Study on 3D CFBG Vibration Sensor and Its Application

PHOTONIC SENSORS / Vol. 6, No. 1, 2016: 90 96 Study on 3D CFBG Vibration Sensor and Its Application Qiuming NAN 1,2* and Sheng LI 1,2 1 National Engineering Laboratory or Fiber Optic Sensing Technology, Wuhan University o Technology, Wuhan, 430070, China 2 Key Laboratory o Fiber Optic Sensing Technology and Inormation Processing, Ministry o Education, Wuhan, University o Technology, Wuhan, 430070, China * Corresponding author: Qiuming NAN E-mail: 197114012@qq.com Abstract: A novel variety o three dimensional (3D) vibration sensor based on chirped iber Bragg grating (CFBG) is developed to measure 3D vibration in the mechanical equipment ield. The sensor is composed o three independent vibration sensing units. Each unit uses double matched chirped gratings as sensing elements, and the sensing signal is processed by the edge iltering demodulation method. The structure and principle o the sensor are theoretically analyzed, and its perormances are obtained rom some experiments and the results are as ollows: operating requency range o the sensor is 10 Hz 500 Hz; acceleration measurement range is 2 m s 2 30 m s 2 ; sensitivity is about 70 mv/m s 2 ; crosstalk coeicient is greater than 22 db; sel-compensation or temperature is available. Eventually the sensor is applied to monitor the vibration state o radiation pump. Seen rom its experiments and applications, the sensor has good sensing perormances, which can meet a certain requirement or some engineering measurement. Keywords: Three-dimensional (3D); matched; chirped iber Bragg grating (CFBG); edge iltering demodulation; crosstalk Citation: Qiuming NAN and Sheng LI, Study on 3D CFBG Vibration Sensor and Its Application, Photonic Sensors, 2016, 6(1): 90 96. 1. Introduction Vibration measurement is one o the most eective methods or mechanical equipment condition monitoring and ault diagnosis. However, vast majority o vibration measurement is based on electromagnetic sensors, although this technology has been used or some o its advantages, its application is greatly limited in some special occasion [1 4]. For example, in the lammable, explosive, and electromagnetic-intererence situation, a large network is required, and the signal is required to transmit over a long distance. The iber Bragg grating sensing technology is a newly minted one, which has such characteristics, such as nonelectric detection, anti-intererence o electromagnetic, ease to use in large-scale network, large amount o inormation, and long-distance transmission. It provides a new technical means or the vibration monitoring o mechanical equipment [5, 6]. According to statistics, there are being done some researches on a one-way iber grating vibration sensor [7], but in practical application, it is requently demanded to detect the spatial vibration o a certain part o the machine at the same time. Due to the limitation o the installation space or Received: 19 October 2015 / Revised: 6 December 2015 The Author(s) 2015. This article is published with open access at Springerlink.com DOI: 10.1007/s13320-015-0292-6 Article type: Regular

Qiuming NAN et al.: Study on 3D CFBG Vibration Sensor and Its Application 91 other reasons, three one-way vibration sensors can be installed in the identical position so diicultly that the authentic state o equipment can not be monitored completely and correctly [8 10]. Consequently, it is very essential to conduct the research on a three-dimensional chirped iber Bragg grating (3D CFBG) vibration sensor. 2. Structure and principle o sensor Fig. 1 Structure o 3D CFBG accelerometer. 2.1 Structure design According to the application requirements, the overall design goals o the sensor are as ollows: (1) Integration and seal design (2) Working requency 500 Hz (3) Acceleration range 20 m s 2 (4) Ambient temperature range: 20 80 (5) Single dimensional size 100 mm According to this goal, we design a structure o the three-dimensional iber grating vibration sensor as shown in Fig. 1. The sensor is composed o three unidirectional sensing units that are mutually vertical. Each sensing unit is composed o a base, an elastomer, a mass block, and two CFBGs, and its structure and principle are shown in Fig. 2. Two CFBGs written in the same optic iber are ormed into precise matching by accurate abrication process, and they are symmetrically ixed between the base and the mass block. In order to avoid dead zone, we give them 2 nm/s pretension. When the sensor receives the vibration signal rom the outside world, the mass supported by the elastic membrane will vibrate up and down to drive the CFBGs stretch along the axial direction, and the relection peaks o CFBG1 and CFBG2 will change, as shown in Fig. 3. The light emitting diode (LED) bandwidth is approximately 40 nm. The detected signal by the photodetector is the change in the envelope spectrum o the two CFBGs, which is negatively correlated with that o the overlapped spectrum. To take the upward acceleration o the mass block as an example, CFBG2 elongates, CFBG1 shrinks, the overlapped area becomes bigger but the detected signal is negative, and vice versa. Fig. 2 Structure and principle o sensing unit. Fig. 3 Principle o matching demodulation. From the structure and principle o the sensor, the sensor has the ollowing two notable advantages. Firstly, when the external temperature changes, the relection spectra o the two gratings will move to the same direction, and the envelope area remains constant, so the sel-compensation or temperature change can be realized. Secondly, when the sensor is orced to vibrate, the moving direction o two relection spectrums are opposite, and the envelope area is increased signiicantly. Accordingly, the sensitivity o the sensor is eectively improved. 2.2 Theoretical analysis In order to urther illustrate the working principle o the sensor, the authors have conducted theoretical calculation. As shown in Fig. 4, the upward exciting orce exerted to the mass block is Fa Ma, and it makes the elastomer deorm, while CFBG2 is elongated to generate a let tension F.

92 Photonic Sensors For the elastomer, the stress state can be transormed into equivalent to the orce F a and a clockwise torque M F d, where d is the distance between mass block and elastic node, namely the distance between A and B in Fig. 4. Fig. 4 Schematic diagram o the orce exerted to the sensor. According to the calculation o mechanics o materials or the deormation o bending beam, the delection o elastic body can be expressed as 3 2 Fl a Ml 3EI 2EI (1) where l is the length o the elastomer, E is the elastic modulus, and I is the moment o inertia. L O A O A 2 2 2 1 OA AB sin AB BB AB cos 2 2 2 1 2 2 1 1 1 2 2 2 OA 2 1. (2) The tension L o the optical iber subjected to external vibration can be expressed as (2). As shown in Fig. 4, is the angle o the elastomer caused by the stress. As the rotation angle is very small, (2) can be expressed as 2 2 L L L (3) where L is the original length o the iber. The axial stress o the iber and the change in wavelength caused by the stress can be expressed respectively as L = = F E A (4) where L E, A, and K are the elastic modulus, cross-sectional area, and stiness o optical iber, respectively. (1 P e ) (5) where P e and are the eective elastic optical coeicient and initial wavelength o iber grating, respectively. From (1) to (5), the sensitivity S o the sensor can be expressed as 2 1 2ml S 1 Pe (6) 2 a L 6EI 3K dl where m is the quality o the mass. From (6), it can be observed that there is a linear relationship between the wavelength change o FBG and the outside acceleration a. The elastomer can be regarded as a uniorm cantilever beam, so its stiness K e is expressed as 3EI Ke. (7) 3 l According to the kinematics equation o the object structure, the irst-order resonant requency o the sensor is calculated as 1 K 1 K 2 e d L K. (8) 2 m 2 m From (6) and (8), i m is larger and l is longer, then S is bigger, and is smaller; vice versa. Thereore, the structure parameters o the sensor should be reasonably designed according to the requirement o the measurement. 3. Perormance tests o sensor In accordance with the above designs, we have made some samples o 3D CFBG vibration sensor. Next, we test the sensing perormances o the sensor by means o the experimental apparatus, as shown in Fig. 5. These sensing perormances mainly include amplitude-requency characteristics, acceleration characteristics, anti-crosstalk, and anti-temperature inluence perormance. Fig. 5 Vibration testing system.

Qiuming NAN et al.: Study on 3D CFBG Vibration Sensor and Its Application 93 3.1 Amplitude-requency characteristics In the amplitude-requency characteristic test, the output acceleration o the exciter is kept constant, 3 m/s 2 as the input vibration signal. The requency o the input signal starts rom 10 Hz, incrementing 50 Hz as a step, and when the requency value reaches 800 Hz, the step size o the requency is adjusted to 20 Hz. The experimental data o each step should be recorded. During the test, the amplitude-requency characteristics o three sensing units (X, Y, Z) are measured, respectively, as shown in Fig. 6. The results show that their irst-order resonant requencies are 910 Hz, 890 Hz, and 890 Hz, respectively, the lat segments o the curves are all the range rom 10 Hz to 500 Hz, namely, and the operating requency range o the sensor is 10 Hz 500 Hz. Amplitude (mv) 1200 1000 800 600 400 X direction Y direction Z direction 0 200 400 600 800 1000 1200 Frequency (Hz) Fig. 6. Amplitude-requency characteristics o 3 sensing units. 3.2 Linear calibration Linear calibration is a good method to test the sensor s perormances like sensitivity, linearity, and linear range. The requency o the input signal is 200 Hz, and measurement acceleration is within 2 m s 2 30 m s 2, which are read out by the piezoelectric standard acceleration sensor, type 4371. Write down the output voltage values o the 3D CFBG vibration sensor at each setting value. The calibration curves are shown in Fig. 7. From Fig. 7, we can see when the input acceleration is within the range o 2 m s 2 30 m s 2, the linear degrees o calibration curves are all more than 0.999, and the three sensitivities are 73 mv/m s 2, 69 mv/m s 2, and 75 mv/m s 2, respectively. With an increase in the acceleration, the output voltage increases slowly, and the sensitivity decreases. When the acceleration is more than 30 m s 2, the output voltage becomes saturated. Thereore, the acceleration measurement range o the sensor can be considered as 2 m s 2 30 m s 2. Voltage (mv) 800 600 400 200 X axial Y axial Z axial 0 0 12 24 36 48 60 Accelerometer (m s 2 ) Fig. 7 Calibration curve o 3D FBG accelerometer. 3.3 Anti-crosstalk perormance First o all, let the exciter vibrate in the X axis direction, the input signal is a sine wave signal, o which the vibration requency is 60 Hz, and the acceleration is 6 m s 2. Three-direction response data are collected at the same time, and the eective values o output voltage are 450 mv, 32 mv, and 34 mv, respectively. Similarly, under the condition o the invariable vibration signal, let Y and Z axes be the main vibration directions, respectively, repeat the above experiments, and the results are shown in and Figs. 8, 9, and 10. From Table 1, the maximum crosstalk coeicient can be calculated about 22 db, which indicates that crosstalk has little eect on the measurement. Table 1 Test results o crosstalk coeicient (mv). Main vibration direction X Y Z X 450 36 38 Output Y 32 462 39 response Z 34 35 470 Fig. 8 Response curves o X shat as main vibrating direction.

94 Photonic Sensors From Fig. 11, it can be seen that the matching state o the double gratings has no change when the temperature varies rom 20 to 80, and the output is stable, which shows that the sensor is insensitive to the change o the temperature and has a strong ability to resist temperature intererence. Fig. 9 Response curves o Y shat as main vibrating direction. Fig. 10 Response curves o Z shat as main vibrating direction. 3.4 Anti-temperature perormance The sensing perormance o the sensor based on edge iltering demodulation is largely determined by the matching state o the two gratings. When the ambient temperature changes, the relection peaks o the two gratings will shit; i the two relection peaks move synchronously, the matching state keeps constant, but i they are not synchronized, the matching state will change. The results o the anti-temperature inluence experiment are shown in Fig. 11. 4. Application 4.1 Application description The 3D CFBG vibration sensor developed in the paper has been used in the vibration monitoring o the radiation pump in petrochemical industry. In the industrial test, 3 pumps were monitored and 2 sensors were installed on each pump. The monitoring parameters included the vibration o the bearing seat in the three directions, the horizontal (V), vertical (H), and axial (A). In the installation, the sensor was installed on the bearing seat by bolts. Site installation conditions are shown in Fig. 12. Fig. 12 Sensor site installation photos. 4.2 Application results Fig. 11 Matching state o double gratings. On Jan 20th, 2014, the online monitoring system issued a warning signal. It was ound through the query that the vibration speed o 3H o the No.3 pump was about 5.2 mm/s, as shown in Fig. 13, and this value was increasing. Further analysis o the vibration data in other directions, which were larger than the normal values, is shown in Fig. 14, which showed that the working condition o the pump was deteriorating. Moreover, urther studies ound that there were the ault characteristic requencies o the inner ring and cage o the bearing in the velocity

Qiuming NAN et al.: Study on 3D CFBG Vibration Sensor and Its Application 95 spectrum. Through the comprehensive analysis o the above test results, the preliminary diagnosis was that there were peeling deect on the inner ring and severe wear on the cage. Fig. 13 Frequency spectrum o 3H measuring point. Fig. 14 Frequency spectrum o multimeasuring points. Fig. 15 Fault bearing physical picture. On Jan 23th, the radiation pump was removed or maintenance and it was ound that there were two larger and a dozen smaller exoliations on the inner ring o the bearing, and some obvious damages on the surace o the shat, as shown in Fig. 15. According to the above situation, the bearing was replaced and the system showed the pump had been into working in the normal state ater maintenance, which adequately demonstrated the accuracy and reliability o the above ault diagnosis. 5. Conclusions A novel variety o 3D vibration sensor based on CFBG is developed to measure 3D vibration in the mechanical equipment ield. Theoretical analysis and experimental tests have been perormed or the sensor, and it has been applied to engineering practice. The main conclusions are as ollows. (1) In order to have a good perormance, the ollowing innovative design is implemented. Firstly, the sensing unit is designed as an improved cantilever structure to increase the operating requency o the sensor. Secondly, the two matched chirped gratings are used as a sensing element to increase the measurement range o acceleration. Finally, the demodulation method based on the edge iltering is used to solve the problem o high requency signal acquisition. (2) The results o perormance test are as ollows. The operating requency range o the sensor is 10 Hz 500 Hz, the acceleration measurement range is 2 m s 2 30 m s 2, the sensitivity is about 70 mv/m s 2, the crosstalk coeicient is greater than 22 db, and it has a strong ability to resist the temperature intererence. These experimental results are in good agreement with the theoretical analysis. (3) The 3D CFBG vibration sensor has been applied to monitor the working state o the radiation pump and the process o the bearing ault has been successully monitored, which may be an important basis or ault analysis. Acknowledgment This research was unded by the Key Project o National Science Foundation o China, Award Number: 61290311. Open Access This article is distributed under the terms o the Creative Commons Attribution 4.0 International License (http:// creativecommons.org/licenses/by/4.0/),

96 Photonic Sensors which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate i changes were made. Reerences [1] Y. Zheng, Vibration monitoring and analysis or rotating machinery, Gas Tribine Technology, 2010, 23(1): 39 48. [2] D. Fu and Y. Wang, The research o on-line vibration monitoring system o rotating machinery, Machine Tool & Hydraulics, 2005, (1): 152 153. [3] Q. M. Nan, Study on dynamic sensing and monitoring methods or petrochemical acilities based on optical iber grating, Ph.D. dissertation, Dept. Wuhan University, China, 2014. [4] Z. Zhou, Q. Liu, Q. Ai, and C. Xu, Intelligentmonitoring and diagnosis or modern mechanical equipment based on the integration o embedded technology and FBGS technology, Measurement, 2011, 44(9): 1499 1511. [5] Q. M. Nan, Study and application o CFBG vibration sensor with symmetrical push-pull coniguration, in the 22nd International Conerence on Optical Fiber Sensor, Beijing, China, 15 19, 2012. [6] G. Wang and B. Xie, Improving the perormance o chirped iber grating with cladding being etched as sinusoidal unction, Optik International Journal or Light and Electron Optics, 2010, 122(6): 557 559. [7] Y. Zhao and Z. Li, Study on tri-axial accelerometer based on FBG, Chinese Journal o Scientiic Instrument, 2006, 27(6): 299 301. [8] J. Wang, C. Chen, D. Tang, C. Zhang, and Y. Cui, Three-component photo-elastic iber optic accelerometer based on the photo-elastic eect, Chinese Journal o Sensors and Acuators, 2006, 19(3): 804 806. [9] L. Ding, J. Zhao, J. Wang, and Q. Sui, Design o FBG vibration sensor based on matching ilter demodulation, Electro-Optic Technology Application, 2009, 24(3): 36 39. [10] Q. Nan, Research on 3D FBG Accelerometer and demodulation method, Chinese Optics Letters, 2014, (A01): 149 153.