A d i r e c t i d e n t i f i c a t i o n procedure i s proposed t o o b t a i n the mass and s t i f f n e s s m a t r i c e s based on the t e s t measured eigenvalues and eigenvectors. The method i s based on t h e theory of m a t r i x p e r t u rb a t i o n i n which t h e c o r r e c t mass and s t i f f n e s s matrices a r e expanded i n terms of a n a l y t i c a l v a l u e s plus a modification matrix. The s i m p l i c i t y of t h e procedure enables r e a l time o p e r a t i o n during t h e s t r u c t u r a l t e s t i n g . I n t r o d u c t i o nOne of t h e p r i n c i p a l o b j e c t i v e s of t h e modal t e s t i s t o v e r i f y t h e a n a l y t i c a l model by comparing t h e experimentally determined modal c h a r a c t e r i s t i c s such a s t h e n a t u r a l frequencies and mode shapes t o those obtained from t h e a n a l y t i c a l model.Frequently, t h e a n a l y t i c a l model does n o t i n i t i a l l y produce c h a r a c t e r i s t i c s t h a t concur with t e s t r e s u l t s , and consequently t h e a n a l y t i c a l model i s adjusted o r improved u n t i l t h e a n a l y s i s and t e s t r e s u l t s agree. With increasing complexity of t h e s t r u c t u r a l system, t h e model adjustment o r improvement becomes d i f f i c u l t and systematic approaches a r e necessary, (Refs. 1 and 2).One of the approaches i s based on t h e estimat i o n theory i n which t h e d i f f e r e n c e s between t h e measured and a n a l y t i c a l l y obtained eigenvectors and eigenvalues a r e used t o e s t i m a t e those model parame t e r s t h a t a f f e c t t h e s e q u a n t i t i e s .The essence of t h i s approach i s t h e establishment of the so-called "Jacobian Matrix" whose elements a r e t h e derivat i v e s of eigenvalues and modal displacements with r e s p e c t t o t h e parameters. These parameters can be t h e elements i n t h e mass matrix and the s t i f f n e s s of t h e s t r u c t u r a l members.Using the Jacobian Matrix the changes i n eigenvalues and eigenvectors a s f u n c t i o n s of parameter changes can be established.The most important advantage of t h e approach i s t h e r e t a i n i n g of physical meanings of estimated parameters.However, t h e approach i s an i t e r a t i v e procedure whose convergence may be slow and dependent on t h e s e l e c t i o n of parameters and t e s t measurements, (Ref. 3 ) . This method was applied on a r e a l i s t i c s t r u c t u r a l system, t h e Viking S p a c e c r a f t , with t e s t measured eigenvalues and e i g e n v e c t o r s , (Ref. 4 ) . I t demonstrated t h a t the convergence of t h e i t e r a t i o n procedure can be slow f o r c e r t a i n cases. On t h e o t h e r hand, methods have been developed t h a t mass and s t i f f n e s s m a t r i c e s can be d i r e c t l y i d e n t i f i e d by using t e s t measured eigenvectors and *Member of Technical S t a f f , AIAA Member **Supervisor, S t r u c t u r e s and Dynamics Technology Group, AIAA Member Index Ca...
Cartoon/map images are synthetic graphics without complicated color and texture variation, which makes the embedding of invisible and robust digital watermarks difficult. In this research, we propose a wavelet-based, thresholdadaptive watermarking scheme (TAWS) which can embed invisible robust watermarks into various kinds of graphical images. TAWS selects significant subbands and inserts watermarks in selected significant coefficients. The inserted watermarks are adaptively scaled by different threshold values to maintain the perceptual integrity of watermarked images and achieve robustness against compression and signal processing attacks. Another major contribution of this work is that the cast watermark is retrieved without the knowledge of the original image. The so-called blind watermark retrieval technique is very useful in managing a large cartoon, trademark and digital map databases. Finally, a company logo that clearly identifies the copyright information can be embedded in cartoon and map images without serious perceptual loss. Experimental results are given to demonstrate the superior performance of TAWS.
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