An investigation into the performance of macro-fiber composites for sensing and structural vibration applications

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Mechanical Systems and Signal Processing Mechanical Systems and Signal Processing 18 (2004) 683–697 www.elsevier.com/locate/jnlabr/ymssp

An investigation into the performance of macro-fiber composites for sensing and structural vibration applications Henry A. Sodano*, Gyuhae Park, Daniel J. Inman Center for Intelligent Material Systems and Structures, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0261, USA Received 22 November 2002; received in revised form 19 May 2003; accepted 27 May 2003

Abstract This paper presents the use of macro-fiber composites (MFC) for vibration suppression and structural health monitoring. The major advantages of the piezoelectric fiber composite actuators are their high performance, flexibility, and durability when compared with the traditional piezoceramic (PZT) actuators. The recently developed MFC actuator provides these advantages and can be used in structural vibration applications. In addition, the ability of MFC devices to couple the electrical and mechanical fields is larger than in monolithic PZT. In this study, we showed that an MFC could be used as a sensor and actuator to find modal parameters of an inflatable structure. This sensor and actuator combination has also been used to reduce vibration in an inflated object. Once the sensing capability was identified, we developed a self-sensing circuit for an MFC. Our experimental results clearly indicate that this strategy can suppress structural vibration, while reducing the number of system components. Finally, the MFC has been implemented as impedance sensor for structural health monitoring (e.g. a of bolted joint connection). The experimental results presented in this paper show the potential of MFC for use in the dynamics and control of flexible structures. r 2003 Elsevier Ltd. All rights reserved.

1. Introduction The use of piezoceramic PZT materials for structural actuation and sensing is a fairly welldeveloped area. Monolithic PZT, however, imposes certain restrictions for its practical use in realworld applications. For instance, the extremely brittle nature of the PZT material requires extra attention during the handling and bonding procedures. In addition, the conformability to curved

*Corresponding author. Fax: +1-540231-2903. E-mail address: [email protected] (H.A. Sodano). 0888-3270/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0888-3270(03)00081-5

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surfaces is extremely poor requiring extra treatment of the surfaces and additional manufacturing capabilities. The idea of using active piezoceramic composite actuators to overcome these limitations has been explored, producing a number of solutions. There are several types of active composites available commercially or under development at research institutes, namely 1–3 composites by Smart Materials Corp. [1], Active Fiber Composite actuator developed by MIT [2], and Macrofiber composite (MFC) actuators constructed at NASA Langley Center [3]. An overview and comparison of these actuators can be found in the literature [4]. The active fiber composite actuators provide not only the required durability and flexibility, but also higher electromechanical coupling by capitalizing on the stronger longitudinal d33 constants by using interdigitated electrodes [5]. This paper presents an experimental investigation of the MFC actuator, seen in Fig. 1, for use in several structural vibration applications, including vibration suppression of an inflated object, self-sensing actuation, and as an impedance sensor for structural health monitoring. The MFC is a revolutionary composite actuator that has been recently developed at the NASA Langley Research Center. An MFC actuator consists of thin PZT fibers imbedded in Kapton film and covered with an interdigitated electrode pattern, this configuration is shown in Fig. 2. Due to the MFC’s construction using piezofibers, the overall strength of the material is greatly increased when compared to that of the base material, while affording the MFC greatly increased flexibility. This has the same principles behind it as the construction of steel cables. Furthermore, the interdigitated electrodes allow the applied electric field to run axially allowing the higher d33 coefficient to come into play, rather than the d31 coefficient active in a monolithic PZT. The result is that the MFC has a substantially larger electromechanical coupling coefficient and produces larger force and free displacement. The MFC has been used primarily for structural actuation [3], while the sensing capabilities have not been fully developed or tested. From our analysis, presented here, the MFC patches were found to possess excellent strain-sensing capabilities compared to conventional piezofilm (PVDF) or piezoceramic-based sensors. This feature enabled us to use MFC for measuring the dynamics of a structure, as well as exciting the structure for vibration testing and control.

Fig. 1. Macro fiber composite actuator [3].

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Fig. 2. Schematic showing the cross-sectional layout of the MFC.

2. Inflatable structures Inflatable structures show significant promise for future space applications. The dynamic behavior of inflated space-based devices is particularly important for satellite structures since they are subjected to a variety of dynamic loadings. However, their extremely lightweight, flexible, and high damping properties pose difficult problems in vibration testing and analysis. The choice of applicable sensing and actuation systems suitable for use with inflated structures are somewhat limited because of the structures low stiffness and high flexibility. In addition, excitation methods have to be carefully chosen since the extremely flexible nature causes point excitation to result in only local deformation. Griffith and Main [6] used a modified impact hammer to excite the global modes of the structure while avoiding local excitation. Slade et al. [7] tested a torus attached to three struts with a lens in a thermal vacuum chamber. They found significant differences in the response between the structure in ambient and vacuum conditions. Park et al. [8] investigated the feasibility of using smart materials, such as PVDF films, to find modal parameters and to attenuate vibration in a flexible inflated structure. Previous results [9] have shown the MFC is an ideal actuator for both exciting the torus and attenuating the vibration of the torus. Here we extend that work to show that MFC patches are also suitable for sensing. In this section, we experimentally investigated the use of MFC to both sense and control the vibration of a very flexible, inflated structure. The MFC can be integrated in an unobtrusive way onto the skin of an inflatable object, which is ideal for inflatable structures. Because of the need for the sensor and actuator materials to be flexible enough to conform to the toroidal shape of an inflatable structure, shown in Fig. 3, and produce significant strain, these materials are extremely compatible with inflatable satellite applications. The variety of sizes, abilities, low mass, fast time response, and negligible added stiffness offered by MFC are additional features ideal for the use of control and dynamics of inflatable structures. The test structure considered here is an inflatable torus made of Kapton with a 1.8-m ring diameter and a 0.15-m tube diameter as shown in Fig. 4. The torus was made of flat sheets of polyimide film Kapton. The internal pressure of the torus was maintained at 4.826 kPa using a

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Fig. 3. The MFC actuator attached to the torus.

Fig. 4. Inflated test object for dynamic analysis.

small aquarium pump causing it to retain it toroidal shape. The testing results of using MFC excitation with a PVDF sensor and miniaturized accelerometer have been reported in our previous efforts [8,9]. Hence, only the results using MFC sensors and actuators for testing and control are presented here. Typical experimental results with an electromagnetic shaker input and MFC sensor are shown in Fig. 5. This illustrates clearly that the MFC can be used successfully as a low-frequency vibration sensor.

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Fig. 5. Out-of-plane frequency response function with shaker input and MFC output.

The conclusion that can be made from our previous results and the results presented in Fig. 4 is that the data measured with the MFC sensors is consistent with data acquired using accelerometers or PVDF sensors. The identified resonant frequencies are almost identical to those found using other methods of measuring the frequency response. In fact, with higher electromechanical coupling, the data measured with MFC have a much clearer and distinct response than does the frequency response obtained with PVDF sensors. With our previous efforts concentrated on the use of MFC as an actuator, the experimental results presented here validate its usefulness as a sensor for measuring the dynamics of inflatable space structures. Furthermore, during testing, the MFC excitation produced less interference with suspension modes of the free–free torus than excitations from the shaker. Without connections to the ground (except for the electrical cable), the MFC actuators and sensors can be considered as an integral part of an inflated structure. Our experiments used a scaled model of the inflatable torus (1.8 m); the full-scale torus intended for use in space would have a diameter as large as 30 m. Although one MFC actuator could globally excite and sense the response of the test structure, it is still questionable if a single MFC patch could produce adequate actuation for modal parameter identification in a larger gossamer satellite. In order to overcome this limitation, experiments were successfully performed with multiple MFC actuators/sensors [10]. The use of multiple actuator/sensor combinations could also be used as control devices of an inflatable structure not only for vibration suppression but also for static shape control. Two pairs of MFC sensor/actuators were attached to the torus. Each pair of MFC consisted of one sensor and one actuator located 180
apart from one another as shown in Fig. 6. The reason for locating the sensor far from its corresponding actuator was because we wanted to control the global vibration of the structure and not the local vibration. A shaker was used to produce point disturbance because it was able to easily excite the global modes of the inflatable structure. In order to control and reduce vibrations of the torus with multiple sensors and actuators, we used positive position feedback methods since they are fairly robust to parameter uncertainties. Our experimental results clearly indicate that this control strategy and the MFC sensors/actuators

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Fig. 6. Location of sensors and actuators for tests performed of the inflatable torus.

Frequency Response of the First Mode Before and After Control 1.8 Uncontrolled One Actuator Two Actuators

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Fig. 7. Vibration control results (first mode) using two MFC actuators.

can reduce and control the vibration in the inflatable torus. A dSPACE control board was used in the control experiment to implement real-time control. Each sensor/actuator pair used a separate controller providing the freedom to control separate modes or frequencies. The sampling frequency was set at 1000 Hz. Control systems using MFC as actuators and sensors were shown to significantly reduce vibration levels in the torus. In one experiment, a control system was designed to control the first out-of-plane mode (12.8 Hz) of the torus. Fig. 7 shows a clear reduction in vibration level (up to 50%) using one MFC actuator. More significant vibration reduction is achieved by using two actuators (up to 70%). The time response of each sensor is shown in Fig. 8. As the bonding condition improves, it is expected that the control authority over the whole structure would increase. In conclusion, the experimental results presented validate the usefulness

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Fig. 8. Time response of vibration before and after control was applied.

and potential of flexible MFC for actuating and measuring the dynamics of inflatable space structures. 3. Self-sensing MFC actuators Due to their electromechanical coupling characteristics, piezoelectric materials are often used as a sensor and actuator simultaneously, termed a self-sensing actuator [11]. A self-sensing actuator is perfectly collocated, and can be used in the field of vibration suppression. Collocated control is advantageous when examining the closed loop stability of the structure [12]. Once the sensing capabilities of MFC were identified, our next investigation was to design a self-sensing circuit for MFC patches. The same circuit design and scheme has been used as in Ref. [11], which is shown in Fig. 9. When implementing the self-sensing actuator, an electrical bridge circuit is used to measure strain. The bridge circuit used in this experiment consisted of three operational amplifiers (model number LM324AN), the capacitance of the MFC in use, a matching capacitance, and various resistors and capacitors. The values of the resistors and capacitors used to balance our bridge are listed below, although any number of values could be used: RP ¼ R1 ¼ 4:8 MO; R2 ¼ 100 KO; CB ¼ C2 ¼ 1 mF; Cmfc ¼ CMatched ¼ 950 pF: The self-sensing tests were performed on an aluminum beam of dimensions 558  50  0.5 mm with the MFC mounted at the root of the beam, as shown in Fig. 10. The frequency response

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Fig. 9. Schematic diagram of self-sensing bridge circuit [10].

function was measured for the aluminum beam, shown in using the MFC self-sensing actuator from 0 to 1000 Hz. It can be seen from Fig. 11 that the self-sensing MFC was capable of measuring the frequency response. The resonant frequencies are clearly identified. After the self-sensing MFC circuit identified the resonant frequencies a positive position feedback control algorithm was designed to suppress the vibration of the beam. A dSPACE controller and one self-sensing actuator were used to control the beam. One test performed on the second mode (63.75 Hz) achieved 90% vibration reduction as shown in Fig. 12. The control signal supplied to the self-sensing MFC actuator was limited to 35 V due to the size of the op-amps used and the vulnerability of the dSPACE to high voltages (MFC of this type can be used up to 1500 V). If larger control voltages were used the vibration reduction would be even more substantial. The MFC self-sensing actuator was able to produce larger control forces on the beam than a monolithic PZT of equal size due to the MFC’s higher electro-mechanical coupling. The MFC patches have been shown to control the dynamics of the torus and they can also be used effectively as self-sensing actuators. Incorporating both of these applications for dynamic control of the inflatable torus would provide significant advantages over the use of separate sensors and actuators. The ability to use half as many MFC patches on the torus as in previous experiments would decrease the complexity of the control methods. Additionally, since the MFCs are more effective on the local level, self-sensing methods would allow the vibration at the point of the MFC to be measured and controlled instead of a separate sensor measuring the vibration at a location that may not be experiencing the same dynamics. For these reasons the effectiveness of a self-sensing MFC actuator needs to be investigated on inflatable structures, in particular the torus.

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Fig. 10. MFC bonded to front of beam.

FRF of an AL beam measured by MFC self-sensing actuator

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Fig. 12. The vibration of the beam measured with a separate PZT sensor, before and after the control signal to MFC is applied.

4. MFC for impedance-based structural health monitoring One of the goals for structural engineers is to develop systems and structures that can monitor their own structural integrity. Besides preventing catastrophic failures, on-line damage detection would reduce costs by minimizing maintenance and inspection cycles. There are dozens of structural health monitoring [13]. Here we examine suitability of MFC sensor for structural health monitoring systems using the impedance technique [14–18]. The basic concept of the impedance method is to use high-frequency vibrations to monitor the local area of a structure for changes in structural impedance that would indicate damage or imminent damage. This is possible using piezoelectric sensor/actuators whose electrical impedance becomes directly related to the structure’s mechanical impedance when they are bonded together. The impedance measurements can easily give information on changing parameters, such as resonant frequencies, that will allow for the quantification of damage. More detailed description on the impedance-based health monitoring technique can be found in Ref. [14]. To date, only the monolithic piezoceramic sensors have been used with the impedance method, but brittle piezoceramic can only withstand very small bending. This brittleness makes it somewhat difficult to handle and bond to the monitored structure. Consequently, there is a need for a more robust sensor that can be easily and quickly applied to a wide variety of structures. As shown in the previous sections, MFC patches have several desirable characteristics due to various features. They are capable of being repeatedly manufactured at low cost, tolerant to damage, capable of conforming to surfaces, and can be embedded in structures. Also the excellent strain sensing capability of MFC is suitable for use in the impedance method, which utilizes the direct and converse versions of the piezoelectric effect simultaneously. Therefore, as a feasibility study, the use of MFC as an impedance sensor/actuator has been investigated. This hybrid type sensor would certainly have the advantage of being robust, reliable, and easily adaptable, as an impedance-based structural health monitoring sensor.

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Fig. 13. A composite beam test setup. composite beam: impedance measured by MFC 350 undamaged 1 bolts real

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Fig. 14. Impedance measured by MFC for composite beam.

Our first experiment includes monitoring of a composite 1-d beam like structure as shown in Fig. 13. A MFC patch (3  1  0.05 cm) has been bonded to the side of a beam, and a PZT patch (5  1.5  0.05 cm) has been embedded inside of the beam for the acquisition of electrical impedance. The beam measures 50  2.5  0.3 cm. An HP4194 electrical impedance analyzer was used for the measurement of the MFC and PZT’s electrical impedance in the frequency range of 20–30 kHz. Damage was induced by loosening connection bolts in the root of the beam. This damage can be considered as a boundary condition in global sense, but also may be considered as damage (such as cracks) in connection parts. The impedance measurements (real part) of the MFC and PZT over the 20–30 kHz frequency range are shown in Figs. 14 and 15. Only the real portion of the electrical impedance is analyzed to predict damage because it is more sensitive to structural change than the imaginary part [14]. As can be seen in the figures, the resonant frequencies shift as increasing levels of damage have been observed, which is clearly indicative of changes in structural integrity. Another observation is that the identified resonant frequencies from MFC and PZT are exactly the same. For instance, when 1 bolt has been loosened, the resonant frequencies are 21.7, 25.5,

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Fig. 15. Impedance measured by PZT for composite beam.

Fig. 16. Bolted joint side and front view.

and 29 kHz measured from both PZT and MFC. This confirms the feasibility of MFC as impedance sensors for use in structural health monitoring. However, the electrical impedance measured by MFC is less distinct compared to a monolithic PZT. It is believed that this occurs due to the PZT being embedded in the composite beam producing better mechanical interaction than that of the bonded MFC. Additionally, the MFC has a much higher electrical capacitance compared to that of the monolithic PZT (4.6 vs 0.6 nF). This occurs due to the PZT’s size, which is related to the electrical capacitance of the PZT, therefore the responses are somewhat influenced by capacitive responses. It should be noted that we applied only 1 V to measure the electrical impedance. If we apply higher voltage and achieved better bonding, it is expected the MFC would produce a more clear response than the figures shown below. Our next experiment includes the monitoring of a bolted joint connection. Bolted connections are commonly found in various civil, mechanical, and aerospace structures. At times bolted connections can be difficult to inspect because of their locations in and around structures. A 550  50  4 mm beam was securely bolted as shown in Fig. 16. A MFC patch was bonded to one

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Fig. 18. Impedance measured by the PZT for the bolted joint.

side of the beam, while a piezoceramic (PZT) from Piezo Systems Inc. was bonded to the other side. In order to simulate damage to the bolted joint the torque applied to the bolt was reduced from its original state at 40.67 Nm, to torque of 27.1 and 13.56 Nm. These impedance measurements, in the frequency range of 40–47 kHz, were then compared to the bolted joint in its original state for both the MFC and the PZT. Plots of impedance measured by the PZT, and by the MFC, shown in Figs. 17 and 18. These plots also show that the peaks are clear and distinct for both the MFC and PZT. The impedance was clearly altered when the torque was changed from its original state to the reduced states.

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It can be clearly seen that as the levels of damage increase, there is a corresponding change in electrical impedance. Therefore, damage can be detected by monitoring electrical impedance from both PZT and MFC. Unlike the previous beam example, the peaks and shape of impedance measurements from MFC and PZT are not exactly the same. It is believed that the PZT, which utilized the d31 constant, measures bending, axial and torsional strains altogether, whereas the MFC measures only the bending and axial modes by utilizing only d33 constants [3]. This feature may simplify the analysis of electrical measurements made with MFC sensors. In conclusion, the MFC demonstrated its potential as impedance sensors for various structural health monitoring applications. 5. Conclusion The Macro Fiber Composite Actuator, recently developed by NASA Langley Research Center, has been investigated here as an alternative sensor for structural dynamics and control applications. The MFC consists of rectangular piezoceramic fibers sandwiched between layers of adhesive and polyimide films and covered with interdigitated electrode patterns allowing it to function in the stronger d33 direction. This unique design provides more desirable characteristics when compared with monolithic PZT-based devices in terms of reliability, robustness to damage, adaptation to many environments, flexibility and higher electromechanical coupling. Previously, the MFC has only been investigated as an actuator. Here we report on the sensing capabilities of this new device. The results presented here have shown that an MFC actuator works exceptionally well as both a sensor as part of a modal-testing system (low-frequency vibration), as part of a structural health monitoring system (high-frequency electrical impedance), as part of a control system (feedback sensor) and as a self-sensing actuator (simultaneously acting as both a sensor and an actuator). Previous tests that have been performed with monolithic piezoceramic devices have been repeated here with MFC sensing and actuation to illustrate the effectiveness and usefulness of the MFC device as a sensor. These experiments include the modal testing of an inflated torus, the selfsensing vibration control of a beam and the structural health monitoring of a bolted joint. Clearly, the MFC forms a useful sensing device. Acknowledgements This work was sponsored by NASA Langley Research Center, grand number LaRC 01-1103 under the direction of Dr. W. Keats Wilkie and by the Air Force Office of Scientific Research under grant numbers F49620-99-1-0231 and F49620-03-0163 under the direction of Dr. Dan Segalman and Dr. Dean Mook, respectively. The authors greatly acknowledge the support. References [1] http://www.smart-materials.com [2] A.A. Bent, Active fiber composites for structural actuation, Doctor of Philosophy Dissertation, Massachusetts Institute of Technology, January 1997.

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[3] W.K. Wilkie, R.G. Bryant, J.W. High, R.L. Fox, R.F. Hellbaum, A. Jalink, B.D. Little, P.H. Mirick, Low-cost piezocomposite actuator for structural control applications, Proceedings of Seventh SPIE International Symposium on Smart Structures and Materials, Newport Beach, CA, March 5–9, 2000. [4] B.R. Williams, G. Park, D.J. Inman, W.K, Wilkie, An overview of composite actuators with piezoceramic fibers, Proceedings of 20th International Modal Analysis Conference, February 4–7, 2002, Los Angles, CA. [5] N.W. Hagood, R. Kindel, R. Ghandi, P. Gaudenzi, Improving transverse actuation of piezoelectrics using interdigitated surface electrodes, SPIE Paper No. 1975-25, Proceedings of the 1993 North American Conference on Smart Structures and Materials, Albuquerque, NM, 1993. [6] D.T. Griffith, J.A. Main, Modal testing of an inflated thin film polyimide torus structures, Proceedings of 18th International Modal Analysis Conference, San Antonio, Texas, February 2000. [7] K.N. Slade, M.L. Tinker, J.O. Lassiter, R. Engberg, Dynamics of an inflatable structure in vacuum and ambient conditions, AIAA Journal 39 (5) (2001) 894–901. [8] G. Park, M. Kim, D.J. Inman, Integration of smart materials into dynamics and control of inflatable space structures, Journal of Intelligent Material Systems and Structures 12 (6) (2001) 423–433. [9] G. Park, E. Ruggiero, D.J. Inman, Dynamic testing of an inflatable structures using smart materials, Smart Materials and Structures 11 (1) (2002) 147–155. [10] E. Ruggiero, G. Park, D.J. Inman, J. Wright, Multi-input, multi-output modal testing techniques for gossamer spacecraft, Proceedings of 2002 ASME International Mechanical Engineering Congress and Exposition, November 11–16, 2002, New Orleans, LA. [11] J.J. Dosch, D.J. Inman, Self-sensing piezoelectric actuator for collocated control, Journal of Intelligent Material Systems and Structures 3 (1992) 166–185. [12] D.J. Inman, H.H. Cudney, Structural and machine design using piezoceramic materials: a guide for structural design engineers. NASA Grant Number NAG-1-1998, 2000. [13] C.R. Farrar, S.W. Doebling, D.A. Nix, Vibration-based structural damage identification, Philosophical Transactions of the Royal Society: Mathematical, Physical & Engineering Sciences 359 (2001) 131–149. [14] G. Park, H. Cudney, D.J. Inman, Impedance-based health monitoring of civil structural components, ASCE/ Journal of Infrastructure Systems 6 (4) (2000) 153–160. [15] G. Park, H. Cudney, D.J. Inman, Feasibility of using impedance-based damage assessment for pipeline systems, Earthquake Engineering & Structural Dynamics Journal 30 (10) (2001) 1463–1474. [16] G. Park, H. Cudney, D.J. Inman, An integrated health monitoring technique using structural impedance sensors, Journal of Intelligent Material Systems and Structures 11 (6) (2000) 448–455. [17] V. Lopes, G. Park, H. Cudney, D.J. Inman, A structural health monitoring technique using artificial neural network and structural impedance sensors, Journal of Intelligent Material Systems and Structures 11 (3) (2000) 206–214. [18] D. Peairs, G. Park, D.J. Inman, Improving accessibility of the impedance-based structural health monitoring method, Journal of Intelligent Material Systems and Structures, 2002, accepted for publication.