Accuracy and robustness of four basic single degree of freedom methods for determining the modal parameters of non-lightly damped systems

An adequate measurement strategy (the combination of measurement method, response parameter, and where applicable, damping estimator) should be used for accurate determination of the natural frequency and damping of non-lightly damped single degree of freedom (SDOF) systems, otherwise, significant measurement bias errors can exist. The accuracy of four commonly used methods is studied and the measurement strategies that are exact are identified. This can depend on the damping mechanism, which if not known can be gleaned from patterns of bias errors between frequency estimations from different strategies. Since in practice a SDOF system rarely exists, or as SDOF techniques are knowingly applied to multiple degree of freedom (MDOF) systems, the robustness of the strategies in the presence of a secondary mode is studied, and the more robust (and therefore preferable) strategies are identified. The measurement method based on the real spectral part is advantageous as it evaluates the damping exactly and is independent of the damping mechanism, in addition the natural frequency obtained is independent of the response parameter. However, this method is not a relatively robust one, and is also sensitive to phase changes accrued during measurements.