FBG sensor system for trailing edge chord-wise hinge ...

FBG sensor system for trailing edge chord-wise hinge rotation measurements M. Ciminello*a, A. Concilioa, D. Flautob, F.Mennellac Italian Aerospace Research Center, Via Maiorisi 81043, Capua (CE), Italy b University of Palermo, Viale delle Scienze 90128, Palermo, Italy c University of Napoli “Federico II”, P.le Tecchio 80100, Napoli, Italy

a

ABSTRACT It is the aim of this paper to present the design of a sensor system based on fiber Bragg gratings (FBG) for the strain monitoring of an adaptive trailing edge (ATE) device. Some of the activities herein showed comes from developments inside the project SARISTU (EU-FP7), funded by the European Union inside the VII Framework Programme and focused on smart aircraft structures. Because the TE is immerged into 3D structural and aerodynamic fields, the sensor system network should have chord- and span-wise features. The ATE device will be equipped with a shape monitoring system using a widely distributed sensors based on fiber optic (FO) elements herein referred to, mainly with the aim of reducing the number of channels (then expense, complexity, etc.). In what follows, the mathematical modelling of a sensor system concept based on FBG is applied to evaluate the chord-wise strain of a trailing edge device. A hinge rotation detection capabilities based on strain measurements is presented. The detection and process of data concerning the in-flight ATE local deformation are necessary to reconstruct the shape produced by the action of a dedicated actuation system. Keywords: fiber optics, rotation angle, shape reconstruction

1. INTRODUCTION Civil aircraft flight profiles are almost standard but it may occur to fly fast or slow, at low or high altitude depending on a number of factors. Lift coefficient can range between 0.08 and 0.4, while aircraft weight reduces by a 30% as fuel burns. Best configuration then changes with reference to actual flight mission and during the flight, fitting specific and mutating conditions. Chord- and span-wise wing camber variations may allow setting and chasing the best lay-out as a function of the particular and transforming reference state, always targeting best aerodynamic and structural performance. Trailing edge modification is an attractive and simple way to attain desired goal. The architecture herein presented is based on a multi-rib SDOF system, each activated by rod-like load-bearing actuators1. Key aspects is the shape control system assuring the specified displacement tolerances while guaranteeing suitable behavior under static and impulse loads. For this purpose, the ATE device is equipped with a shape monitoring system using a widely distributed sensors based on FBG elements. Almost all airplanes operate under stringent environmental requirements related to temperature and moisture. The sensors in the vicinity of an engine must withstand a temperature of about 170 ◦C. The sensors on the other structural parts must endure a temperature of about −50 ◦C. A FBG sensor can satisfy the requirements related to the operating temperature. In contrast to electrical sensors with electrical cables, the optical fiber with index gratings is not affected by moisture. In addition, the existing electric cable uses the method of shielding to avoid susceptibility to electro-magnetic interference and high-intensity radiated fields. This brings about an increase of the cable weight. On the other hand the optical fiber minimizes weight and saves space for fly-by-light application. It is worthwhile noting that multiplexing OF based on extrinsic Fabry–Perot (FP) interferometry were proposed for monitoring airflow pressure over smart wings in the mid-1990s and for applications involving actuator- and shape-memory-alloy- (SMA-) controlled airfoils and multiparameter skin friction measurements2-4. FBG has also been widely applied in optical fiber sensing as it provides other

Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2013, edited by Jerome Peter Lynch, Chung-Bang Yun, Kon-Well Wang, Proc. of SPIE Vol. 8692, 869221 · © 2013 SPIE · CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2012017 Proc. of SPIE Vol. 8692 869221-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/02/2013 Terms of Use: http://spiedl.org/terms

important advantages, such as high sensitivity, capabilities of multiplexing and networking, and wavelength absolute encoding5,6. Utilizing the characteristic that the Bragg wavelength of FBG shifts due to the influence of temperature and strain7, many kinds of FBG-based sensors have been manufactured, whereas, the rotational angle sensing of FBG is still rare among the reported literatures to our knowledge8,9. FBG is here recommended as a sensor to monitor the local hinge rotation of the trailing edge structure of an aircraft. In what follows, the mathematical model describing the transfer function of the system in the hypothesis of small angle of rotation is described and the sensing device concept is implemented in finite element code for validation. Some of the technical information herein reported can be retrieved from the official web page of Saristu1 project.

1.1 Architecture description A classic schematic of the ATE is sketched in Figure 1. The architecture herein presented is based on a multi-rib SDOF system, each activated by rod-like load-bearing actuators1. Because the ATE is immerged into 3D structural and aerodynamic fields, the sensor system network has chord- and span-wise features. The device will be then equipped with a shape monitoring system using a widely distributed optical sensors opportunely located, according to a strain field map identification.

Figure 1. Schematic of the ATE concept1.

In particular the ATE sensor network is hence designed in order to ensure the monitoring of bending chord-wise effect by detecting hinges rotations of the rib sub-component. A simplified schematic of the investigated architecture is reported in Figure 2. The rib architecture is described as made of single elastic hinge connecting a fixed section to a rotating one. The straight lines representing FO running in a region close to the hinge.

Figure 2. Schematic of the morphing rib.

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Figure 3 shows the mechanical principle of the sensing device. It consists of two rod segment hinged at their midpoint looking like a “scissor” and to be coupled, somehow, to the hosting structure, at the hinge location. The 4 pins placed in the arms of the arches constrain the device to both the rotating sections of the rib. Of course, based on the hosting geometry, it will be possible to suitably adjust the geometry of the bows. The extremes of these latter provide housings equipped with notches, to allow optical fibers fixation. It is necessary to mount the fibers with a suitable pre-strain so that compression inducing instability (fiber buckling) is avoided. It is evident that the proposed structure possesses a "self-restraint" geometry for the rotation; also it is possible to provide the system with suitable strokes to preserve the integrity of the optical fiber itself. The FO must be deformable within the hinge region and pretty stiff otherwise, according to the concept illustrated above.

Fiber Optic

Holder Screws

Figure 3. Schematic of the “scissor” sensing device.

During the operational life of the sensor system, each rotation will correspond to a relative strength of one sensor and to a relative compression of the opposite one. The simultaneous reading of two sensors provides a greater accuracy of measurement, and this also allows for the self-compensated temperature measurements. It is indeed true that working in opposite way, "common mode" can be rejected due to the effect of temperature changing.

2. MATHEMATICAL MODEL 2.1 Hinges rotation from strain measurements Referring to Figure 4, which represent the hinge detail taken from Figure 2, soon after the rotation, the fibre will elongate in the un-bonded segment that will be deformed uniformly, and the segment AB will stretch into the segment AC. Let the COB triangle be considered. The relation among its angles is: α + 2β = 180° Î β=(180°-α)/2

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(1)

Considering instead the OKB triangle, it follows: α0/2 + 90° + γ = 180° Î γ = 90° - α0/2

(2)

Therefore, the flat angle ABH, may be expressed as: γ + β + δ = 180° Î δ = (α0+α)/2

(3)

Let H be the orthogonal projection of C over the straight line AB. If small angles are considered, arc length coincides with its chord length. From trigonometric relations, it follows:

(4)

BH = BC cos (δ) = Rα cos (δ)

A

α/2

B

K γ α0 α

R

H δ β Rα β C

O

Figure 4. Geometrical scheme linking hinge rotation to longitudinal strain component.

For small angles, the centre angles are double circumference angles insisting on the same arch (in this case, BOC and BAC, respectively), it holds: AH = AC cos (α/2) Î AH ≈ AC

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(5)

Finally, from the definition of strain, the following relation comes out: ε = (AC-AB) / AB

(6)

ε = (AH-AB) / AB = BH / AB

(7)

hence

From equations (4) and (7), the expression linking the angle of rotation to the longitudinal strain, is given. The length AB is the extension of the un-bonded fibre that should be considered in the design process. ε = Rα cos (δ) /AB

(8)

This value of AB is irrespective of the FBG length; in fact, it can be assumed that the strain is uniform along the unbonded region. Such a value depends on α0, the distance from the hinge which is function of the structural design and the maximum expected rotation angle α.

3.

NUMERICAL VALIDATION

3.1 Hinges rotation from FEM strain measurements In the finite element model, the rib structure is simulated using solid elements while FO is simulated by using a cylindrical rod of 125μm in diameter and with a Young modulus of 70GPa. A maximum ATE rotation ≤ 5 deg, (i.e. α ≤ 0.02914 rad) is assumed. While α0 is now equal to 160 deg (design hypothesis) and R is equal to 60 mm.

Figure 5. FEM sketch of the sensorized hinge.

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In Figure 6, some results comparing analytical and numerical approaches are reported. The dotted line corresponding to the finite element data while the straight line to the analytical ones. The rib rotation is imposed, from 1 deg to 5 deg each step monitoring the fiber longitudinal strain.

0,008

ii

O,0D7

0,006 0,005

-

O,OD4

0,003 O,0D2

0,001 05

1,5

1

2,5

2

3,5

3

4,5

4

5,5

5

alpha (deg)

Figure 6. Analytical and numerical comparison. Straight line: Analytical approach, Dotted line: FEM approach.

Table 1. Analytical and numerical error estimation.

α (deg) Analytical (ε) FEM (ε) Abs Err (%)

1

2

3

4

5

0,001658 0,001575 0,008

0,003240 0,003150 0,009

0,004745 0,004725 0,002

0,006174 0,006300 0,013

0,007527 0,007876 0,035

4.

CONCLUSION

The mechanical principle of a sensing device consisting of a “scissor” has been introduced. The mathematical modeling of the concept, based on FBG sensors, has been applied to evaluate the relation between the chord-wise strain of a trailing edge device with its rotation. The rib has been forced to rotate from 1 deg to 5 deg. Then the analytical results have been compared with the numerical ones on the base of a FE formulation. This preliminary evaluation shows a good correlation between data using two different modeling approaches. An experimental test campaign is planned to verify the reliability of the proposed concept and to highlight structural design limitations eventually.

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5.

ACKNOWLEDGMENTS

Part of the activities herein showed were supported by the project SARISTU (EU-FP7), funded by the European Union inside the VII Framework Programme and focused on smart aircraft structures. The authors would express their gratitude to Mr. Antonio Alfano of University of Naples “Federico II” for his contribution in the realization of the experimental setup assessment.

REFERENCES

[1] http://www.saristu.eu/project/activities/as-02-structural-tailoring-of-wing-trailing-edge-device/ [2] Jones M. E., Duncan P. G., Crotts R., Shinpaugh K., Grace J. L., Murphy K. A. and Claus R. O., “Multiplexing optical fiber-based pressure sensors for smart wings”, Proc. SPIE 2838 230–6. [3] Duncan P. G., Jones M. E., Shinpaugh K. A., Poland S. H., Murphy K. A. and Claus R. O., “Optical fiber pressure sensors for adaptive wings”, Proc. SPIE 3042 320–31. [4] Jung-Ryul L., Chi-Young R., Bon-Yong K., Sang-Guk K., Chang-Sun H. and Chun-Gon K., “In-flight health monitoring of a subscale wing using a fiber Bragg grating sensor”, Smart Mater. Struct. 12 (2003) 147–155 PII: S0964-1726(03)56154-7. [5] Liu D., Ngo N. Q., Tjin S. C., and Dong X., “A dual-wavelength fiber laser sensor system for measurement of temperature and strain”, IEEE Photon. Technol. Lett., vol. 19, no. 15, pp. 1148–1150, (Aug. 2007). [6] Kirkendall C. K. and Anthony D. A., “Overview of high performance fibre-optic sensing”, J. Physics D: Appl. Phys., vol. 37, no. 18, pp. 197–216, (2004). [7] Lin G. C. et al., “Thermal performance of metal-clad fiber Bragg grating sensors”, IEEE Photon. Technol. Lett, vol. 10, no. 3, pp. 406–408, (Mar. 1998). [8] Hui Y., Xiufeng Y., Zhengrong T., Ye C. and Ailing Z.; “Temperature-indipendent rotational angle sensor based on fiber Bregg grating”, IEEE sensors journal, vol. 11, no.5, Pages 1233-1235, (May 2011). [9] D'Emilia, G.; Iaconis, F., "A simple fiber optic sensor for angle measurement," Instrumentation and Measurement Technology Conference, 1994. IMTC/94. Conference Proceedings. 10th Anniversary. Advanced Technologies in I & M. 1994 IEEE, vol., no., pp.295-299 vol.1, 10-12 (May 1994).

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