Chapter 11 Use of Continuous Scanning LDV for Diagnostics Dario Di Maio Abstract Nowadays, diagnostic is an important assessment of integrity of structural components. Amongst the technologies capable of monitoring structural integrity the continuous scanning (CS) measurement method can be a valid alternative in quasi real-time acquisition methods. Its major feature is the capacity of continuously recording vibration response by sweeping a laser beam over a surface. The LDV output signal will be amplitude modulated because of the periodicity of the scanning for a given vibration oscillation. The spectral content of the LDV output signal is made of sidebands, which can be used for representing an Operational Deflection Shape of the structure. This paper proposes a numerical study focused on the changes of the spectral signature of the sidebands when a damage occurs in a structure. The major objective is to map these changes against an undamaged spectral signature and evaluate if this approach can be used for diagnostic purposes. Keywords Continuous scanning • ODS • Diagnostics • Damage • Real-time 11.1 Introduction Diagnostic is important for assessing the structural integrity either at component or at system level. There are many technologies researched and available in literature some of which generated useful outcomes and, eventually, used for monitoring structural performances. It is not the scope of this paper to discuss such a broad topic but the final objective is to show how a novel measurement technology, the continuous scanning LDV measurement methods, can be used for performing diagnostics. Researchers have been using ODS technology for damage assessment as presented in [1, 2] and some of those used the continuous scanning technology as presented in [3, 4]. The continuous scanning measurement method is based on a contactless sensor, like a laser beam, which can be scanned continuously over a vibrating path. The laser beam will measure an amplitude modulated time signal, the modulation of which depends on the scanning rate. The set of spectral sidebands generated by such a modulation can be used for recovering Operational Deflection Shape (ODS) information. This approach was used by some authors for different applications one of which is damage detection [5]. In fact, the process of recovering ODS by CSLDV technique is rather fast and accurate thus allowing rapid implementation of diagnostics based on ODSs. The process of recovering the ODS is based on a transformation matrix which allows to transform the amplitudes of the spectral sidebands in polynomial coefficients thus a polynomial function describing the ODS. The novelty in this paper regards the sidebands as indicator of structural integrity. The sidebands can be considered as good as measured without any further signal processing. In fact, one can say that for an undamaged, or pristine, structure the spectral sidebands for a given excitation frequency and force level would stay unchanged as long as the structure would not be subjected to any structural deterioration. These spectral sidebands can be assumed as the pristine signature of a healthy structure. In the presence of structural deterioration the original signature will change and the new sidebands will not correlate any longer with the pristine case. This can be repeated for any excitation frequency and any forcing level. The most immediate outbreak of using spectral sidebands is to enable a quasi real-time assessment of structural changes. The CSLDV measurement technique allows fast scanning of the laser beam which can continuously measure modulation LDV time signal. This work on diagnostic was exploited for assessing damage during vibration testing of structures under fatigue. An example of experimental data will show what is expected to observe when a structure changes its response shape while fatiguing. This observation led the investigation of this matter by virtual testing where measurement parameters can be more easily controlled. D. DiMaio ( ) University of Bristol, University walk, BS8 1TR Bristol, UK e-mail: dario.dimaio@bristol.ac.uk © The Society for Experimental Mechanics, Inc. 2016 J. De Clerck, D.S. Epp (eds.), Rotating Machinery, Hybrid Test Methods, Vibro-Acoustics & Laser Vibrometry, Volume 8, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-30084-9_11 127
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