فهرست مطالب

Journal of Theoretical and Applied Vibration and Acoustics
Volume:1 Issue: 1, Winter & Spring 2015

  • تاریخ انتشار: 1394/09/20
  • تعداد عناوین: 6
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  • Abdolreza Ohadi, Ali Reza Seyed Kanani Pages 1-9
    The aim of this paper is robust identification of smart foam, as an electroacoustic transducer, considering unmodeled dynamics due to nonlinearities in behaviour at low frequencies and measurement noise at high frequencies as existent uncertainties. Set membership estimation combined with model error modelling technique is used where the approach is based on worst case scenario with unknown but bounded uncertainties. The outcome is a robust identified model which consists of a nominal model with its uncertainty bounds that fits exactly the H_∞ robust control scheme which has been utilized in active noise control in recent years. While the nominal model has the desired physical characteristics as cut-off frequency and the anticipated slope and flatness before and after this frequency, respectively, it is maintained in the acceptably tight uncertainty upper and lower limits, thus validating the identification procedure. Looseness and tightness of uncertainty strip has also been discussed regarding nonlinearities and measurement noise in low and high frequency regions. Meanwhile the identified nominal model can also be utilized in non-robust noise control methods due to its lower order, reflecting the advantage of the applied identification approach.
    Keywords: Smart Foam, Set Membership Estimation, Model Error Modeling, Robust Identification
  • Ali Gholami, Farhang Honarvar, Hamid Abrishami Moghaddam Pages 10-20
    Time-Delay-Estimation (TDE) has been a topic of interest in many applications in the past few decades. The emphasis of this work is on the application of model-based estimation (MBE) for TDE of ultrasonic signals used in ultrasonic thickness gaging. Ultrasonic thickness gaging is based on precise measurement of the time difference between successive echoes which reflect back from the back wall of the test piece. The received echoes are modelled by Gaussian pulses and the desired system response is estimated using Gauss-Newton and Space Alternating Generalized Expectation Maximization (SAGE) algorithms. In addition to the model-based estimation approach, five other TDE techniques including peak-to-peak measurement, cross-correlation, cross-correlation with interpolation, phase-slope, and cross-correlation with Wiener filtering are also considered and compared with the SAGE. The main advantage of the SAGE algorithm, in addition to its higher accuracy, is its ability to deconvolve the overlapping echoes.
    Keywords: Ultrasonic, Time, Delay, Estimation, Model, Based, SAGE algorithm
  • Morteza Dardel, Sadeq Yaqubi, Hamidreza Mohammadi Daniali, Mohammad Hassan Ghasemi Pages 21-31
    In the current study, behavior of crank-slider mechanism with single and multiple clearance joints are analyzed. Using Lankarani-Nikravesh theory for estimating discontinuous contact forces in clearance joints, relevant systems have been mathematically modeled. Through numerical simulations, perturbations in response of mechanisms with clearance joints have been analyzed. Effects of increasing number of clearance joints have been addressed. From comparisons between responses of crank-slider mechanism with a single clearance joint and multiple clearance joints, it is concluded that perturbations intensify as the number of clearance joints in mechanism increases. Nonlinear dynamics of system are analyzed, using Poincare maps and bifurcation diagrams. Effects of joint friction on the response of the mechanism are investigated. Subsequently, a control scheme for providing continuous contact in clearance joints and maintaining a more stable mechanism is pro-posed. Obtained results demonstrate the effectiveness of proposed control method on reducing effect's of clearance and maintaining continuous contact in clearance joint.
    Keywords: Keywords: Multiple clearance joints, Joint friction, Nonlinear dynamics, Control
  • Mohammad Mehdi Meshki, Ali Salehzadeh Nobari Pages 32-40
    In this paper dynamic damping properties of a nominated flexible structural adhesive have been identified using an extended-direct modal based joint identification method. It has been revealed that damping characteristics of adhesive are correlated to both frequency and mode shape. Young’s and shear moduli increase with frequency but damping on the other hand, decrease. The results showed that mode shape has an important role on the dynamic mechanical properties of adhesive. Modes that dominantly create normal stress on the adhesion surface represent higher stiffness and lower loss factor compared to shear modes. The different level of loss factors between bending modes and shear modes are notable, but damping property of bending modes and shear modes on the other hand, is in the same order and decreases with frequency. It has been shown that the effective, viscoelastic, mechanical properties of the adhesive can be identified successfully, using the suggested method of identification.
    Keywords: identification, Dynamic Damping properties, Youngs, shear moduli, Experimental modal analysis
  • Constantin Daniel Comeaga, Cristinel Ilie, Marius Popa Pages 41-47
    The purpose of this paper is to present a new type of dynamic balancing system, having a driving solution of the rotating part based on magnetic interactions. The magnetic system also plays the role of an elastic bearing. In the first part of the article is presented the technical solution which allows the dynamic balancing evaluation depending on the radial displacement between two disks with permanent magnets, creating a magnetic coupling. It is presented the results obtained on the experimental way, that validated both the numerical simulation as well as the analytic calculation. Using a 2D model, the resultant magnetic force was analytically calculated, whose value depends on the misalignment of the balanced part, against the equilibrium position. Due to the specific geometry, to validate the 2D analytical calculation model, it was necessary to create a FEM model of the magnetic system. A simulation was performed to evaluate the dependence between the radial displacements and the magnetic forces. It was used a 3D simulation software, specific for these kind of problems - the INFOLYTICA software. The final results show that there is a similarity between 2D analytical calculation model, 3D simulation and practical measurements. In the second part of the article it is presented a practical application for this type of balancing system, used to build a two plane dynamic balancing machine for cardan shafts. It is also shown how it can be eliminated the disturbing unbalance introduced by the clamping system of the balanced part using a software method.
    Keywords: balancing system, magnetic interaction, perturbation removal
  • Mehdi Hashemi, Mohsen Asghari Pages 48-61
    In this paper, the flexural free vibrations of three dimensional micro beams are investigated based on strain gradient theory. The most general form of the strain gradient theory which contains five higher-order material constants has been applied to the micro beam to take the small-scale effects into account. Having considered the Euler-Bernoulli beam model, governing equations of motion are written by utilizing the Hamilton’s principle. Then, the state-space solution technique is used to find some solutions for natural frequencies of the beam under various boundary conditions. The numerical results show that the resonant frequencies are significantly dependent on the length scale parameter of the micro beam. The less the non-dimensional length scale is, the more deviation appears between results obtained for natural frequencies of micro shaft by strain gradient theory and classical continuum theory. Moreover, except for a micro shaft which is simply supported at both ends, the extra type of boundary conditions emerges from using strain gradient theory significantly affects the results.
    Keywords: Micro, beams, Strain gradient elasticity theory, Non, classical continuum theory, Size effect, Free vibrations