Functional Design

Engineering of complex systems requires decomposing system-level functionality so that it can be assigned to components.  The Hughes Aircraft developed an approach for this for General Motors Systems Engineering.  The figure below shows the relationship between system requirements, functional/performance analysis and component design.  

Main principles behind the diagram:

• The system should perform valuable functions consistent with the environment where it is deployed (Car provides transportation)

• A system can be viewed as a two forms of composites:  functional partitioning vs physical partitioning.

• Some of the system requirements can be allocated directly to a physical component

• Some of the system requirements are complex functions and require analysis and tradeoffs (Range of an EV)

• Results of a functional  analysis are recorded in a performance spec (Range is affected by mass, battery, tires, drag)

• Functional (performance) specs allocate derived requirements to components.

• A baseline is established when all system requirements have been decomposed and allocated to components. (typically the PDR baseline)

• Iteration of component design and implement with revised requirements continues during product development.

This process was applied to engineering of GM’s EV1 electric vehicle.  Range was the most challenging system function requirement.  The attached 3/5 card with the parameters affecting range was used by the EV-1 team.  The card is a summary of a Range functional analysis/spec.