Browsing by Author "Liu, C. Y."

Now showing 1 - 7 of 7

Application of Cauchy wavelet transformation to identify time-variant modal parameters of structures
(2016-12-01)
Huang, C. S.
;
Liu, C. Y.
;
Su, W. C.
;
交大名義發表
;
National Chiao Tung University
This work proposes a procedure for accurately identifying instantaneous modal parameters of a linear time-varying system using a time-varying autoregressive with exogenous input (TVARX) model with the continuous Cauchy wavelet transform (CCWT). An appropriate TVARX model is established using the velocity and displacement responses of the system under consideration. The time-varying coefficients of the TVARX are expanded as piecewise polynomial functions. CCWTs with various scale parameters are then applied to the TVARX model to evaluate the instantaneous modal parameters of different modes. The CCWTs of the velocity and displacement responses are analytically obtained from the CCWT of the measured acceleration responses. The effectiveness and accuracy of the proposed procedure are validated by numerical simulations of single and multiple degrees of freedom systems that have periodically varying and sharply varying stiffness and damping coefficients. The effects of noise, the Cauchy wavelet function and the order of the polynomial on the evaluation of the modal parameters are explored in processing the numerically simulated acceleration responses of systems with a single degree of freedom subjected to base excitation. Finally, the proposed procedure is adopted to determine the modal parameters of a five-story symmetric steel frame from its measured acceleration responses in a shaking table test. The measured strains reveal the yielding of columns in the first story. The variations of the identified instantaneous natural frequencies and modal damping ratios with time are consistent with the physical phenomena that are observed from the measured strains and base excitation acceleration. (C) 2016 Elsevier Ltd. All rights reserved.
Characterization study of GaN-based epitaxial layer and light-emitting diode on nature-patterned sapphire substrate
(2012-03-01)
Lin, H. Y.
;
Chen, Y. J.
;
Chang, C. L.
;
Li, X. F.
;
Kuo, C. H.
Chemical wet etching on c-plane sapphire wafers by three etching solutions (H3PO4, H2SO4, and H3PO4/H2SO4 mixing solution) was studied. Among these etching agents, the mixing H3PO4/H2SO4 solution has the fastest etching rate (1.5 mu m/min). Interestingly, we found that H2SO4 does not etch the c-plane sapphire wafer in thickness; instead, a facet pyramidal pattern is formed on the c-plane sapphire wafer. GaN light-emitting diode (LED) epitaxial structure was grown on the sapphire wafer with the pyramidal pattern and the standard flat sapphire wafer. X-ray diffraction and photoluminescence measurement show that the pyramidal pattern on the sapphire wafer improved crystalline quality but augmented the compressive stress level in the GaN LED epilayer. The horizontal LED chips fabricated on the pyramidal-patterned sapphire wafer have a larger light output than the horizontal LED chips fabricated on the standard flat sapphire wafer by 20%.
Identification of Instantaneous Modal Parameter of Time-Varying Systems via a Wavelet-Based Approach and Its Application
(2014-04-01)
Su, W. C.
;
Liu, C. Y.
;
Huang, C. S.
;
交大名義發表
;
National Chiao Tung University
This work presents an efficient approach using time-varying autoregressive with exogenous input (TVARX) model and a substructure technique to identify the instantaneous modal parameters of a linear time-varying structure and its substructures. The identified instantaneous natural frequencies can be used to identify earthquake damage to a building, including the specific floors that are damaged. An appropriate TVARX model of the dynamic responses of a structure or substructure is established using a basis function expansion and regression approach combined with continuous wavelet transform. The effectiveness of the proposed approach is validated using numerically simulated earthquake responses of a five-storey shear building with time-varying stiffness and damping coefficients. In terms of accuracy in determining the instantaneous modal parameters of a structure from noisy responses, the proposed approach is superior to typical basis function expansion and regression approach. The proposed method is further applied to process the dynamic responses of an eight-storey steel frame in shaking table tests to identify its instantaneous modal parameters and to locate the storeys whose columns yielded under a strong base excitation.
Identifying the Modal Parameters of a Structure from Ambient Vibration Data via the Stationary Wavelet Packet
(2014-11-01)
Su, W. C.
;
Huang, C. S.
;
Chen, C. H.
;
Liu, C. Y.
;
Huang, H. C.
Ambient vibration tests are conducted widely to estimate the modal parameters of a structure. The work proposes an efficient wavelet-based approach to determine the modal parameters of a structure from its ambient vibration responses. The proposed approach integrates the time series autoregressive (AR) model with the stationary wavelet packet transform. In addition to providing a richer decomposition and allowing for an improved time-frequency localization of signals over that of the discrete wavelet transform, the stationary wavelet packet transform also has significantly higher computational efficiency than the wavelet packet transform in terms of decomposing time-shifted signals because the former has a time-invariance property. The correlation matrices needed in determining the coefficient matrices in an AR model are established in subspaces expanded by stationary wavelet packets. The formulation for estimating the correlation matrices is shown for the first time. Because different subspaces contain signals with different frequency subbands, the fine filtering property enhances the ability of the proposed approach to identify not only the modes with strong modal interference, but also many modes from the responses of very few measured degrees of freedom. The proposed approach is validated by processing the numerically simulated responses of a seven-floor shear building, which has closely spaced modes, with considering the effects of noise and incomplete measurements. Furthermore, the present approach is employed to process the velocity responses of an eight-storey steel frame subjected to white noise input in a shaking table test and ambient vibration responses of a cable-stayed bridge.
Identifying the modal parameters of structure from Seismic Response in time-frequency domain
(2015-01-01)
Su, W. C.
;
Huang, C. S.
;
Liu, C. Y.
;
土木工程學系
;
Department of Civil Engineering
This work presents the use of a short time Fourier transform to determine the natural frequencies, damping ratios, and mode shapes of a structure from its free vibration or earthquake response data. The short time Fourier transform is applied to the measured acceleration responses of a structural system, and to reconstruct the autoregressive with exogenous input (ARX) model in time-frequency domain. The accuracy of this procedure is numerically confirmed; the effects of the length of frequency band and noise on the ability to accurately estimate the dynamic characteristics are also investigated. The feasibility of the present procedure to elucidate real structures is demonstrated through processing the measured responses of steel frames in shaking table tests.
Room-temperature ferromagnetism in Zn and Mn codoped SnO(2) films
(2009-01-01)
Liu, S. J.
;
Liu, C. Y.
;
Juang, J. Y.
;
Fang, H. W.
;
電子物理學系
Room-temperature ferromagnetism was observed in Mn and Zn codoped SnO(2) films grown on c-cut sapphires by using pulsed laser deposition technique. The valence of Mn ions in the codoped films is determined to be 2+ from x-ray absorption near edge spectroscopy. Moreover, the ferromagnetism is affected by the carrier concentration of the films. This result is consistent with the carrier-mediated model. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3056374]
Room-temperature ferromagnetism in Zn and Mn codoped SnO2 films
(2009-01-01)
Liu, S. J.
;
Liu, C. Y.
;
Juang, J. Y.
;
Fang, H. W.
;
電子物理學系
Room-temperature ferromagnetism was observed in Mn and Zn codoped SnO2 films grown on c-cut sapphires by using pulsed laser deposition technique. The valence of Mn ions in the codoped films is determined to be 2+ from x-ray absorption near edge spectroscopy. Moreover, the ferromagnetism is affected by the carrier concentration of the films. This result is consistent with the carrier-mediated model. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3056374]