- Email: [email protected]
Some engineering issues of an FBG sensor with the dual gratings parallel matched interrogation method
ARTICLE IN PRESS
Optik
Optics
Optik 120 (2009) 20–23 www.elsevier.de/ijleo
Some engineering issues of an FBG sensor with the dual gratings parallel matched interrogation method Xichun Yanga,b,, Jincheng Peia,b, Yage Zhanc, Rude Zhua, Hong Hea, Shiqing Xianga a
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China Graduate School of the Chinese Academy of Sciences, Beijing 100039, China c Applied Physics Department, College of Science, Donghua University, Shanghai 201602, China b
Received 29 January 2007; accepted 15 June 2007
Abstract The key issues of engineering application of the dual gratings parallel matched interrogation method are expanding the measurable range, improving the usability, and lowering the cost by adopting a compact and simple setup based on existing conditions and improving the precision of the data-processing scheme. A credible and effective data-processing scheme based on a novel divisional look-up table is proposed based on the advantages of other schemes. Any undetermined data is belonged to a certain section, which can be confirmed at first, then it can be looked up in the table to correspond to microstrain by the scheme. It not only solves inherent problems of the traditional one (double value and small measurable range) but also enhances the precision, which improves the performance of the system. From the experimental results, the measurable range of the system is 525 me, and the precision is 71 me based on normal matched gratings. The system works in real time, which is competent for most engineering measurement requirements. r 2007 Elsevier GmbH. All rights reserved. Keywords: Dual gratings parallel matched; Bragg grating; Strain; Look-up table
1. Introduction Research on the theory and application of a fiber Bragg grating (FBG) has made great progress in the sensing field since Morey reported the possibility of using an FBG for sensing at 1989 [1]. An FBG is a wavelength-coded component, and it has all the outstanding advantages of the traditional fiber sensor such as good reliability, EMI immunity, light weight, high accuracy, etc. There have been great requirements of an
Corresponding author. Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China. E-mail address: [email protected] (X. Yang).
0030-4026/$ - see front matter r 2007 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2007.06.012
FBG sensor in civil engineering, the spaceflight industry, intelligent traffic, etc. Wavelength interrogation is the most important task in research on the FBG sensor system. Throughout more than 20 years of development, there have been several mature interrogation methods such as optical spectrometer method, edge filter method, tunable fiber F-P filter method, interferometer scanning method, matched FBG-based filter method, etc. [2]. The matched FBG-based filter method has advantages such as low cost, simple configuration and working in real time, which has been considered as the best solution for a commercial system. A novel dual gratings parallel matched interrogation method has been proposed and studied seriously [3]. The
ARTICLE IN PRESS X. Yang et al. / Optik 120 (2009) 20–23
novel method preserves the advantages of the traditional matched FBG-based filter method such as high resolution, fast interrogation speed, good repetition and low cost, and solves inherent problems of the traditional ones: double value and small measurable range. In our previous research, we paid attention to the theoretical and experimental possibility of the novel dual gratings parallel matched interrogation method, but did not study its engineering performance. A novel divisional look-up table scheme is proposed based on a deep study on data-processing schemes, which plays a key role in improving the engineering performance of the sensing system.
2. Theory and experimental setup Fig. 1 shows a schematic diagram of the experimental setup used in the proposed sensor system. Fig. 2 shows a schematic diagram of spectra of the sensor FBG and the matched FBGs, in which the detectable signal corresponds to the hatching part. There are two matched FBGs and a sensor FBG in the setup, the reflective center wavelengths of which are Sensor FBG BBS FBG PD1
D A Q
PD2
Computer
FBG
Fig. 1. Schematic diagram of the parallel matched grating interrogation scheme.
21
l1 ¼ 1549:78 nm, l2 ¼ 1550:01 nm, lB ¼ 1549:56 nm; respectively. The principle is as follows. The backreflected light of the sensor FBG is split into two beams by means of a 50:50 coupler, and then they are reflected by two matched FBGs and detected by photodetectors, respectively. The strain on the sensor FBG will be tuned by a cantilever; in the meantime, the center wavelength of back-reflected light will shift, which can be reflected on a photodetector current that is in proportion to the convolutional result of spectra between the sensor FBG and the corresponding matched FBG. The temperature shift characteristic of sensor FBG and matched FBGs is uniform for they are the same type of FBG. In the experimental setup, they are installed in identical temperature surroundings. Even if temperature shifts, the relative position of spectra between sensor FBG and matched FBGs will not be affected. So the error caused by temperature shift is restrained.
3. Experimental results The experimental result can be seen in Fig. 3, in which there are two curves representing the voltage signals output by PD1 and PD2. The central wavelength of the sensor FBG will red shift if there is a positive strain on it, and the outputting voltage signals will change. In experimental results, there are differences in peak value and line profile between the two curves, which results from differences in the reflected spectra profile and reflectivity between the two matched FBGs. 1000 g poise will produce 130 me on the cantilever according to its parameter. The voltage signal of PD responds linearly while the cantilever is being loaded from 10 to 4050 g. The sensor FBG is stuck to the center of the cantilever. If the cantilever works within the elastic range, there is no slippage between the sensor
0.9
9.0
0.8
8.5
Chanel 1 Chanel 2
0.7 8.0
FBG1
0.5
FBG
Voltage V
Reflectivity
0.6 FBG2
0.4
7.5 7.0
0.3 6.5
0.2 6.0
0.1 0 1548.5
0
1549
1549.5 1550 1550.5 Wavelength/nm
1551
1551.5
Fig. 2. Spectra of the sensor FBG and the matched FBGs.
1000
2000 Weight g
3000
4000
Fig. 3. Experimental results for the output of the two PDs V1 and V2 vs. the weight of poise.
ARTICLE IN PRESS 22
X. Yang et al. / Optik 120 (2009) 20–23
FBG and the cantilever. Therefore, the strain on the sensor FBG is equal to the strain on cantilever, and the corresponding strain value is as follows:
1.3
D ¼ ½ðG max G min Þ=1000 130 ¼ 525 me:
1.1
1.2
From Eq. (1), the measurable range is up to 525 me by taking two normal FBGs to demodulate, which can satisfy most measurement requirements of bridge and building strain monitoring system.
V1/V2
(1)
1 Section1
Section2
Section3
0.9
0.8
4. Data-processing scheme design for expanding the measurable range
0.7 0
There is a non-linear voltage array V 1 ; V 2 ; . . . ; V n corresponding to a strain array m1 ; m2 ; . . . ; mn . If a voltage value V x ðV i pV x pV iþ1 Þ, corresponding strain value mx , wants to be calculated accurately, it can be realized by the look-up table scheme as Eq. (2): miþ1 mi mx ¼ ðV x V i Þ þ mi . (2) V iþ1 V i The essence of the scheme is that a non-linear curve is separated into an N section linear curve. The section to which any pending data belong to is firstly determined by looking up the table; then the corresponding result can be calculated by linear arithmetic. There are two curves of voltage signals output by PDs according to different loads as shown in Fig. 3. In our former research, a Gauss theoretical model solving double value was proposed according to the experimental results [4]. The appropriate thresholds PL and PH were selected to divide the whole experimental data into four sections; then microstrain values corresponding to voltage signals could be realized by simple arithmetic. But the voltage signal output by PD is not extreme according to the Gauss distribution, and the linearity of voltage values between PL and PH is not so good. So there will be biggish error calculated by this model. A similar experimental result as shown in Fig. 3 was reported by Nunes et al [5]. They only processed the voltage signal between the peak and the trough and obtained wide enough measurable range, because the bandwidth of the filter components they used was big enough. Although the method avoided the problem of double value, the measurable range of the system was not expanded furthest. In our experimental setup, the 3-dB bandwidth of normal FBG is only 0.2 nm, and the measurement range of the system is about 300 me. There is random fluctuation in signal output by PD because of some unstable factors such as optical power fluctuation of laser, etc. To this problem, the output of the FBG sensing system is given as the ratio (V1/V2) of the voltage signals measured by the PDs as shown in Fig. 4, which reduces voltage fluctuation caused by
1000
2000 Weight g
3000
4000
Fig. 4. Experimental results for system output (V1/V2) vs. the weight of poise.
optical power fluctuation and yields a curve made up of three almost linear sections. For the character of small bandwidth of normal FBG, the whole curve is processed to expand the measurable range. First the section to which the pending data belong is determined; then it is looked up in the table to correspond to microstrain. The linearity of two border upon points is very good after subdividing the curve shown in Fig. 4. So any pending data can correspond to microstrain by linear arithmetic. The advantage of this method to the Gauss theoretical model is that the calculation error induced by the linear error of original data is reduced. The divisional look-up table method not only solves the problem of double value but also expands the measurable range. The limitation of narrow demodulation range induced by the small bandwidth of normal FBG is compensated. The curve shown in Fig. 4 is divided into three sections according to the location of the peak and trough by programming. Any data in Fig. 4 correspond to only one section according to the relation of two voltage curves output by PDs in Fig. 3, which solves the problem of double value. A whole curve is obtained from experiments to be the reference curve. Because of the bad linearity of curves in peak and trough areas, more experimental data are inserted in these areas to improve the precision of the look-up table. So any pending data are able to correspond to the only microstrain in the reference curve by the division lookup table scheme. The experimental results shown in Fig. 5 support the theoretical results. From the comparison between ameliorated experimental results and former ones [4], the ameliorated experimental results are smooth, and the precision is improved to 71 me, which is better than the former 72.6 me. The influence on experimental results from optical power fluctuation and environment temperature
ARTICLE IN PRESS X. Yang et al. / Optik 120 (2009) 20–23 Theoretical Experimental
600 500
Strain με
400
23
could satisfy the rigorous requirements of field engineering, and play a key role in the engineering process. From the aspect of system cost, our interrogation system adopts normal commercial FBGs only; compared to the interferometer scanning interrogation system, the cost is reduced rapidly.
300 200
6. Conclusion
100 0 0
1000
2000 Weight g
3000
4000
Fig. 5. Curve of theoretical and experimental results for the system.
shift and the error introduced by data processing are restrained to a great extent.
The novel dual gratings parallel matched interrogation system has a simple configuration, which improves system usability and reduces the system cost. A credible and effective data-processing scheme based on a novel divisional look-up table is proposed based on the advantages of other schemes. The best measurable range of the system is 525 me, and the precision is 71 me based on normal matched gratings. The improvement of data processing mines potential of the system and improves engineering performance.
5. Improving usability and lowering the system cost
References
There are some disadvantages in the existing commercial interferometer scanning interrogation system. For example, the signal output of the interferometer is unstable because the interferometer is sensitive to temperature and shake; the system cannot work in real time because of the scanning module. All these factors limit the performance of the interferometer scanning interrogation system. The novel dual gratings parallel matched interrogation system we proposed is composed of normal commercial FBGs only, which reduces the complexity of the interrogation system and works in real time without a mechanical module. The simpler the system, the more reliable the signal output. All these advantages
[1] W.W. Morey, G. Meltz, W.H. Glen, Fiber optic Bragg grating sensors, Proc. SPIE 1169 (1989) 98–107. [2] Y. Zhao, Y. Liao, Discrimination methods and demodulation techniques for fiber Bragg grating sensors, Opt. Lasers Eng. 41 (2004) 1–18. [3] Z. Yage, L. Qing, X. Shiqing, et al., Study on the optimization of matched grating interrogation technique of fiber grating sensor, Acta Photon. Sin. 33 (6) (2004) 711–715 (in Chinese). [4] L. Qing, Z. Yage, X. Shiqing, Two-values question in signal detecting of strain sensor based on fiber Bragg gratings, Chin. J. Lasers 31 (8) (2004) 988–992. [5] L.C.S. Nunes, L.C.G. Valente, A.M.B. Braga, Analysis of a demodulation system for fiber Bragg grating sensors using two fixed filters, Opt. Lasers Eng. 42 (2004) 529–542.
Optik
Optics
Optik 120 (2009) 20–23 www.elsevier.de/ijleo
Some engineering issues of an FBG sensor with the dual gratings parallel matched interrogation method Xichun Yanga,b,, Jincheng Peia,b, Yage Zhanc, Rude Zhua, Hong Hea, Shiqing Xianga a
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China Graduate School of the Chinese Academy of Sciences, Beijing 100039, China c Applied Physics Department, College of Science, Donghua University, Shanghai 201602, China b
Received 29 January 2007; accepted 15 June 2007
Abstract The key issues of engineering application of the dual gratings parallel matched interrogation method are expanding the measurable range, improving the usability, and lowering the cost by adopting a compact and simple setup based on existing conditions and improving the precision of the data-processing scheme. A credible and effective data-processing scheme based on a novel divisional look-up table is proposed based on the advantages of other schemes. Any undetermined data is belonged to a certain section, which can be confirmed at first, then it can be looked up in the table to correspond to microstrain by the scheme. It not only solves inherent problems of the traditional one (double value and small measurable range) but also enhances the precision, which improves the performance of the system. From the experimental results, the measurable range of the system is 525 me, and the precision is 71 me based on normal matched gratings. The system works in real time, which is competent for most engineering measurement requirements. r 2007 Elsevier GmbH. All rights reserved. Keywords: Dual gratings parallel matched; Bragg grating; Strain; Look-up table
1. Introduction Research on the theory and application of a fiber Bragg grating (FBG) has made great progress in the sensing field since Morey reported the possibility of using an FBG for sensing at 1989 [1]. An FBG is a wavelength-coded component, and it has all the outstanding advantages of the traditional fiber sensor such as good reliability, EMI immunity, light weight, high accuracy, etc. There have been great requirements of an
Corresponding author. Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China. E-mail address: [email protected] (X. Yang).
0030-4026/$ - see front matter r 2007 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2007.06.012
FBG sensor in civil engineering, the spaceflight industry, intelligent traffic, etc. Wavelength interrogation is the most important task in research on the FBG sensor system. Throughout more than 20 years of development, there have been several mature interrogation methods such as optical spectrometer method, edge filter method, tunable fiber F-P filter method, interferometer scanning method, matched FBG-based filter method, etc. [2]. The matched FBG-based filter method has advantages such as low cost, simple configuration and working in real time, which has been considered as the best solution for a commercial system. A novel dual gratings parallel matched interrogation method has been proposed and studied seriously [3]. The
ARTICLE IN PRESS X. Yang et al. / Optik 120 (2009) 20–23
novel method preserves the advantages of the traditional matched FBG-based filter method such as high resolution, fast interrogation speed, good repetition and low cost, and solves inherent problems of the traditional ones: double value and small measurable range. In our previous research, we paid attention to the theoretical and experimental possibility of the novel dual gratings parallel matched interrogation method, but did not study its engineering performance. A novel divisional look-up table scheme is proposed based on a deep study on data-processing schemes, which plays a key role in improving the engineering performance of the sensing system.
2. Theory and experimental setup Fig. 1 shows a schematic diagram of the experimental setup used in the proposed sensor system. Fig. 2 shows a schematic diagram of spectra of the sensor FBG and the matched FBGs, in which the detectable signal corresponds to the hatching part. There are two matched FBGs and a sensor FBG in the setup, the reflective center wavelengths of which are Sensor FBG BBS FBG PD1
D A Q
PD2
Computer
FBG
Fig. 1. Schematic diagram of the parallel matched grating interrogation scheme.
21
l1 ¼ 1549:78 nm, l2 ¼ 1550:01 nm, lB ¼ 1549:56 nm; respectively. The principle is as follows. The backreflected light of the sensor FBG is split into two beams by means of a 50:50 coupler, and then they are reflected by two matched FBGs and detected by photodetectors, respectively. The strain on the sensor FBG will be tuned by a cantilever; in the meantime, the center wavelength of back-reflected light will shift, which can be reflected on a photodetector current that is in proportion to the convolutional result of spectra between the sensor FBG and the corresponding matched FBG. The temperature shift characteristic of sensor FBG and matched FBGs is uniform for they are the same type of FBG. In the experimental setup, they are installed in identical temperature surroundings. Even if temperature shifts, the relative position of spectra between sensor FBG and matched FBGs will not be affected. So the error caused by temperature shift is restrained.
3. Experimental results The experimental result can be seen in Fig. 3, in which there are two curves representing the voltage signals output by PD1 and PD2. The central wavelength of the sensor FBG will red shift if there is a positive strain on it, and the outputting voltage signals will change. In experimental results, there are differences in peak value and line profile between the two curves, which results from differences in the reflected spectra profile and reflectivity between the two matched FBGs. 1000 g poise will produce 130 me on the cantilever according to its parameter. The voltage signal of PD responds linearly while the cantilever is being loaded from 10 to 4050 g. The sensor FBG is stuck to the center of the cantilever. If the cantilever works within the elastic range, there is no slippage between the sensor
0.9
9.0
0.8
8.5
Chanel 1 Chanel 2
0.7 8.0
FBG1
0.5
FBG
Voltage V
Reflectivity
0.6 FBG2
0.4
7.5 7.0
0.3 6.5
0.2 6.0
0.1 0 1548.5
0
1549
1549.5 1550 1550.5 Wavelength/nm
1551
1551.5
Fig. 2. Spectra of the sensor FBG and the matched FBGs.
1000
2000 Weight g
3000
4000
Fig. 3. Experimental results for the output of the two PDs V1 and V2 vs. the weight of poise.
ARTICLE IN PRESS 22
X. Yang et al. / Optik 120 (2009) 20–23
FBG and the cantilever. Therefore, the strain on the sensor FBG is equal to the strain on cantilever, and the corresponding strain value is as follows:
1.3
D ¼ ½ðG max G min Þ=1000 130 ¼ 525 me:
1.1
1.2
From Eq. (1), the measurable range is up to 525 me by taking two normal FBGs to demodulate, which can satisfy most measurement requirements of bridge and building strain monitoring system.
V1/V2
(1)
1 Section1
Section2
Section3
0.9
0.8
4. Data-processing scheme design for expanding the measurable range
0.7 0
There is a non-linear voltage array V 1 ; V 2 ; . . . ; V n corresponding to a strain array m1 ; m2 ; . . . ; mn . If a voltage value V x ðV i pV x pV iþ1 Þ, corresponding strain value mx , wants to be calculated accurately, it can be realized by the look-up table scheme as Eq. (2): miþ1 mi mx ¼ ðV x V i Þ þ mi . (2) V iþ1 V i The essence of the scheme is that a non-linear curve is separated into an N section linear curve. The section to which any pending data belong to is firstly determined by looking up the table; then the corresponding result can be calculated by linear arithmetic. There are two curves of voltage signals output by PDs according to different loads as shown in Fig. 3. In our former research, a Gauss theoretical model solving double value was proposed according to the experimental results [4]. The appropriate thresholds PL and PH were selected to divide the whole experimental data into four sections; then microstrain values corresponding to voltage signals could be realized by simple arithmetic. But the voltage signal output by PD is not extreme according to the Gauss distribution, and the linearity of voltage values between PL and PH is not so good. So there will be biggish error calculated by this model. A similar experimental result as shown in Fig. 3 was reported by Nunes et al [5]. They only processed the voltage signal between the peak and the trough and obtained wide enough measurable range, because the bandwidth of the filter components they used was big enough. Although the method avoided the problem of double value, the measurable range of the system was not expanded furthest. In our experimental setup, the 3-dB bandwidth of normal FBG is only 0.2 nm, and the measurement range of the system is about 300 me. There is random fluctuation in signal output by PD because of some unstable factors such as optical power fluctuation of laser, etc. To this problem, the output of the FBG sensing system is given as the ratio (V1/V2) of the voltage signals measured by the PDs as shown in Fig. 4, which reduces voltage fluctuation caused by
1000
2000 Weight g
3000
4000
Fig. 4. Experimental results for system output (V1/V2) vs. the weight of poise.
optical power fluctuation and yields a curve made up of three almost linear sections. For the character of small bandwidth of normal FBG, the whole curve is processed to expand the measurable range. First the section to which the pending data belong is determined; then it is looked up in the table to correspond to microstrain. The linearity of two border upon points is very good after subdividing the curve shown in Fig. 4. So any pending data can correspond to microstrain by linear arithmetic. The advantage of this method to the Gauss theoretical model is that the calculation error induced by the linear error of original data is reduced. The divisional look-up table method not only solves the problem of double value but also expands the measurable range. The limitation of narrow demodulation range induced by the small bandwidth of normal FBG is compensated. The curve shown in Fig. 4 is divided into three sections according to the location of the peak and trough by programming. Any data in Fig. 4 correspond to only one section according to the relation of two voltage curves output by PDs in Fig. 3, which solves the problem of double value. A whole curve is obtained from experiments to be the reference curve. Because of the bad linearity of curves in peak and trough areas, more experimental data are inserted in these areas to improve the precision of the look-up table. So any pending data are able to correspond to the only microstrain in the reference curve by the division lookup table scheme. The experimental results shown in Fig. 5 support the theoretical results. From the comparison between ameliorated experimental results and former ones [4], the ameliorated experimental results are smooth, and the precision is improved to 71 me, which is better than the former 72.6 me. The influence on experimental results from optical power fluctuation and environment temperature
ARTICLE IN PRESS X. Yang et al. / Optik 120 (2009) 20–23 Theoretical Experimental
600 500
Strain με
400
23
could satisfy the rigorous requirements of field engineering, and play a key role in the engineering process. From the aspect of system cost, our interrogation system adopts normal commercial FBGs only; compared to the interferometer scanning interrogation system, the cost is reduced rapidly.
300 200
6. Conclusion
100 0 0
1000
2000 Weight g
3000
4000
Fig. 5. Curve of theoretical and experimental results for the system.
shift and the error introduced by data processing are restrained to a great extent.
The novel dual gratings parallel matched interrogation system has a simple configuration, which improves system usability and reduces the system cost. A credible and effective data-processing scheme based on a novel divisional look-up table is proposed based on the advantages of other schemes. The best measurable range of the system is 525 me, and the precision is 71 me based on normal matched gratings. The improvement of data processing mines potential of the system and improves engineering performance.
5. Improving usability and lowering the system cost
References
There are some disadvantages in the existing commercial interferometer scanning interrogation system. For example, the signal output of the interferometer is unstable because the interferometer is sensitive to temperature and shake; the system cannot work in real time because of the scanning module. All these factors limit the performance of the interferometer scanning interrogation system. The novel dual gratings parallel matched interrogation system we proposed is composed of normal commercial FBGs only, which reduces the complexity of the interrogation system and works in real time without a mechanical module. The simpler the system, the more reliable the signal output. All these advantages
[1] W.W. Morey, G. Meltz, W.H. Glen, Fiber optic Bragg grating sensors, Proc. SPIE 1169 (1989) 98–107. [2] Y. Zhao, Y. Liao, Discrimination methods and demodulation techniques for fiber Bragg grating sensors, Opt. Lasers Eng. 41 (2004) 1–18. [3] Z. Yage, L. Qing, X. Shiqing, et al., Study on the optimization of matched grating interrogation technique of fiber grating sensor, Acta Photon. Sin. 33 (6) (2004) 711–715 (in Chinese). [4] L. Qing, Z. Yage, X. Shiqing, Two-values question in signal detecting of strain sensor based on fiber Bragg gratings, Chin. J. Lasers 31 (8) (2004) 988–992. [5] L.C.S. Nunes, L.C.G. Valente, A.M.B. Braga, Analysis of a demodulation system for fiber Bragg grating sensors using two fixed filters, Opt. Lasers Eng. 42 (2004) 529–542.