Validation of Vibro Acoustic Modulation of wind turbine blades for structural health monitoring using operational vibration as a pumping signal Sungmin Kim, Douglas E. Adams, Hoon Sohn, Gustavo Rodriguez_Rivera, Jan Vitek, Scott Carr, Ananth Grama Purdue University ABSTRACT This paper presents a crack detection technique for wind turbine blades using Vibro Acoustic Modulation. This technique identifies the nonlinear characteristics of a blade by utilizing the wind turbine operational vibration as a pumping signal. The underlying theory of the technique is described and then an experiment with a 900 watt small scale wind turbine is conducted using a wind tunnel. Two other conventional Vibro Acoustic Modulation tests were also performed with same blades to compare the results from these offline tests with the proposed online Vibro Acoustic Modulation test using the turbine operational vibration as the pumping signal. It is shown that the structural vibration of the blades during operation lead to the same trends in the measured Vibro Acoustic Modulation spectra for the cracked blade as when the impact and piezoelectric actuator narrowband pumping signals are used. INTRODUCTION Wind power is considered by many to be the most feasible renewable power source due to the competitive cost of energy compared to other options including solar power; therefore, the wind power industry has grown rapidly in recent decades [1][2]. As wind turbines have been installed and operated, economic data has shown that maintenance costs consume a significant portion of the total energy costs that are borne by wind farm owners/operators [3]. To minimize unscheduled maintenance costs (which are typically much higher than scheduled maintenance costs), it is important to detect initial defects on the turbines before they lead to a catastrophic failure of the system. Sungmin Kim and Douglas E. Adams, School of Mechanical Engineering, Purdue University, 1500 Kepner Drive, Lafayette, IN 47905 Hoon Sohn, Dept. of Civil and Environmental Engineering, KAIST, 291 Deahak ro, Yuseong gu, Daejeon, Republic of Korea Gustavo Rodriguez Rivera, Jan Vitek, Scott Carr and Ananth Grama, Computer Science Department, Purdue University, Third and University Streets, West Lafayette, IN 47906
In particular, wind turbine blades require robust structural health monitoring technologies not only because of their higher costs of replacement but also because the incipient and final failure of a wind turbine blade can cause other subsystems in the turbine to fail such as the tower and drive train [1]. The failure of a wind turbine blade in operation can also introduce hazards for neighboring facilities and the people nearby [4]. It is also reported that blade failure is the one of the most frequent failures in wind turbines [5]. In this study, a new Vibro Acoustic Modulation method to detect cracks on wind turbine blades was investigated. This method was discussed in a previous paper that focuses on aerospace structural health monitoring [6]. There are several benefits to using this technique for crack detection in wind turbine blades. First, Vibro Acoustic Modulation is the result of nonlinear characteristics, which are introduced by cracks in the composite material [7]. Second, the sensitivity of this technique is greater than many other damage detection techniques [8]. Third, studies have reported that Vibro Acoustic Modulation can effectively detect cracks in heterogeneous materials from which wind turbine blades are constructed [9][10]. Fourth, this technique is less affected by environmental conditions and loading conditions, and this attribute improves the robustness of the technique for structural health monitoring of wind turbines because these turbines are expected to operate in variable wind conditions. Lastly, this technique can detect cracks while the turbine is in operation [6]. This paper reviews the basic theory of Vibro Acoustic Modulation for crack detection and describes how the Vibro Acoustic Modulation technique was implemented on a 900 watt wind turbine. Vibro Acoustic Modulation tests on an operating wind turbine in a wind tunnel are presented. The result of the test was then validated by comparing the Vibro Acoustic Modulation spectra with the results obtained using other sources of vibration for the pumping signal. THEORETICAL REVIEW OF VIBRO ACOUSTIC MODULATION It is known that certain kinds of defects such as cracks in materials increase the nonlinear behavior of the material. Vibro Acoustic Modulation is used to detect defects by measuring this nonlinear behavior [9][11]. The Vibro Acoustic Modulation test measures the response of a specimen when it is excited by two sinusoidal signals at different frequencies, which are referred to as the pumping signal and the probing signal. To produce modulation that can be separated from operational responses, the pumping frequency,, and probing frequency,, are supplied in the low and high frequency range, respectively.
Figure 1. Frequency spectrum of a response of a nonlinear system when two excitation signals at different frequencies were applied to the system If the structural specimen is linear, the steady state response is the linear superposition of the forced response of each signal. However, when cracks or other defects cause nonlinear behavior within the specimen, additional nonlinear components appears in the response [6]. Equation (1) describes the stress that the specimen undergoes when two sinusoidal strain signals are applied to the specimen and it contains a defect that introduces nonlinear behavior: is the local material stress, is the effective Young s Modulus, is a quadratic nonlinear parameter, is the probing signal strain amplitude, is the pumping signal strain amplitude, and and are the frequencies of the two signals. The first term contains the linear response components. These are the only components that linear systems exhibit. The components in the second term contain frequencies of and, which are forced harmonics. The third term contains frequencies such as, which are called sideband modulations. Vibro Acoustic Modulation observes these sideband modulation components at. The key idea of the technique presented in this paper is to utilize the low frequency structural vibration of the blades as the pumping signal for the Vibro Acoustic modulation test. It is shown that if the structural vibration caused by the rotation of the rotor is used as a pumping signal, nonlinear modulation that is comparable to that obtained using conventional pumping signals is observed. (1) EXPERIMENTAL SETUP Wind Tunnel and Wind Turbine The wind tunnel used in the experiment consists of 6 industrial fans pulling the air through a honeycomb material to achieve uniform flow at the inlet to the rotor. A 900 watt wind turbine, Whisper 100 manufactured by Southwest Windpower, was used for the tests. The diameter of the turbine rotor is 2.1m and the rotor contains 3 blades. Two healthy blades (Blade 1 and Blade 2) and a damaged blade (Blade 3) were used. The instrumentation installed on the damaged blade (Blade 3) is shown in Figure 2(b). A crack was introduced at the trailing edge of the blade. This location was determined through stress analysis in a previous paper [6]. Two MFC (Micro Fiber Composite) transducers were installed on the blade on both sides of the crack. One transducer was used as an actuator and the other was used as a sensor. For this study, a remote controlled laptop and small amplifier were installed on the nacelle rather than delivering the signals through a slip ring.
Crack A0 S1 (b) (a) Figure 2. Experimental setup for Vibro Acoustic Modulation tests. (a) Whisper 100 wind turbine in the wind tunnel tested. (b) Sensors and actuator installed on the blade. A0: actuator, S1: sensor Blade Properties Blades of the Whisper 100 are made of reinforced carbon fiberglass. Since none of the other specific properties of the material are given by manufacturer, tensile tests were performed to meausre the material properties. The tests were conducted with two small specimens cut from the root of extra blades. Specimen 1 had a cross section area 16.0 mm X 6.4 mm and specimen 2 had a cross section area 12.81 mm X 6.55 mm. The force was measured using the pressure of the tensile machine and the strain was measured from the strain gauges attached to the specimens. Figure 3(a) is the stress strain curve measured from the test. Even though these curves indicate a nonlinear relationship between stress and strain before blade failure, they are nearly linear for the lowest stress level. In the Vibro Acoustic Modulation tests, the amplitude of the vibration is much smaller than the failure strength. Therefore, this nonlinear behavior under large loading conditions does not affect the sidebands level that are measured in the Vibro Acoustic Modulation tests. Young s modulus, which was estimated from the linear part of this curve, was 4.749 GPa for Specimen 1 and 4.945 GPa for Specimen 2. The failure stress was 57.52 MPa for Specimen 1 and 29.62 MPa for Specimen 2.
(a) (b) Figure 3. (a) Stress strain curves of the tensile tests. (b) Failure surface of the tensile test specimen These results indicate that even though the Young s Modulus of the specimen is relatively consistent, the failure strength of the specimen varies considerably. This variation in failure strength illustrates the need for a structural health monitoring technique that detects damage in the blade as early as possible. Figure 3(b) shows one of the failure surfaces of Specimen 2. As shown in the picture, the blade material has fiberglass oriented randomly and has many cavities within it. This indicates that the material of the blade is not homogenous. These randomly located cavities might also introduce nonlinear behavior in the undamaged blades. VIRBO ACOUTIC MODULATION TESTS Vibro Acoustic Modulation Test in Operating Conditions Vibro Acoustic Modulation tests were conducted on the wind turbine operating in the wind tunnel. While the wind turbine was rotating, a MFC actuator was excited and another MFC sensor was used to measure the response of the blades. The rotational speed of the wind turbine was 3.0 3.2 Hz. Sinusoidal probing signal excitations to the MFC actuator at frequencies of 5 khz 10 khz were used. Figure 4 shows one of the response spectra that were acquired. The peak at 3.1Hz is the rotational speed of the rotor and is observed in the low frequency range. This signal provided the pumping signal for the Vibro Acoustic Modulation test. Around the peak at the probing frequency, the sideband peaks due to the modulation with this pumping signal are observed at. Figure 4 shows that the probing signal is also modulated by all 0~15Hz components including second and third harmonics of the pumping signal. This means any low frequency vibration of the structure can be utilized as a pumping signal in the Vibro Acoustic Modulation test. Figure 5 shows the response of the probing signal and the sideband level (A p1 + A n1 ) with different probing frequencies used in the tests. In these tests, only Blade 3 (which is cracked) shows high sideband levels.
Figure 4. Frequency spectrum of the data measured from Vibro Acoustic Modulation test on the operating wind turbine (Hz, Hz) Figure 5. Vibro Acoustic Modulation test results with different probing frequencies. Probing signal response (top) and First sideband level (bottom) Since the probing signal response is affected by the different frequency response functions of the different blades, each blade exhibits a different probing frequency response. This result corresponds well with the results of researchers [13] the sideband level is affected by the frequency response function of the specimen in the neighborhood of the probing frequency. Impact Modulation Test The Impact Modulation test is a nonlinear defect detection technique that uses vibration at the natural frequencies of the material excited by an impact as the pumping signal [14]. In this test, the tips of the blades were impacted to excite the first modes of the blades in flap motion rather than by operating the turbine. The same transducer setup
as described previously was used to supply the probing signal excitation. The test was repeated using the same probing frequencies used in Vibro Acoustic Modulation test. Figure 6. Result of Impact Modulation test with different probing frequencies Figure 7. Result of Vibro Acoustic Modulation test on the blades off turbine with Swept probing signal The first natural frequency of the blade was approximately 9.4Hz. The free vibration at this frequency was used as the pumping signal in this test. The measured probing signal response and sideband level were plotted in Figure 6 as in Figure 5. Even though the pumping signal was different from the previous test, the sideband level changes are similar to the results of the Vibro Acoustic Modulation test suggesting that the phenomenon being measured due to the crack is not sensitive to the pumping signal that is used. Conventional Vibro Acoustic Modulation Off Turbine Test with Swept Signal The wind turbine blades were taken off the turbine and the roots of the blades were clamped onto a fixture. To provide the pumping signal, a PCB disc actuator was installed and operated at 400 Hz. The probing signal was excited by a MFC attached to the
blades and the signal was measured by the MFC used in the previous test. While the disc actuator produced the pumping signal, a swept signal between 5 khz 20 khz was applied as the probing signal. The result of this test was plotted in Figure 7. Similar to Figure 5 and Figure 6, the response at the probing frequency and sideband level over the different probing frequencies were plotted. Figure 7 also shows similar curves to those in Figure 5. The results indicate that the proposed technique, consisting of the Vibro Acoustic Modulation test on the operating wind turbine blades utilizing the structural vibration of the blades as the pumping signal, can measure the nonlinear characteristic of the blades as well as the conventional types of pumping signals. CONCLUSION In this paper, a crack detection method using Vibro Acoustic Modulation was investigated. The key idea of this technique is to utilize the structural vibration as a pumping signal. It is shown that this technique measures the nonlinear characteristics of the blades as well as other methods by comparing with the results from two other conventional Vibro Acoustic Modulation tests. It is also shown that the use of a single probing frequency might not be sufficient to detect the difference between a healthy blade and a damaged blade since the response of the sidebands varies due to the frequency response differences in the blades. This method should be more effective in larger wind turbine blades because the structural vibrations in operation are larger due to the lower resonant frequencies. The larger the pumping signal, the clearer the sideband signals that are acquired. This technique is promising because it can be applied to the structural health monitoring of other rotating machinery such as turbines in power plants and rotary wing aircraft as well. Further studies such as stochastic data analysis to minimize false alarms, investigation of initial material nonlinearity of the blade and modal analysis of the blade are recommended to improve the application of this technique. REFERENCES 1. M. A. Rumsey and A. Paquette, Structural health monitoring of wind turbine blades, Smart Sensor Phenomena, Technology, Networks, and Systems (2008). 2. C. C. Ciang, J. Lee, and H. Bang, Structural health monitoring for a wind turbine system: a review of damage detection methods, Measurement Science and Technology, Vol. 19 (2008). 3. European Wind Energy European Wind Energy Association, [Wind Energy The Facts, Voume.2 Costs & Prices], Earthscan (2012). 4. M. Ragheb, Safety of wind systems (2011) 5. Caithness Windfarm Information Forum, Summary of Wind Turbine Accident data to 31 December 2012, www.caithnesswindfarms.co.uk 6. S. Kim, D. E. Adams, and H. Sohn, Crack detection on wind turbine blades in an operating environment using Vibro Acoustic Modulation technique, AIP Conf. Proc. 1511, pp.286 293 (2012). 7. Salwan Obeed Waheed, Nawras Haidar Mostafa, and Dhyai Hassan Jawad, Nonlinear Dynamic
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