55,807 research outputs found
Design of reliable aerospace system architecture
Reliability and redundancy of safety-critical network systems is a paramount issue in system
engineering. Be it in evaluating existing network systems or solving optimization problems for
designing network systems, it is important to consider reliability and redundancy. This dissertation
is in collaboration with AIRBUS Group, France, and they are very interest in the optimal
design of safety-critical aircraft architecture systems which have to consider reliability and redundancy.
To address the problem of optimally designing such systems, we chose to focus on
one specific aircraft architecture system the door management system. It checks if all doors are
properly closed and the cabin has the correct pressure. It is a safety-critical system since it is
part of the pressurization system of an aircraft.
To optimally design the DMS while considering reliability, a suitable reliability evaluation algorithm
is necessary. In this dissertation, we begin by proposing a suitable reliability evaluation
algorithm for a type of non series-parallel network system which includes the DMS and which
can be used in an optimization model. The reliability evaluation algorithm is based on a simplification of the probability principle of inclusion-exclusion formula for intersections of unions. The
simplification exploits the presence of many repeated events and has many fewer terms, which
significantly reduces the number of operations needed. We compare its computational efficiency
against the sum of disjoint products method KDH88 for a simple artificial example and for the
DMS.
Afterwards, we introduce the first MILP model for the DMS with k-redundancy. As the
model is too difficult to be solved efficiently by standard MILP solvers, we discuss the issues
of solving the model with general solving methods such as branch-and-bound and branch-and-price.
We introduce specialized branching rules and new heuristics to solve the DMS problem
with k-redundancy more efficiently and show results of computational tests which compare the
specialized solving algorithms with general solving algorithms for example instances of the DMS
problem.
Lastly, we discuss the problems of considering reliability in MI(N)LP models for the DMS
and how the new reliability evaluation algorithm can be used. In this discussion, we give different
MI(N)LP models for the DMS problem with redundancy and reliability. Moreover, we propose
a new heuristic for the DMS problem with redundancy and reliability. It is based on branch-and-bound, the Dantzig-Wolfe decomposition and on the new reliability evaluation algorithm.
We show results of computational tests of the new heuristic for example instances of the DMS
problem and discuss its validity
Exact two-terminal reliability of some directed networks
The calculation of network reliability in a probabilistic context has long
been an issue of practical and academic importance. Conventional approaches
(determination of bounds, sums of disjoint products algorithms, Monte Carlo
evaluations, studies of the reliability polynomials, etc.) only provide
approximations when the network's size increases, even when nodes do not fail
and all edges have the same reliability p. We consider here a directed, generic
graph of arbitrary size mimicking real-life long-haul communication networks,
and give the exact, analytical solution for the two-terminal reliability. This
solution involves a product of transfer matrices, in which individual
reliabilities of edges and nodes are taken into account. The special case of
identical edge and node reliabilities (p and rho, respectively) is addressed.
We consider a case study based on a commonly-used configuration, and assess the
influence of the edges being directed (or not) on various measures of network
performance. While the two-terminal reliability, the failure frequency and the
failure rate of the connection are quite similar, the locations of complex
zeros of the two-terminal reliability polynomials exhibit strong differences,
and various structure transitions at specific values of rho. The present work
could be extended to provide a catalog of exactly solvable networks in terms of
reliability, which could be useful as building blocks for new and improved
bounds, as well as benchmarks, in the general case
Taming Uncertainty in the Assurance Process of Self-Adaptive Systems: a Goal-Oriented Approach
Goals are first-class entities in a self-adaptive system (SAS) as they guide
the self-adaptation. A SAS often operates in dynamic and partially unknown
environments, which cause uncertainty that the SAS has to address to achieve
its goals. Moreover, besides the environment, other classes of uncertainty have
been identified. However, these various classes and their sources are not
systematically addressed by current approaches throughout the life cycle of the
SAS. In general, uncertainty typically makes the assurance provision of SAS
goals exclusively at design time not viable. This calls for an assurance
process that spans the whole life cycle of the SAS. In this work, we propose a
goal-oriented assurance process that supports taming different sources (within
different classes) of uncertainty from defining the goals at design time to
performing self-adaptation at runtime. Based on a goal model augmented with
uncertainty annotations, we automatically generate parametric symbolic formulae
with parameterized uncertainties at design time using symbolic model checking.
These formulae and the goal model guide the synthesis of adaptation policies by
engineers. At runtime, the generated formulae are evaluated to resolve the
uncertainty and to steer the self-adaptation using the policies. In this paper,
we focus on reliability and cost properties, for which we evaluate our approach
on the Body Sensor Network (BSN) implemented in OpenDaVINCI. The results of the
validation are promising and show that our approach is able to systematically
tame multiple classes of uncertainty, and that it is effective and efficient in
providing assurances for the goals of self-adaptive systems
Integration of tools for the Design and Assessment of High-Performance, Highly Reliable Computing Systems (DAHPHRS), phase 1
Systems for Space Defense Initiative (SDI) space applications typically require both high performance and very high reliability. These requirements present the systems engineer evaluating such systems with the extremely difficult problem of conducting performance and reliability trade-offs over large design spaces. A controlled development process supported by appropriate automated tools must be used to assure that the system will meet design objectives. This report describes an investigation of methods, tools, and techniques necessary to support performance and reliability modeling for SDI systems development. Models of the JPL Hypercubes, the Encore Multimax, and the C.S. Draper Lab Fault-Tolerant Parallel Processor (FTPP) parallel-computing architectures using candidate SDI weapons-to-target assignment algorithms as workloads were built and analyzed as a means of identifying the necessary system models, how the models interact, and what experiments and analyses should be performed. As a result of this effort, weaknesses in the existing methods and tools were revealed and capabilities that will be required for both individual tools and an integrated toolset were identified
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