11,861 research outputs found
Energy-based trajectory tracking and vibration control for multilink highly flexible manipulators
In this paper, a discrete model is adopted, as proposed by Hencky for elastica based on rigid bars and lumped rotational springs, to design the control of a lightweight planar manipulator with multiple highly flexible links. This model is particularly suited to deal with nonlinear equations of motion as those associated with multilink robot arms, because it does not include any simplification due to linearization, as in the assumed modes method. The aim of the control is to track a trajectory of the end effector of the robot arm, without the onset of vibrations. To this end, an energy-based method is proposed. Numerical simulations show the effectiveness of the presented approach
Drag cancellation by added-mass pumping
A submerged body subject to a sudden shape-change experiences large forces
due to the variation of added-mass energy. While this phenomenon has been
studied for single actuation events, application to sustained propulsion
requires studying \textit{periodic} shape-change. We do so in this work by
investigating a spring-mass oscillator submerged in quiescent fluid subject to
periodic changes in its volume. We develop an analytical model to investigate
the relationship between added-mass variation and viscous damping and
demonstrate its range of application with fully coupled fluid-solid
Navier-Stokes simulations at large Stokes number. Our results demonstrate that
the recovery of added-mass kinetic energy can be used to completely cancel the
viscous damping of the fluid, driving the onset of sustained oscillations with
amplitudes as large as four times the average body radius . A quasi-linear
relationship is found to link the terminal amplitude of the oscillations ,
to the extent of size change , with peaking at values from 4 to 4.75
depending on the details of the shape-change kinematics. In addition, it is
found that pumping in the frequency range of
is required for
sustained oscillations. The results of this analysis shed light on the role of
added-mass recovery in the context of shape-changing bodies and
biologically-inspired underwater vehicles.Comment: 10 pages, 6 Figures, under review in JFM Rapid
Conservative effects in spin-transfer-driven magnetization dynamics
It is shown that under appropriate conditions spin-transfer-driven
magnetization dynamics in a single-domain nanomagnet is conservative in nature
and admits a specific integral of motion, which is reduced to the usual
magnetic energy when the spin current goes to zero. The existence of this
conservation law is connected to the symmetry properties of the dynamics under
simultaneous inversion of magnetisation and time. When one applies an external
magnetic field parallel to the spin polarization, the dynamics is transformed
from conservative into dissipative. More precisely, it is demonstrated that
there exists a state function such that the field induces a monotone relaxation
of this function toward its minima or maxima, depending on the field
orientation. These results hold in the absence of intrinsic damping effects.
When intrinsic damping is included in the description, a competition arises
between field-induced and damping-induced relaxations, which leads to the
appearance of limit cycles, that is, of magnetization self-oscillations.Comment: 5 pages, 3 figure
Performance evaluation of an open distributed platform for realistic traffic generation
Network researchers have dedicated a notable part of their efforts
to the area of modeling traffic and to the implementation of efficient traffic
generators. We feel that there is a strong demand for traffic generators
capable to reproduce realistic traffic patterns according to theoretical
models and at the same time with high performance. This work presents an open
distributed platform for traffic generation that we called distributed
internet traffic generator (D-ITG), capable of producing traffic (network,
transport and application layer) at packet level and of accurately replicating
appropriate stochastic processes for both inter departure time (IDT) and
packet size (PS) random variables. We implemented two different versions of
our distributed generator. In the first one, a log server is in charge of
recording the information transmitted by senders and receivers and these
communications are based either on TCP or UDP. In the other one, senders and
receivers make use of the MPI library. In this work a complete performance
comparison among the centralized version and the two distributed versions of
D-ITG is presented
Anomalous vortex ring velocities induced by thermally-excited Kelvin waves and counterflow effects in superfluids
Dynamical counterflow effects on vortex evolution under the truncated
Gross-Pitaevskii equation are investigated. Standard longitudinal mutual
friction effects are produced and a dilatation of vortex rings is obtained at
large counterflow. A strong temperature-dependent anomalous slowdown of vortex
rings is observed and attributed to the presence of thermally exited Kelvin
waves. This generic effect of finite-temperature superfluids is estimated using
energy equipartition and orders of magnitude are given for weakly interacting
Bose-Einstein condensates and superfluid
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