141 research outputs found
Optimizing low-order controllers for haptic systems under delayed feedback
Cataloged from PDF version of article.In this paper, a PD controller design for haptic systems under delayed feedback is considered. More precisely, a complete stability analysis of a haptic system where local dynamics are described by some second-order mechanical dynamics is presented. Next, using two optimization techniques (H∞ and stability, margin optimization) an optimal choice for the controller gains is proposed. The derived results are tested on a three degree-of-freedom real-time experimental platform to illustrate the theoretical results. © 2013 Elsevier Ltd
Duality Invariant M-theory: Gauged supergravities and Scherk-Schwarz reductions
We consider the reduction of the duality invariant approach to M-theory by a
U-duality group valued Scherk-Schwarz twist. The result is to produce
potentials for gauged supergravities that are normally associated with
non-geometric compactifications. The local symmetry reduces to gauge
transformations with the gaugings exactly matching those of the embedding
tensor approach to gauged supergravity. Importantly, this approach now includes
a nontrivial dependence of the fields on the extra coordinates of the extended
space.Comment: 22 pages Latex; v2: typos corrected and references adde
Low-order controller design for haptic systems under delayed feedback
In this paper, we consider PD controller design for haptic systems under delayed feedback. More precisely, we present a complete stability analysis of a haptic system where local dynamics are described by some second-order mechanical dynamics. Next, using two optimization techniques (H ∞ and stability margin optimization) we propose an optimal choice for the controller gains. The derived results are tested on a three degree of freedom real-time experimental platform to illustrate the theoretical results. © 2012 IFAC
Massive Type II in Double Field Theory
We provide an extension of the recently constructed double field theory
formulation of the low-energy limits of type II strings, in which the RR fields
can depend simultaneously on the 10-dimensional space-time coordinates and
linearly on the dual winding coordinates. For the special case that only the RR
one-form of type IIA carries such a dependence, we obtain the massive
deformation of type IIA supergravity due to Romans. For T-dual configurations
we obtain a `massive' but non-covariant formulation of type IIB, in which the
10-dimensional diffeomorphism symmetry is deformed by the mass parameter.Comment: 21 page
Ramond-Ramond Cohomology and O(D,D) T-duality
In the name of supersymmetric double field theory, superstring effective
actions can be reformulated into simple forms. They feature a pair of vielbeins
corresponding to the same spacetime metric, and hence enjoy double local
Lorentz symmetries. In a manifestly covariant manner --with regard to O(D,D)
T-duality, diffeomorphism, B-field gauge symmetry and the pair of local Lorentz
symmetries-- we incorporate R-R potentials into double field theory. We take
them as a single object which is in a bi-fundamental spinorial representation
of the double Lorentz groups. We identify cohomological structure relevant to
the field strength. A priori, the R-R sector as well as all the fermions are
O(D,D) singlet. Yet, gauge fixing the two vielbeins equal to each other
modifies the O(D,D) transformation rule to call for a compensating local
Lorentz rotation, such that the R-R potential may turn into an O(D,D) spinor
and T-duality can flip the chirality exchanging type IIA and IIB
supergravities.Comment: 1+37 pages, no figure; Structure reorganized, References added, To
appear in JHEP. cf. Gong Show of Strings 2012
(http://wwwth.mpp.mpg.de/members/strings/strings2012/strings_files/program/Talks/Thursday/Gongshow/Lee.pdf
New families of interpolating type IIB backgrounds
We construct new families of interpolating two-parameter solutions of type
IIB supergravity. These correspond to D3-D5 systems on non-compact
six-dimensional manifolds which are T^2 fibrations over Eguchi-Hanson and
multi-center Taub-NUT spaces, respectively. One end of the interpolation
corresponds to a solution with only D5 branes and vanishing NS three-form flux.
A topology changing transition occurs at the other end, where the internal
space becomes a direct product of the four-dimensional surface and the
two-torus and the complexified NS-RR three-form flux becomes imaginary
self-dual. Depending on the choice of the connections on the torus fibre, the
interpolating family has either N=2 or N=1 supersymmetry. In the N=2 case it
can be shown that the solutions are regular.Comment: 20 page
Gauged Double Field Theory
We find necessary and sufficient conditions for gauge invariance of the
action of Double Field Theory (DFT) as well as closure of the algebra of gauge
symmetries. The so-called weak and strong constraints are sufficient to satisfy
them, but not necessary. We then analyze compactifications of DFT on twisted
double tori satisfying the consistency conditions. The effective theory is a
Gauged DFT where the gaugings come from the duality twists. The action,
bracket, global symmetries, gauge symmetries and their closure are computed by
twisting their analogs in the higher dimensional DFT. The non-Abelian heterotic
string and lower dimensional gauged supergravities are particular examples of
Gauged DFT.Comment: Minor changes, references adde
On the Riemann Tensor in Double Field Theory
Double field theory provides T-duality covariant generalized tensors that are
natural extensions of the scalar and Ricci curvatures of Riemannian geometry.
We search for a similar extension of the Riemann curvature tensor by developing
a geometry based on the generalized metric and the dilaton. We find a duality
covariant Riemann tensor whose contractions give the Ricci and scalar
curvatures, but that is not fully determined in terms of the physical fields.
This suggests that \alpha' corrections to the effective action require \alpha'
corrections to T-duality transformations and/or generalized diffeomorphisms.
Further evidence to this effect is found by an additional computation that
shows that there is no T-duality invariant four-derivative object built from
the generalized metric and the dilaton that reduces to the square of the
Riemann tensor.Comment: 36 pages, v2: minor changes, ref. added, v3: appendix on frame
formalism added, version to appear in JHE
Large Gauge Transformations in Double Field Theory
Finite gauge transformations in double field theory can be defined by the
exponential of generalized Lie derivatives. We interpret these transformations
as `generalized coordinate transformations' in the doubled space by proposing
and testing a formula that writes large transformations in terms of derivatives
of the coordinate maps. Successive generalized coordinate transformations give
a generalized coordinate transformation that differs from the direct
composition of the original two. Instead, it is constructed using the Courant
bracket. These transformations form a group when acting on fields but,
intriguingly, do not associate when acting on coordinates.Comment: 40 pages, v2: discussion of dilaton added, to appear in JHE
Asymmetric Orbifolds, Non-Geometric Fluxes and Non-Commutativity in Closed String Theory
In this paper we consider a class of exactly solvable closed string flux
backgrounds that exhibit non-commutativity in the closed string coordinates.
They are realized in terms of freely-acting asymmetric Z_N-orbifolds, which are
themselves close relatives of twisted torus fibrations with elliptic
Z_N-monodromy (elliptic T-folds). We explicitly construct the modular invariant
partition function of the models and derive the non-commutative algebra in the
string coordinates, which is exact to all orders in {\alpha}'. Finally, we
relate these asymmetric orbifold spaces to inherently stringy Scherk-Schwarz
backgrounds and non-geometric fluxes.Comment: 30 page
- …