Laser Doppler vibrometry on rotating structures in coast-down: resonance frequencies and operational deflection shape characterization

In rotating machinery, variations of modal parameters with rotation speed may be extremely
important in particular for very light and undamped structures, such as helicopter rotors or
wind turbines. The natural frequency dependence on rotation speed is conventionally
measured by varying the rotor velocity and plotting natural frequencies versus speed in the
so-called Campbell diagram. However, this kind of analysis does not give any information
about the vibration spatial distribution i.e. the mode shape variation with the rotation speed
must be investigated with dedicated procedures. In several cases it is not possible to fully
control the rotating speed of the machine and only coast-down tests can be performed. Due to
the reduced inertia of rotors, the coast-down process is usually an abrupt transient and
therefore an experimental technique, able to determine operational deflection shapes (ODSs)
in short time, with high spatial density and accuracy, appears very promising. Moreover
coast-down processes are very difficult to control, causing unsteady vibrations. Hence, a very
efficient approach for the rotation control and synchronous acquisition must be developed. In
this paper a continuous scanning system able to measure ODSs and natural frequencies excited
during rotor coast-down is shown. The method is based on a laser Doppler vibrometer (LDV)
whose laser beam is driven to scan continuously over the rotor surface, in order to measure the
ODS, and to follow the rotation of the rotor itself even in coast-down. With a single
measurement the ODSs can be recovered from the LDV output time history in short time and
with huge data saving. This technique has been tested on a laboratory test bench, i.e. a rotating
two-blade fan, and compared with a series of non-contact approaches based on LDV:
• traditional experimental modal analysis (EMA) results obtained under non-rotating conditions
by measuring on a sequence of points on the blade surface excited by an impact hammer,
• continuous scanning LDV measuring the ODS of the structure excited by an impact hammer
modulating the laser output,
• tracking laser Doppler vibrometry (TLDV) operating at different rotation speeds under
stationary conditions,
• tracking continuous scanning laser Doppler vibrometry (TCSLDV) operating at different
rotation speeds under stationary conditions.
EMA and TLDV have been performed over the same grid of points sufficiently dense to have
ODSs with adequate spatial resolution, it requiring long measurement time. The application of different techniques allowed us to completely characterize the tested bladed rotor and to
validate the continuous scanning application to transient rotator processes.