Exploring human rhythmic gait movement in the role of cerebral cortex signal

The rhythmic movement is a spontaneous behavior due to the central pattern generator(CPG).At present,the CPG model only shows the spontaneous behavior,butdoes not refer to the instruction regulation role of the cerebral cortex.In this paper,a modified model based on the Matsuoka neural oscillator theory is presented to better show the regulation role of the cerebral cortex signal to the CPG neuronal network.The complex interaction between the in put signal and other parameters in the CPG networkis establish...     

Exploring human rhythmic gait movement in the role of cerebral cortex signal

The rhythmic movement is a spontaneous behavior due to the central pattern generator (CPG). At present, the CPG model only shows the spontaneous behavior, but does not refer to the instruction regulation role of the cerebral cortex. In this paper, a modified model based on the Matsuoka neural oscillator theory is presented to better show the regulation role of the cerebral cortex signal to the CPG neuronal network. The complex interaction between the input signal and other parameters in the CPG network is established, making all parameters of the CPG vary with the input signal. In this way, the effect of the input signal to the CPG network is enhanced so that the CPG network can express the self-regulation movement state instead of being limited to the spontaneous behavior, and thus the regulation role of the cerebral cortex signal can be reflected. Numerical simulation shows that the modified model can generate various movement forms with different modes, frequencies, and interchanges between them. It is revealed in theories that the cerebral cortex signal can regulate the mode and frequency of the gait in the course of the gait movement.     

Stability of quadruped robots’ trajectories subjected to discrete perturbations

In this paper, we study the stability of a mathematical model for trajectory generation of a qua-druped robot. We consider that each movement is composed of two types of primitives: rhythmic and discrete. The discrete primitive is inserted as a perturbation of the purely rhythmic movement. The two primitives are modeled by nonlinear dynamical systems. We adapt the theory developed by Golubitsky et?al. in (Physica D 115: 56?C72, 1998; Buono and Golubitsky in J. Math. Biol. 42:291?C326, 2001) for quadrupeds gaits. We conclude that if the discrete part is inserted in all limbs, with equal values, and as an offset of the rhythmic part, the obtained gait is stable and has the same spatial and spatiotemporal symmetry groups as the purely rhythmic gait, differing only on the value of the offset.     

旋转流场下模型大幅俯仰运动实验研究

在中航气动院FL-8低速风洞中,采用单自由度振荡机构进行了旋转流场下大幅俯仰运动的气动特性实验研究。模型在绕风速轴连续旋转的同时,进行给定频率和振幅绕体轴的俯仰振荡运动。测量了模型的动态气动特性,着重分析了不同运动参数对模型气动特性的影响。实验结果表明,旋转速度的存在使大振幅俯仰振荡实验中的滚转力矩和偏航力矩产生了明显的迟滞特性,但对俯仰力矩和法向力的迟滞特性影响不大。     

Fractional central pattern generators for bipedal locomotion

Locomotion has been a major research issue in the last few years. Many models for the locomotion rhythms of quadrupeds, hexapods, bipeds and other animals have been proposed. This study has also been extended to the control of rhythmic movements of adaptive legged robots. In this paper, we consider a fractional version of a central pattern generator (CPG) model for locomotion in bipeds. A fractional derivative D ^α f(x), with α non-integer, is a generalization of the concept of an integer derivative, where α=1. The integer CPG model has been proposed by Golubitsky, Stewart, Buono and Collins, and studied later by Pinto and Golubitsky. It is a network of four coupled identical oscillators which has dihedral symmetry. We study parameter regions where periodic solutions, identified with legs’ rhythms in bipeds, occur, for 0<α≤1. We find that the amplitude and the period of the periodic solutions, identified with biped rhythms, increase as α varies from near 0 to values close to unity.     

Numerical simulation of the influence of fabric’s motion on protective clothing performance during flash fire exposure

The motion of a person wearing protective clothing induces the clothing to move periodically towards the skin causing a cyclic variation in the air gap between the fabric and the skin. At the same time, the clothing movement causes cooling air to periodically flow into the air gap between the fabric and the skin. This paper uses a finite volume model to investigate these two effects and the resultant effect of the protective clothing movement on its performance during flash fire exposure. Special attention is drawn to the air gap model since it responds directly to the clothing movement. A parametric study is carried out to investigate the influence of a wider range of clothing movement. Specifically, the effect of the variation in the periodic movement frequency and amplitude on the clothing performance was investigated. The results show that increasing the movement frequency improves the clothing protective performance, while increasing the movement amplitude worsens the clothing performance.     

Bifurcation and chaos of a simple walking model driven by a rhythmic signal

The walk of animals is achieved by the interaction between the dynamics of their mechanical system and the central pattern generator (CPG). In this paper, we analyze dynamic properties of a simple walking model of a biped robot driven by a rhythmic signal from an oscillator. In particular, we examine the long-term global behavior and the bifurcation of the motion that leads to chaotic motion, depending on the model parameter values. The simple model consists of a hip and two legs connected at the hip through a rotational joint. The joint is driven by a rhythmic signal from an oscillator, which is an open loop. In order to analyze the bifurcation, we first obtained approximate solutions of the walking motion and then constructed discrete dynamics using the Poincaré map. As a result, we found that consecutive period-doubling bifurcations occur as the model parameter values change, and that the walking motion leads to chaotic motion over the critical value of the model parameters. Moreover, we approximately obtained the period-doubling solutions and the critical value by employing a Newton-Raphson method. Our analytical results were verified by the numerical simulations.     

Exploiting the subharmonic parametric resonances of a buckled beam for vibratory energy harvesting

The potential of harvesting vibratory energy via a bistable beam subjected to subharmonic parametric excitations is investigated. The vibrating structure is a buckled beam with two stable equilibria separated by a potential barrier. The beam is subjected to a superposition of a static axial load beyond its buckling load and a harmonic axial excitation whose frequency is around twice the frequency of the buckled beam’s first vibration mode. A macro-fiber composite patch is attached to one side of the beam to convert the strain energy resulting from the beam’s oscillation into electricity. The study considers two regimes of excitations: an amplitude sweep and a frequency sweep. In the first regime, the amplitude of excitation is quasi-statically varied while the excitation frequency is tuned at twice the natural frequency of the first vibration mode. In the second regime, the excitation frequency is swept forward and backward around the subharmonic resonant frequency while the amplitude of excitation is kept constant. A theoretical model which governs the electromechanical coupling of the transverse vibrations of the beam and the output voltage is used to monitor the response as the excitation parameters are changed. An experimental setup is also built and a series of tests is performed to validate the theoretical findings. It is shown that, depending on the amplitude and frequency of excitation, the harvester can perform small-amplitude periodic intra-well motion, intra- and inter-well chaotic motions, as well as periodic inter-well motions. Experimental results also show that, as compared to the classical linear resonance, utilizing the sub-harmonic resonance of a bistable energy harvesters can result in a broadband frequency response.     

A theoretical study of the mechanism with variable compression ratio and expansion ratio

In recent years, due to global energy crisis, many governments have reduced emission of carbon dioxide by regulations, which make improving e?ciency of engines an important topic. Carmakers have focused on an energy-saving measure called variable compression ratio (VCR) because it suits related technologies with high e?ciency easily. Among numerous VCR mechanisms, one with eccentric sleeve should draw people’s attention, because it could vary both compression ratio and expansion ratio simultaneously, and also bring excellent energy-saving performance with Atkinson effect, which make the mechanism a rather promising measure in fuel reduction. This paper presents a theoretical study of the mechanism piston movement process, then a discussion about effects of the eccentric phase and bore-to-stroke ratio. The result shows that the mechanism has similar piston movement processes with conventional engines. A proper eccentric phase and a lower bore-to-stroke ratio are suitable to the mechanism because of a larger VCR range and enhanced Atkinson effect. The conclusion could be a necessary reference for the original dimension selection and quite essential to further improve the competitiveness of the mechanism.     

直升机弹性多支点传动轴的主共振分析

传动轴主共振的最大振幅、稳定性、振幅突变性是其主要特性。在根据质心运动定理、Galer-kin法和Dirac函数求得非惯性移动系下直升机的倾斜弹性多支点传动轴的弯曲运动方程基础上,用多尺度法求得稳态下主共振的一次近似定常解,再分析了主共振的最大振幅、支点的位置及数量对主共振的影响、主共振的稳定性及振幅突变性等。弹性中间支点有提高传动轴的运动稳定性、增大相邻阶主共振之间的频率范围及限幅器的功用。提出通过减小偏心距、降低主共振的阶数、加大传动轴中及中间支点处的阻尼减小主共振的最大振幅及消除振幅突变。     

扰动风作用下微型旋翼流场的数值模拟及流动控制研究

对于处在低雷诺数下的微型旋翼,扰动风对其流场影响很大。本文基于动态嵌套网格技术的双时间法,采用网格速度法模拟扰动风,研究了不同频率和幅值的正弦式扰动风下三维微型旋翼上的拉力系数响应情况,并采用变形网格方法模拟了旋翼桨叶的周期性边距运动对扰动风的流动控制,为今后旋翼机的主动控制研究提供了线索。研究结果表明,低雷诺数下,旋翼在周期性扰动风作用下的拉力响应曲线拟合函数与扰动风函数形式相同,响应曲线函数同样具有周期性变化趋势,波动幅值与扰动风幅值成正比例关系;对不同飞行状态下的旋翼,采用周期等角变距方式,能够找到一个适合的控制参数使得旋翼在正弦式扰动风环境下旋翼的拉力波动在一定范围内得到有效抑制。     

Modeling Human Motor Systems in Nonlinear Dynamics: Intentionality and Discrete Movement Behaviors

Attempts to understand human movement systems from the perspective of nonlinear dynamics have increased in recent years, although research has almost exclusively focused on modeling rhythmical movements as limit cycle oscillators. Only a limited amount of work has been undertaken on discrete movements, generally only in the form of numerical simulations and mathematical models. In this paper we briefly overview the key findings from previous research on movement systems as nonlinear dynamical systems, and report data from a behavioral experiment on the coordination observed in a prehension movement under both discrete and rhythmical conditions. In a rhythmical condition subjects grasped dowels in time to a metronomic beat, whereas in a discrete condition a target dowel was grasped within a predetermined movement time. A scanning procedure was implemented to monitor changes in the time of relative final hand closure during hand transport to the dowel. For each condition, a pre-test and post-test of 10 trials were also conducted either side of the scanning trial block. No effects between condition or trial block were noted and there was a large amount of within-subject variability in the coordination data. The findings support previous theoretical modeling suggesting that subject intentionality acts as a more powerful constraint on the intrinsic dynamics of the movement system in discrete compared to rhythmical conditions. The high levels of individual variability were interpreted as being due to the competition between specific and non-specific control parameters (e.g., the subject's intentionality and the metronomic beat). It is concluded that discrete prehension movements appear amenable to a nonlinear dynamical analysis. The data also point to the innovative use of within-subject analyses in future work modeling motor systems as nonlinear dynamical systems.     

Three-dimensional dynamic imaging of sand particles under wheel via gamma-ray camera system

In recent years, attempts have been made to deploy robots for use in various activities such as planetary exploration, post-tsunami seashore reconnaissance, and volcano investigations. These robots may have to move on soft terrain. The movement of sand or soil particles under the wheels or tracks greatly affects the robot’s ability to maneuver. There is a simple but difficult problem with measuring particle movement: the sand and soil particles beneath the surface are not visible. Only 2D visualization techniques that take a surface picture of the ground or use transparent boards are available. A nuclear 3D imaging technique called positron emission particle tracking (PEPT) was developed at the University of Birmingham for this purpose. PEPT detects pairs of gamma rays emitted by a positron-emitting radionuclide of a tracer particle, which produces an image of the tracer. Thus, the overarching goal of this study was to explore the 3D terramechanics between terrain particles and a wheel or track using PEPT. As an initial step, this paper introduces an imaging technique for standard sand under a rotating wheel using PEPT and presents some images of sand particles under various conditions. Absolute displacements along the longitudinal, vertical, and lateral axes are presented.     

Hydrodynamics of a flexible cylinder under modulated vortex-induced vibrations

Both amplitude modulation and frequency modulation of Vortex-induced Vibration (VIV) are observed in a recent model test of a flexible cylinder under oscillatory flow, but its hydrodynamics has not yet been broached in detail. This paper employs the Forgetting Factor Least Squares (FF-LS) method for identification of time-varying hydrodynamics of a flexible cylinder under modulated VIV. The FF-LS method’s applicability to accurately identify time-varying hydrodynamic coefficients is demonstrated through an elastically mounted rigid cylinder under flow with a given modulated motion. Furthermore, we propose a framework to predict instantaneous amplitude (envelope) and frequency using time-varying hydrodynamic coefficients to establish their analytical relationship. This prediction method is further extended to a highly tensioned flexible cylinder through Fourier series expansion in the spatial domain. By performing the identification procedure for all sampled data of a flexible cylinder undergoing oscillatory flow, we obtain the corresponding time-varying hydrodynamics in the cross-flow direction considering the amplitude and frequency modulation. The results show that, under modulated VIV, hydrodynamic coefficients of the flexible cylinder also show time-varying characteristics. We further investigate differences between identified hydrodynamic coefficients and those obtained from the database of a cylinder with modulated motion under flow. Prediction results using these identified time-varying coefficients reveal that the time-varying excitation coefficients mainly influence the amplitude modulation, and the time-varying added-mass coefficients contain the major information of frequency modulation. These results further suggest including the temporal derivative of the instantaneous amplitude as one determining parameter in building databases to improve the prediction of modulated VIV.     

Application of the finite segment method to stabilisation of the force in a riser connection with a wellhead

This paper presents a spatial model of riser dynamics formulated using the segment method and its applications. The model has been validated by comparison of the authors’ own results with those obtained from experimental measurements and Abaqus on the basis of forced vibration with large amplitude for the riser submerged in water. The influence of the sea environment is considered. Correctness and numerical effectiveness of the model enable us to formulate and solve the force stabilisation problem. A dynamic optimisation problem is formulated and solved. As a result vertical courses of movement of the upper end of the riser are obtained which compensate the horizontal movement of the base and stabilise the force in the connection of the riser with a wellhead.     

Mechanism of mode selection in Rayleigh—Bénard convection with free—rigid boundaries

Higher harmonic resonances with wavenumber ratio of 1:2, 1:2:3 and so on are shown to take place in Rayleigh—Bénard convection under free—rigid boundary condition. Bifurcation diagrams for two-dimensional motion are obtained for the Prandtl number P = 7. The subharmonic instability is explained by a couple of amplitude equations obtained from weakly nonlinear stability theory. A straightforward extension of the coupled amplitude equations leads to a model which consists of n amplitude equations. The mechanism of mode selection is illustrated by numerical simulations of the model equations.     

Transparency enhancement for an active knee orthosis by a constraint-free mechanical design and a gait phase detection based predictive control

This paper proposes a novel mechanical design of a lower limb exoskeleton device which prevents the residual stresses due to arthro-kinematics movements of synovial joints and by the way allows effective compensation for dynamic disturbances in osteo-kinematic movements of the wearer. Here, the exoskeleton is only actuated at the knee joints to provide assistive torques, which are required to assist the anatomical joint motion and to increase the transparency of the device. Dynamic simulations of a virtual human equipped with this exoskeleton are used to quantify the disturbances induced by the device during locomotion and to show the benefit of passive mechanisms introduced in the mechanical attaches as well. The authors also demonstrated how the device’s transparency can be improved by providing the motor torques in order to compensate the inertial and gravitational effects. This can be done rely on the knowledge of the locomotion movement phases. A robust gait phase detection method was implemented on the experimental device in order to identify specific gait phases in real time. This method exploits the K-nearest neighbors algorithm to identify the k-closest trained vectors, coupling with a discrete time Markov chain to determine the phases shift probability during the gait cycle. This gait detection algorithm was tested with a percentage of success of more than 95% when the subjects walked with constant and variable stride lengths.     

Arm motion control model based on central pattern generator

According to the theory of Matsuoka neural oscillators and with the consideration of the fact that the human upper arm mainly consists of six muscles, a new kind of central pattern generator(CPG) neural network consisting of six neurons is proposed to regulate the contraction of the upper arm muscles. To verify effectiveness of the proposed CPG network, an arm motion control model based on the CPG is established.By adjusting the CPG parameters, we obtain the neural responses of the network, the angles of joint and hand of the model with MATLAB. The simulation results agree with the results of crank rotation experiments designed by Ohta et al., showing that the arm motion control model based on a CPG network is reasonable and effective.     

Linear and non-linear vibration and frequency response analyses of microcantilevers subjected to tip-sample interaction

Despite their simple structure and design, microcantilevers are receiving increased attention due to their unique sensing and actuation features in many MEMS and NEMS. Along this line, a non-linear distributed-parameters modeling of a microcantilever beam under the influence of a nanoparticle sample is studied in this paper. A long-range Van der Waals force model is utilized to describe the microcantilever-particle interaction along with an inextensibility condition for the microcantilever in order to derive the equations of motion in terms of only one generalized coordinate. Both of these considerations impose strong nonlinearities on the resultant integro-partial equations of motion. In order to provide an understanding of non-linear characteristics of combined microcantilever-particle system, a geometrical function is wisely chosen in such a way that natural frequency of the linear model exactly equates with that of non-linear model. It is shown that both approaches are reasonably comparable for the system considered here. Linear and non-linear equations of motion are then investigated extensively in both frequency and time domains. The simulation results demonstrate that the particle attraction region can be obtained through studying natural frequency of the system consisting of microcantilever and particle. The frequency analysis also proves that the influence of nonlinearities is amplified inside the particle attraction region through bending or shifting the frequency response curves. This is accompanied by sudden changes in the vibration amplitude estimated very closely by the non-linear model, while it cannot be predicted by the best linear model at all.     

横倾状态下地板表面形貌对人体步进摩擦的影响

人体在行走过程中足底与地面之间产生的间歇性摩擦现象称为步进摩擦，地板表面形貌作为步进摩擦的重要影响因素之一而受到广泛关注. 现有研究成果多是基于水平路面，对坡度道路，特别是横倾状态下地板表面形貌对人体步进摩擦的影响研究较少. 本文作者以普通碳钢作为地板材料，机械加工出不同的表面形貌，利用止滑试验机研究了水平状态下地板表面形貌与其能够提供的最大临界摩擦系数的关系，利用步进摩擦试验平台研究了横倾状态下地板表面形貌对人体步进摩擦的影响. 结果表明:水平状态下，随着表面粗糙度的增大和表面纹理间距的减小，临界摩擦系数与人体行走实际所产生的有效摩擦系数均增加，防滑性能提高；横倾状态下，随着路面横倾角度的增大，有效摩擦系数及其侧向分量增大，侧滑风险提高；地板表面纹理排列方向对有效摩擦系数的影响明显，横向分布的防滑性能优于纵向.