Characterization of rotating structures in coast-down by means of continuous tracking laser doppler vibrometer

Abstract

In rotating machinery variations of modal parameters with rotation speed may be extremely important in particular if very light and undamped structures are taken into account, like helicopters rotors or wind turbines. The relation between natural frequencies and rotation speed is expressed in the form of Campbell diagrams. However it could be required to know also the deviation of operational or mode shapes. In several cases it is not possible to fully control the rotating speed of the machine and only coast-down tests can be performed. Such kind of tests is often fast due to the reduced inertia of rotors: for this reason, an experimental technique able to determine Operational Deflection Shapes (ODSs) in short time and with sufficient accuracy, appears very promising. Moreover coast-down processes are very difficult to be controlled, they causing unsteady vibrations. The need to obtain ODSs from coast-down experiments requires the measurement over a large number of points and therefore a very efficient approach for the rotation control and synchronous acquisition must be developed. In this paper a continuous scanning system operating on rotating structures has been developed, that allows to measure ODS and natural frequencies excited in rotating conditions at different rotation speed during a coast down. This techniques has been tested on a laboratory test bench and compared with traditional Experimental Modal Analysis (EMA) results obtained in non-rotating conditions and with data from Tracking Laser Doppler Vibrometry (TLDV) operating in coast down and at consecutive constant rotation speeds (i.e. each measurement was performed in steady conditions). EMA and TLDV have been performed over a grid of points in order to have ODSs with adequate spatial resolution, it requiring long measurement time. However these data has been used as reference to validate the continuous scanning approach.