A film pressure sensor based on optical fiber Bragg ...

A Film Pressure Sensor Based on Optical Fiber Bragg Grating Zhichun Zhang*a, Gang Dengb, Yongbo Daia, Yanju Liub, Jinsong Lenga a Centre for Composite Materials and Structures, Harbin Institute of Technology (HIT), No. 2 YiKuang Street, P. O. Box 3011, Harbin 150080, People’s Republic of China; b Department of Aerospace Science and Mechanics, Harbin Institute of Technology (HIT), No. 92 West Dazhi Street, P. O. Box 301, Harbin 150080, People’s Republic of China ABSTRACT The measurement of pressure is essential for the design and flying controlling of aircraft. In order to measure the surface pressures of the aircraft, the common pressure tube method and Pressure sensitive paint measurement method have their own disadvantages, and are not applicable to all aircraft structures and real time pressure monitoring. In this paper, a novel thin film pressure sensor based on Fiber Bragg Grating (FBG) is proposed, using FBG measuring the tangential strain of the disk sensing film. Theoretical circle strain of the disk sensing film of the pressure sensor under pressure and temperature variation are analyzed, and the linear relationship between FBG center wavelength shift and pressure, temperature variation is gotten. The pressure and temperature calibration experiments prove the theoretical analysis. But the calibration sensing parameters are small than the calculating ones, which is caused by the constraint of optical fibre to the thin sensing film. Keywords: Film pressure sensor, Fibre Bragg Gratting, Sensing film, Tangential strain, Uniform strain field, Thin plate theory

1. INTRODUCTION Pressure and its distribution on the surface of aircraft provide the elevation force, and they are the important design parameters of aircraft wing, especially for large wing plane. With the morphing aircraft grown, the pressure and its distribution are not only needed in the designing of the aircraft, but also important controlling parameter in aircraft flying control. So the pressure and its distribution measurement on the surface are very important in airplane design and control. The most common used pressure measurement technology is the pressure tube measurement, in which need install pressure tubes in the hole of the body or surface of the plane. And this method cannot be applied to a thin or complicated curved body because of insufficient spaces to install the sensor or the tube by making a hole and it is not easy to measure the multipoint pressures. And this method destroys the integration of airplane, which can’t real time measure the pressure of flying plane. Pressure sensitive paint is another method to measure pressure, which is based on the pressure inducing luminescent effect, and use CCD cameras to take photos of body to measure the pressure distribution. But this method is not suitable for large area pressure measurement and real-time pressure monitoring [1, 2]. For its small size and light weight, relative high resolution, immunity to electromagnetic interference, ability to multiplexed and good durability, Fibre Optical Bragg Gratting (FBG) sensor becomes an ideal sensor for strain, temperature, deformation, pressure and others parameters measurement in the application of aerospace, medicine, civil and electron engineering [3-5]. Madoka Nakajima developed a FBG pressure sensor shown in Fig. 1, which transits the out vertical pressure by a spacer to the horizontal strain of FBG sensor, the calibration results show that there was good linear relationship between the out pressure and the FBG wavelength shift [6]. But in his study, there is no theoretical relationship between the pressure and the horizontal strain of the FBG. And at the same time, FBG sensor is only applicable in uniform strain field measurement. But his sensor, the spacer causes the non-uniform strain distribution along the FBG length, and leads the spectrum deterioration, and the center wavelength can’t reflect the strain. In this paper, a film pressure sensor is developed, in which the FBG measures the tangential strain of the round sensing film. The theoretical relationship between the pressure and tangential strain, and sensing parameter of the sensor were analyzed. And the pressure and temperature calibration experiment were done to prove the sensor design principle.

Smart Sensor Phenomena, Technology, Networks, and Systems 2010, edited by Kara J. Peters, Wolfgang Ecke, Theodore E. Matikas, Proc. of SPIE Vol. 7648, 764809 · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.847826 Proc. of SPIE Vol. 7648 764809-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 03/16/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

Fig 1 FBG pressure sensor developed by Madoka Nakajima[6]

2. FBG FILM PRESSURE SENSOR DESIGNING THEORY Because the FBG sensor is only applicable to measure the uniform strain field, and sensitive to axial strain, a new type of film pressure sensor is developed, shown in Fig. 2. The pressure sensor is composed of sensing film layer, base layer, adhesion layer, circular glued FBG and reference pressure tube. There is round hole in the base layer. By bonding the sensing film, base layer, adhesion layer and reference pressure tube together, a cavity is formed with a circular sensing film. A reference pressure tube connects the cavity to the out atmosphere, which can keep the constant reference pressure in the cavity. The FBG fibre glues circular in the lower surface of the sensing film with the same center of the base layer hole. In the circumference of circular of the FBG fibre on the sensing film, there is the same tangential strain, presuming that the FBG deforms with sensing film, and the strain can be measured by the FBG sensor. FBG Sensor

Sensing Film Base Layer

Adhesion Layer Reference Pressure Tube

FBG Fiber

Fig. 2 FBG film pressure sensor

2.1 Film Pressure Sensor Structure Mechanical Analysis Because the sensing film is glued to the boundary of the base layer hole, so the sensing film can be simplified using thin circle plate with clamped boundary condition. t and R are the thickness and radius of the sensing film respective, q is the outer pressure. The strain, stress under pressure and temperature on the circumference of the sensing film with random radius r are shown in Fig. 3.

Proc. of SPIE Vol. 7648 764809-2 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 03/16/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

q t

σ θ q , σ θ T , εθ q , εθ T

σ rq , σ rT , ε rq , ε rT R r

Fig.3. Mechanical model of the sensing film

According to the thin plate theory under clamped boundary condition:

σ rq =

σθ q =

Where:

3q 2 [ R (1 + υ ) − r 2 (3 + υ )] 8t 2

3q 2 [ R (1 + υ ) − r 2 (1 + 3υ )] 2 8t α E ΔT σ rT = σ θ T = 1 −υ

(1)

(2) (3)

σ rq , σ θ q ----the radial and tangential stress on the circumference with radius of r under out pressure respective; σ rT , σ θ T ---- the radial and tangential thermo stress on the circumference with radius of r with temperature variation respective;

ν ----- the Poisson's ratio of the sensing film; α -----the coefficient of thermal expansion of the sensing film And the total radial and tangential stress σ r and σ θ are caused by the out pressure and temperature, which is 3q α E ΔT σ r = σ rT + σ rq = 2 [ R 2 (1 + υ ) − r 2 (3 + υ )] + (4) 8t 1−υ 3q α E ΔT (5) σ θ = σ θ T + σ θ q = 2 [ R 2 (1 + υ ) − r 2 (1 + 3υ )] + 8t 1−υ According the plastic deformation theory, the strain in two the two directions ε r , ε θ under out pressure and temperature variation can be expressed:

1 3(1 − υ 2 )( R 2 − 3r 2 ) (σ r − υσ θ ) = q + αΔT E 8t 2 E 1 3(1 − υ 2 )( R 2 − r 2 ) εθ = (σ θ − υσ r ) = q + αΔT 8t 2 E E

εr =

(6)

(7) Where, E-----the tensile modulus of the sensing film From equation 6 and 7, the stain of ε r , ε θ on the circumference with radius of r of sensing film with the radius of R are linear to pressure and temperature variation respectively.


2.2 FBG FILM PRESSURE SENSOR SENSING PRINCIPLE According FBG basic sensing principle

λ0 = 2neff Λ

(8)

Δλ = α ε ε + αT ΔT

(9)

Where:

α ε αT ----the strain and temperature sensing parameter of bare Fibre Bragg Grating respectively Because the FBG is only sensitive to axial strain, strain measured by the FBG in the film pressure sensor is ε θ , and the relation between FBG wavelength shift and pressure and temperature variation is,

Δλθ = αε εθ + αT ΔT = αε (

3αε (1−υ 2 )(R2 − r 2 ) 3(1−υ 2 )(R2 − r 2 ) q + α Δ T ) + α Δ T = q + (αεα + αT )ΔT T 8t 2 E 8t 2 E

(10)

And the above equation can be simplified as,

Δλθ = K q q + KT ΔT

(11)

Where:

Kq =

3α ε (1 − υ 2 )( R 2 − r 2 ) 8t 2 E

(12)

KT = α ε α + α T

(13)

K q , KT are the pressure and temperature sensing parameter of the film pressure sensor respectively. To a film pressure sensor with defined sensing film radius R and FBG installed radius r, the above two parameters are constant, so there are linear relationship between wavelength shift and pressure, temperature variation respectively. The K q is sensitive to sensing film material and dimension properties, which increases with sensing film radius increasing, and decreases with the increasing of FBG installed radius, the KT is only sensitive to sensing film coefficient of thermo expansion and bar FBG sensing parameter.

3. FBG FILM PRESSURE SENSOR CALIBRATION EXPERIMENTS 3.1 FBG Film Pressure Sensor Fabrication According to the theoretical analysis, the FBG film pressure sensor was made, as shown in Fig. 4. The sensing film layer was made of polyimide film, in which the thickness was 0.1mm, Possion’s ratio was 0.3, tensile elastic modulus was 2.8GPa, coefficient of thermal expansion was 18ppm/℃. The base layer was made of plastic plate, which 0.5mm in thickness and with hole of 9mm in radius. Adhesion layer was made of aluminum plate with the thickness of 0.1mm. The FBG fibre installed radius was 6mm, the α ε , α T are 1.2 pm / με and 10.2 pm / ℃ respectively. According equation 12 and 13, the calculation pressure and temperature sensing parameters of the film pressure sensor are K q = 658.125pm / kpa , KT = 31.8pm/℃ respectively.


Fig.4. Photo of film pressure sensor

3.2 FBG Film Pressure Sensor Calibration Experiments From equation 11, the FBG film pressure sensor is sensitive to pressure and temperature. To get the sensing parameter of the pressure sensor, the pressure and temperature calibration experiments were done respectively. The MOI SI425 FBG interrogator was used to measure the FBG center wavelength, within 1pm in wavelength measurement resolution. For the pressure calibration experiment, a pressure calibration system was developed, shown in Fig. 5 and the pressure calibration experiment setup shown in Fig.6. And the pressure control resolution is 0.01kpa. In the pressure calibration experiment, the temperature kept constant. In the temperature calibration experiment, the pressure sensor was put in the oven, which inner pressure was constant, and the reference pressure tube connected to outside atmosphere. The temperature controlling resolution was 1 ℃. In this experiment, the pressure kept constant, and aimed to got the K T of pressure sensor.

Manometer

Film Pressure Sensor Switch Optical Conector

Air In

FBG Interrogator

Switch Air Out

Reference Pressure Tube

Fig. 5 Diagram of FBG film pressure sensor pressure calibration system

Fig. 6 FBG film pressure sensor pressure calibration experiment setup

3.3 FBG Film Pressure Sensor Calibration Results and Discussion Fig. 7 shows the temperature and pressure calibration results of the FBG film pressure sensor, and the FBG film pressure sensor has good sensitivity. For the temperature calibration, there is linear relationship between FBG wavelength shift and temperature, and the temperature sensing parameter is 14.6pm/℃ . And there is also well linear relation of wavelength shift and pressure, the measurement pressure sensing parameter is 34.4pm/kpa. And the FBG film pressure sensor can distinguish the pressure direction, which positive pressure causes the FBG wavelength increase, and negative pressure causes the wavelength decrease.


But comparing the FBG film pressure sensor sensing parameter of calculating and calibration, which is 31.8pm/℃ and 658.125pm/kpa for calculation, and 14.6 pm/℃ and 34.4pm/kpa for calibration, there is big difference. This is caused by the big modulus difference between sensing film and FBG fibre, which are 2.8 and 78GPa, respective. The sensing film deformation can’t lead the same value deformation of FBG, which will cause non-coordination deformation of sensing film and FBG. And the FBG will restrict the sensing film deform. But in the theoretical analysis, the modulus difference is ignored, and without thinking about the constraint action of FBG fibre to the sensing film. The theoretical analysis of constraint with FBG fibre to the sensing film should be investigated in future. 800 80

700 60

FBG wavelength shift(pm)

FBG wavelength shift (pm)

600 500 400 300 200

Measurement Result linear Fit

100

20 0 -20

Measurement Result linear Fit

-40 -60

KTM=14.6pm/c

0

40

KqM=34.4pm/kpa

-80

-100

20

30

40

Temperature

50

60

70

-2000

-1500

-1000

c

-500

0

500

1000

1500

2000

Pressure(Pa)

Fig. 7 Temperature and pressure calibration results of the FBG film sensor

4. CONCLUSION In this paper, we developed a new kind of FBG film pressure sensor. Theoretical analysis and calibration experiment were studied, the results shown the linear relationship between FBG wavelength shift and pressure, temperature variation respectively. But there were big difference of sensing parameter between theoretical calculating and calibration results, which was caused by the constraint of FBG to the sensing film deformation.

ACKNOWLEDGMENT This paper is supported by key laboratory opening funding of defense key laboratory of special environmental composites materials technology (Project: HIT.KLOF. 2009031)

REFERENCES [1] Zhou Qiang, Liu Bo, Gao Limin, Chen Liusheng, Shi Miao, "Pressure Measurement on Suction Surface of a Single Vane Using Pressure-sensitive Paint," Chinese Journal of Aeronautics, 22, 138-144(2009) [2] Erickson G. E., Gonzalez, H. A, "Pressure-Sensitive Paint Investigation of Double-Delta Wing Vortex Flow Manipulation," 43rd AIAA Aerospace Sciences Meeting and Exhibit, AIAA, 1059(2005) [3] George T., Kanellos George, Papaioannou Dimitris Tsiokos, Christos Mitrogiannis, George Nianios, Nikos Pleros, "Two dimensional polymer-embedded quasi-distributed FBG pressure sensor for biomedical applications," Optical express, 18(1), 179-186(2010) [4] Jun Hong Ng, Xiaoqun Zhou, Xiufeng Yang, Jianzhong Hao, "A simple temperature-insensitive fiber Bragg grating displacement sensor," Optics Communications, 273, 398-401(2007)


[5] Bremer K., lewis E., Moss B., Leen Lochmann, G. S, "Conception and preliminary evaluation of an optical fibre sensor for simultaneous measurement of pressure and temperature," Journal of physics: conference series. 178, 012016(2009) [6] Madoka Nakajima, Masahito Wakahara, Toichi Fukasawa, Shota Numata, "Development of a Film-type Pressure Sensor using Optical Fiber with Multiple Fiber Bragg Gratings," 26th AIAA Aerodynamic Measurement Technology and Ground Testing Conference. 23 - 26 June 2008, Seattle, Washington, 3949(2008)
