Effect Of Random Excitation Research Articles

Effects of Random Excitations on the Dynamical Response of Duffing Systems

We study the dynamics of a Duffing oscillator excited by correlated random perturbations for both fixed and periodically modulated stiffness. In the case of fixed stiffness we see that Poincare map gets distorted due to the random excitation and, the distortion increases with the increase of correlation of the field. In a strongly correlated field, however, the map becomes purely random. We analyse the maximum value of the Lyapunov exponent and see that the random response competes with the chaotic motion to increase the stability of the system. In the case of periodically modulated stiffness, the periodic parametric excitation causes the Duffing system to execute dynamics of two fixed-point attractors. These attractors remain non-chaotic even in the presence of random field but can get merged due to induced fluctuation in the trajectory. It is seen that the random field can change the status of the system from transit to stable state.

Improving the read/write performance of hard disk drives under external excitation sources based on multi-objective optimization

Hard disk drives (HDDs) are sensitive devices responsible for storing and retrieving digital information in computer systems and electronic products. In particular, HDDs of laptop personal computers (LPCs) are very sensitive especially in harsh mechanical environments. Accordingly, it is necessary to protect them against damages attributable to shock and vibration so as to reach better magnetic read/write performance. In this paper, the nonlinear multi-objective optimization method is utilized to enhance the passive control performance of rubbers through minimizing destructive effects of shock and random excitations. As a matter of fact, this investigation will focus on improving the shock and vibration isolation system with special emphasis on the role of rubber feet. A zero mean, stationary process with Gaussian distribution is applied to the base in order to consider effects of random excitation. Moreover, a half-sine pulse of acceleration is assumed as the base shock excitation. According to limitations of geometrical parameters and lumped modal parameters, twelve inequality constraints are presented. Modal parameters of rubber mounts and rubber feet are selected as design variables of optimization, and the multi-objective optimization problem is solved for three commercial cases of HDD by using non-dominated sorting genetic algorithm II (NSGA-II), which was proposed based on Pareto-optimality concept. Therefore, new characteristics of rubber mounts and rubber feet are recommended to be manufactured for the purpose of protecting HDDs against shock and random excitations.

Covariance-driven wavelet technique for structural damage assessment

In this study, a wavelet-based covariance-driven system identification technique is proposed for damage assessment of structures under ambient excitation. Assuming the ambient excitation to be a white-noise process, the covariance computation is shown to be able to separate the effect of random excitation from the response measurement. Wavelet transform (WT) is then used to convert the covariance response in the time domain to the WT magnitude plot in the time-scale plane. The wavelet coefficients along the curves where energy concentrated are extracted and used to estimate the modal properties of the structure. These modal property estimations lead to the calculation of the stiffness matrix when either the spectral density of the random loading or the mass matrix is given. The predicted stiffness matrix hence provides a direct assessment on the possible location and severity of damage which results in stiffness alteration. To demonstrate the proposed wavelet-based damage assessment technique, a numerical example on a 3 degree-of-freedom (DOF) system and an experimental study on a three-story building model, which are all under a broad-band excitation, are presented. Both numerical and experimental results illustrate that the proposed technique can provide an accurate assessment on the damage location. It is however noted that the assessment of damage severity is not as accurate, which might be due to the errors associated with the mode shape estimations as well as the assumption of proportional damping adopted in the formulation.