Structural Degrees Of Freedom Research Articles

Gait Kinematic Modeling of a Hexapod Robot

The hexapod robot is one of the most important types of robots dedicated to complex tasks. The importance given to this kind of robots is due to its high stability and precision, particularly, in irregular and dangerous areas. In this article, a hexapod robot with rectangular architecture body and six legs symmetrically distributed along two sides is considered. In order to obtain greater mobility, legs with three degrees of freedom structure are adopted. The aim of this work is to plan several gaits for the hexapod robot, by elaborating a general kinematic modeling approach. It concerns the lifting and the propelling phases of any gait adopted by the robot. In order to obtain clear representation and efficient generation of the system equations, appropriate coordinate frames are attached to the body and the legs. Simulation results confirm the kinematic model solution and show main steps of the generated gaits: (i) wave gait,(ii) ripple gait, and (iii) tripod gait.

Thermo‐analytical Investigations on the Superoxides AO2 (A = K, Rb, Cs), Revealing Facile Access to Sesquioxides A4O6

Rb4O6 and Cs4O6 represent open shell p electron systems, featuring charge, spin, orbital and structural degrees of freedom, which makes them unique candidates for studying the ordering processes related, otherwise exclusively encountered in transition metal based materials. Probing the physical responses has been restrained by the intricacy of synthesizing appropriate amounts of phase pure samples. Tracing the thermal decomposition of respective superoxides has revealed that at least the rubidium and cesium sesquioxides exist in thermodynamic equilibrium, appropriate p‐T conditions given. These insights have paved the way to highly efficient and convenient access to Rb4O6 and Cs4O6.

Bifurcation Analysis with Aerodynamic-Structure Uncertainties by the Nonintrusive PCE Method

An aeroelastic model for airfoil with a third-order stiffness in both pitch and plunge degree of freedom (DOF) and the modified Leishman–Beddoes (LB) model were built and validated. The nonintrusive polynomial chaos expansion (PCE) based on tensor product is applied to quantify the uncertainty of aerodynamic and structure parameters on the aerodynamic force and aeroelastic behavior. The uncertain limit cycle oscillation (LCO) and bifurcation are simulated in the time domain with the stochastic PCE method. Bifurcation diagrams with uncertainties were quantified. The Monte Carlo simulation (MCS) is also applied for comparison. From the current work, it can be concluded that the nonintrusive polynomial chaos expansion can give an acceptable accuracy and have a much higher calculation efficiency than MCS. For aerodynamic model, uncertainties of aerodynamic parameters affect the aerodynamic force significantly at the stage from separation to stall at upstroke and at the stage from stall to reattach at return. For aeroelastic model, both uncertainties of aerodynamic parameters and structure parameters impact bifurcation position. Structure uncertainty of parameters is more sensitive for bifurcation. When the nonlinear stall flutter and bifurcation are concerned, more attention should be paid to the separation process of aerodynamics and parameters about pitch DOF in structure.

Light control of orbital domains: case of the prototypical manganite La0.5Sr1.5MnO4

Control of electronic and structural ordering in correlated materials on the ultrafast timescale with light is a new and emerging approach to disentangle the complex interplay of the charge, spin, orbital and structural degree of freedom. In this paper we present an overview of how orbital order and orbital domains can be controlled by near IR and THz radiation in the layered manganite La0.5Sr1.5MnO4. We show how near-IR pumping can efficiently and rapidly melt orbital ordering. However, the nanoscale domain structure recovers unchanged demonstrating the importance of structural defects for the orbital domain formation. On the contrary, we show that pulsed THz fields can be used to effectively orientate the domains. In this case the alignment depends on the in-plane electric field polarisation and is induced by an energy penalty that arises from THz field induced hopping of the localised charges.

Self-amplified photo-induced gap quenching in a correlated electron material.

Capturing the dynamic electronic band structure of a correlated material presents a powerful capability for uncovering the complex couplings between the electronic and structural degrees of freedom. When combined with ultrafast laser excitation, new phases of matter can result, since far-from-equilibrium excited states are instantaneously populated. Here, we elucidate a general relation between ultrafast non-equilibrium electron dynamics and the size of the characteristic energy gap in a correlated electron material. We show that carrier multiplication via impact ionization can be one of the most important processes in a gapped material, and that the speed of carrier multiplication critically depends on the size of the energy gap. In the case of the charge-density wave material 1T-TiSe2, our data indicate that carrier multiplication and gap dynamics mutually amplify each other, which explains—on a microscopic level—the extremely fast response of this material to ultrafast optical excitation.

Interferroelectric transition as another manifestation of intrinsic size effect in ferroelectrics

In general, crystallite size reduction is known to suppress the ferroic order (polarization or magnetization) in ferroelectric and magnetic systems. Using free particles of a giant tetragonality $(c/a--1=0.18)$ ferroelectric alloy $\mathrm{BiFe}{\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{PbTi}{\mathrm{O}}_{3}$ as the model system, here we show that the intrinsic size effect in ferroelectrics can as well manifest in terms of switching the ground state from one ferroelectric phase (tetragonal with polarization along [001]) to another ferroelectric phase (rhombohedral with polarization along [111]). In this particular case, because of the strong coupling of the magnetic and structural degrees of freedom, a magnetic order also sets in below the critical size, making it a size induced magnetoferroelectric transformation. The driving force for this unusual transformation is argued to be the large depolarizing and domain wall energies in the tetragonal phase.

Study on LQR control algorithm using superelement model

The conventional linear quadratic regulator (LQR) control algorithm is one of the most popular active control algorithms. One important issue for LQR control algorithm is the reduction of structure’s degrees of freedom (DOF). In this work, an LQR control algorithm with superelement model is intended to solve this issue leading to the fact that LQR control algorithm can be used in large finite element (FE) model for structure. In proposed model, the Craig-Bampton (C-B) method, which is one of the component mode syntheses (CMS), is used to establish superelement modeling to reduce structure’s DOF and applied to LQR control algorithm to calculate Kalman gain matrix and obtain control forces. And then, the control forces are applied to original structure to simulate the responses of structure by vibration control. And some examples are given. The results show the computational efficiency of proposed model using synthesized models is higher than that of the classical method of LQR control when the DOF of structure is large. And the accuracy of proposed model is well. Meanwhile, the results show that the proposed control has more effects of vibration absorption on the ground structures than underground structures.

Control of Multipolar and Orbital Order in Perovskite-like [C(NH2)3]CuxCd1-x(HCOO)3 Metal-Organic Frameworks.

We study the compositional dependence of molecular orientation (multipolar) and orbital (quadrupolar) order in the perovskite-like metal-organic frameworks [C(NH2)3]CuxCd1-x(HCOO)3. Upon increasing the fraction x of Jahn-Teller-active Cu(2+), we observe an orbital disorder/order transition and a multipolar reorientation transition, each occurring at distinct critical compositions xo = 0.45(5) and xm = 0.55(5). We attribute these transitions to a combination of size, charge distribution, and percolation effects. Our results establish the accessibility in formate perovskites of novel structural degrees of freedom beyond the familiar dipolar terms responsible for (anti)ferroelectric order. We discuss the implications of cooperative quadrupolar and multipolar states for the design of relaxor-like hybrid perovskites.

Pair distribution function analysis: The role of structural degrees of freedom in the high-pressure insulator to metal transition ofVO2

The evolution of the local structure of ${\mathrm{VO}}_{2}$ was investigated across the pressure-induced insulator to metal transition (IMT) by means of pair distribution function measurements. The pressure behavior of the V-V and V-O bond lengths have been determined. The data demonstrated that the pressure-driven IMT is not activated by the suppression of the Peierls-type distortion. A clear octahedra symmetrization is observed in the metallic phase, suggesting a link between structural degree of freedom and the metallization process.

Premartensitic transition and relevant magnetic effects in Ni50Mn34In15.5Al0.5 alloy.

Resistance measurement, in situ optical microscopic observation, thermal and magnetic measurements have been carried out on Ni50Mn34In15.5Al0.5 alloy. The existence of a pronounced premartensitic transition prior to martensitic transition can be characterized by microstructure evolution as well as exothermic peak and smooth decrease of resistance and magnetization with obvious hysteresis over a wide temperature range upon cooling. Consequently, the alloy undergoes two successive magneto-structural transitions consisting of premartensitic and martensitic transitions. Magnetoelastic coupling between magnetic and structural degrees of freedom would be responsible for the appearance of premartensitic transition, as evinced by the distinct shift of transitions temperatures to lower temperature with external applied field of 50 kOe. The inverse premartensitic transition induced by magnetic field results in large magnetoresistance, and contributes to the enhanced inverse magnetocaloric effect through enlarging the peak value and temperature interval of magnetic entropy change ΔSm.

Failure of the Volume Function in Granular Statistical Mechanics and an Alternative Formulation.

We first show that the currently accepted statistical mechanics for granular matter is flawed. The reason is that it is based on the volume function, which depends only on a minute fraction of all the structural degrees of freedom and is unaffected by most of the configurational microstates. Consequently, the commonly used partition function underestimates the entropy severely. We then propose a new formulation, replacing the volume function with a connectivity function that depends on all the structural degrees of freedom and accounts correctly for the entire entropy. We discuss the advantages of the new formalism and derive explicit results for two- and three-dimensional systems. We test the formalism by calculating the entropy of an experimental two-dimensional system, as a function of system size, and showing that it is an extensive variable.

Temperature-pressure phase diagram of cubic Laves phaseAu2Pb

The temperature ($T$) as a function of pressure ($P$) phase diagram is reported for the cubic Laves phase compound Au$_2$Pb, which was recently proposed to support linearly dispersing "topological" bands, together with conventional quadratic bands. At ambient pressure, Au$_2$Pb exhibits several structural phase transitions at $T_1$ $=$ 97 K, $T_2$ $=$ 51 K, and $T_3$ $=$ 40 K with superconductivity below $T_{\rm{c}}$ $=$ 1.2 K. Applied pressure results in a rich phase diagram where $T_1$, $T_2$, and $T_3$ evolve strongly with $P$ and a new phase is stabilized for $P$ $>$ 0.64 GPa that also supports superconductivity below 1.1 K. These observations suggest that Au$_2$Pb is an ideal system in which to investigate the relationship between structural degrees of freedom, band topology, and resulting anomalous behaviors.

Role of Barrier Modification and Inelastic Surface Excitations in Sub-Barrier Fusion of 16 32 S + 40 94 Zr Reaction

The fusion dynamics of 16 S+40 Zr reaction at near and sub-barrier energies is investigated within the context of different theoretical approaches. The various theoretical models like one-dimensional Wong formula, l-summed ex- tended Wong formula, the energy-dependent Woods-Saxon potential model (EDWSP model), and coupled channel for- mulation have been used to address the impacts of nuclear structure degrees of freedom of the colliding pairs. The roles of different Skyrme forces along with Wong formalism are also tested in the analysis of the sub-barrier fusion dynamics of the 16 S+40 Zr reaction. The influence of the low-lying sur- face vibrational states of the collision partners is investigated within the framework of coupled channel calculations per- formed by the code CCFULL. In the present work, it has been observed that the EDWSP model introduces barrier modifica- tion effects somewhat similar to those of the coupled channel approach, as well as those of using different Skyrme forces and hence it reasonably addresses the observed fusion data of 16 S+40 Zr reaction in the close vicinity of the Coulomb barrier.

ANALISIS DINAMIS SISTEM STRUKTUR DENGAN SKEMA MASSA KONSISTEN

The paper deals with frequency analysis of irreguler framed structures. The analysis used finite element method cast in matrix formulation. Apart from frequency analysis of framed structures that assumed to be of frame with relative rigid floor system, and the mass of structure is lumped at each floor, the analysis adopted consistent mass formulation. To reduce structural degrees of freedom, static condensation and multi-point constraint algorithms where used. The natural frequency resulted out of proposed analysis was then compared to that obtained by assuming rigid floor. The difference was due to the different schemes used in the consideration of inertial mass forces. Key Words : dynamic analysis, finite element method, multi-point constraints, static condensation, natural frequency.

A predictive structural model for bulk metallic glasses.

Great progress has been made in understanding the atomic structure of metallic glasses, but there is still no clear connection between atomic structure and glass-forming ability. Here we give new insights into perhaps the most important question in the field of amorphous metals: how can glass-forming ability be predicted from atomic structure? We give a new approach to modelling metallic glass atomic structures by solving three long-standing problems: we discover a new family of structural defects that discourage glass formation; we impose efficient local packing around all atoms simultaneously; and we enforce structural self-consistency. Fewer than a dozen binary structures satisfy these constraints, but extra degrees of freedom in structures with three or more different atom sizes significantly expand the number of relatively stable, ‘bulk' metallic glasses. The present work gives a new approach towards achieving the long-sought goal of a predictive capability for bulk metallic glasses.

A bidirectional tuned liquid column damper for reducing the seismic response of buildings

In this article, a new bidirectional tuned liquid column damper (BTLCD) is proposed for controlling the seismic response of structures. The device acts as two independent and orthogonal tuned liquid column dampers (TLCDs), but due to its configuration, it requires less liquid than two equivalent independent TLCDs. The equations of motion of the system formed by the BTLCD and the primary structure to be controlled are obtained by means of Lagrangian dynamics explicitly considering the non-symetrical action of the damping forces. First, the primary structure was assumed to have two degrees of freedom (DOFs). Assuming that the system is excited by a base acceleration that can be considered to be a white noise random process, the optimum design parameters of the device were obtained to minimise the response of the primary structure. The optimum design parameters are presented as expressions covering a wide range of possible configurations for the device in a controlled structure. The use of a BTLCD to control the seismic response of several DOF structures was also studied, showing that if the structural response occurs mainly in two perpendicular modes, then the optimum design parameters for two DOF structures can be used. Experimental analyses of the BTLCD are developed in order to verify its dynamical properties. Finally, the device is designed for controlling the seismic response of a six DOF scale model. Numerical analyses are developed in order to verify the effectiveness and accuracy of the equations and design procedures proposed herein. Copyright © 2015 John Wiley & Sons, Ltd.

Magnetic charges and magnetoelectricity in hexagonal rare-earth manganites and ferrites

Magnetoelectric (ME) materials are of fundamental interest and show broad potential for technological applications. Commonly the dominant contribution to the ME response is the lattice-mediated one, which is proportional to both the Born electric charge $Z^{\rm e}$ and its analogue, the dynamical magnetic charge $Z^{\rm m}$. Our previous study has shown that exchange striction acting on noncollinear spins induces much larger magnetic charges than those that depend on spin-orbit coupling. The hexagonal manganites $R$MnO$_3$ and ferrites $R$FeO$_3$ ($R$ = Sc, Y, In, Ho-Lu) exhibit strong couplings between electric, magnetic and structural degrees of freedom, with the transition-metal ions in the basal plane antiferromagnetically coupled through super-exchange so as to form a 120$^\circ$ noncollinear spin arrangement. Here we present a theoretical study of the magnetic charges, and of the spin-lattice and spin-electronic ME constants, in these hexagonal manganites and ferrites, clarifying the conditions under which exchange striction leads to an enhanced $Z^{\rm m}$ values and anomalously large in-plane spin-lattice ME effects.

Electric field induced short range to long range structural ordering and its influence on the Eu+3 photoluminescence in the lead-free ferroelectric Na1/2Bi1/2TiO3

Eu+3 was incorporated into the lattice of a lead-free ferroelectric Na1/2Bi1/2TiO3 (NBT) as per the nominal formula Na0.5Bi0.5−xEuxTiO3. This system was investigated with regard to the Eu+3 photoluminescence (PL) and structural behaviour as a function of composition and electric field. Electric field was found to irreversibly change the features in the PL spectra and also in the x-ray diffraction patterns below the critical composition x = 0.025. Detailed analysis revealed that below the critical composition, electric field irreversibly suppresses the structural heterogeneity inherent of the host matrix NBT and brings about a long range ferroelectric state with rhombohedral (R3c) distortion. It is shown that the structural disorder on the nano-scale opens a new channel for radiative transition which manifests as a new emission line branching off from the main 5D0→7F0 line along with a concomitant change in the relative intensity of the other crystal field induced Stark lines with different J values. The study suggests that Eu+3 luminescence can be used to probe the relative degree of field induced structural ordering in relaxor ferroelectrics and also in high performance piezoelectric alloys where electric field couples very strongly with the lattice and structural degrees of freedom.

A practical and efficient coupling method for large scale soil–structure interaction problems

A practical and efficient coupling method for performing nonlinear static pushover or time history analysis for soil–structure interaction (SSI) systems is presented. The method combines the advantages of efficient analysis of a half-space soil medium represented as discrete filters and powerful modeling capabilities of finite element analysis (FEA) software for large scale nonlinear structural systems, thus is potentially useful for solving large scale realistic civil infrastructure problems. The boundary conditions of displacement continuity and force equilibrium between soil and structure are satisfied by using Newton׳s method. The coupling between the two substructures is based on a real-time data communication technique called the client–server (CS) integration technique. A comprehensive study is made regarding the newly developed coupling method by using a single- and a multi- degree of freedom structure and soil systems, as well as a real world SSI example. Several details are discussed, including the effect of simulation time step sizes, comparison of implicit and explicit methods, effects of increasing nonlinearity in the SSI system, and the nonlinear seismic responses of the SSI systems in cases of considering vs. not considering SSI effect. This paper proposes a practical and efficient method for nonlinear static pushover or seismic analysis of large scale SSI systems, and part of the research results provides valuable insight for engineering practice.

Interplay between electronic and structural degrees of freedom in quarter-filled low dimensional conductors

We review the basic aspects of the charge density wave (CDW) and bond order wave (BOW) instabilities observed in one dimension (1D) organic conductors at either the 2kF and/or 4kF critical wave vectors. We start by recalling the main features of the coupled structural/electronic Peierls instabilities observed in donor–acceptor (D–A) charge transfer (CT) salts. Then we consider the specific case of 2:1 salts D2X where X is a monovalent anion. We show that the incipient CDW/BOW instabilities of the Bechgaard and Fabre salts are those of the parent quarter-filled CT salts TMTSF-DMTCNQ and TMTTF-DMTCNQ respectively. We also consider more specifically the influence of specific features of D2X salts such as the stack dimerization, the Fermi surface warping and the coupling to the anions. Then we discuss more generally the role of the anions in the Bechgaard and Fabre salts by pointing out the influence of polarization and charge displacement induced by the anion shift. Finally we show that some of these features are also relevant to understand the subtle interplay between structural and electronic degrees of freedom in 2D quarter-filled organic salts such as the (BEDT-TTF)2X series.