碩士論文公告區

年度: 92
姓名: 留鼎傑(Ding-Jie Liu)
論文題目(中): 以小波轉換分析混凝土版內鋼筋及管線之敲擊回音反應
論文題目(英): Wavelet analysis of the impact echo response with respect to reinforcement and piplines inside concrete slabs
摘要(中):
傳統的訊號分析是建立在傅立葉轉換的基礎之上,但由於傅立葉無法表達訊號的時頻局部性質,因此便有了小波轉換的發展,小波轉換相較於傅立葉轉換最大的不同是在於小波轉換是一種時間窗與頻率窗都可改變的時頻局部化分析方法,在低頻部分具有較寬時間域,而頻率域上。反之,在高頻部分具有較寬頻率域,而時間域上窄。
本研究使用敲擊回音法偵測混凝土版內異質管線訊號,由於混凝土本身的材質不均勻加上內含不同材質管線,所接收到訊號無法在時域中加以判讀,因此就利用頻譜分析之技術,將時域訊號轉換成頻域訊號,而在頻譜中找到一個主要的尖峰,但有時不易在頻譜中選定主要的尖峰。因此就針對這個問題,嘗試利用小波轉換法來改善所接收到的訊號。利用此種技術,將所接收到的時域之敲擊回音訊號,做連續小波轉換,並將所得之結果顯示在二維的比例圖像上。原本在時域訊號不易辨認的多次背面回,在這圖上會清晰浮現出來,偵測混凝土版內含異質管線的工作變得較為準確與可行。
摘要(英):
Fourier transform is frequently used in analyzing the frequency content of a transient signal. The drawback of Fourier transform is that the time- domain characteristic cannot be shown simultaneously. Wavelet transform is one of the time-frequency dual transforms that preserves both frequency content and time information of a transient signal. The length of both time window and frequency window can be varied in wavelet transform. As a result, the time resolution is better in high frequency band while the frequency resolution is better in low frequency band.
A typical problem in the processing of impact-echo signals arises from peak detection based solely on frequency spectrum. Current study aims to improve the feature identification of impact-echo signals obtained from buried objects in concrete. Rebars, steel tubes, and PVC tubes of different diameters at various depths from the concrete surface are tested.
Numerical simulations are carried out based on models constructed using the finite element method. The received signals, both experimental and simulated, are analyzed using both fast Fourier transform and continuous wavelet transform. Multiple back-wall reflections can be easily detected from the wavelet coefficient plots (WCPs). Low-frequency signals due to rebar at a greater distance is clearer shown in the WCP than those due to rebar at a smaller distance from the side wall of concrete specimens. Such findings provide a more effective way to pick up rebar signals using the impact-echo method.
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相關連結: http://ethesys.lib.cyut.edu.tw/ETD-db/ETD-search-c/view_etd?URN=etd-1128103-155859

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