مقالات رزومه:
دکتر محمد احمدی بلوطکی
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International Journal of Advanced Design and Manufacturing Technology, Volume:14 Issue: 4, Dec 2021, PP 37 -44The majority of underactuated systems are nonholonomic, due to non-integrable differential constraints. Therefore, controlling an underactuated system is considered as a challenging problem. In this study, an adaptive controller based on super-twisting sliding mode controller is proposed for a class of robust underactuated systems subjected to uncertainties and external disturbances. The adaptive compensator was designed so that there would be no need to the upper bound of the external disturbance. The controller parameters of adaptive sliding mode control are tuned based on a multi-objective non-dominated sorting of genetic optimization algorithm. The results of simulation and the demonstration of the effectiveness and applicability of the proposed scheme are presented.Keywords: Adaptive Sliding Mode Control, Multi-Objective Non-Dominated Sorting, Robust controller, Stability Analysis, Underactuated Systems
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International Journal of Advanced Design and Manufacturing Technology, Volume:14 Issue: 1, Mar 2021, PP 59 -71
In this paper, the problems arising from determining the modal properties of large and complex structures are investigated. For this purpose, the free interface component mode synthesis method has been used. In the following, Singular-Value Decomposition (SVD) is applied as a powerful mathematical tool to determine the appropriate coordinates to participate in the coupling process. Also, the effective error resources including modal shear error and the continuous systems overlapping error and their solution are introduced. Initially, a discrete system has been employed to investigate the free interface component mode synthesis method. Eventually, the studied main samples in this research are beam, plate and cylindrical shell. It is worth noting that the application of this method on the cylindrical shell has not been observed in previous researches.
Keywords: Component Mode Synthesis, Free Interface Method, Modal Analysis, Substructures, Singular-Value Decomposition Method -
In this study, torsional wave propagation is investigated in a rod that are made of one and two dimensional functionally graded material. Firstly, the governing equations of the wave propagation in the functionally graded cylinder derived in polar coordinate. Secondly, finite difference method is used to discretize the equations. The Von Neumann stability approach is used to obtain the time step size. Two states are assumed for material distribution, in first state it’s considered that the material variation occurred only in radial direction(Ti6A14V and Al2O3) and in second state the material properties vary in radial and length directions(BN, Al 1100, Ti6A14V and Al2O3). Moreover, the effect of cutoff frequency and boundary condition in wave propagation is studied. The results was validated by comparing the analytical and numerical solutions for an isotropic rode subjected to a torsional impulsive load. The results show that the torsional wave propagation in FGM rod evidently effects by material composition variation.Keywords: Functionally Graded Materials, Finite Difference Method, Wave Propagation, torsional wave
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International Journal of Advanced Design and Manufacturing Technology, Volume:13 Issue: 2, Jun 2020, PP 109 -117
In analytical studies, step (Heaviside) function is used to simulate an impact load. However, in real behaviour of materials, loading and unloading take a short time. The present study discusses analytically the effects of pulse shape and rising time of an impact load on dynamic stress intensity factor. Firstly, a pulse load with positive slip (linear and non-linear) is applied on a cracked plate and the amount of dynamic stress intensity factor on the crack tip is obtained. Then the effects of pulse time are discussed. Results show that increasing the rise time decreases the stress intensity factor because of reduction of inertia effects. Moreover, the duration of rise time plays the main role in dynamic stress intensity factor changes and how the variations are not matter.
Keywords: Dynamic Stress Intensity Factor, Fracture, Impact, Pulse Shape -
اندازه گیری نیروی های تحریک تصادفی سازه های در حین حمل و نقل تحت تحریک های جاده ای و یا ماهواره برهای پرتاب شده تحت تحریک های محیطی جهت شناسایی پارامترهای مودال امری غیرممکن است، از این رو در این پژوهش چالش و هدف ما تعیین خواص مودال سازه ها بدون داشتن نیروی تحریک ورودی و تنها با داشتن پاسخ ها می باشد. در پژوهش هایی که تاکنون در زمینه آنالیز مودال صورت گرفته، توجه کمتری به مقایسه روش های حوزه زمان و فرکانس به خصوص در تعیین میرایی شده است. همچنین تاکنون آنالیز حساسیتی بین تحریک های مختلف نویزسفید، جاروب سینوسی و بررسی تاثیر این تحریک ها بر روش های حوزه زمان و فرکانس آنالیز مودال کارکردی صورت نگرفته است. ابتدا این روش ها بر روی یک مدل گسسته پنج درجه آزادی (جرم، فنر و دمپر) پیاده سازی و صحه سنجی شده است و در نهایت بر روی یک سازه واقعی پیاده سازی صنعتی شده است. این پژوهش نشان می دهد که این روش های تجزیه قادر به استخراج پارامترهای مودال مودهای ابتدایی یک نمونه واقعی هستند. در پایان مقایسه ای بین نتایج به دست آمده صورت گرفته و کارایی هر یک از روش ها مورد ارزیابی قرار گرفته است.کلید واژگان: آنالیز مودال کارکردی, میرایی, حوزه زمان, شناسایی, حوزه فرکانسIt is impossible to measure the random excitation loads of structures during transportation under road excitations or satellite carriers thrown under environmental excitations in order to identify modal parameters; thus in this research, our challenge and purpose is to determine the modal properties of structures without having the input excitation load and, but only by having the response. In the studies have been performed so far in the field of modal analysis, less attention has been paid to comparing time and frequency methods, especially in the determination of damping. In addition, no sensitivity analysis between different excitations of white noise, sinusoidal sweep and the effect of these excitations on time and frequency domain methods of modal functional analysis have been performed so far. At first, these methods are implemented and validated on a 5-DOF discrete model (mass, spring, and damper), and finally they are industrially implemented on an actual structure. This paper shows that the decomposition methods are capable to extract the modal parameters of the primary modes of an actual sample. At the end, a comparison is made between the results and the effectiveness of each method is evaluated.Keywords: Operational Modal Analysis, Damping, Time Domain, Identification, Frequency Domain
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در این تحقیق عمل کرد روش های مختلف آنالیز مودال محیطی در حضور نیروهای تحریک هارمونیک مطالعه شده است. یک تیر تحت آزمون مودال تجربی قرار گرفته و پارامترهای مودال آن استخراج شده است. سپس آزمون های آنالیز مودال محیطی بر روی تیر تحت تحریک های هم زمان تصادفی و هارمونیک انجام گردیده است. استخراج پارامترهای مودال به روش های مختلف آنالیز مودال محیطی انجام شده و نتایج با پارامترهای حاصل از آنالیز مودال کلاسیک مقایسه شده است. در این تحلیل ها سعی شده اثر تحریک هارمونیک بر نتایج حذف گردد. نتایج این بررسی تجربی نشان می دهد که روش زیر فضای اتفاقی در مقایسه با روش های تجزیه حوزه فرکانس و تجزیه حوزه فرکانس بهبود یافته از دقت بالاتری برخوردار است. زمانی که میرایی سیستم در یکی از مودها کم باشد، روش های تجزیه حوزه فرکانس تخمین بهتری را ارائه می کند.کلید واژگان: آنالیز مودال کارکردی, مولفه هارمونیک, پارامترهای مودال, حوزه فرکانسIn the present research, performance of harmonic components identification and elimination techniques in practical operational modal data is investigated. Firstly, classical modal testing has been carried out on a cantilever steel beam and modal data are extracted. Four bending modes were available in the frequency band of interest. Afterwards, the same structure underwent operational modal tests with simultaneous random and harmonic excitation forces. In OMA, measurement data obtained from the operational responses are used to estimate the parameters of models that describe the system behavior. Extracted modal parameters from these data compared with baseline parameters. Extraction of modal parameters from operational data was fulfilled through different common OMA techniques. Each set of obtained results was evaluated in comparison with reference results. Accuracy and capability of each method in eliminating spurious modes caused by harmonic input components have been studied. For data analysis, in addition to developed MATLAB codes, commercial software of PULSE has been utilized in parallel as a tool for verification. The results of this survey demonstrate that the accuracy of the Stochastic Subspace Identification method is higher compared to Frequency Domain Decomposition and Enhanced Frequency Domain Decomposition methods. However, when the system has low modal damping, Frequency Domain Decomposition methods provide better estimates.Keywords: Operational Modal Analysis, Harmonic Component, Modal Parameters, Frequency Domain
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