在模態參數識別的過程中，模態干涉常會影響識別結果的精度甚至造成可識別性的問題。造成模態干涉的主要原因通常有相近頻率及高阻尼比等等。在相近模態的干涉因素下，重根模態又為最嚴重的狀況，在此模態干涉的影響下，識別時容易造成可識別性的問題，導致識別上的困難。本文利用模態分析理論的頻率響應函數說明當系統具有重根模態的狀況下，需利用多輸入的識別方法才能完整地識別出重根模態。吾人可藉由複模態指示函數曲線圖來確定系統的模態數進而判斷是否具有相近模態或重根模態，再進一步搭配各模態的增強頻率響應函數進行模態參數識別，能夠提高該模態的響應並且降低其他模態的干涉與雜訊的影響。吾人也可直接利用奇異值的數據與其關係式進行模態參數識別，可省略計算增強頻率響應函數的過程，計算較為快速。

In the process of identification of modal parameters, modal interference often affects the accuracy of the result of identification and even causes the problem of identifiability. Major causes of modal interferences include closely spaced modal frequencies, high damping ratios, and so on. Regarding closely spaced modes, the repeated modes are the most severe situation, which may lead to the problem of identifiability of modal parameters. This thesis explains why we need to use multiple references to identify repeated modes according to the formulation of frequency response function in modal analysis theory. We can determine the number of significant modes of a system by the Complex Mode Indicator Function, even when there are close modes or repeated modes. The Complex Mode Indication Function has been combined with the Enhanced Frequency Response Function to be a modal parameter estimation method. We can also use directly the data of singular values to identify modal parameters, and this method does not need to calculate the Enhanced Frequency Response Function.

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