Updating modal models from response measurements

A method for updating modal models from response measurements is extended from the case of a single impulsive excitation to a more general broadband excitation in the presence of secondary excitations. The original technique was based on analysis of the cepstrum of the response, as forcing function and transfer function effects are additive in the response cepstrum, and also separated if the force log spectrum is reasonably smooth and flat. Use is made of principal components analysis by singular value decomposition to separate the autospectrum of the response at each point to the dominant excitation, which is then curve fitted in the cepstral domain for its poles and zeros to give updated estimates of the FRFs. The resulting FRFs are scaled (because of including the information on zeros) and in the study gave reasonable estimates of the mode shapes when the dominant force was four times larger than the next largest.