Modal Testing of Rotating Discs Using a Scanning LDV

Abstract

Abstract Modal testing of rotating structures is complicated by difficulty of access and by the fact that, if the source of excitation is fixed in space, responses occur at frequencies other than that of the excitation. This paper demonstrates the use of, and derives output equations for, a Laser Doppler Vibrometer (LDV) for response measurement. Examples are given of measurements using the LDV beam aimed (i) at a point fixed in space, (ii) at a fixed point on the disc, and (iii) rotating at a non-synchronous speed. The third technique is perhaps the most useful, enabling the circumferential spatial disc response, expressed in terms of travelling waves or orthogonal standing modes, to be directly derived at any speed, frequency or state of disc mistuning. The same technique, applied to a non-rotating disc, provides a simple qualitative measurement of the mode shape associated with each natural frequency. An example based on a centrifugal impeller is included.