A model (glossary) is an abstraction of a system that offers insight about one or more of the the system's aspects, such as its function, behavior, structure, properties, or cost. Modeling is a common practice that is shared by most engineering disciplines, such as (1) electrical engineering, which uses electrical circuit design models, (2) mechanical engineering, which uses three-dimensional computer-aided design models, and (3) software engineering, which uses software design models. Each of these disciplines has their own language, with its syntax and semantics, serving as a means of communication among professionals in that discipline. Analytic models are used to support power, thermal, structural, and embedded real-time analysis.
Modeling of systems as holistic, value-providing entities, has been gaining recognition as a central process of systems engineering (glossary). Modeling serves to make concepts concrete and formal, enhance quality, productivity, documentation, and innovation, as well as reduce the cost and risk of systems development. Different types of models may be needed to represent systems in support of the analysis, specification, design, and verification of systems. This knowledge area provides an overview of models used to represent different aspects of systems.
What is a Model?
This topic provides foundational concepts, such as definitions of a model and a modeling language, and expresses their relationships to modeling tools (glossary) and model-based systems engineering (MBSE).
Definition of a Model
There are many definitions of the word model. In the context of systems engineering, a model that represents a system and its environment is of particular importance to the system engineer who must analyze, specify, design, and verify systems, as well as share information with other stakeholders. A variety of system models are used to represent different types of systems for different modeling purposes. The modeling standards topic refers to some of the standard system modeling languages and other modeling standards that support MBSE.
A physical model can be a mockup that represents an actual system, such as a model airplane.
A mathematical model may represent possible flight trajectories in terms of acceleration, speed, position, and orientation. A logical model may represent logical relationships that describe potential causes of airplane failure, such as how an engine failure can result in a loss of power and cause the airplane to lose altitude, or how the parts of the system are interconnected. It is apparent that many different models may be required to represent a system-of-interest (SoI).
Modeling Tools
Models are created by a modeler using modeling tools. For physical models, the modeling tools may include drills, lathes, and hammers. For more abstract models, the modeling tools are typically software programs running on a computer. These programs provide the ability to express modeling constructs using a particular modeling language.
A word processor can be viewed as a tool used to build text descriptions using natural language. In a similar way, modeling tools are used to build models using modeling languages. The tool often provides a tool palette to select symbols and a content area to construct the model from the graphical symbols or other concrete syntax. A modeling tool typically checks the model to evaluate whether it conforms to the rules of the language, and enforces such rules to help the modeler create a well-formed model. This is similar to the way a word processor checks the text to see that it conforms to the grammar rules for the natural language.
Some modeling tools are commercially available products, while others may be created or customized to provide unique modeling solutions. Modeling tools are often used as part of a broader set of engineering tools which constitute the systems development environment. There is increased emphasis on tool support for standard modeling languages that enable models and modeling information to be interchanged among different tools.
Relationship of Model to Model-Based Systems Engineering
The International Council of Systems Enginnering (INCOSE) INCOSE Systems Engineering Vision 2020 (2007) defines MBSE as “the formalized application of modeling to support system requirements, design, analysis, verification, and validation activities beginning in the conceptual design phase and continuing throughout development and later life cycle phases.” In MBSE, the models of the system are primary artifacts of the systems engineering process, and are managed, controlled, and integrated with other parts of the system technical baseline.
This contrasts with the traditional document-centric approach to systems engineering, where text-based documentation and specifications are managed and controlled. Leveraging a model-based approach to systems engineering is intended to result in significant improvements in system specification and design quality, lower risk and cost of system development by surfacing issues early in the design process, enhanced productivity through reuse of system artifacts, and improved communications among the system development team.
In addition to creating models, the MBSE approach typically includes methods for model management which aim to ensure that models are properly controlled and methods for model validation which aim to ensure that models accurately represent the systems being modeled.