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

Abstract

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.

Laser Doppler Vibrometry Measurements in Structural Dynamics

“…Such concept, addressed in literature as Tracking CSLDV (TCSLDV), consists in performing a continuous scan over the target surface while tracking the target rigid body motion. A deeper insight into the method, given by application cases, can be found in [57][58][59][60].…”

Modal shape measurement of fiber-reinforced composite plate with high efficiency and precision based on laser linear scanning method

“…The experimental data were used to calculate the eigenfunctions and the eigenvalues of the plate. Martarelli et al 18 measured the modal shapes and operational deflection shapes of a two-bladed rotor by applying different methods based on the laser Doppler vibrometer. Ji et al 19 studied the measurement technique of sine vibration in an unknown direction with an SPLV, and applied it into the in-plane vibration test of the ultrasonic motor.…”

Investigating the vibrational behaviour of a rotating two-blade propeller by using a self-tracking method

“…A self-tracking method also eliminates the limitation of using a mirror system at low rotational speeds. 12 Martarelli et al 13 proposed a continual scanning system based on laser doppler vibrometry (LDV), whose goal was to measure operational eflection shapes (ODSs) and natural frequencies by scanning upon the surface of the rotor. Considerable data and time saving are among the distinct features of this study.…”