Pieter J. Mosterman

Diagnosis of continuous valued systems in transient operating regions

The complexity of present day embedded systems (continuous processes controlled by digital processors), and the increased demands on their reliability motivate the need for monitoring and fault isolation capabilities in the embedded processors. This paper develops monitoring, prediction, and fault isolation methods for abrupt faults in complex dynamic systems. The transient behavior in response to these faults is analyzed in a qualitative framework using parsimonious topological system models. Predicted transient effects of hypothesized faults are captured in the form of signatures that specify future faulty behavior as higher order time-derivatives. The dynamic effects of faults are analyzed by a progressive monitoring scheme till transient analysis mechanisms have to be suspended in favor of steady state analysis. This methodology has been successfully applied to monitoring of the secondary sodium cooling loop of a fast breeder reactor.

An Overview of Hybrid Simulation Phenomena and Their Support by Simulation Packages

Hybrid systems combine continuous behavior evolution speci fied by differential equations with discontinuous changes specified by discrete event switching logic. Numerical simulation of continuous behavior and of discrete behavior is well understood. However, to facilitate simulation of mixed continuous/discrete systems a number of specific hybrid simulation issues must be addressed. This paper presents an overview of phenomena that emerge in simulation of hybrid systems, reported in previously published literature. They can be classified as (i) event handling, (ii) run-time equation processing, (iii) discontinuous state changes, (iv) event iteration, (v) comparing Dirac pulses, and (vi) chattering. Based on these phenomena, numerical simulation requires the implementation of specific hybrid simulation features. An evaluation of existing simulation packages with respect to these features is presented.

Computer Automated Multi-Paradigm Modeling: An Introduction

Modeling and simulation are quickly becoming the primary enablers for complex system design. They allow the representation of intricate knowledge at various levels of abstraction and allow automated analysis as well as synthesis. The heterogeneity of the design process, as much as of the system itself, however, requires a manifold of formalisms tailored to the specific task at hand. Efficient design approaches aim to combine different models of a system under study and maximally use the knowledge captured in them. Computer Automated Multi-Paradigm Modeling (CAMPaM) is the emerging field that addresses the issues involved and formulates a domain-independent framework along three dimensions: (1) multiple levels of abstraction, (2) multiformalism modeling, and (3) meta-modeling. This article presents an overview of the CAMPaM field and shows how transformations assume a central place. These transformation are, in turn, explicitly modeled themselves by graph grammars.

A THEORY OF DISCONTINUITIES IN PHYSICAL SYSTEM MODELS

Abstract Physical systems are by nature continuous, but often display nonlinear behaviors that make them hard to analyze. Typically, these nonlinearities occur at a time scale that is much smaller than the time scale at which gross system behavior needs to be described. In other situations, nonlinear effects are small and of a parasitic nature. To achieve efficiency and clarity in building complex system models, and to reduce computational complexity in the analysis of system behavior, modelers often abstract away any parasitic component parameter effects, and analyze the system at more abstract time scales. However, these abstractions often introduce abrupt, instantaneous changes in system behavior. To accommodate mixed continuous and discrete behavior, this paper develops a hybrid modeling formalism that dynamically constructs bond graph model fragments that govern system behavior during continuous operation. When threshold values are crossed, a meta-level control model invokes discontinuous state and model configuration changes. Discontinuities violate physical principles of conservation of energy and continuity of power , but the principle of invariance of state governs model behavior when the control module is active. Conservation of energy and continuity of power again govern behavior generation as soon as a new model configuration is established. This allows for maximally constrained continuous model fragments. The two primary contributions of this paper are an algorithm for inferring the correct new mode and state variable values in the hybrid modeling framework, and a verification scheme that ensures hybrid models conform to physical system principles based on the principles of divergence of time and temporal evolution in behavior transitions . These principles are employed in energy phase space analysis to verify physical consistency of models.

A Combined Qualitative/Quantitative Approach for Fault Isolation in Continuous Dynamic Systems

Abstract The TRANSCEND system for fault detection and isolation in continuous systems uses qualitative reasoning methods to analyze transients caused by abrupt faults. Qualitative transient analysis avoids some of the computational difficulties associated with numerical schemes, but they lack discriminating power. This paper presents the formal basis for qualitative transient analysis, and then establishes the limits of the discriminatory power of this methodology. An integrated scheme that starts with qualitative fault isolation to narrow down possible fault hypotheses, and then uses a focused quantitative parameter estimation scheme to identify the true fault is developed. This approach provides a number of advantages over purely quantitative FDI schemes.

A comprehensive methodology for building hybrid models of physical systems

Abstract This paper describes a comprehensive and systematic framework for building mixed continuous/discrete, i.e., hybrid physical system models. Hybrid models are a natural representation for embedded systems (physical systems with digital controllers) and for complex physical systems whose behavior is simplified by introducing discrete transitions to replace fast, often nonlinear dynamics. In this paper we focus on two classes of abstraction mechanisms, viz., time scale and parameter abstractions, discuss their impact on building hybrid models, and then derive the transition semantics required to ensure that the derived models are consistent with physical system principles. The transition semantics are incorporated into a formal model representation language, which is used to derive a computational architecture for hybrid systems based on hybrid automata. This architecture forms the basis for a variety of hybrid simulation, analysis, and verification algorithms. A complex example of a colliding rod system demonstrates the application of our modeling framework. The divergence of time and behavior analysis principles are applied to ensure that physical principles are not violated in the definition of the discrete transition model. The overall goal is to use this framework as a basis for developing systematic compositional modeling and analysis schemes for hybrid modeling of physical systems. Preliminary attempts in this area are discussed, with thoughts on how to develop this into a more general methodology.

Zero-Crossing Location and Detection Algorithms For Hybrid System Simulation

Abstract Computational models of embedded control systems often combine continuous-time with discrete-event behavior, mathematically representing hybrid dynamic systems. An essential element of numerical simulation of a hybrid dynamic system is the generation of discrete events from continuous variables that exceed thresholds. In particular, the occurrence of such an event has to be detected and the point in time where the threshold is first exceeded has to be located. This paper presents a number of problems that are encountered in event detection and location when using existing techniques. Solution strategies that balance efficiency and robustness are presented to address: (i) repeated detection of a zero-crossing event at consecutive time steps, (ii) masked zero-crossing events because of multiple zero-crossing functions, and (iii) chattering and Zeno behavior.

Computer-automated multiparadigm modeling in control systems technology

The use of model-based technologies has made it imperative for the development of a feedback control system to deal with many different tasks such as: plant modeling in all its variety; model reduction to achieve a complexity or level of abstraction suitable for the design task at hand; synthesis of control laws that vary from discrete event reactive control to continuous model predictive control, their analyses, and testing; design of the implementation; modeling of the computational platform and its operating system; analysis of the implementation effects; software synthesis for different platforms to facilitate rapid prototyping, hardware-in-the-loop simulation, etc. Throughout these tasks, different formalisms are used that are very domain specific (e.g., tailored to electrical circuits, multibody systems, reactive control algorithms, communication protocols) and that often need to be coupled, integrated, and transformed (e.g., a block diagram control law in the continuous domain has to be discretized and then implemented in software). Significant improvements in many aspects (performance, cost, development time) of the development process can therefore be achieved by: 1) relating and integrating these different formalisms; 2) automatic derivation of different levels of modeling abstractions; and 3) rigorous and tailored design of the different formalisms by capturing the domain (or meta) knowledge. The emerging field of computer automated multiparadigm modeling (CAMPaM), presented in this paper in the context of control system design, aims to develop a domain-independent formal framework that leverages and unifies different activities in each of these three dimensions.

HYBRSIM—a modelling and simulation environment for hybrid bond graphs

Abstract Bond graphs are a powerful formalism to model continuous dynamics of physical systems. Hybrid bond graphs introduce an ideal switching element, the controlled junction, to approximate continuous behaviour that is too complex for numerical analysis (e.g. because of non-linearities or steep gradients). HYBRSIM is a tool for hybrid bond graph modelling and simulation implemented in Java and is documented in this paper. It performs event detection and location based on a bisectional search, handles run-time causality changes, including derivative causality, performs physically consistent (re-)initialization and supports two types of event iteration because of dynamic coupling. It exports hybrid bond graph models in Java and C/C++ code that includes discontinuities as switched equations (i.e. pre-enumeration is not required).

Design and implementation of an electronics laboratory simulator

This paper describes the design and implementation of a computer-simulated laboratory for use in undergraduate engineering education. The simulated laboratory is implemented in a Windows environment. Several forms of tutorials and other assistance are available to the user to complete the laboratory. Evaluations indicate that when the simulation is used with class lectures, there is a significant improvement in both theory and lab knowledge. Use of the simulation significantly cuts subsequent time and requests for assistance in the physical lab.