6 research outputs found
Investigating Bayesian optimization for rail network optimization
Optimizing the operation of rail networks using simulations is an on-going task where heuristic methods such as Genetic Algorithms have been applied. However, these simulations are often expensive to compute and consequently, because the optimization methods require many (typically >104) repeat simulations, the computational cost of optimization is dominated by them. This paper examines Bayesian Optimization and benchmarks it against the Genetic Algorithm method. By applying both methods to test-tasks seeking to maximize passenger satisfaction by optimum resource allocation, it is experimentally determined that a Bayesian Optimization implementation finds ‘good’ solutions in an order of magnitude fewer simulations than a Genetic Algorithm. Similar improvement for real-world problems will allow the predictive power of detailed simulation models to be used for a wider range of network optimization tasks. To the best of the authors’ knowledge, this paper documents the first application of Bayesian Optimization within the field of rail network optimization
Accelerating computational discovery of porous solids through improved navigation of energy-structure-function maps
Whilst energy-structure-function (ESF) maps are a powerful new tool for in silico materials design, the cost of acquiring an ESF map for many properties is too high for routine integration into high-throughput virtual screening workflows. Here, we propose the next evolution of the ESF map. This uses parallel Bayesian optimization to selectively acquire energy and property data, generating the same levels of insight at a fraction of the computational cost. We use this approach to obtain a two orders of magnitude speedup on an ESF study that focused on the discovery of molecular crystals for methane capture, saving over 500,000 CPUh from the original protocol. By accelerating the acquisition of insight from ESF maps, we pave the way for the use of these maps in automated ultra-high throughput screening pipelines greatly reducing the opportunity risk associated with the choice of system to calculate
No-Regret Constrained Bayesian Optimization of Noisy and Expensive Hybrid Models using Differentiable Quantile Function Approximations
This paper investigates the problem of efficient constrained global
optimization of hybrid models that are a composition of a known white-box
function and an expensive multi-output black-box function subject to noisy
observations, which often arises in real-world science and engineering
applications. We propose a novel method, Constrained Upper Quantile Bound
(CUQB), to solve such problems that directly exploits the composite structure
of the objective and constraint functions that we show leads substantially
improved sampling efficiency. CUQB is a conceptually simple, deterministic
approach that avoid constraint approximations used by previous methods.
Although the CUQB acquisition function is not available in closed form, we
propose a novel differentiable sample average approximation that enables it to
be efficiently maximized. We further derive bounds on the cumulative regret and
constraint violation under a non-parametric Bayesian representation of the
black-box function. Since these bounds depend sublinearly on the number of
iterations under some regularity assumptions, we establis bounds on the
convergence rate to the optimal solution of the original constrained problem.
In contrast to most existing methods, CUQB further incorporates a simple
infeasibility detection scheme, which we prove triggers in a finite number of
iterations when the original problem is infeasible (with high probability given
the Bayesian model). Numerical experiments on several test problems, including
environmental model calibration and real-time optimization of a reactor system,
show that CUQB significantly outperforms traditional Bayesian optimization in
both constrained and unconstrained cases. Furthermore, compared to other
state-of-the-art methods that exploit composite structure, CUQB achieves
competitive empirical performance while also providing substantially improved
theoretical guarantees
A methodology for passenger-centred rail network optimisation
Optimising the allocation of limited resources, be they existing assets or
investment, is an ongoing challenge for rail network managers. Recently,
methodologies have been developed for optimising the timetable from the
passenger perspective. However, there is a gap for a decision support tool
which optimises rail networks for maximum passenger satisfaction, captures
the experience of individual passengers and can be adapted to different
networks and challenges. Towards building such a tool, this thesis develops a
novel methodology referred to as the Sheffield University Passenger Rail
Experience Maximiser (SUPREME) framework. First, a network assessment
metric is developed which captures the multi-stage nature of individual
passenger journeys as well as the effect of crowding upon passenger
satisfaction. Second, an agent-based simulation is developed to capture
individual passenger journeys in enough detail for the network assessment
metric to be calculated. Third, for the optimisation algorithm within SUPREME,
the Bayesian Optimisation method is selected following an experimental
investigation which indicates that it is well suited for ‘expensive-to-compute’
objective functions, such as the one found in SUPREME. Finally, in case studies
that include optimising the value engineering strategy of the proposed UK High
Speed Two network when saving £5 billion initial investment costs, the
SUPREME framework is found to improve network performance by the order
of 10%. This thesis shows that the SUPREME framework can find ‘good’
resource allocations for a ‘reasonable’ computational cost, and is sufficiently
adaptable for application to many rail network challenges. This indicates that a
decision support tool developed on the SUPREME framework could be widely
applied by network managers to improve passenger experience and increase
ticket revenue. Novel contributions made by this thesis are: the SUPREME
methodology, an international comparison between the Journey Time Metric
and Disutility Metric, and the application of the Bayesian Optimisation method
for maximising the performance of a rail network