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Higher-order frequency response functions (FRFs) are important to the analysis and identification of structural nonlinearities. Though much research effort has been devoted recently to their potential applications, practical issues concerning the difficulty and accuracy of higher-order FRF measurement have not been rigorously assessed to date. This paper presents a new method for the accurate measurement of higher-order FRFs. The method is developed based on sinusoidal input, which is ideal for exciting a nonlinear structure into desired regimes with flexible control, and the correlation technique, which is a novel signal processing method capable of extracting accurate frequency components present in general nonlinear responses. The correlation technique adopted is a major improvement over Fourier transform based existing methods since it eliminates leakage and aliasing errors altogether and proves to be extremely robust in the presence of measurement noise. Extensive numerical case studies have been carried out to critically assess the capability and accuracy of the proposed method and the results achieved are indeed very promising. Interesting nonlinear behavior such as frequency shift and jump have been observed in first-, second- and third-order FRFs, as well as solitary islands which have been identified over which higher-order FRFs virtually do not change as input force amplitude varies. Higher-order FRFs over such solitary islands are essentially their theoretical counterparts of Volterra transfer functions which can be measured with very low input force and can be profitably employed for the identification of physical parameters of structural nonlinearities. Subsequently, a nonlinear parameter identification method has also been developed using measured higher-order FRFs and results are presented and discussed. MOE (Min. of Education, S’pore)

Two types of cracks are often encountered in engineering structural systems and these are the open cracks and the breathing cracks. Existence of open cracks often leads to the loss of physical stiffness, resulting in a mostly linear structure with reduced load bearing capacity and vibration frequencies. The development of breathing cracks not only reduces structural stiffness, but tends to render the otherwise linear structure to become nonlinear, due to their bilinear stiffness characteristics associated with open and closed states. The nonlinear structural vibration responses can then be investigated and potentially employed to detect and identify breathing cracks within a structural system. In the present study, breathing cracks are modeled based on fracture mechanics from which bilinear stiffness values are obtained. These values are then incorporated into finite element models to compute the first- and second-order frequency response functions (FRFs) based on a proposed correlation technique which is both very accurate and resilient against measurement uncertainties. The existence of well-defined second-order FRFs has been firmly established for the bilinear oscillator, as well as a cantilevered beam with breathing cracks. By expressing the bilinear restoring force as a polynomial series, analytical derivation of second-order FRFs of general structural systems such as the GARTEUR AG11 structure with breathing cracks has been established for the first time. Further, a method of identification has been developed to identify the physical parameters of breathing cracks using second-order FRFs. With these new developments that are presented in this paper, a solid foundation has been laid for the potential applications of higher-order FRFs to damage identification and assessment of general real structural systems. MOE (Min. of Education, S’pore)

A new effective and efficient time domain modal analysis is presented in this paper. Based on the theoretical expansion of free decay response functions of damped structures, the method is formulated to utilize measured free-decay response data to efficiently form an equivalent derived eigensystem. Rigorous mathematical derivation will show that modal parameters (natural frequencies, damping ratios and mode shapes) of a damped test structure can be determined from the eigenvalues and eigenvectors of the derived equivalent eigensystem. Scaling of the resulting mode shapes so that they become so called A-normalized will be explored. General guidelines for selection of user-selectable algorithm parameters are also discussed to maximize the benefit of the proposed method. Another important theoretical development associated with the new method is a new concept of modal confidence factor (MCF) for the examination of identification results. The practical applicability and validity of the proposed method and the new MCF will be demonstrated through various numerical case studies. The concepts of equivalent derived eigensystem and MCF associated with the proposed method are new and have been demonstrated to be very effective. The identification results obtained are very accurate as compared with their exact counterparts and the method is very efficient in computational implementations.

The temperature dependence of the photoluminescence (PL) emission spectra of self-organized InAs/GaAs quantum dots (QDs) grown under different growth conditions in the range 20-300K has been investigated. Three InAs QD samples were grown on (100) 2º-tilted toward (111)A Si-doped GaAs substrates by metal-organic chemical vapour epitaxy (MOVPE), with various size uniformities and dot densities. Observing the measured PL spectra at 20K, the differences caused by size uniformities among the three samples were obvious. The PL spectra were simulated with rate equations, taking into account the carrier relaxation between the first excited state and the ground state. Interestingly, the calculated relaxation lifetimes at 20K were 198ps, 139ps and 54ps for the samples. The temperature dependent PL spectra were also simulated using the same model. Based on the calculated values of temperature dependent relaxation lifetimes, the differences in changes with temperature among the three samples are discussed. The results are consistent with the thermal redistribution effect.

A single wafer silicon condenser microphone with a novel single deeply corrugated diaphragm is presented in this paper. The microphone diaphragm with corrugation depth of 100 μm is only 1 mm2 in area, while the open‐circuit sensitivity as high as 9.8 mV/Pa under a bias voltage of 6 V has been obtained. The recorded frequency bandwidth is about 20 kHz. The measurements show reasonable agreements with the theoretical predictions.

Calreticulin proteins play essential roles in regulating various metabolic processes and in molecular signal transduction in animals and plants. Using homologous PCR, we screened a cDNA library of the wheat resistance gene Yr5 from a near-isogenic line in the susceptible common wheat variety Taichung 29, which was inoculated with an incompatible race CYR32 of Puccinia striiformis. We isolated a novel full-length cDNA encoding calreticulin protein, which we named TaCRT1. Sequence analyses indicated that TaCRT1 contains an open reading frame of 1287 bp in length; it was deduced to encode 428 amino acids. Clustering analysis showed that TaCRT1 belongs to group III of the calreticulin protein family. Semi-quantitative RT-PCR was used to analyze expression profiles of the isolated gene under biotic and abiotic stresses. Expression of TaCRT1 was suppressed by exogenous application of phytohormones, such as abscisic acid and methyl jasmonate, and by dehydration; but it was induced by CYR32 infection and cold treatment. Based on the expression patterns, we propose that TaCRT1 participates in regulatory processes involved in defense responses and stress resistance in wheat.

Sensitivity analysis, the study of how a structure's dynamic characteristics change with design variables, has been used to predict structural modification effects in design for many decades. In this paper, methods for calculating the eigensensitivity, frequency response function sensitivity and its modified new formulation are presented. The implementation of these sensitivity analyses to the practice of finite element model improvement using vibration test data, which is one of the major applications of experimental modal testing, is discussed. Since it is very difficult in practice to measure all the coordinates which are specified in the finite element model, sensitivity based methods become essential and are, in fact, the only appropriate methods of tackling the problem of finite element model improvement. Comparisons of these methods are made in terms of the amount of measured data required, the speed of convergence and the magnitudes of modelling errors. Also, it is identified that the inverse iteration technique can be effectively used to minimize the computational costs involved. The finite element model of a plane truss structure is used in numerical case studies to demonstrate the effectiveness of the applications of these sensitivity based methods to practical engineering structures.

The purpose of this paper is to develop both numerical and experimental way that can be effectively used for vibration interaction prediction of the disk-spindle systems in HDD. First, the work focuses on the effective modeling method for the analysis of the vibration interaction of whole disks-spindle system. Second, the analytical results had been verified through the proposed experimental method using externally induced excitations for disk-spindle system in product assembly. The two set results agree very well. As shown, the numerical modeling and experiment methods can provide much information on design before to make hardware and verify it in lab.

The disturbances acting on the track-following servo system of an optical disk drive inherently contain significant periodic components that cause track error of a periodic nature. In this paper, discrete-time sliding mode repetitive control (DSMRC) was developed, which can reject such disturbances. The developed approach ensures the robustness of the sliding mode dynamics to parameter uncertainties and exogenous disturbances and the complete rejection of the periodic disturbance component. The satisfied simulation results as well as the experimental results using an optical disk drive are given to illustrate the validity of the proposed method.

The paper presents a study on the problem of disturbance rejection, specifically the periodic disturbance, by applying discrete-time sliding mode control methods for hard disk drives. For perturbation such as modeling errors and external disturbances, their compensation is formulated using the designed sliding mode control. The convergence rate between the disturbance and its compensation has been shaped by an additional parameter. Computation time delay is presented to address the perturbation effects. The approach developed ensures the robustness of the sliding mode dynamics to parameter uncertainties and exogenous disturbances, and the rejection of the periodic disturbance component. Satisfactory simulation and experimental results have been achieved based on a servo system of a hard disk drive (HDD) to illustrate the validity of the controller for repeatable run-out (RRO) compensation.

In this study, a novel principle "electret" microphone is proposed and implemented by a single-chip fabrication technique. The microphone uses a charged floating electrode surrounded by highly insulated materials as "electret" to excite electric field. Net free-electric-charges (not "bonded" charges as in traditional electret) in the floating electrode can be easily charged by use of the "hot" electron technique, available using the so-called avalanche-breakdown technique of p/sup +/-n junction. Thus the electret microphone is rechargeable, which can greatly increase the lifetime of the microphone. Preamplifier has been on-chip integrated in a JFET type by use of ion implantation. "Electret" charges are bonded in a deep potential trap, different from that of traditional electret of which the charges are in a surface shallow potential trap, thus this microphone can operate at high temperature (high as 300/spl deg/C) and has high stability and reliability. Experiments show that the prototype microphone has a 3 mV/Pa sensitivity and 2l kHz frequency bandwidth in a 1 mm /spl times/1 mm diaphragm area. Microphone performance can be further improved by use of the optimized processes and designs. The fabrication is completely IC-compatible, hence the microphone is promising in the integrated acoustic systems.

In order to understand the dynamic properties and develop a valid finite element model of a plastic ball grid array (PBGA) PCB assembly, a series of experimental modal tests have been conducted and reported in this paper. Firstly, the purpose and importance of conducting these experimental modal tests are discussed. Secondly, detailed test results of our PBGA PCB assembly are presented, which can serve as a valuable reference for those who are interested in the vibration fatigue reliability of PBGA PCB assemblies. Lastly, some factors which have an important influence on the modal test results of the PCB assembly are discussed.

In this paper, a distributed self-sensing actuator (DSSA) is presented for active disk vibration/flutter suppression in hard disk drives (HDDs). The effectiveness of the disk vibration/flutter suppression by DSSA under different designed shapes has been investigated numerically. Both the sensing and actuating effects of the active piezoceramic layer were evaluated through experiments. Then, the out-of-plane vibrations of top disk were measured with and without the feedback control of DSSA. The magnitudes of disk vibration and flutter that are induced by the surrounding airflow, the ball bearing, and the resonances of the system, could be significantly suppressed to a large percent.