Stable Periodic States Research Articles

Stability analysis for passive robots walking on inclined surfaces with local angles

For common-legged robots with a kind of stiff joints, the human-like and consecutive gait shown by passive walking robots is a bit difficult to achieve because of varying degrees of control, which is usually accompanied by computational cost. Of course, passive dynamic walking undoubtedly has its inevitable disadvantages because of the lack of control, which is stability. At the same time, the significant nonlinearity of the passive gait increases the difficulty of realizing the walking stability of the passive robot. Therefore, the passive robot still needs certain control to achieve stable walking even on the natural plane or smooth slope. In this paper, a passive robot model walking on an inclined plane with a local angle is studied. First of all, the approximate solution of the nonlinear dynamic equation is given by the perturbation method, and the conditions for the robot to achieve stable walking without external forces are obtained. Further, the input-output feedback linearization control based on hybrid zero dynamics is employed to carry out virtual constraints on the passive robot during the swing stage of walking, facilitating the transition from the unstable state to the stable periodic state. From the result of the experiments, the walking stability of the passive robot is improved over a larger range of walking status values compared with the condition without external force control, and the use of this method reduces the complicated work of solving the fixed point. These findings possibly have reference value in passive walking stabilization control. Meanwhile, the simulation results obtained by designing and studying the minimum controlled walking model show that human walking is based on the uncontrolled mechanical process. The exploration of the mechanism of passive gait can provide some enlightenment to the research of human texture and the application of exoskeleton.

Extreme multistability in symmetrically coupled clocks

Extreme multistability (EM) is characterized by the emergence of infinitely many coexisting attractors or continuous families of stable states in dynamical systems. EM implies complex and hardly predictable asymptotic dynamical behavior. We analyze a model for pendulum clocks coupled by springs and suspended on an oscillating base and show how EM can be induced in this system by specifically designed coupling. First, we uncover that symmetric coupling can increase the dynamical complexity. In particular, the coexistence of multiple isolated attractors and continuous families of stable periodic states is generated in a symmetric cross-coupling scheme of four pendulums. These coexisting infinitely many states are characterized by different levels of phase synchronization between the pendulums, including anti-phase and in-phase states. Some of the states are characterized by splitting of the pendulums into groups with silent sub-threshold and oscillating behavior, respectively. The analysis of the basins of attraction further reveals the complex dependence of EM on initial conditions.

Simplicial epidemic model with birth and death.

In this paper, we propose a simplicial susceptible-infected-susceptible (SIS) epidemic model with birth and death to describe epidemic spreading based on group interactions, accompanying with birth and death. The site-based evolutions are formulated by the quenched mean-field probability equations for each site, which is a high-dimensional differential system. To facilitate a theoretical analysis of the influence of system parameters on dynamics, we adopt the mean-field method for our model to reduce the dimension. As a consequence, it suggests that birth and death rates influence the existence and stability of equilibria, as well as the appearance of a bistable state (the coexistence of the stable disease-free and endemic states), which is then confirmed by extensive simulations on empirical and synthetic networks. Furthermore, we find that another type of the bistable state in which a stable periodic outbreak state coexists with a steady disease-free state also emerges when birth and death rates and other parameters satisfy the certain conditions. Finally, we illustrate how the birth and death rates shift the density of infected nodes in the stationary state and the outbreak threshold, which is also verified by sensitivity analysis for the proposed model.

Pattern formation in a four-ring reaction-diffusion network with heterogeneity.

In networks of nonlinear oscillators, symmetries place hard constraints on the system that can be exploited to predict universal dynamical features and steady states, providing a rare generic organizing principle for far-from-equilibrium systems. However, the robustness of this class of theories to symmetry-disrupting imperfections is untested in free-running (i.e., non-computer-controlled) systems. Here, we develop a model experimental reaction-diffusion network of chemical oscillators to test applications of the theory of dynamical systems with symmeries in the context of self-organizing systems relevant to biology and soft robotics. The network is a ring of four microreactors containing the oscillatory Belousov-Zhabotinsky reaction coupled to nearest neighbors via diffusion. Assuming homogeneity across the oscillators, theory predicts four categories of stable spatiotemporal phase-locked periodic states and four categories of invariant manifolds that guide and structure transitions between phase-locked states. In our experiments, we observed that three of the four phase-locked states were displaced from their idealized positions and, in the ensemble of measurements, appeared as clusters of different shapes and sizes, and that one of the predicted states was absent. We also observed the predicted symmetry-derived synchronous clustered transients that occur when the dynamical trajectories coincide with invariant manifolds. Quantitative agreement between experiment and numerical simulations is found by accounting for the small amount of experimentally determined heterogeneity in intrinsic frequency. We further elucidate how different patterns of heterogeneity impact each attractor differently through a bifurcation analysis. We show that examining bifurcations along invariant manifolds provides a general framework for developing intuition about how chemical-specific dynamics interact with topology in the presence of heterogeneity that can be applied to other oscillators in other topologies.

Long-wave equation for a confined ferrofluid interface: periodic interfacial waves as dissipative solitons

We study the dynamics of a ferrofluid thin film confined in a Hele-Shaw cell, and subjected to a tilted non-uniform magnetic field. It is shown that the interface between the ferrofluid and an inviscid outer fluid (air) supports travelling waves, governed by a novel modified Kuramoto–Sivashinsky-type equation derived under the long-wave approximation. The balance between energy production and dissipation in this long-wave equation allows for the existence of dissipative solitons. These permanent travelling waves’ propagation velocity and profile shape are shown to be tunable via the external magnetic field. A multiple-scale analysis is performed to obtain the correction to the linear prediction of the propagation velocity, and to reveal how the nonlinearity arrests the linear instability. The travelling periodic interfacial waves discovered are identified as fixed points in an energy phase plane. It is shown that transitions between states (wave profiles) occur. These transitions are explained via the spectral stability of the travelling waves. Interestingly, multi-periodic waves, which are a non-integrable analogue of the double cnoidal wave, are also found to propagate under the model long-wave equation. These multi-periodic solutions are investigated numerically, and they are found to be long-lived transients, but ultimately abruptly transition to one of the stable periodic states identified.

Modelling of time-varying meshing stiffness under tooth wear condition for nonlinear dynamics in compound planetary gear set

Under tooth profile wear condition, with running time increasing, the tooth chord length of the gear tooth is thinner and the tooth profile is not involute profile any more, which results in the changes in amplitude and fluctuation of time-varying meshing stiffness. In this work, in order to reveal the influence of tooth profile wear on time-varying meshing stiffness, an analytical model for calculating the time-varying meshing stiffness under tooth wear condition is built and incorporated with a “translational-rotational” dynamic model of compound planetary gear set. Poincare section, the phase trajectory and bifurcation diagram are employed for analysis of periodic and chaotic motion. The results indicate that, tooth wear accumulation result in the reduction in the amplitude of meshing stiffness at the meshing region of double-pair teeth, but makes no difference in meshing stiffness for the meshing region of single-pair teeth. In order to avoid the system being in Chaos motions, under large tooth wear condition, the input speed should be avoided away from 3700 r/min-6725 r/min, so as to keep the system in a stable periodic motion state.

Research on the impact response and chaos characteristics in the dynamic damage evolution process of concrete material

The impact response of concrete material under different dynamic loading conditions (single impact loading with different velocity and cyclic impact loading with different amplitude) have been tested and analyzed by split Hopkinson bar (SHPB) equipment, and the chaos characteristics of sample damage evolution process have also been discussed. Experimental results show that the mechanical properties of concrete samples show obvious strain rate effect, the peak stress increased by 750% with the single impact loading pressure increases from 0.2 MPa to 0.7 MPa. With the increase of the cyclic loading amplitude, the loading times and strain value of the sample decrease when the macroscopic failure occurs. Furthermore, it can be found that the process of dynamic response of concrete sample has experienced stable state, periodic state and chaotic state three stages in the process of damage evolution; the phase proportion of the stable state and the periodic state display negative correlation as the increase of the loading velocity (or the increase of the cyclic amplitude), conversely, the phase proportion of the chaotic state display positive correlation.

Fluid dynamics of a self-propelled biomimetic underwater vehicle with pectoral fins

Fluid dynamics of a self-propelled biomimetic underwater vehicle (BUV) with pectoral fins is investigated by an immersed boundary (IB) method. Typically, the BUV with a pair of pectoral fins starts from rest and attains a constant mean velocity as the mean longitudinal force is zero. The capability and accuracy of the IB method to deal with the interaction between the fluid and complex moving body are firstly validated. Then we carry out a parametric study to understand the effect of key governing parameters on the dynamic response of the BUV. It is found that with the increase of motion frequency or rolling amplitude, the pectoral fin propulsors can induce larger forward velocity so that the BUV takes less time to attain its stable periodic swimming state. Although the pectoral fin is a very complicated lifting surface, a linear relationship between forward Reynolds number (final swimming velocity is used as velocity scale) and frequency Reynolds number (product of motion frequency and fin chord length is used as velocity scale) can be established when the frequency Reynolds number is above a critical value. A linear relationship between forward Reynolds number and rolling amplitude is also found within the studied range of rolling amplitude. Furthermore, a small-density-ratio BUV is sensitive to the surrounding flow with more rapid evolution process of self-propulsion. Whereas, BUV with a large density ratio is more stable. The implications of the hydrodynamic analysis on the bio-inspired engineering design of BUV with pectoral fins are also discussed.

Robust computation with rhythmic spike patterns

Information coding by precise timing of spikes can be faster and more energy efficient than traditional rate coding. However, spike-timing codes are often brittle, which has limited their use in theoretical neuroscience and computing applications. Here, we propose a type of attractor neural network in complex state space and show how it can be leveraged to construct spiking neural networks with robust computational properties through a phase-to-timing mapping. Building on Hebbian neural associative memories, like Hopfield networks, we first propose threshold phasor associative memory (TPAM) networks. Complex phasor patterns whose components can assume continuous-valued phase angles and binary magnitudes can be stored and retrieved as stable fixed points in the network dynamics. TPAM achieves high memory capacity when storing sparse phasor patterns, and we derive the energy function that governs its fixed-point attractor dynamics. Second, we construct 2 spiking neural networks to approximate the complex algebraic computations in TPAM, a reductionist model with resonate-and-fire neurons and a biologically plausible network of integrate-and-fire neurons with synaptic delays and recurrently connected inhibitory interneurons. The fixed points of TPAM correspond to stable periodic states of precisely timed spiking activity that are robust to perturbation. The link established between rhythmic firing patterns and complex attractor dynamics has implications for the interpretation of spike patterns seen in neuroscience and can serve as a framework for computation in emerging neuromorphic devices.

Propagation phenomena for partially degenerate nonlocal dispersal models in time and space periodic habitats

This paper is concerned with propagation phenomena of a general class of partially degenerate nonlocal dispersal cooperative systems in time and space periodic habitats. We first show that such system has a finite spreading speed interval in any direction and there is a spreading speed for the partially degenerate system under certain conditions. Next, we prove that if the wave speed is greater than the spreading speed, there exists a time and space periodic traveling wave solution connecting the stable positive time and space periodic steady state and 0. Finally, we apply these results to two species partially degenerate competition systems and a partially degenerate epidemic model with nonlocal dispersal in time and space periodic habitats.

Hybrid multisynchronization of coupled multistable memristive neural networks with time delays

In this paper, we focus on synchronization issue of coupled multistable memristive neural networks (CMMNNs) with time delay under multiple stable equilibrium states. First, we build delayed CMMNNs consisting of one master subnetwork without controller and N−1 identical slave subnetworks with controllers, and every subnetwork has n nodes. Moreover, this paper investigates multistability of delayed CMMNNs with continuous nonmonotonic piecewise linear activation function (PLAF) owning 2r+2 corner points. By using the theorems of differential inclusion and fixed point, sufficient conditions are derived such that master subnetwork of CMMNNs can acquire (r+2)n exponentially stable equilibrium points, stable periodic orbits or hybrid stable equilibrium states. Then, this paper proposes hybrid multisynchronization of delayed CMMNNs related with various external inputs under multiple stable equilibrium states for the first time. There exist (r+2)n hybrid multisynchronization manifolds in CMMNNs with different initial conditions and external inputs. Finally, two numerical simulations are given to illustrate the effectiveness of the obtained results.

Discrete-Time Modeling and Symmetrical Dynamics of V²-Controlled Buck Converters With Trailing-Edge and Leading-Edge Modulations

Through analyzing the operating principles of $V^{2}$ controlled buck converters, 2-D discrete-time models and accurate switched models of $V^{2}$ controlled buck converters with trailing-edge and leading-edge modulations are established, respectively. The discrete-time models demonstrate the advantages of simple modeling and less computational complexity. Based on this, the dynamical behaviors of $V^{2}$ controlled buck converters are revealed through bifurcation diagrams and the boundary equations are derived by using the Jacobian matrix. Meanwhile, the operating regions with varying output capacitor time-constants are divided effectively by the boundary equations. The time-domain waveforms and phase portraits are also obtained through time-domain simulation. The research results indicate that trailing-edge- and leading-edge-modulated buck converters have symmetrical dynamical behaviors, such as symmetrical bifurcation, symmetrical time-domain waveforms, symmetrical phase portraits, and symmetrical period-doubling bifurcation boundaries. A symmetrical axis (SA) or a symmetrical point (SP) can be found in bifurcation diagrams, time-domain waveforms, phase portraits, and period-doubling bifurcation boundaries with varying output capacitor time-constants and duty ratios. By ramp compensation, the operating states of $V^{2}$ controlled buck converters with trailing-edge and leading-edge modulations could be effectively stabilized from unstable to stable periodic state. Experimental results are further given to verify the theoretical analysis and simulation results.

Asymptotic stability for spectrally stable Lugiato-Lefever solitons in periodic waveguides

We consider the Lugiato-Lefever model of optical fibers in the periodic context. Spectrally stable periodic steady states were constructed recently in the studies of Delcey and Haragus [Philos. Trans. R. Soc., A 376, 20170188 (2018)]; [Rev. Roumaine Math. Pures Appl. (to be published)]; and Hakkaev et al. (e-print arXiv:1806.04821). The spectrum of the linearization around such solitons consists of simple eigenvalues 0, −2α < 0, while the rest of it is a subset of the vertical line {μ:Rμ=−α}. Assuming such a property abstractly, we show that the linearized operator generates a C0 semigroup and, more importantly, the semigroup obeys (optimal) exponential decay estimates. Our approach is based on the Gearhart-Prüss theorem, where the required resolvent estimates may be of independent interest. These results are applied to the proof of asymptotic stability with phase of the steady states.

Prandtl number dependence of convective fluids in tall laterally heated slots

The influence of the Prandtl number on the stable stationary and periodic flows of the fluids contained in laterally heated slots under realistic conditions, namely non-slip boundaries, insulated top and bottom horizontal limits and perfectly conducting lateral sides, is analyzed by using continuation methods. The branches of solutions are computed by decreasing the Prandtl number, for four Rayleigh numbers. The dynamical behavior depends strongly on both parameters. For a Rayleigh number, Ra = 103, the steady flow remains stable in the wide range of Prandtl numbers computed. At Ra = 104 and O(105) the first bifurcations are of Hopf type giving rise to a type of oscillations that affects the bulk of the fluid, alternating from a general circulation to multi-vortex solutions, or the boundary layer, respectively. However, it is found that, in any case, the location of the shear determines the type of the oscillations. Moreover, at Ra = 104 the critical multipliers at the secondary bifurcations on the main branch of POs are real, giving rise to different kinds of very bounded stable periodic states of different symmetries and periods. At Ra of order 105 the instability of the periodic orbits gives rise directly to quasi-periodic flows.

Low-Index Equilibrium and Multiple Period-Doubling Cascades to Chaos of Atmospheric Flow in Beta-Plane Channel

The nonlinear dynamical behavior of an atmospheric circulation in a beta-plane channel is examined on a five-spectral mode model, truncated from the Charney and DeVore quasi-geostrophic equation. Bifurcation and chaos are observed when subjected to a topographic driving disturbance and a thermally driving zonal source. An equilibrium state undergoes supercritical Hopf bifurcation and becomes a stable periodic state with respect to the magnitude of the thermally driving source, whereas the periodic state undergoes a subcritical Hopf bifurcation and transforms into a low-index equilibrium state with respect to the increasing topographic driving disturbance. The stable periodic state further develops into a pair of stable periodic states when increasing the thermally driving source. The first one with the period of 4.3 days exhibits an oscillation of strong and weak zonal flow patterns, whereas the second one with the period of 6.8 days demonstrates a fluctuation amongst weak zonal disturbance flow patterns. Moreover, the two periodic states transform respectively into chaos through separate period-doubling cascades with the further development of the thermally driving source.

Suppression of dynamics and frequency synchronization in coupled slow and fast dynamical systems

We present our study on the emergent states of two interacting nonlinear systems with differing dynamical time scales. We find that the inability of the interacting systems to fall in step leads to difference in phase as well as change in amplitude. If the mismatch is small, the systems settle to a frequency synchronized state with constant phase difference. But as mismatch in time scale increases, the systems have to compromise to a state of no oscillations. We illustrate this for standard nonlinear systems and identify the regions of quenched dynamics in the parameter plane. The transition curves to this state are studied analytically and confirmed by direct numerical simulations. As an important special case, we revisit the well-known model of coupled ocean-atmosphere system used in climate studies for the interactive dynamics of a fast oscillating atmosphere and slowly changing ocean. Our study in this context indicates occurrence of multi stable periodic states and steady states of convection coexisting in the system, with a complex basin structure.

Pulsating waves of a partially degenerate reaction–diffusion system in a periodic habitat

We investigate a partially degenerate reaction–diffusion system in a periodic habitat and prove the existence and stability of pulsating waves. More specifically, we show that if the wave speed is greater than the spreading speed, then there exists a pulsating wave connecting the stable positive periodic steady state to the unstable trivial one. Further, this pulsating wave attracts exponentially in time all solutions with initial functions in its bounded neighborhood with respect to a weighted maximum norm.

Microscopic instability in recurrent neural networks.

In a manner similar to the molecular chaos that underlies the stable thermodynamics of gases, a neuronal system may exhibit microscopic instability in individual neuronal dynamics while a macroscopic order of the entire population possibly remains stable. In this study, we analyze the microscopic stability of a network of neurons whose macroscopic activity obeys stable dynamics, expressing either monostable, bistable, or periodic state. We reveal that the network exhibits a variety of dynamical states for microscopic instability residing in a given stable macroscopic dynamics. The presence of a variety of dynamical states in such a simple random network implies more abundant microscopic fluctuations in real neural networks which consist of more complex and hierarchically structured interactions.

Co-existing hidden attractors in a radio-physical oscillator system

The term ‘hidden attractor’ relates to a stable periodic, quasiperiodic or chaotic state whose basin of attraction does not overlap with the neighborhood of an unstable equilibrium point. Considering a three-dimensional oscillator system that does not allow for the existence of an equilibrium point, this paper describes the formation of several different coexisting sets of hidden attractors, including the simultaneous presence of a pair of coinciding quasiperiodic attractors and of two mutually symmetric chaotic attractors. We follow the dynamics of the system as a function of the basic oscillator frequency, describe the bifurcations through which hidden attractors of different type arise and disappear, and illustrate the form of the basins of attraction.

Global nonlinear dynamic characteristics of rod-fastening rotor supported by ball bearings

Flexible rod-fastening rotor-ball bearing system (FRRBS) is usually adopted in aircraft engines. In this paper, a simplified model for the FRRBS is proposed, and the global nonlinear dynamic characteristics of FRRBS are investigated. The FRRBS has two kinds of special structural features: rods and interfaces. The circumferentially distributed rods are modelled as a constant stiffness matrix with an additional moment (when unequal pre-tightening occurs) without mass effect. The interfaces between disks are modelled as stiffness matrices with normal and tangential contact stiffness, which are determined by pre-tightening force. Then, the shaft is discretized by Timoshenko elements and supported by ball bearings with radial internal clearance. Periodic motions and bifurcation sets are obtained by shooting method. The result shows that the insufficient pre-tightening brings about abnormal vibrations through quasi-periodic instability in high speed region, and the unequal pre-tightening leads to severe vibrations in stable periodic operation state. Moreover, the mass eccentricity would be the most nonlinear source for the nonlinear instabilities. Generally, this paper presents a method for modelling a FRRBS to study the nonlinear dynamic characteristics of the FRRBS.