Research on Fiber Bragg Grating Acoustic Emission Technology Applied in Helicopter Bearing Detection

Transcription

1 Available online at ScienceDirect Procedia Engineering 00 (2014) APISAT2014, 2014 Asia-Paciic International Symposium on Aerospace Technology, APISAT2014 Research on Fiber Bragg Grating Acoustic Emission Technology Applied in Helicopter Bearing Detection Gao Xuan a, *, Zhang Xiao-peng a, Li Ning a, and Pei Xin b a Department o Flight Simulation Training; Army Aviation Institute; Beijing,101123, China b Department o Aviation and Mechanical Engineering; Army Aviation Institute; Beijing,101123, China Abstract Fiber bragg grating acoustic emission technology can be applied in bearing detection against on the issue o existing nondestructive testing or helicopter structural components mostly not realizing in-situ on-line monitoring. Description o Fiber grating acoustic emission detection principle, with the strengths and weaknesses o iber bragg grating sensor; detailed analysis o ault mechanism and requency o helicopter bearing, and study o the resonance demodulation algorithm or signal postprocessing; proposing a case o bearing on-line detection compared with other detection technologies against consistent analysis result The Authors. Published by Elsevier Ltd. Peer-review under responsibility o Chinese Society o Aeronautics and Astronautics (CSAA). Keywords: Acoustic Emission; Fiber Bragg Grating; Rolling Bearing; 1. Preace The key structural parts o helicopter occur deormation, racture and corrosion, accompanied by acoustic emission (reerred to as AE) signal generation. Thereore, the damage o key structural parts o helicopter can be monitored by detecting the acoustic emission signal. As a kind o emerging nondestructive testing technology, acoustic emission can directly relect the micro crack generation and diusion within the materials; and acoustic emission technology can directly detect the ault o original stage, thus eectively improve the service lie o * Corresponding author. Tel.: ; address: gaoxuan-1981@163.com The Authors. Published by Elsevier Ltd. Peer-review under responsibility o Chinese Society o Aeronautics and Astronautics (CSAA).

2 2 Gao Xuan/ Procedia Engineering 00 (2014) helicopter, with prevention o harmul accidents, ruly nipping in the bud. At present, piezoelectric ceramic sensor based on piezoelectric eect (reerred to as PZT) is relatively mature acoustic emission sensor in domestic and oreign market. But the piezoelectric sensor generally has great limitation with its big volume, heavy weight and being susceptible to electromagnetic intererence. While, the helicopter is normally in complex working conditions, restricting the application o piezoelectric sensor in helicopter structural health monitoring. Since the birth o Fiber Bragg Grating, it has been widely applied in the optical iber sensing and communication with its anti-electromagnetic intererence, small size, high plasticity, corrosion resistance, etc. However, the research at home on this aspect is relatively less so ar. Fiber bragg grating sensor is practically applied to the acoustic emission detection in the paper with reaching the expected eect. 2. Fiber Bragg Grating Acoustic Emission Detection Principle 2.1. Acoustic Emission Technology Acoustic emission is deined as a kind o nature phenomena in the orm o elastic waves to release part o strain energy within a material or structure under the action o external or internal orce at the same time o deormation or damage, is a relaxation process transitioning rom unstable upper state to a stable state, caused by uneven stress distribution within the material. Acoustic emission technology is to detect the acoustic emission signal by using special acoustic emission instrument and to iner the variation produced inside o the material through the detection signal or detection parameter analysis. The key structural parts o helicopter occur deormation, racture and corrosion, accompanied by acoustic emission signal (AE) generation all the time, thereore, the damage o key structural parts o helicopter can be monitored by detecting the acoustic emission signal Fiber Bragg Grating Acoustic Emission Detection Principle When acoustic emission wave propagates in a homogeneous medium, the signal amplitude will gradually decay with the increase o propagation distance; the acoustic emission wave will occur scattering and diraction with uneven transmission medium, thus resulting in the attenuation o signal strength; acoustic emission wave may happen transmission, relection and scattering etc. in the two mediums interace i transiting rom one medium to another medium, which also can cause the decay o signal strength. Acoustic emission signal itsel belongs to a kind o weak signal, and the signal amplitude will greatly decay again through the material transer, so it is necessary to detect the acoustic emission signal by using high sensitivity sensor (Fig. 1.). Fiber bragg grating is used as the sensor in the paper, comparing with the conventional electrical sensor, it has the advantages o small size, light weight, easy networking anti-electromagnetic intererence and etc. Fig.1. Acoustic Emission Detection Principle

3 Gao Xuan/ Procedia Engineering 00 (2014) Fig.2. Fiber Bragg Grating Acoustic Emission Sensor Schematic Diagram The tunable narrow-band light source can proceed the wavelength demodulation o sensing grating as shown in Fig.2: the narrow linewidth laser generated rom the tunable narrow-band light source connects with A port o coupler, coupler B port connects with the photoelectric conversion circuit part o signal processing, coupler C port is linked to FBG sensor; the light rom the light source goes through the isolator, accesses to rom Fiber coupler A port and goes out rom C port, then arrives FBG sensor, causes the drit o central emission wavelength B by receipt o the external acoustic emission signal thus resulting in the changes o light signal relected by FBG, and then comes back iber coupler rom Port C. Hal o light rom Port A is blocked by isolator, and the other hal o light outputs rom Port B and is converted into electrical signals by going into the photoelectric receiver, and then the electrical signal successively goes through the ampliying circuit, ilter circuit and analog-to-digital conversion circuit, enters into the FPGA or integrated data processing. At last, the inal data enters into industrial control computer through PCI bus. 3. Advantages and Disadvantages o Fiber Bragg Grating Sensor So ar, most o the acoustic emission detectors are piezoelectric ceramic PZT sensor based on the piezoelectric eect. But the piezoelectric sensor generally has great limitation, while the helicopter is normally in complex working conditions, restricting the application o piezoelectric sensor in helicopter structural health monitoring. Whereas the iber bragg grating with its lexible structure, small size and light weight, can be easy to embed into the structure itsel or make surace mounting without impacting on structure s completeness. The working principle o iber Bragg grating (FBG) is to modulate the wavelength o Bragg through AE wave, and then detect AE signal by relected light wavelength changes. Fiber bragg grating (FBG) sensor has unique characteristics as below: Strong anti-intererence ability. On one hand, normal transmission iber does not aect the requency characteristic o transmited light wave(neglecting the nonlinear eect in iber); on the other hand, Fiber Bragg

4 4 Gao Xuan/ Procedia Engineering 00 (2014) Grating sensor system essentially removes the intererence caused by various kind o light intensity luctuations, or instance, light source intensity luctuation, random luctuation caused by iber micro-bend eect and coupling loss and etc., all o them cannot aect the wavelength characteristics o sensing signal. The sensing probe has simple structure and small size, and can be made into various o shapes according to the object to be detected, moreover, it can be embedded into the object to be detected. Non-conductivity and little inluence on the detected medium. With corrosion-resistance and immunity rom electromagnetic intererence, suitable or working in harsh environment. Be easy to orm sensing network. Fiber bragg grating writing process is relatively mature so that it easy to orm large scale production. High sensitivity and high resolution. It is worth mentioning that the tunable narrow-band light source method can demodulate ideal wavelength change o iber grating at room temperature or little temperature change. However, at greatly temperature change, iber grating could be aected by the temperature, leading to its central relection wavelength change, thus inluencing the sensing accuracy o iber grating. 4. Fiber Bragg Grating Acoustic Emission Detection Technology Applied in the Rolling Bearing As an important rotating part in the mechanical equipments o helicopter, rolling bearing is one o the important ault sources o helicopter mechanical equipments. Comparing with other mechanical parts, the great eature o rolling bearing is great lietime discreteness; some bearings are still in good condition although they have greatly exceeded the design lietime, while some bearings appear various aults even though they don t reach the design lietime. For helicopter, it is impossible to take down various kinds o rolling bearing installed on itsel or all kinds o detections, so in situ online detection is in the need o army Failure mechanism and Failure Frequency o Helicopter Bearing There are so many reasons or rolling bearing aults, such as materials deects, processing and assembling without conorming to the speciication, poor lubrication, moisture and oreign mater intrusion, all o them can lead to early damage o rolling bearing at dierent extent. During the long-time operation, even i all other conditions are normal, the rolling bearing may occur aults like atigue spalling and wear, thus aecting the normal operation o mechanical equipments. Generally speaking, the ault orms o rolling bearing mainly include: a)wear The surace wear o rolling bearing is generally caused by the relative motion between bearing raceway and rolling body. Bearing wear will lead to increased clearance and make the bearing rolling surace roughness increased as well, resulting in reducing the running accuracy, thus lower the accuracy o the whole set o system with great noise and vibration, meanwhile the service lie o bearing will be greatly restricted. b)fatigue Spalling Fatigue spalling is reerred to: when the rolling bearing is running, the raceway and rolling body interact on each other, at the beginning the cracks may appear under the surace by maximum stress, but with the long time running and rotation, the crack is gradually serious, then extends to the contact surace and appears spalling pit, inally resulting in large spalling. The impact load generated by atigue spalling will increase the vibration and noise o bearing running. Thereore, the atigue spalling is the main reason or rolling bearing ault i no other actors. The bearing atigue lietime is actually the lietime o bearing as people usually said. c)corrosion Corrosion can cause a lot o equipment aults, there is no exception or rolling bearing. Water invading bearing can cause corrosion. I the temperature is relatively low, the water vapor in the air will condense into water droplets on the bearing surace, which also can cause corrosion. In addition, sometimes the electric current passes through the internal o bearing, the sparks will generate when the electric current goes through very thin oil ilm, which can make the bearing surace melt and leave some uneven corrugated traces. d)plastic Deormation

5 Gao Xuan/ Procedia Engineering 00 (2014) When mechanical system is overloaded at working, the rolling bearing also can correspondingly suer rom excessive impact load, and it also could cause an additional load with high operating temperature. I high-hardness oreign maters enter into the raceway, it probably orms a dent on the surace o raceway. The ault impact load caused by all above reasons will urther lead to the spalling o other near surace i not controlled. Bearing Diagram as shown in Fig.3, the inner ring diameter o bearing is d i, the outer ring diameter o bearing is d o, Rolling body diameter is d, the central diameter o bearing rolling body is D, the contact angle between rolling body and inner and outer rings (the intersection angle between the contact point and the axis o inside and outside diameter) is. Fig.3. Bearing Diagram Fig.4. Time-requency Domain Waveorm o Impact Signal I there is no sliding riction between rolling body and raceway at bearings rotation and there is no deormation or each part o bearing when operating, the rotating requency o bearing inner ring is i, the rotating requency o 0 bearing outer ring is o g (outer ring ixed), the revolution requency o rolling body is, the number o rolling body is N, then seen as ollows: The speed o point A on the inner ring o bearing is: v d ( D d cos ) A (1) i i i The speed o point B on the outer ring o bearing is: So the speed o rolling body is: So the revolution requency o rolling body is: v v B d ( D d cos ) 0 (2) o o v v 1 ( D d cos ) D 2 2 o A B g i g (3) 1 d g (1 cos ) 2 D (4) i

6 6 Gao Xuan/ Procedia Engineering 00 (2014) I there is a ault or the outer ring o bearing, the ailure requency or bearing outer ring o single rolling body (the dierence between revolution requency o rolling body and rotation requency o bearing outer ring) is: 1 d 1 d os g o (1 cos ) i 0 (1 cos ) 2 D 2 D (5) In that way, the ailure requency or bearing outer ring o N pieces o rolling body is: on 1 d D d cos N os N (1 cos ) i N i (6) 2 D 2D In the same way, the ailure requency or bearing inner ring o N pieces o rolling body is: in 1 d D d cos N is N ( i g ) N (1 cos ) i N i (7) 2 D 2D Because the rolling body contacts with the bearing inner ring (or example A point), the rotation requency o them is inversely proportional to the diameters, thereore the rotation requency o rolling body (that s the ailure requency o rolling body) is: d d D d cos 1 d 2 d D i gg is (1 cos ) i i (8) 4.2. Research on Resonance Demodulation Algorithm D d cos 2Dd When bearings occur aults (e.g. pitting corrosion), periodic collision will happen in the process o bearing rotation, each collision will generate a transient pulse signal, namely impact signal. Because the impact signal is in very short o duration, it has a wide bandwidth in the requency domain. As shown in Fig. 4. Figure 4 illustrates that the impact signal not only has low requency inormation, but also contains relatively abundant high requency inormation. At low requency range, the signal is drowned by the noise o bearing operation and is not easy to detect (the noise o bearing basically ocus on the low requency range); at high requency range, the noise intererence or signal is small and weak, so bearing ailures can be analyzed through the high-requency signal detection o impact signal. Resonance demodulation algorithm is based on this principle. i

7 Gao Xuan/ Procedia Engineering 00 (2014) Fig.5.Resonance Demodulation Algorithm Diagram Fig.6. Rolling Bearing Detection Experiment Diagram Fig.5 is the diagram o resonance demodulation algorithm. High Frequency signal o impact signal also belongs to the weak signal, so it is not easy to directly detect. Hence a high requency resonator can be used here. According to the resonance principle, the high requency signal o impact signal can be ampliied, meanwhile, a band-pass ilter by centering on the resonance requency o high requency resonator will be set up to extract the high requency signal, and then the signal waveorm will be obtained by carrying on the envelope detection; At the end o the envelope detection, the envelope signal containing ailure inormation is gained. By analyzing, the ailure requency o bearing inner ring, outer ring, rolling body and etc., is concentrated at low requency range, generally rom dozens o Hz to several hundreds o Hz. Thereore, low-pass ilter can be used here to ilter the envelope signal and remove the high requency but decay signal; in general, the threshold o low-pass ilter is set to 1 khz. Ater receiving the low requency envelope signal, FFT (Fast Fourier Transorm) can be used or its requency spectrum analysis. The requency spectrum will be gained ater carrying on FFT calculation or the low requency envelope signal. According to the bearing ailure requency pre-calculated, it needs to observe whether there is corresponding ailure requency on the spectrum graph, such as one times requency, double requency, triple requency, quadruplicated requency and etc. I yes, it illustrates the bearing occurs this kind o ault, at that time several groups o data should be analyzed again or other detecting methods should be adopted to veriy; i no, several groups o data also needs to be analyzed to conirm the bearing without any ailure or others detecting methods can be used to test Research on the On-ling Detection Experiment or the Tail Drive Bearing o A Certain Type o Helicopter Experiment Conditions Fiber bragg grating rolling bearing acoustic emission detection experiment is carried on in the paper to observe the perormance o iber grating detection rolling bearing acoustic emission signal. The principle o experiment device as shown in Fig.6, it is adopted single iber bragg grating acoustic emission sensing system. Ater talking with the mechanist, with their many years o experience or maintenance and other nondestructive tests, they choose a tail drive bearing which they doubt existing trouble to run the experiment. The tail drive bearing o the helicopter is rolling ball bearing, the diameter o the rolling body d is 10.7mm, the diameter o center o bearing rolling body D is 53.5mm, the number o rolling body N is 9, the contact angle is 0. The theoretical ailure requency o bearing detected ater calculation is as ollows (Table 1.):

8 8 Gao Xuan/ Procedia Engineering 00 (2014) Table 1. The theoretical ailure requency o bearing detected Rotate Speed o Bearing n(r/min) 1200 Rotation Frequency r (Hz) 20 Failure Frequency o Inner Ring in (Hz) 108 Failure Frequency o Outer Ring (Hz) 72 on Failure Frequency o Rolling Body gg (Hz) Experimental Result and Analysis The helicopter goes through in turn power-on, idle speed, ull throttle, idle speed and shut down. During the data collection, randomly take one group o data at stable idle speed phase or FFT, Hilbert transorm and resonance demodulated analysis. The experimental data is as shown in Fig.7 and Fig.8. Fig.7. Graph or Bearing Signal Time-Domain Detected at the Rotation Speed o 1200RPM

9 Gao Xuan/ Procedia Engineering 00 (2014) Fig.8. Graph or Bearing Signal Frequency-Domain Detected at the Rotation Speed o 1200RPM Fig.7 shows that FBG detect 14 impact signals in 204ms, the signal period is t o / ms, which coincide with the theoretical analysis result, 13.89ms. That shows FBG detects the ailure signal with high signal noise ratio; the resonance demodulation algorithm is used to process the time-domain signal and obtain the bearing signal requency-domain graph o Fig.8; the igure shows that FBG sensor detects the Hz o signal requency and its double requency. It can be concluded that the ailure theoretical requency o outer ring on (Hz) is 72Hz, and when analyzing the acoustic emission detection signal o tail drive bearing, the ailure requency is very close to the ailure requency o outer ring and its double requency. It is preliminarily judged rom that the outer ring o the bearing has ault or damage. It provides the evidence or mechanist s judgment through the experiment, but it also needs to adopt other detection methods to veriy urther. 5. Conclusion Fiber bragg grating acoustic emission technology is applied on the detection o helicopter s bearing in the paper. By using the eatures o iber bragg grating, a FBG sensor is installed near the bearing to obtains the strain inormation, thus realize on-line in-situ detection. As a commonly used sensor o acoustic emission detector, comparing with piezoelectric sensor, iber bragg grating sensor has higher sensitivity and response speed, which makes the iber bragg grating acoustic emission detection system have more widely application in helicopter structure cracks, such as the damage detection or clamps, bolts, rivets, welding studs and other asteners, the damage detection or the stress concentration point in the body structure, the leakage/leak detection or helicopter s oil tank and landing gear actuating cylinder as well as the corrosion detection or bearing strength parts. Reerences [1]Dunegan H.L.,Green A.T.. Factors Aecting Acoustic Emission Response rom Materials[J].Materials Research and Standard,1972,MTRSA.11(3):pp21~24. [2]Shen Gongtian,Dai Guang, Liu Shieng. Acoustic Emission Progress in China Celebration or the 25th anniversary o Chinese Society or NDT [A].Nondestructive Testing,2003. [3]Wei Wenhui. The Application o Acoustic Emission Technique in Crack Detection During Fatique Test o Aircrat Componments. [A]. The 2nd National Acoustic Emission Conerence Collected Papers. [ C].Guilin:1983. [4]Gen Rongsheng, etc..identiication o Acoustic Emission Signals Produced by Fatique crack Initiation in Wing Spars o Aircrat. [J]. Acta Aeronautica Et Astronautica Sinica, 1996,17( 3):

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