One of the key features of vibration characteristics (e.g., critical speed, damping ratio, Q-value, instability margin, etc.) of a rotor-bearing system can be described by their eigenvalues. The eigenvalues are conventionally evaluated by feed forward excitations, e.g., hammering test, harmonic sweep excitation, and unbalance response measurement. However it has been difficult to identify their damping ratios under various conditions. In this paper, we introduce a new method using a modal open loop transfer function. This method provides a more accurate Q-value and can help to identify the damping ratio by considering only the mode which we want to measure. We applied this method to a few models and some test rigs: 1) 3DOF model (simulation), 2) A flexible rotor equipped with active magnetic bearings (experiment). As a result, we concluded: 1) As the number of sensors is increased, the accuracy of the measurement also increases. 2) Our new method is more effective in terms of the accuracy of modal damping ratio than the conventional method is.

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