264 research outputs found
Learning context-aware adaptive solvers to accelerate quadratic programming
Convex quadratic programming (QP) is an important sub-field of mathematical
optimization. The alternating direction method of multipliers (ADMM) is a
successful method to solve QP. Even though ADMM shows promising results in
solving various types of QP, its convergence speed is known to be highly
dependent on the step-size parameter . Due to the absence of a general
rule for setting , it is often tuned manually or heuristically. In this
paper, we propose CA-ADMM (Context-aware Adaptive ADMM)) which learns to
adaptively adjust to accelerate ADMM. CA-ADMM extracts the
spatio-temporal context, which captures the dependency of the primal and dual
variables of QP and their temporal evolution during the ADMM iterations.
CA-ADMM chooses based on the extracted context. Through extensive
numerical experiments, we validated that CA-ADMM effectively generalizes to
unseen QP problems with different sizes and classes (i.e., having different QP
parameter structures). Furthermore, we verified that CA-ADMM could dynamically
adjust considering the stage of the optimization process to accelerate
the convergence speed further.Comment: 9 pages, 4 figure
Genetic Algorithms with Neural Cost Predictor for Solving Hierarchical Vehicle Routing Problems
When vehicle routing decisions are intertwined with higher-level decisions,
the resulting optimization problems pose significant challenges for
computation. Examples are the multi-depot vehicle routing problem (MDVRP),
where customers are assigned to depots before delivery, and the capacitated
location routing problem (CLRP), where the locations of depots should be
determined first. A simple and straightforward approach for such hierarchical
problems would be to separate the higher-level decisions from the complicated
vehicle routing decisions. For each higher-level decision candidate, we may
evaluate the underlying vehicle routing problems to assess the candidate. As
this approach requires solving vehicle routing problems multiple times, it has
been regarded as impractical in most cases. We propose a novel
deep-learning-based approach called Genetic Algorithm with Neural Cost
Predictor (GANCP) to tackle the challenge and simplify algorithm developments.
For each higher-level decision candidate, we predict the objective function
values of the underlying vehicle routing problems using a pre-trained graph
neural network without actually solving the routing problems. In particular,
our proposed neural network learns the objective values of the HGS-CVRP
open-source package that solves capacitated vehicle routing problems. Our
numerical experiments show that this simplified approach is effective and
efficient in generating high-quality solutions for both MDVRP and CLRP and has
the potential to expedite algorithm developments for complicated hierarchical
problems. We provide computational results evaluated in the standard benchmark
instances used in the literature
MIMO active vibration control of magnetically suspended flywheels for satellite IPAC service
Theory and simulation results have demonstrated that four, variable speed flywheels
could potentially provide the energy storage and attitude control functions of existing
batteries and control moment gyros (CMGs) on a satellite. Past modeling and control
algorithms were based on the assumption of rigidity in the flywheel’s bearings and the
satellite structure.
This dissertation provides simulation results and theory which eliminates this
assumption utilizing control algorithms for active vibration control (AVC), flywheel
shaft levitation and integrated power transfer and attitude control (IPAC) that are
effective even with low stiffness active magnetic bearings (AMB), and flexible satellite
appendages.
The flywheel AVC and levitation tasks are provided by a multi input multi output
(MIMO) control law that enhances stability by reducing the dependence of the forward
and backward gyroscopic poles with changes in flywheel speed.
The control law is shown to be effective even for (1) Large polar to transverse inertia ratios which increases the stored energy density while causing the poles to
become more speed dependent and, (2) Low bandwidth controllers shaped to suppress
high frequency noise. These two main tasks could be successfully achieved by MIMO
(Gyroscopic) control algorithm, which is unique approach.
The vibration control mass (VCM) is designed to reduce the vibrations of flexible
appendages of the satellite. During IPAC maneuver, the oscillation of flywheel spin
speeds, torque motions and satellite appendages are significantly reduced compared
without VCM. Several different properties are demonstrated to obtain optimal VCM.
Notch, band-pass and low-pass filters are implemented in the AMB system to
reduce and cancel high frequency, dynamic bearing forces and motor torques due to
flywheel mass imbalance. The transmitted forces and torques to satellite are
considerably decreased in the present of both notch and band-pass filter stages.
Successful IPAC simulation results are presented with a 12 [%] of initial attitude
error, large polar to transverse inertia ratio (IP / IT), structural flexibility and unbalance
mass disturbance.
Two variable speed control moment gyros (VSCMGs) are utilized to demonstrate
simultaneous attitude control and power transfer instead of using four standard pyramid
configurations. Launching weights including payload and costs can be significantly
reduced
Metabolite concentrations, fluxes and free energies imply efficient enzyme usage.
In metabolism, available free energy is limited and must be divided across pathway steps to maintain a negative ΔG throughout. For each reaction, ΔG is log proportional both to a concentration ratio (reaction quotient to equilibrium constant) and to a flux ratio (backward to forward flux). Here we use isotope labeling to measure absolute metabolite concentrations and fluxes in Escherichia coli, yeast and a mammalian cell line. We then integrate this information to obtain a unified set of concentrations and ΔG for each organism. In glycolysis, we find that free energy is partitioned so as to mitigate unproductive backward fluxes associated with ΔG near zero. Across metabolism, we observe that absolute metabolite concentrations and ΔG are substantially conserved and that most substrate (but not inhibitor) concentrations exceed the associated enzyme binding site dissociation constant (Km or Ki). The observed conservation of metabolite concentrations is consistent with an evolutionary drive to utilize enzymes efficiently given thermodynamic and osmotic constraints
Generating Dispatching Rules for the Interrupting Swap-Allowed Blocking Job Shop Problem Using Graph Neural Network and Reinforcement Learning
The interrupting swap-allowed blocking job shop problem (ISBJSSP) is a
complex scheduling problem that is able to model many manufacturing planning
and logistics applications realistically by addressing both the lack of storage
capacity and unforeseen production interruptions. Subjected to random
disruptions due to machine malfunction or maintenance, industry production
settings often choose to adopt dispatching rules to enable adaptive, real-time
re-scheduling, rather than traditional methods that require costly
re-computation on the new configuration every time the problem condition
changes dynamically. To generate dispatching rules for the ISBJSSP problem, we
introduce a dynamic disjunctive graph formulation characterized by nodes and
edges subjected to continuous deletions and additions. This formulation enables
the training of an adaptive scheduler utilizing graph neural networks and
reinforcement learning. Furthermore, a simulator is developed to simulate
interruption, swapping, and blocking in the ISBJSSP setting. Employing a set of
reported benchmark instances, we conduct a detailed experimental study on
ISBJSSP instances with a range of machine shutdown probabilities to show that
the scheduling policies generated can outperform or are at least as competitive
as existing dispatching rules with predetermined priority. This study shows
that the ISBJSSP, which requires real-time adaptive solutions, can be scheduled
efficiently with the proposed method when production interruptions occur with
random machine shutdowns.Comment: 14 pages, 10 figures. Supplementary Material not include
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