178,569 research outputs found
Microgravity vibration isolation: An optimal control law for the one-dimensional case
Certain experiments contemplated for space platforms must be isolated from the accelerations of the platforms. An optimal active control is developed for microgravity vibration isolation, using constant state feedback gains (identical to those obtained from the Linear Quadratic Regulator (LQR) approach) along with constant feedforward (preview) gains. The quadratic cost function for this control algorithm effectively weights external accelerations of the platform disturbances by a factor proportional to (1/omega)(exp 4). Low frequency accelerations (less than 50 Hz) are attenuated by greater than two orders of magnitude. The control relies on the absolute position and velocity feedback of the experiment and the absolute position and velocity feedforward of the platform, and generally derives the stability robustness characteristics guaranteed by the LQR approach to optimality. The method as derived is extendable to the case in which only the relative positions and velocities and the absolute accelerations of the experiment and space platform are available
Robust process-controller for Nd:YAG welding
A robust process-controller was developed which maintains a fully penetrated weld. This feedback-controller is robust for various process disturbances that may occur, like variations in sheet thickness, welding velocity, focal position, shielding gas variations, etc. The signals from an optical sensor are used to design a so-called "switching" controller, which enables fully penetrated welding at a minimum amount of laser power. It is shown that the working principle of the controller is applicable to different material types, more specific different steels and stainless steels. Experiments have been carried out to show the ability to cope with varying sheet thickness and\ud
welding velocity on FeP04 mild steel. Furthermore a method is described which can be used to easily find a good set of control parameters from a single identification experiment
Haptic guidance improves the visuo-manual tracking of trajectories
BACKGROUND: Learning to perform new movements is usually achieved by
following visual demonstrations. Haptic guidance by a force feedback device is
a recent and original technology which provides additional proprioceptive cues
during visuo-motor learning tasks. The effects of two types of haptic
guidances-control in position (HGP) or in force (HGF)-on visuo-manual tracking
("following") of trajectories are still under debate. METHODOLOGY/PRINCIPALS
FINDINGS: Three training techniques of haptic guidance (HGP, HGF or control
condition, NHG, without haptic guidance) were evaluated in two experiments.
Movements produced by adults were assessed in terms of shapes (dynamic time
warping) and kinematics criteria (number of velocity peaks and mean velocity)
before and after the training sessions. CONCLUSION/SIGNIFICANCE: These results
show that the addition of haptic information, probably encoded in force
coordinates, play a crucial role on the visuo-manual tracking of new
trajectories
Can gravitational infall energy lead to the observed velocity dispersion in DLAs?
The median observed velocity width v_90 of low-ionization species in damped
Ly-alpha systems is close to 90 km/s, with approximately 10% of all systems
showing v_90 > 210 km/s at z=3. We show that a relative shortage of such
high-velocity neutral gas absorbers in state-of-the-art galaxy formation models
is a fundamental problem, present both in grid-based and particle-based
numerical simulations. Using a series of numerical simulations of varying
resolution and box size to cover a wide range of halo masses, we demonstrate
that energy from gravitational infall alone is insufficient to produce the
velocity dispersion observed in damped Ly-alpha systems, nor does this
dispersion arise from an implementation of star formation and feedback in our
highest resolution (~ 45 pc) models, if we do not put any galactic winds into
our models by hand. We argue that these numerical experiments highlight the
need to separate dynamics of different components of the multiphase
interstellar medium at z=3.Comment: 12 Pages, 9 Figures, accepted to ApJ, printing in colour recommende
Experimental study of contact transition control incorporating joint acceleration feedback
Joint acceleration and velocity feedbacks are incorporated into a classical internal force control of a robot in contact with the environment. This is intended to achieve a robust contact transition and force tracking performance for varying unknown environments, without any need of adjusting the controller parameters, A unified control structure is proposed for free motion, contact transition, and constrained motion in view of the consumption of the initial kinetic energy generated by a nonzero impact velocity. The influence of the velocity and acceleration feedbacks, which are introduced especially for suppressing the transition oscillation, on the postcontact tracking performance is discussed. Extensive experiments are conducted on the third joint of a three-link direct-drive robot to verify the proposed scheme for environments of various stiffnesses, including elastic (sponge), less elastic (cardboard), and hard (steel plate) surfaces. Results are compared with those obtained by the transition control scheme without the acceleration feedback. The ability of the proposed control scheme in resisting the force disturbance during the postcontact period is also experimentally investigated
A flow disturbance estimation and rejection strategy for multirotors with round-trip trajectories
This paper presents a round-trip strategy of multirotors subject to unknown
flow disturbances. During the outbound flight, the vehicle immediately utilizes
the wind disturbance estimations in feedback control, as an attempt to reduce
the tracking error. During this phase, the disturbance estimations with respect
to the position are also recorded for future use. For the return flight, the
disturbances previously collected are then routed through a feedforward
controller. The major assumption here is that the disturbances may vary over
space, but not over time during the same mission. We demonstrate the
effectiveness of this feedforward strategy via experiments with two different
types of wind flows; a simple jet flow and a more complex flow. To use as a
baseline case, a cascaded PD controller with an additional feedback loop for
disturbance estimation was employed for outbound flights. To display our
contributions regarding the additional feedforward approach, an additional
feedforward correction term obtained via prerecorded data was integrated for
the return flight. Compared to the baseline controller, the feedforward
controller was observed to produce 43% less RMSE position error at a vehicle
ground velocity of 1 m/s with 6 m/s of environmental wind velocity. This
feedforward approach also produced 14% less RMSE position error for the complex
flows as well
The baryonic self similarity of dark matter
The cosmological simulations indicates that the dark matter haloes have
specific self similar properties. However the halo similarity is affected by
the baryonic feedback, the momentum injected by the supernovae re-shape the
dark matter core and transform it to a flat density core, with a scale length
imposed by the baryonic feedback. Additionally the baryon feedback impose also
an equilibrium condition, which when coupled with the imposed baryonic scale
length induce a new type of similarity. The new self similar solution implies
that the acceleration generated by dark matter is scale free, which in turns
implies that the baryonic acceleration at a reference radius is also scale
free. Constant dark matter and baryonic accelerations at a reference radius
have effectively been observed for a large class of different galaxies, which
is in support of this approach. The new self similar properties implies that
the total acceleration at larger distances is scale free, the transition
between the dark matter and baryons dominated regime occurs at a constant
acceleration, and the maximum of the velocity curve which defines the amplitude
of the velocity curve at larger distances is proportional to .
These results demonstrates that in this self similar model, cold dark matter is
consistent with the basics of MOND phenomenology for the galaxies. In agreement
with the observation the coincidence between the self similar model and MOND is
expected to break at the scale of clusters of galaxies. Some numerical
experiments shows that the behavior of the density near the origin is closely
approximated by a Einasto profile.Comment: Last versio
Robotic Force Control using Observer-based Strict Positive Real Impedance Control
This paper presents theoretical and experimentalresults on observer-based impedance control for forcecontrol without velocity feedback. As the velocity may notavailable to measurement, which is often the case for industrialrobots, an observer was designed to reconstruct velocityin such a way that it be useful for stabilizing feedbackcontrol and to modification of the damping in the impedancerelationship. A good model of the robot joint used wasobtained by system identification. Experiments were carriedout on an ABB industrial robot 2000 to demonstrate resultson observer-based SPR feedback applied in the design.Stability was shown via a modified Popov criterion
Self-sustained oscillations of a shock wave interacting with a boundary layer on a supercritical airfoil
A theory is proposed of the self-sustaining oscillations of a weak shock on an airfoil in steady, transonic flow. The interaction of the shock with the boundary layer on the airfoil produces displacement thickness fluctuations which convect downstream and generate sound by interaction with the trailing edge. A feedback loop is established when this sound impinges on the shock wave, resulting in the production of further fluctuations in the displacement thickness. The details are worked out for an idealized mean boundary layer velocity profile, but strong support for the basic hypotheses of the theory is provided by a comparison with recent experiments involving the generation of acoustic "tone bursts' by a supercritical airfoil section
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