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1.
ContextDemonstrating compliance of critical systems with safety standards involves providing convincing evidence that the requirements of a standard are adequately met. For large systems, practitioners need to be able to effectively collect, structure, and assess substantial quantities of evidence.ObjectiveThis paper aims to provide insights into how practitioners deal with safety evidence management for critical computer-based systems. The information currently available about how this activity is performed in the industry is very limited.MethodWe conducted a survey to determine practitioners’ perspectives and practices on safety evidence management. A total of 52 practitioners from 15 countries and 11 application domains responded to the survey. The respondents indicated the types of information used as safety evidence, how evidence is structured and assessed, how evidence evolution is addressed, and what challenges are faced in relation to provision of safety evidence.ResultsOur results indicate that (1) V&V artefacts, requirements specifications, and design specifications are the most frequently used safety evidence types, (2) evidence completeness checking and impact analysis are mostly performed manually at the moment, (3) text-based techniques are used more frequently than graphical notations for evidence structuring, (4) checklists and expert judgement are frequently used for evidence assessment, and (5) significant research effort has been spent on techniques that have seen little adoption in the industry. The main contributions of the survey are to provide an overall and up-to-date understanding of how the industry addresses safety evidence management, and to identify gaps in the state of the art.ConclusionWe conclude that (1) V&V plays a major role in safety assurance, (2) the industry will clearly benefit from more tool support for collecting and manipulating safety evidence, and (3) future research on safety evidence management needs to place more emphasis on industrial applications.  相似文献   
2.
ContextEnterprise software systems (e.g., enterprise resource planning software) are often deployed in different contexts (e.g., different organizations or different business units or branches of one organization). However, even though organizations, business units or branches have the same or similar business goals, they may differ in how they achieve these goals. Thus, many enterprise software systems are subject to variability and adapted depending on the context in which they are used.ObjectiveOur goal is to provide a snapshot of variability in large scale enterprise software systems. We aim at understanding the types of variability that occur in large industrial enterprise software systems. Furthermore, we aim at identifying how variability is handled in such systems.MethodWe performed an exploratory case study in two large software organizations, involving two large enterprise software systems. Data were collected through interviews and document analysis. Data were analyzed following a grounded theory approach.ResultsWe identified seven types of variability (e.g., functionality, infrastructure) and eight mechanisms to handle variability (e.g., add-ons, code switches).ConclusionsWe provide generic types for classifying variability in enterprise software systems, and reusable mechanisms for handling such variability. Some variability types and handling mechanisms for enterprise software systems found in the real world extend existing concepts and theories. Others confirm findings from previous research literature on variability in software in general and are therefore not specific to enterprise software systems. Our findings also offer a theoretical foundation for describing variability handling in practice. Future work needs to provide more evaluations of the theoretical foundations, and refine variability handling mechanisms into more detailed practices.  相似文献   
3.
The successful operation of any petrochemical plant is dependent on the use of several utilities which may include electricity, steam, compressed air, cooling media, refrigeration media, nitrogen, condensate and fuel gas. These utilities form a significant portion of the fixed cost associated with running a plant. Utility optimisation has not received much attention until recently, driven by rising energy costs, stricter environmental policies, more competitive markets, and the threat of climate change. The generation, preparation, and transportation of utilities require energy and therefore should be optimised to reduce losses and improve operating efficiency. One example of such a utility is a cooling water system. This paper describes the modelling of a dual circuit induced draft cooling water system for control and optimisation purposes. The derived model is verified with plant data indicating promising results. The model is represented in a steady-state algebraic form as well as a dynamic state-space form. This provides a convenient basis for simulation studies and controller/optimiser design.  相似文献   
4.
Engineered transport of material at the nano/micro scale is essential for the manufacturing platforms of the future. Unlike conventional transport systems, at the nano/micro scale, transport has to be achieved in the presence of fundamental sources of uncertainty such as thermal noise. Remarkably, it is possible to extract useful work by rectifying noise using an asymmetric potential; a principle used by Brownian ratchets. In this article a systematic methodology for designing open-loop Brownian ratchet mechanisms that optimize velocity and efficiency is developed. In the case where the particle position is available as a measured variable, closed loop methodologies are studied. Here, it is shown that methods that strive to optimize velocity of transport may compromise efficiency. A dynamic programming based approach is presented which yields up to three times improvement in efficiency over optimized open loop designs and 35% better efficiency over reported closed loop strategies that focus on optimizing velocities.  相似文献   
5.
Control of self-assembling systems at the micro- and nano-scale provides new opportunities for the engineering of novel materials in a bottom-up fashion. These systems have several challenges associated with control including high-dimensional and stochastic nonlinear dynamics, limited sensors for real-time measurements, limited actuation for control, and kinetic trapping of the system in undesirable configurations. Three main strategies for addressing these challenges are described, which include particle design (active self-assembly), open-loop control, and closed-loop (feedback) control. The strategies are illustrated using a variety of examples such as the design of patchy and Janus particles, the toggling of magnetic fields to induce the crystallization of paramagnetic colloids, and high-throughput crystallization of organic compounds in nanoliter droplets. An outlook of the future research directions and the necessary technological advancements for control of micro- and nano-scale self-assembly is provided.  相似文献   
6.
Model predictive control (MPC) has been effectively applied in process industries since the 1990s. Models in the form of closed equation sets are normally needed for MPC, but it is often difficult to obtain such formulations for large nonlinear systems. To extend nonlinear MPC (NMPC) application to nonlinear distributed parameter systems (DPS) with unknown dynamics, a data-driven model reduction-based approach is followed. The proper orthogonal decomposition (POD) method is first applied off-line to compute a set of basis functions. Then a series of artificial neural networks (ANNs) are trained to effectively compute POD time coefficients. NMPC, using sequential quadratic programming is then applied. The novelty of our methodology lies in the application of POD's highly efficient linear decomposition for the consequent conversion of any distributed multi-dimensional space-state model to a reduced 1-dimensional model, dependent only on time, which can be handled effectively as a black-box through ANNs. Hence we construct a paradigm, which allows the application of NMPC to complex nonlinear high-dimensional systems, even input/output systems, handled by black-box solvers, with significant computational efficiency. This paradigm combines elements of gain scheduling, NMPC, model reduction and ANN for effective control of nonlinear DPS. The stabilization/destabilization of a tubular reactor with recycle is used as an illustrative example to demonstrate the efficiency of our methodology. Case studies with inequality constraints are also presented.  相似文献   
7.
A variable structure control (VSC) method for unstable industrial processes is proposed. The proposed control method is able to provide a highly satisfactory system performance and to tackle with robustness issues of the processes in the presence of uncertainties. An ITAE-based numerical tuning algorithm for acquiring optimal control parameters, and a direct auto-tuning mechanism for the proposed controller are also provided. The performance of the proposed VSC method is illustrated on some unstable process models including a continuous stirred tank reactor (CSTR), in order to show its effectiveness, validity and feasibility.  相似文献   
8.
The development of Jacobian-free software for solving problems formulated by nonlinear partial differential equations is of increasing interest to simulate practical engineering processes. For the first time, this work uses the so-called derivative-free spectral algorithm for nonlinear equations in the simulation of flows in porous media. The model considered here is the one employed to describe the displacement of miscible compressible fluid in porous media with point sources and sinks, where the density of the fluid mixture varies exponentially with the pressure. This spectral algorithm is a modern method for solving large-scale nonlinear systems, which does not use any explicit information associated with the Jacobin matrix of the considered system, being a Jacobian-free approach. Two dimensional problems are presented, along with numerical results comparing the spectral algorithm to a well-developed Jacobian-free inexact Newton method. The results of this paper show that this modern spectral algorithm is a reliable and efficient method for simulation of compressible flows in porous media.  相似文献   
9.
We introduce a new architecture for the design of a tool for modeling and simulation of continuous and hybrid systems. The environment includes a compiler based on Modelica, a modular and a causal standard specification language for physical systems modeling (the tool supports models composed using certain component classes defined in the Modelica Standard Library, and the instantiation, parameterization and connection of these MSL components are described using a subset of Modelica). Models are defined in Modelica and are translated into DEVS models. DEVS theory (originally defined for modeling and simulation of discrete event systems) was extended in order to permit defining these of models. The different steps in the compiling process are show, including how to model these dynamic systems under the discrete event abstraction, including examples of model simulation with their execution results.  相似文献   
10.
In this paper, the presence of limit cycles in the behaviour of antagonistically coupled magneto-rheological (MR) actuators is investigated. The actuator considered in this paper offers high torque-to-mass and torque-to-inertia ratios, for inherent safe actuation. While the antagonistic arrangement is beneficial in improving the actuator performance and eliminating backlash, it may result in limit cycles when the actuator operates in a position control loop. The occurrence of a limit cycle depends on the parameters of the actuator as well as the controller. An in-depth analysis is carried out in this paper to establish a connection between the system parameters and the limit cycle occurrence. Moreover, sufficient conditions for avoiding limit cycles are derived specifically for a proportional-derivative (PD) controller. Simulations and experimental results validate the analysis and provide insights into conditions under which limit cycles are observed in the operation of antagonistic MR actuators.  相似文献   
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