2,618 research outputs found
Design and Experimental Validation of an Explicit MPC Controller for Regulating Temperature in PEM Fuel Cell Systems
This paper proposes a temperature controller for PEM fuel cell systems with an air blower as thermal
circuit. The objective of this controller is to maintain the stack temperature over a given set-point which
is obtained from the results of a real-time optimization algorithm with the goal of minimizing the stack
degradation and maximizing the global efficiency. An Explicit MPC is proposed to deal with this control
problem which presents delays, the critical sampling time, constraints and disturbances. The simulation
results show good performance of the controller which accurately tracks the temperature reference over
the overall range of operating conditions. Furthermore, the controller is implemented in real-time on a
PEM fuel cell test-bench which is installed in the Fuel Cell Laboratory at the University of Seville
Coarse Brownian Dynamics for Nematic Liquid Crystals: Bifurcation Diagrams via Stochastic Simulation
We demonstrate how time-integration of stochastic differential equations
(i.e. Brownian dynamics simulations) can be combined with continuum numerical
bifurcation analysis techniques to analyze the dynamics of liquid crystalline
polymers (LCPs). Sidestepping the necessity of obtaining explicit closures, the
approach analyzes the (unavailable in closed form) coarse macroscopic
equations, estimating the necessary quantities through appropriately
initialized, short bursts of Brownian dynamics simulation. Through this
approach, both stable and unstable branches of the equilibrium bifurcation
diagram are obtained for the Doi model of LCPs and their coarse stability is
estimated. Additional macroscopic computational tasks enabled through this
approach, such as coarse projective integration and coarse stabilizing
controller design, are also demonstrated
Smooth Inequalities and Equilibrium Inefficiency in Scheduling Games
We study coordination mechanisms for Scheduling Games (with unrelated
machines). In these games, each job represents a player, who needs to choose a
machine for its execution, and intends to complete earliest possible. Our goal
is to design scheduling policies that always admit a pure Nash equilibrium and
guarantee a small price of anarchy for the l_k-norm social cost --- the
objective balances overall quality of service and fairness. We consider
policies with different amount of knowledge about jobs: non-clairvoyant,
strongly-local and local. The analysis relies on the smooth argument together
with adequate inequalities, called smooth inequalities. With this unified
framework, we are able to prove the following results.
First, we study the inefficiency in l_k-norm social costs of a strongly-local
policy SPT and a non-clairvoyant policy EQUI. We show that the price of anarchy
of policy SPT is O(k). We also prove a lower bound of Omega(k/log k) for all
deterministic, non-preemptive, strongly-local and non-waiting policies
(non-waiting policies produce schedules without idle times). These results
ensure that SPT is close to optimal with respect to the class of l_k-norm
social costs. Moreover, we prove that the non-clairvoyant policy EQUI has price
of anarchy O(2^k).
Second, we consider the makespan (l_infty-norm) social cost by making
connection within the l_k-norm functions. We revisit some local policies and
provide simpler, unified proofs from the framework's point of view. With the
highlight of the approach, we derive a local policy Balance. This policy
guarantees a price of anarchy of O(log m), which makes it the currently best
known policy among the anonymous local policies that always admit a pure Nash
equilibrium.Comment: 25 pages, 1 figur
Improving the Price of Anarchy for Selfish Routing via Coordination Mechanisms
We reconsider the well-studied Selfish Routing game with affine latency
functions. The Price of Anarchy for this class of games takes maximum value
4/3; this maximum is attained already for a simple network of two parallel
links, known as Pigou's network. We improve upon the value 4/3 by means of
Coordination Mechanisms.
We increase the latency functions of the edges in the network, i.e., if
is the latency function of an edge , we replace it by
with for all . Then an
adversary fixes a demand rate as input. The engineered Price of Anarchy of the
mechanism is defined as the worst-case ratio of the Nash social cost in the
modified network over the optimal social cost in the original network.
Formally, if \CM(r) denotes the cost of the worst Nash flow in the modified
network for rate and \Copt(r) denotes the cost of the optimal flow in the
original network for the same rate then [\ePoA = \max_{r \ge 0}
\frac{\CM(r)}{\Copt(r)}.]
We first exhibit a simple coordination mechanism that achieves for any
network of parallel links an engineered Price of Anarchy strictly less than
4/3. For the case of two parallel links our basic mechanism gives 5/4 = 1.25.
Then, for the case of two parallel links, we describe an optimal mechanism; its
engineered Price of Anarchy lies between 1.191 and 1.192.Comment: 17 pages, 2 figures, preliminary version appeared at ESA 201
Towards More Practical Linear Programming-based Techniques for Algorithmic Mechanism Design
R. Lavy and C. Swamy (FOCS 2005, J. ACM 2011) introduced a general method for
obtaining truthful-in-expectation mechanisms from linear programming based
approximation algorithms. Due to the use of the Ellipsoid method, a direct
implementation of the method is unlikely to be efficient in practice. We
propose to use the much simpler and usually faster multiplicative weights
update method instead. The simplification comes at the cost of slightly weaker
approximation and truthfulness guarantees
High-Temperature Activated AB2 Nanopowders for Metal Hydride Hydrogen Compression
A reliable process for compressing hydrogen and for removing all contaminants
is that of the metal hydride thermal compression. The use of metal hydride
technology in hydrogen compression applications though, requires thorough
structural characterization of the alloys and investigation of their sorption
properties. The samples have been synthesized by induction - levitation melting
and characterized by Rietveld analysis of the X-Ray diffraction (XRD) patterns.
Volumetric PCI (Pressure-Composition Isotherm) measurements have been conducted
at 20, 60 and 90 oC, in order to investigate the maximum pressure that can be
reached from the selected alloys using water of 90oC. Experimental evidence
shows that the maximum hydrogen uptake is low since all the alloys are
consisted of Laves phases, but it is of minor importance if they have fast
kinetics, given a constant volumetric hydrogen flow. Hysteresis is almost
absent while all the alloys release nearly all the absorbed hydrogen during
desorption. Due to hardware restrictions, the maximum hydrogen pressure for the
measurements was limited at 100 bars. Practically, the maximum pressure that
can be reached from the last alloy is more than 150 bars.Comment: 9 figures. arXiv admin note: text overlap with arXiv:1207.354
A New Lower Bound for Deterministic Truthful Scheduling
We study the problem of truthfully scheduling tasks to selfish
unrelated machines, under the objective of makespan minimization, as was
introduced in the seminal work of Nisan and Ronen [STOC'99]. Closing the
current gap of on the approximation ratio of deterministic truthful
mechanisms is a notorious open problem in the field of algorithmic mechanism
design. We provide the first such improvement in more than a decade, since the
lower bounds of (for ) and (for ) by
Christodoulou et al. [SODA'07] and Koutsoupias and Vidali [MFCS'07],
respectively. More specifically, we show that the currently best lower bound of
can be achieved even for just machines; for we already get
the first improvement, namely ; and allowing the number of machines to
grow arbitrarily large we can get a lower bound of .Comment: 15 page
Simulations of the Poynting--Robertson Cosmic Battery in Resistive Accretion Disks
We describe the results of numerical "2.5--dimensional" MHD simulations of an
initially unmagnetized disk model orbiting a central point--mass and responding
to the continual generation of poloidal magnetic field due to a secular source
that emulates the Poynting--Robertson (PR) drag on electrons in the vicinity of
a luminous stellar or compact accreting object. The fluid in the disk and in
the surrounding hotter atmosphere has finite electrical conductivity and allows
for the magnetic field to diffuse freely out of the areas where it is
generated, while at the same time, the differential rotation of the disk twists
the poloidal field and quickly induces a substantial toroidal--field component.
The secular PR term has dual purpose in these simulations as the source of the
magnetic field and the trigger of a magnetorotational instability (MRI) in the
disk. The MRI is especially mild and does not destroy the disk because a small
amount of resistivity dampens the instability efficiently. In simulations with
moderate resistivities (diffusion timescales up to 16 local dynamical
times) and after 100 orbits, the MRI has managed to transfer outward
substantial amounts of angular momentum and the inner edge of the disk, along
with azimuthal magnetic flux, has flowed toward the central point--mass where a
new, magnetized, nuclear disk has formed. The toroidal field in this nuclear
disk is amplified by differential rotation and it cannot be contained; when it
approaches equipartition, it unwinds vertically and produces episodic jet--like
outflows. The poloidal field in the inner region cannot diffuse back out if the
characteristic diffusion time is of the order of or larger than the dynamical
time; it continues to grow linearly in time undisturbed and without saturation,
as the outer sections of many poloidal loops are being drawn radially outward.Comment: 27 pages, 55 figure
Designing cost-sharing methods for Bayesian games
We study the design of cost-sharing protocols for two fundamental resource allocation problems, the Set Cover and the Steiner Tree Problem, under environments of incomplete information (Bayesian model). Our objective is to design protocols where the worst-case Bayesian Nash equilibria, have low cost, i.e. the Bayesian Price of Anarchy (PoA) is minimized. Although budget balance is a very natural requirement, it puts considerable restrictions on the design space, resulting in high PoA. We propose an alternative, relaxed requirement called budget balance in the equilibrium (BBiE).We show an interesting connection between algorithms for Oblivious Stochastic optimization problems and cost-sharing design with low PoA. We exploit this connection for both problems and we enforce approximate solutions of the stochastic problem, as Bayesian Nash equilibria, with the same guarantees on the PoA. More interestingly, we show how to obtain the same bounds on the PoA, by using anonymous posted prices which are desirable because they are easy to implement and, as we show, induce dominant strategies for the players
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