Fiber Bragg Grating-Based Acceleration Sensors for Civil and Building Structures

Fiber Bragg Grating-Based Acceleration Sensors for Civil and Building Structures A. Mita Graduate School of Science and Technology, Keio University 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan email: mita@sd.keio.ac.jp Abstract Acceleration sensors are the key components in a health monitoring system for civil and building structures especially for detecting damages due to earthquakes and wind loads. Optical acceleration sensors have many advantages over conventional electrical acceleration sensors such as their immunity to electromagnetic interference and their capability to transmit signals over long distance without any additional amplifiers. We have recently developed a new optical acceleration sensor that uses fiber Bragg grating elements. The acceleration sensor was designed to have high sensitivity of 1 pm/gal in low frequency range. The cross-axis sensitivity was minimized by the use of a leaf spring. The mechanism employed for this sensor ensures uniform strain distribution in the Bragg grating element so that the Bragg reflection peak will not be deteriorated. On the other hand, the Bragg grating element needed to apply post-tension to induce appropriate bias strain. This bias strain is much higher than the operational limit of the optical fibers in communication industry. The high strain level is harmful and results in failure within a short service period. To overcome this shortcoming, the direct writing procedure without removal process and the use of high screening strain in a proof-test are considered. Employing these measures, the lifetime of a Bragg element is found to be extended to more than 50 years even at 8,000 microstrain applied to the Bragg grating element. The advantage of multiplexing capability is demonstrated by proposing a liquefaction sensor consisting of an FBG acceleration sensor and FBG pressure sensors. 1

Introduction A health monitoring system started getting strong attentions in Japan after the 1995 Hyogo-Ken Nanbu (Kobe) Earthquake in which more than 6,000 people were killed and 40,000 buildings were destroyed. As was the case after the 1994 Northridge Earthquake, many steel buildings suffered sever damages mainly at their beam-column joints. It was a surprising fact that the damages were not found until removing fire-protection coatings on beam-column joints. In most cases, it was not possible to find the correct degree of the damages by a simple eye-inspection of the structure surface because there were no major visible damages on the surface of fire-protection material. This fact prompted strong demands in real-time nondestructive assessment systems for steel buildings. One possible system currently under study is the one utilizing the analysis of dynamic response of a building. Although this method is not suited for detecting small damages, the method is excellent to identify the global health of a building. Acceleration sensors play a key role in such nondestructive assessment systems. This method is considered to be feasible for detecting damages due to large earthquake or wind load. However, the current electric acceleration sensors have certain limitations to apply for such a purpose. Among others, heavy cabling labor and sensitivity to electromagnetic fields often becomes an obstacle when installing them into a building. Optical acceleration sensors have many advantages over conventional electrical sensors such as their immunity to electromagnetic interference and their capability to transmit signals over long distance without any additional amplifiers. In addition, fiber Bragg grating (FBG) system has a multiplexing capability that reduces a cabling labor drastically. This research has been motivated by needs for optical acceleration sensors that have high sensitivity especially in a low frequency range (<100 Hz) for civil and building application. In the early stage in development, the optical acceleration sensors have been typically configured within an interferometric architecture (e. g. [1]). Recently, several acceleration sensors based on FBG have been proposed for structural monitoring. Berkoff et al. ([2]) proposed an FBG acceleration sensor. They embedded a Bragg grating element into a commercially available elastomer that is attached to a mass. The natural frequency of the sensor was set at about 2 KHz or higher to detect high frequency components. This sensor, however, suffers from cross-axis sensitivity and birefringence-splitting of the Bragg reflection peak. Todd et al. ([3]) improved the performance of an FBG acceleration sensor by using beam-plates. They were able to minimize the cross-axis sensitivity to less than 1 % of the primary axis sensitivity. Though their system has many desirable features, the resolution of 212.5 microstrain/g is not enough for civil and building application. Moreover the distribution of strain along the beam-plate to which a Bragg grating element is glued is not uniform so that the Bragg reflection peak may be broadened to result in reduced resolution. In this paper, certain design aspects of a new FBG-based acceleration sensor are discussed. The system consists of a cantilevered beam and a mass. A Bragg grating element is not directly glued to the cantilever to avoid possible non-uniform strain in the element. Instead, the Bragg element is tensioned after placed onto the system to achieve a uniform strain distribution. The cross-axis sensitivity is minimized by the use of a leaf spring. The prototype FBG acceleration sensor with its natural frequency of 45 Hz could achieve the sensitivity of 1 pm/gal or 0.83 microstrain/gal. Shake table tests have been extensively conducted to evaluate its dynamic characteristics. The response values were compared with those obtained by a servo-type acceleration sensor to find a good agreement. The FBG-based acceleration sensor studied here requires bias strain. The level of base strain is much higher than the value normally allowed for fiber cables used for communication. The allowable strain is typically less than 0.5 %. The high strain level is harmful and results in failure within a short service period. To overcome this shortcoming, the direct writing procedure ([4]) without removal process and the use of high screening strain in a proof-test ([5]) are considered. Employing these measures, the lifetime of a Bragg element is found to be extended to more than 50 years even at 8,000 microstrain applied to the Bragg grating element. This lifetime is sufficient for most application in civil and building structures. A liquefaction sensor consisting of an FBG acceleration sensor and FBG pressure sensors is presented as an example of multiplexing capability. 2

Design of FBG Acceleration sensor The schematic of an FBG acceleration sensor is presented in Fig. 1. The acceleration sensor consists of an L-shaped rigid cantilever beam, a concentrated mass and a spring. To detect strains due to applied acceleration, a Bragg grating element is placed at the center of points A and B. The element is bonded to the acceleration sensor at points A and B as indicated in Fig. 1. The Bragg element is post-tensioned to use only the tension side of stress conditions. By employing this configuration, the Bragg grating element is always subject to uniform strain distribution along its measuring length resulting in a sharp reflection characteristics with no broadening. This feature is attractive to keep a good resolution of the sensor in a wide amplitude range. The plan and side views of a prototype acceleration sensor are shown in Fig. 2. The interior view is shown in Fig. 3. A leaf spring is used in the sensor to minimize cross-axis sensitivity. To achieve high sensitivity in low frequency region, the natural frequency was tuned to be 45Hz. In the following analysis, the acceleration sensor is modeled by a single-degree-of-freedom system with no damping. The stiffness of the Bragg grating element is therefore considered to be included in a spring element K as indicated in Fig. 1. y L A Bragg grating B yg d l mass M spring K Acceleration Fig.1 Mechanism of FBG acceleration sensor 40mm 25mm Bragg grating 12mm Fiber cable Leaf spring Mass Case Fig. 2 Plan and side view of FBG acceleration sensor 3

Fig. 3 Interior view of FBG acceleration sensor The equation of motion for the system depicted in Fig. 1 subject to the ground acceleration a g is written as M& y + Ky = Ma g (1) where Dividing both sides by the mass M results in a = & (2) g y g & y ω y = (3) + 2 0 a g where K ω 0 = (4) M represents the natural frequency of the acceleration sensor. Considering a harmonic ground acceleration of frequency ω a g iωt = Age (5) allows us to express motion of the mass in the form The amplitude Y is obtained by substituting eqs. (5) and (6) into eq. (1) as y iωt = Ye (6) 1 1 = (7) Y 2 2 1 ( ω ω0 ) ω0 A g 4

It is understood that the ground acceleration is proportional to the displacement motion of the mass Y in a low frequency range and that the amplitude is in inverse proportion to squared natural frequency. The strain value ε induced in the Bragg grating element can be approximately expressed in the form d / l 1 ε A 2 g (8) L ω The natural frequency of the prototype system is 45 Hz. The effective length of the Bragg grating element is 30mm. The ration d/l is 0.2. Under this condition, 1.0 Gal (=cm/s 2 ) produces 0.83 microstrain in the Bragg grating element. When a 1550 nm-wavelength Bragg grating is used, the strain-to-wavelength sensitivity is 1.2 pm/ microstrain (see [6]). Therefore, the acceleration amplitude of 1.0 Gal corresponds to 1.0 pm wavelength shift in the Bragg wavelength. The relation between Bragg wavelength shift λ (pm) and the acceleration amplitude A g (Gal) can be simply expressed in the form 0 λ =1.0A g (9) The dynamic property of the prototype FBG acceleration sensor was tested by conducting a shake table test as shown in Fig. 4. The wavelength shift was detected by an interrogation system FBG-IS made by Micron Optics, Inc. The resolution of this system is 1.0 pm so that the resolution for this acceleration sensor is about 1.0 Gal. Test results for a 3Hz test with the sampling frequency of 34Hz are shown in Fig. 5. The response values in time domain and frequency domain agree well with those obtained by a servo acceleration sensor. Fig. 4 Shake table test Fig. 5 Comparison of time histories and Fourier amplitudes 5

Lifetime Estimation The Bragg grating element in the prototype acceleration sensor is always subject to a bias strain due to post-tensioning. When a measurement range of plus and minus 2,500 Gal is necessary, at least 2,500 microstrain should be applied to the element as the bias strain. However, most production process for Bragg grating elements involves removal of coating material to write the grating to ensue enough reflectivity. Though recoating is made after writing the grating, the tension strength of the fiber is drastically reduced. In some case, Bragg grating elements do not tolerate the strain level as low as 5,000 microstrain. The use of such an element to this type of acceleration sensor is not feasible at all. Recently, however, a direct writing method over coating material has been established (see [4]). By this production method, the strength of the Bragg grating element is kept almost the same with its original strength. In addition, Komachiya et al. ([5]) have demonstrated that the long lifetime can be achieved by increasing the screening strain for a proof-test. For telecommunication fibers screening strain of 0.5% is normally applied as a proof-test. Although increasing the screening strain results in several failures in a long fiber, the sound fiber length is still long enough for the use of sensors. In the following, lifetime estimation is made for Bragg grating elements produced by the direct writing method proposed by Imamura et al. ([4]). The failure of glass fibers is caused by growth of a surface flaw or a microcrack in the glass fibers. When the screening strain ε p was applied to a sensor fiber for t p seconds as a proof-test, the probabilistic lifetime is expressed in the form (see [5]) ln(1 F ) ( n+ 1) / m s n T = 1 1 ( εp / εs ) LN p t p (10) where L is the length of a sensor fiber, F s is the failure probability in time T, and ε s is the constant strain in its service situation. The fatigue constant n and a characteristic parameter m describing a distribution of the crack size are determined from tests and are given as 23.9 and 4, respectively, for a single -mode glass fiber (see [5]). The number of failures (per kilometer) N p was also experimentally determined. To estimate the lifetime of an FBG sensor system, a sensor system with ten FBG acceleration sensors multiplexed on one fiber cable is considered. Each FBG acceleration sensor utilizes a sensing fiber of FBG length of 30mm. Therefore, the total length L of the sensor fiber under large strains that should be considered in the lifetime estimation is 300mm or 3 10-4 km. The failure probability F s for this system is set very small. It was set here as small as 10-7. To see the effects of screening strain levels, two levels of screening strain, 2.5% and 3.5% were considered. The parameters used for lifetime estimation for a Bragg grating sensor system are summarized in Tab. 1. In Fig. 6, the lifetime estimation is shown. From the figure, it is concluded that a long lifetime of more than 50 years could be achieved even for a FBG acceleration sensor with the service strain level of 5,000 microstrain by producing the element by the direct writing and using large screening strain of 2.5 %. Table 1 Parameter values for lifetime estimation Parameters Unit Level 1 Level 2 Screening strain Loading time Failure probability Fiber length No. of failures per kilometer m n % s km 2.5 1.0 1.0x10-7 3.0x10-4 1 4 23.9 3.5 1.0 1.0x10-7 3.0x10-4 6 4 23.9 6

Fig. 6 Lifetime estimation of an FBG sensor system consisting of ten multiplexed sensors Application to Liquefaction Sensor Utilizing multiplexing capability of FBG-based sensors, an FBG acceleration sensor can be integrated into a liquefaction sensor by multiplexing FBG pressure sensors (see Fig. 7) as schematically shown in Fig. 8. An FBG acceleration sensor on the ground surface could be used to detect the acceleration response of the ground, while the multiple FBG pressure sensors to measure the pore water pressure in the ground. Combining the pore water pressure data with the acceleration data, liquefaction condition of the ground is immediately assessed during an earthquake. As indicated in this example, multiplexing several kinds of sensors will be an attractive feature of FBG-based sensory system. Fig. 7 FBG pressure sensor FBG accelerometer FBG pressure sensor Earthquake Fig. 8 Liquefaction sensor consisting of FBG acceleration sensor and FBG pressure sensor 7

Concluding Remarks Optical acceleration sensors have many advantages over conventional electrical acceleration sensors such as their immunity to electromagnetic interference and their capability to transmit signals over long distance without any additional amplifiers. The developed new optical acceleration sensor based on fiber Bragg grating elements has high sensitivity of 1 pm/gal in low frequency range. In addition, employing a leaf spring minimizes the cross-axis sensitivity. The mechanism employed for this sensor ensures uniform strain distribution in the Bragg grating element so that the Bragg reflection peak will not be broadened. On the other hand, the Bragg grating element needed to apply post-tension to induce an appropriate bias strain. The shortcomings resulting from this bias strain can be overcome by the direct writing procedure and the use of high screening strain in a proof-test. By doing so, the lifetime of a Bragg grating element is extended to more than 50 years even at 8,000 microstrain in the Bragg grating element. The advantage of multiplexing capability highlighted by a liquefaction sensor will enhance the use of FBG-based sensors for structural monitoring. The proposed liquefaction sensor consists of an FBG acceleration sensor and FBG pressure sensors multiplexed in one optical fiber cable. Acknowledgement The author is grateful to Mr. Yokoi at Tokyo Sokushin Co. Ltd. for continuing support and to Mr. Iwaki at Shimizu Corporation for his valuable discussions. References 1. A. D. Kersey, D. A. Jackson and M. Corke, High Sensitivity Fiber-Optic Acceleration sensor, Electronics Letters, 18, 559-561 (1982) 2. T. A. Berkoff and A. D. Kersey, Experimental Demonstration of a Fiber Bragg Grating Acceleration sensor, IEEE Photonics Technology Letters, 8, 1677-1679 (1996) 3. M. D. Todd, G. A. Johnson, B. A. Althouse and S. T. Vohra, Flexural Beam-Based Fiber Bradd Grating Acceleration sensors, IEEE Photonics Technology Letters, 10, 1605-1607 (1998) 4. K. Imamura, T. Nakai, K. Moriura, Y. Sudo and Y. Imada, Mechanical Strength Charanteristics of Tin-Codoped Germanosilicate Fibre Bragg Gratings by Writing Through UV-Transparent Coating, Elecronics Letters, 34, 1016-107 (1998) 5. M. Komachiya, R. Minamitani, T. Fumino, T. Sakaguchi and S. Watanabe, Proof-testing and probabilistic lifetime estimation of glass fibers for sensor applications, Applied Optics, 38, 2767-2774 (1999) 6. Y.-J. Rao, In-Fibre Brag Grating Sensors, Meas. Sci. Technol., 8, 355-375 (1997) 8