4 Preliminary System Design

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4 Preliminary System Design

• also known as advance development

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Preliminary System Design
Preliminary system design extends the translation
of system-level requirements into design
requirements for the subsystem level,
configuration-item level, and below. Included is
an extension of functional analysis and
requirements allocation from the baseline
described in Section 3.6, to the depth needed to
identify specific requirements for hardware,
software, people, facilities, logistic support
elements, and related resources.
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Preliminary System Design
Trade-off studies are conducted and the results
described in the form of an allocated baseline
(i.e., development, product, process, and
material speci-fications). This chapter continues
the top-down approach and presents refined
functional analysis and allocation (refer to
Figure 2.4, Blocks 1.1 and 1.2), detailed
trade-off studies (Block 1.3), synthesis and
evaluation at the subsystem level (Blocks 1.4 and
1.5), and a final description of the major
elements of the system.
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Functional Analysis

• As described previously functional analysis is
  the iterative process of breaking down (or
  decomposing) requirements from the system level,
  to the subsystem level, and as far down the
  hierarchical system structure as necessary to
  describe functional interfaces and identify
  resource needs adequately. These resources are in
  the form of hardware, software, people,
  facilities, data or combinations thereof. The
  functional analysis evolves from an identified
  need, the results of the feasibility analysis,
  the definition of system operational
  requirements, and the maintenance concept.

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Functional Analysis

• Functional flow diagrams (or functional block
  diagrams) are developed to describe the system in
  functional terms. These are developed to reflect
  both operational and support activities as they
  occur throughout the system life cycle, and they
  are structured in a manner that illustrates the
  hierarchal aspects of a system (see Figure 3.11).

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Functional Analysis
The general sequence of steps leading from the
identification of a need to the functional
analysis is illustrated in Figure 4.3. Although
the functional block diagram covers events
through the entire life cycle, Block 9 has been
expanded to cover the operational scenario
illustrated (i.e.. fly aircraft from point A to
point B). This scenario may be decomposed into
segments, indicating the whats at a lower
level. Eventually, one needs to convert to the
"hows" resulting in the identification of a
specific UHF communication subsystem (refer to
Block 9.5.1).
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Requirement Allocation

• Lower-level elements of the system are defined
  by combining (or grouping) similar functions into
  logical subdivisions, identifying major
  subsystems, configuration units, and so forth
  (refer to Figure 3.13). Figure 4.6 presents a
  summary of this process, evolving from a
  functional definition of System XYZ to the
  packaging of the system into three units (i.e..
  Unit A, Unit B and Unit C).

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Requirement Allocation

• Given the "packaging" concept shown in Figure
  4.6, it is now appropriate to apply the TPMs to
  the units. The objective is to allocate the
  system-level "design-to" requirements down to
  Unit A, Unit B, and Unit C, and to develop
  specific qualitative and quantitative "design-to"
  requirements for each of the units (i.e., DDPs).
  Figure 4.8 shows the results of an allocation of
  System XYZ requirements to the unit level and the
  assembly level. The objective is to establish
  criteria or goals for the design, or procurement
  of the three units and the assemblies.

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Requirement Allocation

• The purpose of allocation is to provide
  guidelines to assist design engineers develop or
  select a product (equipment, software, personnel,
  data, etc.) that will ultimately be compatible
  with, and contribute to meeting system-level
  requirements. That is, when combining and
  integrating the units in Figure 4.8, the results
  must be in compliance with system requirements.
  This usually requires an iterative process
  because it may not always be possible to meet the
  allocated parameter values with the available
  technology exactly. Trade-offs are made, the
  allocations are revised as necessary, and the
  best approach is defined as a model.

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Engineering Design Technologies

• To design is to project and evaluate ideas for
  bringing a needed system or product into being.
  For the design process to yield a good result,
  numerous abstractions (design alternatives) must
  be formulated and evaluated. This must be
  accomplished accurately, with relative ease, and
  in a limited time. The design team needs to
  converge to a final design which meets the need
  effectively and efficiently. Several
  computer-based technologies are emerging to
  facilitate this design assurance.
• CAD, CAED, CAD / CAM and CALS

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Synthesis and Design Definition

• Synthesis refers to the combining and
  structuring of parts and elements in such a
  manner so as to form a functional entity. System
  synthesis is achieved when sufficient preliminary
  design progress has been made and trade-off
  studies have been accomplished to confirm and
  assure the completeness of system performance and
  other design requirements (as allocated for
  detail design).

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System Design Reviews

• Design is a progression from a defined need to
  an entity that will perform a useful function in
  a satisfactory manner. Design involves through a
  series of stages conceptual design, preliminary
  system design, and detail design and development.
  In each major stage of the design process, an
  evaluative function is accomplished to ensure
  that the design is in compliance with the
  specified requirements before proceeding with the
  next stage. Deviations are noted, and the
  necessary corrective action should be initiated
  as appropriate.