PHOTONICS 2010: 10th International Conference on Fiber Optics & Photonics December 11-15, 2010, IIT Guwahati, India
STRUCTURAL MONITORING OF LAUNCHERS WITH FIBER OPTIC SENSORS T. C. Buck1, M. S. Müller1, A. W. Koch1, A. Reutlinger2, A. Boesz2, M. Froevel3, I. McKenzie4 1 Technische Universität München, Institute for Measurement Systems and Sensor Technology 2 KayserThrede GmbH, 3Instituto Nacional de Técnica Aeroespacial, 4European Space Agency
[email protected] Abstract: Within the scope of an ESA-funded project the capabilities of fiber optic measurement systems for structural monitoring during launcher lift-off were evaluated. For this goal, a highly integrated fiber Bragg grating interrogator based on an arrayed waveguide grating, specially designed for the acquisition of dynamic structural loads, was developed.
1. INTRODUCTION Currently, the number of sensors on launchers for structural monitoring is limited due to the weight and volume of the electrical sensor systems and the harsh environmental conditions during lift-off. Fiber optic sensors based on fiber Bragg gratings (FBGs) have the potential to overcome these restrictions and thus to provide an on flight structural monitoring of launchers during lift off.
wavelength selective reflectivity at the FBG. Incident light within a narrow wavelength band around the Bragg wavelength λB is reflected. Light at wavelengths out of this narrow reflection band passes the FBG unaffected. The Bragg wavelength is given by
λB = 2ΛB ⋅ ne ff ,
(1)
where ΛB is the modulation period of the refractive index modulation and neff is the effective index of refraction at the FBG [2]. Physical quantities like temperature T or strain εz affect ΛB and neff, respectively. The Bragg wavelength shift is given by
(
)
∆λB = λB 1 + α Λ + α n dT + λB ⋅ (1 − pe ) ⋅ ε z e
Figure 1: Application of fiber optic sensors to the ARIANE launcher
Within the scope of an ESA-funded project a FBG interrogator for dynamic acquisition of structural loads during launch conditions was developed. The application of measurement systems on launchers sets tight restrictions towards equipment size, power consumption and weight. These restriction can not be fulfilled by existing FBG interrogators [1].
2. FIBER BRAGG GRATING SENSORS FBG sensors are formed by a periodic modulation of the refractive index within the core of a singlemode fiber. This periodic modulation leads to a
where λB is the Bragg wavelength, αL is the thermoelastic coefficient, αne is the thermooptic coefficient and pe is the effective Pockels constant. Several transducer structures have been developed that allow the acquisition of other quantities like acceleration, etc. This is achieved by transforming these quantities into a change of the strain in the singlemode fiber at the FBG [6].
3. MEASUREMENT SYSTEM Currently available FBG measurement systems do not fulfill the tight restrictions towards dynamic signal acquisition and environmental loads, as they occur during launcher lift-off. A highly integrated FBG measurement system based on a specially designed arrayed waveguide grating (AWG) was developed within the project [3], [4]. The system is capable of detecting structural loads with a cut-off frequency of 20 kHz while providing the required strong anti-aliasing capabilities. For the intended measurement task, an interrogator based on an AWG as an optical edge filter was
(2)
developed. AWGs are integrated optical dispersive components. Light coupled into the AWG is split into a number of output channels, each of which exhibits a characteristic transmission characteristic. In a first approximation, the transmission characteristic of an AWG output channel can well be approximated by a Gaussian shape. T
Ti
Ti+1
Figure 3 Schematic of the designed AWG-based FBGinterrogator
Figure 2 Functional principle of the FBG interrogator; the narrow peak indicates the spectral FBG reflection, the broad curves indicate adjacent AWG output channel transmission spectra
The employed measurement scheme is depicted in Figure 1. A spectral FBG sensor reflection is indicated by the narrow spectral peak. The transmission spectra of two adjacent AWG output channels are indicated by the two spectrally broad curves Ti and Ti+1. Depending on the center wavelength of the FBG, this gives rise to light intensities in two adjacent output channels. The measurement task is to evaluate the center wavelength of the FBG sensor. The FBG center wavelength can be determined by the logarithmic ratio of the measured intensities in the adjacent channels [5]
Figure 4 Output channel transmission spectra of the AWG 10
T / a.u.
λ
A schematic of an AWG based FBG interrogator is presented in figure 3. A series of serial multiplexed FBG sensors are illuminated by a spectrally broad light source. Amplified spontaneous emission (ASE) light sources exhibit superior characteristics regarding the degree of polarization of the emitted light. That makes ASE light sources suited candidates for this application. The reflected light from the FBG sensors is guided to the AWG via a fiber optic circulator. The reflected light from the FBGs is processed in the AWG and it is measured by the photodetectors connected to a signal processing unit (SPU).
10
10
ρ=
log( Ii − Ib ) , log( Ii +1 − I b )
1
0
−1
(3)
where Ii and Ii+1 denote the light intensities in the adjacent channels caused by the light reflected from the FBG and Ib denotes the intensity of the background stray light in the fiber and parasitic reflections, e.g. from fiber couplers, etc. [5]. A large number of output channels can be incorporated within a single AWG. Thus, a large number of FBGs can be simultaneously interrogated by a single AWG. The size of such an interrogator can be minimized by integration of photodetectors onto the AWG chip.
1535
1540
1545 1550 Wavelength / nm
1555
1560
for linear polarized light
Vibration monitoring of launcher structures during lift-off requires a measurement uncertainty of about 10 pm at a maximum signal frequency of 20 kHz. The described measurement uncertainty of 10 pm corresponds to a measurement uncertainty of about 8.3 µstrain. Considering this restriction in combination with the desired spectral measurement range per sensor, simulations according to [3] were performed and a customized AWG with a channel spacing of 3.33 nm and a channel FWHM of 2.33 nm based on these simulations was developed. Photodiodes were directly integrated onto the AWG, reducing the device size to a minimum. Figure 4 shows the measured transmission profiles for linear polarized light coupled into the AWG for the center output channels. Thermal stabilization of the AWG is
achieved by use of a thermoelectric cooler in the AWG packaging. A characteristic signal determination function for a FBG sensor is exemplarily shown in figure 5. The evaluation function exhibits good linearity over the measurement range of approximately 4.5 nm. The optoelectronic signal amplification and signal shaping circuit was designed to exhibit an amplification of 106 V/A at a -3 dB cutoff frequency of 20 kHz.
Figure 6 Physical layout of FBG acceleration sensor
The test setup is shown in figure 7. The test structure is mounted at an angle of 20 degree onto a vertical shaker platform. A series of mechanical load spectra, as they are also employed during structure qualifications for the ARIANE launcher, were applied to the shaker platform. The mechanical response of the structure was recorded by the developed fiber optic interrogator. Shaking tests were performed at different load levels and different shaking directions.
Figure 5 Exemplary interrogation function for a FBG sensor
3. EXPERIMENTAL SETUP A carbon honeycomb lightweight structure similar to the ARIANE structure was equipped with a series of FBG sensors. Draw tower FBGs were embedded into the structure for measuring internal dynamic strains during testing. Commercial MicronOptics fiber optic os-3100 strain gages were applied onto the structure. Dynamic structural loads can also be well evaluated by measuring the acceleration at the structure. Therefore, proprietary FBG acceleration sensors [6] were applied to the structure. The acceleration sensors were designed to exhibit a sensitivity of 1 pm/g at a resonance frequency of 6.0 kHz. The mechanical layout of the employed FBG acceleration sensors is shown in figure 6. The sensor exhibits a height of 17.7 mm and a diameter of about 8 mm. The acceleration sensors are designed to exhibit a minimum cross talk regarding acceleration normal to the active direction of the sensor. The objective of the performed test campaign was to simulate loads on a launcher structure as they occur during real lift off. The lightweight structure equipped with the sensors was mounted onto an electromechanic shaker platform at INTA (Instituto Nacional de Técnica Aeroespacial, Madrid).
Figure 7 Carbon lightweight structure equipped with FBG strain and vibration sensors
4. MEASUREMENT RESULTS The FBG measurement system showed good performance during dynamic load evaluation. Figure 8 exemplarily shows the evaluated strain response during a linear frequency sweep applied to the shaker platform. The strain signals were acquired at a sample rate of 80 kHz. Two low intensity mechanic resonances with a strain amplitude of 60 µstrain are observable.
REFERENCES
Figure 8 Recorded strain trend with FBG sensor during dynamic load test.
Evaluated signals from the fiber optic acceleration sensors for one shaker test are exemplarily presented in figure 9. The above presented mechanic resonances in the strain signal are also clearly identified by the acceleration sensor.
Figure 9 Evaluated spectral acceleration response of fiber optic acceleration sensor. Two mechanic resonances can be identified clearly
5. CONCLUSION We present the development of a FBG interrogator designed for monitoring of structural loads during launcher lift-off. The interrogator shows good performance on the acquisition of dynamic mechanical loads on the structure. The recorded dynamic signals are in good agreement with theoretical predictions from finite element simulations. Fiber optic sensors show huge potential to work as in-flight monitoring systems for launchers, as, compared to conventional electrical sensors, they offer advantages regarding equipment size and instrumentation weight. The TRL (technology readiness level) of the FBG interrogator has yet to be increased.
[1] Zhao Y., Liao Yanbiao, Discrimination methods and demodulation techniques for fiber Bragg grating sensors, Optics and Lasers in Engineering, 41, pp. 1-18, 2004 [2] A. Othonos, Fiber Bragg Gratings, Review of Scientific Instruments, 68:4309-4341, 1997 [3] T. C. Buck, M. S. Mueller, A. W. Koch, Compact FBG Interrogator based on a customized integrated optical arrayed waveguide grating, 15th European Conference on Integrated Optics, Cambridge, 2009 . [4] T. C. Buck, M. S. Mueller, A. W. Koch Performance analysis of interrogators for FiberBragg-grating sensors based on arrayed waveguide gratings. Proceedings of the SPIE Europe Optical Metrology Conference, ICMInternational Conference Centre Munich, 2009, Munich, Germany. [5] Y. Sano, T. Yoshino, Fast optical wavelength interrogator employing arrayed waveguide grating for distributed fiber Bragg grating sensors, Journal of Lightwave Technology, Vol 21, No 1, pp. 132-139. [6] M. S. Müller, T. C. Buck, A. W. Koch, Fiber Bragg grating-based acceleration sensor, Optomechatronic Technologies, 2009. ISOT 2009. International Symposium on, pp. 127-132, 2009.
