Impedance control of a cable-driven SEA with mixed H2/H∞H_2/H_\infty synthesis

Purpose: This paper presents an impedance control method with mixed $H_2/H_\infty$ synthesis and relaxed passivity for a cable-driven series elastic actuator to be applied for physical human-robot interaction. Design/methodology/approach: To shape the system's impedance to match a desired dynamic model, the impedance control problem was reformulated into an impedance matching structure. The desired competing performance requirements as well as constraints from the physical system can be characterized with weighting functions for respective signals. Considering the frequency properties of human movements, the passivity constraint for stable human-robot interaction, which is required on the entire frequency spectrum and may bring conservative solutions, has been relaxed in such a way that it only restrains the low frequency band. Thus, impedance control became a mixed $H_2/H_\infty$ synthesis problem, and a dynamic output feedback controller can be obtained. Findings: The proposed impedance control strategy has been tested for various desired impedance with both simulation and experiments on the cable-driven series elastic actuator platform. The actual interaction torque tracked well the desired torque within the desired norm bounds, and the control input was regulated below the motor velocity limit. The closed loop system can guarantee relaxed passivity at low frequency. Both simulation and experimental results have validated the feasibility and efficacy of the proposed method. Originality/value: This impedance control strategy with mixed $H_2/H_\infty$ synthesis and relaxed passivity provides a novel, effective and less conservative method for physical human-robot interaction control.Comment: 11 pages, already published in Assembly Automatio

Inferring Gene Regulatory Network Using An Evolutionary Multi-Objective Method

Inference of gene regulatory networks (GRNs) based on experimental data is a challenging task in bioinformatics. In this paper, we present a bi-objective minimization model (BoMM) for inference of GRNs, where one objective is the fitting error of derivatives, and the other is the number of connections in the network. To solve the BoMM efficiently, we propose a multi-objective evolutionary algorithm (MOEA), and utilize the separable parameter estimation method (SPEM) decoupling the ordinary differential equation (ODE) system. Then, the Akaike Information Criterion (AIC) is employed to select one inference result from the obtained Pareto set. Taking the S-system as the investigated GRN model, our method can properly identify the topologies and parameter values of benchmark systems. There is no need to preset problem-dependent parameter values to obtain appropriate results, and thus, our method could be applicable to inference of various GRNs models.Comment: 8page

Universality of Heisenberg-Ising chain in external fields

Motivated by the recent surge of transverse-field experiments on quasi-one-dimensional antiferromagnets Sr(Ba)Co$_2$V$_2$O$_8$, we investigate the quantum phase transition in a Heisenberg-Ising chain under a combination of two in-plane inter-perpendicular transverse fields and a four-period longitudinal field, where the in-plane transverse field is either uniform or staggered. We show that the model can be unitary mapped to the one-dimensional transverse-field Ising model (1DTFIM) when the $x$ and $y$ components of the spin interaction and the four-period field are absent. When these two terms are present, following both analytical and numerical efforts, we demonstrate that the system undergoes a second-order quantum phase transition with increasing transverse fields, where the critical exponents as well as the central charge fall into the universality of 1DTFIM. Our results naturally identify the 1DTFIM universality of 1D quantum phase transitions observed in the existed experiments in Sr(Ba)Co$_2$V$_2$O$_8$ with transverse field applied along either [100] or [110] direction. Upon varying the tuning parameters a critical surface with 1DTFIM universality is determined and silhouetted to exhibit the general presence of the universality in a much wider scope of models than conventional understanding. Thus our results provide a broad guiding framework to facilitate the experimental realization of 1DTFIM universality in real materials.Comment: 8 pages, 4 figure

Online Energy Management for a Sustainable Smart Home with an HVAC Load and Random Occupancy

In this paper, we investigate the problem of minimizing the sum of energy cost and thermal discomfort cost in a long-term time horizon for a sustainable smart home with a Heating, Ventilation, and Air Conditioning (HVAC) load. Specifically, we first formulate a stochastic program to minimize the time average expected total cost with the consideration of uncertainties in electricity price, outdoor temperature, renewable generation output, electrical demand, the most comfortable temperature level, and home occupancy state. Then, we propose an online energy management algorithm based on the framework of Lyapunov optimization techniques without the need to predict any system parameters. The key idea of the proposed algorithm is to construct and stabilize four queues associated with indoor temperature, electric vehicle charging, and energy storage. Moreover, we theoretically analyze the feasibility and performance guarantee of the proposed algorithm. Extensive simulations based on real-world traces show the effectiveness of the proposed algorithm.Comment: 14 pages, 21 figure

Photon-mediated electronic correlation effects in irradiated two-dimensional Dirac systems

Periodically driven systems can host many interesting and intriguing phenomena. The irradiated two-dimensional Dirac systems, driven by circularly polarized light, are the most attractive thanks to intuitive physical view of the absorption and emission of photon near Dirac cones. Here, we assume that the light is incident in the two-dimensional plane, and choose to treat the light-driven Dirac systems by making a unitary transformation to capture the photon-mediated electronic correlation effects, instead of using usual Floquet theory. In this approach, the electron-photon interaction terms can be cancelled out and the resultant effective electron-electron interactions can produce important effects. These effective interactions will produce a topological band structure in the case of 2D Fermion system with one Dirac cone, and can lift the energy degeneracy of the Dirac cones for graphene. This method can be applicable to similar light-driven Dirac systems to investigate photon-mediated electronic effects in them.Comment: 5 pages, 4 figure

Saving Gradient and Negative Curvature Computations: Finding Local Minima More Efficiently

We propose a family of nonconvex optimization algorithms that are able to save gradient and negative curvature computations to a large extent, and are guaranteed to find an approximate local minimum with improved runtime complexity. At the core of our algorithms is the division of the entire domain of the objective function into small and large gradient regions: our algorithms only perform gradient descent based procedure in the large gradient region, and only perform negative curvature descent in the small gradient region. Our novel analysis shows that the proposed algorithms can escape the small gradient region in only one negative curvature descent step whenever they enter it, and thus they only need to perform at most $N_{\epsilon}$ negative curvature direction computations, where $N_{\epsilon}$ is the number of times the algorithms enter small gradient regions. For both deterministic and stochastic settings, we show that the proposed algorithms can potentially beat the state-of-the-art local minima finding algorithms. For the finite-sum setting, our algorithm can also outperform the best algorithm in a certain regime.Comment: 31 pages, 1 tabl

Computation Load Balancing Real-Time Model Predictive Control in Urban Traffic Networks

Owing to the rapid growth number of vehicles, urban traffic congestion has become more and more severe in the last decades. As an effective approach, Model Predictive Control (MPC) has been applied to urban traffic signal control system. However, the potentially high online computation burden may limit its further application for real scenarios. In this paper, a new approach based on online active set strategy is proposed to improve the real-time performance of MPC-based traffic controller by reducing the online computing time. This approach divides one control cycle into several sequential sampling intervals. In each interval, online active set method is applied to solve quadratic programming (QP) of traffic signal control model, by searching the optimal solution starting at the optimal solution of previous interval in the feasible region. The most appealing property of this approach lies in that it can distribute the computational complexity into several sample intervals, instead of imposing heavy computation burden at each end of control cycle. The simulation experiments show that this breakthrough approach can obviously reduce the online computational complexity, and increase the applicability of the MPC in real-life traffic networks

Passivity guaranteed stiffness control with multiple frequency band specifications for a cable-driven series elastic actuator

Impedance control and specifically stiffness control are widely applied for physical human-robot interaction. The series elastic actuator (SEA) provides inherent compliance, safety and further benefits. This paper aims to improve the stiffness control performance of a cable-driven SEA. Existing impedance controllers were designed within the full frequency domain, though human-robot interaction commonly falls in the low frequency range. We enhance the stiffness rendering performance under formulated constraints of passivity, actuator limitation, disturbance attenuation, noise rejection at their specific frequency ranges. Firstly, we reformulate this multiple frequency-band optimization problem into the $H_\infty$ synthesis framework. Then, the performance goals are quantitatively characterized by respective restricted frequency-domain specifications as norm bounds. Further, a structured controller is directly synthesized to satisfy all the competing performance requirements. Both simulation and experimental results showed that the produced controller enabled good interaction performance for each desired stiffness varying from 0 to 1 times of the physical spring constant. Compared with the passivity-based PID method, the proposed $H_\infty$ synthesis method achieved more accurate and robust stiffness control performance with guaranteed passivity.Comment: 10 pages, already published in Mechanical Systems and Signal Processin

Hidden chiral symmetry protected Z⊕Z\mathbb{Z}\oplus\mathbb{Z} topological insulators in a ladder dimer model

We construct a two-leg ladder dimer model by using two orbitals instead of one in Su-Schrieffer-Heeger (SSH) dimer chain and find out a chiral symmetry in it. In this model, the otherwise-hidden additional chiral symmetry allows us to define two chiral massless fermions and show that there exist interesting topological states characterized by $\mathbb{Z}\oplus\mathbb{Z}$ and corresponding zero mode edge states. Our complete phase diagram reveals that there is an anomalous topologically nontrivial region in addition to the normal one similar to that of SSH model. The anomalous region features that the inter-cell hopping constants can be much smaller than the intra-cell ones, being opposite to the normal region, and the zero mode edge states do not have well-defined parity each. Finally, we suggest that this ladder dimer model can be realized in double-well optical lattices, ladder polymer systems, and adatom double chains on semiconductor surfaces.Comment: 6 pages, 5 figure

Floquet Weyl fermions in circularly-polarised-light-irradiated three-dimensional stacked graphene systems

Using Floquet theory, we illustrate that Floquet Weyl fermions can be created in circularly-polarised-light-irradiated three-dimensional stacked graphene systems. One or two semi-Dirac points can be formed due to overlapping of Floquet sub-bands. Each pair of Weyl points have a two-component semi-Dirac point parent, instead of a four-component Dirac point parent. Decreasing the light frequency will make the Weyl points move in the momentum space, and the Weyl points can approach to the Dirac points when the frequency becomes very small. The frequency-amplitude phase diagram is worked out. It is shown that there exist Fermi arcs in the surface Brillouin zones in circularly-polarised-light-irradiated semi-infinitely-stacked and finitely-multilayered graphene systems. The Floquet Weyl points emerging due to the overlap of Floquet sub-bands provide a new platform to study Weyl fermions.Comment: 7 pages, 4 figures (minor improvement