Keywords Functional modeling behavioral modeling systems engineering requirements flowdown
Standards groups

Functional Modeling During Design A functional model is a graphical representation of the transformation of energy, material or information flows as they pass through a system. Functional models are frequently used during the
design of systems under the guise of schematics, flow-charts and process diagrams (all of which are graphical representations of the flows through a system along with the operations that are performed on them). For example, if you told an engineer with some experience in the field of hydraulics to design an open reservoir system that extends a single piston (with an external return force) and is actuated by a single manual control input, they would probably start by creating a schematic like the one in Figure 1. Such a schematic would be developed early in the design of a system to be used as a tool for identifying the basic functionality of the system and to drive specific component selection.

The problem with using diagrams such as this early in the design of systems is that their creation requires too many assumptions about the form of the solution. Rather than indicating, in general, what the system should do, such a diagram represents what basic component families will be used to solve
the problem (potentially before the problem has been fully understood). Instead, a less form-specific representation should be used during the early stages of design.

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Function-based Systems Engineering (FuSE) is a design method that uses functional modeling throughout the first three phases of engineering design: product planning, conceptual design and
embodiment design. The objective of the method is to implement existing and newly developed functional modeling based tools throughout the design of systems. Specific improvements over
current design methods include: A standardized functional modeling method that is applicable throughout the design process, conceptual functional models that limit form-specific assumptions and are used for identifying potential solutions to product functionality, form-specific functional models
that assist detailed behavioral model identification, behavioral model development based on functional models, well defined methods for identifying, modeling and evaluating solutions and improved identification and representation of auxiliary functions. The design method is introduced along with a motivating example based on an automotive powertrain.

Ryan S. Hutcheson,Daniel A. McAdams,Robert B.Stone,Irem Y. Tumer
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