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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 />

References<br />

[1] S.W. James, S. Khaliq, R.P. Tatam, A long period grating liquid <strong>level</strong> <strong>sensor</strong>, in:<br />

15th Optical Fiber <strong>sensor</strong> Conference 1, 2002, pp. 127–130.<br />

[2] C. Yang, S. Chen, G. Yang, Fiber optical liquid <strong>level</strong> <strong>sensor</strong> under cryogenic environment,<br />

Sens. <strong>Actuators</strong> A 94 (2001) 69–75.<br />

[3] M. Lomer, J. Arrue, C. Jaurequi, P. Aiestraran, J. Zubia, J.M. Lopez-Higuera, Lateral<br />

polishing of bends in plastic optical fibers applied to a multipoint liquid-<strong>level</strong><br />

measurement <strong>sensor</strong>, Sens. <strong>Actuators</strong> A 137 (2007) 68–73.<br />

[4] T. Guo, Q. Zhao, Q. dou, H. Zhang, L. Xue, G. Huang, X. Dong, Temperatureinsensitive<br />

fiber Bragg grating liquid-<strong>level</strong> <strong>sensor</strong> based on bending cantilever<br />

beam, IEEE Photon Technol. Lett. 17 (11) (2005) 2400–2402.<br />

[5] X. Dong, Y. Liu, Z. Liu, X. Dong, Simultaneous displacement <strong>and</strong> temperature<br />

measurement with cantilever-based fiber Bragg grating <strong>sensor</strong>, Opt. Commun.<br />

192 (2001) 213–217.<br />

[6] Y. Zhao, C. Yu, Y. Liao, Differential FBG <strong>sensor</strong> for temperature-compensated highpressure<br />

(or displacement) measurement, Opt. Laser Technol. 26 (2004) 39–42.<br />

[7] W. Zhang, X. Dong, Q. Zhao, G. Kai, S. Yuan, FBG-based <strong>sensor</strong> for simultaneous<br />

measurement of force (or displacement) <strong>and</strong> temperature based on bilateral<br />

cantilever beam, IEEE Photon. Technol. Lett. 13 (12) (2001) 1340–1342.<br />

[8] Y. Zhao, J. Yang, B.-J. Peng, S.-Y. Yang, Experimental research on a fiber-optic<br />

cantilever-type inclinometer, Opt. Laser Technol. 37 (2005) 555–559.<br />

[9] Y. Zhao, Y. Zhao, M. Zhao, Novel force based on a coupling of fiber Bragg gratings,<br />

Measurement 38 (2005) 30–33.

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