In Operational Modal Analysis applications, it is assumed that the structure is excited by white noise. However, in some cases, the operational vibration data are acquired while rotating equipment is active in the background or while it is even the main source of excitation. The structural responses will then consists of a broadband response from which the structural modes can be determined and additional harmonic response at discrete frequencies, which are disturbing the parameter identification process. Sometimes, the harmonic response is dominating and the Operational Modal Analysis methods only find poles at these harmonic frequencies. Therefore, it is desired to try to remove the disturbing harmonics from the data before applying Operational Modal Analysis.
In this paper, a method that serves this purpose will be discussed. If the fundamental frequency of the disturbing harmonics is not known, it will be estimated by applying a "tacho-less rpm extraction" procedure. Using the (possibly fluctuating) rpm, the data can be converted to the angle domain and, then time (or better: angle) synchronous averaging is applied to remove the harmonics. This procedure will be illustrated using simulated data as well as real industrial operational data from an in-flight helicopter test and from a running large diesel engine.
