分数阶双头分子马达的欠扩散输运现象

研究具有幂律记忆性的细胞液中双头分子马达的定向输运现象,选取幂函数作为广义Langevin方程的阻尼核函数,建立了分数阶过阻尼耦合Brown马达模型,讨论了阶数及耦合系数对双头分子马达定向输运速度的影响. 仿真结果表明,分数阶过阻尼双头分子马达也会产生定向输运现象,并且在某些阶数下会产生整数阶情形所不具有的反向定向流. 当噪声强度固定时,输运速度随着阶数以及耦合系数的变化均会出现广义随机共振现象. 特别地,研究发现双头分子马达在记忆闪烁棘轮势中具有某些单头分子马达所不具备的运动特性,定向流的大小和方向由噪声与双头间作用力相互耦合控制. 关键词： 分数阶双头分子马达 欠扩散 定向输运 广义随机共振     

A random walker on a ratchet

We analyze a model for a walker moving on a ratchet potential. This model is motivated by the properties of transport of motor proteins, like kinesin and myosin. The walker consists of two feet that are represented as two particles coupled nonlinearly through a bistable potential. In contrast to linear coupling, the bistable potential admits a richer dynamics, where the ordering of the particles can alternate during the walking. The transitions between the two stable states on the bistable potential correspond to a walking with alternating particles. We distinguish between two main walking styles: alternating and no alternating, resembling the hand-over-hand and the inchworm walking in motor proteins, respectively. When the equilibrium distance between the two particles divided by the periodicity of the ratchet is an integer, we obtain a maximum for the current, indicating optimal transport.     

Controllable 3D atomic Brownian motor in optical lattices

We study a Brownian motor, based on cold atoms in optical lattices, where atomic motion can be induced in a controlled manner in an arbitrary direction, by rectification of isotropic random fluctuations. In contrast with ratchet mechanisms, our Brownian motor operates in a potential that is spatially and temporally symmetric, in apparent contradiction to the Curie principle. Simulations, based on the Fokker-Planck equation, allow us to gain knowledge on the qualitative behaviour of our Brownian motor. Studies of Brownian motors, and in particular ones with unique control properties, are of fundamental interest because of the role they play in protein motors and their potential applications in nanotechnology. In particular, our system opens the way to the study of quantum Brownian motors.     

Drift coefficients of motor proteins moving along sidesteps

In the paper, we investigate two motor proteins moving along the sidesteps： a motor protein moving along a two- dimensional sidestep and another protein moving along a three-dimensional sidestep. The drift coefficients （or stationary average velocities） of these two motor proteins are calculated. We believe that our investigation of the motor proteins moving along the sidesteps in the present paper can benefit the investigation of the transport of the motor proteins to some extent.     

A simplified tether model for molecular motor transporting cargo

Molecular motors are proteins or protein complexes which function as transporting engines in biological cells. This paper models the tether between motor and its cargo as a symmetric linear potential. Different from Elston and Peskin's work for which performance of the system was discussed only in some limiting cases, this study produces analytic solutions of the problem for general cases by simplifying the transport system into two physical states, which makes it possible to discuss the dynamics of the motor--cargo system in detail. It turns out that the tether strength between motor and cargo should be greater than a threshold or the motor will fail to transport the cargo, which was not discussed by former researchers yet. Value of the threshold depends on the diffusion coefficients of cargo and motor and also on the strength of the Brownian ratchets dragging the system. The threshold approaches a finite constant when the strength of the ratchet tends to infinity.     

Traveling wave ultrasonic motor: coupling effects in free stator

Generally a stator of traveling wave ultrasonic motor (TWUM) consists of piezoelectric transducers (annular plate or rods) coupled by the way of a metallic ring. These transducers divided into halves are excited independently by two electrical signals with different phases of about 90 degrees. So an elastic traveling wave propagates along the circumference of the ring and a rotor pressed on this vibrating surface is then driven by the stator via contact forces. Many difficulties appear in developing TWUM because the contact between the stator and the rotor via a frictional material is very important. However that may be, the first stage consists in obtaining a vibrating stator with optimum characteristics with two symmetrical phases. The aim of this paper is to discuss some coupling effects in a free stator through an enhanced equivalent circuit model. A simple experimental method based on impedance measurements is performed to estimate the coupling characteristics at a low driving voltage. This paper reports results obtained with the free stator of the well known piezoelectric ultrasonic motor "USR60" by Shinsei Co. Ltd. Since the stator behaves as an elastic body, interactions between the two electrical inputs might be described by the introduction of a coupling oscillator. The comparison of experimental and theoretical results leads to validate the new equivalent circuit of the free stator. The presence of coupling impedance could imply a change of electrical supply condition to optimize the TWUM efficiency. The effects of unbalanced features for each electrical input and the applicability of the proposed model to actual operating condition are discussed in the paper.     

Processive motor protein as an overdamped brownian stepper

The two headed motor protein kinesin appears to "walk" along the biopolymer microtubule in 8 nm steps. There is ample justification for a model where the motion of the detached head to the next docking site on the biopolymer is described as ratcheted diffusion. The forward reorientation of an attached head can be conceived of as a power stroke. A model that is based on these premises can accurately predict parameters of motor protein motion.     

Magnetic islands modelled by a phase-field-crystal approach

Using a minimal model based on the phase-field-crystal formalism, we study the coupling between the density and magnetization in ferromagnetic solids. Analytical calculations for the square phase in two dimensions are presented and the small deformation properties of the system are examined. Furthermore, numerical simulations are conducted to study the influence of an external magnetic field on various phase transitions, the anisotropic properties of the free energy functional, and the scaling behaviour of the growth of the magnetic domains in a crystalline solid. It is shown that the energy of the system can depend on the direction of the magnetic moments, with respect to the crystalline direction. Furthermore, the growth of the magnetic domains in a crystalline solid is studied and is shown that the growth of domains is in agreement with expected behaviour.     

Chimeras in leaky integrate-and-fire neural networks: effects of reflecting connectivities

The effects of attracting-nonlocal and reflecting connectivity are investigated in coupled Leaky Integrate-and-Fire (LIF) elements, which model the exchange of electrical signals between neurons. Earlier investigations have demonstrated that repulsive-nonlocal and hierarchical network connectivity can induce complex synchronization patterns and chimera states in systems of coupled oscillators. In the LIF system we show that if the elements are nonlocally linked with positive diffusive coupling on a ring network, the system splits into a number of alternating domains. Half of these domains contain elements whose potential stays near the threshold and they are interrupted by active domains where the elements perform regular LIF oscillations. The active domains travel along the ring with constant velocity, depending on the system parameters. When we introduce reflecting coupling in LIF networks unexpected complex spatio-temporal structures arise. For relatively extensive ranges of parameter values, the system splits into two coexisting domains: one where all elements stay near the threshold and one where incoherent states develop, characterized by multi-leveled mean phase velocity profiles.     

Symmetric linear potential and imperfect Brownian ratchet in molecular motor function

Biomolecular motors are tiny engines that transport materials at the microscopic level within biological cells. In recent years, Elston and Peskin et al have investigated the effect of the elastic properties of the tether that connects the motor to its cargo at the speed of the motor. In this paper we extend their work and present a tether in the form of symmetric linear potential.Our results show that when the driving mechanism is an imperfect Brownian ratchet, the average speed decreases as the stiffness of the tether increases in the limit of large motor diffusion coefficient, which is similar to the results of Elston and Peskin.However, a threshold for the stiffness of the tether connecting the motor to its cargo is found in our model. Only when the tether is stiffer than the threshold can the motor and its cargo function co-operatively, otherwise, the motor and its cargo depart from each other. This result is more realistic than that of the spring model of Elston and Peskin.     

Phase ordering induced by defects in chaotic bistable media

The phase ordering dynamics of coupled chaotic bistable maps on lattices with defects is investigated. The statistical properties of the system are characterized by means of the average normalized size of spatial domains of equivalent spin variables that define the phases. It is found that spatial defects can induce the formation of domains in bistable spatiotemporal systems. The minimum distance between defects acts as parameter for a transition from a homogeneous state to a heterogeneous regime where two phases coexist The critical exponent of this transition also exhibits a transition when the coupling is increased, indicating the presence of a new class of domain where both phases coexist forming a chessboard pattern.     

Double-temperature ratchet model and current reversal of coupled Brownian motors

On the basis of the transport features and experimental phenomena observed in studies of molecular motors, we propose a double-temperature ratchet model of coupled motors to reveal the dynamical mechanism of cooperative transport of motors with two heads, where the interactions and asynchrony between two motor heads are taken into account. We investigate the collective unidirectional transport of coupled system and find that the direction of motion can be reversed under certain conditions. Reverse motion can be achieved by modulating the coupling strength, coupling free length, and asymmetric coefficient of the periodic potential, which is understood in terms of the effective potential theory. The dependence of the directed current on various parameters is studied systematically. Directed transport of coupled Brownian motors can be manipulated and optimized by adjusting the pulsation period or the phase shift of the pulsation temperature.     

Investigation of magnetically coupled ferromagnetic stripe arrays

We studied the magnetic coupling of ferromagnetic, submicron-sized stripes in the material system MnAs on GaAs. A specific coupling state, determined by stripe period and stripe width, can be tuned via film thickness and temperature, respectively. Micromagnetic imaging – in combination with micromagnetic simulations – reveals two coupling regimes. Strong magnetic coupling between the stripes creates micromagnetic domains extending across several stripes, whereas weak coupling allows for demagnetization within individual stripes. This behavior is observed for all investigatedfilm thicknesses, since a stripe geometry leading to a given coupling scenario is a function of temperature. PACS 68.37.Rt; 68.35.Rh; 75.70.-i; 75.70.Kw     

电机温度时滞耦合系统自抗扰控制仿真研究

电机温度过高会造成绝缘性能老化,电机安全性能下降。电机控制系统是典型的非线性系统,电机温度也因此具有时滞性和耦合性的特点,难以建立准确的数学模型。传统的方法对电机温度的控制精度较差,从而导致电机温度失控。为此,提出基于BP神经网络自抗扰控制算法的电机时滞耦合关系下温度控制方法。将BP神经网络与PID控制方法相结合建立电机温度网络自抗扰控制器模型,利用梯度下降法修正电机温度控制器模型的权值系数,从而实现了BP神经网络自抗扰控制器参数的实时调整。实验结果表明,利用BP神经网络自抗扰算法进行电机时滞耦合关系下温度调整,能够有效提高控制的精确度,缩短了控制过程中的时间延时。     

Anisotropy of sliding friction on the triglycine sulfate (010) surface

The frictional properties of freshly cleaved (010) surfaces of the ferroelectric TriGlycine Sulfate (TGS) were investigated by combined scanning and friction force microscopy under ambient conditions. A frictional contrast could be observed between domains with different electrical polarity, as well as between terraces inside individual domains which are separated by steps of half of the unit-cell height or an odd multiple of this value. The latter contrast mechanism originates from the arrangement of the molecules at the surface which is chemically homogeneous, but structurally rotated by 180° between different terraces. The resulting asymmetric surface potential gives rise to a frictional anisotropy in different directions that can be detected by the force microscope, as well as to a change of the frictional force between forward and backward scan direction.     

某光电吊舱内方位电机组件低温摩擦特性分析研究

为提升产品可靠性和环境适应性，需对某光电吊舱内方位电机组件在-50℃的摩擦特性进行改善。分析了可能影响电机组件摩擦特性的因素，并针对各因素提出相应的改善措施，借助仿真与试验相结合的研究方法，对改善措施的效果进行了验证。经研究:-50℃时，电机组件摩擦力矩的增大既有低温单独作用的结果，也有低温和电机组件潜在因素共同作用的结果。电机组件在低温下的微变形不足以引起摩擦力矩的增大，通过增加电机保护罩和在电机与电机座之间增加厚度为1 mm的软磁合金薄片，能使电机的摩擦力矩降低到0.063 N·m。该改善措施适用于光电吊舱电机组件的设计及类似产品的开发研制。     

The dynamic behavior of a cantilever beam coupled to a non-ideal unbalanced motor through numerical and experimental analysis

An excitation force that is not influenced by the system state is said to be an ideal energy source. In real situations, a direct and feedback coupling between the excitation source and the system must always exist at a certain level. This manifestation of the law of conservation of energy is known as the Sommerfeld effect. In the case of obtaining a mathematical model for such a system, additional equations are usually necessary to describe the vibration sources with limited power and its coupling with the mechanical system. In this work, a cantilever beam and a non-ideal DC motor fixed to its free end are analyzed. The motor has an unbalanced mass that provides excitation to the system which is proportional to the current applied to the motor. During the coast up operation of the motor, if the drive power is increased slowly, making the excitation frequency pass through the first natural frequency of the beam, the DC motor speed will remain the same until it suddenly jumps to a much higher value (simultaneously its amplitude jumps to a much lower value) upon exceeding a critical input power. It was found that the Sommerfeld effect depends on some system parameters and the motor operational procedures. These parameters are explored to avoid the resonance capture in the Sommerfeld effect. Numerical simulations and experimental tests are used to help gather insight of this dynamic behavior.