Modified SSTD method to account for harmonic excitations during operational modal analysis

In-operation modal testing is a procedure, which allows to extract modal parameters of a structure in operational condition. It is based on the assumption that input to the structure is stationary white noise. In practice however, many structure's input force cannot be assimilated to white noise input. In some of the cases, input force can be compared to the combination of white noise and harmonic excitations. Harmonic excitations can occur due to components like unbalanced rotors or fluctuating forces in electric actuators. The usual way to compute modal parameters in the presence of harmonic excitations is to treat harmonically excited frequencies to be virtual eigenfrequencies of the structure. However, if the frequency of the harmonic input is close to an eigenfrequency of the system, operational modal analysis procedures fail to identify the modal parameters properly. Correlation functions of the response signals can be treated as a impulse response functions as stated in the Natural Excitation Technique (NExT). Time-domain method like Single Station Time Domain (SSTD) can be used to process the correlation functions to compute modal parameters. In this paper a modified SSTD method is proposed, which can be applied to include the effect of purely harmonic vibrations, assuming the harmonic frequencies are known a priori. We illustrate the efficiency of the proposed approach on the experimental example of a plate structure excited by multi-harmonic loads superposed on random excitation.