結構系統的動態特性可藉由其自然頻率、阻尼比及模態振形加以描述，而一般模態參數識別法須同時利用激勵及響應資料來識別模態參數。但許多工程結構在環境振動作用下，僅能獲得其響應資料。因此無需激勵信號的量測而直接由響應資料識別模態參數，為本文探討之重點。本文考慮當線性結構系統在環境振動下，若激勵信號可近似為非定常白訊，其響應信號間的相關函數與系統的脈衝響應或自由振動下的衰減響應有相同的數學形式。因此可將響應信號間的相關函數視為自由振動下的響應信號，從而識別模態參數，並利用模態可信度(MAC)的方法來確認識別模態。本文採用Ibrahim時域參數識別法，探討其應用於無激勵信號之模態參數識別的適用性。由數值模擬顯示，此法對即使有相近模態的系統，亦能有效地識別出模態參數。此外，考慮結構受一般非定常激勵信號，應用相關函數法與濾波器概念可求得「合成系統」的自由衰減信號函數，進而使用Ibrahim時域法識別模態參數。並可利用結構模態正交性將響應信號中假想激勵信號模態與結構模態加以區分。

　　Dynamical systems can be characterized by their modal parameters, which include natural frequencies, damping ratios and mode shapes. Identification of system characteristics is usually accomplished using both input and output data from the structural system. In many cases, however, only output measurements are available for structures under ambient vibration conditions. It can be shown that if the input signals can be modeled as non-stationary white noise, which is a product of white noise and an deterministic envelope function, the theoretical auto- and cross-correlation functions of structural response have the same mathematical form as free vibration of the structure. In this thesis, the Ibrahim Time Domain method is employed as the modal identification schemes to extract modal parameters from vibration data. Through numerical simulation, the applicability of the modal parameter identification method proposed is confirmed. Furthermore, the numerical simulation is considered modal parameter identification using ambient data excited by non-stationary color noise. This is accomplished via adding, in cascade, a pseudo-force system to the structure’s system under consideration. The input to the pseudo-force system is non-stationary white noise and the output of which is the actual force(s) applied to the structure. The non-stationary white noise input(s) and the structure’s responses are then used to identify the combined system. Identification results are then sorted as either structural parameters or inputs force(s) characteristics.

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