11,322 research outputs found
Performance Analysis of Series Elastic Actuator based on Maximum Torque Transmissibility
The use of the Series Elastic Actuator (SEA) system as an actuator system
equipped with a compliant element has contributed not only to advances in human
interacting robots but also to a wide range of improvements in the robotics
area. Nevertheless, there are still limitations in its performance; the elastic
spring that is adopted to provide compliance is considered to limit the
actuator performance thus lowering the frequency bandwidth of force/torque
generation, and the bandwidth decreases even more when it is supposed to
provide large torque.
This weakness is in turn owing to the limitations of motor and motor drives
such as torque and velocity limits. In this paper, mathematical tools to
analyze the impact of these limitations on the performance of SEA as a
transmission system are provided. A novel criterion called Maximum Torque
Transmissibility (MTT)is defined to assess the ability of SEA to fully utilize
maximum continuous motor torque. Moreover, an original frequency bandwidth
concept, maximum torque frequency bandwidth, which can indicate the maximum
frequency up to which the SEA can generate the maximum torque, is proposed
based on the proposed MTT. The proposed MTT can be utilized as a unique
criterion of the performance, and thus various design parameters including the
load condition, mechanical design parameters, and controller parameters of a
SEA can be evaluated with its use. Experimental results under various
conditions verify that MTT can precisely indicate the limitation of the
performance of SEA, and that it can be utilized to accurately analyze the
limitation of the controller of SEA
Representations of integral quadratic polynomials
In this paper, we study the representations of integral quadratic
polynomials. Particularly, it is shown that there are only finitely many
equivalence classes of positive ternary universal integral quadratic
polynomials, and that there are only finitely many regular ternary triangular
forms. A more general discussion of integral quadratic polynomials over a
Dedekind domain inside a global field is also given
Towards Accurate Force Control of Series Elastic Actuators Exploiting a Robust Transmission Force Observer
This paper develops an accurate force control algorithm for series elastic
actuators (SEAs) based on a novel force estimation scheme, called transmission
force observer (TFOB). The proposed method is designed to improve an inferior
force measurement of the SEA caused by nonlinearities of the elastic
transmission and measurement noise and error of its deformation sensor. This
paper first analyzes the limitation of the conventional methods for the SEA
transmission force sensing and then investigates its stochastic
characteristics, which indeed provide the base to render the accurate force
control performance incorporated with the TFOB. In particular, a tuning
parameter is introduced from holistic closed-loop system analyses in the
frequency domain. This gives a guideline to attain optimum performance of the
force-controlled SEA system. The proposed algorithm is experimentally verified
in an actual SEA hardware setup
Black Hole Entropy and Exclusion Statistics
We compute the entropy of systems of quantum particles satisfying the
fractional exclusion statistics in the space-time of 2+1 dimensional black hole
by using the brick-wall method. We show that the entropy of each effective
quantum field theory with a Planck scale ultraviolet cutoff obeys the area law,
irrespective of the angular momentum of the black hole and the statistics
interpolating between Bose-Einstein and Fermi-Dirac statistics.Comment: REVTeX file, 12 page
The Effect of Anisotropic Extra Dimension in Cosmology
We consider five dimensional conformal gravity theory which describes an
anisotropic extra dimension. Reducing the theory to four dimensions yields
Brans-Dicke theory with a potential and a hidden parameter which implements
the anisotropy between the four dimensional spacetime and the extra dimension.
We find that a range of value of the parameter can address the current dark
energy density compared to the Planck energy density. Constraining the
parameter and the other cosmological model parameters using the recent
observational data consisting of the Hubble parameters, type Ia supernovae, and
baryon acoustic oscillations, together with the Planck or WMAP 9-year data of
the cosmic microwave background radiation, we find for Planck data
and for WMAP 9-year data at 95\% confidence level. We also obtained
constraints on the rate of change of the effective Newtonian constant~() at present and the variation of since the epoch of
recombination to be consistent with observation.Comment: Revised version. arXiv admin note: text overlap with arXiv:1512.0054
Massive Photon and Dark Energy
We investigate cosmology of massive electrodynamics and explore the
possibility whether massive photon could provide an explanation of the dark
energy. The action is given by the scalar-vector-tensor theory of gravity which
is obtained by non-minimal coupling of the massive Stueckelberg QED with
gravity and its cosmological consequences are studied by paying a particular
attention to the role of photon mass. We find that the theory allows
cosmological evolution where the radiation- and matter-dominated epochs are
followed by a long period of virtually constant dark energy that closely mimics
CDM model and the main source of the current acceleration is provided
by the nonvanishing photon mass governed by the relation . A
detailed numerical analysis shows that the nonvanishing photon mass of the
order of eV is consistent with the current observations. This
magnitude is far less than the most stringent limit on the photon mass
available so far, which is of the order of eV.Comment: 28 pages, 14 figures, Revised version, accepted for publication in
PR
A quantum field theoretical model of neutrino oscillation without external wave packets
We develop a general and consistent model of neutrino oscillation based on
the quantum field theoretical description of the neutrino production and
detection processes. Emphasis is placed on the locality of the interactions of
these processes, where on top of the usual application of the four fermion
local Hamiltonian, we assume that weak interactions switched on only when the
wave functions of the particles involved are overlapping and switched off upon
their separation. A key assumption in our treatment is that the wave packet
sizes of the particles, in particular, the neutrino producing source particles
and the neutrino absorbing detector particles, are taken to be negligible
compared with their mean free path in their respective medium. With this
assumption, and taking into considerations of the finite time of neutrino
production, neutrino wave packets with well-defined edges are generated. This
fact, together with the locality of weak interactions, enable us to relate the
propagation time to the propagation distance, thus doing away with the ad hoc
time averaging procedure normally employing in derivations of neutrino
oscillation formula. No assumptions on the particular forms of particle wave
functions; for example, Gaussians, need to be made. A good feature of our
approach is that the neutrino oscillation formula is automatically normalised
if the in-going states of the production and detection processes are
normalised. We also show that causality and unitarity cannot both be satisfied
in virtual neutrino models.Comment: 32 pages, 10 figure
Dark aspects of massive spinor electrodynamics
We investigate the cosmology of massive spinor electrodynamics when torsion
is non-vanishing. A non-minimal interaction is introduced between the torsion
and the vector field and the coupling constant between them plays an important
role in subsequential cosmology. It is shown that the mass of the vector field
and torsion conspire to generate dark energy and pressureless dark matter, and
for generic values of the coupling constant, the theory effectively provides an
interacting model between them with an additional energy density of the form
. The evolution equations mimic CDM behavior up to
term and the additional term represents a deviation from CDM. We show
that the deviation is compatible with the observational data, if it is very
small. We find that the non-minimal interaction is responsible for generating
an effective cosmological constant which is directly proportional to the mass
squared of the vector field and the mass of the photon within its current
observational limit could be the source of the dark energy.Comment: 15 pages, 1 figure. Accepted for publication in JCA
Neutron Star Structure in Ho\v{r}ava-Lifshitz Gravity
We present interesting aspects of a modified theory of gravity called
Ho\v{r}ava-Lifshitz (HL) gravity that can be constrained by the
phenomenological and observational prospects in terms of neutron star (NS)
structure and equation-of-state models. The deformation in HL gravity from
general relativity (GR) can change typical features of the NS structure. In
this Letter, we investigate the NS structure by deriving
Tolman-Oppenheimer-Volkoff equation in HL gravity. We find that a NS in HL
gravity with larger radius and heavier mass than a NS in GR remains stable
without collapsing into a black hole.Comment: 6 pages, 4 figures, 1 tabl
Topological quantum phase transitions in the spin-singlet superconductor with Rashba and Dresselhaus (110) spin-orbit couplings
We examine the topological properties of a spin-singlet superconductor with
Rashba and Dresselhaus (110) spin-orbit couplings. We demonstrate that there
are several topological invariants in the Bogoliubov-de Gennes (BdG)
Hamiltonian by symmetry analysis. We use the Pfaffian invariant
for the particle-hole symmetry to demonstrate all the possible phase diagrams
of the BdG Hamiltonian. We find that the edge spectrum is either Dirac cone or
flat band which supports the emergence of the Majorana fermion in this system.
For the Majorana flat bands, a higher symmetric BdG Hamiltonian is needed to
make them topologically stable. The Pfaffian invariant and
the winding number are used in determining the location of
the Majorana flat bands
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