Video Processing Techniques for the Contactless Investigation of Large Oscillations

Abstract

The experimental acquisition of large vibrations presents various technical difficulties. Especially in the case of geometric nonlinearities, dealing with very flexible, very light structures causes minimal variations in mass or stiffness to affect severely the dynamical response. Thus, sensors’ added masses change the behaviour of the structure with respect to the unloaded condition. Moreover, the most common tools regularly employed for acquisition in vibration analysis - that is to say, laser vibrometers and accelerometers - are often designed with small amplitudes in mind. Their recordings are known to lack accuracy when the investigated structure undergoes large or very large motions, due to geometrical reasons. Image-based measurement techniques offer a valid solution to this problem. Here, an ensemble of three video processing techniques are benchmarked against each other and tested as viable options for the non-contact dynamic characterisation of slender beam-like structures. The methods have been applied to the case study of an aluminium spar for a highly-flexible airwing prototype and compared to the measurements recorded by a laser velocimeter and several Raspberry PI Inertial Measurement Units (IMUs), which also proved to be minimally invasive.

The Teager-Kaiser Energy Cepstral Coefficients as an Effective Structural Health Monitoring Tool

“…The second case study concerns the spar of a prototype wing, the linear and nonlinear dynamics of which have been investigated in recent studies [61,62]. Importantly, the prototype skin is supposed to completely transfer the aerodynamic loads to the spar, making the structural behaviour of the latter a good approximation of the dynamical response of the whole spar-skin ensemble.…”

“…Therefore, this value has been set as the new cut-off frequency. The centre frequencies for MO-MMFCCs were defined from the modal analysis of the wing spar and benchmarked and validated against experimental data obtained in [61,62]; they correspond, in order of increasing natural frequency, to the 1st, 2nd, 3rd, and 4th flapwise bending mode; the 1st torsional mode; the 5th flapwise flexural; the 1st chordwise bending mode; the 2nd torsional; and, the 6th flexing modes. The tenth mode (the 7th flapwise) fell over the maximum investigable frequency at 333.88 Hz and was therefore replaced by adding '1 to the begin of the array, thus resulting in To conclude, the low frequency range of LO-MMFCCs was not changed and it remains defined between 0 and f s /2, which here becomes 128 Hz.…”

An experimental study of the feasibility of phase‐based video magnification for damage detection and localisation in operational deflection shapes

“…Other non-contact alternatives exist: laser scanning is the main competitor of video-based techniques. A complete discussion will be too long to be reported here; an exhaustive comparison can be found in [14] and some related studies recently made by the Authors are reported in [15]. For the aim of this work, it is enough to say that high-speed cameras are much more common and easy to access than multi-point laser scanners, especially for untrained labourers.…”

Using Video Processing for the Full-Field Identification of Backbone Curves in Case of Large Vibrations

“…This has been proven here and in previous studies to not affect the quality of measured accelerations, while being minimally invasive. Other specifications about the Raspberry PI ® IMUs utilised for the air wing spar here investigated are reported in [69]; for completeness’ sake, the most relevant technical details about them and the rest of the instrumentation are reported in Table 2.…”