32,181 research outputs found
Impedance control of a cable-driven SEA with mixed synthesis
Purpose: This paper presents an impedance control method with mixed
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
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
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)CoVO, 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 and 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)CoVO 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 negative curvature
direction computations, where 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 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 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 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 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
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