From operating deflection shapes towards mode shapes using transmissibility measurements

A transmissibility measurement is the traditional way of making a frequency response measurement when the excitation force cannot be measured. It is calculated in the same way as the admittance function, or transfer function. Whereas the admittance function is the ratio of response divided by force, transmissibility is the ratio of a response divided by a reference response. It is well known that operating deflection shapes can be displayed from a set of two or more transmissibilities. The use of transmissibility functions in the field of operational modal analysis has only recently been introduced by the authors of this paper. In general the “model” obtained from transmissibility functions is only valid for the concerned operating condition and is not related to any system parameter. In this paper a new post-processing method based on transmissibility measurements is introduced that allows the estimation of the modal parameters and in particular the unscaled mode shapes. The new method is introduced and illustrated with a numerical test.