Rongming Lin

Structural damage detection using measured FRF data

Abstract The underlying principle behind structural damage detection techniques is that vibration signature, e.g. modal properties or frequency response function (FRF) data, is a sensitive indicator of structural physical integrity and thus can be used to detect damage. Since indirectly-measured modal data contain accumulative errors incurred in modal parameter extraction and provide much less information than FRF data, it is more reasonable and reliable to use directly-measured FRF data for structural damage detection. In this paper, a new damage detection algorithm is formulated to utilize an original analytical model and FRF data measured prior and posterior to damage for structural damage detection. Based on nonlinear perturbation equations of FRF data, an algorithm has been derived which can be used to determine a damage vector indicating both location and magnitude of damage from perturbation equations of FRF data. An additional development with respect to the proposed technique is an effective technique introduced for weighting perturbation equations of FRF data at selected locations and frequencies so as to reduce influence of measurement errors on accuracy of damage detection to the minimum. For extension of the proposed algorithm to cases of incomplete measurement in terms of coordinates, an iterative version of the proposed algorithm has been introduced. The validity and applicability of the proposed damage detection algorithm have been demonstrated through numerical and experimental studies on a practical plane 3-bay frame structure, respectively.

Design considerations in micromachined silicon microphones

We present and compare the different designs of micromachined silicon condenser microphones. The aim is to develop the microphones with high sensitivity and low fabrication cost. Slotted and corrugated diaphragms have been designed and fabricated in order to increase the mechanical sensitivity of microphones. At the same time, we developed the fabrication process for the low stress or stress-free multilayers polysilicon used as the microphone diaphragms. To increase the microphone chip density on one wafer and avoid the sticking problem during the wet release process, a new process design using deep reactive ion etching is proposed, which is available in our laboratory.

Large deflection analysis of plates under thermal loading

Abstract A global method of generalized differential quadrature is presented to solve the problems of large deflections of thin plates under thermal loading. By applying the generalized differential quadrature, the drawbacks of existing analytical and numerical solution techniques for the problem of non-linear bending of heated plates, such as complexity, lack of generality and computational inefficiency, have been overcome. The weighting coefficients for the approximation of derivatives required in differential quadrature formulation are calculated in a very simple way yet without any restriction on the choice of grid points. Numerical implementation of the method is straightforward and different boundary conditions can be easily incorporated. Examples are given to demonstrate the accuracy and efficiency of the proposed method. To validate the method, a case of large deflection of plates without thermal loading is also considered and the results compare well with existing analytical and numerical solutions.

Study of deep silicon etching for micro-gyroscope fabrication

Deep silicon etching technique can offer many advantages in microelectromechanical system (MEMS) fabrication because of its high etch rate, high aspect ratio and high anisotropic etching behavior comparing with the conventional reactive ion etching technique. In this study, the deep reactive ion etcher produced by Surface Technology Systems Limited (STS, UK) has been used to fabricate the micro-gyroscope. The effects of feature size of the structure, platen power and process pressure on etch rate and reactive ion etching lag have been studied. Micro-gyroscope has been fabricated by using the optimum etching conditions.

Deflection of plates with nonlinear boundary supports using generalized differential quadrature

Abstract A global method of generalized differential quadrature has been presented in this paper to solve the problems of the deflections of plates with general nonlinear elastic boundary supports. The proposed method has been found to be computationally very efficient and numerically very accurate, especially in the case where nonlinear problems are to be solved. The weighting coefficients for the approximation of the derivatives required in differential quadrature formulation are calculated in a very simple way yet without any restriction on the choice of grid points. Numerical implementations of the method is straightforward and different boundary conditions can be easily incorporated. Numerical results demonstrate the great potential of the method for nonlinear analysis of structural components.

Control of stress in highly doped polysilicon multi-layer diaphragm structure

Polysilicon films can be used as stress regulating (or compensating) films to achieve zero resultant stress or low resultant tensile stress in a multi-layer diaphragm structure. Influence of LPCVD deposition condition, substrate, film thickness, crystallized degree and pre-annealing on residual stress in LPCVD polysilicon films was studied. The polysilicon deposited on PSG substrate shows the lowest residual stress. The relationship between crystallized degree of polysilicon films and the film thickness was investigated with the aid of Raman Scattering Spectrometry. The residual stress shows a significant dependence on the film thickness because the crystallized degree raises with the film thickness. The test results show that: (1) for a thinner film (0.20 μm), even if a higher deposition temperature is used (630°C), its crystallized degree is still quite low and a quite higher residual tensile stress results in the film; and (2) for a thicker film (4 μm), even if an amorphous deposition temperature (580°C) is used, significant crystallization will still occur in as-deposited films and a residual tensile stress results in the films. The stress control test of highly boron doped polysilicon-oxide diaphragm structure was carried out. The result shows that the property and magnitude of the stresses in a highly boron doped polysilicon-oxide diaphragm can be arbitrarily changed in a certain range by varying the holding time of final annealing.

A practical algorithm for the efficient computation of eigenvector sensitivities

Abstract Derivatives of eigenvalues and eigenvectors have become increasingly important in the development of modern numerical methods for areas such as structural design optimization, dynamic system identification and dynamic control, and the development of effective and efficient methods for the calculation of such derivatives has remained to be an active research area for several decades. In this paper, a practical algorithm has been developed for efficiently computing eigenvector derivatives of generalized symmetric eigenvalue problems. For eigenvector derivative of a separate mode, the computation only requires the knowledge of eigenvalue and eigenvector of the mode itself and an inverse of system matrix accounts for most computation cost involved. In the case of two close modes, the modal information of both modes is required and the eigenvector derivatives can be accurately determined simultaneously at minor additional computational cost. Further, the proposed method has been extended to the case of practical structural design where structural modifications are made locally and the eigenderivatives of the modes concerned before are still of interest. By combining the proposed algorithm together with the proposed inverse iteration technique and singular value decomposition theory, eigenproperties and their derivatives can be very efficiently computed. Numerical results from a practical finite element model have demonstrated the practicality of the proposed method. The proposed method can be easily incorporated into commercial finite element packages to improve the computational efficiency of eigenderivatives needed for practical applications.

A new complex inverse eigensensitivity method for structural damping model identification

Abstract Damping plays an important role in the structural stability and vibration control. However, damping properties are the most difficult to model analytically but can only be revealed by vibration test. The work presented in this paper provides an effective method to identify a structural damping model by correlating the analytical mass and stiffness matrices which are developed by finite element methods and are assumed to contain modeling errors, with measured complex eigenvalues and eigenvectors of a test structure. The method employs both analytical and measured complex modal data to calculate the required eigensensitivity coefficients which are then used to identify the damping model of a structure based on complex inverse eigensensitivity formulation. The method is developed based on the practical assumption that the measured coordinates are incomplete and the identified damping matrix is physically meaningful and possesses the same physical connectivity as that of the stiffness matrix. Numerical examples are given to demonstrate the practical applicability of the proposed method and the results shown are very promising.

A study on micromachined bimetallic actuation

Abstract This paper presents a theoretical and experimental study of bimetallic actuation in micro-electro-mechanical systems (MEMS). The finite element method (FEM) has been used to predict and optimize the performance of bimetallic membranes, including maximum deflection, actuation force under definite temperature load, and their natural frequencies with fluidic (air or liquid) damping. The study involves micromachined pumps, valves and some other structures. Heat transfer analysis is also conducted to evaluate the temperature field distribution and the transient behavior of the microstructures. Some structures have been obtained for high displacements under low driving power, and also for quick response of bimetallic pumps. The results show that the experiments agree well with theoretical predictions.

A study on corrugated diaphragms for high-sensitivity structures

In this paper, the corrugated diaphragm technique is studied for applications to micromachined highly sensitive devices. Both static and dynamic behaviours of square corrugated diaphragms with various structural parameters and material properties are analysed using the finite-element method (FEM). The simulations show that very high performance (e.g., high mechanical sensitivity, small static deflection and high resonance frequency) of the diaphragms can be achieved using this technique. For the proposed corrugated diaphragms with anisotropically etched corrugations, much better reproducibility of the device performance can be expected. Different types of square diaphragm with rectangular corrugations have been fabricated using the silicon anisotropic etching technique and the optimized processes are presented. The measurements have shown reasonable agreement with the analyses. A capacitive pressure sensor is realized by the corrugation technique, with which sensitivity as high as 0.33% has been achieved.