Identification of natural frequencies and damping ratios of machine tool structures by the dynamic data system approach

Modal parameter identification of machine tool structures is conventionally achieved by first obtaining frequency response curves from experimental data and then implementing a graphical approximation or curve fitting procedure. The principal shortcomings of this procedure originate from the inherent limitations of the Fourier Transform method, used in obtaining the frequency responses, and from the subjective judgement of the analyst attempting to obtain the modal parameters by curve fitting. As an alternative to the conventional method a direct modal parameter identification method, based on the Dynamic Data System approach, has been proposed. It utilizes data in the form of time series to develop discrete difference equations representing the dynamic properties of the machine tool structure at the points of measurement. The subsequent use of the models allows the determination of the global dynamic properties of the system such as the modal natural frequencies and damping ratios. The theoretical relationships between the continuous vibratory system and its discrete representation in the form of Autoregressive Moving Average (ARMA) models has been introduced and discussed. The proposed method has been applied to the analysis of experimental data obtained from a drilling machine. Subsequently a comparative assessment is given between these results and results obtained by the conventional Fourier Transform based method.