Performance analysis of continuous tracking laser Doppler vibrometry applied to rotating structures in coast-down

In this paper a performance analysis of the so-called tracking continuous scanning laser
Doppler vibrometry (TCSLDV) exploited in coast-down has been performed. This non-contact
measurement system is able to scan continuously over a rotating surface during coast-down
and to determine vibration operational deflection shapes (ODSs) and natural frequencies in
short time, i.e. the temporal extent of the coast-down. The method is based on a laser Doppler
vibrometer (LDV) whose laser beam is driven to scan continuously over the whole rotor
surface synchronously with its rotation, so that the LDV output is modulated by the structure’s
ODSs. This technique has a full-field nature that enables it to measure simultaneously the time
and spatial dependence of the vibration in a unique measurement. However, the TCSLDV
presents some criticalities in practical applications, especially when applied to rotary transient
and fast processes. In fact, if the vibration is transient and decays very fast, then the laser beam
could not have had the time to scan the complete structure surface and the modulation of the
ODS could be partial. An analytical model reproducing a representative experiment has been
developed in order to evaluate the sensitivity of results to testing conditions. The laser beam
trajectory in both the fixed and rotating reference systems has been synthesized showing its
dependence on experimental parameters as the rotation speed variation during coast-down. It
has been demonstrated the decrease in speed induces the deformation of the laser trajectory
influencing the LDV output time history, spectrum and consequently the recovered ODS.