On the Accuracy of the Modal Parameter Estimates INOMA

Modal identification procedures are used to evaluate the vibration properties of structures and can be divided into two branches, that is Input-Output and Output-Only analyses. Several methods are developed to assess the accuracy of the modal parameter estimates in the Input-Output framework, whereas deeper investigation is still needed to assess the confidence boundaries of Output-Only methods. Such accuracy of the estimates is generally given by comparing them with the expected set of modal parameters provided by a numerical model. In this paper, the accuracy of the modal parameters estimated using Output-Only techniques is investigated aided by the use of both time and frequencybased estimating methods coupled to a sensitivity analysis when varying data acquisition settings. Furthermore, insights into the robustness of the estimating process to the measuring chain is obtained from several independent tests and changes to the boundary conditions. Indeed, the experimental boundary conditions are altered by using materials with different elastic properties and different connection configurations. The uncertainties arising from the estimating process and the experimental setup mounting are evaluated for the natural frequencies, damping ratios, mode shapes through several modal assurance criterion formulations. The tests show that the accuracy of modal parameter estimates can be assessed and increased by weighting the similarity between multiple estimates of the same vibration mode. The accuracy depends on mounting configurations due to changes of structural properties. An evaluation of the overall accuracy of the modal parameter estimates finds that the accuracy is increased when information from multiple OMA algorithms is used.