><br />
varies by 36 cm, the Bragg wavelength changes by as much as<br />
5.4 nm. The wavelength-shift sensitivity to the liquid-<strong>level</strong> is about<br />
0.15 nm/cm.<br />
Plots of the Bragg wavelength shift versus the liquid-<strong>level</strong> change<br />
are shown in Fig. 3. The relationship between the two parameters<br />
is quite linear. Regardless of the direction of the liquid-<strong>level</strong> varia-
250 K.-R. Sohn, J.-H. Shim / <strong>Sensors</strong> <strong>and</strong> <strong>Actuators</strong> A 152 (2009) 248–251<br />
Fig. 3. Experimental results for the shift in the Bragg wavelength versus the liquid<strong>level</strong><br />
changes at room temperature.<br />
tion, the datum traces almost coincide at the same liquid-<strong>level</strong>. The<br />
cantilever-based FBG <strong>sensor</strong>s thus possess reversibility.<br />
Fig. 4 shows the interrogation system based on intensity demodulation.<br />
In this experiment, LabVIEW software is used for data<br />
acquisition, signal processing, <strong>and</strong> as the user interface. An OBPF is<br />
applied as an interrogation element to determine the wavelengthdependent<br />
loss.<br />
The principle of the demodulation mechanism was illustrated<br />
as follows. The optical spectrum was measured after the incident<br />
light had been reflected by the FBG <strong>and</strong> passed through the OBPF,<br />
<strong>and</strong> typical results are depicted in Fig. 5. The Bragg wavelength<br />
of the FBG must be at an optical wavelength at which the OBPF<br />
curve has an approximately constant slope (when measured in<br />
logarithmic units of intensity, dBm) over a wide range of wavelengths.<br />
The broadb<strong>and</strong> optical source must have a constant output<br />
over this wavelength range. Under these conditions, the narrowb<strong>and</strong><br />
reflected power from the FBG passed through the OBPF with<br />
transmission power approximately linearly dependent upon the<br />
wavelength. The accuracy of this linearity depends upon both the<br />
linearity of the OBPF response <strong>and</strong> the uniformity of the broadb<strong>and</strong><br />
source over the wavelength range concerned.<br />
For this <strong>sensor</strong> structure, the temperature dependence of the<br />
peak wavelengths was calculated as 0.035 nm/ ◦ C using values of the<br />
coefficient of thermal expansion (TEC) of the component materials<br />
Fig. 4. Intensity-based interrogation system.<br />
Fig. 5. OBPF curve <strong>and</strong> intensity demodulation measured at room temperature.<br />
(e.g., polypropylene <strong>and</strong> cured ultraviolet epoxy resin). The TECs<br />
of these materials are known to be around 200 ppm/ ◦ C, which are<br />
several hundred times larger than those of glass fibers. As shown in<br />
Fig. 6, as the ambient temperature increases from 27 to 60 ◦ C, the<br />
reflection spectrum of the FBG (dotted line) shifts to longer wavelengths<br />
(dashed line) by as much as 1.1 nm. The optical intensity<br />
measured by the optical power meter (Wavepower 345R, Fiberpia<br />
Inc.) varied from −50.8 to −46 dBm despite there being no change<br />
in the liquid-<strong>level</strong>.<br />
It is necessary to compensate for the temperature dependence<br />
in the intensity-based interrogation, so that the spectral responses<br />
obtained at different temperature are the same. Here, we manually<br />
tune the OBPF by magnitude of the shift in the reflection spectrum<br />
of the FBG caused by the temperature change. As a result, the spectrum<br />
drawn as a dashed line is shifting toward that drawn as a solid<br />
line. However, the peak wavelength of the FBG does not change. The<br />
spectrum drawn as a solid line nearly coincides with that drawn as<br />
a dotted line. The power reduces from −46 to −50.4 dBm, which<br />
is nearly equal to that measured at 27 ◦ C. Thus, by means of our<br />
demodulating scheme using a tunable OBPF, we have achieved a<br />
temperature-compensation method for achieving the same power<br />
Fig. 6. Temperature compensation by OBPF tuning.
Fig. 7. (a) Temporal response to the liquid-<strong>level</strong> rise <strong>and</strong> fall <strong>and</strong> (b) relative power<br />
versus liquid-<strong>level</strong> at room temperature.<br />
intensity for the same liquid-<strong>level</strong> at different ambient temperatures.<br />
The temporal response of the cantilever-based liquid-<strong>level</strong> <strong>monitoring</strong><br />
system at room temperature is illustrated in Fig. 7(a).<br />
The relative optical power is continuously recorded in real time<br />
for different liquid <strong>level</strong>s in a range from 0 to 36 cm. The figure<br />
shows the performance of the interrogation system for tracking the<br />
K.-R. Sohn, J.-H. Shim / <strong>Sensors</strong> <strong>and</strong> <strong>Actuators</strong> A 152 (2009) 248–251 251<br />
Bragg wavelength shift according to the liquid-<strong>level</strong> variation. It is<br />
observed that the acquiring power is directly proportional to the liquid<br />
<strong>level</strong>. As long as the liquid <strong>level</strong> is maintained, the output power<br />
does not change. Fluctuations at the lower <strong>level</strong>s are probably due<br />
to vibrations in the liquid tank. The relative power changes with<br />
respect to the liquid <strong>level</strong> are shown in Fig. 7 (b). The demodulated<br />
output signal linearly responds to the liquid <strong>level</strong> with a sensing<br />
resolution of approximately 0.1 dB/cm. With the liquid <strong>level</strong> varying<br />
repeatedly from the top of the tank to the bottom <strong>and</strong> back to<br />
the top again, the <strong>sensor</strong> system has good stability <strong>and</strong> is able to<br />
provide consistent results.<br />
4. Conclusion<br />
We have presented an in-line fiber liquid-<strong>level</strong> <strong>monitoring</strong> <strong>sensor</strong><br />
system using an FBG embedded in a cantilever rod. Bending of<br />
the cantilever rod induces expansion or compression in the FBG <strong>and</strong><br />
leads to a shift in the Bragg wavelength. To measure the wavelengthencoded<br />
liquid-<strong>level</strong> change, an OBPF is used as an interrogation<br />
element to determine the wavelength-dependent loss. Experimental<br />
results showed that the FBG-based <strong>sensor</strong> system, which has<br />
a simple structure, exhibited good linearity, reproducibility, <strong>and</strong><br />
reversibility in response to liquid-<strong>level</strong> changes. For compensation<br />
of the temperature dependence of the <strong>sensor</strong> head, we have suggested<br />
that the OBPF tuning method should take into account the<br />
temperature variation.<br />
We expect that the proposed <strong>sensor</strong> systems will have potential<br />
applications especially in combustible environments such as<br />
oil tanks owing to the electrical passiveness of the fiber materials.<br />
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