Functional Dependency Models
A model for Diagnostics, Prognostics, Testability and System Engineering

Areas of Applicability: (comparison chart)
 Diagnostics / Prognostics  System Engineering  Maintainability
 Testability  Reliability  Life Cycle Cost
 Design-for-Test  Safety  Other

What - Functional Dependency Modeling refers to the approach whereby the functionality of a system is captured in a model, rather than trying to describe its failures. This concept follows the logic that it is easier to describe how a system works than to describe how a system fails.

Why - Functional Dependency Models are the first step towards allowing conceptual designs to be entered, without having to rebuild the whole model for later analyses. The drawback to a functional-only model is that it does not allow failure modes to be added, which can affect how the model is built and the degree to which diagnostics can match real-world testing.

When - Functional Dependency Models are easily built during concept phase for up-front optimization. The ability for the functional dependency model to support analysis throughout the design process depends on the degree to which failure mode testing is performed. Failure mode testing mandates that the functional model be changed in some way that could obfuscate the model. Also, because failure modes can not be directly handled, FMECAs are not easily integrated, making this model less amenable to support reliability engineering efforts.

History - The Functional Dependency Model was first formalized in the 1990's by a product called eXpress. This product was another product of the Dependency Modeling approach, its predecessor being called "STAT"--a pure dependency modeling tool. It is noteworthy that due to the limitation of not being able to enter failure modes, the eXpress tool has already moved into a Hybrid Model approach.

Related Links - Failure Mode Dependency ModelsHybrid Models