Time for Reactive System Modeling

Reactive systems interact with their environment by reading inputs and computing and feeding back outputs in reactive cycles that are also called ticks. Often they are safety critical systems and are increasingly modeled with highlevel modeling tools. The concepts of the corresponding modeling languages are typically aimed to facilitate formal reasoning about program constructiveness to guarantee deterministic output and are explicitly abstracted from execution time aspects. Nevertheless, the worst-case execution time of a tick can be a crucial value, where exceedance can lead to lost inputs or tardy reaction to critical events. This thesis proposes a general approach to interactive timing analysis, which enables the feedback of detailed timing values directly in the model representation to support timing aware modeling. The concept is based on a generic timing interface that enables the exchangeability of the modeling as well as the timing analysis tool for the flexible implementation of varying tool chains. The proposed timing analysis approach includes visual highlighting and modeling pragmatics features to guide the user to timing hotspots for timing related model revisions

Rule-based simulation models

Procedural modeling systems, rule based modeling systems, and a method for converting a procedural model to a rule based model are described. Simulation models are used to represent real time engineering systems. A real time system can be represented by a set of equations or functions connected so that they perform in the same manner as the actual system. Most modeling system languages are based on FORTRAN or some other procedural language. Therefore, they must be enhanced with a reaction capability. Rule based systems are reactive by definition. Once the engineering system has been decomposed into a set of calculations using only basic algebraic unary operations, a knowledge network of calculations and functions can be constructed. The knowledge network required by a rule based system can be generated by a knowledge acquisition tool or a source level compiler. The compiler would take an existing model source file, a syntax template, and a symbol table and generate the knowledge network. Thus, existing procedural models can be translated and executed by a rule based system. Neural models can be provide the high capacity data manipulation required by the most complex real time models

A global method for coupling transport with chemistry in heterogeneous porous media

Modeling reactive transport in porous media, using a local chemical equilibrium assumption, leads to a system of advection-diffusion PDE's coupled with algebraic equations. When solving this coupled system, the algebraic equations have to be solved at each grid point for each chemical species and at each time step. This leads to a coupled non-linear system. In this paper a global solution approach that enables to keep the software codes for transport and chemistry distinct is proposed. The method applies the Newton-Krylov framework to the formulation for reactive transport used in operator splitting. The method is formulated in terms of total mobile and total fixed concentrations and uses the chemical solver as a black box, as it only requires that on be able to solve chemical equilibrium problems (and compute derivatives), without having to know the solution method. An additional advantage of the Newton-Krylov method is that the Jacobian is only needed as an operator in a Jacobian matrix times vector product. The proposed method is tested on the MoMaS reactive transport benchmark.Comment: Computational Geosciences (2009) http://www.springerlink.com/content/933p55085742m203/?p=db14bb8c399b49979ba8389a3cae1b0f&pi=1

Timing diagrams add Requirements Engineering capability to Event-B Formal Development

Event-B is a language for the formal development of reactive systems. At present the RODIN toolkit [15] for Event-B is used for modeling requirements, specifying refinements and doing verification. In order to extend graphical requirements modeling capability into the real-time domain, where timing constraints are essential, we propose a Timing diagram (TD) [13] notation for Event-B. The UML 2.0 based notation provides an intuitive graphical specification capability for timing constraints and causal dependencies between system events. A translation scheme to Event-B is proposed and presented. Support for model refinement is provided. A partial case study is used to demonstrate the translation in practice

Prototype of Fault Adaptive Embedded Software for Large-Scale Real-Time Systems

This paper describes a comprehensive prototype of large-scale fault adaptive embedded software developed for the proposed Fermilab BTeV high energy physics experiment. Lightweight self-optimizing agents embedded within Level 1 of the prototype are responsible for proactive and reactive monitoring and mitigation based on specified layers of competence. The agents are self-protecting, detecting cascading failures using a distributed approach. Adaptive, reconfigurable, and mobile objects for reliablility are designed to be self-configuring to adapt automatically to dynamically changing environments. These objects provide a self-healing layer with the ability to discover, diagnose, and react to discontinuities in real-time processing. A generic modeling environment was developed to facilitate design and implementation of hardware resource specifications, application data flow, and failure mitigation strategies. Level 1 of the planned BTeV trigger system alone will consist of 2500 DSPs, so the number of components and intractable fault scenarios involved make it impossible to design an `expert system' that applies traditional centralized mitigative strategies based on rules capturing every possible system state. Instead, a distributed reactive approach is implemented using the tools and methodologies developed by the Real-Time Embedded Systems group.Comment: 2nd Workshop on Engineering of Autonomic Systems (EASe), in the 12th Annual IEEE International Conference and Workshop on the Engineering of Computer Based Systems (ECBS), Washington, DC, April, 200

An Investigative Study on Impact of Frequency Dynamics in Load Modeling

Load modeling plays a significant impact in assessing power system stability margin, control, and protection. Frequency in the power system is desired to be kept constant, but in a real sense, it is not constant as loads continually change with time. In much literature, frequency dynamics are ignored in the formulation of load models for the basic assumption that it does not affect the models.  In this paper, the composite load model was formulated with Voltage-Frequency Dependency (V-FD) on real and reactive powers and applied to estimate the load model. 2- Area network 4- machines Kundur test network was used for testing the developed model.  The model was trained with measurements from a low voltage distribution network supplying the Electrical Engineering department at Ahmadu Bello University, Zaria. Both training and testing data were captured under normal system operation (dynamics). To evaluate the V-FD model performance, Voltage-Dependent (VD) model was examined on the same measured data. The work makes use of the Feed Forward Neural Network (FFNN) as a nonlinear estimator. Results obtained indicate that including frequency dynamics in modeling active power reduces the accuracy of the model. While in modeling reactive power the model performance improves. Hence, it can be said that including frequency dynamics in load modeling depends on the intended application of the model

Modeling Chemical Reactivity in Aqueous and Organic Systems: From Electronic Structure Methods to Force Field Development

Modeling reactivity in chemical systems has evolved dramatically in line with the capabilities of modern computing. Despite the advances in computational ability, the level in which one can model a system depends on a number of factors including the region of reactivity, size of the system, level of sophistication required in the molecular description, and so on. Electronic structure methods allow for a detailed description of the potential energy surface and inherently include all essential physics required for reactivity to occur, however these methods are limited by their computational expense. On the other hand, force fields allow for an atomistic description of the interactions and drastically reduce the simulation time, yet typical force fields are dependent on a fixed bond topology, and as such, cannot model bond cleavage and formation. This dissertation addresses modeling reactivity from electronic structure methods to force field development for reactive systems. The first section of the dissertation will focus on the hydrated HCl system. Accurately modeling covalent HCl, as well as ionization and subsequent proton shuttling, is essential in systems such as gas-liquid nucleation in the atmosphere, concentrated acid solutions, and HCl at the air-water interface. The amount of sampling required for gas-liquid nucleation pathways, or simulation time for large system sizes in the case of concentrated acid simulations necessitates an expedient description of the potential energy surface. To this end, a reactive force field has been developed. In order to determine the solvent environment factors required for an accurate force field description, ab initio molecular dynamics and metadynamics have been performed on HCl(H2O)n(n=1-22). These simulations will be discussed in chapter two, while the development and performance of a reactive force field based on the multi-state empirical bond formalism will be described in chapter three. The second section of the dissertation will focus on modeling reactivity with electronic structure methods for two organic systems. The systems range from determining the factors guiding the regioselectivity of silyloxyallyl cations by analyzing reaction profiles, SAPT energy decomposition, and molecular orbital analysis (chapter four), to the formation of an EDA complex and the corresponding charge transfer (chapter five)

Steepest-entropy-ascent nonequilibrium quantum thermodynamic framework to model chemical reaction rates at an atomistic level

The steepest entropy ascent (SEA) dynamical principle provides a general framework for modeling the dynamics of nonequilibrium (NE) phenomena at any level of description, including the atomistic one. It has recently been shown to provide a precise implementation and meaning to the maximum entropy production principle and to encompass many well-established theories of nonequilibrium thermodynamics into a single unifying geometrical framework. Its original formulation in the framework of quantum thermodynamics (QT) assumes the simplest and most natural Fisher-Rao metric to geometrize from a dynamical standpoint the manifold of density operators, which represent the thermodynamic NE states of the system. This simplest SEAQT formulation is used here to develop a general mathematical framework for modeling the NE time evolution of the quantum state of a chemically reactive mixture at an atomistic level. The method is illustrated for a simple two-reaction kinetic scheme of the overall reaction F + H2 HF + F in an isolated tank of fixed volume. However, the general formalism is developed for a reactive system subject to multiple reaction mechanisms. To explicitly implement the SEAQT nonlinear law of evolution for the density operator, both the energy and the particle number eigenvalue problems are set up and solved analytically under the dilute gas approximation. The system-level energy and particle number eigenvalues and eigenstates are used in the SEAQT equation of motion to determine the time evolution of the density operator, thus, effectively describing the overall kinetics of the reacting system as it relaxes towards stable chemical equilibrium. The predicted time evolution in the near-equilibrium limit is compared to the reaction rates given by a standard detailed kinetic model so as to extract the single time constant needed by the present SEA model

Modeling and analysis of variable reactive power limits of a Doubly Fed Induction Generator (DFIG) used in variable speed wind turbines

In this thesis, the mathematical modeling of variable reactive power limits for a Doubly Fed Induction Generator (DFIG) based on its capability curves is presented. Reactive power limits have been adjusted dynamically based on the capability curves when the system was subjected to a disturbance. This ensures that the operation of the DFIG is always within safe limits and utilizes the available capability of the DFIG to improve the performance of the system.The small signal stability of the system is studied by considering the load as a variable parameter. The differential-algebraic model of the DFIG, synchronous generators and their associated controllers and the power system network is linearized and bifurcation analysis considering the load as the bifurcation parameter has been performed. The PV curves, the stator and rotor current magnitudes, and the eigenvalue trajectories are plotted as the load is varied. Time domain simulations are performed to observe the change in stator and rotor currents when the system is subjected to a load change and a change in the wind speed.The system considered for testing is the IEEE 9 bus system which is modified to include a wind farm consisting of 5 wind turbines. The variable reactive power limits are implemented in the reactive power controller of the DFIG and the performance of the system is compared to that of the system with fixed limits in the reactive power controller. From the bifurcation analysis, it was observed that the stator and rotor currents were at the maximum limits when the lower and upper limits of the controller were reached. Also, the Hopf bifurcation was found to occur at a lower load level compared to the system with fixed reactive power limits. From the time domain simulations, it was observed that the stator and rotor currents did not exceed the maximum limits in the system with variable reactive power limits when the system was subjected to a change in the load and a change in the wind speed. Hence, the problem of over/under estimating the reactive power capability of the DFIG based wind farm was avoided