5,243 research outputs found
Integral MRAC with Minimal Controller Synthesis and bounded adaptive gains: The continuous-time case
Model reference adaptive controllers designed via the Minimal Control Synthesis (MCS) approach are a viable solution to control plants affected by parameter uncertainty, unmodelled dynamics, and disturbances. Despite its effectiveness to impose the required reference dynamics, an apparent drift of the adaptive gains, which can eventually lead to closed-loop instability or alter tracking performance, may occasionally be induced by external disturbances. This problem has been recently addressed for this class of adaptive algorithms in the discrete-time case and for square-integrable perturbations by using a parameter projection strategy [1]. In this paper we tackle systematically this issue for MCS continuous-time adaptive systems with integral action by enhancing the adaptive mechanism not only with a parameter projection method, but also embedding a s-modification strategy. The former is used to preserve convergence to zero of the tracking error when the disturbance is bounded and L2, while the latter guarantees global uniform ultimate boundedness under continuous L8 disturbances. In both cases, the proposed control schemes ensure boundedness of all the closed-loop signals. The strategies are numerically validated by considering systems subject to different kinds of disturbances. In addition, an electrical power circuit is used to show the applicability of the algorithms to engineering problems requiring a precise tracking of a reference profile over a long time range despite disturbances, unmodelled dynamics, and parameter uncertainty.Postprint (author's final draft
Vortex and translational currents due to broken time-space symmetries
We consider the classical dynamics of a particle in a -dimensional
space-periodic potential under the influence of time-periodic external fields
with zero mean. We perform a general time-space symmetry analysis and identify
conditions, when the particle will generate a nonzero averaged translational
and vortex currents. We perform computational studies of the equations of
motion and of corresponding Fokker-Planck equations, which confirm the symmetry
predictions. We address the experimentally important issue of current control.
Cold atoms in optical potentials and magnetic traps are among possible
candidates to observe these findings experimentally.Comment: 4 pages, 2 figure
Bayesian Optimization for Probabilistic Programs
We present the first general purpose framework for marginal maximum a
posteriori estimation of probabilistic program variables. By using a series of
code transformations, the evidence of any probabilistic program, and therefore
of any graphical model, can be optimized with respect to an arbitrary subset of
its sampled variables. To carry out this optimization, we develop the first
Bayesian optimization package to directly exploit the source code of its
target, leading to innovations in problem-independent hyperpriors, unbounded
optimization, and implicit constraint satisfaction; delivering significant
performance improvements over prominent existing packages. We present
applications of our method to a number of tasks including engineering design
and parameter optimization
Effective squirmer models for self-phoretic chemically active spherical colloids
Various aspects of self-motility of chemically active colloids in Newtonian
fluids can be captured by simple models for their chemical activity plus a
phoretic slip hydrodynamic boundary condition on their surface. For particles
of simple shapes (e.g., spheres) -- as employed in many experimental studies --
which move at very low Reynolds numbers in an unbounded fluid, such models of
chemically active particles effectively map onto the well studied so-called
hydrodynamic squirmers [S. Michelin and E. Lauga, J. Fluid Mech. \textbf{747},
572 (2014)]. Accordingly, intuitively appealing analogies of
"pusher/puller/neutral" squirmers arise naturally. Within the framework of
self-diffusiophoresis we illustrate the above mentioned mapping and the
corresponding flows in an unbounded fluid for a number of choices of the
activity function (i.e., the spatial distribution and the type of chemical
reactions across the surface of the particle). We use the central collision of
two active particles as a simple, paradigmatic case for demonstrating that in
the presence of other particles or boundaries the behavior of chemically active
colloids may be \textit{qualitatively} different, even in the far field, from
the one exhibited by the corresponding "effective squirmer", obtained from the
mapping in an unbounded fluid. This emphasizes that understanding the
collective behavior and the dynamics under geometrical confinement of
chemically active particles necessarily requires to explicitly account for the
dependence of the hydrodynamic interactions on the distribution of chemical
species resulting from the activity of the particles.Comment: 26 pages, 11 figure
Considerations in the design of tip-coupled air-transport systems
It is shown that the lift-drag ratio of tip-coupled systems can be expressed as a simple multiple of the lift-drag ratio of the isolated units comprising the system. When operated for maximum lift-drag ratio, the extent of the coupled system is limited by maximum lift coefficient, high-altitude engine characteristics, and degraded performance of the isolated unit climbing to couple into the system. When operated at constant altitude, the gain from coupling is severely limited. If the cruise altitude is that for best performance of the isolated unit, the system lift-drag ratio can be no better than twice that of the isolated unit even when an infinite number of units are coupled. System performance may be further degraded since span-load distributions which yield good performance for the individual units reduce the efficiency of the coupled system. Coupling a pair of modern transport aircraft results in only about half the expected gain because of a poor span-distribution across the coupled pair. The control deflections required to maintain roll and pitch equilibrium further degrade the possible gain
Price of Competition and Dueling Games
We study competition in a general framework introduced by Immorlica et al.
and answer their main open question. Immorlica et al. considered classic
optimization problems in terms of competition and introduced a general class of
games called dueling games. They model this competition as a zero-sum game,
where two players are competing for a user's satisfaction. In their main and
most natural game, the ranking duel, a user requests a webpage by submitting a
query and players output an ordering over all possible webpages based on the
submitted query. The user tends to choose the ordering which displays her
requested webpage in a higher rank. The goal of both players is to maximize the
probability that her ordering beats that of her opponent and gets the user's
attention. Immorlica et al. show this game directs both players to provide
suboptimal search results. However, they leave the following as their main open
question: "does competition between algorithms improve or degrade expected
performance?" In this paper, we resolve this question for the ranking duel and
a more general class of dueling games.
More precisely, we study the quality of orderings in a competition between
two players. This game is a zero-sum game, and thus any Nash equilibrium of the
game can be described by minimax strategies. Let the value of the user for an
ordering be a function of the position of her requested item in the
corresponding ordering, and the social welfare for an ordering be the expected
value of the corresponding ordering for the user. We propose the price of
competition which is the ratio of the social welfare for the worst minimax
strategy to the social welfare obtained by a social planner. We use this
criterion for analyzing the quality of orderings in the ranking duel. We prove
the quality of minimax results is surprisingly close to that of the optimum
solution
- …