Optimal reorientation of underactuated spacecraft using genetic algorithm with wavelet approximation

The optimal attitude control of an underactuated spacecraft is investigated in this paper. The flywheels of the spacecraft can somehow only provide control inputs in two independent directions. The dynamic equations are formulated for the spacecraft under a nonholonomic constraint resulting from the constant time-rate of the total angular momentum of the system. The reorientation of such underactuated spacecraft is transformed into an optimal control problem. A genetic algorithm is proposed to derive the control laws of the two flywheels angle velocity inputs. The control laws are approximated by the discrete orthogonal wavelets. The numerical simulations indicate that the genetic algorithm with the wavelet approximation is an effective approach to deal with the optimal reorientation of underactuated spacecraft.

Chaotic attitude and reorientation maneuver for completely liquid-filled spacecraft with flexible appendage

The present paper investigates the chaotic attitude dynamics and reorientation maneuver for completely viscous liquid-filled spacecraft with flexible appendage. All of the equations of motion are derived by using Lagrangian mechanics and then transformed into a form consisting of an unperturbed part plus perturbed terms so that the system＇s nonlinear characteristics can be exploited in phase space. Emphases are laid on the chaotic attitude dynamics produced from certain sets of physical parameter values of the spacecraft when energy dissipation acts to derive the body from minor to major axis spin. Numerical solutions of these equations show that the attitude dynamics of liquid-filled flexible spacecraft possesses characteristics common to random, non- periodic solutions and chaos, and it is demonstrated that the desired reorientation maneuver is guaranteed by using a pair of thruster impulses. The control strategy for reorientation maneuver is designed and the numerical simulation results are presented for both the uncontrolled and controlled spins transition.

Numerical investigation on the effect of depletion-induced stress reorientation on infill well hydraulic fracture propagation

Depletion-induced stress change causes the redistribution of stress field in reservoirs,which can lead to the reorientation of principal stresses.Stress reorientation has a direct impact on fracture propagation of infill wells.To understand the effect of stress reorientation on the propagation of infill well’s fractures,an integrated simulation workflow that combines the reservoir flow calculation and the infill well hydraulic fracturing modeling is adopted.The reservoir simulation is computed to examine the relationship between the extent of stress reversal region and reservoir properties.Then,the hydraulic fracturing model considering the altered stress field for production is built to characterize the stress evolution of secondary fracturing.Numerical simulations show that stress reorientation may occur due to the decreasing of the horizontal stresses in an elliptical region around the parent well.Also,the initial stress difference is the driving factor for stress reorientation.However,the bottom hole pressure,permeability and other properties connected with fluid flow control timing of the stress reorientation.The decrease of the horizontal stresses around the parent well lead to asymmetrical propagation of a hydraulic fracture of the infill well.The study provides insights on understanding the influence of stress reorientation to the infill well fracturing treatment and interference between parent and infill wells.

Magnetization reorientation induced by spin-orbit torque in YIG/Pt bilayers

In this work,we report the reorientation of magnetization by spin-orbit torque(SOT)in YIG/Pt bilayers.The SOT is investigated by measuring the spin Hall magnetoresistance(SMR),which is highly sensitive to the direction of magnetic moment of YIG.An external in-plane rotating magnetic field which is applied to the YIG/Pt bilayers,and the evolutions of SMR under different injected currents in the Pt layer,result in deviation of SMR curve from the standard shape.We conclude that the SOT caused by spin accumulation near the interface between YIG and Pt can effectively reorient the inplane magnetic moment of YIG.This discovery provides an effective way to modulate YIG magnetic moments by electrical methods.

Temperature-driven spin reorientation transition of magnetron sputtered nickel thin film

The temperature-driven spin reorientation transition of magnetron sputtered Ni/Si （111） systems has been studied. The relationship between ac initial susceptibility and temperature of nickel films with different thicknesses shows that the magnetization orientation changes from in-plane to out-of-plane with the increase of temperature. The temperature dependence of mugnetoelastic, magneto-crystalline, and magnetostatic anisotropies determines the direction of the reorientation transition. The temperature-driven spin reorientation transition is supported by Hall coefficient measurements which show that its temperature dependence is similar to that of susceptibility.

Comparative Analysis of Hydration Layer Reorientation Dynamics of Antifreeze Protein and Protein Cytochrome P450

Antifreeze proteins(AFPs)inhibit ice recrystallization by a mechanism remaining largely elusive.Dynamics of AFPs’hydration water and its involvement in the antifreeze activity have not been identified conclusively.We herein,by simulation and theory,examined the water reorientation dynamics in the first hydration layer of an AFP from the spruce budworm,Choristoneura fumiferana,compared with a protein cytochrome P450(CYP).The increase of potential acceptor water molecules around donor water molecules leads to the acceleration of hydrogen bond exchange between water molecules.Therefore,the jump reorientation of water molecules around the AFP active region is accelerated.Due to the mutual coupling and excitation of hydrogen bond exchange,with the acceleration of hydrogen bond exchange,the rearrangement of the hydrogen bond network and the frame reorientation of water are accelerated.Therefore,the water reorientation dynamics of AFP is faster than that of CYP.The results of this study provide a new physical image of antifreeze protein and a new understanding of the antifreeze mechanism of antifreeze proteins.

THEORETICAL EXPLANATION OF SPIN REORIENTATION IN DyTiFe_(11) COMPOUND

DyTiFe_(11) compound is a ferromagnetic substance.It has tetragonal body-centered ThMn_(12)-type crystallographic structure.At room temperature,the easy magnetization direction is the c-axis.A spin reorientation begins to appear at about 175K.The contribution of Fe sublattice to magnetocrystalline anisotropy was determined by experiments and that of Dy sublattice was obtained by using single ion model calculation.Results show that the spin reorientation arises from the competition of anisotropy between Fe and Dy sublattices.

LOW ENERGY PATHS AND REORIENTATION OF SIDE-GROUPS OF POLYMERS DURING CONFORMATIONAL STATE TRANSITION

INTRODUCTION The conformational state transition of polymer chains relates to crystallization processes, migration ofthe chains in solution, fluctuation of the end-to-end distance of random coils, and the relaxation and phasetransitions of polymers. A description of the conformational state transition requires questions about; 1) howmany stable conformational states for a specific σ bond; 2) the barriers between the states; 3) the mechanismof the conformational transition; 4) any cooperative behavior during the transition. Flory and his coworkers

Magnetocrystalline anisotropy and spin reorientation transition of DyMn_6Sn_6 compound

The spin-reorientation transition of intermetallic compound DyMn6Sn6 was investigated by applying the molecular field theory. The temperature dependence of easy magnetization direction of compound and the magnetic moment directions of Dy and Mn ions were theoretically calculated and they have good agreement with the experimental data. In the framework of single ion model, the temperature dependence of magnetocrystalline anisotropic constants K1R and K2R of Dy ion were also calculated. The results show that the fourth-order crystal field parameter, B 40, and the corresponding second-order magnetocrystalline anisotropic constant, K2R, of Dy ion must be taken into account in order to explain the spin-reorientation transition satisfactorily. The competition between K2R and K1R plays a key role in the spin-reorientation transition of DyMn6Sn6.

Reorientation and obstacle avoidance control of free-floating modular robots using sinusoidal oscillator

This paper presents that a serpentine curve-based controller can solve locomotion control problems for articulated space robots with extensive flight phases,such as obstacle avoidance during free floating or attitude adjustment before landing.The proposed algorithm achieves articulated robots to use closed paths in the joint space to accomplish the above tasks.Flying snakes,which can shuttle through gaps and adjust their landing posture by swinging their body during gliding in jungle environments,inspired the design of two maneuvers.The first maneuver generates a rotation of the system by varying the moment of inertia between the joints of the robot,with the magnitude of the net rotation depending on the controller parameters.This maneuver can be repeated to allow the robot to reach arbitrary reorientation.The second maneuver involves periodic undulations,allowing the robot to avoid collisions when the trajectory of the global Center of Mass(CM)passes through the obstacle.Both maneuvers are based on the improved serpenoid curve,which can adapt to redundant systems consisting of different numbers of modules.Finally,the simulation illustrates that combining the two maneuvers can help a free-floating chain-type robot traverse complex environments.Our proposed algorithm can be used with similar articulated robot models.

考虑吸收、散射损耗时向列液晶电光特性的偏振测试探究

对于厚度达几十微米以上或掺杂染料分子的向列液晶样品而言,各向异性的散射和吸收将对偏振检测结果产生显著甚至剧烈的影响,此时不考虑吸收、散射损耗的理想偏振分析模型将不再适用.本文设计出一种更为普适的测量向列液晶指向矢取向方向的检偏测试装置,给出考虑各向异性吸收和散射时的偏振分析理论,并结合具体的实验测试给出可靠稳定的测试方案和具体的测量电致向列液晶指向矢偏转角的数据处理方法.

磁畴壁拓扑结构研究进展

拓扑磁性斯格明子作为信息载体单元具备高可靠性、高集成度、低能耗等优势,有望提高数据读写精度、降低功耗,从而研发新型拓扑自旋电子学材料与原理型器件,为信息技术、5G通信和大数据等的高速发展提供材料与技术支持.但磁性斯格明子同时存在需要磁场稳定以及电流驱动下斯格明子霍尔效应引起偏转等缺点,严重阻碍了其在实际器件中的应用,因此探索新型拓扑磁畴结构和适宜应用的材料体系成为研究的关键.本文将重点介绍自2013年理论预言磁畴壁斯格明子以来,利用高分辨率洛伦兹透射电子显微镜原位实空间发现并研究磁畴壁拓扑麦纫和磁畴壁斯格明子的实验工作.首次在范德瓦耳斯Fe_(5–x)GeTe_(2)二维磁性材料中发现温度诱发的180°磁畴壁转变为拓扑麦韧链,研究了磁畴壁麦纫态在外界电场、磁场作用下的集体运动行为,揭示了基于自旋重取向、磁畴壁限域效应以及弱相互作用下生成磁畴壁拓扑态的机制.在该机制指导下,设计制备了具有自旋重取向的GdFeCo非晶亚铁磁薄膜,不仅获得了磁畴壁麦纫,验证了生成机制的普适性,还成功实现了畴壁麦韧对到畴壁斯格明子的可逆拓扑转变,开辟了基于磁畴壁等内禀限域效应开展拓扑磁性物态探索和研究的新方向.

A study of magnetostriction, spin reorientation, and Mssbauer spectra of Tb_(0.3)Dy_(0.6)Pr_(0.1)(Fe_(1-x)Al_x)1.95 alloys with substitution of Fe by Al

The effects of Al substitution for Fe on the structure, magnetics, magnetostriction, anisotropy and spin reorientation of a series of Tb0.3Dy0.6Pr0.1(Fe1-xAlx)1.95 alloys (x=0.05, 0.1, 0.15, 0.2, 0.25, 0.3) at room temperature have been investigated. The alloys of Tb0.3Dy0.6Pr0.1(Fe1-xAlx)1.95 substantially retain MgCu2-type C-15 cubic Laves phase structure when x【0.2. The mixed phases appear with x = 0.2, and cubic Laves phase decreases with increasing x. The magnetostriction of the Tb0.3Dy0.6Pr0.1(Fe1-xAlx)1.95 alloys decreases drastically with increasing x and the giant magnetostrictive effect disappears for x 】 0.15. Fortunately, a small amount of Al substitution is beneficial to a decrease in the magnetocrystalline anisotropy. The spin reorientation temperature decreases with increasing x. The analysis of the Mssbauer spectra indicates that the easy magnetization direction in the {110} plane deviates slightly from the main axis of symmetry with the increase of Al concentration x, namely, spin reorientation, resulting in the change of macroscopical magnetic properties and magnetostriction. The hyperfine field decreases, but the isomer shifts increases with Al concentration increasing and the quadruple splitting QS shows a weak concentration dependence.

A CONSTITUTIVE MODEL FOR TRANSFORMATION, REORIENTATION AND PLASTIC DEFORMATION OF SHAPE MEMORY ALLOYS

A constitutive model is developed for the transformation, reorientation and plastic deformation of shape memory alloys （SMAs）. It is based on the concept that an SMA is a mixture composed of austenite and martensite, the volume fraction of each phase is transformable with the change of applied thermal-mechanical loading, and the constitutive behavior of the SMA is the combination of the individual behavior of its two phases. The deformation of the martensite is separated into elastic, thermal, reorientation and plastic parts, and that of the austenite is separated into elastic, thermal and plastic parts. Making use of the Tanaka＇s transformation rule modified by taking into account the effect of plastic deformation, the constitutive model of the SMA is obtained. The ferroelasticity, pseudoelastieity and shape memory effect of SMA Au-47.5 at.%Cd, and the pseudoelasticity and shape memory effect as well as plastic deformation and its effect of an NiTi SMA, are analyzed and compared with experimental results.

Water reorientation in the hydration shells of hydrophilic and hydrophobic solutes

We discuss some key aspects of our recent theoretical work on water reorientation dynamics,which is important in a wide range of phenomena,including aqueous phase chemical reactions,protein folding,and drug binding to proteins and DNA. It is shown that,contrary to the standard conception that these dynamics are diffusional,the reorientation of a water molecule occurs by sudden,large amplitude angular jumps. The mechanism involves the exchange of one hydrogen bond for another by the reorienting water,and the process can be fruitfully viewed as a chemical reaction. The results for reorientation times,which can be well described analytically,are discussed in the context of the molecular level interpretation of recent ultrafast infrared spectroscopic results,focusing on the concepts of structure making/breaking and solvent 'icebergs'.

Ethylene-induced microtubule reorientation is essential for fast inhibition of root elongation in Arabidopsis

Microtubule reorientation is a long-standing observation that has been implicated in regulating the inhibitory effect of ethylene on axial elongation of plant cells. However, the signaling mechanism underlying ethylene-induced microtubule reorientation has re- mained elusive. Here, we reveal, by live confocal imaging and kinetic root elongation assays, that the time courses of ethylene-induced microtubule reorientation and root elongation inhibition are highly correlated, and that microtubule reorientation is required for the full responsiveness of root elongation to ethylene treatment. Our genetic analysis demonstrated that the effect of ethylene on microtubule orientation and root elongation is mainly transduced through the canonical linear ethylene signaling pathway. By using pharmacological and genetic analyses, we demonstrate further that the TIR1/AFBs-Aux/IAAs-ARFs auxin signaling pathway, but not the ABP1-ROP6-RlC1 auxin signaling branch, is essential for ethylene-induced microtubule reorientation and root elongation inhibition. Together, these findings offer evidence for the functional significance and elucidate the signaling mechanism for ethylene-induced microtubule reorientation in fast root elongation inhibition in Arabidopsis.

In-situ synchrotron high energy X-ray diffraction study of spontaneous reorientation of R phase upon cooling in nanocrystalline Ti_(50)Ni_(45.5)Fe_(4.5)alloy

This study investigated a peculiar phenomenon of self-reorientation of thermally formed R phase in nanocrystalline Ti_(50)Ni_(45.5)Fe_(4.5)by means of in-situ syn-chrotron high energy X-ray diffraction(HE-XRD).Two samples with different average grain sizes of 40 and 90 nm were investigated.R phase in the 40-nm grain size sample was found to self-reorient gradually upon cooling,whereas the same phenomenon did not occur in the 90-nm grain size sample.This self-reorientation process is attributed to the development and evolution of an internal stress anisotropy caused by the second order continuous lattice distortion of R phase upon further cooling in the small nanograined matrix,which lacks the self-accommodation mechanism for internal stress cancellation.

Effect of additive elements on exchange coupling and spin reorientation transition of nanocrystalline single-phase Nd-Fe-B alloy

Nd12.3Fe81.7-xZrxB6.0 (x=0,1.5) and (NdDyTb)12.3(FeZrNbCu)81.7-yCoyB6.0 (y=0,12) ribbons were prepared by melt spinning at 22 m/s and subsequent annealing.The influences of Zr,Dy/Tb and Co substitutions on magnetic properties and spin reorientation transitions of nanophase Nd2Fe14B have been systematically investigated.Compared with Zr-free sample,the remanence,intrinsic coercivity and maximum energy product for Nd12.3Fe80.2Zr1.5B6.0 ribbon increase by 10.8 %,17.8 % and 60.2 %,respectively.The significant improvement of magnetic properties originates from the finer grains of the sample by introducing Zr,which leads to the stronger exchange coupling between neighboring grains.The intrinsic coercivity for (DyTb)-substituted ribbon is significantly increased although the remanence is reduced,which could be compensated by the substitution of Co for Fe.The spin reorientation temperature Tsr of nanocrystalline Nd2Fe14B alloys was determined by measuring the ac magnetic susceptibility.It was found to be lower than that of bulk Nd2Fe14B.The substitutions of Zr,Dy/Tb and Co result in reduction of Tsr.The smaller the grain size,the lower the Tsr will be.Influence of spin reorientation on magnetization characteristics of nanophase Nd2Fe14B was discussed.

Thermal control magnetic switching dominated by spin reorientation transition in Mn-doped PrFeO_(3) single crystals

Spin reorientation transition (SRT) has attracted substantial attention due to its important role in the ultrafast control of spins. However, the transition temperature is usually too low for its practical applications. Here, we demonstrate the ability to modulate the SRT temperature in PrFe_(1−x)Mn_(x)O_(3) single crystals from 196 K to 317 K across the room temperature by varying the Mn concentration. Interestingly, the Γ_(4) to Γ_(1) spin reorientation of the Mn-doped PrFeO_(3) is distinct from the Γ_(4) to Γ_(2) spin reorientation transition as in the parent material. Because of the coupling between rare-earth ions and transition-metal ions in determining the SRT temperature, the demonstrated control scheme of spin reorientation transition temperature by Mn-doping is expected to be used in temperature control magnetic switching devices and applicable to many other rare-earth orthoferrites.

磁场诱导的TmFeO_(3)单晶自旋重取向

TmFeO_(3)具有磁光效应、多铁性和自旋重取向等丰富的物理特性,在凝聚态物理和材料物理领域具有重要的研究价值.本文利用时域太赫兹低温磁光谱,研究TmFeO_(3)单晶在1.6 K温度下自旋共振频率随外加磁场的变化规律,并表征其内部复杂的相互作用.结果表明,随磁场增加TmFeO_(3)单晶的准铁磁共振向高频移动,而准反铁磁共振在临界磁场(2.2—3.6 T)转变为准铁磁共振,通过磁结构分析和理论拟合,证实单晶磁矩发生了磁场诱导的自旋重取向.本研究有助于深入理解稀土正铁氧体在外磁场、温度场综合作用下,内部磁结构的调控机制,开发相关的自旋电子学器件.