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Song, Yan; Xiao, Wen; Qi, Xiaoyu
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5 Citations
A predator–prey system with stage structure and time delay for the prey is investigated. By analyzing the corresponding characteristic equations, the local stability of a positive equilibrium and two boundary equilibria of the system is discussed, respectively. By using persistence theory on infinite dimensional systems and comparison argument, respectively, sufficient conditions are obtained for the global stability of the positive equilibrium and one of the boundary equilibria of the proposed system. Further, the existence of a Hopf bifurcation at the positive equilibrium is studied. Numerical simulations are carried out to illustrate the main results.
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Ma, Guiyu; Li, Zhongxiao; Qi, Yingqun; Wu, Ti
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A reversible additionfragmentation chain transfer (RAFT) agent, 2((2(((dodecylthio)carbonothioyl)thio)propanoyl)oxy)ethyl acrylate (DTEA), was synthesized through the condensation reaction of 2(dodecylthiocarbonothioylthio) propanoic acid with 2hydroxyethyl acrylate. DTEA is a bifunctional molecule which is characterized by having one double bond and one thiocarbonylthio group. Branched poly(methyl acrylate)s (BPMAs) were prepared through onestep radical polymerization of methyl acrylate (MA) in the presence of DTEA. ^{1}H NMR spectra of the polymers were used to study the degree of branching (DB). The DB values of the three prepared BPMAs were calculated to be 0.46, 0.18 and 0.087, respectively. It was found that the DB value could be adjusted by controlling the ratio of MA to DTEA.
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Chang, Liang; Jia, Yingmin
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This paper considers the multiobjective reliable robust output feedback control problem for receiver stationkeeping in boom and receptacle refueling (BRR), which considers the main features of BRR, i.e., mass and inertia variation of receiver aircraft, sensors failure, input constraints and disturbance attenuation. A new receiver aircraft model is firstly established in terms of those main features of BRR. Then, a new multiple objectives robust output feedback controller is designed for this control problem. The controller’s existence is derived by using the Lyapunov method and linear matrix inequalities (LMIs) technique; and then the desired controller can be achieved by the LMI tools. A practice example is presented to demonstrate that the proposed controller design method can successfully solve the multiobjective control problem.
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Zhou, Zhidong; Jiang, Quan
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The thin stiff films on prestretched compliant substrates can form wrinkles, which can be controlled in micro and nanoscale systems to generate smart structures. Recently, buckled piezoelectric/ferroelectric nanoribbons have been reported to show an enhancement in the piezoelectric effect and stretchability, which can be applied in energy harvesting devices, sensors and memory devices instead of polymeric polyvinylidine fluoride (PVDF). This paper studies the buckling and postbuckling process of ferroelectric thin films bonded to the prestretched soft layer, which in turn lies on a rigid support. Nonlinear electromechanical equations for the buckling of thin piezoelectric plates are deduced and employed to model the ferroelectric film poled in the thickness direction. Two buckling modes are analyzed and discussed: partially deadhered buckling and fully adhered buckling. Transition from one buckling mode to the other is predicted and the effect of piezoelectricity on the critical buckling condition of piezoelectric film is examined.
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Nan, Yang; Sun, Xin; Zhang, Li
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Thank to the observation that in massive multiinput multioutput systems, the channels associated with different base station antennas may share common sparse support, the significant path delays can be accurately captured by only few pilots, leading to a reduction of pilot overhead. However, when the number of pilots is small, the path gains can not be accurately estimated and this limits the system performance. To solve this problem, in this paper we propose a decision aided compressive sensing based channel estimation scheme, which utilizes the decoded data to refine the channel estimation. This scheme can effectively improve the channel estimation without increasing the length of pilot sequence, which is confirmed by both analyses and simulation results.
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He, ShuMing; Lin, Yun
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It is well known that the zeroattracting least mean square (ZALMS) algorithm and reweighted zeroattracting LMS (RZALMS) algorithm outperform the standard LMS algorithm in sparse systems. However, because the ZALMS algorithm does not distinguish the size of the tap coefficients, its performance in lowsparse or nonsparse systems declines rapidly. Although RZALMS selectively attracts taps with small magnitudes, there is extra attraction to large tap coefficients, which can increase the steadystate mean square error (MSE). In this paper, a Cauchy distribution functionpenalized LMS (CLMS) algorithm is proposed. The proposed algorithm changed the penalty term of the cost function into Cauchy distribution function, which can decrease the attraction to large tap coefficients and enhances the force to the small tap coefficients. The simulation results indicate that the CLMS can achieve lower steadystate MSE than other algorithms in sparse systems and demonstrates similar performance to the conventional LMS algorithm in nonsparse system.
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Wang, Dongdong; Liang, Qingwen; Zhang, Hanjie
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8 Citations
A superconvergent isogeometric formulation is presented to compute the eigenvalues for three dimensional wave equation. This three dimensional superconvergent isogeometric formulation is characterized by a higher order mass matrix formulation with particular reference to the quadratic basis functions. The three dimensional higher order mass matrix is built upon an optimal combination of the reduced bandwidth mass matrix and the consistent mass matrix. The frequency error associated with the isogeometric discretization of three dimensional wave equation is derived in detail. In particular, the optimal mass combination parameter for higher order mass matrix is devised as a function of the two spatial wave propagation angles, which enables that arbitrary frequency corresponding to a given wave propagation direction can be computed in a superconvergent way. Two extra orders of accuracy, i.e., 6th order of accuracy, are attained by the proposed higher order mass matrix than the consistent mass matrix for the frequency computation of three dimensional wave equation. The dispersion property of the present three dimensional higher order mass matrix formulation is examined as well. The accuracy of the proposed three dimensional superconvergent isogeometric formulation is testified by several numerical examples.
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Wang, Chong; Lai, Yuanming; Zhang, Mingyi; Li, Shuangyang
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Knowledge of particle shape and configurationdependent thermal conductivity is necessary to investigate heat or water transfer in geomaterials, especially under freezing states. Thermal conductivity of a porous medium is affected by its matrix components and particle shapes. However, the particle shapes in geomaterials are various, and the effect of the ice phase on the thermal conductivity may increase substantially as ice content increases from unfrozen to freezing. In this study, a generalized thermal conductivity model for geomaterials is proposed based on phase transition theory and geometry approximation with respect to unfrozen and freezing states. Volumetric contents of each component and shape factors are required to predict thermal conductivities by the model. In order to evaluate the model, test results from both ours and previous literatures are employed to evaluate the calculated ones, and they match very well. In addition, compared with the other two models, i.e., a physical model and an empirical model, the proposed model is more reasonable and effective.
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Staicu, Stefan; Liu, XinJun; Wang, Jinsong
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44 Citations
Recursive matrix relations for kinematics and dynamics of the HALF parallel manipulator are presented in this paper. The prototype of this robot is a spatial mechanism with revolute actuators, which has two translation degrees of freedom and one rotation degree of freedom. The parallel manipulator consists of a base plate, a movable platform and a system of three connecting legs, having wide application in the fields of industrial robots, simulators, parallel machine tools and any other manipulating devices where high mobility is required. Supposing that the position and the motion of the moving platform are known, an inverse dynamics problem is solved using the principle of virtual powers. Finally, some iterative matrix relations and graphs of the torques and powers for all actuators are analysed and determined. It is shown that this approach is an effective means for kinematics and dynamics modelling of parallel mechanisms.
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Mao, Xiaobo; Mao, Kuanmin; Wang, Fengyun; Yan, Bo; Lei, Sheng
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The convective heat transfer coefficient (CHTC) is a key parameter for thermal analysis. The calculation method of empirical equations cannot accurately reflect the real situation of an object, especially when the fluid temperature is a variable, making the CHTC calculation more difficult. Based on finite element (FE) thermal analysis, an iterative algorithm of FE thermal analysis combined with experimental data was proposed. Furthermore, in the thermal analysis process, the temperature of the object itself changes and the heat is not uniform. The adopted CHTCs dynamically varied with the uneven wall temperature and ambient temperature. A CHTC gradual change “dynamic boundary condition” application mode was used. The interactive iterative calculation was completed by the FE software ABAQUS 6.14 and MATLAB 2016b, and a program was written to improve the calculation accuracy of the CHTC using multiple iterations. Taking the machine tool ball screw feed system as an example, a large number of experiments were conducted over 1 year under different ambient temperatures to verify that the method described in this paper was accurate and feasible. In addition, the relationship between the CHTC and ambient temperature change was found determined, and an explanation was provided.
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