24 Replicating Aerodynamic Excitation in the Laboratory 271 When carrying out a physical MIMO test, it may beneficial to use a mixture of shaker sizes simultaneously. It may be more appropriate to have some large shakers to apply the low frequency excitation and some smaller shakers to apply the high frequency excitation. In addition, using piezoelectric patches or stack actuators may be ideal for exciting the high frequency excitation and would be far simpler to use that rigging up electrodynamic shakers. It is only in recent years that the technology associated with physical MIMO vibration controllers has matured sufficiently, allowing the concept of a MIMO random vibration test, as detailed in this paper, to be considered viable. In addition, recent advances in computing, FE modelling and SLDV allows virtual testing to be an integral part of the process and can allow a transition away from the traditional rigid shaker testing for replicating aerodynamic excitation. 24.8 Conclusions This paper has demonstrated issues with the common methods of replicating aerodynamic excitation in the laboratory. These issues come in two main forms: (I) Quality of replication (II) Efficiency of the test A new and simple approach based on MIMO vibration control of multiple shakers has been demonstrated that could yield significant benefits in both of the areas above and utilises technology which has only recently become available. The new approach could help alter the perception of environmental testing, which some see as slow, inefficient and non-representative, to a perception of tests that are highly representative of in-service environments and which are carried out in an efficient manner. The use of virtual testing and modern technology could allow testing practices to close the gap in capability on computer modelling. A little thought up front could result in far better, faster and cost-effective tests, where less time is wasted post-test investigating spurious results and redundant data. 24.9 Further Work Currently, no physical testing has been carried out as part of this research. It is unknown if current MIMO vibration controllers can achieve the desired results or if one has to be developed. An LMS MIMO vibration controller is currently being acquired and it is proposed that research is carried out to ascertain its suitability for the MIMO testing methodology detailed in this paper. If a new system has to be developed then this may take significant time. In addition, mature MIMO vibration controllers will have mechanisms in place to deal with inverting near singular matrices and abort limits to protect the structure which may make development of a new system highly complicated. Once a suitable system is in place, physical testing can be carried out on the Dummy Air Launched Missile. The dummy missile shall be subjected to a wind tunnel test at the University of Bristol and this shall be replicated with a MIMO vibration test in the laboratory. The results for the two shall then be compared and an assessment made about the viability of using the MIMO technique to replicate distributed aerodynamic excitation. None of the calculations in this paper have included noise. It is imperative that the sensitivity of the calculations to noise be understood as there may be significant levels of noise in physical test measurements. References 1. Himelblau H, Kern DL, Manning JE, Piersol AG, Rubin S (2001) Dynamic environmental criteria. NASA Technical Handbook (NASAHDBK-7005) 2. Kroeger RC, Hasslacher GJ III (1965) Relationship of measured vibration data to specification criteria. Acoust Soc Am 37(1):43–53 3. Piersol AG (1966) The development of vibration test specifications for flight vehicle components. J Sound Vib 4(1):88–115 4. Piersol AG (1974) Criteria for the optimal selection of aerospace component vibration test levels. Proc Inst Environ Sci 88–94 5. Ewins DJ (2006) A future for experimental structural dynamics. In: Proceedings of ISMA 2006: international conference on noise and vibration, Leuven 6. Smallwood DO (2000) The correct balance between analysis and test. J Sound Vib 34(4):6–7 7. Wilkinson P (2007) The changing role of physical testing in vehicle development programmes. J Terramechanics 44:15–22 8. Kammer DC (1991) Sensor placement for on-orbit modal identification and correlation of large space structures. J Guid Control Dyn 14(2):251–259
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