Optimal Control Applications and Methods (10991514)46(2)pp. 602-614
The present work revolves around employing the benefits of neighboring extremal approach in the nonlinear model predictive control of mechanical systems evolving on SE(3). In the NMPC process, necessary conditions of optimality are extracted based on some discrete-time version of the equations of motion referred as LGVI and the obtained TPBVP equations are solved using simplified sensitivity derivatives by means of an indirect shooting method. Taking into consideration that the repetitive optimization process of the NMPC is time consuming, making its implementation challenging, a method of neighboring extremal is proposed here for recalculating the responses of the systems in scenarios that face some unexpected changes in their parameters or are encountering some last-minute re-planning schemes. Based on the existing responses of the system to the first set of initial conditions, the reaction of the system to the altered set of initial conditions can be reconstructed without the need to solve the whole optimization process from scratch by utilizing the features of the NE method. A spacecraft model evolving on SE(3) with actuation constraints confirms the efficiency of the whole process in term of accuracy versus computation burden. Forasmuch as actuator failure is a probable yet important event in aerial/spatial missions, the performance of the control system in dealing with such situations is examined. The algorithm robustness and its capability to compensate the effects of the actuator loss is checked. © 2024 John Wiley & Sons Ltd.
International Journal of Machine Learning and Cybernetics (1868808X)
This study presents a novel method for predicting the dynamic behavior of a system using a type of recurrent neural network as a state observer. This neural observer, which is trained with input–output data, is implemented on a washing machine. The problem under investigation is the detection of a coupled imbalanced state in washing machines, which is a difficult challenge. The proposed detection method in this study is named Nonlinear Auto-regressive exogenous Deep Deterministic Policy Gradient (NAR3PG). It has been shown that converting time series to images and teaching images to a convolutional neural network has acceptable results close to those of NAR3PG. Because of its inherent features and good results in detecting balance and coupled imbalance states, the NAR3PG neural method is the most suitable neural method used in this study. However, one of its main disadvantages is the long training time in the presence of many observations of the past, but it is the simplest method for converting the NAR3PG neural method to hardware code. © The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2025.
Haptic interface devices help users to have a better perception when they interact with a virtual reality (VR) environment. In this context, ungrounded torque and force feedback devices can greatly enhance users' experience by transferring physical interactions between the virtual environment and reality in a more tangible way. This article introduces a handheld haptic device that uses two propellers to apply feedback torques and forces to users' hands, aiming to provide a more immersive experience. The utilization of a servo motor is considered in the design of this haptic device to make the rotation of propellers around the roll axis possible, eliminating the dependence of generated torques and forces direction on users' hand roll angle. Powered by a LiPo battery and communicating with the VR environment through Bluetooth, the device has no physical connections to the external environment. An experiment was conducted to evaluate the device's potential as a haptic device by comparing its immersion accuracy to an identical real setup. As the results demonstrate, this haptic device can offer participants a sense of the virtual environment, very close to reality. © 2023 IEEE.
Amirkabir Journal of Civil Engineering (2588297X)54(12)pp. 4729-4750
Advances in Nano Research (2287237X)15(5)pp. 467-484
Despite the significant features of fiber-reinforced cementitious composites (FRCCs), including better mechanical, fractural, and durability performance, their high content of cement has restricted their use in the construction industry. Although ground granulated blast furnace slag (GGBFS) is considered the main supplementary cementitious material, its slow pozzolanic reaction stands against its application. The addition of nano-sized mineral modifiers, including nano-silica (NS), is an alternative to address the drawbacks of using GGBFS. The main object of this empirical and numerical research is to examine the effect of NS on the strain-hardening behavior of cementitious composites; ten mixes were designed, and five levels of NS were considered. This study proposes a new method, using a four-point bending test to assess the use of nano-silica (NS) on the flexural behavior, first cracking strength, fracture energy, and micromechanical parameters including interfacial friction bond strength and maximum bridging stress. Digital image correlation (DIC) was used for monitoring the initiation and propagation of the cracks. In addition, to attain a deep comprehension of fiber/matrix interaction, scanning electron microscope (SEM) analysis was used. It was discovered that using nano-silica (NS) in cementitious materials results in an enhancement in the matrix toughness, which prevents multiple cracking and, therefore, strain-hardening. In addition, adding NS enhanced the interfacial transition zone between matrix and fiber, leading to a higher interfacial friction bond strength, which helps multiple cracking in the composite due to the hydrophobic nature of polypropylene (PP) fibers. The findings of this research provide insight into finding the optimum percent of NS in which both ductility and high tensile strength of the composites would be satisfied. As a concluding remark, a new criterion is proposed, showing that the optimum value of nano-silica is 2%. The findings and proposed method of this study can facilitate the design and utilization of green cementitious composites in structures. © 2023 Techno-Press, Ltd.
Robotica (02635747)41(4)pp. 1313-1334
The issue of implementing nonlinear model predictive control (NMPC) on mechanical systems evolving on special orthogonal group (SO(3)) is taken into consideration in the first place. Necessary conditions of optimality are extracted based on Lie group variational integrators, leading to a two-point boundary value problem (TPBVP) which is solved using sensitivity derivatives and indirect shooting methods. Fast Newton-like methods referred to as fast solvers which are commonly used to solve the TPBVP are established based on the repetition of a nonlinear process. The numerical schemes employed to alleviate the computation burden consist of eliminating some constraint-related but non-essential terms in the trend of sensitivity derivatives calculation and for solving the TPBVP equations. As another claim, assuming that a first attempt to resolve the NMPC problem is accessible, the problem subjected to some changes in its initial conditions (due to some re-planning schemes) can be resolved cost-effectively based on it. Instead of solving the whole optimization process from the scratch, the optimal control inputs and states of the system are updated based on the neighboring extremal (NE) method. For this purpose, two approaches are considered: applying NE method on the first solution that leads to a neighboring optimal solution, or assisting this latter by updating the NMPC-related optimization using exact TPBVP equations at some predefined intermediate steps. It is shown through an example that the first method is not accurate enough due to error accumulations. In contrast, the second method preserves the accuracy while reducing the computation time significantly. © The Author(s), 2023. Published by Cambridge University Press.
In this research, the problem of controlling a one-link robot with joint flexibility by the non-linear model predictive control method (NMPC) is considered. The issue concerning the input-to-state stability (ISS) of the NMPC has been considered. Through using the cost function of the NMPC problem in order to play the role of the Lyapunov function, the ISS stability of the system in the presence of disturbances and uncertainties, such as robot joint flexibility, is reached. Two modeling examples for a single-link robot have been investigated, which include unmeasurable variables as a portion of the system state variables. These unmeasured states are representative of the unmodeled dynamics, standing for either the flexibility at the joint or the link itself. In the first example, the control input is the actuation torque, and in the second one, the voltage to the motor. Simulation results demonstrate the effectiveness and stability of the proposed approach in the presence of disturbances and system uncertainties. © 2023 IEEE.
International Journal of Robotics and Automation (19257090)37(3)pp. 280-287
In this paper, we address the online smooth motion modulation for hybrid cyclic motions. For this purpose, we present an architecture for online trajectory generation from a library of desired periodic trajectories including impacts. The proposed architecture is a Central Pattern Generator (CPG) consisting of a synchronized network of multiple novel oscillatory systems. The proposed CPG provides limit cycle tracking of the desired multidimensional periodic trajectory from any initial condition irrespective of the CPG parameters. The proposed architecture is applicable for hybrid motions including impacts and continuous motions. The soundness of the proposed architecture is verified by numerical simulation on IIWA Kuka arm in a hitting drum scenario. © 2022 Acta Press. All rights reserved.
Advances in Materials Research (South Korea) (2234179X)11(4)pp. 351-374
Softening function is the primary input for modeling the fracture of concrete when the cohesive crack approach is used. In this paper, based on the laboratory data on notched beams, an inverse algorithm is proposed that can accurately find the softening curve of the concrete. This algorithm uses non-linear finite element analysis and the damage-plasticity model. It is based on the kinematics of the beam at the late stages of loading. The softening curve, obtained from the corresponding algorithm, has been compared to other softening curves in the literature. It was observed that in determining the behavior of concrete, the usage of the presented curve made accurate results in predicting the peak loads and the load-deflection curves of the beams with different concrete mixtures. In fact, the proposed algorithm leads to softening curves that can be used for modeling the tensile cracking of concrete precisely. Moreover, the advantage of this algorithm is the low number of iterations for converging to an appropriate answer. © 2022 Techno-Press, Ltd.
Multibody System Dynamics (13845640)56(3)pp. 189-219
Despite extensive studies on motion stabilization of bipeds, they are still stymied by the lack of disturbance coping capability on slippery surfaces. In this paper, a novel controller for stabilizing a bipedal motion in its sagittal plane is developed with regard to the surface friction limitations. By taking into account the physical limitation of the surface in the stabilization trend, a more advanced level of reliability is achieved that provides higher functionalities such as push recovery on low-friction surfaces and prevents the stabilizer from overreacting. The discrete event-based strategy consists of modifying the step length and the time period at the beginning of each footstep in order to reestablish the necessary stability conditions while taking into account the surface friction limitation as a constraint to prevent slippage. Adjusting footsteps to prevent slippage in confronting external disturbances is perceived as a novel strategy for keeping stability, quite similar to human reaction. The developed methodology consists of rough closed-form solutions utilizing elementary mathematical operations to obtain the control inputs, allowing to reach a balance between convergence and computational cost, which is quite suitable for real-time operations even with modest computational hardware. Several numerical simulations, including push recovery and switching between different gaits on low-friction surfaces, are performed to demonstrate the effectiveness of the proposed controller. In correlation with human-gait experience, the results also reveal some physical aspects favoring stability and the fact of switching between gaits to reduce the risk of falling in confronting different conditions. © 2022, The Author(s), under exclusive licence to Springer Nature B.V.
In the present work, nonlinear model predictive control of a spacecraft equipped with reaction wheels as the actuators that generate the driving torque for the system is studied. The spacecraft body is also exposed to external disturbance originated from the existence of off-center in the system structure. The system configuration manifold is Lie group SO(3). Discrete equations of motion of the system, referred to as Lie group variational integrators and the NMPC formulation are much more complicated due to the terms added to the equations considering the effects of speed of the wheels and the angular momentum exchanges between the wheels and the spacecraft body and also due to the effects of external disturbances. However, the model of the system and consequently, the obtained results are much more realistic in return. The necessary conditions of optimality are extracted and solved using indirect shooting method based on sensitivity derivatives. Since the equations of motion are established based on Lie group properties, the geometric structure of the system is preserved in long time integration. In addition, the extracted equations are symplectic and momentum preserving and show good energy behavior. The system of a spacecraft with three reaction wheels with the off-center in the z-direction which impose external disturbance to the system is simulated using the extracted equations for NMPC control of spacecraft on SO(3) and the simplification of sensitivity equations is used to reduce the computation time of optimizations. The obtained results show that our method is able to bring the system to the zero position even when there is an off-center in the yaw direction of the spacecraft by structure. © 2022 IEEE.
Mechanical Systems and Signal Processing (08883270)164
A stochastic interpretation of the stick/slip mechanism is proposed to model and mitigate the complicated friction interaction that occurs between surfaces, with direct implications on the stability of dynamic mechanical systems involved. Most investigations dealt with the estimation of such stability regions in a deterministic framework. A realistic approach is to consider random aspects in the analysis of complex dynamic behaviors. In the present article, a stick–slip nonlinear model possessing multiple periodic solutions is considered under fluctuating random excitations. The time evolution of the probability density function is studied by employing an adaptive path integration method and then compared through Monte-Carlo simulations. A parametric study is performed on characteristics such as noise intensity and initial distribution. The physical example employed will permit to assume the entrance of noise through a first-order linear filter. Results reveal that the randomness factor extremely affects the probability of occurrence of each solution, justifying the application of such analyses in reliability-related studies and situations where sensitivity to randomness is acute. © 2021 Elsevier Ltd
Structural Engineering and Mechanics (12254568)81(5)pp. 575-589
In order to enhance the greenness in the strain-hardening composites and to reduce the high cost of typical polyvinyl alcohol fiber reinforced engineered cementitious composite (PVA-ECC), an affordable strain-hardening composite with green binder content has been proposed. For optimizing the strain-hardening behavior of cementitious composites, this paper investigates the effects of polypropylene fibers on the first cracking strength, fracture properties, and micromechanical parameters of cementitious composites. For this purpose, digital image correlation (DIC) technique was utilized to monitor crack propagation. In addition, to have an in-depth understanding of fiber/matrix interaction, scanning electron microscope (SEM) analysis was used. To understand the effect of fibers on the strain hardening behavior of cementitious composites, ten mixes were designed with the variables of fiber length and volume. To investigate the micromechanical parameters from fracture tests on notched beam specimens, a novel technique has been suggested. In this regard, mechanical and fracture tests were carried out, and the results have been discussed utilizing both fracture and micromechanical concepts. This study shows that the fiber length and volume have optimal values; therefore, using fibers without considering the optimal values has negative effects on the strain-hardening behavior of cementitious composites. Copyright © 2022 Techno-Press, Ltd.
In this paper, nonlinear model predictive control of mechanical systems evolving on Lie group SE(3) is studied. Discrete-time equations of motion referred to as LGVI are used in order to extract the necessary conditions of optimality. Then, TPBVP equations are solved using sensitivity derivatives to find Lagrange multipliers. A fast solver for NMPC is used to solve the problem. Extracting necessary conditions of optimality based on LGVI leads to group structure preservation and good convergence behavior of the system under study. Subsequently, some modifications based on removing non-essential nonlinear terms are applied to sensitivity derivatives and TPBVP equations, aiming to reduce the computation time of solving the NMPC problem. The presented method is applied on a spacecraft evolving on SE(3) with constraints as an example and the simulation results show the efficiency of the proposed method. Also, it is shown that introducing the aforementioned modifications reduces the computation time by a considerable amount. © 2021 IEEE.
International Journal of Dynamics and Control (2195268X)9(3)pp. 872-884
Manipulation and secure transportation of loads by robotic manipulators along predetermined paths is still an open challenge in the robotics field. In this article, the problem of pushing and driving an object on a surface employing a robot to reach a specific destination is considered under a-priori unknown friction conditions. For this purpose, the equations of motion are derived according to the limit surface conceptualization of friction force. Although friction parameters may be considered locally constant, nonetheless they gradually vary over large surfaces. The friction term is thus faced with a slowly time-varying indeterminacy and needs an online estimation scheme. By defining a mission consisting of pushing an object via a robot to find a path among obstacles, an optimum path is firstly planned then a non-linear model predictive controller is used to track the desired path. In order to alleviate the indeterminacy due to friction, the corresponding terms in the equations of motion are set as a disturbance which is estimated by a purposely designed observer and then eliminated in a feedforward manner. There is also no need to perform preliminary offline tests. © 2020, Springer-Verlag GmbH Germany, part of Springer Nature.
International Journal of Non-Linear Mechanics (00207462)131
In this study, the stochastic dynamic response of a spur gear pair model under the excitation of filtered noise is investigated. The spur gear pair is modeled as a single degree of freedom (SDOF) system in which nonlinear and non-smooth backlash and time-varying mesh stiffness as well as stochastic excitation are concurrently considered. Four cases are addressed, based on how the noise is incorporated in the loading terms. A first order filter is employed to generate various filtered noises with the same energy but different power spectrum. The combination of the SDOF gear model and the shaping filter leads to a (3D) Markov model. The numerical path integration (PI) technique is adopted to obtain the probabilistic response of the gear model using an adaptive time-stepping method in order to increase the accuracy of the time integration. A 4D system is also considered by applying a second order filter to model a narrow-band noise. The results are verified by comparing with Monte Carlo simulations. The effect of the noise spectrum on the probabilistic response is evaluated for different loading cases. The response PDF is of key importance in relation to reliability assessment of gear systems. © 2021 Elsevier Ltd
Digital-enabled manufacturing systems require a high level of automation for fast and low-cost production but should also present flexibility and adaptiveness to varying and dynamic conditions in their environment, including the presence of human beings; however, this presence of workers in the shared workspace with robots decreases the productivity, as the robot is not aware about the human position and intention, which leads to concerns about human safety. This issue is addressed in this work by designing a reliable safety monitoring system for collaborative robots (cobots). The main idea here is to significantly enhance safety using a combination of recognition of human actions using visual perception and at the same time interpreting physical human–robot contact by tactile perception. Two datasets containing contact and vision data are collected by using different volunteers. The action recognition system classifies human actions using the skeleton representation of the latter when entering the shared workspace and the contact detection system distinguishes between intentional and incidental interactions if physical contact between human and cobot takes place. Two different deep learning networks are used for human action recognition and contact detection, which in combination, are expected to lead to the enhancement of human safety and an increase in the level of cobot perception about human intentions. The results show a promising path for future AI-driven solutions in safe and productive human–robot collaboration (HRC) in industrial automation. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
Mechanical Systems and Signal Processing (08883270)139
The steady state response of a particular harvesting system is investigated under the condition of external and internal resonance, with emphasis on the double jump phenomenon. Following the trend of recent researches on exploring nonlinearity aspects for broadening the frequency resonance bandwidth, the problem of effective energy harvesting from a broadband source is dealt with. The proposed harvester can be embedded within vibrational structures and can dually act as a vibration absorber. The governing non-linear partial differential equations are truncated into a set of perturbed equations via Galerkin method. The method of multiple scales is applied to derive the modulated amplitude versus frequency at the vicinity of flapwise and chordwise primary resonances, as well as around other internal resonance frequencies. The amplitude-frequency response plot reveals resonance peaks bending to the left and right, i.e., splitting into two different tongues in contrast to conventional jumps which lean only toward higher or lower frequency directions. Numerical results demonstrate that this internal resonance-based harvesting design can produce sufficient power for the consumption of typical MEMS devices. © 2020 Elsevier Ltd
International Journal of Engineering, Transactions A: Basics (17281431)32(4)pp. 608-616
In this article, a fast and reliable map-merging algorithm is proposed to produce a global two dimensional map of an indoor environment in a multi-robot simultaneous localization and mapping (SLAM) process. In SLAM process, to find its way in this environment, a robot should be able to determine its position relative to a map formed from its observations. To solve this complex problem, simultaneous localization and mapping methods are required. In large and complex environments, using a single robot is not reasonable because of the error accumulation and the time required. This can explain the tendency to employ multiple robots in parallel for this task. One of the challenges in the multi-robot SLAM is the map-merging problem. A centralized algorithm for map-merging is introduced in this research based on the features of local maps and without any knowledge about robots initial or relative positions. In order to validate the proposed merging algorithm, a medium scale experiment has been set up consisting of two heterogeneous mobile robots in an indoor environment equipped with laser sensors. The results indicate that the introduced algorithm shows good performance both in accuracy and fast map-merging. © 2019 Materials and Energy Research Center. All rights reserved.
International Journal Of Civil Engineering (17350522)17(5)pp. 597-605
This paper deals with the challenging problem of predicting the load carrying capacity of reinforced concrete shear-critical beams. To simulate the cracking behavior of concrete, the discrete crack approach based upon non-linear fracture mechanics is used. An algorithm with two pathways of implementation has been proposed so as to implement fictitious crack model to analyze reinforced concrete beams using finite-element method. The merit of the proposed algorithm is its capability to recognize and to incorporate them in the analysis procedure. The proposed method is capable of predicting simultaneous multiple shear cracks, load-deformation behavior, and ultimate shear capacity of reinforced concrete beam. The obtained results show a good compliance with the available experimental benchmark studies on reinforced concrete beams failed in shear. As one of the important aspects of shear capacity is the size effect issue, some large-scaled test beams have been numerically simulated using the proposed algorithm and the results have been compared with that of ACI 318-11 building code as well as the well-known modified compression field theory (MCFT). The comparison corroborated the robustness of the proposed algorithm in detecting the well-known shear-scaling phenomenon. In the same time, the comparison emphasized on the weakness of the current codes of practice to overestimate the shear capacity of large-scaled beams owing to not taking the size effect into account. © 2018, Iran University of Science and Technology.
Journal of Sound and Vibration (10958568)447pp. 170-185
This study is purposed to enhance conventional spur gear models by moving from a deterministic to a stochastic representation, based on adding a Gaussian white noise in the loading terms while including the effect of both backlash and time-varying mesh stiffness. Meanwhile, an efficient numerical path integration (PI) technique is used to obtain the probability distribution functions (PDFs) of the response. A novel adaptive time-stepping scheme is proposed for the PI method that momentarily increases its accuracy in order to detect the edge of transition when dealing with the non-smoothness due to backlash. The results demonstrate good agreement with Monte Carlo simulation (MCS). It is observed that if the initial distribution covers a region containing the basins of attraction of coexisting solutions, the stationary PDF will be formed around both of their deterministic solutions. The resulting response PDF contains subtle information about the system, which is essential in the design of its parts and its reliability assessment. The generality of the approach is such that the new path integration method can be implemented for the stochastic analysis of other non-smooth random dynamical systems. © 2019 Elsevier Ltd
Journal of Sound and Vibration (10958568)422pp. 300-317
The robust attitude and vibration control of a flexible spacecraft trying to perform accurate maneuvers in spite of various sources of uncertainty is addressed here. Difficulties for achieving precise and stable pointing arise from noisy onboard sensors, parameters indeterminacy, outer disturbances as well as un-modeled or hidden dynamics interactions. Based on high-order sliding-mode methods, the non-minimum phase nature of the problem is dealt with through output redefinition. An adaptive super-twisting algorithm (ASTA) is incorporated with its observer counterpart on the system under consideration to get reliable attitude and vibration control in the presence of sensor noise and momentum coupling. The closed-loop efficiency is verified through simulations under various indeterminate situations and got compared to other methods. © 2018 Elsevier Ltd
Molecular Physics (00268976)116(13)pp. 1659-1669
This research focuses on numerically investigating the self-diffusion coefficient and velocity autocorrelation function (VACF) of a dissipative particle dynamics (DPD) fluid as a function of the conservative interaction strength. Analytic solutions to VACF and self-diffusion coefficients in DPD were obtained by many researchers in some restricted cases including ideal gases, without the account of conservative force. As departure from the ideal gas conditions are accentuated with increasing the relative proportion of conservative force, it is anticipated that the VACF should gradually deviate from its normally expected exponentially decay. This trend is confirmed through numerical simulations and an expression in terms of the conservative force parameter, density and temperature is proposed for the self-diffusion coefficient. As it concerned the VACF, the equivalent Langevin equation describing Brownian motion of particles with a harmonic potential is adapted to the problem and reveals an exponentially decaying oscillatory pattern influenced by the conservative force parameter, dissipative parameter and temperature. Although the proposed model for obtaining the self-diffusion coefficient with consideration of the conservative force could not be verified due to computational complexities, nonetheless the Arrhenius dependency of the self-diffusion coefficient to temperature and pressure permits to certify our model over a definite range of DPD parameters. © 2018 Informa UK Limited, trading as Taylor & Francis Group.
Robotica (02635747)36(4)pp. 588-606
This paper presents a model-based controller consisting of a feedback linearization scheme and a state-dependent proportional derivative (PD) controller adapted to a parallel flight simulator Stewart mechanism. This parallel robot is considered to emulate motions of highly maneuverable aircrafts, which require well-trained pilots. The simulations are based upon a reduced-model prototype built in order to verify kinematic design aspects and control laws. Indeterminacies in the mass distribution of the system will generally affect model-based controllers, necessitating compensation or the employment of robust control methods. Through introducing the pilot's sensorial feedback of acceleration, the pilot's behavior in giving commands is emulated via an optimization process, which tunes the controller coefficients accordingly. Stability of the designed control system is guaranteed via the Lyapunov approach. To further explore the system through perilous flight scenarios, three pre-designed maneuvers are selected as test cases. It is expected that closed-loop control tasks in which a pilot tracks a target, while at the same time the controller rejects disturbances and adapts itself to the pilot's progressive skills, are ameliorated through this arrangement. Numerical results show that the proposed method is found robust in the training process in conditions of parameters indeterminacy. © 2017 Cambridge University Press.
International Journal of Mechanical Sciences (00207403)142pp. 191-215
This paper deals with the induced instability due to parametric resonance of rectangular plates traversed by inertial loads and lying on elastic foundations. The extended Hamilton's principle is employed to derive the partial differential equation associated with the transverse motion of the plate. Subsequently, this equation is transformed into a set of ordinary differential equations by the Galerkin method. Including vertical, centripetal and Coriolis acceleration terms related to the moving mass transition in the analysis leads to governing equations with time-varying mass, damping and stiffness coefficients. Particularly, the intermittent passage of masses along rectilinear paths, or the motion of an individual mass along an orbiting path, permits to subcategorize the problem as a parametrically excited system with periodic coefficients. By applying the incremental harmonic balance (IHB) method, the stability of the induced plate vibrations is investigated, revealing an emersion of instability tongues in the parameters plane. Semi-analytical results are provided for various boundary conditions of the plate which got verified through direct numerical simulations and other results reported in the literature. © 2018 Elsevier Ltd
Nonlinear Dynamics (0924090X)89(3)pp. 2141-2154
In this paper, the dynamic stability of a simply supported beam excited by the transition of circulating masses is investigated by preserving nonlinear terms in the analysis. The intermittent loading across the beam results in a time-varying periodic equation. The effects of convective mass acceleration besides large deformation beam theory are both considered in the derivation of governing equations which is performed through adopting a variable-mass-system approach. In order to deal with the coupling between longitudinal and transversal deflections, the inextensibility assumption is implicitly introduced into the Hamiltonian formulation to reduce the model order. An appropriate interpretation is presented in order to maintain this approximation reasonable. Different semi-analytical methods are implemented to find the domains of stability and instability of the problem in a parameter space. By accounting the non-autonomous form of the governing equations, a qualitative change in behavior due to nonlinear terms is demonstrated which has not been addressed in previous studies. © 2017, Springer Science+Business Media Dordrecht.
Acta Mechanica (16196937)227(4)pp. 1213-1224
The problem of an elastic beam under the periodic loading of successive moving masses is investigated as a pragmatic case for studying dynamic stability of linear time-varying systems. This model serves to highlight the odds of multi-solutions coexistence, a form of hidden instability which reveals dangerous as it may be precipitated by the slightest disturbance or variation in the model. Since no engineering model perfectly represents a physical system, such situations for which Floquet theory naively predicts stability are potentially inevitable. The harmonic balancing method is used in order to thoroughly explore the stability diagrams for detecting these instability gaps. Although this phenomenon has also been described in other physical systems, it has not been addressed for beam–moving mass systems. This result may find particular importance in applications involving self-induced vibrations of elastic structures and hence also appears of practical relevance. © 2016, Springer-Verlag Wien.
Robotica (02635747)33(9)pp. 2001-2024
In this research, a Stewart parallel platform with rotary actuators is simulated and a prototype is tested under different operative conditions. The purpose is to make the robot robust against inertia variations considering the fact that different payloads of unknown size may be transported. Due to the complexity issued by expressing the equations of motion with independent variables, the governing equations are derived by Lagrange's method using Lagrange multipliers for imposing the kinematic constraints imposed on this parallel robot. Eliminating Lagrange multipliers by projecting the equations onto the orthogonal complement of the space of constraints, the equations of motion are transformed to a reduced form suitable for the purpose of controller design. The control approach considered here is based on a neuro-fuzzy interference method. As a first step, each revolute arm link are individually trained under different loadings and diverse maneuvers. It is purposed that once employed together, the links will have learned how to collaborate with each others for performing a common task. Training data are divided to several clusters by using a subtractive clustering algorithm. For every cluster, a fuzzy rule is derived so that the output follows the desired trajectory. In the last stage, these rules are employed by utilizing back propagation algorithms and the effectiveness of the neuro-fuzzy system becomes approved by performing multiple maneuvers and its robustness is checked under various inertia loads. The controller has ultimately been implemented on a prototype of the Stewart mechanism in order to analyze the reliability and feasibility of the method. Copyright © Cambridge University Press 2014.
Acta Mechanica (16196937)226(4)pp. 1241-1253
A Timoshenko beam excited by a sequence of identical moving masses is studied as a time-varying problem. The effects of centripetal and Coriolis accelerations besides the vertical component of acceleration of the moving mass are considered. Using Galerkin procedure, the partial differential equations of motion which are derived by Hamilton’s principle are transformed to ordinary differential equations. The incremental harmonic balance method is implemented to determine the boundary curve of instability and other companion curves of resonance in the parameter plane. A new approach for identifying the conditions of resonance is investigated by presenting an intuitive definition of resonance for time-varying systems. The influence of employing different deformation theories on the critical parameter values of stability and resonance curves is studied. The validity of the instability and resonance curves is examined by numerical simulations and also ascertained through comparing with those reported in the literature. © 2014, Springer-Verlag Wien.
Journal of Vibroengineering (13928716)16(6)pp. 2779-2789
In this paper, the dynamic stability analysis of a simply supported beam excited by a sequence of moving masses is investigated. All components of the mass acceleration including the centripetal, the Coriolis and the vertical one are considered. The periodical traverse of masses across the beam results to a linear time-periodic problem. The Floquet theory and the Incremental Harmonic Balance (IHB) method are implemented to obtain the boundary between stable and unstable regions in the parameters plane. A new approach for identifying the conditions of resonance is investigated by presenting an intuitive definition of resonance for time-varying systems. This approach enables the IHB method to determine inherent curves of resonance conditions besides its ability to find the boundary curve separating the stable and unstable regions. Numerical simulations confirm the correctness of resulted curves. © JVE INTERNATIONAL LTD. JOURNAL OF VIBROENGINEERING 2014.
Nonlinear Dynamics (0924090X)77(3)pp. 699-727
The flexural vibration of a symmetrically laminated composite cantilever beam carrying a sliding mass under harmonic base excitations is investigated. An internally mounted oscillator constrained to move along the beam is employed in order to fulfill a multi-task that consists of both attenuating the beam vibrations in a resonance status and harvesting this residual energy as a complementary subtask. The set of nonlinear partial differential equations of motion derived by Hamilton's principle are reduced and semi-analytically solved by the successive application of Galerkin's and the multiple-scales perturbation methods. It is shown that by properly tuning the natural frequencies of the system, internal resonance condition can be achieved. Stability of fixed points and bifurcation of steady-state solutions are studied for internal and external resonances status. It results that transfer of energy or modal saturation phenomenon occurs between vibrational modes of the beam and the sliding mass motion through fulfilling an internal resonance condition. This study also reveals that absorbers can be successfully implemented inside structures without affecting their functionality and encumbering additional space but can also be designed to convert transverse vibrations into internal longitudinal oscillations exploitable in a straightforward manner to produce electrical energy. © 2014 Springer Science+Business Media Dordrecht.
Journal of Aerospace Engineering (08931321)27(2)pp. 249-261
In this research, the applicability and feasibility of using a spherical-shaped platform as an experimental facility that emulates the dynamics of on-orbit conditions in the laboratory environment is investigated. Indeed, this will permit evaluation of path planning and feedback control algorithms for performing precise satellite maneuvers on an actual system in local situ. The correct operation of this system requires that rolling motion be maintained; nonetheless, some practical issues may occur because of passage to surfaces with different roughnesses, causing an irregular transient performance from a sliding to pure rolling state or vice versa. Noting that an impulse in momentum is verified on reaching the separating limits of those regions, nevertheless the designed attitude controller is observed to be able to reestablish its function despite such disturbances and change of operational conditions. This study is a preliminary step toward final concretization by the build of a prototype in the near future. © 2014 American Society of Civil Engineers.
Journal of Mechanics in Medicine and Biology (17936810)14(2)
Atherosclerosis, as the leading cause of mortality, is usually regarded as a systemic disease and several well-identified risk factors have been implicated in its pathogenesis. Low or highly oscillatory wall shear stress has mainly been linked to the development of atherosclerosis. Conditions under which human blood can be considered Newtonian for the purpose of arterial flow modeling are investigated with emphasis on near wall shear stresses. The Lattice Boltzmann method is implemented in parallel for both Newtonian and non-Newtonian models of blood and then examined in the context of steady and oscillatory flows. As the lattice method permits to adjust the morphology of the computational domain during the solving process, the artery walls are reshaped in a recursive manner by the progressive accumulation of deposits according to the conventional OSI criterion. Regions subjected to partial obstructions identified qualitatively well with those susceptible to atherosclerosis in the in vivo sample, thereby approving this criterion by verifying its accumulative effect. The present work demonstrates the suitability of LB method for studying flows across geometries that transform due to atherosclerotic progression and permits to explain the trend of deposit distribution across time. © 2014 World Scientific Publishing Company.
Berahmand, A.A.,
Panahi, A.G.,
Sahabi, H.,
Feizi, H.,
Moghaddam, P.R.,
Shahtahmassebi, N.,
Fotovat, A.,
Karimpour, H.,
Gallehgir, O. Biological Trace Element Research (15590720)149(3)pp. 419-424
Two experiments were done in 2008 and 2009 to study the effects of magnetic field and silver nanoparticles on fodder maize (Zea mays L.). These experiments were done with seven treatments based on a randomized complete block design in four replications. The treatments were as follows: magnetic field and silver nanoparticles+Kemira fertilizer (T1), magnetic field and silver nanoparticles+Humax fertilizer (T2), magnetic field and silver nanoparticles (T3), Kemira fertilizer (T4), Librel fertilizer (T5), Humax fertilizer (T6), and a control (T7). Results showed that fresh yield was higher in treatments T3 and T4. Treatments T3 and T4 had increased maize fresh yields of 35 and 17.5 % in comparison to the control, respectively. The dry matter yield of those plants exposed to magnetic field and silver nanoparticles was significantly higher than that from any of the other treatments. Magnetic field and silver nanoparticle treatments (T3 and T1) showed higher percentages for ears, and the lowest percentages were found in treatments T7 and T5. In general, the soil conditions for crop growth were more favorable in 2009 than in 2008, which caused the maize to respond better to treatments tested in the study; therefore, treatments had more significant effects on studied traits in 2008 than in 2009. © Springer Science+Business Media, LLC 2012.
Nonlinear Dynamics (0924090X)69(4)pp. 2221-2235
Equations of motion for a special system pertaining to the class of mixed nonholonomic mechanical systems are studied in the modern setting of geometric mechanics. The presented attitude control test bed is intended to provide an experimental facility that, in certain senses, emulates the dynamics of on-orbit conditions in the laboratory site, allowing the evaluation of path planning and feedback control algorithms. This paper demonstrates the feasibility of the approach and proposes a concurrent solution to the attitude tracking control problem that, due to uncertainties of the parameters, is likely to require effective adaptive aptitudes. Moreover, the invariance with respect to Lie group actions of governing dynamics and measurable output readings has allowed the investigation of controllability and observability in an intrinsic manner. © Springer Science+Business Media B.V. 2012.
Communications in Nonlinear Science and Numerical Simulation (10075704)17(12)pp. 4901-4916
Equations of motion for a special system, intended to provide an experimental facility for application of spatial attitude control schemes, are studied in the modern setting of geometric mechanics. Imposed constraints and inherited symmetry existing in the system's dynamics structure help to resolve the Lagrange-D'Alembert principle into a set of reduced-order equations of motion.On-orbit conditions are mimicked, permitting to evaluate feedback control algorithms for precise satellite manoeuvres in a laboratory situ but also to investigate stability issues due to complex rotational dynamics and interactions with flexible components. It is demonstrated that the same implications concerning gyro stability of the spatial system can be replicated as well on this prototype. © 2012 Elsevier B.V.
Physics Letters, Section A: General, Atomic and Solid State Physics (03759601)376(12-13)pp. 1137-1145
This work concerns with viscous and nonlocal effects on the structural stability of single wall carbon nanotube (SWCNT) conveying flowing fluid. First, the nonlinear Donnell shallow shell formulations are developed to model nonlocal effects. Then, the effect of different viscosity coefficients across the nanotube, due to the depletion layer near the wall, on the structure stability is studied. Results show that viscosity has strong stabilization effect for 40 nm diameters SWCNTs, but may be ignored for sub-7 nm diameters. Simulation results reveal non-stabilizing behavior of nonlocal effects for sub-3 nm diameters SWCNTs. © 2012 Elsevier B.V. All rights reserved.
Systems and Control Letters (01676911)61(4)pp. 495-505
Several concepts and results in geometric mechanics are used to analyze and control the locomotion system of an unconventional robot encapsulated in a sphere shell, assumed to roll without slipping on the floor and internally equipped with a set of inertia gyros as indirect driving devices. Lie group symmetries intrinsic to this problem, i.e., invariance of the system's Lagrangian and velocity distribution to some group of motions, allows the reduction of the equations of motion. This system whose motion ability is based on angular momentum conservation is established as a controllable nonholonomic system for which the attitude/position cannot be stabilized by smooth feedback laws. Pursuing the reduction process permits us to design a feedback law extensible to both kinematic and dynamic levels of actuation, enabling the robot to execute finite-time reorientation and repositioning maneuvers while confined to move in corridor-like domains. The derivation of the underlying nonlinearity contents via the geometric approach helps the analysis not to rely on a specific choice of coordinates and allows taking profit of the vector structure of the equations for further investigations. © 2012 Elsevier B.V. All rights reserved.
The present attitude control testbed is intended to provide an experimental facility that, in certain senses, emulates the dynamics of in-orbit conditions and permits to evaluate path planning and feedback control algorithms for precise satellite manoeuvres in laboratory situ. This paper shows the feasibility of the approach and demonstrates how attitude control rules become compatible for both realms. Equations of motion for this internally and externally constrained nonholonomic system are studied in the modern setting of geometric mechanics. © 2011 IEEE.
