2,497 research outputs found
Solving DCOPs with Distributed Large Neighborhood Search
The field of Distributed Constraint Optimization has gained momentum in
recent years, thanks to its ability to address various applications related to
multi-agent cooperation. Nevertheless, solving Distributed Constraint
Optimization Problems (DCOPs) optimally is NP-hard. Therefore, in large-scale,
complex applications, incomplete DCOP algorithms are necessary. Current
incomplete DCOP algorithms suffer of one or more of the following limitations:
they (a) find local minima without providing quality guarantees; (b) provide
loose quality assessment; or (c) are unable to benefit from the structure of
the problem, such as domain-dependent knowledge and hard constraints.
Therefore, capitalizing on strategies from the centralized constraint solving
community, we propose a Distributed Large Neighborhood Search (D-LNS) framework
to solve DCOPs. The proposed framework (with its novel repair phase) provides
guarantees on solution quality, refining upper and lower bounds during the
iterative process, and can exploit domain-dependent structures. Our
experimental results show that D-LNS outperforms other incomplete DCOP
algorithms on both structured and unstructured problem instances
Cost Function Networks to Solve Large Computational Protein Design Problems
International audienc
Reinforcement Learning and Tree Search Methods for the Unit Commitment Problem
The unit commitment (UC) problem, which determines operating schedules of
generation units to meet demand, is a fundamental task in power systems
operation. Existing UC methods using mixed-integer programming are not
well-suited to highly stochastic systems. Approaches which more rigorously
account for uncertainty could yield large reductions in operating costs by
reducing spinning reserve requirements; operating power stations at higher
efficiencies; and integrating greater volumes of variable renewables. A
promising approach to solving the UC problem is reinforcement learning (RL), a
methodology for optimal decision-making which has been used to conquer
long-standing grand challenges in artificial intelligence. This thesis explores
the application of RL to the UC problem and addresses challenges including
robustness under uncertainty; generalisability across multiple problem
instances; and scaling to larger power systems than previously studied. To
tackle these issues, we develop guided tree search, a novel methodology
combining model-free RL and model-based planning. The UC problem is formalised
as a Markov decision process and we develop an open-source environment based on
real data from Great Britain's power system to train RL agents. In problems of
up to 100 generators, guided tree search is shown to be competitive with
deterministic UC methods, reducing operating costs by up to 1.4\%. An advantage
of RL is that the framework can be easily extended to incorporate
considerations important to power systems operators such as robustness to
generator failure, wind curtailment or carbon prices. When generator outages
are considered, guided tree search saves over 2\% in operating costs as
compared with methods using conventional reserve criteria
An efficient memetic, permutation-based evolutionary algorithm for real-world train timetabling
Train timetabling is a difficult and very tightly constrained combinatorial
problem that deals with the construction of train schedules. We focus on the
particular problem of local reconstruction of the schedule following a small
perturbation, seeking minimisation of the total accumulated delay by adapting
times of departure and arrival for each train and allocation of resources
(tracks, routing nodes, etc.). We describe a permutation-based evolutionary
algorithm that relies on a semi-greedy heuristic to gradually reconstruct the
schedule by inserting trains one after the other following the permutation.
This algorithm can be hybridised with ILOG commercial MIP programming tool
CPLEX in a coarse-grained manner: the evolutionary part is used to quickly
obtain a good but suboptimal solution and this intermediate solution is refined
using CPLEX. Experimental results are presented on a large real-world case
involving more than one million variables and 2 million constraints. Results
are surprisingly good as the evolutionary algorithm, alone or hybridised,
produces excellent solutions much faster than CPLEX alone
A Partially Randomized Approach to Trajectory Planning and Optimization for Mobile Robots with Flat Dynamics
Motion planning problems are characterized by huge search spaces and complex obstacle structures with no concise mathematical expression. The fixed-wing airplane application considered in this thesis adds differential constraints and point-wise bounds, i. e. an infinite number of equality and inequality constraints.
An optimal trajectory planning approach is presented, based on the randomized Rapidly-exploring Random Trees framework (RRT*).
The local planner relies on differential flatness of the equations of motion to obtain tree branch candidates that automatically satisfy the differential constraints. Flat output trajectories, in this case equivalent to the airplane's flight path, are designed using BĂ©zier curves. Segment feasibility in terms of point-wise inequality constraints is tested by an indicator integral, which is evaluated alongside the segment cost functional.
Although the RRT* guarantees optimality in the limit of infinite planning time, it is argued by intuition and experimentation that convergence is not approached at a practically useful rate. Therefore, the randomized planner is augmented by a deterministic variational optimization technique. To this end, the optimal planning task is formulated as a semi-infinite optimization problem, using the intermediate result of the RRT(*) as an initial guess. The proposed optimization algorithm follows the feasible flavor of the primal-dual interior point paradigm. Discretization of functional (infinite) constraints is deferred to the linear subproblems, where it is realized implicitly by numeric quadrature. An inherent numerical ill-conditioning of the method is circumvented by a reduction-like approach, which tracks active constraint locations by introducing new problem variables. Obstacle avoidance is achieved by extending the line search procedure and dynamically adding obstacle-awareness constraints to the problem formulation.
Experimental evaluation confirms that the hybrid approach is practically feasible and does indeed outperform RRT*'s built-in optimization mechanism, but the computational burden is still significant.Bewegungsplanungsaufgaben sind typischerweise gekennzeichnet durch umfangreiche SuchrĂ€ume, deren vollstĂ€ndige Exploration nicht praktikabel ist, sowie durch unstrukturierte Hindernisse, fĂŒr die nur selten eine geschlossene mathematische Beschreibung existiert.
Bei der in dieser Arbeit betrachteten Anwendung auf FlĂ€chenflugzeuge kommen differentielle Randbedingungen und beschrĂ€nkte SystemgröĂen erschwerend hinzu.
Der vorgestellte Ansatz zur optimalen Trajektorienplanung basiert auf dem Rapidly-exploring Random Trees-Algorithmus (RRT*), welcher die SuchraumkomplexitĂ€t durch Randomisierung beherrschbar macht. Der spezifische Beitrag ist eine Realisierung des lokalen Planers zur Generierung der Ăste des Suchbaums. Dieser erfordert ein flaches Bewegungsmodell, sodass differentielle Randbedingungen automatisch erfĂŒllt sind. Die Trajektorien des flachen Ausgangs, welche im betrachteten Beispiel der Flugbahn entsprechen, werden mittels BĂ©zier-Kurven entworfen. Die Einhaltung der Ungleichungsnebenbedingungen wird durch ein Indikator-Integral ĂŒberprĂŒft, welches sich mit wenig Zusatzaufwand parallel zum Kostenfunktional berechnen lĂ€sst.
Zwar konvergiert der RRT*-Algorithmus (im probabilistischen Sinne) zu einer optimalen Lösung, jedoch ist die Konvergenzrate aus praktischer Sicht unbrauchbar langsam. Es ist daher naheliegend, den Planer durch ein gradientenbasiertes lokales Optimierungsverfahren mit besseren Konvergenzeigenschaften zu unterstĂŒtzen. Hierzu wird die aktuelle Zwischenlösung des Planers als InitialschĂ€tzung fĂŒr ein kompatibles semi-infinites Optimierungsproblem verwendet. Der vorgeschlagene Optimierungsalgorithmus erweitert das verbreitete innere-Punkte-Konzept (primal dual interior point method) auf semi-infinite Probleme. Eine explizite Diskretisierung der funktionalen Ungleichungsnebenbedingungen ist nicht erforderlich, denn diese erfolgt implizit durch eine numerische Integralauswertung im Rahmen der linearen Teilprobleme.
Da die Methode an Stellen aktiver Nebenbedingungen nicht wohldefiniert ist, kommt zusÀtzlich eine Variante des Reduktions-Ansatzes zum Einsatz, bei welcher der Vektor der Optimierungsvariablen um die (endliche) Menge der aktiven Indizes erweitert wird.
Weiterhin wurde eine Kollisionsvermeidung integriert, die in den Teilschritt der Liniensuche eingreift und die Problemformulierung dynamisch um Randbedingungen zur lokalen BerĂŒcksichtigung von Hindernissen erweitert.
Experimentelle Untersuchungen bestĂ€tigen, dass die Ergebnisse des hybriden Ansatzes aus RRT(*) und numerischem Optimierungsverfahren der klassischen RRT*-basierten Trajektorienoptimierung ĂŒberlegen sind. Der erforderliche Rechenaufwand ist zwar betrĂ€chtlich, aber unter realistischen Bedingungen praktisch beherrschbar
Learning Large-Scale Bayesian Networks with the sparsebn Package
Learning graphical models from data is an important problem with wide
applications, ranging from genomics to the social sciences. Nowadays datasets
often have upwards of thousands---sometimes tens or hundreds of thousands---of
variables and far fewer samples. To meet this challenge, we have developed a
new R package called sparsebn for learning the structure of large, sparse
graphical models with a focus on Bayesian networks. While there are many
existing software packages for this task, this package focuses on the unique
setting of learning large networks from high-dimensional data, possibly with
interventions. As such, the methods provided place a premium on scalability and
consistency in a high-dimensional setting. Furthermore, in the presence of
interventions, the methods implemented here achieve the goal of learning a
causal network from data. Additionally, the sparsebn package is fully
compatible with existing software packages for network analysis.Comment: To appear in the Journal of Statistical Software, 39 pages, 7 figure
- âŠ