139 research outputs found
A remark about linear switched systems in the plane
No full textIn this note we prove that if a switched system F formed by a pair of linear vector fields of R2 is asymptotically controllable, then the discrete time operator associated to F admits at least one real eigenvalue p, with |p| < 1. For the particular case at hand, this is an improvement of previous existing results
Stability control and recurrent switching rules
No full textRecently, it has been enlightens the interest of a class of switching rules with nice properties, called eventually periodic: more precisely, it is proven that a finite family F of linear vector fields of R^d can be stabilized by means of eventually periodic switching rules, provided that it is asymptotically controllable and satisfies an additional finite time controllability condition. Unfortunately, simple examples point out that in general, eventually periodic switching rules are not robust with respect to state measurement errors. In this paper, we introduce a new type of switching rules with improved robustness properties, called recurrent switching rules. They are subject to the construction of a finite sequence of complete cones Gamma_1,...,Gamma_H of R^d. We shown that under natural assumptions, if a stabilizing eventually periodic switching rule for F is known, then Gamma_1,...,Gamma_H can be constructed in such a way that F is stabilized by any recurrent switching rule subject to Gamma_1,...,Gamma_
Closed loop stabilization of planar bilinear switched systems
No full textIn this paper we address the closed loop switched stabilization problem for planar bilinear systems under the assumption that the control is one dimensional and takes only the values 0 and 1. We construct a class of state-static-memoryless stabilizing feedback laws which preserve the properties of open loop switching signals. In order to prove the stability of the implemented system, we use Lyapunov techniques for differential equations with discontinuous righthand side. Finally we point out some possible extensions of our result and compare it with related results previously proven by other author
L_2 gain stabilizability with respect to Filippov solutions
No full textWe give a sufficient condition for discontinuous L2-gain stabilizability of a nonlinear affine system with respect to Filippov solutions. Our condition requires the existence of a viscosity supersolution of a suitable Hamilton-Jacobi equation
Bounded-input-bounded-state stabilization of switched processes and periodic asymptotic controllability
No full textThe main result of this paper is a sufficient condition for the existence of periodic switching signals which render asymptotically stable at the origin a linear switched process defined by a pair of real matrices. The interest of this result is motivated by the application to the problem of bounded-input-bounded-state (with respect to an external input) stabilization of linear switched processes
The kinematic relationship between disk and jet in the DG Tauri system
No full textWe present high angular resolution millimeter wavelength continuum and \cii observations of the circumstellar disk surrounding the T Tauri star DG Tauri. We show that the velocity pattern in the inner regions of the disk is consistent with Keplerian rotation about a central 0.67 M_sun star. The disk rotation is also consistent with the toroidal velocity pattern in the initial channel of the optical jet, as inferred from HST spectra of the first de-projected 100 AU from the source. Our observations support the tight relationship between disk and jet kinematics postulated by the popular magneto-centrifugal models for jet formation and collimation
Exploring the feedback of asymmetric jets on the orbital motions in protoplanetary disks
No full textProtoplanetary disks are often associated with powerful bipolar jets. In most cases the two jet lobes carry a different amount of linear momentum. We investigate the dynamical feedback of such an asymmetric jet on its launch region in the disk. We adopt a Hamiltonian formulation and solve for the departures from the initial Keplerian orbits with a symplectic integrator. The back-reaction effect produces a shift in the position of the orbits toward the weaker jet lobe, deforming the shape of the inner disk. The loci of the orbits oscillate quasiperiodically, alternating radial and vertical displacements. The amplitude is a small fraction of the disk thickness, and is proportional to the momentum imbalance. Such motions can contribute to the onset of turbulence, and to the mixing of molecular material
On the use of shape memory alloys for deployable passive heat radiators in space satellites
No full textThe present work presents a multifunctional structure for space engineering application part of the TOPDESS project, funded by ESA.
The main aim of the project is the design of a thermal control device able to deploy through passive actuation. A combined device has been designed, made up of a Pulsating Heat Pipe (PHP) foldable heat exchanger and Shape Memory Alloy (SMA) wire. The deployment of the SMA wire is conceived to be controlled by thermal contact with the heat source and by conduction along the wire. Since the heat sources are lumped and the wire is subject to convection, a temperature gradient develops along the wire.
A monodimensional mode able to predict the behavior of an SMA wire subjected to a spatial temperature gradient, is presented in this paper.
The results show that the system can carry out folding and unfolding cycles with rotation angles greater than 80° only if the wire is subjected to uniform temperature distribution; in the case of temperature gradient, the achievable rotation angle is about 20°.
The analysis states the feasibility of the actuation system, highlighting the critical technological aspects, to lay the groundwork for the future development of the whole system
A resolved outflow of matter from a brown dwarf
No full textThe birth of stars involves not only accretion but also, counter-intuitively, the expulsion of matter in the form of highly supersonic outflows. Although this phenomenon has been seen in young stars, a fundamental question is whether it also occurs among newborn brown dwarfs: these are the so-called `failed stars', with masses between stars and planets, that never manage to reach temperatures high enough for normal hydrogen fusion to occur. Recently, evidence for accretion in young brown dwarfs has mounted, and their spectra show lines that are suggestive of outflows. Here we report spectro-astrometric data that spatially resolve an outflow from a brown dwarf. The outflow's characteristics appear similar to, but on a smaller scale than, outflows from normal young stars. This result suggests that the outflow mechanism is universal, and perhaps relevant even to the formation of planets
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