For more than 20 years Laser Doppler Vibrometry (LDV) is an established tool in vibration engineering for non-contact vibration measurements. The LDV method is not only limited to measurements on single test points. Utilizing a pair of galvanoelectrical mirrors for steering the laser beam in x and y directions across the surface the vibration pattern of full areas can be mapped. Such scanning LDVs (SLDV) have found many applications in the automotive industry as well as in institutional research. However the use of the SLDV technology is limited to stationary objects and rotating objects with smooth uninterrupted structures such as brake discs or tires. Up until now measurements on objects with complex noncontinuous structures such as bladed discs, fans, propellers have not been possible yet. Furthermore, interpretation of travelling non-stationary modes which can be found on rotating tires, saw blades, etc. has been difficult with conventional SLDV technology.
A Derotator is an optical unit overcoming this limitation by maintaining a fixed position of the SLDV laser beam in the rotating coordinate system of the test object. By “derotating” the object with a rotating optical unit a stationary image of the object is achieved and a standard SLDV measurement can be done where the laser beam location on the objects remains stationary. Like in usual SLDVs, the system is acquiring data in one point then moves to the next scanning point and so on.
By means of measuring the vibration behaviour of a rotating object during an engine run up critical rotational speeds and resonance frequencies of the rotating object can be identified. Furthermore material stiffness changes caused by centrifugal forces can be visualized analyzing the Eigenfrequency changes in a waterfall or Campbell diagram.
In this paper the basic principle of a derotated SLDV measurement is explained. In the first application presented in this paper the Operational Deflection Shape (ODS) of a rotating disk is measured with and without derotating and the differences in the results are analyzed. In the second application a typical measurement procedure doing a trouble shooting on a pump rotor will be elucidated. Finding the critical speed and the corresponding ODS clearly shows the reason for the failure in operation conditions.