Page 171 - Design for Six Sigma a Roadmap for Product Development

144 Chapter Five

The team will study the datum entities against the functional require-
ments generated by phase 2 QFD in order to check whether at least one
solution, a design entity, exists that is the approximate physical trans-
lation of the functional requirements. If the answer is “Yes” (Fig. 5.2),
then the selected entity should be within a slight variation from the
pursued design. The team may declare the project as an incremental
design problem and work toward improvements in order to satisfy the
customer requirements, progressing from the datum design as a start-
ing point. Adding, replacing, or eliminating design parameters (DPs)
without altering the FRs is our definition of an “incremental design.”
The “creative design” includes alterations made in the incremental
design case plus alterations made to the FR array. In the incremental
design scenario, if more than one datum entity exists, the best entity
could be selected using the Pugh concept selection method. In the
absence of datum entities, the only option is the creative design, which
requires more conceptualizing work and, therefore, more extensive
deployment of TRIZ and axiomatic design methods.
In summary, the objective of specifying nominal and tolerances of the
FRs and the DPs is to verify structure choices for functional solution
entity elements and interfaces. A structure can be defined as an input-
output or cause-and-effect relationship of functional elements.
Mathematically, it can be captured by design mappings such as QFD
and the zigzagging method of axiomatic design. Graphically, it is
depicted in a “block” diagram that is composed from nodes connected by
arrows depicting the relationships. A structure should capture all
design elements within the scope and ensure correct flowdown to criti-
cal parameters. A structure is captured mathematically using mapping
matrices, and matrices belonging to the same hierarchical level are
clustered together. Hierarchy is built by the decomposing design into a
number of simpler functional design matrices that collectively meet the
high-level functional requirements identified in step 2 of the DFSS
algorithm. Two structures are recognized in the DFSS algorithm:

■ The physical structure between the functional requirements (FRs)
and the design parameters (DPs)
■ The process structure between the DPs and the process variables
(PVs)

The physical structure is usually developed first to define the
design concept. Preliminary concepts are best selected using the
Pugh selection method. The preliminary work to verify structural
choices should help the DFSS team get started on concept genera-
tion. The team needs to select the best solution entity element tech-
nologies in terms of design parameters (DPs) to meet or exceed